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Trees, 21 (3), ISSN:0931-1890, ISSN:1432-2285

The success of habitat enhancements is typically assessed by subsequent monitoring of the focal taxonomic group. However, enhancement actions are likely to affect other, non-targeted species. On a south-facing slope in the Swiss Jura mountains, a mixed-forest stand was thinned out by irregular removal cuttings to improve the habitat conditions for reptiles. We used this enhancement action as a case study to monitor changes in the macrofungal community that came along with it. During 3 years before and after forest thinning, the site was visited between six and twelve times per year. Thereby, all apparent fungal species were recorded along a ringlike transect, split into 32 transect sections. We used site-occupancy models to estimate fungal species richness and abundance. These models allow to separately estimate occurrence probability and detection probability of species, and to account for differences in detection probability, depending on habitat and season. After the forest thinning, the occurrence probabilities of ectomycorrhizal and saprobic fungi were significantly higher than before. As a result, we estimated a mean increase in overall species richness by 4.4% (median 4.3%, CI 2.1–6.8%) and an increase in abundance by 20.0% (median 19.9%, CI 14.8–25.7%). The two major habitat changes associated with forest thinning, the decrease in living wood and the increase in dead wood on most transect sections, could not explain the whole extent of the estimated increase in species richness and abundance. We believe that forest thinning may have fostered fungal species richness by creating a larger density and diversity of suitable microhabitats. With some caution, we conclude that the small-scale habitat enhancement for reptiles at the Bolberg, creating islands of open forest, did not negatively affect species richness and abundance of macrofungi, a non-targeted species group.

Recent biodiversity-ecosystem functioning experiments in temperate grasslands have shown that productivity positively correlates with plant species richness. Resource partitioning (in particular, nitrogen [N] partitioning) has been proposed as one possible mechanism to explain this pattern. There is evidence for interspecific differences in chemical form, soil depth, and timing of N uptake. However, it has rarely been tested whether such differences result in increased N exploitation at the plant community level. Using 15N-labeled litter that was mixed into different soil layers, we tested whether eight common grasses and forbs grown in communities of one, two, or four species differ with respect to the proportions of N taken up. from different soil depths (N niche), and how this affects the total N uptake of plant communities. We calculated proportional similarity between species (niche overlap) with regard to N uptake from the labeled soil layers; we further calculated an a priori measure of community N uptake based on species N uptake in monoculture (community niche). Interestingly, however, plant community N uptake was not affected by species richness, possibly because community-level N uptake was determined by (diversity-independent) soil N mineralization rates. We nevertheless observed a positive effect of species richness on productivity due to increased aboveground biomass: N ratios. This may indicate increased competition for light, resulting in increased amounts of comparably N-poor stem tissue. However, community N content and biomass were positively correlated with the community niche, a measure which is strongly linked to species composition. Thus, our results suggest that the studied species are generalists rather than specialists regarding N uptake depth, and that species composition was more important than species richness in determining community N uptake. Overall, N partitioning may be a less important driver of positive biodiversity-productivity effects in temperate grasslands than previously assumed.

Stefanie von Felten, Nina Buchmann, and Michael Scherer-Lorenzen Harrison et al. (2007) reported on an interesting N labeling study. Under field conditions, they assessed whether coexisting plant species of temperate grasslands show preferences for different chemical forms of nitrogen (N), including ammonium nitrate (inorganic N) and three amino acids of varying complexity (organic N). The authors found that all plant species were able to take up the full range of amino acids offered to them, as shown by N and C enrichment in plant tissues. However, plants all preferred inorganic over organic N, indicated by higher N enrichments after ammonium nitrate compared to organic N labeling. We do not object to the general interpretation of the results and the authors’ main conclusions. Yet, we would like to comment on the plant uptake of intact amino acids. When testing for significant relationships between excess C and N of plants to infer direct uptake of amino acids (Nasholm et al. 1998), Harrison et al. (2007) should have accounted for the different C:N ratios of the amino acids used. The amino acid tracers were U-C2N-glycine, U-C3N-serine, and U-C9N-phenylalanine (all N 98% and C 98%), and their ratios of C:N atoms are 2:1, 3:1, and 9:1 respectively. While the authors point out that these differences in available C may affect the preferences of plants and microbes, they omitted to consider the methodological consequences. One common problem (see e.g., Jones et al. 2005) when using dual-labeled amino acids to study organic N uptake by plants is detecting the C label in plants. Due to the high C:N ratio of plants and the high abundance of C (;1.08 atom % in C3 plants), the dilution of C is usually 60–150 times higher than that of N (Nasholm and Persson 2001). Finding a significant relationship between excess C and N requires separating the shift in C resulting from direct amino acid uptake from natural variation and analytical error. However, this is often not possible, due to rather low concentrations of tracer C. As a solution, Nasholm and Persson (2001) suggested to concentrate the labeled fraction of the plant material studied, by extracting the soluble fraction containing the label. For assessing the uptake of intact amino acids using the dual-labeling approach, the critical step is to assure that there is a theoretical possibility of detecting this uptake. From the measured values of dN (after labeling with N) the theoretical shift in dC corresponding to 100% intact uptake can be calculated (Nasholm and Persson 2001). Thereby it can be determined whether this shift is distinguishable from ‘‘noise.’’ Given the high amount of C in phenylalanine, it is not surprising that Harrison et al. (2007) found a significant relationship between excess C and N across all species for this amino acid, but not for glycine and serine. In their paper, Fig. 2A shows that shoot N enrichment over all plant species was highest for glycine and lowest for phenylalanine (among organic N forms), while shoot C enrichment was similar for all amino acids (Fig. 2C). This almost opposite pattern for C and N enrichment also applies for single species (Fig. 1), roots (Fig. 3), and microbes (Fig. 4). In the latter, C enrichment was actually highest when labeled with phenylalanine, and lowest in the case of glycine. We think that these results are due to the different C:N ratios of the three amino acids rather than indicating higher uptake of phenylalanine compared to glycine and serine, which is particularly unlikely given that phenylalanine is the largest and most complex amino acid tested. However, without significant relationships between excess C and N in plant tissues, the proportion of amino acids taken up as intact molecule cannot be estimated for glycine and serine. Moreover, although no data on amino acid concentration in the soil solution are shown, it is likely that phenylalanine is the least abundant of the three amino acids, and glycine the most abundant. Thus, the dilution of the added N tracer (equal for all N forms) by the natural abundance pool was probably least for phenylalanine and strongest for glycine, again leading to an overestimation of phenylalanine uptake when assessed by N labeling. We fully agree with Harrison et al. (2007), that a rigorous test to detect organic N uptake by plants requires compound specific isotope analysis (a combination of gas chromatography with isotope ratio mass spectrometry; see e.g., Persson and Nasholm 2001). But clearly, the results of Harrison et al. (2007) demonstrate that the use of the Nasholm et al. (1998) method to infer Manuscript received 21 June 2007; accepted 27 August 2007. Corresponding Editor: P. M. Groffman. 1 Institute of Plant Sciences, ETH Zurich, Zurich CH-8092 Switzerland. 2 Institute of Environmental Sciences, University of Zurich, Zurich CH-8057 Switzerland. E-mail: stefanie.vonfelten@ipw.agrl.ethz.ch