Multi-scale factors affecting bird use of isolated remnant oak trees in agro-ecosystems

Abstract

With recent emphasis on sustainable agriculture, conservation of native biota within agricultural systems has become a priority. Remnant trees have been hypothesized to increase biological diversity in agro-ecosystems. We investigated how remnant Oregon white oak (Quercus garryana) trees contribute to conserving bird diversity in the agro-ecosystem of the Willamette Valley, Oregon, U.S.A. We compared bird use of isolated oak trees in three landscape contexts -croplands, pastures, and oak savanna reserves -and ranked the relative importance of four factors thought to influence bird use of individual trees: (i) tree architecture; (ii) tree isolation; (iii) tree cover in the surrounding landscape; and (iv) landscape context, defined as the surrounding land use.We evaluated species-specific responses and four community-level responses: (i) total species richness; (ii) richness of oak savanna-associates; (iii) tree forager richness; and (iv) aerial and ground forager richness. We documented 47 species using remnant oaks, including 16 species typically occurring in oak savanna. Surprisingly, landscape context was unimportant in predicting frequency of use of individual trees. Tree architecture, in particular tree size, and tree cover in the surrounding landscape were the best predictors of bird use of remnant trees. Our findings demonstrate that individual remnant trees contribute to landscape-level conservation of bird diversity, acting as keystone habitat structures by providing critical resources for species that could not persist in otherwise treeless agricultural fields. Because remnant trees are rarely retained in contemporary agricultural landscapes in the United States, retention of existing trees and recruitment of replacement trees will contribute to regional conservation goals.

Tree decline and the future of Australian farmland biodiversity

“…For example, in the United States, tree recruitment in oak savannas (Quercus spp.) is widely believed to be below replacement levels (11,12). Although the reasons for this recruitment failure are sometimes unclear (37), like in Australia, tree recruitment is particularly low on private land (12), and birds are among the animals predicted to be negatively affected by ongoing tree decline (11).…”

“…is widely believed to be below replacement levels (11,12). Although the reasons for this recruitment failure are sometimes unclear (37), like in Australia, tree recruitment is particularly low on private land (12), and birds are among the animals predicted to be negatively affected by ongoing tree decline (11). In Europe, anthropogenic systems with scattered oak trees are facing a similar situation, with insufficient recruitment reported in Spanish dehesas (9, 10) and wood pastures in the United Kingdom (29,30) and Romania (28).…”

“…For example, birds and bats control insect pests (4)(5)(6), and trees provide shade for livestock (7,8). Scattered trees occurring throughout the farmland matrix are prominent features of agricultural landscapes around the world, including in southern Europe (9,10), North America (11,12), Central America (13)(14)(15), and Australia (16). Farmland trees often represent relicts of largely cleared forest or woodland ecosystems (17,18) and are believed to play important roles in maintaining ecosystem function and farmland biodiversity (7,13,16).…”

Vegetation Restoration and Other Actions to Enhance Wildlife in European Agricultural Landscapes

“…7.2). Planting isolated trees may also provide a disproportionate positive value for wildlife and potential for seed dispersal (DeMars et al 2010;Fischer et al 2010).…”

Comparison of the current state of non-forest woody vegetation in two contrasted case study areas in Central Europe

“…They distinguished eight classes of NFWV based on all three attributes combined. By contrast, many studies are only concerned with tree vegetation -scattered trees, isolated trees or trees outside forests Levin et al, 2009;Manning et al, 2009;DeMars et al, 2010). Other studies concentrate on linear elements such as hedgerows (Burel, 1992;Barr and Gillespie, 2000;McCollin et al, 2000;Sklenička et al, 2009;Sánchez et al, 2010).…”