DID BIODIVERSITY PLAY A ROLE?
Investigated aspects of the forest hydrological cycle that correlated significantly with the tree species diversity gradient expressed as Shannon diversity index (H’, Shannon and Weaver, 1949) were throughfall in some periods, stemflow during the whole study period (Chapter 2) and soil water extraction in a desiccation period in summer 2006 (Chapter 3). Also stand deposition of K+, Ca2+, Mg2+, Cl-, Mn2+, and H+ correlated significantly with the Shannon index of the study plots (Chapter 4).
Following the definition by Loreau and Hector (2001), a biodiversity effect is either a complementarity or a selection effect. However, the complementarity effect is often supposed to be the ‘real’ biodiversity effect. Since we did not have the possibility to investigate all species in monocultures, it is not possible to distinguish clearly between complementarity and species selection effects. However, several indications for either of these effects on the above mentioned aspects exist and will be discussed.
Throughfall was significantly positively related to the tree species diversity gradient in summer 2005, autumn 2006, and summer 2007 (Chapter 2). In addition, it tended to increase along the investigated gradient in all studied periods. Supposedly, many stand and especially canopy characteristics together influenced throughfall differences among the study plots and were well represented by the Shannon diversity index. Since for example no single tree species explained throughfall differences more frequently than tree species diversity, a selection effect is unlikely. A complementarity effect would result for example from higher crown occupancy in mixed stands compared to monospecific beech stands, as it was shown by Frech et al. (2003) for a forest stand in the Hainich close to our study plots. Larger crown overlap could lead to a higher LAI (leaf area index) with increasing tree species diversity.
However, LAI did not show a directed trend along the investigated tree species gradient (Jacob et al., 2010; M. Jacob personal communication) and throughfall increased rather than decreased with increasing tree species diversity. Although a diversity effect on throughfall is likely, it is not possible to identify the exact reason for the throughfall differences among the study plots. The relative importance of certain forest stand characteristics for throughfall seemed to be influenced by abiotic meteorological conditions.
The observed decrease in stemflow along the investigated tree species diversity gradient from monospecific beech to mixed tree species study plots was a clear species associated effect (Chapter 2). The higher stemflow amounts in the beech dominated plots can be related to a negative selection effect (Loreau and Hector, 2001), since many species with low stemflow amounts were included in the mixed species study plots and beech proportion decreased with increasing tree species diversity. Although hornbeam had high stemflow amounts as well, its proportion did not compensate for decreasing beech proportion on the study plots along the tree species diversity gradient.
At the beginning of a desiccation period in summer 2006 (June), soil water was much faster extracted with increasing tree species diversity of the study plots (Chapter 3). This resulted in low soil water reserves at the species rich plots one month later. Since water extraction of the study plots in June correlated better with tree species diversity than with any single tree species, a complementarity effect is more likely than a selection effect. Even though no indications of a spatial segregation of the fine roots of the coexisting species were found in 0 to 40 cm soil depth (Meinen et al., 2009c), roots exert different activity. Furthermore, water extraction of the herb layer probably played an important role in addition to the water uptake by the trees in the mixed study plots. This would support the complementarity effect, however, not only regarding diversity in the tree layer but also on an ecosystem level. A more rapid and more complete exploitation of soil water was also observed in highly diverse grasslands compared to species-poor grasslands and resulted in an earlier or more severe drought stress for the species in the diverse communities (Hooper and Vitousek, 1998; van Peer et al., 2004; Kreutziger, 2006; Verheyen et al., 2008).
The nutrient input by stand deposition (throughfall and stemflow) increased with increasing tree species diversity whereas the acid input decreased along this gradient and was highest in monospecific beech stands. This was related to two processes: interception deposition of ions and canopy exchange processes (leaching or uptake; Chapter 3). Deposition through gross precipitation was the same for all study plots. Interception deposition of all investigated ions
(except for SO42-) decreased with increasing tree species diversity (Chapter 4). This could be related to the higher beech dominated stands and their rougher canopies. Probably, the species specific leaf surface properties played a role as well. The outcome corresponds to the results of the throughfall investigation (Chapter 2), where also many stand characteristics probably acted together and were not possible to be separated clearly. Thereby, it is also difficult to divide a selection and a complementarity effect here. The canopy exchange rates of most of the ions (K+, Ca2+, Mg2+, Fe2+, Cl-, and PO43-) increased with increasing tree species diversity, which means that canopy leaching of these ions was highest at the most diverse study plots (Chapter 4). In contrast, the leaching of Mn2+ and H+ was highest at the beech dominated plots. Canopy exchange is affected by the wettability, physiology, and ion status of foliage which differ between the tree species and thereby between forest stands. The differences in canopy exchange rates were therefore most likely a selection effect, although a complementa-rity effect can not be excluded. Besides, the differences in canopy exchange of some ions (Mn2+, K+, Ca2+, Mg2+, and PO43-) could arise from differences in soil properties, which are likely to have an impact on the foliar ion status (Chapter 4).
Both species effects as well as complementarity effects played a role in this study. We are aware that the investigated tree species diversity gradient corresponds to single species gradients at the same time. Although beech had the opposite gradient to the tree species diversity gradient, Shannon index of the study plots was in many cases a better explanatory variable than beech proportion of the study plots.
Various (environmental) factors can act as hidden sources of variability in biodiversity experiments (Healy et al., 2008). Such ‘hidden treatments’ (Huston, 1997) are difficult to avoid as well in experiments as in observational studies like the present one. This is most likely the reason why never one factor alone clearly explained the differences among the study plots in the present study. Besides the soil characteristics as for example clay content also some stand characteristics such as stand height, stand age, and canopy roughness provided hidden treatments. Furthermore, possible reasons for different stand characteristics can not only origin from differences in species composition but also from former management practices.
Observations along the studied tree diversity gradient are important for detecting patterns that can be investigated later in experiments. The subsequent second project phase (start in spring 2008) investigates clusters of tree species combinations including monospecific clusters of all tree species in the Hainich (Leuschner et al., 2009), trying to disentangle relations of certain
species other than beech and neighbour effects. Furthermore, a tree species diversity experiment was established not far away from the Hainich area in 2003-2004 (Scherer-Lorenzen et al., 2007). This so called BIOTREE experiment will become very interesting for comparison with the results of the studies in the Hainich because of the spatial vicinity and the similar tree species composition. All species were planted in monocultures and different combinations.