Secondary diversity: Ecological and spectral dimensions of secondary succession following smallholder cultivation in the southern Yucatan.

摘要

This dissertation examines the properties of secondary forest succession in the southern Yucatan following smallholder shifting cultivation. Specifically it addresses two aspects of these forests: (1) the feasibility of detecting multiple age classes of secondary forest through remote sensing analysis with the goal of accurately mapping secondary forests dynamics in the region and (2) the ecological changes occurring in these forests both through time (succession) and with ongoing shifts in agricultural practices.;Differentiation of successional forests from mature forests was possible using linear spectral unmixing of Landsat Enhanced Thematic Mapper Plus imagery, but only the broad patterns of succession were consistently detectable. High degrees of variability in leaf cover due to the range in phenological timing that is common to tropical dry forests created unique classification challenges both spatially and temporally. Cultivation does have a significant effect on the species diversity and composition of forests, and recovery rates may vary dramatically for small and large trees. For large stems, species composition and richness will not recover to the level of mature forest within the first 20 years of succession. For small stems, species richness and diversity do reach levels of mature forest within the first 10 years, but this is likely due to the overlap in successional stages and not a maturation of the forest as mature forest species are still not present. Species composition does not recover in the first 20 years of succession in either group. Further, the type of cultivation has a significant effect on the diversity of secondary successional forests, and that regeneration after milpa cultivation (maize-based) has higher species richness and abundance than after mixed cultivation plots. The number of cropping cycles has a less clear effect, but there may be a trend towards declining species richness with increasing cropping cycle.