Title

Regeneration dynamics of a Madagascar rainforest and their relationship to human disturbances

Date of Completion

January 2001

Keywords

Biology, Ecology|Agriculture, Forestry and Wildlife

Degree

Ph.D.

Abstract

Because of human disturbance, the majority of today's tropical forests are secondary forests. Understanding how forest regeneration dynamics are influenced by human impacts is a prerequisite to formulating management actions needed to conserve tropical forest biodiversity. The goals of this study were to: (1) Characterize the effects of different land uses on the structure, composition and site variables of a littoral forest in eastern Madagascar. (2) Parameterize juvenile mortality and growth models for a suite of native trees as a function of light, soil moisture and soil type. (3) Calibrate a predictive model of regeneration potential for these native trees as a function of human impact. ^ A range of human disturbances created a heterogeneous mosaic of forest types, both in terms of structure and species composition. The thickness of the O horizon and understory light were the two site variables that were most associated with the variation in species composition along gradients of human impacts. Axis 1 of a CCA ordination analysis was found to represent a gradient of human impact, from low to severe. ^ Species-specific tradeoffs in mortality and growth were observed among seedlings of four native tree species transplanted in a forest environment along light, soil moisture and soil type gradients. Species partitioned the light-soil moisture continuum into discrete niches based on their modeled relative performance as seedlings and saplings. ^ Densities of wild seedlings and saplings of the four native tree species used in the transplant trials did not always reflect the densities predicted by the niche partitioning model. However, predictive modeling of regeneration based on that model predicted seedling presence/absence among 21 forest plots sampling the gradient of human impact. ^ Seed dispersal was a predictor of natural regeneration for shade-tolerant species in a fragmented forest landscape, while seedling mortality was a predictor of regeneration for Harungana madagascariensis, a pioneer tree species adapted to disturbed sites. Harungana regeneration was not a good indicator of human disturbance but regeneration of Ocotea cymosa , a very shade-tolerant, slow growing native tree species, was found to be a good indicator of human disturbance. ^