Time lags in functional response to management regimes - evidence from a 26-year field experiment in wet meadows

Abstract

Questions (i) How does functional composition and diversity respond to different timings and frequencies of mowing or fallow treatment? (ii) To which species assembly and related ecosystem processes do these developments correspond? (iii) What is the time course of these developments, and do they reach a stable state? Location North-western Germany Methods In 1987, a field experiment in nine wet meadow sites formerly exploited as fertilized meadows or pastures was established with four management treatments: (i) mowing twice a year in June/July and in September, (ii) mowing once a year in June/July, (iii) mowing once a year in September, and (iv) leaving fallow except shrub removal. Vegetation was recorded at least every second year and the functional composition (community weighted means) and diversity (Rao's quadratic entropy) were analysed in the course of the experiment. We selected eight plant functional traits (Canopy height, SLA, LDMC, Seed mass, Shoot cyclicity, Clonal Index, Onset of flowering, Leaf distribution) capturing the main plant life strategies. Results Temporal changes, fluctuations and divergences between treatments could be observed in both functional composition and diversity. In particular, the vegetation of the fallow treatment showed strong competition for light and a higher investment into internal nutrient cycling. Over time, all treatments respond towards more stressful conditions due to generally less intensive management compared with the former management and respective nutrient depletion, leading to a more nutrient retentive and patch holding strategy and little investment into competitive seedlings. High competition and loose abiotic filters in the fallow treatment led to a niche separation in foliar traits. Mowing on the other hand enforced a divergence of reproduction mechanisms. Conclusion We detected long-term fluctuations and ongoing changes in functional composition and diversity even after 26 years. Especially early succession was characterized by a transient loss of functional diversity, caused by strong time lags in immigration and extinction of species. Our findings stress the importance of long-term experimental studies to avoid precarious misinterpretations. This article is protected by copyright. All rights reserved.