Associations between canopy and understory species increase along a rainshadow gradient in the Alps: habitat heterogeneity or facilitation?

摘要

Spatial associations among overstory and understory species tend to increase on gradients from wet to dry climates. This shift in the strength of spatial associations has usually been attributed to shared abiotic requirements between canopy species and understory assemblages within communities and/or to an increase in habitat heterogeneity in dry climates and therefore higher beta diversity. On another hand, more important positive effects of tree canopies on understory species in drier climates may also explain stronger associations and higher beta diversity. We examined these three hypotheses along a strong rainshadow gradient that occurs from the wet external Alps to the dry inner Alps by analyzing with correspondence analysis and canonical correspondence analysis the species composition of 290 releves of forests dominated to different degrees by Abies alba and Picea abies. We found important differences in climatic requirements for Abies and Picea, with Abies occurring in warmer and drier habitats than Picea. The understory species associated with these two species showed similar correlations with temperature but not with moisture, with understory species of Picea-communities having stronger xeric affinities than understory species of Abies-communities. We found no significant associations between canopy species and understory composition in the external Alps despite the fact that Abies and Picea occurred in substantially different environments. In contrast, Abies and Picea occurred in more similar environments in the inner Alps, but the understory assemblages associated with either Abies or Picea were significantly different. This increase in canopy-understory associations was in part determined by strong differences in moisture between southern and northern aspects in the inner Alps, which affected both canopy and understory species distributions. However, differences between the canopy effects of Picea and Abies also appeared to contribute to stronger associations between canopy and understory species, and consequently to increase beta diversity. This pattern only occurred on southern aspects of the inner Alps but was highly significant. Our results suggest that species distributions may be continuous on the wet ends of moisture gradients but discrete on dry ends. Relatively discrete communities at stressful ends of gradients appear to develop as a result of both habitat differentiation and the positive effects of overstory species.