Dynamic niche sharing in dry acidic grasslands -a 15N-labeling experiment
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In this study we investigated the temporal variability of N-source utilization of pioneer plant species in different early successional stages of dry acidic grasslands. Current theory states that plant species occupy distinct ecological niches and that there are species-specific, temporal N-uptake patterns. We hypothesized that small-scale dynamics in the natural habitat may affect niche differentiation among plant species. We investigated N-uptake patterns of two co-occurring plant species from different functional groups (Corynephorus canescens, Rumex acetosella) under natural conditions using 15N-labeled nitrate and ammonium in three different early successional stages during early and late summer. We found (1) marked seasonal dynamics with respect to N-uptake and N-source partitioning, and (2) different uptake rates across successional stages but a similar N-form utilization of both species. Nitrate was the main N-source in the early and later successional stages, but a shift towards enhanced ammonium uptake occurred at the cryptogam stage in June. Both species increased N-uptake in the later successional stage in June, which was associated with increasing plant biomass in C. canescens, whereas R. acetosella showed no significant differences in plant biomass and root/shoot-ratio between successional stages. Ammonium uptake decreased in both species across all stages with increasing drought. Nevertheless, the peak time of N-uptake differed between the successional stages: in the early successional site, with the lowest soil N, plants were able to extend N-uptake into the drier season when uptake rates in the other successional stages had already declined markedly. Hence, we found a pronounced adjustment in the realized niches of co-occurring plant species with respect to N-uptake. Our results indicate that ecological niches can be highly dynamic and that niche sharing between plant species may occur instead of niche partitioning.
KeywordsEcological niches Nitrate Ammonium Succession Stable isotopes
We would like to thank N. Krämer, M. Seither, T. Assaf, S. Unger, K. Ortmeier and others for help in the field; B. Teichner for laboratory assistance and K. G. Rascher for help with statistical questions and proofreading. Further we are grateful to A. Leßmann (LRA Fürth, Untere Naturschutzbehörde) for the possibility to do fieldwork at the nature protection area “Hainberg”. Helpful comments of A. Kahmen, M. Bartelheimer, B. Hartard and S. Unger and two anonymous referees are gratefully acknowledged.
All experiments comply with the current laws of the country in which they were performed.
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