Text and pic by Liina Saar
Recently published paper by Liina Saar, Meelis Pärtel and Aveliina Helm together with Francesco de Bello from Czechia (University of South Bohemia) in Oecologia revealed that grasslands with regular and long management history are assembled differently from abandoned grasslands and young developing grasslands, although these habitats seem visually similar. Results of this study showed that long-term management of grasslands allows small-scale coexistence of many species with different life-history traits and habitat requirements, whereas in abandoned or young grasslands, species with more similar characteristics and requirements co-exist. Understanding the mechanisms behind species assembly and their relationship with land-use history is vital for habitat conservation and restoration, as well for mitigating the effects of global change.
Citation: Saar, L., de Bello, F., Pärtel, M., & Helm, A. (2017). Trait assembly in grasslands depends on habitat history and spatial scale. Oecologia, DOI: 10.1007/s00442-017-3812-9 (link to full text)
Although the theory used to infer the mechanisms behind assembly patterns has been widely discussed in the literature, the empirical results vary across different spatial scales, species pool sizes, habitat types or across studied traits. Less is known how assembly mechanisms vary in dynamic landscapes subjected to changes in land-use history.
What was done?
We studied the community assembly in grassland habitats with differing land-use history, from historical grasslands to dynamic ones, focusing on principal life-history traits and combining several spatial scales. We used novel functional species pool framework proposed by de Bello et al. (2012). This method tests the trait assembly patterns (divergence or convergence) at finer scales in comparison with the patterns at broader scales (species pool), which better distinguishes biotic and dispersal-related effects from environmental filtering. By analysing the trait distribution and mean trait values it is possible to detect main processes driving assembly. For different scales of observations, we used (1) plot scale, consisting of species from the 2×2 m sample plots, (2) local community scale, consisting of species in the area surrounding the 2×2 m plots, and (3) habitat-specific species pool at the broadest scale, compiled as cumulative list of species sampled from 35 sites of respective grassland type.
What was found and what it means?
Our study showed that species assembly patterns in grasslands depend both on habitat history and on the spatial scale considered (Figure 1). Mostly random or divergent co-occurrence patterns characterised diverse fine-scale vegetation in grasslands with long continuous management history. Strong convergence patterns were detected in former and developing grasslands, indicating filtering effects of biotic interactions (exclusion of weaker competitors) and dispersal limitation. Dominance of certain trait values in dynamic grasslands indicate the need for changes in management activity and necessity for more active conservation in order to improve habitat environmental conditions for allowing more species from species pool to co-exist in a habitat.
At broader scales, mostly convergence patterns prevailed in all grassland types (Figure 1b). Differing trait patterns at finer and broader spatial scales indicate different mechanisms governing species assembly at different scales and development stages.
Figure 1 The mean trait dissimilarity (indicated with the effect size) between species a filtered from the local community to 2 × 2 m plots and b from the habitat species pool to local communities in grasslands with different development histories across all analysed traits (dispersal mechanism, life span, main pollen vector, mean plant height, mean seed weight, mode of reproduction, specific leaf area, terminal velocity). The dotted line indicates that effect size is zero, results above zero indicate ‘divergence’, below zero ‘convergence’. Grey boxplots indicate that the effect size significantly differs from zero (P < 0.05). In box plots, the median for each data set is indicated by the heavy central line, and the first and third quartiles are the lower and upper edges of each box, which is known as the interquartile range (IQR). Individual points indicate outliers (within 1.5 times the interquartile range from the upper or lower quartile)
During the past century, grasslands in Europe have undergone marked changes in land-use, leading to a decline in plant diversity both at local and regional scales, thus possibly also affecting the mechanisms of species sorting into local communities. We studied plant species assembly in grasslands with differing habitat history and hypothesized that trait divergence prevails in historical grasslands due to niche differentiation and trait convergence prevails in more dynamic grasslands due to competitive exclusion and dispersal limitation. We tested these hypotheses in 35 grassland complexes in Estonia, containing neighbouring grassland habitats with different land-use histories: continuously managed open historical grassland, currently overgrown former grassland and young developing grassland. We assessed species assembly patterns in each grassland type for finer scale – a 2×2 m plot scale from a local community pool and for broader scale – a local community from the habitat species pool for that grassland stage and observed changes in trait means at finer scale. We found that grasslands with long management history are assembled differently from former grasslands or young developing grasslands. In historical grasslands, divergence or random patterns prevailed at finer scale species assembly while in former or developing grasslands, mostly convergence patterns prevailed. With increasing scale convergence patterns become more prevalent in all grassland types. We conclude that land-use history is an important factor to consider when assessing grassland functional trait assembly, particularly at small scales. Understanding the mechanisms behind species assembly and their relationship with land-use history is vital for habitat conservation and restoration.