Background/Question/Methods Range-margin plant communities are likely highly sensitive to global change drivers such as climate. However, such drivers can act both directly and indirectly through, for example, altering the strength of biotic interactions. The relative importance of these processes is unclear, but understanding this is crucial to understanding how communities will respond to environmental change. Unfortunately, most field studies addressing these issues are comparatively short-term, whilst valuable long-term studies are rare.
We investigated the long-term dynamics of a range margin steppic plant community occurring on porphyritic outcrops near Leipzig, Germany. Study plots were mapped annually from 1980 to 2005. We examined changes over time in the composition, species richness and cover of the community, as well as the spatial relationships between individuals (an indicator of changes in plant-plant interactions). Our aim was to explore whether different species groups (i.e. annuals and perennials) showed different long-term dynamics and differential influence on community characteristics. Spatial pattern analysis allowed us to assess whether such responses are driven by external pressures or indirectly via interactions within the plant community, and whether interactions were changing in relation to climate in the manner expected from current theory.
Results/Conclusions During the study period local temperature increased whilst cover and species richness declined. Different climatic drivers were correlated with different components of the community. Perennials slowly declined in line with increased temperatures. As perennials dominate the cover in these communities reduced perennial cover led to lower community cover. Annuals responded positively to rainfall – there was no long-term rainfall trend at the site, and hence no long-term trend in the cover and number of annuals and overall species richness. Although we expected long-term changes in climatic conditions to be associated with changes in plant interactions and hence spatial pattern (for example increasing facilitation and aggregation in hotter, drier conditions) we did not see this response. Instead we observed a step-change in the relative spatial patterning of annuals and perennials driven by the influx and expansion of the annual Centaurea stoebe which appears to be suited to the warmer climatic conditions now occurring at this site.
Our results demonstrate that these arid range margin systems are responsive to climate, that different climate drivers regulate different groups of species and hence community characteristics (cover and species richness), but that spatial patterning within the community may be more strongly influenced by species turnover than gradual changes in interactions between extant species.