Global Change and Forest Disturbances in the Mediterranean Basin: Breakthroughs, Knowledge Gaps, and Recommendations

Abstract

Forest ecosystems in the Mediterranean Basin are mostly situated in the north of the Basin (mesic). In the most southern and dry areas, the forest can only exist where topography and/or altitude favor a sufficient availability of water to sustain forest biomass. We have conducted a thorough review of recent literature (2000–2021) that clearly indicates large direct and indirect impacts of increasing drought conditions on the forests of the Mediterranean Basin, their changes in surface and distribution areas, and the main impacts they have suffered. We have focused on the main trends that emerge from the current literature and have highlighted the main threatens and management solution for the maintenance of these forests. The results clearly indicate large direct and indirect impacts of increasing drought conditions on the forests of the Mediterranean Basin. These increasing drought conditions together with over-exploitation, pest expansion, fire and soil degradation, are synergistically driving to forest regression and dieback in several areas of this Mediterranean Basin. These environmental changes have triggered responses in tree morphology, physiology, growth, reproduction, and mortality. We identified at least seven causes of the changes in the last three decades that have led to the current situation and that can provide clues for projecting the future of these forests: (i) The direct effect of increased aridity due to more frequent and prolonged droughts, which has driven Mediterranean forest communities to the limit of their capacity to respond to drought and escape to wetter sites, (ii) the indirect effects of drought, mainly by the spread of pests and fires, (iii) the direct and indirect effects of anthropogenic activity associated with general environmental degradation, including soil degradation and the impacts of fire, species invasion and pollution, (iv) human pressure and intense management of water resources, (v) agricultural land abandonment in the northern Mediterranean Basin without adequate management of new forests, (vi) very high pressure on forested areas of northern Africa coupled with the demographic enhancement, the expansion of crops and higher livestock pressure, and the more intense and overexploitation of water resources uses on the remaining forested areas, and (vii) scarcity and inequality of human management and policies, depending on the national and/or regional governments and agencies, being unable to counteract the previous changes. We identified appropriate measures of management intervention, using the most adequate techniques and processes to counteract these impacts and thus to conserve the health, service capacity, and biodiversity of Mediterranean forests. Future policies should, moreover, promote research to improve our knowledge of the mechanisms of, and the effects on, nutrient and carbon plant-soil status concurrent with the impacts of aridity and leaching due to the effects of current changes. Finally, we acknowledge the difficulty to obtain an accurate quantification of the impacts of increasing aridity rise that warrants an urgent investment in more focused research to further develop future tools in order to counteract the negative effects of climate change on Mediterranean forests.

1. Background. Mediterranean Forest: A System Shaped by Water, Nutrient Limitations, and Human Management. The Mediterranean Basin Case

Five coastal regions of the continents between latitudes of 34 and 45° in both hemispheres have Mediterranean climates: Mediterranean Basin, California, central Chile, the Cape region of South Africa and southwestern and southern Australia [1,2]. This Mediterranean climate has typically been associated with wet and cold winters and dry and hot summers, but with high variability, mainly in the intensity of the drier periods that increases from high to low latitudes in these areas [1,2]. The Mediterranean Basin has a large range of ecosystems, socioeconomic conditions, governmental policies, and long-term human influence [3,4]. The Mediterranean Basin is a global hotspot of biological diversity and the most diverse biome in Europe. Models, however, have projected a loss of area of about 11–25% by the end of this century [5,6], mainly due to the advance of desertification [6]. However, Alessandri et al. (2014) [7] using 25 different General Circulation Models projected that the Mediterranean climate could also be expanded at the end of the 21th century towards north-east by 740,000 km², a 19 ± 1% of the current Mediterranean are in the Mediterranean Basin. The current legacy of Mediterranean forests has nevertheless been modulated by historical socioeconomic pressures, leading to several unsustainable forest practices and frequently to the neglect by human society of forested land by human society [8]. The combination of climate change, anthropogenic disturbance (overexploitation of forest resources, human-induced fires, and deforestation) and other aspects of global change (particularly inappropriate land uses and pollution) will continue to affect Mediterranean forest vegetation [9]. Forests are, however, estimated to currently cover about 10% of the total land surface of the Mediterranean Basin [10]. This means that the total Mediterranean Basin has more than 25 million ha of forest plus 50 million ha of other wooded lands mainly associated to crop abandonment [11]. Between 2010 and 2015, forests in the Mediterranean Basin have increased their cover by 2%, but the level of forest degradation, and the vulnerabilities to climate change, population expansion, wildfires and aridity have also increased [11].

The Mediterranean Basin is especially sensitive to climate change due to the nature of transition between arid and temperate regions [12]. The increase in aridity in the basin has been widely demonstrated throughout the last decades [13,14,15,16]. Aridity is even projected to increase in the future under a projected 3–4 °C increase in temperature and 20% decrease in rainfall in forested areas [5,10,17,18]. Recent data indicate a temperature increase in the last century of about 0.85 °C globally and 1.3 °C in the Mediterranean area compared to temperatures for 1880–1920 [19]. Apart from the lower water content during most of the year, Mediterranean soils often suffer nutrient deficiencies, especially of nitrogen (N) and phosphorus (P) [18,20,21,22,23]. This low fertility is especially critical in forests due to their higher demands for water and nutrients [5,18]. The effects of higher CO₂ levels are also increasingly affecting foliar composition and P and N contents, further decreasing nutrient availability [24]. Although, the Mediterranean forest is still a global sink of carbon [10]. In 2005, the Mediterranean Basin forest accumulated 4.6 Mt y⁻¹ and this value can change between 3.3 to 5.97 Mt y⁻¹ in 2050 depending on the future socioeconomic scenarios [10].

The increase in forested area in the last 100–150 years is mainly due to forest regeneration in rural areas following continuous abandonment, as observed in several Mediterranean countries, and more recently it is also due to the European Common Agriculture Policy (as in Spain) [18,25]. Remote-sensing studies focusing on the overall Mediterranean Basin region have more recently indicated that the area of forests in the Mediterranean region has remained stable or slowly increase in the last few decades (see the “Depopulation and abandonment of rural areas” section below). A stable or increasing forest area, as defined by the FAO, though, tells us nothing about forest degradation [10]. To all these circumstances, we must add in Mediterranean forested areas the long-term influence of certain extensive use of forests to obtain several products such as food, cork, wood, etc. at once [18,25]. For example, the dehesas are agroforestry systems that have been managed in the long-term by humans endeavoring several activities such as raising pigs, bulls, and goats or growing cereals that coexist with an open forest dominated frequently by evergreen oaks, mainly Quercus robur L., Quercus pubescens Willd., Quercus ilex L., and Quercus rotundifolia Lam. [10,18]. These systems mostly occupy locations that without human intervention would mostly become evergreen holm oak forests, which would increase the risk of great and frequent fires [10]. These areas occupy 17.7% of the Mediterranean region [10] and are currently threatened by human abandonment and agricultural intensification which drives them to an uncertain future [10].

The high plant diversity that characterizes Mediterranean ecosystems is associated with the success of coexisting species in avoiding competition for soil resources by differential exploitation of space (soil layers) and time (yearly and daily) [18]. The long-term evolutionary adaptation of Mediterranean plants to drought thus allows them to cope with moderate increases in drought without excessive losses of production and survival in some species [18,26,27,28,29,30]. For example, increasing the allocation of resources to roots providing a deep root system helps capture soil nutrient and water from deep soil layers and protect against soil erosion [18]. This together with high translocation and resorption of foliar nutrients, large contents of recalcitrant compounds, and high conservative and efficient use of soil resources provide Mediterranean plants a high resistance to drought stress [18]. The current increase in aridity and its interactions with other disturbances such as fire, expansion of pest ranges, and plant invasion, however, may decrease this capacity for resistance, and may degrade soils, as it is already occurring in some areas of the southern Iberian Peninsula, France, and Italy [18,27,30,31,32,33,34,35].

This study thus aimed to (i) identify the current status of the diverse human and natural disturbances in Mediterranean forests over recent decades, (ii) identify the main causes of the changes in these forested areas responsible for the current situation, which can provide clues for projecting a future perspective for forested areas in the Mediterranean Basin, (iii) propose actions that could help improve the survival of the forest biome in the Mediterranean Basin, and (iv) identify obscure issues from all levels that help advance our knowledge for obtaining a global overview of the present and future evolution of this biome. The sites with observational/experimental data finally used in this study are highlighted in Figure 1.

2. Several Concurrent Risks

2.1. Present and Future Aridity: The Larger Threat

The current increase in aridity in the Mediterranean Basin in recent decades is associated with higher temperatures in southern Europe and northern Africa [35,36,37]. Most models project larger increases in aridity in the Mediterranean Basin [37,38,39,40] and a shift of current arid conditions towards northern latitudes [5]. Based on various scenarios of future climate projection, Barredo et al. (2016) [5] reported that the current climatic area of the basin would shift to other areas by the end of the 21st century and that the current area would decrease by 15–23%. Central and southern areas of the Iberian Peninsula, southern Italian Peninsula, some Mediterranean islands, central Turkey, North Africa, and southeastern Greece are the most affected areas, with larger shifts in the area of Mediterranean climate towards hotter and drier climates. Extreme precipitation and a general decrease in total precipitation and increase in temperature, however, are also expected [41,42]. A general increase in aridity, coupled with stronger and potentially destructive storms causing flood damage and extreme heatwaves, are thus possible [41,42,43]. These changes have already had several impacts on plant-soil systems in Mediterranean forested areas [18,44,45] and are expected to continue in the near future [18].

For example, a long-term (20 years) climatic manipulation in a holm oak (Quercus ilex) forest has produced a continuously lower availability of soil water similar to that projected for the near future and has demonstrated that the capacity of the trees to adapt to this level of drought is higher than previously expected [46]. Most species of tall evergreen shrubs, however, have increased in coverage and biomass more than Q. ilex [47,48], and the forest structure can be transformed into a shrubland structure under more frequent intense heatwaves and droughts [10,18,46,49].

To complete this scenario, we must consider that the Mediterranean climate has frequent torrential rains [50] that are projected to increase [51] and thus produce larger nutrient losses in the near future. In this context, the rise in drought periods together with more frequent extreme rain events can exert a strong impact on Mediterranean soil C:N:P ratios due to the different cycling and solubility/volatilization of these three elements [18]. This can in turn also have asymmetrical effects favoring more some species than others [18]. A cascade of feedback effects among increased aridity, more frequent torrential rains, more intense and frequent fires, and changes in land use have thus exceeded the capacity of Mediterranean plants to cope with drought and other stresses [52,53] (Figure 2). For example, several areas of central and southern Spain suffer from advanced stages of degradation and losses of biomass and soil, ultimately leading to desertification [18,54,55]. This scenario is not favorable for maintaining the current area of forests in the basin and thus for conserving their current structures, species compositions, ranges and diversities [18,56,57,58]. In fact, forests have clearly moved towards higher altitudes of the basin in the last few decades [59,60].

Several studies have consistently reported that annual growth and seed production in Mediterranean forests in recent decades are strongly correlated with annual drought intensity [61,62]. Warmer and drier conditions linked to increases in Atlantic multidecadal oscillations (AMO) are associated with the increase in post-1990 defoliation in the forests of Spain [63]. Experimental, observational, and metadata studies have clearly indicated that the decreases (dieback, defoliation, and lower growth) in Mediterranean oaks (Quercus spp.) and pines (Pinus spp.) in southern Europe are mainly due to more frequent drought, often interacting with higher temperatures (higher water demand) and pathogenic attack [64,65,66,67,68,69,70], all of which are associated with further decreases in nutrient-cycling rates, changes to soil trophic-web structure, and lower soil fertility [71,72].

Studies have also widely observed that tree dieback under drought conditions is associated with a depletion of reserves of nonstructural carbohydrates [73] and with a general increase in tree dormancy [74] and decrease in gross primary production (GPP) [75]. Mediterranean trees in their current distributions are thus frequently at the limit of their capacity of positive net primary production, so a further increase in aridity would be catastrophic [76,77,78]. The suitable period of growth of Q. ilex forests has been advanced by approximately 10 days by advancing the onset of spring by winter warming, but an early cessation of growth in spring and summer by approximately 26 days has reduced tree production and growth capacity [76]. This limiting situation has even been detected in the most xeric tree in the Mediterranean Basin, Pinus halepensis, whose populations in the semi-arid limit of its distribution in southern Europe have already been strongly affected by drought in the last two decades [77,78]. The overall Mediterranean domain is projected to shift to North in the coming decades [5,79].

Several Mediterranean forested areas are on steep slopes and thus frequently have shallow soils, so they are more sensitive to drought as observed in climate models in scenarios of atmospheric CO₂ concentrations affected, with reductions in growth during and after the severe droughts of the 1990s and rising [80]. Therefore, forests tend to be replaced with shrubland and even steppe vegetation as projected in climate models for scenarios of rising atmospheric CO₂ concentrations [81] (Figure 2). A recent study with satellite data found that an expansion of slow-growing evergreen hardwood Mediterranean species, forest dieback and defoliation of mainly non-Mediterranean species, and diseases and pests could be observed along the gradient between temperate wet and Mediterranean climates in western Europe [82]. Even replanting with the most drought-resistant Mediterranean tree species in desert areas of southeastern Spain has been negatively affected by the increasing frequency and intensity of drought, with more defoliation and decreased growth [83].

Some studies have observed how drought can limit the potential positive response to rising atmospheric CO₂ concentrations [84]. The expansion of shrublands at the expense of forests is thus expected from the interaction of more intense and extreme recurrence of droughts associated with warming and large fires [85,86,87,88]. Interestingly though, the increasing patchiness of forests, however, can increase the capacity of forests to resist future drought. Flores-Renteria et al. (2015) [89] reported that decreasing the fragment size of holm oak forests in central and southern Spain improved resistance to further drought by improving local environmental conditions, with higher water-use efficiency and availabilities of soil water and nutrients.

The situation of Mediterranean forest associated to global warming is especially critical in the North Africa Mediterranean forest. The impact of drought-aridity rise in the last few years of forest dieback in North Africa has been especially intense from 1970s–1980s and up to the current days [90,91,92,93,94,95,96,97,98]. This process is especially intense in several areas where the overgrazing pressure, the expansion of crops, and the urbanization, together with wood exploitation have continuously risen [98,99,100,101,102]. These strong and fast multiple effects of human related activities have proven to favor the most stress-tolerant species such as Quercus suber and be detrimental for species with higher water needs such as Cedrus atlantica in areas where they coexist [100].

2.1.1. Drought Is Currently Shaping the Structure of Mediterranean Forests

An increase in drought, however, may also have different effects in different forest Mediterranean species and communities. Mediterranean plants have developed various morphological and physiological strategies to adapt to drought [36], but the largest trees in a community are frequently more sensitive and less resistant and resilient to increases in aridity [36,103]. Sperlich et al. (2015) [104] observed that semi-deciduous and deciduous species responded differently from evergreen species to severe summer drought. The impact of drought on defoliation and dieback can asymmetrically affect individual trees based on size, even in the same species, shorter trees are more resistant and resilient to increases in drought than taller trees, both in intra- and interspecific comparisons [105]. Tall shrub species defoliate less than dominant canopy trees under more intense drought, so tall shrubs tend to produce more leaves in the upper canopy levels in a clear step towards replacing forests with shrubland [106,107]. The effects of drought across current environmental gradients of aridity may differ more between current xeric sites than mesic sites. Pasho et al. (2012) [108] reported that a specific decrease in annual precipitation was responsible for canopy defoliation in most xeric forests, dominated mainly by species such as P. halepensis Mill. and Q. ilex, whereas the worsened annual water balance was responsible for defoliation in more mesic forests, dominated by species such as P. sylvestris L., P. nigra Arnold or Abies alba Mill. Pasho et al. (2011) [109] found that the growth of species at xeric sites responded to the amount of spring and summer precipitation, whereas the growth of species at mesic sites responded more directly to the amount of summer precipitation only.

Most studies have nevertheless found that the effects of drought on the fitness of various species of Mediterranean forest trees were much more detrimental to mesic than to more xeric species [110,111,112]. Genera that have evolved under Mediterranean climatic conditions, mostly shrubs, however, were more damaged by the severe drought in 1994 in Spanish woodland areas than pre-Mediterranean species, mostly trees, that evolved earlier such as Pinus sp, Quercus sp. or Olea europea L. [49]. Contrasting results, though, have also been reported [113]. These asymmetrical effects are even more evident when comparing the Mediterranean with temperate and/or boreal tree species coexisting in mountainous areas within Mediterranean latitudes where the Mediterranean species are expanding to sites previously occupied by wet-temperate forest species [114].

Tall shrubs such as Phillyrea sp. have a higher capacity than trees to adapt and resist intensive droughts [115,116] and have a higher capacity than sympatric trees to maintain their foliage and concentrations of nonstructural carbohydrates after droughts [116]. Experimental and observational studies have clearly demonstrated a scenario of gradual replacement in communities from dominance by trees to dominance by tall shrubs [115,116].

2.1.2. Grazing Pressure under Drought Enhancement

Disturbance by livestock and over-exploitation have frequently increased, thus aggravating the negative effect of the reduction in natural precipitation [117,118]. These disturbances have been especially observed in savannoid woodlands and dehesas, the agroforestry human managed ecosystem where the Mediterranean evergreen genus Quercus is dominant [54,119]. This savonnoid system combines livestock exploitation with pigs and bovine with grain crops mainly located in the central and southern Iberian and Italian Peninsulas [54,119]. These systems are currently frequently accompanied by encroachment that has even worsened the tree status by increasing the competition for soil water between deep-rooted shrubs and trees [120].

However, the livestock pressure is decreasing very fast in large parts of the northern Mediterranean basin, to such an extent, that herbivory has fallen to levels unnaturally low [119]. This is leading to dense pioneer forests going hand in hand with massive loss of botanical diversity of small light demanding species that need levels of grazing [119]. The increasing frequency of annual extreme summer droughts in recent decades has been directly monitored at the plot level [121,122] or by multitemporal imagery from satellites such as Sentinel-2 [123]. These studies have found that extreme droughts can have more detrimental effects on Mediterranean forests than a long-term constant increase in drought intensity [122], and that the increasing frequency of extreme droughts decreases resistance and resilience of forest ecosystems, exacerbating the recovery of Mediterranean forests [121].

2.1.3. Aridity and Warming: An Open Door for Pests. The Case of “La Seca” (Phytophthora cinnamomi Rands) and Insect Outbreaks

The increases in heatwaves, warming, and droughts have been associated with pathogens in Mediterranean forests, such as the pine processionary moth (Thaumetopoea pityocampa Schiff), which has been expanding even more to northern forests and that has been correlated with a reduction in the growth of pine trees in several pine forests in the Mediterranean Basin [124,125,126].

The aggressive Phytophthora fungi (mainly P. cinnamomi), widely known in the Iberian Peninsula as “La seca” [127], however, is causing the largest areas of mortality. Infestation by P. cinnamomi is currently the main direct cause of declines in Q. ilex and Q. suber forests in several areas of southern Europe [128], with a large capacity to kill these two species [129]. The increasing spread of this pathogen has been favored by warmer temperatures and increased aridity [130,131,132,133,134], making it very difficult to discern between drought, rise in temperature or P. cinnamomi as the main cause of Quercus forest decline [135]. This pathogen is expected to spread further with the projected warmer temperatures and increase in drought in Mediterranean and sub-Mediterranean areas of southern Europe, mainly in Quercus forests [134,135,136,137]. Recent satellite images and photointerpretation studies in locations of affected trees in southwestern Spain from 2001 to 2016 have identified spatial patterns consistent with the hypothesis that Phytophthora is the main cause of open oak forests (dehesas) defoliation and decline in the study areas that may also be influenced by increased drought [138]. Infestation by P. cinnamomi can interfere with and suppress the expression of tree genes involved in chemical defenses such as tannins [139], and the metabolomic shifts of trees affected by this pathogen are similar to those for drought [140]. The early stages of infestation by Phytophthora spp., however, are difficult to detect [140].

Many recent studies and experiments have provided more information about Phytophthora spp. infestation. The decline of Quercus forests caused by Phytophthora spp. is concurrent with losses in the biological activity and nutrient availability in the soil [136], with large impacts on the trophic web of soil invertebrates [141]. The deleterious effects of P. cinnamomi depends on site traits and species [142]. Quercus species in the Mediterranean Basin are generally more sensitive to this pathogen than are Pinus species, and the two most xeric Pinus species, P. halepensis and P. pinea L., are more sensitive than more mesic species such as P. nigra, P. pinaster Ait., and P. sylvestris [143,144,145,146]. The decline of P. pinaster in some areas of Spain has been mainly driven by drought and other biotic factors such as infestation by Viscum album L., and not by Phytophthora [144]. Moreira et al. (2018) [145] reported that the resistance of Q. suber and Q. faginea Lam. saplings to P. cinnamomi depended on different genotypic groups associated with a geographic origin and that Q. faginea saplings were generally more resistant than Q. suber saplings, suggesting that Q. faginea could be used as a rootstock for Q. suber reforestation. The common habit of planting plants such as wheat, oat, vetch or lupin among the trees in open Quercus woodland can accelerate the spread of P. cinnamomi, depending on the host species. Wheat and oats are not infested, but lupin can be infested, contributing to higher densities of zoospores and thus the expansion of this tree disease [146].

Insect outbreaks may also have strong impacts on the Mediterranean forest [147,148,149,150]. The interaction with aggravating biotic factors, such as insect attack, determines the tree’s resistance to drought. Several studies have reported warming-arid periods related to high levels of defoliation by an insect attack [150,151,152,153,154,155]. Outbreaks produced by the gypsy moth (Lymantria dispar dispar) have been observed after dry-warm seasons in Pinus radiate and Castanea sativa forests in Spain [155]. A rise in the dieback associated to fungus has been observed to be related with the aridity rise in the last few decades [91,97,148,156,157,158,159,160]. Attacks by pathogenic fungi are especially intense in some forests of the North Africa Mediterranean range affecting intensely Q. ilex and Q. suber in Algeria mountains [156]. The Argania spinosa forests that are most able to resist dry conditions in the extreme arid limit with the Sahara Desert have been especially threatened by the continuous rise of warming [161,162]. In contrast, some forests in wetter sites such as Central Atlas have been more stable during the last decades [163].

2.1.4. The Particular Case of Temperate and Boreal “Forest Islands”

Mediterranean species are expanding into areas containing non-Mediterranean forest species [88,114,144]. Mediterranean forest species are more resilient to extreme and chronic droughts than species at wetter sites throughout the Mediterranean Basin [164,165,166]. This resilience is especially common in Mediterranean varieties of two typical non-Mediterranean species, Fagus sylvatica L. [45,88,167,168] and P. sylvestris [144,165,169,170,171,172,173,174], that occupy large areas of Mediterranean mountains and provide considerable forest ecosystem services in the landscape. Some recent studies have focused on the effects of current and past droughts on P. sylvestris in mountains in southern Europe, with clear and increasing symptoms of declining crowns and increased defoliation and mortality, mainly in stands at low altitudes [144,171,172,173].

Microclimatic conditions driven by topographic features allow the establishment of sub-Mediterranean, wet temperate, and even boreal forests in the Mediterranean biome, particularly in southern Europe [175]. The effects of drought have been largely detrimental to the forest populations of those species in the border of its species distribution area. There is an increased mortality and a decreased recruitment [169], e.g., in P. sylvestris and P. uncinata Ramond in Iberian areas [176], decreases in forest density in F. sylvatica forests in the Iberian Peninsula, southern France, and Italy [114,175]. The increase in aridity is the main but not the only cause of the decline and increased mortality of these communities. Other variables sometimes associated with aridity, such as the amount of atmospheric ozone, are also affecting these non-Mediterranean forests at high altitudes [176]. The dieback and substitution of temperate/boreal species by Mediterranean forest species is even faster where browsing pressure by ungulates is high, as in Sierra Nevada mountains (South Spain), where ungulates prefer to feed on P. sylvestris than P. pinaster saplings [177].

2.2. Species Invasion

Forests are generally the least invaded habitats globally [178], particularly in Europe and the Mediterranean Basin, where forests are currently not especially affected by species invasion [10,179]. Peri-urban forests, however, are the most vulnerable to invasion due to the presence and activity of humans that apply pressure for dispersing invasive species [180]. Human perturbation of open ecological spaces allows new species to invade while maintaining high resource availability [181]. A greenhouse experiment conducted by Erskine-Ogden et al. (2016) [182] found that a further increase in aridity and/or N deposition would increase the growth of invasive Mediterranean woody species relative to native competitor species, suggesting that projected increases in aridity would favor the spread of some current invasive forest species in the Mediterranean Basin.

The intentional introduction of Acacia and Eucalyptus species in Portugal, Turkey, and Spain is perhaps the best example of the large impact of invasive forest species in forested areas of the basin [183]. These species have spread to sites other than those where they were planted for afforestation and silvicultural management [183]. For example, Acacia species native to Australia were introduced to stabilize sand dunes but have become dominant, displacing native species [183]. Species of both of these genera have recently taken advantage of forest openness after fires to spread into several forested areas of Portugal [184,185]. Acacia longifolia Andrews is an aggressive species, invading forests and open forests (dehesas) at several sites in Portugal, with notable success at more xeric sites [186]. These invasions could be due to the higher capacity of these species to tolerate high temperatures under water stress compared to their most common native tree competitors [187].

Robinia pseudoacacia L., and Ailanthus altissima Mill. are two of the most invasive and well-established non-native tree species. They have mainly spread from introduced plantations, affecting abandoned land, riverine forests, and road margins, but their capacity to invade established woodlands is lower [188,189,190]. A. altissima, however, has spread to forested areas [191]. R. pseudoacacia can fix N₂ and thus gain advantage in N-limited soils [192]. Invasion by A. altissima and R. pseudoacacia into riverine forests have increased litter production and nutrient cycling, favoring their own spread [190,193].

2.3. Forest Wildfires

Fire is caused by the integration of climate, topography, changes in land cover (fuel characteristics) and use, and socioeconomic factors. Fire is a frequent and historical long-term endemic disturbance in Mediterranean climates [57]. Large seed banks and the presence of savannoid woodlands are associated with the high resilience of some forest communities that conserve and retain nutrients and where soils quickly recover after a fire [194,195]. The changes in climate and soil in several Mediterranean areas have already increased the risk of fire in the Mediterranean Basin [41,196]. The increasing movement of human populations from rural to urban areas frequently tends to favor the build-up of fuel and thus larger and more intense fires. Decreases in the presence and density of human populations, however, reduce the probability of fires caused by humans [197,198]. The decrease in forest biomass due to harvesting by humans for energy likely decreases the risk of fire in rural areas undergoing depopulation, since this practice reduces the fire hazard [199]. Depopulation provides a scenario of fewer but larger and more intense fires. We are thus assisting to the intensification of the classical role of fires in Mediterranean landscapes, with extensive and sudden destruction of forests and other ecosystems such as shrubland, grassland, and cropland, greatly impeding natural regeneration [200,201].

Drought has a negative synergy with the increase in more extreme wildfires and mainly extreme summer droughts [5,202,203,204,205,206,207,208,209]. This synergy, when fires follow droughts, promotes shifts in dominance from seeder to resprouter species [205]. Droughts after fires, however, strongly affect post-fire effects on the status of soil nutrients [208]. One hundred years of observational data from 77 sites with and without fires in Spain and France indicated that water stress converted forests in open shrubland by hampering recovery after a fire [209]. Thus, the high fire frequency in some Mediterranean areas can transform forest to shrubland [210,211], and the combined effects of aridity and fire constitute another important positive synergy favoring the conversion of forest to shrubland [209]. The area damaged by fires has increased in the last few decades in several parts of the Mediterranean Basin [212]. Trade-offs between resistance to drought versus resistance to fire have recently been observed, e.g., resistance to embolism (a good adaptation to drought damage) has been negatively correlated with fire tolerance in conifers, which increases flammability when the water content of soil is low [213]. Forest fires in areas with Mediterranean climates greatly degrade the soil, mainly due to an increased erosion on steep slopes from torrential rain [214,215]. Fire is a driver of desertification that is linked to a continual positive feedback of higher frequency and intensity of torrential rainfall (associated to climatic change) with an increase of soil erosion, which in turn leads to a loss of soil fertility and thus plant cover. These can also be aggravated by human activities such as excessive livestock pressure, and by increasing the frequency and extend of fires [18]. Fires briefly increase the concentrations of carbon (C) and N in the upper soil layers [216], but soils frequently lose C and N long after a fire [217,218]. Modeling has strongly linked past fires with immediately previous droughts [219] and has projected high and important risks of fire in the near future in areas of the Mediterranean Basin where aridity is projected to increase [220,221]. The soil temperatures reached during a fire in Mediterranean forested areas is another factor determining the effects of fire on the regeneration of soil quality. Temperatures <220 °C have little impact on soil conditions, enabling rapid recovery of vegetation, whereas the effects of temperatures >220 °C on soil degradation are more intense, causing a slower recovery of vegetation [222].

2.4. Soil Fragility and Forest Maintenance and Regeneration

As discussed previously, low and irregular precipitation is the main factor limiting tree establishment and growth under Mediterranean conditions, which is particularly harmful in areas where the soil has a poor capacity to retain water [223]. The synergistic effect of the unstable balance between the trees and soil water content, together with anthropogenic actions on forest environments and their degradation, which affect floristic composition, act against soil stability and fertility [224,225,226,227]. This difficulty in regenerating and re-establishing forests after clear-cutting or fire accelerates the conversion of forests to pastures, degraded areas, and ultimately to desertification [228], thus complicating forest sustainability [226]. We must also consider that excessive livestock pressure is still occurring in several forested or potentially forested Mediterranean areas, mainly in the south part, also favoring soil degradation that generally contributes to forest degradation and/or slow regeneration [229,230]. Soils, though, can quickly regenerate naturally after a single fire [231], and natural regeneration can be improved by the application of organic amendments to the soil surface after a fire [232], which however cannot be applied on steep slopes where erosion following fires increases after heavy rains, mainly when the fires have also affected the litter cover [233].

The quality of soil is a key aspect ensuring the health and regenerative capacity of Mediterranean forests [228]. Forests are the best types of ground cover for maintaining soil quality and avoiding leaching and degradation in Mediterranean climates [229,234,235]. Conserving and managing forest cover to avoid deforestation as much as possible is thus the best strategy to avoid desertification [224,234]. Various Mediterranean species and even genotypes within species can also control soil processes mediated by microbes [236].

2.5. High Diversity at Risk

The Mediterranean Basin is a focus of biological diversity [237,238,239]. Forests covered an estimated 82% of the landscape before the first human impacts thousands of years ago [239]. Forest trees and shrubs contribute directly and indirectly to the plant diversity, directly since the basin has a high diversity of tree species [237,240]. For example, the European Mediterranean region includes an unexpectedly high number of tree taxa, almost 200 more taxa than in central Europe [239]. Trees have indirectly contributed to diversity since Mediterranean forests are foci of diversity of taxa other than plants [241]. Mediterranean forests, however, are not only key ecosystems for maintaining Mediterranean plant diversity but are also foci of high diversities of microbes [242,243,244], fungi [245,246], lichens [247], and animals [247,248,249], all of which are very sensitive to the impacts of drought, fire, and other disturbances [250,251]. Tree forest diversity controls soil invertebrate diversity, because of more mixed-litter coming from distinct tree species is associated to more diverse soil invertebrate communities [252].

Late-successional forests dominated by Quercus species sustain a higher diversity of higher plants than do secondary forests dominated by Pinus species [253], but Pinus forests can be associated with higher fungal diversity than Quercus forests [245]. Kouba et al. (2015) [254] reported that communities dominated by early-, intermediate-, and late-successional species have similar levels of plant diversity at different stages of regeneration due to historical land uses and perturbations. This study also reported that historical land management has driven the current lack of late successional species in forested areas of the Mediterranean Basin. Alvarez et al. (2009) [255] reported similar results. Another study found that the replacement of Q. suber with P. pinaster was associated with a loss of plant diversity [253]. The richness of bird species (both generalists and specialists) tends to increase in more advanced successional forest stages when the diversity of tree species increases, but richness tends to decrease when the forest cover is very closed and dense [256]. Birds consistently maintain richer communities with higher diversity in areas with forest patches of different sizes and species compositions that provide many habitats [257]. A management strategy avoiding landscape homogenization and excessive fuel accumulation and thus fire risk is the best scenario for reducing fire frequency and thus preserving taxonomic diversity [258,259,260]. Biodiversity in Mediterranean areas has been widely monitored and quantified, so identifying species indicative of different levels of diversity is feasible in Mediterranean landscapes [261]. Moreover, current warming has had positive effects on angiosperm Mediterranean forests and a negative effect on gymnosperms Mediterranean forest as observed in Spain due to hydraulic traits and reproductive success [262,263].

2.6. Depopulation and Abandonment of Rural Areas

The abandonment of rural areas in several parts of the Mediterranean Basin in the last 80–100 years has been associated with an increase in forest cover [264]. For example, forest cover was continuously reduced in Catalonia from 1868 to the 1950s, but forest cover has since continuously expanded with the abandonment of traditional forest activities and mainly with the abandonment of cropland [265]. These secondary new forests in the initial-medium stages of the succession process, however, have frequently poor age structure, and frequently non adequate density, diversity, and maturity and are thus more prone to wildfires [265]. The global strategy to overcome this situation has been a combination of re-farming establishment and extensive livestock and sustainable forest management [265,266].

3. Human Management. How Can it Help Mediterranean Forest Health and Conservation?

We have accumulated a large amount of data in recent years from a variety of studies providing an overview of how and what type of land management could realistically and sustainably help conserve the diversity of Mediterranean forested areas (Figure 1). More information on how to manage the forest structure could help us improve the capacity of Mediterranean forests to cope with drought conditions and how and where the control of forest density could help us reach the desired management objectives [267]. Different management and economic activities can have very different levels of impact. For example, sustainable cork production in Q. suber forests is completely compatible with forest cover, health, and regeneration, whereas cattle grazing and establishing herbaceous crops among trees degrades the soil and impedes the maintenance of forest cover in Q. suber communities [268].

The maintenance of forest patches between croplands in rural areas can be a suitable management strategy throughout territories with adequate planning [269,270,271]. Positive relationships between forest-patch area and species richness can be optimal for patches of about 10 ha, when diversity remains stable [269]. The appropriate management of livestock and logging ensures the equal representation of different stages of forest succession among forest patches of different sizes, which would be the best tool to maintain the diversity of woody plants and forest services among landscapes, as some studies strongly suggest [270,272]. Gonzalez-Moreno et al. (2011) [270] reported that patches of secondary forest dominated by pines contained more plant species than patches of mature forest dominated by evergreen oak species. Moreover, the more diverse the tree species of a Mediterranean forest, the more diverse the soil trophic webs, as a result of a larger litter composition variability [241].

The extensive management of livestock as a potential alternative tool for maintaining biodiversity and avoiding the risk of fire in Mediterranean landscapes has recently been debated [273]. The results are not completely satisfactory and do not currently allow a general conclusion. Cattle grazing in a Mediterranean forest preferred woody plants, potentially reducing the number of flammable taxa, but having generally detrimental effects on plant-community health, and the cattle had to be supplied with supplementary food in the long term [273]. Decreasing forest and woodland density to increase the vitality and survival of individual trees under drought conditions must be preceded by accurately evaluating the present situation and a realistic final objective in the line to preserve a community with the maximum of diversity and biomass and also resistance to environmental stress. Late successional Mediterranean forest species such as Q. suber mainly need the presence of old large individuals to successfully recruit seedlings [247]. The increased extension of open bare soil does not allow the establishment and/or permanence of these late-successional Mediterranean tree species.

Reforestation and afforestation are the most direct human actions that can counteract the progressive degradation of forests in Mediterranean landscapes. These actions can currently recuperate savannoid and/or forest structures in several Mediterranean areas from the practical loss of natural forest regeneration by the increasing extensive replacement of forests by shrub communities and even by desertification [18,274,275]. Reforestation and afforestation, mainly in semi-arid and/or burned areas, can conserve soil, C, and nutrients in the system but must be appropriately managed to avoid as many impacts as possible [276,277].

Reforestation and afforestation with more than one species, accounting for microspatial variability, have been successful for creating diverse landscapes [259,278,279]. The combination of Pinus and Quercus species or Quercus species alone have successfully ensured maximal biodiversity [279,280] and soil conservation [280,281] at mesic sites. P. halepensis strongly competes for water, mainly in the upper soil layers, impeding the establishment of shrubs with shallow root systems [282,283,284]. Reforestation and afforestation with oaks alone or with Pinus are thus advisable to ensure maximal diversity of understory plants allowed by the climatic conditions.

The use of nursery species that provide shade can allow the replantation of semi-deciduous oaks when the repopulation combines them with evergreen oaks such as Q. ilex [285] or planting shrubs in reforestations with mesic pines such as P. nigra [286]. P. halepensis, however, has been the best species in most studies, reforestations and afforestation in semi-arid areas with a mean annual precipitation of 300–400 mm [287,288,289,290]. The success of reforestation and afforestation in semi-arid areas can be improved by fertilization [291]. The diversity of understory plants in P. halepensis forests can be improved by planting appropriate shrubs in stands with specific pine densities [277]. Some animal communities have also been improved in stands afforested with P. halepensis in semi-arid Mediterranean areas [292]. Another problem, mainly in countries in phases of economic development and expansion, is the necessity to manage forests to reduce CO₂ emissions in the framework of the Kyoto protocol [293], which increases the need for appropriate forest management, reforestation, and afforestation in the Mediterranean Basin.

The use of mulches and organic amendments [294,295,296] and even of soil conditioners with water-absorbing polymers [297,298] substantially favors the early establishment of tree seedlings. The use of composite amendments (sewage sludge plus green waste) can also be useful if the amounts of P and trace elements are controlled to avoid environmental pollution [200]. Organic amendments can generally efficiently counteract the most common first step of soil degradation by water erosion and wildfires: The loss of organic matter [299,300,301]. We recommend choosing the best species identified by previous studies to ensure the success of reforestation and afforestation [279,302,303].

Management practices after reforestation or afforestation are also important for increasing the probability of success [304]. Periodic reduction of vegetation, such as shrubs and grasses, can improve the survival and growth of planted trees [304]. Appropriate forest management, with a good combination of forest patches of different sizes and successional stages together with other ecosystems, such as cropland, is also an adequate tool to improve the capacity to store C [305]. Appropriate management is also necessary to maintain natural Mediterranean forests in their current structure and areas [306]. An adequate level of thinning a few years after a fire can enhance forest regeneration and allow the control of fuel load for possible future forest fires, both by seeder species such as P. halepensis and by resprouter species such as Q. ilex [307].

Forest conservation can thus be improved by the good management of burned areas or areas with degraded soil. We should first reduce or avoid some practices. Salvage logging is common in several post-fire areas but has been very detrimental to forest regeneration due to soil degradation [308]. Conserving a specific amount and density of logging remnants after a fire has been associated with the better conservation of bird abundance and richness [309]. Various practices of forest management can have positive or negative effects on the post-fire regeneration of Q. suber trees, e.g., traditional coppicing is considered necessary, but branch pruning should be avoided [310]. The use of fire retardants in the dry season to prevent and/or easily control forest fires can increase the alteration of leachates (high cation concentrations and pH) during periods of rain, suggesting a risk to the quality of freshwater bodies [311].

Studies thus suggest that an integrative approach would be the most efficient global management strategy to prevent fire, ensure adequate forest cover, maintain forest quality, and favor regeneration in current Mediterranean forested areas. This strategy should be based on optimal and moderate forest thinning and livestock pressure and on the appropriate management of the distribution of forest species based on the best native species at each site. This strategy should be complemented in non-urban areas with the maintenance of cropland and adequate and sustainable socioeconomic conditions for rural populations, avoiding depopulation and land abandonment and should be the key global strategy to conserve soil quality and biodiversity, preserve forests, and prevent fire [312]. We must, however, also account for the different realities of southern Europe and northern Africa, that are driven towards an extreme opposed situation. Management in southern Europe should be addressed based on the need to avoid excessive rural abandonment and to maintain a diverse landscape [313], but the scenario in northern Africa is currently different. The exponential increase in population in the Maghreb region led to the expansion of cropland in marginal areas initially dominated by woodland, leading to the ploughing of slopes for livestock in the remaining natural non-cropland areas, with the consequent degradation of soil [298].

Land abandonment followed by forest re-establishment, however, should not be a problem, despite the increased risk of fire. First, the conversion of abandoned cropland to forest has been associated with improved soil stability [314,315]. Second, the increase in forest fires can be counteracted by the appropriate management of new forested areas if the global economic exploitation of rural areas, including forest management (with all the above measures), allow the maintenance of sustainable human activities in rural Mediterranean areas.

In summary, aridity is the main climatic constraint for Mediterranean forests, that should be taken into account in forest management. These drought-driven alterations can become stronger if climate change, its associated disturbances (e.g., by floods, droughts, heat waves, and forest fires), and changes in other components of global change (especially the changes of land use, pollution, and overexploitation of resources) continue at current rates or are enhanced. Therefore, we need to know more about properly managing forests to increase their resistance to drought and the interacting disturbances. Information is currently available for appropriate species composition, plant density, and the use of shrubs, but less is known about other important factors such as improving soil conditions to ensure a good supply of nutrients.

Land-use planning is a key “pending question” in several areas of the Mediterranean Basin. A strong alliance to fight against the loss of forest quantity and quality is among the main challenges to be addressed both to avoid excessive depopulation of rural forested areas an unmanaged secondary forest expansion in Mediterranean Europe, and on the other extreme, to the progressive population expansion implying over-exploitation and forest degradation and loss in North Africa. All the data strongly suggest that maintaining biodiversity, as well as a variety of goods and services from cropland and forested areas will be necessary to reach this objective, combining forest patches of different sizes with other communities, including the most appropriate crops for maintaining environmental quality and food security as much as possible, allowing economic sources for the maintenance of an optimal human density. The management of forested areas, and of natural areas in general, should incorporate a hierarchical landscape planning at different scales, including a large-scale plan that considers the combination of areas of different types, multiple users, and the effects of disturbances, such as forest fires, floodings, and especially drought, to a minor-scale action such as the improvement of stakeholders education and tools availability for an adequate management of their territories.