Late Holocene forest and savanna diversity and dynamics across an Amazonian ecotone
Panfil, Steven Nelson
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I used floristic inventory, plant demographic, and paleoecological methods to examine the modern and historical distribution of vegetation at Noel Kempff Mercado National Park in northeastern Bolivia. This park covers 1.5 M ha and spans the ecotone between moist evergreen forest, dry forest, and savanna formations at the southern margin of the Amazon Basin. Floristic analysis of the forest and savanna formations of the region shows that high -diversity is responsible for very high numbers of species. Forest formations have low to moderate richness at the 1 ha plot scale, but species overlap between plots is also low. Twenty-nine 1 ha plots contained 896 species of trees 10 cm dbh. Savanna formations were sampled with the line intercept technique and have habitat heterogeneity similar to the forests. Nine savanna plots contained 662 herbaceous and woody plant species. I investigated the history of vegetation change at my site with carbon isotope analysis of soil organic matter. This technique compares the isotopic composition of carbon in the soil with that of standing vegetation. My results indicate that areas that are presently forested have been continuously forested for more than 3,000 years. The savanna is near its maximum extent, and it has experienced at least one episode of increased woodiness during this time. Plant demographic plots at the edge of forest islands show that the boundary between forest and savanna is abrupt and that the density of woody stems in the savanna rapidly increases in the absence of fire. Natural burns occur at 2-3 year intervals at some locations, and these fires cause high levels of mortality of small woody stems in the savanna. These results suggest that reduced fire frequency would allow the establishment of large trees and shrubs that are resistant to fire, eventually leading to a greater density of woody stems in the savanna and the possible conversion to forest.