Sunken roads as habitats for forest plant species in a dynamic agricultural landscape: effects of age and isolation

Aim This study tests the hypothesis that linear, woody habitat patches surrounding small, sunken rural roads not only function as an unstable sink but also as a true, sustainable habitat for forest plants. Furthermore, factors affecting the presence of forest plant species in sunken roads are determined. Finally, the implications of these findings for the overall metapopulation dynamics of forest plant species in fragmented agricultural landscapes are assessed. Location The study area, c. 155 km(2) in size, is situated in a fragmented agricultural landscape within the loamy region of central Belgium. Methods Forest species presence-absence data were collected for 389 sunken roads. The effect of area, depth, age and isolation on sunken road species richness was assessed using linear regression and analysis of variance (anova). Analysis of covariance was employed to study the interaction between age and isolation. Differences in plant community dispersal spectra in relation to sunken road age and isolation were analysed by means of linear regression and anova. Results Sunken roads proved to function as an important habitat for forest plants. The sink-hypothesis was falsified by a clear species accumulation in time: sunken road species richness significantly increased with the age of the elements. Sunken road age mainly affected species richness through effects on both area and depth, affecting habitat quality and diversity. Furthermore, sunken road isolation had a significant impact on species richness as well, with the number of forest species decreasing with increasing isolation of the elements, indicating dispersal limitation in sunken road habitats. Moreover, a significant age x isolation interaction effect was demonstrated. Differences in regression slopes for isolation between age classes revealed that the effect of isolation intensified with increasing age of the elements. Differential colonization in relation to forest species dispersal capacities probably account for this, as confirmed by the analysis of sunken road plant community dispersal spectra, with the fraction of species with low dispersal capacities increasing with increasing age and decreasing isolation of the elements. Main conclusions During sunken road development, area and depth increase and, gradually, suitable habitat conditions for forest plant species arise. Depending on their ecological requirements and dispersal capacities, forest species progressively colonize these habitats as a function of the element's isolation. The functioning of sunken roads as a sustainable habitat for forest species enhances the metapopulation viability of forest plants in agricultural landscapes and has important consequences for forest restoration practices. Moreover, the results of this work call for integrating the presence of forest species in small-scaled linear habitat patches in forest fragmentation studies.