Colonization credit of post-agricultural forest patches in NE Germany remains 130-230Â years after reforestation

In many temperate regions worldwide, a large portion of deciduous forest grows on former agricultural land, while a smaller portion is 'ancient' forest on sites with no historical record of agricultural land use. The differences in species diversity between ancient and post-agricultural forests have been well documented. However, in regions where forest fragmentation occurred only a few centuries ago, it remains unclear whether these differences are due to an extinction debt in ancient forests (i.e., a delay in local species extinction), a colonization credit in post-agricultural forests (i.e., species are yet to colonize a patch), or both. Additionally, our knowledge on how soil conditions and landscape configuration interact with species' traits to determine the colonization credit is limited. Here, we surveyed ancient and post-agricultural forest patches in NE Germany to quantify the magnitude of the colonization credit and identify its determinants. The colonization credit in an average forest patch amounted to 4.7 forest specialist species and ranged up to 9 species in highly isolated patches. In contrast, we found more species than predicted in patches better connected to ancient forests. The colonization credit was not smaller in older patches than it was in younger ones. Species with a low dispersal potential and a low seed output contributed most to the colonization credit. Our study demonstrates that in a landscape where the extinction debt has already been paid and only a small fraction of ancient forest is left, the recovery of forest specialist diversity in post-agricultural forests may take several centuries.