1. Introduction
From the 1970s, the theory of “territorial systems of ecological stability” has been formulated in the former Czechoslovakia. The aim was, through instruments of regional and spatial planning, to secure and enhance landscape biodiversity [
1]. In its theory, the ecological stability system issues from the biogeographical theory of islands [
2], concept of ecological stability published by I. Míchal [
3], and geobiographic differentiation of landscape in context of geobiocenology [
4,
5]. The territorial systems, referred to with the Czech acronym as ÚSES (hereinafter as ÚSES), incorporate also a concept of landscape patches, corrirors, matrix and networks [
6]. The concept of ecological networks has been developed since the early 1990s in other countries of Europe [
7]. The concept of ecological networks includes also the “green belts/ways” (greenways) [
8] or interconnecting of landscapes for the purpose of protecting biological diversity [
9].
In the 1990s, the concept laid down by Löw [
1] was further developed in the Czech Republic through other methodological documents, e.g., Lepeška et al. [
10]. At the same time there was some first evidence of actual measures delivered in the landscape, as the first wildlife corridors (biological corridors) were planted on farmland [
11,
12,
13]. Plantings of trees and shrubs are delivered by different means and technologies. There are various scientific projects dealing with this issue [
11,
12]. However, the results of the research often tend to be unclear and difficult to compare, as different parameters are evaluated in individual projects, there are different methodological approaches, and there tend to be initial differences from the very beginning of each project (local conditions, used planting materials or technologies). As early as at that time, one question has never been clearly addressed, that is, whether some of the planting methods are more advantageous in terms of cost and prosperity of the planted trees [
14]. In 2007, the evolution of the ecological networks (ÚSES) in the Czech Republic was summarized by Buček, Maděra and Úradníček [
15].
In general, establishing woody stands in the landscape can be delivered by two core methods. The first one is the so-called forestry method, when less mature stock is being planted in higher densities, and their further development and control is anticipated through natural processes and additional energies added. This method is often applied for afforestation of farmland with the aim to establish economically managed forest stands. This topic is covered by a number of research papers, e.g., by Nizinski [
16], Friedrichsdorf [
17], Bartoš and Kacálek [
18], Jylhä and Hytönen [
19], Leugner [
20], Vacek et al. [
21] and Deptula et al. [
22]. Forestry method of establishing woody stands is also used in the reclamation of land after mining and mineral extraction. Research on such areas has been delivered, e.g., by Dimistrovský and Vesecký [
23] or Štýs [
24].
The second way is the so-called landscaping (horticultural) method; when mature planting stock is used, planting is delivered in the final spacing grids, and much lower cost on the management of the established stand is anticipated, which is referred to also by Jahn [
25].
In practice, many aspects affect the selection of one of these methods, but mostly, it is a reflection of the owner’s or investor’s interest, without any exact assessment of the method’s suitability or convenience. Expert discussion about the convenience and suitability of both the mentioned methods is currently rather empirical and goes on rather in the practitioners’ domain than in the research and scientific community. Here, it is possible to refer to many papers from the regular workshops “ÚSES—zelená páteř krajiny” (ÚSES—the green backbone of landscape) [
26].
Regardless, exact knowledge in this area could significantly contribute to a more effective implementation of these features in the landscape, both in terms of their faster onset of delivering their ecosystem functions and in terms of cost-effectiveness (saving funds and capacities).
This paper aims to present the results of four-year-long research within a site of linear planting in the farmland (so-called wildlife corridor), which was intentionally established using both mentioned methods (the forestry method and landscaping method) in segments to allow for such a study. The authors’ idea is to exactly evaluate in the given conditions the effectiveness of both methods of establishing woody stands in terms of their ecosystem functions and to compare the results against some partial results from the assessment of plantings outside forests in the Czech Republic [
27,
28,
29,
30,
31,
32,
33,
34,
35,
36,
37,
38,
39].
4. Discussion
A study of the Czech and foreign literature has revealed that there are basically no studies in which the results could be compared with. The published studies:
Only some studies contain some partial data that could be compared against the results of this study, and rather, they are input data or by-products that were not the aim of the research. These studies are mentioned in the text below.
The species can be evaluated as successful. As per mortality, in 2019, it achieved 11.36% (3rd lowest rating) while demonstrating good vitality, with both height and trunk diameter values increasing; in 2019, these values made a significant leap (height 72.67–83.75–96.26–142.59 cm, i.e., increased by 96.21% between 2016 and 2019; trunk diameter: 9.35–10.43–13.18–23.19 mm, between years 2016 and 2019, the value increased by 147.96%). This finding is consistent with the description of the species as provided by Chmelař [
45] and also corresponds with the data given for the planting material by Dostálek et al. [
31].
An unwritten experience from practice has been confirmed: this species thrives less when planted on agricultural land, which is demonstrated by the continuous and regularly increasing mortality; from 6.9 in 2016, it increased to the ultimate 37.93% in 2019. Although in absolute numbers this species mortality is not the highest one, its growth is the fastest. This empirical knowledge is not supported by sufficient scientific data, so one can only speculate about the reasons, even though Dostálek et al. [
31] also recorded a decrease in height in small
Carpinus betulus seedlings in the first two years after planting, and only in the fourth year were the recorded values higher than at the time of planting.
When it comes to vitality expressed by the trunk diameter and plant height, in 2016–2018, the values stagnated, or there was even a slight decrease. This can be explained by the death of larger seedlings and survival of smaller ones. In 2019, there is already a significant increase in values, which could indicate the acclimatization of the seedlings on the site.
The species demonstrated the lowest mortality of all the tree species (only two individuals died since planting, which indicates 5.13% ultimate mortality).
While the trunk diameter increment was rising relatively regularly throughout the entire measurement period, in 2016–2018, the height increased at a slower rate; in 2019 it made a significant leap, which reflects in the steepness of growth also supported by the linear curve on
Figure 7 (the highest of all the tree species). Chmelař [
45] states that, on one hand, juvenile seedlings grow faster, but on the other hand, they require shading at a young age. This could explain why, until 2017, the growth was weaker and shot up in 2019 when the seedlings had already adapted while being partially shaded by other species.
In the forestry plantings, this species demonstrated the highest mortality since 2016 when it reached 46.15% to increase to 73.08% in 2019. However, as it is a species that generally establishes and thrives well, we can only speculate that the problem could have occurred as early as during the planting, when poor planting material was used. This hypothesis is also supported by the fact that both the surviving and later added individuals demonstrated the highest values in trunk diameter increment and the second highest height increment (after
Acer campestre) of all the tree species. Trunk diameter increased from 8.89 mm in 2016 to 22.39 mm (151.8%); height increased from 102.9 cm (semi-matured stock was planted) to 184.42 cm (79.23%). The species is a so-called pioneer tree, which is capable of quickly occupying open sites and is typical in its vigorous growth when young. Chmelař [
45] adds that it is one of the fastest-growing indigenous tree species in Central Europe.
Another unwritten knowledge proved to be true with this species: in the first years following planting on agricultural land it suffers a relatively high mortality, especially if care is poor (mainly insufficient or absence watering). On the other hand, oak demonstrates good sprouting ability [
45,
46]; therefore, some individuals with the dead stem could regenerate from the root base. These individuals were classified as alive.
The fact that the sprouts were classified as living plants and their height was measured partially influenced the overall height of the species (in particular in years 2016 and 2017) that stagnated (37.66 to 38.18 cm) until 2019 when a more significant increase in the value occurred (46.19 cm, i.e., by 22.66% compared to the initial state and by 21.29% inter-annually). This increase is again partially influenced by the faster-growing sprouts. Stagnation in the height increase after planting and consequent gradation was also recorded by Jelínek and Úradníček [
37], in whose study the average height of oak seedlings in their monitored wildlife corridor in Stříbrnice was just 33.6 cm four years after planting, whereas after seven years, it had grown to 62.4 cm (by 86%).
The trunk diameter average increased regularly (6.15–6.35–6.49–7.08 mm, increase by 15.16% from the beginning and by 9.63% inter-annually), which was also due to the fact that new and non-woody sprouts were not measured, so no distortion of this characteristic has occurred. The smaller leap in 2019 is probably due to the greater growth dynamic of the already established seedlings.
In this species, just like in Carpinus betulus, a gradual increase in mortality has been recorded, but it was not very steep (in 2016, the mortality was 6.25%; in 2019, it was 17.5%).
The trunk diameter increase was slight at first, but in 2019, a significant leap has been recorded (from 5.05% and 7.95% to the ultimate 26.12%). In the two measurements taken in 2017, the height increment was actually negative (−3. 84 % and −0.63 %) while in 2016, a significant increase to 36.53% occurred. These results are consistent with the findings of Dostálek et al. [
29], who studied the impact of different types of mulch on the thriving of seedlings (an exception was an all-over mulching with straw, in which the increment value continued to increase from the second year on). The slow growth rate in the first years is also mentioned by Chmelař [
45].
A species with the second lowest mortality recorded (after Fraxinus excelsior), which increased from 5.56% in 2016 to 8.33% in 2019 (this increase is the lowest of all the tree species from the forestry plantings in the monitored period).
The success of this species is further highlighted by the good values of trunk diameter and height growth. Its trunk diameter increment was the third best (from 5.19% in 2017 to 89.43% in 2019), as per the height increment (from 12.74% in 2017 to 65.82% in 2019), it was more of a middle of the notional chart (4th place), very close behind Fraxinus excelsior and Populus alba (only the Acer campestre on the 1st place demonstrates a significantly higher value) and with a significant distance from the other species.
Looking at
Figure 7, it is obvious that the individual species split into two groups. The species with regular trunk diameter and height increment are
Populus alba,
Ulmus glabra and
Acer campestre. From the chart, it is clear that, already, the initial values from 2016 signal an increment from the time of planting, where the higher value of
Populus alba is due to planting of a different category of seedlings (semi-mature ones). The success of these species is characterized by the length of the interpolated linear curve; the identical slope of all the three curves is interesting as well.
The second group comprises the other species: their curves are short and clustered in a space with significantly lower values of the trunk diameter and height increment. There is a visible stagnation of or even drop in the values in 2016–2018 and a later leap in 2019. Although it might seem that the vitality of these trees is low, it is exactly that leap in 2019 that shows that even these trees are capable of thriving on the site and that their initial poor results were also caused by their general biological characteristics (see the comments on the individual species above).
Due to the fact that the target community of the wildlife corridor is a stand of trees, the below comments on the shrubs are just general, pointing out just some of the significant abnormalities of the individual species.
In general, the mortality of the shrub species could be evaluated as adequate and non-threatening in the occurrence of the species on the site (the overall mortality of all the shrubs was 25%). An example is
Corylus avellana, the ultimate mortality of which was recorded as 94.29%. Higher mortality was also recorded with
Cornus sanguinea (32.14%) and
Rhamnus cathartica (27.78%). Regarding all these three species, it is necessary to point out that major dying has already occurred between the planting and start of the measurements in 2016 (
Corylus—80%,
Cornus—23.81%,
Rhamnus—22.22%). The reasons are not clear; it could have been a poor-quality planting material, incorrect handling before planting or improper implementation of the actual planting. This hypothesis can be supported by the fact that an identical situation occurred among shrubs in the landscaping plantings (see below). A thriving shrub species is the
Ligustrum vulgare (in 2019, the mortality was 4.55%) and mainly
Euonymus europaeus (ultimate mortality 1.85%, which means only one piece of all the planted ones). For a comparison with the results of other authors, it was only possible to use the study of Jelínek and Úradníček [
34]. Consistency only occurs with certain species and only in initial phases of growth; later, the authors describe a frequent reduction of the shrubs, which they link with the development of the tree layer, the state of the tree canopy and inadequate conditions for shrub growth in permanent shade.
The height increment of the seedlings is initially stagnant or in the lower percentage units of (2017). Some of the seedlings have lost their terminal shoots due to drying and damage by spring frosts. In 2018, half of the species have clearly shown height increment, and in 2019, they all demonstrated a more significant growth. From
Figure 10, it is clear that the average height of all the shrubs was increasing (see the interpolated linear curve). Upon comparison with the results of Jelínek and Úradníček [
34], who monitored growth increments in a wildlife corridor Vracov between 1993–2007 (relevant data for years 1993 and 1996), it follows that comparable species demonstrated an increment but that its dynamics were, with some exceptions, significantly higher than in Šardice.
As this type of planting shares some common characteristics for all the tree species, they are not commented individually.
In general, the mortality is very low; for the
Acer campestre, Carpinus betulus, Tilia cordata and
Ulmus glabra species, it is zero. For
Fraxinus excelsior, Populus alba and
Quercus robur, it reaches 4.17/12.5/9.09%, which, with the low numbers of the planted trees, represents one dead piece of
Fraxinus excelsior and
Populus alba each and four pieces of
Quercus robur. As per the
Populus alba and
Quercus robur, these deaths occurred before the start of the measurements (see
Table 1 and
Figure 4). These results correspond with the finding of Jelínek and Úradníček [
37], who, in their partial plots in the Radějov wildlife corridor, recorded zero or just a minimum mortality of one dead piece of
Tilia platyphyllos and
T. cordata in the form of landscaping stock plants.
The values of the trunk diameter and plant height parameters do show that only
Populus alba had a significant dynamic. The interpolated linear curve in
Figure 8 is similar to that of the forestry seedlings of this species, which is caused by the use of the same planting material—matured seedlings. All other species are characterized by the stagnating height of the plants. Individual measurements show a minimum height increment or even a decrease. Decreasing or fluctuating average values of height in the particular measurements can be explained by the death of the terminal shoots of some individuals in the respective years, either due to late frosts in spring or due to periods of heavy drought during the vegetation season (in particular
Fraxinus excelsior and
Quercus robur). To some degree, the measured values could have also been impacted by an inaccuracy of measuring (+/−10 cm) and weather conditions on the measurement days (on windy days, some of the bent terminals were bending down even more, and per measurements in the leafless state, bent terminals could have given other values than with leaves). The stagnating height increment in large seedlings in the first few years after planting was also observed by Dostálek et al. [
30]. Jelínek and Úradníček [
35] describe a similar stagnation with a reference to post-planting shock. On the other hand, it is necessary to point out that, at the time of the measurement, the individual seedlings were in solitary positions (full sun on the crowns, the absence of canopy and, therefore, the absence of competition from the surrounding seedlings). Due to this, the plants did not have to invest energy in developing height, but rather on developing the assimilation apparatus in their crowns and in the trunk diameter increment, which is consistent with the recorded values per all the species: these values keep increasing consistently. The most significant trunk diameter increase has again been recorded for
Populus alba (216% in 2019 compared to the initial value in 2016), whereas
Fraxinus excelsior was the poorest-thriving of all (its ultimate increment in 2019 was only 7%, which can be interpreted as stagnation of the species). In 2019, the increment of the other species ranged between 1/5 and 1/3 of the initial value from 2016. An exception was only
Carpinus betulus, which reached an increment of 16.88% at the end of the measurement.
Although the found relative increment values of the landscaping seedlings are lower than those of the forestry seedlings, it does not mean that the landscape seedlings were not vigorous. From the perspective of mortality, on the opposite side, they are more successful than the forestry seedlings.
Looking at the mortality values, it is obvious that the situation is not much different from the shrub plantings in forestry enclosures. The high mortality situation repeats with Corylus avellana, which was over 80% in 2016, and in 2019, it was 100%. This similarity between both types of the plantings indicates a poor planting stock or technological error during the planting. Very low values were again reached by Euonymus europaeus (2.08%) and Ligustrum vulgare (3.28%). Even though wildlife damage significantly impacted the landscaping plantings (see below), in the comparison of both the types, it did not affect the overall mortality.
In the evaluation of the seedlings’ height, there is a marked difference compared to shrubs in forestry plantings. In the landscaping plantings, there is visible stagnation, sometimes even a major drop in the measured values. That is determined mainly by browsing damage caused by wildlife on seedlings that were not protected by a fence or any individual protection (the greatest damage was observed generally on Euonymus europaeus); then, there were dead terminal shoots (more commonly of Rhamnus cathartica and Cornus sanguinea) or, to a limited degree, spring frost damage. Summing up, the most vigorously growing shrub was Corylus avellana, but due to the high mortality, these data are not conclusive. The biggest drop was that of Cornus sanguinea (−9.73% in 2019): it has initially shown an impact of dead terminal shorts or entire shoot growth altogether, followed by wildlife damage. A great decrease in Rhamnus cathartica in spring 2017 was caused mainly by the dead terminal shoots and all growth, but it was also caused by high mortality, mainly on plots B1 and B2, when partial replanting (new small seedlings) was delivered. Except Rhamnus cathartica, all other species reacted to damage with very vigorous growth and a large quantity of new shoots.
5. Conclusions
The main and unique aim of the project was to compare and evaluate the suitability of two methods of planting a wildlife corridor on agricultural land, in the identical conditions of single sites (identical soil and climate characteristics of the site, identical influence of the current weather). This initial assignment suggested that every comparison with the results of similar experiments will just be a generalising one, and so, in the search of possible experiments in similar habitat conditions, the selection of potential sites was narrowed down to the Central European region. Due to the historical development and occurrence of large blocks of arable land in some post-socialist countries, this space further narrows down to the former East Germany, Czech Republic, Slovakia, Hungary and, in a limited degree, also to Poland.
Further requirements for comparable works were the used species (or at least genera) of ligneous plants and the age of the plantings at the time of the measurements not exceeding 10 years.
Leaving aside the requirement for comparable conditions of the environment, the requirement for the age of the planting has become significantly limiting for the selection of other works for comparison. Many studies focus on the youngest seedlings as per nursery production, but monitoring of fresh plantings in the countryside de facto does not take place at all. Forestry research deals mainly with large-scale plantings for forest management purposes or for land reclamation purposes. The study of plantings delivered as part of ecological measures in agricultural landscape deals rather with the effects of the established stands on the surrounding environment (mainly regarding soil erosion) or evaluates the success of plantings in relation to a certain factor of the environment (content of a certain element in the soil, hydrological regime, soil conditioning before planting or the type and intensity of care for freshly planted areas).
Due to the above factors, it was very difficult to find comparable experiments within the Czech Republic and within Central Europe.
The plantings in the regional wildlife corridor LBK2 in Šardice can be considered viable regardless of the size of the planting material used.
How does one answer the question in the title of the paper? The answer will not be a definite one. Higher mortality was that of seedlings planted the forestry way, but at the same time, these had a greater dynamic of both height and trunk diameter increments compared to the landscaping stock.
As for the landscaping stock, the empirical experience had been partially confirmed, which was also demonstrated through the measurements: reduction of the root system does affect the increment in the first years following planting. The height increment values were minimal. However, trunk diameter increments were clearly visible on all the species throughout the entire period of measurements. This leads to the question of why a trunk increases but the terminal does not. An answer could perhaps be provided by some experiment that would evaluate the development of these parameters in an area with a significantly denser spacing of the plants, where the seedlings across the planted area would not have solitary positions but would compete against each other.
In the shrub plantings, there was not such a significant difference in mortality between the plantings in areas with forestry seedlings and landscaping stock. There was a significant difference in the measured height, when the absence of protection against wildlife browsing among the landscaping stock was clearly demonstrated. In both the cases, the plantings of Corylus avellana proved to be completely unsuccessful, probably due to an unknown error in the planting technology.
For a successful planting, several important points need to be addressed throughout the process of establishing a vegetation element:
The proposed species composition must be appropriate for the site conditions;
Quality planting material (appropriate height or size category, absence of growth defects and seedling damage) must be used;
Proper handling of planting material before and during planting must occur;
Well executed planting operations, including protection of seedlings (fencing or individual trunk protection), must occur;
Good aftercare, including the correct timing of individual operations (especially watering, cutting seedlings, checking fencing or stem protection and the correct and functional anchoring of seedlings), must be provided.