The biogeography of plant species with disjunct distributions between eastern Asia (EA) and eastern North America (ENA) suggests evolutionary stasis (because sister taxa on the two continents tend to occur in similar climates) and a tendency towards drought-intolerance (e.g., disappearance from western North America since the late Tertiary due to increased aridity). These observations suggest that the ecological traits and dynamics (e.g., population trends) of EA-ENA disjuncts may be distinct from co-occurring non-disjunct species. We compared the ecological traits and recent population trends of EA-ENA disjunct and non-disjunct tree species in ENA using a database of ecological traits and systematically sampled forest inventory data collected by the USA Forest Service in the 1980s and 2000s.
Results/Conclusions
Disjuncts tended to be less drought-tolerant and more shade-tolerant than non-disjuncts. A majority of disjunct species were classified as being pyrophobic (negative population response to fire), whereas a majority of non-disjuncts were classified as being pyrophilic (positive population response to fire). These and other ecological/functional traits displayed a moderate degree of phylogenetic conservatism when examined in the context of a phylogeny of eastern USA tree species. Despite ecological trait differences between disjunct and non-disjunct species, the two groups had similar population trends from the 1980s to 2000s in the north-central, northeastern, and southeastern USA. We suggest that trait differences had counter-acting effects on disjunct and non-disjunct species, so that the two groups had similar population trends despite being ecologically distinct. For example, drying trends over recent decades in the north-central and southeastern USA would be expected to adversely affect drought-intolerant disjunct species, but the expected population declines may have been counter-balanced by fire-suppression, which favors pyrophobic disjuncts over pyrophilic non-disjuncts.