The concept of plant available water capacity (AWC) of soil was proposed by Veihmeyer & Hendrickson (1931), who defined it as the water content of soil between an upper limit, called field capacity, and a lower limit or permanent wilting point. Thus, AWC characterizes the reservoir of the soil, which holds water that may be withdrawn by plants. AWC is a central property for soil fertility and thus of great relevance for forestry planning (AK STOK, 1996; Silva et al., 2014).

The influence of soil fertility on tree growth is a central paradigm of forest site classification (e.g. Barnes et al., 1982; Pyatt et al., 2001). Since the effort for soil examination is quite high, height growth has served as a major indicator for site fertility in forest classification systems in Europe and North America for a long time (Tesch, 1980). The foundation for this method was the observation that dominant height versus age relationship remains quite stable even at different management regimes (Huang & Titus, 1993). Although soil effects are obvious and undisputed, only a few more recent studies quantify soil effects on height growth of adult trees in Central Europe. Investigations on the country-level imply only a low contribution of soil parameters for describing the height growth compared to temperature and precipitation (Brandl et al., 2014; Burggraef et al., 2016; but see Boncina et al., 2023). On the other hand, some studies show marked soil effects along a large climatic gradient in the Bavarian Alps (Mellert & Ewald, 2014a,b), while others highlight effects of stand density and water availability on the height growth of trees (Toraño Caicoya & Pretzsch, 2021). Investigations involving microsite effects on single tree growth are scarce (Rabbel et al., 2018; Hartl et al., 2021). To our knowledge our investigation is the first study about the effect of small-scale variation in AWC on height growth of single trees.

However, single tree growth in seed crop stands is an important issue especially for forest genetics and therefore a central motivation of our study. To broaden the research base, the project sensFORclim aims to locate climate resilient populations of important main tree species by various methods (climate envelopes, dendroecology, physiology, and genetics) at different spatial scales (continent, region, stand) (Schmied et al., 2023). Within the framework of sensFORclim the current study encompasses actual and potential seed crop stands in Southern Germany. Working at the single tree level, it is important to know whether phenotypic differences, e.g. in tree growth, are based on individual genotype performance (Rellstab et al., 2015; Pfenninger et al., 2021) or microsite effects due to small-scale variation in soil parameters. This issue is not only interesting within population genomics but also of practical relevance for forest seed harvesting and breeding (Clark & Wilson, 2005), since soil effects could confound the effectiveness of plus-tree selection conducted by tree breeders (Cornelius, 1994).

Therefore, we investigated possible small-scale soil effects on achieved tree height in more or less even-aged monospecific stands of three important tree species (Abies alba (Aalba), Fagus sylvatica (Fsylv), Picea abies (Pabie)) within their niche space. For the characterisation of the niche zone (NZ), we used the climatic marginality towards the rear edge (Mellert et al., 2016), i.e. the dry and warm border of species distributions (Hampe & Petit, 2005), as within this climatic zone important demographic and genetic processes are taking place (Fady et al., 2016). This study comprises a large gradient from the cold and wet mountainous climate to the warmer and drier temperate climate in the lowlands of Southern Germany (Schmied et al., 2023). We surveyed the soil in close proximity to trees sampled by estimation of soil texture and gravel content, following the German soil survey guidelines (KA5, AG Boden, 2005; AK STOK, 1996). Based on these data, we calculated the AWC of the pedons (Johnson, 1963) associated with the specific target trees.

We hypothesised that trees growing on pedons with higher AWC reach greater heights and assume that this effect interacts with the NZ of the species. According to our assumption, achieved tree height should increasingly benefit from higher microsite-AWC with increasing marginality of the NZ. This effect should be most obvious at shallower soils with a high heterogeneity in AWC. If such an effect should exist this would be of general relevance in forest research. In our research area, genetic evaluations that focus on single trees and plus-tree selection would be most affected.

Regarding the site trophy, the traditional expectation is that height growth increases among sites from acidic to base-rich soils (Neebe & Hofmann, 1982). However, on base-rich calcareous soils with high carbonate content in the whole soil profile and a low loam content, tree growth can be limited due to a shortage of N, P, K and trace elements (Mellert et al., 2014a; AK STOK, 1996; Kolb et al., 2018).

The mixed regression model for testing AWC effects by NZ on height growth explained 43% of the total variance (marginal R², Table 1). Mean tree growth was similar at the first three niche zones (NZ: 1_O – 3_M) levels but significantly lower at rear edge (4_R) (Fig. 2a). This reflects the low soil moisture regime at rear edge (4_R) stands. With respect to studied hypothesis, we observed significant differences in achieved height between NZs when accounting for different levels of AWC at the small-scale. We found a negative effect of AWC on growth at optimal sites, but a significant positive effect on growth of stands at intermediate sites (2_I) and the rear edge (4_R) with unfavourable soil conditions. For marginal sites with zonal soil conditions (3_M), we did not find a significant influence of individual AWC.

As we did not see a stepwise increase in the AWC influence with increasing marginality along the NZs from optima (1_O) to the rear edge (4_R), the result only partially confirms our hypothesis. Additionally, we did not expect a negative effect of AWC at optimal sites. However, under the most unfavourable site conditions with low water regime and water storage due to larger gravel contents and/or sandy soil texture (4_R), the promoting effect of AWC on single tree growth was maximal. The estimated mean partial effect on height growth over the whole range of AWC values was almost 8 m at those sites (Fig. 2). The marked contribution of the hypothesised interaction effect (21% of explained variance) indicates that AWC was particularly affecting growth on sites in the optimal zone (1_O) and the rear edge (4_R) representing sites with low water regime.

The growth performance of the three tree species was significantly different, with fir and spruce equal to each other and beech with markedly lower growth performance (Table 1, Fig. 3). The NR was not statistically significant.

The main result of our study that trees growing on soil patches with higher AWC reach greater heights on sites with a low water regime generally confirms our assumption. This small-scale effect should be most pronounced in warm-dry climates where such microsites with higher AWC provide a water-flux for a sustainable growth and buffer against dry periods. Our results widely support this assumption, as height growth increased significantly with AWC at patches on the sites with the lowest water regime. However, on sites with better water supply the effect is not so clear.

Effects of AWC on tree growth have been increasingly investigated in recent years (Mellert et al., 2018; Rohner et al., 2018; Chakraborty et al., 2021). However, studies considering the small-scale effects of AWC are rare (Rabbel et al., 2018; Hartl et al., 2021). The study from Thomas et al. (2018) deducted fine-scale soil effects on single trees (P. abies) indirect from wood isotopic signature and found a consistent stomatal control over gas exchange along the soil moisture gradient.

We show that small-scale soil variability is a significant factor for the modification of height growth of single trees in Southern Germany, especially at stony sites with low water regime. Genetic evaluations (Pfenninger et al., 2021) and plus-tree selection (Cornelius, 1994; Clark & Wilson, 2005) focusing on height growth may be confounded by soil effects in stands growing on such sites. The large contribution of the interaction between local AWC and NZ to the overall explained variance supports the view that soil water storage capacity is generally important at the rear edge of species distribution (Mellert et al., 2018; Chakraborty et al., 2021).

Only a few dendroecological studies consider small-scale soil effects on tree species of the temperate zone (Rabbel et al., 2018; Hartl et al., 2021). Whereas the results of the German study (Rabbel et al., 2018) supports the view of an increasing importance of soil moisture on the small-scale at dryer sites, Hartl et al. (2021) showed a negative effect of moist microsites on radial growth of Pinus sylvestris in Scandinavian forests due to a shortening of the growing season at such cold patches. This might also be responsible for the negative effect of AWC in our study. In spring, higher water storage in pedons with high AWC may delay soil warming and tree growth compared to drier sites also in the cool-temperate and humid climate of the optimal NZ (cf. Hartl et al., 2021).

As our study address the question of small-scale soil effects on the pedon level along a national climatic gradient, recent studies carried out on the site level involve the more specific question whether AWC provide a significant buffer for trees to bridge dry periods, an issue of increasing importance in the face of climate change. Although we have not investigated temporal aspects, we investigated climate soil-interaction along a gradient from cold-wet to warm-dry condition. The results of gradient studies can be related to temporal studies through the concept of spatiotemporal substitution. Chakraborty et al. (2021) investigated stands along transects in the transition zone of beech to oak. From the pairwise analysis of the radial growth on dry plots (AWC < 60 L m^-2 vs less dry plots (AWC > 60 L m^-2) they concluded that changes in soil water conditions could influence beech trees’ growth on their drought limit under the changing climate even at the microsite level (plots with 8 to 12 trees). A study based on national forest inventory data and the European soil data base (Mellert et al., 2018) supports the hypothesis that important European tree species maintain their water supply in the more arid southern margin of their distribution range by shifting to sites with higher water storage than in their humid core distribution range. Such effects at the plot level are also reported from a study in France (Lebourgeois et al., 2005), in which soil water capacity strongly modulates ring characteristics and climate-growth relationships. On the contrary, Lévesque et al. (2016) described that AWC was a poor predictor for beech growth in the Swiss alps at sites where mean annual precipitation was at least 1000 mm. However, at the southern latitudinal distribution limit of the species in Spain, Jump et al. (2006) showed that a shortage in soil water supply explained the growth decline of beech trees. Effects of AWC on radial growth are not only described for beech, but also for spruce, fir and other species (Rohner et al., 2018). It would be very interesting to study whether such effects found at the site level apply also for the small-scale. Should this be the case, this would be another factor to explain the patchy pattern of forest drought damages that have been found in practice and were considered in some studies (Pfenninger et al., 2021; Frei et al., 2022; West et al., 2022).

The NR was not statistically significant in our regression model (Table 1). However, the trend of single tree growth implied that the effect of site trophy is rather optimal at intermediate sites. This is in accordance with the view that optimal growth occurs rather on nutrient rich but slightly acidic sites (Brandl et al., 2014; Burggraef et al., 2016), whereas on calcareous soil with maximum base saturation growth can be reduced due to unbalanced nutrition and reduced water supply (Mellert & Ewald, 2014a).

Overall, the result supports the main assumption, as the height growth increased significantly with the AWC of the microsites on the sites with the lowest water regime. Comparable studies about small-scale effects of AWC on height growth of single trees are lacking. A major cause for this lack probably lies in methodological difficulties and the effort required for studies estimating the water balance of a stand at the small-scale (Wilson et al., 2001). Also, the precision in determining basic physical soil parameters is generally low (Mellert et al., 2008). An AWC estimation based on one sample per pedon remains necessarily vague. Additionally, the estimation of height growth entails further uncertainties. Therefore, further investigations are needed to substantiate the results of this pilot study.

In summary, small-scale soil variability appears to be a significant factor for the modification of height growth of single trees in Southern Germany, especially at sites with a low soil water regime. Consequently, any study focusing on single tree growth, but does not consider small scale variability in AWC, neglects an important factor affecting single tree growth in such sites. In our research area, genetic evaluations focusing on single trees and plus-tree selection can be particularly affected by such soil effects. However, soil influences can already be assessed with simple soil texture estimation in the field as shown with this study. Therefore, small-scale soil variability should be considered more often in scientific studies and practical evaluation involving single tree performance, especially in genetic environmental associations studies and tree selection for tree breeding. The methods and results of our study should be transferable to temperate forests growing on a wide range of soils.