Structural and compositional change in a old-growth subtropical forest: using functional traits to identify probable drivers

MALIZIA A.; EASDALE, T.A.; GRAU, H.R.

Lima

Taller; Reunión Regional sobre Construcción de un protocolo extendido para el monitoreo a largo plazo de bosques andinos; 2012

Consorcio para el desarrollo Andino (CONDESAN), Instituto de Ecologia Regional (IER, ex LIEY)

Recent studies have shown directional changes in mature tropical forests, but patterns of floristic and functional change are complex and diverse, and their causes are unclear. We assessed structural, compositional and functional changes in a old-growth subtropical forest and tested hypothetical drivers including recovery from past disturbances, reduction in ungulate grazing, CO2 fertilization, and increases in rainfall and temperature. We took advantage of 15 years of permanent plot data for 27 common trees species within 8 ha of subtropical montane forest in Argentina. During this period, stem density increased by 50% (12 stems ha-1 y-1) and basal area by 6% (0.13m2 ha-1 y-1). Understory treelets (Piper tucumanum, Eugenia uniflora, Allophyllus edulis) were the main contributors to stem density increase. Species population growth (recruitment increase not balanced by mortality) was negatively correlated with tree size and longevity, and positively correlated with leaf size and leaf nutrient content. These associations are inconsistent with predictions derived from past disturbances (no increase in shade-tolerant long-living species), and rainfall or temperature increase (no increase in evergreen or deciduous species, respectively). In contrast, the increase in nutrient-rich soft-leaved species is consistent with the reduction in ungulates herbivory. CO2 fertilization may have a minor additional effect and population size may provide an additional explanation. Reduction in large vertebrates populations have been observed in many otherwise undisturbed tropical forests, and our results suggest they may have been overlooked as a potentially significant driver of old-growth forest structural, functional and compositional shifts.