Immigration credit of temperate forest herbs in fragmented landscapes – implications for restoration of habitat connectivity

1. In many agricultural landscapes, it is important to restore networks of
forests to provide habitat and stepping stones for forest specialist taxa.
More knowledge is, however, needed on how to facilitate the immigration of
such taxa in restored forest patches. Here, we present the first
chronosequence study to quantify the dynamics of immigration credits of
forest specialist plants in post-arable forest patches. 2. We studied the
distribution of herbaceous forest specialist plant species in 54
post-arable broadleaved forest patches along gradients of age (20-140
years since forest establishment), distance from ancient forest (0-2600 m)
and patch area (0.5-9.6 ha). With Linear Mixed Models we estimated the
effects of these factors on species richness, patch means of four
dispersal-related plant traits and with Generalized Linear Models on the
occurrence of 20 individual species. 3. Post-arable forest patch age and
spatial isolation from ancient forest, but not patch size, were important
predictors for species richness of forest specialists, suggesting that
also small patches are valuable for habitat connectivity. Compared to
species richness in ancient forest stands, the immigration credit was
reduced by more than 90% after 80 years in post-arable forest patches
contiguous to ancient forest compared to 40% after 80 years and 60% after
140 years in isolated patches (at least 100 m to next forest).
Tall-growing species with adaptations to long-distance dispersal were
faster colonizers while species with heavy diaspores and clonal growth
were slower to colonize. 4. Synthesis and applications: We show that
post-arable oak plantations have a high potential for restoration of
forest herb vegetation. Dispersal-related plant traits play a key role in
explaining interspecific differences among forest specialists. To
facilitate forest herb immigration across all functional groups in
agricultural landscapes, we suggest to create clusters of relatively small
new forest patches nearby older forest with source populations.