Previous research about the relationship between canopy structure and productivity of forests was mostly limited to theLAIor light interception of the canopy. Both of them were mostly found to be positively related to theNPP (e.g, Hardiman et al., 2011; Pretzsch et al., 2015). The data presented in chapter 3 did not follow this pattern. This can be explained by a generally high averageLAIeon all plots of the present study. A steeperLAIe-gradient and the presence of plots
with low leaf areas as well would have been necessary to statistically detect such a relationship.
On the other hand there was a strong and significant positive link between two measures for canopy structural diversity andANPPwood: (1) the heterogeneity of theLAIein the upper canopy and (2) a dense forest understorey below 4.5 m (highLAIeof the lower canopy). Both parameters are related to each other (r=0.66, p<0.001), which suggests that the effect is not cumulative and that the combination of both types of canopy structural diversity is responsible for the promoting effect onANPPwood.
A positive influence of biodiversity (including structural diversity) on ecosystem functioning is usually attributed to the presence of beneficial complementary effects on resource use efficiency (Naeem et al., 1994). In mixed stands complementarity often originates from tree species diversity (Liang et al., 2016). In the case of pure F. sylvaticaforests, trees of different age classes are functionally different (shade-tolerant saplings and young trees vs. light demanding overstorey trees), so complementarity can arise from a high adaption to different light regimes within the upper and lower canopy. One possible interpretation of the results is that an increased structural diversity is only a side effect of a greater tree functional diversity, which actually is the underlying cause for the promotion ofANPPwood. On the other hand, functional diversity is probably unable to unfold its full potential in forests where stand development led to a low structural diversity. So the underlying cause in a productivity promoting effect probably lies with both factors: the combination of functional and structural diversity.
One obvious reason for the productivity promoting effect of structural diversity is the higher light transmittance to lower canopy strata, where the presence of a dense understory adapted to low light levels enables an efficient use of most of the remaining light (Hardiman et al., 2013). If microclimatic differences between upper and lower canopy like a reduced vapor pressure deficit (VPD) and lower wind speeds (Larcher, 2001) are considered as well, additional complementary effects are thinkable. The more favorable within-canopy climatic conditions might cause leaves of the lower canopy to suffer less from drought stress and to maintain photosynthesis under conditions when leaves of the upper canopy are already forced to close their stomata. An increasingδ13C-content of leaves of tall beech trees along a vertical gradient between 20–40 m in Haveˇsov´a is supporting this hypothesis in indicating greater stomatal
limitation of gas exchange at greater canopy heights (Gottschall et al., unpublished). More research about the microclimate within beech forest canopies, its dependence on the stand structure in general, and canopy structural diversity in particular and implications for stomatal closure and photosynthetic activity of leaves is needed to validate this hypothesis.
The present study was carried out in a montane environment between 550–990 m a.s.l. At this elevation, growth of beech forests is limited by temperature constrains, whereas in lower elevations water is the main limiting factor (Dulamsuren et al., 2016). As long as the physiological background of the productivity promoting effect of canopy structural diversity remains unclear, it is difficult to make predictions about the generalizability of the detected productivity promoting effect of canopy structural diversity. The effect of stand and canopy structure on the light regime will probably not be as relevant for tree growth when water availability is the most important growth limiting factor. Interactive influences between drought stress and structural diversity on productivity have to be analyzed by studies with a water availability gradient and for different stand structures simultaneously.
The relevance for forestry practice of a productivity promoting effect of canopy structural diversity has to be evaluated with care. The lack of knowledge concerning the generalizability of the findings to other sites and elevations has to be eliminated before implications can be stated clearly. Stand structure and composition in general is very different in primeval and production forests. It may well be that sole alteration of a single ecosystem attribute will not have the desired effect because preconditions are too different in both systems. Additionally, increased structural diversity (especially greater vertical layering) will most likely facilitate the development of more and thicker branches at the lower trunk and reduce timber quality which is not desirable if high quality wood is the silvicultural production target. But if other aims like production of energy wood and biodiversity conservation have priority, timber quality is less important. A positive effect of structural diversity of forests on the diversity of different taxonomic groups is frequently discussed, so an additional facilitative effect on the productivity may further encourage the increase of structural diversity of forests by silvicultural means.
Future research should aim for an assessment of the effect of canopy structural diversity on ecosystem functioning within production forests of different silvicultural regimes including
alternative systems which are avoiding mono-layered even-aged stands on a broad range of sites and elevations (see for example Pukkala and von Gadow, 2012 and literature cited).