Inferring shifts in tree species distribution using asymmetric distribution curves: a case study in the Iberianmountains
Résumé
The objectives of this study were to examine altitudinal shifts in tree species distributions over one decade to quantify the potential for tree migration. We analysed presence-absence data using two successive surveys of the Spanish Forest Inventory in five Fagaceae tree species (two temperate: Fagus sylvatica and Quercus petraea, one sub-Mediterranean: Q. faginea and two Mediterranean: Q. suber and Q. ilex) in two mountain ranges (the Pyrenees and the Iberian system). Half of the fitted altitudinal distributions were skewed and required use of an asymmetricmodel for unbiased estimates of optimumaltitude and changes in the probability of presence along the altitudinal gradient. For each species andmountain range, shifts were considered to have occurred when the difference in optimum altitude was significant or when differences in probability of occurrence between the two surveys demonstrated the occurrence of colonization or extirpation events.