Contrasting habitat and landscape effects on the fitness of a long-lived grassland plant under forest encroachment : do they provide evidence for extinction debt?

1. Habitat loss, fragmentation and transformation threaten the persistence of many species worldwide. Population and individual fitness are often compromised in small, degraded and isolated habitats, but extinction can be a slow process and extinction debts are common. - 2. Long-lived species are prone to persist as remnant populations in low quality habitats for a long time, but the population and individual-level mechanisms of extinction debt remain poorly explored so far. - 3. We here investigate the mechanisms involved in the long-term persistence of the common grassland specialist, long-lived, clonal plant Aphyllanthes monspeliensis L. (Asparagaceae) after forest encroachment into semi-natural Mediterranean calcareous grasslands in Catalonia (NE Iberian Peninsula). For this purpose we assess vegetative (aboveground and belowground) and reproductive plant performance indicators and their habitat and landscape (current and historical) drivers. - 4. We confirm the existence of an extinction debt for this species, since current plant frequency is related to historical but not current connectivity, and we also find a positive effect of historical connectivity on seed set. In addition, current tree cover negatively affects individual size and aboveground/belowground biomass ratio, and biotic soil acidification leads to a reduction in the flowering probability of individuals and stems. - 5. However, we also find that current connectivity negatively affects flowering and that tree cover enhances seed set. The forestation process, thus, also exerts a positive effect on some fitness traits, probably by providing a moister environment. - 6. Synthesis. Habitat loss and deterioration result in a decreased vegetative performance of Aphyllanthes monspeliensis, a grassland specialist, but show contrasting effects on its reproductive performance. This suggests that the species would perform better in open forests than grasslands in a context of climate change. However, further forest encroachment would increase light competition and soil acidification, threatening its persistence and promoting the payment of the extinction debt if no conservation measures are taken, like opening gaps in forests and enhancing grassland connectivity.