Predicting mechanisms across scales: amplified effects of abiotic constraints on the recruitment of yew Taxus baccata

Efforts to disentangle the mechanisms underlying large-scale spatial patterns need to rely on multi-scale approaches. We illustrate this key issue by analyzing the spatial consistency across scales of the effects of abiotic constraints on the regeneration of English yew Taxus baccata in Europe. We hypothesized that the recruitment rates in a given population would be strongly affected by water availability, which should result in a predictable pattern of regeneration success at regional and continental scales. Accordingly, we predicted: 1) at the regional scale water availability should be higher in sites occupied by yew populations than in random locations; 2) at the regional scale regeneration success should decrease when water availability is lower; and 3) at the continental scale, regeneration success should also decrease where water availability is lower, resulting in decreasing regeneration southwards. To test these predictions we first monitored seedling emergence and survival in two central Spanish populations over two years, and confirmed that yew recruitment is limited by water availability. Additionally, our analysis supported predictions 1 and 2: water availability strongly affected yew presence and regeneration success. At the continental scale (prediction 3), our results confirmed lower regeneration in southern European populations. Assessing the effect of climatic constraints across scales in key population parameters can help to improve large-scale assessments of impacts of climate change on biodiversity.