Early measures for forest stand establishment within the context of climate change in Europe

Schlüsselbegriffe: mechanische Bodenvorbereitung, Trockenheit, Waldbau­maßnahme, biogeografische Regionen

Available at https://doi.org/10.53203/fs.2303.1

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Abstract

Under exacerbated environmental conditions due to climate change and more frequent and more severe disturbances the establishment of forest stands has become a challenge for forest managers. This study aims at comparing stand establishment methods on a European scale. We differentiated between five biogeographic regions to detect similarities and differences regarding current practices as well as trends considering the climatic background using an expert survey. Our results allowed identifying two clusters, one boreal cluster and one European mainland cluster. In both cases mechanical soil preparation and measures against drought are seen as key factors. Experts from the boreal region give high importance to appropriate plant material and fertilization for growth acceleration, and consider insects and fungi as a primary threat. In the mainland cluster, experts rely mainly on silvicultural measures and expect damage by game to be highly detrimental for stand establishment. Across Europe the preferable countermeasure to face drought is not irrigation but a combination of strategies related to mechanical site preparation, plant material, planting method, and silviculture. This study identifies three focal areas for forest practitioners to consider when establishing new forest stands: 

(1) careful selection of the plant material; 

(2) suitable site preparation and 

(3) prudent deliberation on beneficial silvicultural options.

Zusammenfassung

Aufgrund der negativen Folgen des Klimawandels ist die forstliche Bestandes­begründung zu einer Herausforderung für Waldmanager geworden. Ziel der vor­liegenden Arbeit ist es, Bestandesbegründungsmaßnahmen auf europäischer Ebene zu untersuchen. Nach Ausscheidung fünf biogeografischer Regionen wurde mithilfe einer Expertenbefragung ermittelt, welche Gemeinsamkeiten und Unterschiede bzw. welche Trends es hinsichtlich aktueller Praktiken zur Bestandesbegründung gibt. Die Ergebnisse zeigen, dass zwei Cluster unterscheidbar sind, ein borealer Cluste­­r und ein europäischer Festlandcluster. In beiden Fällen gelten mechanische Bodenbearbeitung und Maßnahmen gegen Trockenheit als Schlüsselfaktoren bei der Bestandesbegründung. ExpertInnen der borealen Region legen großen Wert auf ge­eig­netes Pflanzenmaterial und Düngung zur Wachstumsbeschleunigung und sehen Insekt­en und Pilze als zentrale Risikofaktoren. ExpertInnen des Festlandclusters setzen haupt­sächlich auf waldbauliche Maßnahmen und betrachten den Wild­einfluss als Hauptschadensfaktor. Für alle Regionen besteht die erfolgversprechendste Maßnahme gegen Trockenheit nicht in Bewässerung, sondern in einer Reihe nachhaltiger Strategien zur Verbesserung der Wasserversorgung durch mechanische Bodenvorbereitung, geeignetes Pflanzenmaterial, geeignete Pflanzmethoden und Waldbau. Unsere Studie erlaubt folgende Empfehlungen für die forstliche Praxis der Bestandesbegründung: 

(1) Wahl von geeignetem Pflanzmaterial, 

(2) Maßnahmen der Bodenvorbereitung und 

(3) wissensbasierte Anwendung waldbaulicher Maßnahmen.

Introduction

Early measures to ascertain the successful establishment of forest crops  play a key role during the forest management cycle (Mc Carthy et al., 2017; Orazio et al., 2019). To achieve successful establishment on a particular site, the young plants need to develop a robust root system for stability as well as for sufficient water and nutrient uptake. During the juvenile stage, trees are highly sensitive to unfavourable site characteristics, to weed competition, and to eventually occurring human treatment deficiencies (Lyr, 1996; Ritchie & Dunlap, 1980). This holds true and is even exacerbated in the context of climate change (Norby & Jackson, 2000).

In Europe, 94% of the forests are semi-natural, 3.9% are plantations and 2.2% are classified as undisturbed. The forests provide the habitat for 454 native tree species (Rivers et al., 2019) out of which 100 to 120 are of relevance for forestry (Alia et al., 2021). In total, the share of conifers and broadleaves is 46% and 37%, respectively, the rest represents mixed forest stands. The average percentage of areas under special rules of conservation with mostly no wood harvest is 24%. The contribution of the single European regions to this figure reveals large differences, e.g. in Central-West Europe 92% of the forests are available for wood supply while in South-East Europe 53% are managed with a prevailing commercial purpose. Out of European forests, 66% are naturally rejuvenated (Forest Europe, 2020a), the rest is artificially reforested.

The choice of early measures for the forest stand establishment depends on the geographic region where the measures are applied (Ammer et al., 2011; Ramantswana et al., 2021). Even though tree species distribution and tree species diversity are driven by nature, the human intervention has an enormous impact on the forest coverage. In particular, the relevance and priority of establishment measures may differ from country to country and between regions from the North to the South and from the West to the East. In the Nordic region, site preparation after final felling is needed for a better seedling survival rate and increased growth (Sikström et al., 2020). Even if mounding has replaced disc trenching as the most common site preparation method for planted seedlings, disc trenching is still in use when sowing seeds (Official Statistics of Finland, 2022). Mounding is also considered an efficient measure against the pine weevil (Hylobius abietis L.) damages (Löf, 2000). For Norway spruce (Picea abies L. Karst.) in Finland Luoranen & Viiri (2021) demonstrated a higher survival rate of the young plants in mechanically prepared spots in comparison to young plants in untreated patches. Polish studies revealed that mechanical site preparation is recommended not only to create optimum conditions for the pine growth in naturally regenerated sites after clear cut, but also when planting and seeding (Aleksandrowicz-Trzcińska et al., 2017). For the successful establishment of sessile oak (Quercus petraea Matt.) and maritime pine (Pinus pinaster Ait.) in central France, mechanical site preparation has been identified as a key factor since it enables to control aggressive weeds such as moor grass (Molinia sp.) and eagle fern (Pteridium aquilinum L. Kuhn) (Auzuret et al., 2014). In Italy, the management of poplar plantations that are expected to assume relevance in the future due to the steadily increasing demand for quality fiber and renewable energy sources (Anderson et al., 2015), face intensive treatment. The key elements in this respect are insect control, fertilization, pruning, and irrigation which always more becomes an essential premise for the plant survival (Marchi et al., 2022). Conversely, chemical weed control and fertilization activities are loosing importance, in full agreement with the FSC and PEFC regulations (Nermin & Francesco, 2021). For poplar plantations in Sweden (Böhlenius & Övergaard, 2015) demonstrated that the choice of plant material and mechanical site preparation are crucial for the stand establishment. Poplar plantations play a significant role also in the French forestry sector, where they are intensively managed, but not irrigated.

The success of one and the same measure may vary with the region of application (Löf et al., 2012). In an experiment with conifer species the primary advantage of soil scarification was weed control (Munson et al., 1993). In contrast, in boreal forests the most beneficial effect of the same measure was seen in the ability to break up the thick humus layer and to trigger the mineralization process, which leads to enhanced nutrient uptake (Schmidt et al., 1996). And again, the same measure can bring yet a different benefit in semi-arid zones, where harrowing is applied mainly to decrease water evaporation and to increase infiltration (Querejeta et al., 2001). 

Available literature (Cortina et al., 2004; Coyle & Coleman, 2005; Fiala et al., 2010; Stanton et al., 2002; Stanturf et al., 2001; Steele et al., 2021) suggest that three main groups of stand establishment measures can be classified:

(1) use of appropriate plant material, 

(2) resistance to biotic damages, 

(3) enhancement of site conditions and increase of resource availability. 

The latter group includes three elements, mechanical site preparation, water supply, and nutrient supply (Cortina et al., 2011). The provision of resources undeniably is a key requirement for plant growth. Apart from the fact that water as well as nutrients can be added from outside to a stand (irrigation, fertilization), there is a second strategy that aims to achieve the same goal by making efficient use of the resources that are already present in a stand. This is called the sustainable way of water and nutrient supply, within the framework of this study. All before-listed measures, if adopted properly, are competent to contribute to this effect: Mechanical site preparation, by loosening up compacted soils, increases infiltration rates, improves hydraulic conductivity, reduces water runoff, prevents water stagnation, slows down capillary evaporation, facilitates root penetration, and removes weed competition (Cortina et al., 2011; Löf et al., 2012; Lowery & Gjerstad, 1991). The plant material, by means of the stomatal behaviour and the root shape of the used exemplars, has impact on the uptake of water and nutrients (del Campo et al., 2020). Right planting techniques might optimize the root geometry and this way also contribute to the successful uptake of resources (Lobet et al., 2014; Steudle, 2001). And silvicultural measures, when directed to create favorable environmental conditions, maximize the use of available resources. This is comparable to techniques in agroforestry targeted to create positive interactions between plant layers, this way reducing transpiration (Padilla & Pugnaire, 2006).

This study aims to collect information about the main stand establishment techniques (measures) currently practised in Europe, and to detect trends in future reforestation strategies. It is of interest to view the captured measures against the climatic background within greater European regions such as north Europe, central west Europe, central east Europe, and the southern part of the continent. In analogy with the Köppen-Geiger climate map (Geiger, 1961) in the updated form according to Peel et al., (2007), these regions in rough outlines exhibit the following climatic conditions: North Europe has a boreal climate with a rather evenly distributed precipitation. More northerly in this region, the characteristic is subarctic with cold winters and cool summers, while the southern part is humid continental with reasonably cold winters and mild summers. The temperature is specific to the climate zone D (cold) which means that the warmest month is beyond 10°C, and the coldest month is equal to or below 0°C. In central west Europe, the prevailing climate type is temperate oceanic with warm summers, no dry season, and a rather equally distributed rainfall. The temperature as being characteristic of the climate zone C (temperate) includes a warmest month beyond 10°C, and a coldest month ranging between 0°C and 18°C. In central east Europe, the dominating climate type is temperate continental which indicates reasonably warm summers, cold winters, no dry season, and also a principally evenly distributed rainfall. Also here, the temperature is typical for the climate zone D as described before. In south Europe where the temperature likewise represents the C-zone, three main climate types are predominating: The warm mediterranean climate includes drought periods with a precipitation of the driest month in summer being below 40 mm. The warm oceanic climate integrates hot summers with a mean temperature of the warmest month greater than 22°C, but no dry summers, according to the updated Köppen-Geiger climate map. The temperate oceanic climate, likewise present in south Europe, was already characterized above. 

The following research questions are posed for the study:

1. What are the key aspects at the establishment phase? 

2. What are the most severe problems/obstacles encountered during stand establishment?

3. Have the priorities of early measures changed during the last ten years in the context of global warming, and how are they expected to change within the upcoming ten years?

4. Are there patterns in the differences and/or similarities between climatic regions, and if so, how can these be interpreted?

2 Methods

The data acquisition for the study was done by a questionnaire survey. This technique was preferred to a literature review because it enables to strive for information about most recent developments in stand establishment practices, which may have not been documented through publication yet. Experts and practitioners tend to publish their methods with a time-lag and rather in national journals (grey literature) – not as international scientific papers. The survey technique provides a high level of uniformity and comparability of the gathered information (Brancato et al., 2006). 

The arrangement of Europe in large-scale biomes was oriented towards the concept of the European biogeographical regions as defined by the European Environment Agency (Cervellini et al., 2020; Walday & Kroglund, 2002). Five regions were differentiated: Boreal, Atlantic, Continental, Atlantic-Continental, and Atlantic-Mediterranean. For each of these regions, practitioners and experts in the field were selected.

 

2.1 Survey design

The workflow for the survey followed the principal guidelines indicated in the specialized literature (Bird, 2009; Brancato et al., 2006; Sarantakos, 2017). After defining the objective of the study, the conceptualization, operationalization, and pilot testing of the survey were achieved through expert group meetings attended by researchers and forest managers with expertise in this field. In agreement with the above-mentioned classification of stand establishment measures, the focus of the questionnaire was on three principal topics: 

i) mechanical site preparation for reducing soil compaction, 

ii) water supply, including irrigation and sustainable alternatives and 

iii) nutrient supply, including fertilization and sustainable alternatives. 

Irrigation was defined as water being carried from an external water source to a forest stand, while sustainable alternatives were defined as all measures to improve the benefit from the water already available in the stand.

 

2.2 Survey structure and implementation

Research questions one and two were intended to assess the key aspects for a successful stand establishment on the one hand (question one), and the principal risks on the other hand (question two). Both were open questions, and extent as well as level of detail of the given answers, was left to the choice of the respondents. Research question three aimed to determine the urgency of measures against drought, measures of fertilization against nutrient deficiency, measures of fertilization to accelerate growth, and measures against soil compaction, for the past (10 years back), the present, and the future (10 years from now). It represented closed and quantitative questions, and an ordinal scale ranging from one (very low) to five (very high) was adopted.

The questionnaire featured a two-part structure, whereby general information was collected in the first part and a description of concrete measures in the second part (supplementary material). The information needed to answer the research questions, was mostly sourced from part one. Part two was designed to contribute to a better understanding of the statements provided in the first part, especially for the aggregation of the answers into categories with the intention to reduce their complexity. The survey was issued in four languages: English, German, Italian, and Polish (see supplementary material). It was delivered as an e-mail attachment and the respondents filled out the form independently so that any bias caused by the presence of the interviewer was excluded.

2.3 Selection of the survey respondents

The respondents were searched within the before-listed five biogeographic regions. Five main countries were selected to represent the regions, Finland, Poland, France, Austria, and Italy. Additional five countries were added later: Norway, Slovakia, Hungary, Bulgaria, and Spain (see Table 1). The data collection was supported by the personal networks of the authors within these countries. This was advantageous since the success of a survey research to high degree depends on the willingness of the respondents to take their time and to properly fill out the questionnaires.

We targeted 65 senior experts (respondents) with a strong professional background in operational practice, public administration, or the academia. All respondents had expertise in the standard management systems (high forest, short-rotation plantation, and nursery) as adopted by FAO (2020).

 

2.4 Evaluation of the survey

Research questions one and two as being open questions elicited a large variety of answers by the respondents. Therefore, the provided answers were grouped to obtain a concise overview on the practices in place, and to make the outcome manageable for further analyses. For that purpose, two aspects out of all listed aspects per question were taken in consideration, respectively. The reason was that all filled questionnaires contained at least two entries for each of the two questions, but some contained not more than two entries. The process of aggregation comprised two steps, leading to several sub-categories which have been further condensed to main categories. An example might illustrate this: One filled questionnaire for question one (What are the key aspects?) provided the following answers: 

1. choice of the appropriate tree species, 

2. mulching the area, 

3. slash removal previous to mulching in order to keep the mulch-layer thin. 

In this case, the first two aspects were taken in consideration. Answer one was attributed to the sub-category choice of the right tree species/clone, and to the main category silviculture, and answer two was matched with the sub-category soil preparation, and with the main category mechanical site preparation (which besides the soil preparation contained mechanized weed control as second sub-category). In case of uncertainty about the meaning of given statements, a clarification was achieved through personal contacts between the lead author and the survey respondents, and through the lead author’s visits to sites in Finland, Poland, Austria and Italy between July and October 2021. 

On the assumption that early measures might have a positive effect on sustainable water/nutrient supply as explained above, in the context of question one (What are the key aspects?) it was possible to determine whether the practices described by the respondents were comprehended as sustainable methods of water or/and nutrient supply or not. This was done based on explicit reference associated with the answers to questions one and/or two, and/or by referring to the context (part two of the survey with description of concrete measures), and personal communications with the respondents. Again, an example might be helpful for the understanding: A respondent listed as a key aspect deep site preparation. As can be seen, there is no explicit statement available indicating that this measure might serve as sustainable method for the water supply. Even so, as principal problem (question two What are the most severe problems?) the respondent mentioned the water scarcity. Moreover, in the second part of the questionnaire (description of concrete measures) the same respondent described deep ploughing as a measure out of the complex mechanical site preparation, and explicitly stated that it should be done at the end of the vegetative season prior to planting so that the soil has the chance to absorb and store the water during the winter. Hence, in this case the listed key aspect deep site preparation was interpreted to serve as sustainable measure for the water supply, in the intention of the respondent.

 

2.5 Statistical analysis

For the evaluation of research questions one and two, the entries were counted (frequencies) per region and category and expressed as percentages of the total number of entries for a respective region and category. A statistical assessment of the region-specific differences was done by a Pearson's Chi square-Test. To that end, the percentages (question one) were converted into corresponding numeric values. For question three, not available values (NAs) were replaced by the respective median of available values, according to region, measure (against soil compaction, against drought, against nutrient deficiency), and time dimension (present, future, past) (Table 2). On that basis, the mean values were calculated for illustration purposes in the following text. Changes in the urgency of specific measures were gauged through the difference in their intensity rating between future and past, separately for each region. A statistical verification was done by a Kruskal Wallis-test (W. H. Kruskal & Wallis, 1952), followed by the Dunn-test as a post-hoc assessment of differences between the single pairings. For the evaluation of research question four, two techniques were applied, the PAM (Partitioning Around Medoids) technique (Kaufman & Rousseeuw, 2009), and the non-metric multidimensional scaling (nmds) algorithm (Borg & Groenen, 2005; Galbraith et al., 2002). PAM is a clustering method of the medoid type (Rai & Singh, 2010), which represents observations as cluster centres (medoids) that best represent a particular cluster. As consequence, the medoid can be used to characterize a cluster in its essential traits. The PAM method has two major advantages: first, that categorical variables can be integrated and, second, that it is less sensitive to outliers. For mixed data types, consisting of numerical and categorical data (see Table 2), the Gower distance was applied to quantify the distance between individual points (Ranalli & Rocci, 2019). The number of clusters adopted for the analysis was selected after determining the Silhouette width. The estimate for the goodness of clustering was the Silhouette coefficient (Rousseew, 1987). For the nmds-plots, the Bray-Curtis dissimilarity measure was chosen. The estimation of the goodness of fit of the nmds model occured by the Kruskal stress type one which was interpreted along the rating scale (J. B. Kruskal & Wish, 1978): 0.2 and below (poor fit), 0.05 and below (good fit), 0.00 (perfect fit). The level of significance for all statistical analyses of this study was α=0.05. All analyses were performed with the statistical software R (R Core Team, 2022).

3 Results

In total, 65 potential respondents were contacted 43 of whom filled out the questionnaire. Table 2 offers an overview of the characteristics of the survey respondents and the answers to questions one and two (two entries per respondent, columns 4 to 7 respectively) and to question three (containing ordinal values from one to five in columns 8-19). 

 

3.1 Most relevant aspects and threats at the establishment phase

The questions about the most relevant aspects (research question one) and threats (research question two) obtained a wide range of answers, eventually grouped into nine large main categories (Table 3). Five categories grouped the answers to question one (aspects), namely: mechanical site preparation, irrigation, plant material, planting method, and silviculture. Four categories grouped the answers to question two (threats) as follows: drought, insects/fungi, game damage, and insufficient silviculture. The preferences for the analyses have been counted as follows. Since two entries per survey were taken into account (see the explanation above) and 43 surveys were filled, in total 86 answers for each research question are available. For the amelioration of site conditions 28 votes were counted, 24 of which included mechanical site preparation, and 4 referred to irrigation. Out of the first 24, 16 referred to soil preparation as the main purpose of mechanical site preparation, and 8 to weed control as its principal scope. And finally, according to 23 out of these 24 votes, the referenced measure represents a sustainable way for the provision of water or/and nutrients, respectively. All remaining category-specific information provided by Table 3 are interpreted in a similar manner.

Drought emerged as the most severe threat to stand establishment, since it was mentioned by 66% (57 out of 86) of the experts (Table 3). However, irrigation (its essential countermeasure) only scored four votes.  Most respondents (i.e., 84% or 72 out of 86) indicated silviculture (24), mechanical site preparation (23) plant material (18), and planting method (7) as the most beneficial measures when establishing a stand, and at the same time identified them as sustainable methods for the provision of water or/and nutrients (Table 3).

The distribution of votes differed significantly among regions (Figures 1A, 1B). When regional distributions were analysed, Pearson's Chi square Test yielded a p < 0.001 for question one (Figure 1A) and p < 0.001 for question two (Figure 1B), respectively.

For the boreal region, the two fundamental aspects of stand establishment are mechanical site preparation and plant material. The most detrimental factor is insects/fungi. In contrast, in all other parts of Europe, silvicultural measures, followed by mechanical site preparation, are considered most essential for a successful stand establishment, and drought is considered the most detrimental factor, followed by game damage (Figures 1A, 1B).

 

3.2 Expected relevance of measures in the future

Research question three targeted to assess the urgency of measures against drought, fertilization activities, measures against soil compaction, and eventually other measures (see the survey in the supplementary material) from the view of the experts. The aspect other measures was scarcely answered (23 respondents completed this part of the questionnaire out of 43, and only one entry was available for some of the five regions), therefore a statistical evaluation for other measures was not feasible. Figure 2A provides a region-wise illustration of future urgencies of the above-listed measures according to the experts. Figure 2A concentrates on the contrast between the urgencies in the future and in the past, again, according to the regions and the estimations of the experts.

In all regions measures against drought are expected to become the most important in the future (Figure 2A). When the future was compared with the past (Figure 2B), the importance of fertilization promoting growth is expected to increase significantly in the boreal region, while in all other regions the largest increase is expected for measures against drought and measures against soil compaction. 

A Kruskal Wallis test and a Dunn test confirmed the statistical significance of the differences for measures against drought (p-value of 0.003), for fertilization in favour of accelerating growth (p-value of 0.016), and for measures against soil compaction (p-value of 0.015) (Table S1 in the supplementary material). Furthermore, significant differences were found to occur mainly in pairings where the Boreal region is represented, suggesting that different trends are present between the Boreal region on the one hand and all other regions on the other hand (Table S1 in the supplementary material).

 

3.3 Identifying biogeographic clusters for early measures

Occurring similarities and dissimilarities in stand establishment practices between the regions are further illustrated and analyzed by two visualization techniques. The nmds algorithm depicts the 43 observations (Table 2) in a multidimensional space for which five dimensions were specified. The integrated convex hulls delineate the minimum areas of each of the biogeographic regions and make these regions evident. The PAM cluster analysis arranges the same 43 observations around the centers of the defined number of clusters. In our case, two clusters have been set since the respective value of Silhouette width for two clusters is the highest. The results of the two visualization procedures are illustrated in Figures 3A and 3B.

The Kruskal stress type 1 as an estimate for the accuracy of the nmds plot (Figure 3A), is 0.04 which signifies a good fit between the plotted and the real values/surveys. According to Figure 3A, Boreal ranges apart from most of the other regions, only with Atlantic-Mediterranean it has a strong overlapping. Continental and Atlantic-Continental to large extent cover the same area. Atlantic ranges far away from Boreal and rather tends to overlap with the two continental regions. The mediterranean region to noticeable extent covers the area between the boreal zone and the continental zone. The goodness of fit of the cluster model (Figure 3B) as quantified by the Silhouette coefficient is 0.15 for cluster one, 0.27 for cluster two, and 0.36 for the total model (see Table S2 in supplementary material), signifying a weak structure. Even so, it is evident from Figure 3B that the two outlined clusters are well differentiated with only two observations (surveys) ranging in the intersection zone. To interpret the abstract cluster landscape against the background of the real biogeographic regions, the results of the cluster landscape from Fig. 3B are linked with the biogeographic regions in Table 4.

Out of the 43 filled surveys, 17 belong to cluster one and 26 to cluster two. All representatives of the boreal region belong to cluster one. Atlantic-Mediterranean seems to be an intermediate region with a strong representation in cluster one (five surveys, see Table 4). Still, the majority of its elements belong to cluster two. Both Atlantic-Continental as well as Continental predominantly represent cluster two. This way, the results of the cluster analysis (Figure 3B, Table 4) are consistent with the outcomes of the nmds plotting (Figure 3A).

The described limitations notwithstanding, in the following sections we go along with the result of the cluster analysis and assume two clusters: a so-called boreal cluster on the one hand, and a European mainland cluster on the other hand. The medoid (center) of the boreal cluster is represented by survey 4, the medoid of the European mainland cluster by the survey 38. The medoids express the main properties of the clusters, according to the PAM clustering method (Table 5). 

For cluster one (boreal cluster with its medoid in survey no. 4) the most important measures are mechanical site preparation and plant material, while the most severe problems are drought and game damage. Conversely, for cluster two (European mainland cluster with medoid in survey no.38) mechanical site preparation and silvicultural measures are the most important measures and drought the most severe problem.

4 Discussion

4.1 The main categories of establishment measures and threats to cope with

This study concentrated on the measures for successful establishment of forest crops and looked for similarities and/or differences between five biogeographic ecoregions (Boreal, Atlantic, Atlantic-Mediterranean, Atlantic-Continental, and Continental) in Europe. Data collection was carried out by a questionnaire survey. For the quantitative evaluation of the results of the survey on the most important measures at the establishment phase and the obstacles during the establishment, the variety of answers provided by the respondents were condensed to four categories including mechanical site preparation, plant material, correct planting, and silvicultural measures as salient aspects for successful stand establishment, and drought, insects/fungi, game damage, inadequate silvicultural strategies as most severe problems at stand establishment.

The overwhelming majority of the experts qualified the measures for successful stand establishment as sustainable strategies to face water or/and nutrient shortages. An equally large majority pointed at drought as the most severe future threat (Table 3). Experts also indicated that irrigation is currently just a secondary measure to overcome water scarcity: much more importance is attributed to silvicultural strategies (i.e., right timing of planting, choice of tree species, and regeneration/cutting method), immediately followed by mechanical site preparation (Table 3). Nevertheless, financial support from the government or other sources for investments in irrigation (e.g., irrigation facilities, storage reservoirs for irrigation purposes) could influence that view and affect future strategies.

 

4.2 Priorities of stand establishment measures in the different biogeographic regions

In answer to the research questions one and two, the study found that mechanical site preparation and plant material are the most important measures for the boreal area (Figure 1A, Table 5). A preference for the plant material (nurseries and tree breeding) of the boreal biome as a distinctive feature was also found by (Kolström et al., 2011) who scrutinized all climate change adaptation measures in forestry, based on the same European regions as adopted for the present study. In all other regions i.e. the European mainland regions, more importance is attached to silvicultural measures, followed again by mechanical site preparation (Figure 1A, Table 5), according to the here achieved results. When addressing the most urgent threats, insects and fungi raise the highest concern in the Boreal region, while game damage and drought are especially alarming in the other European regions (Figure 1B, Table 5). The boreal concern with insects and fungi likely reflects the current pine weevil (Hylobius abietis L.) infestation that is hitting the Scandinavian forests, favoured by the monospecific character of the stands in that area that nowadays are largely reforested by planting (Lalik et al., 2021). Low tree species diversity in general, and specifically the small number of tree species used in the Nordic countries (Official Statistics of Finland, 2022) might decrease biodiversity, which is conducive to the mass spread of pests (Lundgren & Fausti, 2015). Conversely, in mainland Europe, drought is considered the main problem, followed by game damage (Figure 1B, Table 5). A further difference between the boreal region and mainland Europe is that respondents from the former area do not consider silviculture deficits to be a problem, while those from the latter one do consider it as an important issue (Figure 1).

When concentrating on the quantitative assessment of how significant measures are expected to be in the future according to the experts, especially when compared to the past (research question three), three measures are found to be different among the European regions at the selected significance level, and namely: measures against soil compaction, measures against drought, and fertilizing for growth acceleration (Table 1S supplementary material). Measures against drought will play a dominant role in the future in all regions, but their relevance will increase much more in mainland Europe than in the Boreal region (Figure 2B). Similarly, activities against soil compaction will become more relevant in mainland Europe than in the Boreal region. The most likely reason for that difference is that mechanical site preparation is already common in the boreal region, and therefore its application is not expected to increase as much as in mainland Europe, where mechanical site preparation is still relatively uncommon. That is not to say that its use will not increase in the Boreal region as well, and figure 2B shows that such increase will occur: it is just that its expected increase is not as large as estimated for mainland Europe.

Moreover, in the boreal region, great emphasis is placed on fertilization for growth acceleration (Figure 2B, Table 5), which is in significant contrast to mainland Europe (Table 4, Table 5) where fertilization is expected to be even less of importance than in the past (Figure 2B). When accomplished at the time of the planting or immediately after, fertilization is beneficial for tree growth and stem quality, depending on the involved tree species, the planting methods and the growth region (Marshall et al., 2022; Smethurst, 2010). The basic understanding of fertilization refers to its application at the stand establishment (Smethurst, 2010). However, when directing the focus on a mere economic profitability, it is more viable to apply fertilization at a later stage of the stand development e.g. in the pole stage wood to shorten the time lag between the investment and the final harvest (Pukkala, 2017). By nature, the firstly listed understanding of the term (fertilization at stand establishment) was valid for the present study since it deals with early measures at stand establishment. In the Scandinavian countries fertilization practices, after introduction and widespread application in the 1960s gradually came out of fashion. Only over the recent years the interest has been increasing again (Lindkvist et al., 2011). The positive expectations towards fertilization from the side of the practicioners of this region, might be motivated by the fact that in both boreal countries taken into consideration by the survey (i.e. Finland and Norway) the use of fertilizers to accelerate growth is encouraged by the respective governments as a way to increase CO2 storage through faster trees growth. Both countries, therefore, have great confidence in this measure as part of their climate change mitigation strategy (see forest.fi, 2020). 

The results of a Dunn test concerning fertilization for enhancing growth disclose the boreal region being present in all pairs where significant differences are detected (Table 1S in supplementary material). Thus, the difference between Boreal and the rest of the regions in this respect is statistically substantiated. In total, when considering the measures against soil compaction, those against drought, and those for fertilization, the pairwise comparison by the Dunn test shows that Boreal is present in eight out of nine differing pairings (Table 1S in supplementary material). In outline, an exceptional position of Boreal in contrast to all other European biomes also reflects from (Kolström et al., 2011): In this study, in four out of eight assessed categories of adaptation measures against climate change, Boreal markedly differs from the other regions.  The identified distinction has bearing on the assumption of two clusters as will be discussed in the in the next paragraph.

 

4.3 The two clusters in stand establishment practices in Europe

In the before-said, two rather homogeneous regions became apparent, a boreal region and an area complecting the European mainland countries.  Research question four explicitly raised the question about a grouping of the European regions in the context of measures for stand establishment. A cluster analysis conducted to this end evidenced, in outlines, two graphically distinguishable clusters (Figure 3B), the here so-called boreal cluster and the European mainland cluster. Since the boreal cluster as delineated by the cluster analysis, comprises elements that pertain to regions other than Boreal (Table 4), and since the associated accuracy coefficient points at a relatively weak cluster model (Table 2S in supplementary material), this distinction admittedly is carried by some abstraction and generalization. Even so, it was adopted out of the following considerations: Firstly, all surveys of the boreal region without exception range within one and the same cluster (cluster one). Secondly, the result of the cluster analysis is not interpreted in isolation but in conjunction with the above-presented statistical evaluations in the context of research question three (Table 1S in supplementary material), that clearly suggest that a difference between the boreal region on the one hand and all other European regions on the other hand, is given. Thirdly, the conformity of both approaches (the analyses to research question three, and the cluster analysis) is corroborated by the fact that the information contained by the medoids (which are a component of the performed cluster analysis) (Table 5) finds a clear correspondence in the (above debated) messages reflecting from the Figures 1A,B and 2A,B.

 

4.4 Interpretation of the identified stand establishment priorities in the context of climate change

According to the European Environment Agency (EEA, 2017) some far-reching trends as a result of climate change are to be expected for the European biogeographic regions. The corresponding main trajectories of relevance for this study, are: the boreal region will experience a temperature rise, an increase in precipitation, a northward movement of species, and an increasing potential for forest growth and increasing risk of forest pests, the continental region will face an increase in heat extremes, an increase in risk of forest fire, and a decrease in summer precipitation, the atlantic part of Europe will be confronted with an increasing winter precipitation and an increasing damage risk from winter storms, and the mediterranean countries will undergo an increase in heat extremes, an increase in risk of droughts, of forest fires and desertification, an increase in risk of biodiversity loss, and a decrease in precipitation.

The increasing risk of forest pests as predicted by the EEA report for the boreal region, finds a direct correspondence in the results of this study (Figure 1B). However, the most positive scenario is predicted for the boreal biome, as can be extracted from the before-referred statements. The expected northward migration of tree species in combination with the increasing potential for forest growth, might partly explain the efforts to improve the genetic stock in the boreal region (Figure 1A). An enlargement of the potential tree species pool by highly prospecting candidates such as Douglas-fir, Sitka spruce, lodgepole pine, and different poplars (Haapanen et al., 2015), opens new possibilities for forestry. Though, the growth, the growth rhythm, and especially the flowering phenology of trees are determined not only by the water availability and the temperature but also by the phototrophic conditions of a region of interest (e.g. the night length governing the bud set etc.), which is much of importance for the performance of young trees in a non-domestic environment (Hannerz, 1998).  In addition, various types of pests and pathogens might appear and compromise the success of introduced tree species (Yanchuk et al., 2009). In this light, the need to elaborate much on the generation of approriate plant material as expressed by the practicioners of the boreal region, is better understandable.

In all other bioclimatic regions i.e. the here so-called European mainland regions, the negative expectations prevail, according to the EEA report. The most pressing scenarios are the rise of temperature and the decline in precipitation during the vegetation period, as can be seen from the assertions referred above. In this view, the statements of the experts of the European mainland are also better traceable since in their opinion the aspect of utmost importance is drought and the corresponding countermeasures. First of all, this holds true for the warm mediterranean region where drought periods by definition occur regularly, according to the Köppen-Geiger climate map (Peel et al., 2007). Still, it is also valid for the other European mainland regions, according to the here achieved results (Figure 1B, Figure 2). To this must be added that the techniques for site preparation and against soil compaction which play a key role for the experts of these regions, overwhelmingly are seen as sustainable measures to face drought (Table 3). The more so, measures out of the category silviculture that are fundamental for the stand establishment in the eyes of the experts of these regions (Figure 1), nowadays play a key role in the fight against risks as contained by the EEA report and listed above (storm, pests, and biodeversity loss) (e.g. Tognetti et al., 2022). The term silviculture in this connection might be considered just a synonym for the expression climate smart forestry which aims to build resilient forest ecosystems (Nabuurs et al., 2018). Also with respect to the found relevance of silvicultural measures, therefore, the results of the study mirror the conditions and constraints that climate change has generated in the forests of the European continent.

Ultimately, the experts consulted for this study identified three groups of measures for stand establishment, which constitute the very core of the forestry strategies devised to face climate change: plant material selection, site preparation, and silviculture. To a large extent, this is in agreement with the suggestions outlined in (Forest Europe, 2020b). That report clearly states that “the best opportunity to enhance the adaptive capacity of forests is during a regeneration period (…) and in the early stand-development stages” (p. 34). In the context of the cited report, the strongest focus is on plant material. In that respect, concrete measures cover: the choice of tree species, genotypes, and provenances; the introduction of new species (non-natives); and the managed relocation of native species i.e. the so-called assisted migration (Williams & Dumroese, 2013). Secondly, that report points to the importance of provisioning water to the stands. As essential sub-strategies leading to this goal, it does promote irrigation methods (e.g. the building and restoring of small water reservoirs) but shows a clear preference for more sustainable alternatives. As such, it recommends specific measures within site preparation (e.g. weeding) and silviculture (e.g. suitable tree species combination). Our study fully agrees with those strategies by zooming in on the specific measures  to be taken at the early development stage (i.e. plantation and establishment). 

5 Conclusion

The evaluations of the survey on forest stand establishment practices in Europe led to the distinction of two regions: Boreal and mainland Europe. For the boreal region, most attention is on mechanical site preparation, followed by plant material. The most severe threat for this biogeographical context comes from the insects, and fertilization for growth acceleration is expected to be the most promising measure in the future compared to the past. Experts from mainland Europe, rate silvicultural measures as most important for successful reforestation, followed by mechanical site preparation. Here, the principal threat is seen in drought, followed by game damage. Measures against drought and against soil compaction, are considered as most beneficial for the stand establishment in the future. In all regions the listed key aspects for stand establishment overwhelmingly are seen as effective sustainable measures for the water procurement.

Based on these results, three guidelines for the future practice are expressed: 

i) The most relevant threat in the future is drought. 

ii) The most promising countermeasure against drought is not irrigation but 

iii) rather any of the following techniques for sustainably increasing water use efficiency and availability, such as mechanical site preparation, appropriate plant material, and silvicultural strategies.

Funding

This research was supported as a project entitled GUIDE (12/21) by the grant of FHP (Forst Holz Papier) cooperation platform in Austria.

Supplementary material

Table S1 shows the results of the Kruskal Wallis-test and the Dunn test on the differences between the biogeographic regions in terms of the importance that by the experts is attributed to the listed early measures for stand establishment in 

i) the future and 

ii) the future in contrast to the past.

Table S2 contains the Silhouette coefficients as measures for the goodness of clustering­­­.

The original version of the survey is also included in the supplementary material.

Conflict of interest statement

The authors declare no conflict of interest.

Author Contributions

Conceptualization, B.E., M.vL.; data collection, B.E., S.B., K.B., T.L., C.O., R.S., M.vL; methodology, 412 B.E., M.vL; software and analyses, B.E., writing – original draft preparation, B.E., M.vL, S.B., K.B., T.L., C.O., writing – 413 review and editing, B.E., S.B., K.B., T.L., C.O., R.S., M.vL; supervision, M.vL, funding acquisition, M.vL. 

Acknowledgements

We want to thank Hannes Schönauer, Federal Research and Training Centre for Forests, Natural Hazards and Landscape (BFW), Institute of Forest Growth, Silviculture and Genetics, Vienna, Austria, for the initial teaching on stand establishment practices in the field, Viktoria Valenta, Federal Research and Training Centre for Forests, Natural Hazards and Landscape (BFW), Institute of Forest Growth, Silviculture and Genetics, Vienna, Austria, for editing the questionnaire, Lucio Taverna, Regione Autonoma FVG, Udine, Italia for the instructive teaching on nursery practices, Fabrizio Nardin, GreenWood Resources Inc., for the illustrative teaching on poplar SRC. Our special appreciation goes to all respondents to the survey.

Supplementary Materials

Supplementary Materials as PDF

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