Microhabitats shift rank in suitability for seedling establishment depending on habitat type and climate

1. Spatial patterns of seedling establishment have been shown to be largely affected by differences at the microhabitat scale. However, it is far from clear whether such differences are consistent both in space and time, and how they influence the spatial structure of plant populations. 2. We analyse the spatiotemporal variability in microhabitat quality for seedling emergence and survival of the Mediterranean tree Acer opalus ssp. granatense, examining seedling dynamics at the microhabitat scale in two maple populations intermingled in pine forests and two established on stony slopes during three consecutive years with different climatic conditions. 3. Seedling establishment varied markedly between microhabitats. In general, seedlings benefited from the presence of pre-established vegetation, with lower emergence and survival of maple seedlings in open interspaces than under adult maples, non-conspecific trees and shrubs. 4. However, the quality of most microhabitats depended on both habitat type and year. Emergence and survival under adult maples, heterospecific trees and in open interspaces was higher in forest than in stony-slope populations, and increased significantly with summer rainfall. Shrubs, on the contrary, showed relatively high values of seedling emergence and survival at all sites and in both dry and wet years. Consequently, the benefits of living under shrubs were comparatively greater at lower precipitation levels. 5. The different microhabitats found in heterogeneous landscapes such as Mediterranean forests vary, over relatively large spatiotemporal scales, in quality for seedling establishment. Shrubs showed the highest consistency in patterns of both seedling emergence and survival. This finding, together with the predominance of dry years in Mediterranean areas, may explain why Acer opalus ssp. granatense has a stable spatial distribution at a geographical scale with most saplings associated with shrubs. Spatiotemporal patterns of seedling dynamics at the microhabitat scale may cascade through later demographic stages, eventually impacting the spatial structure of plant populations.