Tropical forest loss may show variable effects on biodiversity, depending on the taxonomic group and on the patterns and history of land-use change. In fact, both linear and non-linear (i.e. extinction thresholds) associations between forest cover and species diversity have been documented, but it is unclear if such taxonomic changes modify the phylogenetic diversity and structure of the remaining assemblages. Also, evaluating these effects on different ontogenetic stages is of upmost importance because it can show consequences for future communities. To address this issue we assessed whether and how (i.e. linear vs. nonlinear) forest loss affects the mean effective number of lineages [qD(T)], mean phylogenetic distance (MPD), and phylogenetic dispersion (NRI-net related index) of plant assemblages in the fragmented Brazilian Atlantic forest. We studied 20 landscapes (5-99% of forest cover) from two regions with contrasting patterns of land-use change, independently considering juveniles (individuals with woody stem, heights ≥ 1.3 m, and DBH ≤ 5 cm) and adult trees (DBH≥5cm) to assess potential extinction debts.
Our findings revealed that all phylogenetic responses to forest loss were linear or null. qD(T)adults was positively related to forest cover when considering rare and common species. qD(T)juveniles was also positively related to forest cover, but such effect differed between regions, except for the dominant species. The most deforested region showed lower values of phylogenetic diversity to the same percentage of forest cover on the most forested region. Forest cover was not associated with MPDadults, but when weighted by abundance, it was positively related to MPDjuveniles. Forest cover loss did not affect NRIadults and NRIjuveniles. The stronger phylogenetic effect on juveniles suggests the existence of extinction debts. Thus, without preventing (and reverting) deforestation in this species-rich tropical region, the phylogenetic diversity of future tree assemblages will be increasingly poorer, potentially limiting the ability of this forest to respond to additional land-use changes.