Herb layer extinction debt in highly fragmented temperate forests – Completely paid after 160 years?

Abstract

a b s t r a c tThe time-delayed extinction of plant species following habitat fragmentation is a well-known phenomenon in ecology. The length of the relaxation time until this 'extinction debt' is paid (i.e., until extinctions cease) depends on the ecosystem, species group and extent of fragmentation. Studies of grassland ecosystems have revealed that plant extirpations after fragmentation can occur rapidly when the degree of fragmentation is high. Studies of extinction debt in highly fragmented forests, however, are lacking. In this study, we evaluated the existence of an extinction debt in the Prignitz, Brandenburg, Germany, where 94% of the semi-natural forests have vanished since 1780. We surveyed the herb-layer species of 104 forest patches and fitted species richness as a function of the historical and present-day patch configurations. Models including the present-day habitat area and connectivity explained the present day species richness better than models including historical patch-configuration variables. There was no significant effect of the historical habitat area on the present day species richness. However, the effect of historical patch connectivity on the richness of forest specialists with short-distance dispersal potential was significant when excluding present-day habitat area from the models and habitat quality and heterogeneity were used as covariables. The extinction debt has largely been paid after approximately 160 years of relaxation time which contrasts with previous studies of temperate forests that have found extinction debts persisting 120-225 years after fragmentation. We demonstrate that extinction debts in temperate forests may be paid off more rapidly if the degree of fragmentation is high.

The relationship between landscape configuration and plant species richness in forests is dependent on habitat preferences of species

“…We did not find any effect of temporal forest continuity on species richness of forest species in our study plots. Even though it could be expected that species richness of forest plants is higher in forests with long temporal continuity (Flinn and Vellend 2005;Honnay et al 2005), production forests may not show this pattern, at least not in a time frame of 100 years (Vellend et al 2006;Kolk and Naaf 2015). In this study, local species richness of the forest species was low even in the forests with long continuity.…”

“…Forest edge length is expected to have a negative effect on the diversity of forest species, so species diversity should decline with increasing forest edge length in metacommunities dominated by forest species. In metacommunities that are dominated by edge species, we predict that forest edge length has a positive effect on species richness of the regional species pool due to the species-area relationship (Holt 2010;Hofmeister et al 2013;Kolk and Naaf 2015).…”

Changes in forest cover in Sudety Mountains during the last 250 years: patterns, drivers, and landscape-scale implications for nature conservation

“…Due to the phenomenon of extinction debt, the actual species richness in a patch can be influenced by past events. In Central European forests, the recent species richness still reflects episodes that occurred in the previous century, such as forest patch shrinkage or loss of connectivity between forest patches [23,24]. Therefore, historical knowledge regarding forest distribution is necessary for understanding the recent species richness distribution, as well as for conservation management planning [11,25].…”

How well is current plant trait composition predicted by modern and historical forest spatial configuration?

“…What we see integrated into the mean trait values is likely to be the trait‐controlled sum of the dynamics of fast‐responding species more rapidly dispersed in time (through persistent seedbanks) and space (through light, slower falling seeds) arriving at different rates from surrounding habitats, coexisting with extinction debt species that are better fitted to historical spatial configurations and hence are likely to decline further. These two processes may occur at different rates however, with extinction debts in forest understorey plants being paid sooner (after around 160 yr) (Kolk and Naaf ) than immigration credits (which can remain for much longer) (Naaf and Kolk ). If extinction debts in forest patches which have lost area have largely been paid in this analysis, this may partly explain why only weak lags were identified here for mean seed terminal velocity and rarity.…”