Effects of sub-canopy weather on understory functional diversity: insights from long-term forest monitoring

Assembly rules are understudied in forest understories, especially at small scales. Partly, this is because a strong and steady filter operates on its biota, resulting in convergent trait syndromes, with a putative quasi-neutral process acting through time. The tree layer provides a distinctive microhabitat for the understory biota, by mediating light availability and through microclimatic buffering, affecting sub-canopy temperature, humidity and nutrient availability. This also affects species coexistence, persistence, and biotic assembly. The question of the effect of local weather on the functional diversity of temperate forest understories remains open. To look for an answer, we applied Bayesian regressions on data from long-term permanent monitoring plots in four old-growth beech forest sites along a latitudinal gradient, including measurements of sub-canopy weather. The availability of time series data allows accounting for time-invariant confounds. Our first finding is a great variability in functional diversity among sampling units at the 1 m2 scale, as well as among years. The second finding is that, even with data across a timespan of 12 years in four sites, each with repeated measurements, there is considerable uncertainty in weather effects, which are potentially large. In general, it is likely that increased sub-canopy air and soil temperatures lead to increased functional diversity, while there is less evidence of an effect of precipitation. The effects are consistent across different sets of above- and below-ground traits, and are stronger in relation to sub-canopy, rather than open-field weather.