Composition and structure of seasonally deciduous forests in the Americas
Forest canopies with leaves shed annually (seasonally deciduous forests) produce a different subcanopy environment than canopies with persistent leaves (evergreen forests). Although causes differ with latitude, seasonality might affect structure and composition of forests similarly. We explored changes in species composition and vertical structure in seasonally deciduous forests from 0°-40° N in the Americas. We measured all woody stems ≥1 cm dbh within replicated 0.0625-ha plots (16 at each of 9 sites, N > 31 000). Numbers of species, as well as density and basal area, were analyzed parametrically for canopy trees, subcanopy trees, shrubs, and lianas. Numbers of species, especially shrubs and subcanopy trees, were greater in warm (30° N) than cool (40° N) temperate forests. Much larger numbers occurred in subtropical (20° N) and tropical (10° N and 0°) forests, but did not vary in relation to latitude. Higher than expected numbers in areas <0.25 ha in all forests, but especially subtropical forests, indicated local heterogeneity in species composition. Few Species occurred exclusively in any structural category, especially at 20° and 30° N. The density of lianas relative to self-supporting plants was highest at 10° N and declined with increasing latitude. The density of woody plants in the canopy, subcanopy, and shrub layers was higher, at 20° N than at other latitudes. In addition, basal area was lowest at 20° N; canopy trees comprised 70% of basal area at 20°, but almost 90% at 0° and 40° N. The subcanopy proportion of basal area was greatest at 30° and least at 40° N. Our study produced general hypotheses. Seasonality influences regeneration: dynamics, affecting forest composition and structure along with gap dynamics. Spatial heterogeneity within seasonal forests reflects local regeneration dynamics, including spatial heterogeneity in site conditions, broad tails of dispersal and establishment under a wide range of environmental conditions, as well as recruitment limitations. Structural characteristics of seasonal forests change latitudinally in ways that reflect differences in hurricane frequency. Hurricane effects interact with seasonality to influence the composition and dynamics of seasonal forests at warm temperate and subtropical latitudes, producing seasonal forests that differ from those at both higher and lower latitudes.