The analysis of both tree water use (Chapter 3) and productivity (Chapter 4) revealed a moderate but comparably strong effect of tree diversity on transpiration rate T as well as on biomass allocation (Bm) and relative growth rates (RGR) under ample water supply. For T and RGR, yield increased on average by ~10% from monocultures to the 3-species mixtures (Fig. 6.1.), although, overyielding was not be linearly enhanced by species richness.
Similarities in the yield of 3- and 5-species assemblages indicate the importance of species mixture per se, while the actual species number in such tree communities seems to be of minor importance. Whether the amount of species might be more effective along a broader tree diversity gradient with higher species numbers or a higher range of diversity levels with accordingly more species combinations cannot be answered by this data. Our findings agree with the idea that functional performances might reach saturation at intermediate levels of species richness, when functional diversity is redundant at higher levels (Cardinale et al.
2006, 2011, Potvin and Gotelli 2008). However, other studies on tree diversity experiments also provide some proof for steady increase in the BEF relationship (e.g. Kunert et al. 2012).
While the increase in stand transpiration in Chapter 3 was mainly interpreted as a selection effect, the additive partitioning approach (AP) after Loreau and Hector (2001) clearly indicated a higher importance of species complementarity in case of tree growth and stand productivity (see Chapter 4). Due to the strong connectivity of growth and transpiration (Law et al. 2002) and the comparably high net diversity effects observed, the involvement of different mechanisms is rather unlikely. Thus, it seems crucial to reappraise the possibility for separating selection and complementarity effects and the applicability of AP for tree diversity studies with such a specific experimental design.
The arithmetic mode of AP quantifies the impact of species selection in overyielding by the covariance of species’ yields in monoculture (M) and their deviations from expected relative yields in mixtures (RE). To consider parallelism between tree water use and growth, it seems plausible to compare species’ rankings of performances in both services. T. cordata 150
Synthesis realized the highest RE values (overyielding) in growth-related traits, but most likely also in transpiration, as indicated by water-use related leaf area (LA) and sapwood area (SA). Albeit T. cordata reached highest transpiration rates of all monocultures, the M-values in growth rates and biomass allocation were exceeded by those of F. excelsior due to higher water-use efficiency. The lower position of T. cordata in the M-ranking for productivity parameters accounts for a lower rating of the selection effect than it could be expected for water consumption. Nevertheless, for our tree sapling experiment the use of AP remains questionable as an overestimation of complementarity effect seems likely. Albeit AP is commonly applied in tree diversity studies (Kunert et al. 2012, Grossiord et al. 2013, 2014), it was originally developed for herbaceous communities, which are highly flexible in terms of replacement and competition-induced alteration of species abundances (Loreau and Hector 2001). In comparison to those short-lived plants, our less dynamic tree sapling assemblages, with fixed numbers of individuals and relatively few species can hardly achieve such clear dominance of a superior species within such a short experimental time frame. Canopy extension of the more productive tree species clearly allows for suppression of inferior species (without replacement or outcompeting only), but the AP procedure might be insensitive to those asymmetries in tree species contribution, which are anyhow prominent for competitive dynamics and yield in young plantations. Therefore, the process of species selection might be underrated and a quantitative separation between both components of the diversity effect is insufficient for a short-running experiment with tree saplings. For instance, overyielding in the 3-species mixture Acer-Carpinus-Fraxinus was realized by the asymmetric performance between the superior F. excelsior and its inferior neighbors. F. excelsior increased RGR by nearly 100% in comparison to the monocultures (reductions in A. pseudoplatanus and C.
betulus by ~40% and ~10%, respectively), which clearly points to the selection effect being the relevant driver in this 3-species mixture (see Fig. 4.4.). Despite of that, the numerical benefit for one out of three species (~33%) reveals a relevant complementarity effect in AP, with comparable impact like species selection on the observed net diversity effect (Fig. 4.1.).
Summarizing the results, evidence for the co-occurrence of species selection and species complementarity in this experiment is obvious. The high contribution of strong-performer species (T. cordata, F.excelsior) to overyielding in water consumption and biomass allocation gives proof for the selection effect. Moreover, the occurrence of complementary behavior is likely, because of (i) transgressive overyielding in some species combinations (higher yield in comparison to the most productive monoculture), and (ii) the synchronous benefit of neighboring species in some other mixed species combinations. Tree diversity effects were
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CHAPTER 6
found to be on average rather low, but the variability among species combinations points to the importance of specific neighbor compositions and tree neighbor identity for favorable interactions. Furthermore, it was shown that species identity was the most important driver beside of soil water availability in tree community functioning. In fact, both tree water consumption and growth performance varied among species by the factor of two, which clearly controlled the stand level performance of the mixtures. These findings on the importance of tree diversity, tree neighbor identity and tree species identity are in agreement with meta-analysis and literature reviews on tree diversity studies (Nadrowski et al. 2010, Scherer-Lorenzen 2014), assuming a relevant but inferior role of the biodiversity-ecosystem functioning (BEF) relationship in forest ecosystems, with dependence on actual species compositions.
It needs to be mentioned that the appearance of diversity effects is not self-evident for such young sapling assemblages. First, tree diversity effects in the BIOTREE experiment (fine root growth) were reported for the sixth year after establishment (Lei et al. 2012a). For the subtropical BEF-China, tree diversity was a bad predictor for sapling growth in two year old
Figure 6.1. Range of relative net diversity effects in stand level performance for water consumption (transpiration rate (T, mm d-1), transpiration rate per leaf area (TLA, ml m-2 d-1)) and productivity (biomass (Bm, g) and relative growth rate (RGRtotal, g g-1 450 d-1)) for assemblages with three (3) or five (5) species, and under moist (left) or dry (right) conditions. Asterisks indicate significance net effects according to the Grand mean over all mixtures (*: p<0.05; **: p<0.01).
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Synthesis plots (Lang et al. 2012, Li et al. 2014); even though positive interactions were already observed. This is in accordance with findings that diversity effects on productivity of plant communities become more prominent over time as the magnitude of complementarity increases when experiments are elongated (Cardinale et al. 2007, Reich et al. 2012). In the pot experiment, eminent interactions among tree saplings occurred already during the second year after establishment. This is due to the specific planting scheme and the confined space for the assembled saplings. Narrow distances (15-20cm) and limited soil volume in the pots (~0.05m3) forced coexisting plants to interact immediately after establishment, or at least during the second year of growth. Such a design is certainly inconvenient with respect to the long-run requirements of tree saplings; but it fits the demand for this short time experiment (2 years), and accounts for the differences in aims and results to other tree diversity experiments (Sardinilla (Potvin et al. 2011), BIOTREE (Scherer-Lorenzen et al. 2007), Forbio (Verheyen et al. 2013, etc.).