Improving Carbon Sequestration in Wetlands Using Native Poplar Genotypes for Reforestation Purposes

Abstract

From the early 2000s, many forestation projects were carried out in the flooding areas of the Po River to preserve abandoned or damaged riverbanks and establish natural populations of Populus nigra L. for species conservation and future seed collection activities. Different clones of P. nigra, belonging to a collection of the Centre for Forestry and Wood (CREA-FL), were planted. The group named ‘POBIA’ comprises 35 selected clones chosen for their survival ability and fast growth. After plantation and a few essential cultural inputs, four establishments were left undisturbed. This study highlights the survival, growth, and performance results of the ‘POBIA’ group compared to other not-selected clones. The ‘POBIA’ clones showed a higher survival than the not-selected clones. Moreover, the ‘POBIA’ groups showed a significantly higher C sink performance in three of four establishments, reaching 278.6 t ha⁻¹ of CO₂ obtained in thirteen years in one site. The management of ‘POBIA’ clones in reforestation projects agrees with the EU recommendations for a vital ecosystem service.

1. Introduction

Forests and trees can retain high amounts of carbon in their wood (aboveground and belowground) and provide various ecosystem services. These services include aiding ecological corridors, enhancing biodiversity, preventing soil erosion, reducing groundwater pollution, and providing valuable products such as small fruits, mushrooms, and firewood [1,2,3]. As pioneer species, poplars (Populus spp.) contribute to multiple ecosystem services thanks to their rapid growth. They provide wood for industry, regulate river floods, reduce soil erosion during flood events, and have a lower environmental impact than crops. Poplars can also filter contaminants in soil and groundwater pollution from fertilizers or other pollutants, reduce greenhouse gases, and create buffer zones between wooded and agricultural lands. From an ecological standpoint, they could create an ecological network, conserve the rural landscape, improve biodiversity, and promote recreational activities like walking, cycling, and horse riding along rivers [4,5].

The Land Use, Land Use Change, and Forests (LULUCF) sector, according to the framework of the Paris Agreement, is expected to play a decisive role in reducing emissions and absorbing CO₂ [6]. While this sector was previously requested to be ‘emission’-neutral, the revised European Regulation [7] requires an additional annual removal of 42 MtCO₂ eq. compared to the EU level’s 2016–2018 sector average [8]. Member States, including Italy, are expected to describe their strategies to achieve the agreement’s objectives, known as Long-Term Strategies (LTSs). The land-based policies and measures for climate change mitigation indicated by Italy include afforestation, reforestation, and forest restoration, as well as restoring essential landscapes/forests and increasing forest adaptation.

During the early 2000s, intense reforestation efforts occurred in Italy along the Po River in Piedmont. These activities aimed to afforest abandoned land and restore areas affected by a severe flood in 2000, protecting them from erosion and future potential devastation [9]; other groups in Europe were engaged in the same activity [2,10,11,12,13,14]. The reforestation project, with the support of the Park of Piedmont Po River [15], had recourse to native poplar species (Populus spp.), in particular, white poplar (P. alba L.) and black poplar (P. nigra L.) [16,17,18].

Black poplar is a pioneer fast-growing tree belonging to Salicaceae. Its natural range covers the center–south of Europe, North Africa, and West Asia, growing mainly along rivers and at the bottom of valleys [19,20]. Due to its reduced natural habitat, black poplar was at risk of extinction [21,22]. There were also concerns about hybridization with non-native species grown for wood production, based on several studies [23,24,25]. The EUFORGEN European Program started in 1994 and considered P. nigra for knowledge exchange among European researchers for species protection and conservation [13,26]. In Italy, the native populations of P. nigra are fragmentary and primarily located near the main rivers due to agricultural expansion and climate change [27,28,29]. P. nigra is a parental male in the principal Italian hybridization program for P. × canadensis clone production, characterized by fast growth, high wood quality, and disease resistance [30]. Due to the EUFORGEN program, particularly the activity of protection and collection of natural genotypes in the ‘Mezzi’ experimental farm of CREA—Research Centre for Forestry and Wood of Casale Monferrato (Italy), there are more than 800 black poplar genotypes useful for environmental and breeding purposes [31,32]. The plantation of native species such as black and white poplar in river flooding areas allowed for the fast reforesting of abandoned lands, protecting soil and settlements from flooding, favoring succession with other native broad-leaved shrubs and trees thanks to the pioneering activity carried out by the poplar, and ensuring rapid absorption of discrete quantities of CO₂ thanks to the poplars’ fast growth. In addition, it allows for the reconstitution of diffusion core groups of these species to recolonize riparian ecosystems typical of these species [9].

In this study, we aim to demonstrate (i) how the selection of faster-growing black poplar genotypes has contributed to the further improvement of carbon sinks without upsetting the natural balance through the use of, for example, hybrid clones; (ii) what the contribution of poplar has in the absorption of CO₂ (in aboveground wood) compared to other broadleaves species; and (iii) what climatic factors in this environment influenced the growth of poplars in this environment.

2. Materials and Methods

Starting in 2003, new plantations were established along the Piedmont banks of the Po River using one- or two-year-old poplar poles; the ‘Mezzi’ farm of the CREA—Research Centre for Forestry and Wood of Casale Monferrato (CREA-FL) provided the necessary poplar trees, while the nursery of the Piedmont Region provided other broadleaf species (shrubs and trees). Different planting projects were developed and adapted to the previous situation of each area where the activities were implemented. The white poplar was introduced using different natural genotypes maintained in nurseries and cataloged in the vegetal archives of the ‘Mezzi’ farm. Over 136 black poplar clones were used at the three sites (Figure 1), with 35 belonging to the experimental clonal mixture named ‘POBIA’. ‘POBIA’ is a group of 35 P. nigra clones coming from different regions of central and northern Italy, consisting of 22 males and 13 females. The selection of this group was focused on rooting ability, fast growth, and resilience. The other native tree and shrub species have been introduced in the planting layout of establishments. The characteristics of each establishment are reported in

Table 1. Characteristics of the plantation sites. Plantation year (Pl. year), poplar planting density in trees per hectare (density) for P. nigra, P. alba, and other species.

Site	Pl. Year	P. nigra Density ha−1	P. alba Density ha−1	Other Species Density ha−1
Ghiaia Grande	2008	204	204	408
Isola Colonia	2005	286	0	285
ValenzaA	2003	417	0	416
ValenzaB	2004	416	417	208

and Supplementary Material S1 (for climate and soil analysis).

The plantation of Ghiaia Grande (45°09′00″ N, 8°19′11″ E) was established in 2008 in a particular natural reserve on the left orographic bank of the Po River. A regional law protects an area of 462 ha. The new plantation aims to reintroduce native black poplar genotypes along the Po River that could be used for seed collection by regional nurseries. The area of the poplar plantation covers about 2.7 ha. The initial plantation density was 3.5 × 3.5 m; black poplar was planted at 7 × 7 m using one- and two-year-old poles of 136 different clones, of which 31 belong to the ‘POBIA’ mixture and alternate to white poplar (P. alba) and other woody species. The plantation includes four poplar trees for each clone without replication (Table 1).

The site of Isola Colonia (45°10′03.62″ N, 8°13′26.36″ E) was established in 2005 and covers a surface of about 2.2 ha on the left orographic bank of the Po River that could be used in the future for seed collection by regional nurseries. Black poplar was planted at 7 × 5 m using one- or two-year poles of 78 different clones, of which 29 belong to the ‘POBIA’ mixture and alternate with other woody species (Table 1).

The site of Valenza hosts different models of plantation on the right orographic bank of the Po River: the area named ValenzaA (45°01′12″ N, 8°38′23″ E) is a black poplar plantation dedicated to seed production. It has a surface of about 1 ha and was established in 2003 using one- and two-year-old poles of 50 black poplar clones, of which 16 belong to the ‘POBIA’ mixture. The initial plantation density was 6 × 4 m. The plantation hosts other woody species, but white poplar is absent (Table 1). The area named ValenzaB (45°01′83″ N, 8°38′29.89″ E) was established in 2004 with black and white poplar and other broadleaves species. It covers a surface of about 3 ha with a plantation layout of 6 × 4 m. Regarding black poplar, the area hosts 20 clones, of which 17 belong to the ‘POBIA’ mixture. The single plots contain 20 trees and are not repeated (Table 1).

Surveys and Statistical Analysis

The results focused on the increase in poplar wood and all trees’ ability to sink carbon, starting from the planting year until 2021. In the spring of 2018, the survival and the diameter at breast height (D) of all of the P. nigra were measured, and total height (H) was assessed to evaluate tree volume (V, m³). For the same period (2003–2017), some climatic data were also collected: annual means were collected for temperature (T, °C), yearly total rain in mm (PAn), total annual potential evapotranspiration in mm (Etp An), total annual hydric deficit (Deficit), and Surplus (mm), based on the Thornthwaite model [33], and the same values for the vegetative period (from April to October): mean temperature in °C of the vegetative season (Tsv), the sum of rain of vegetative season in mm (Psv), Deficit and Surplus, in mm, during the vegetative period (Defsv and Sursv), and evapotranspiration in mm for the annual period (EtpAn).

In autumn 2021, the goal of the surveys was to identify all of the trees that were alive, both poplars (black and white poplars) and other species, to evaluate the current situation in unmanaged natural stands and measure the diameter at breast height (D) and total height (H) to estimate the aboveground volume (V) of all species, applying the equation described in Coaloa and Chiarabaglio [34]:

V = 0.2535 × D^2.093 (m) × H^1.0277 (m) × s^0.0275 (m²) × a^0.0820

where

• V is the total stem volume plus the branches up to 10 cm in diameter;
• D is the diameter at breast height over bark;
• H is the total height;
• s is the average spacing at planting;
• a is the age of plantation.

The total volume (aboveground and belowground) was then estimated by multiplying the aboveground volume by the coefficient of 1.26 obtained from the literature [35,36]. The total wood dry matter weight was calculated by considering the wood density value and using an average value for each species obtained from the literature [37]. The carbon content in dry wood matter is estimated to be 50% internationally [38]. The conversion from carbon content to CO₂ (kilograms) was calculated by the following equation:

CO₂ = C content (kg) × 44.01 (Da)/12.01 (Da)

This formula is used to determine how much CO₂ corresponds to the carbon we have [38]. Since C has an atomic mass of 12.01 Dalton and CO₂ has a molecular mass of 44.01 Dalton, every gram of C corresponds to 3.67 g of CO₂. The tree species analyzed at the end of the experiment are listed in Supplementary Material S2.

All of the estimations were reported for 1 ha of area reforested; poplar’s growth and carbon sequestration were compared to those of other species, and those of the ‘POBIA’ mixture were compared to those of non-POBIA clones. R (version 4.0.0, CRAN) was used for statistical analysis. After analysis of variances, the survival of black poplar between the two groups was tested with Wilcoxon’s non-parametric test; Tukey’s HSD test was used to compare the difference among groups within each site. Finally, by applying a linear model analysis with R (package “stats”), we estimated the potential influence of climatic factors on poplar growth.

3. Results

3.1. Poplar Growth and C Sink: ‘POBIA’ Mixture vs. Non-POBIA Clones

At the end of the experiment, we assessed the difference between ‘POBIA’ and non-POBIA clones regarding mortality (Figure 2). In the grouped sites of Ghiaia Grande and Isola Colonia (due to the total absence of wind damage and water stress), after ten years, 88% of trees belonging to the ‘POBIA’ mixture were alive compared to 70% of non-POBIA clones. The difference in mortality between the ‘POBIA’ clones and non-POBIA clones was significant (W = 3485.5, p-value = 0.0012).

At the Ghiaia Grande site, after ten years, 83% of trees belonging to the ‘POBIA’ mixture were alive compared to 73% of non-POBIA clones. At the Ghiaia Grande site, the ‘POBIA’ mixture reached an average aboveground wood volume of 128.8 m³ ha⁻¹; the total volume (aboveground and belowground) was estimated at 162.3 m³ ha⁻¹, corresponding to mean annual production of 16.2 m³ ha⁻¹ y⁻¹; the non-POBIA clones produced an average aboveground volume of 97.8 m³ ha⁻¹ and 140.5 m³ ha⁻¹ total volume, corresponding to mean annual production of 14.1 m³ ha⁻¹ y⁻¹. The ‘POBIA’ mixture absorbed 134.1 t ha⁻¹ of CO₂ in ten years (13.4 t ha⁻¹ y⁻¹) compared to 101.7 t ha⁻¹ of CO₂ (10.2 t ha⁻¹ y⁻¹) for non-POBIA average clones, as reported in Figure 3A.

At the Isola Colonia site, after thirteen years, 90% of trees belonging to the ‘POBIA’ mixture were alive, compared to 65% of non-POBIA clones. At this site, the ‘POBIA’ mixture reached an average aboveground wood volume of 267.8 m³ ha⁻¹; the total volume (aboveground and belowground) was estimated at 337.4 m³ ha⁻¹, corresponding to an annual production of 25.9 m³ ha⁻¹ y⁻¹ of total wood. The average of the non-POBIA clones’ aboveground production was 115 m³ ha⁻¹; the total volume was estimated at 144.9 m³ ha⁻¹, corresponding to 11.1 m³ ha⁻¹ y⁻¹ of annual wood production. The ‘POBIA’ mixture absorbed 278.6 t ha⁻¹ of CO₂ in thirteen years (21.4 t ha⁻¹ y⁻¹) compared to 119.7 t ha⁻¹ of CO₂ for non-POBIA clones (9.2 t ha⁻¹ y⁻¹), as reported in Figure 3B.

In the ValenzaA plantation, 67% of trees in the ‘POBIA’ mixture were alive after fifteen years, compared to 65% of non-POBIA clones. At this site and model, the ‘POBIA’ mixture reached an average aboveground wood volume of 200.4 m³ ha⁻¹ and a total wood volume of 252.5 m³ ha⁻¹, corresponding to 16.8 m³ ha⁻¹ y⁻¹. The non-POBIA clones reached an average aboveground volume of 172.6 m³ ha⁻¹; the total volume was estimated at 217.5 m³ ha⁻¹, corresponding to 14.5 m³ ha⁻¹ y⁻¹. At this site, the ‘POBIA’ group absorbed an average of 208.5 t CO₂ ha⁻¹ in fifteen years (13.9 t ha⁻¹ y⁻¹), and the non-POBIA clones absorbed an average of 179.6 t CO₂ ha⁻¹ (12.0 t ha⁻¹ y⁻¹), as reported in Figure 4A.

At the ValenzaB site, 62% of trees belonging to the ‘POBIA’ mixture were alive after fourteen years compared to 59% of non-POBIA clones. At this site and model, the ‘POBIA’ mixture reached an average aboveground wood volume of 161.2 m³ ha⁻¹ and a total wood volume of 203.1 m³ ha⁻¹, corresponding to 14.5 m³ ha⁻¹ y⁻¹. The non-POBIA clones reached an average aboveground volume of 145.3 m³ ha⁻¹; the total volume was estimated at 183.1 m³ ha⁻¹, corresponding to 13.1 m³ ha⁻¹ y⁻¹. At this site, the ‘POBIA’ group absorbed an average of 167.7 t CO₂ ha⁻¹ in fourteen years (12.0 t ha⁻¹ y⁻¹), and the non-POBIA clones absorbed an average of 151.2 t CO₂ ha⁻¹ (10.8 t ha⁻¹ y⁻¹), as reported in Figure 4B.

3.2. Poplar C Sink vs. Other Species

Due to its fast growth and survival ability, poplar can capture a very high C amount. We obtained the results in

Table 2. Average CO₂ captured (kg CO₂ y⁻¹) in the wood of poplars and other species.

Site	Species	Average per Tree
Ghiaia Grande	Other	4
	P. alba	36
	P. nigra	73
Isola Colonia	Other	3
	P. nigra	49
ValenzaA	Other	5
	P. nigra	88
ValenzaB	Other	9
	P. alba	62
	P. nigra	57

by estimating the total wood volume per tree per year obtained at the end of 2021 (three years after the last measurements on poplar genotypes) by poplars (white poplar and black poplar) and the other species.

At the ValenzaB site, the area’s natural evolution, made possible by the poplars’ pioneering activity, has allowed the development of other broadleaved species. At the same time, senescence and progressive mortality have begun on the poplars, as shown in Table 2.

3.3. Influence of Climatic Factors on Unmanaged Plantations’ Performance

In the plantations described, the trees only received limited cultural input, such as irrigation and weed control, during the first years to enhance rooting. From the fourth year after establishment, the plantations were unmanaged by conventional forest management practices. Therefore, irrigation, weeds, and disease controls have been suspended. Natural environmental conditions, particularly climatic conditions, have influenced the health and growth of the trees and, consequently, the evolution of tree stands.

Table 3. Estimates of linear model applied to diameter growth at all sites and climatic factors from 2003 to 2017; Intercept = growth in Ghiaia Grande, Site IC = growth in Isola Colonia, Site VA = growth in ValenzaA, Site VB = growth in ValenzaB, Pan = total annual rain (mm), EtpAn = total annual evapotranspiration in mm, Tsv = average temperatures of vegetative period in °C, Defsv = deficit of vegetative period in mm, Sursv = surplus of vegetative period in mm (** p < = 0.01, *** p < 0.001).

	Estimate	Std. Error	t Value	Pr (>|t|)
Intercept	−82.415156	19.260119	−4.279	<0.001 ***
SiteIC	−0.890619	0.749378	−1.188	0.235
Site VA	−3.993094	0.789902	−5.053	<0.001 ***
Site VB	−2.760110	0.848865	−3.252	0.001 **
PAn	0.015304	0.002586	5.919	<0.001 ***
EtpAn	−0.068710	0.040420	−1.700	0.090
Tsv	7.617842	1.882087	4.048	<0.001 ***
Defsv	−0.015844	0.008429	−1.880	0.061
Sursv	0.004230	0.008783	0.482	0.630

reports the results of the linear model.

4. Discussion

In the forestation, afforestation, and reforestation programs of floodplain areas, the use of native poplars represents a successful solution. These species are chosen for their ability to reproduce vegetatively, the availability of many different genotypes, their fast growth, and their pioneer behavior. These features make them suitable for low-cost activities with rapid results. Large plantations were set up to obtain seeds for future propagation and to reintroduce species with reduced distribution due to human pressures in northern Italy’s lowland areas. In the areas considered, an experimental mixture of P. nigra clones named ‘POBIA’ was tested against other non-selected ones to evaluate the survival and growth ability under the problematic natural conditions present in floodplain areas or areas subject to renaturalization. Due to their high survival ability and fast growth, these establishments ensure multiple ecosystem services: the success of the trees in a renaturalization project and the improvement of carbon sink capacity. The ‘POBIA’ mixture clones showed a significantly higher C sink performance in three of four trials than the average non-POBIA clones. The best performance was achieved at 278.6 t ha⁻¹ of CO₂ captured over thirteen years (Isola Colonia). Čomić [39] reported 9.60 t CO₂·ha⁻¹·year⁻¹ sequestered by a plantation of the Populus × canadensis (Moench) clone ‘I-214’. The results indicated that, in a renaturalization project, our clones could reach 21.4 t CO₂·ha⁻¹·year⁻¹. Riccioli [40] states that selected plant material could fix higher CO₂.

Furthermore, poplar’s pioneer ability supports the reintegration of other tree and shrub species towards a renaturalization of the area; at the same time, by providing shelter and new ecological niches in previously abandoned lands for insects, birds, and mammals, it improves biodiversity [41]. As Xu reported [42], high plant diversity will increase C uptake and storage by the ecosystem. Moreover, high plant diversity is related to soil NH₄⁺ total N soil. After thirteen years, the poplar trees are starting to die and settle towards a lower level of presence than the original planting, particularly at the ValenzaB site; these observations are in accordance with Alimpić [22]. Future observations will be focused on these aspects.

In such unmanaged plantations, the choice of native material is insufficient to guarantee the success and continuity of restoration projects; environmental and climatic factors can strongly influence the success and growth of the trees. In our analysis, covering 15 years of climatic and dendrometric data, we found that the total annual rain (Pan) and average temperatures of the vegetative season (Tsv) positively influenced growth, as reported by Vacek [43]. This result underlines the strong dependence of these systems on the effects of climate change in the geographical areas considered, which translate into hot summers and a general reduction in rainfall and extreme events [28,44,45]. These results partially agree with the literature [46,47].

We also found some differences between sites, probably due to other factors not considered, such as soil type and water availability. Moreover, it is possible to identify high spatial variability in soil conditions in these areas subject to flooding. The poplar species (P. nigra and P. alba) clearly distinguished themselves in terms of fast growth compared to other species. Based on our findings, one black poplar tree converted between 49 and 88 kg of CO₂ into biomass per year. For the other species, the range was from 3 to 9 kg of CO₂ per year. Our results are consistent with Toochi [48]: a 25-year-old maple–beech–birch forest can sequester 1.3 kg of CO₂ per tree per year, with a plant density of about 1700 trees ha⁻¹. However, due to a lack of data, we could not assess another significant pool influenced by tree planting: the soil. The presence of roots, root turnover, litter, and the microclimate generated by foliage positively influence the soil’s carbon accumulation activity [49]. The process is generally prolonged, influenced by many factors, and quickly reversible in the event of deforestation due to anthropic or natural events [50]. Soil protection and improvement represent a further ecosystem service [51]. Furthermore, the European Community wants to quantify this accumulation capacity [52].

5. Conclusions

Poplar is a fast-growing pioneer species with many different genotypes. Black and white poplar are native species typical of river environments and naturally resistant to certain diseases. These characteristics make them an excellent choice in river redevelopment projects to encourage the natural settlement of secondary species. Among the various ecosystem services that these trees ensure, the absorption of CO₂ is important and is related to climate change. Introducing poplar species, particularly ‘POBIA’ groups, has proven to be an excellent solution for the tested site, with a design tailored to the specific river conditions. Using a selected mixture of clones has improved growth and, therefore, both the carbon absorption capacity and the resistance to future flood events. Therefore, these initial observations confirm the effectiveness of the ‘POBIA’ clonal mixture as a valid alternative for river redevelopment projects, at least in the Po Valley areas. This study also reveals that both annual rainfall and the average temperatures during the vegetative season could significantly contribute to growth, highlighting the vulnerability of these systems to climate change impacts in this geographical context.

It is necessary to conduct additional research on how forest restoration efforts can be effectively carried out, considering the interplay between environmental elements, the socioeconomic environment, the techniques and practices of forest regeneration and silviculture, and the aspects concerning the production of plant material.