Effects of land-use history on size structure of holm oak stands in Spanish dehesas: implications for conservation and restoration

摘要

Dehesas, rangelands occupied by scattered oak trees and characterized by silvopastoral uses, cover about 3.1 million ha in south-western Spain. There is considerable debate about the long-term persistence of holm oak (Quercus ilex) populations in dehesas, since most stands are overaged and seedlings and saplings are sparse. The forest cycle has been disrupted in most dehesas. Regeneration has been inhibited since stands were opened for agriculture and grazing. Oak diameters from three land-use groups (young dehesa [YD], middle-aged dehesa [MD], and old dehesa [OD]) in Caceres Province, Spain, were compared. These groups differed in the age of the land-use system, i.e. time since the original Mediterranean forest was cleared. The dehesa systems were established about 80 (YD), 150 (MD) and 500 (OD) years ago. An analysis of 66 holm oak cross sections revealed a close correlation (r~2 = 91.2%) between tree rings and diameters, so that diameter seems to be a reliable indicator of tree age. Nested analysis of variance showed significant variation in diameters between the land-use groups. There is generally a positive relationship between tree age and the age of agrosilvopastoral use of the dehesas. Sparse holm oaks in the dehesas are primarily remnants from the first forest cycle. Local differences in growth conditions (for example soil quality and tree density) contribute further significant diameter variation on a between-plot level. Diameter structure of abandoned dehesas showed two peaks and a high proportion of trees in the smallest size class. This indicates that the forest degradation process is reversible. An effective regeneration policy should promote a rotating 20- to 30-year set-aside of dehesa parcels.