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Natural regeneration of canopy trees in a tropical dry forest in Bolivia

Natural regeneration of canopy trees in a tropical dry forest in Bolivia
BONIFACIO MOSTACEDO

2007

Department of Biology,  University of Florida, Gainesville, FL 32611, UNITED STATES OF AMERICA.

ABSTRACT

Fruit production, seedling establishment, and sprouting of canopy trees were studied in a lowland tropical dry forest in the Department of Santa Cruz, Bolivia. Fruit production by reproductively mature trees was monitored over a 5 y period to assess variability among species, trees and years. The effects of tree size, crown area, crown position, and liana infestation on fruit production were also assessed. In a companion study, I assessed the effects of lianas on fruit production by Caesalpinia pluviosa with a liana cutting experiment. To determine how logging disturbances affect seedling recruitment, I monitored seedlings for 3 y in different microsites in permanent plots in two selectively logged plots and an unlogged control plot. I also experimentally assessed the effects of bromeliad cover, drought stress, and seed/seedling predators on seedling recruitment, survival, and growth. Finally, I monitored the emergence, survival, and growth rates of stump and root sprouts over a range of microsites.

Percentages of trees fruiting and numbers of fruits produced varied among species and years. In most species, trees that did and did not fruit did not differ in size or crown position, but in a few cases, the likelihood of fruiting increased with crown area. Contrary to my expectation, there no effect of liana cutting on Caesalpinia pluviosa fruit production was detected 3 yr after cutting. Overall, the effect of logging on the proportion of trees fruiting and fruiting intensity varied among species.

Seedling densities 5 y after selective logging were higher in control than logged plots but this finding was greatly influenced by the most common species, Acosmium cardenasii (43% of seedlings enumerated). At the microsite level, Acosmium was found in highest densities in undisturbed areas while Centrolobium microchaete, another common species, was more common on log extraction paths. Seedling recruitment rates were higher in the unlogged plot and in the undisturbed portions of the logged forest plots, but seedling mortality rates were also higher in these areas. Mortality rates of naturally established seedlings varied greatly among species. Seven of 22 species suffered no mortality during the 2-y monitoring period, whereas relatively high mortality rates were observed for Caesalpinia (26%/y), Sweetia fruiticosa (25%/y), and Machaerium scleroxylon (22%/y).

Results of the experimental study on seedlings suggest that bromeliad competition and seed/seedling predators greatly affected tree seedling establishment. Soil moisture availability also affected seedling establishment, but only as an interaction with the bromeliad removal or predator exclosure treatments. The primary effect of the drought treatment was delayed germination. Despite these general trends, species varied substantially in their sensitivity to bromeliads, drought stress, and predators.

Root and stump sprouts constituted about 50% of the individuals <2 m tall of the canopy tree species studied, but the proportions of sprouts and true seedlings varied among species. Stump sprouting was common, but the probability of sprouting was not consistently related to stump diameter or height. Sprout growth rates were consistently high, at least initially, and sprouting is obviously important to post-logging regeneration in this dry tropical forest.