Breeding bird communities in pine plantations of the Spanish plateaux: biogeography, landscape and vegetation effects.
A study was conducted during 1994 at 2 locations (near Carpio del Campo in the northern Spanish plateau and San Clemente in the southern plateau), to investigate the factors influencing the richness and composition of breeding bird communities in the pine plantations that are now prevalent (Pinus pinaster and, to a lesser extent, P. pinea and P. halepensis) after the elimination of most Quercus ilex forest. It is suggested that this re-afforestation may be encouraged by changes in the subsidy policies of the Common Agricultural Policy, which aims to decrease food production and restore the environmental diversity previously lost through agricultural intensification. The complement of bird species breeding in 88 plantations ranging in size from 0.1 to 6775 ha was determined. Plantations were characterized according to size, distance to the nearest large plantation, vegetation structure and geographical location (northern moist vs. southern xeric Iberian plateaux). Plantation size accounted for 67-75% of the variation in species richness, and was significantly related to the patterns of presence/absence of most individual bird species. Plantations smaller than 25 ha maintained only 50% of the regional pool of forest birds during breeding, whereas this proportion increased to 69-86% for plantations of 25-100 ha. Geographic location, degree of isolation and vegetation structure were also significant. Bird species richness decreased with distance to a large plantation, and increased with prevalence of undergrowth shrubs and with plant species richness. Both species richness and the incidence of individual species in plantations were affected by geographical location, being greater in plantations of the northern plateau. It is suggested that the re-afforestation of former arable land in the Spanish plateau is unlikely to increase species richness in forest bird communities, and that given the regional scarcity of many forest birds and the small scale of new plantations, re-afforestation schemes are unlikely to promote rich forest bird communities. It is concluded that since plantations are unsuitable habitats for those bird species breeding in Spanish dry cereal croplands, and such birds have a high conservation value because of their small and declining populations, the overall environmental benefits of large-scale afforestation programmes will not include an increased diversity of birds.