Community-weighted means and functional dispersion of plant functional traits along environmental gradients on Mount Kilimanjaro

Questions: How do community-weighted means of traits (CWM) and functional dispersion (FDis), a measure of trait variability, change in response to gradients of temperature, precipitation, soil nutrients and disturbance? Is the decrease in trait similarity between plots continuous or discontinuous? Is species turnover between plots linked to trait turnover? Location: Mount Kilimanjaro, Tanzania, Africa. Methods: Sixty plots were established in 12 major vegetation types on Mount Kilimanjaro, covering large gradients of temperature, precipitation, soil nutrients and anthropogenic disturbance representing the dominant ecosystems in East Africa. Environmental data, plant abundances and plant traits were recorded for each plot. Trait CWM and FDis were related to environmental factors with partial least squares regressions. Trait similarity between pairs of plots was assessed with a null model approach. Results: Both CWM and FDis of most traits responded strongly to environmental factors, particularly to precipitation and disturbance. FDis of traits associated with growth and reproduction mostly increased with temperature and precipitation, and decreased with disturbance. Pair-wise plot comparisons revealed an inverse relationship of trait similarity with differences in temperature, precipitation and anthropogenic disturbance. However, changes in similarity were often discontinuous rather than continuous. Several vegetation types differed strongly in species composition but not in traits. Conclusions: Trait dispersion indicating functional niches increased with productivity and temperature. Conversely, low-productivity conditions were characterized by trait convergence. Discontinuous changes in trait similarity between plots suggested tipping points at which trait expressions change strongly to adjust to environmental conditions. Large sections of the temperature gradient were characterized by species turnover with only minor changes in traits, indicating that the functional composition may be resilient to gradual environmental changes until a tipping point is reached.