Grol 2005 (Separate Publ.) Pseudananas, Ananas, Bolivia

Effect of bromeliad (Pseudananas sagenarius) cover on seed predation and tree regeneration in tropical dry forest INPA, Bolivia

Author(s):—M.G.G. Grol

Publication:—ibifbolivia.org.bo (2005).

Abstract:—Dry tropical forests are among the most threatened ecosystems at this moment and large areas have been converted rapidly into grass lands, secondary forest, savanna or agricultural land. Knowledge of seed germination and seedling establishment is required for the success of these forests and for a sustainable management of these forests. Fieldwork of this study was conducted in tropical dry forest INPA, eastern Bolivia, during a period of four months (November 2004 – February 2005). A spiny bromeliad (Pseudananas sagenarius) covers large areas in this forest and is believed to reduce tree regeneration of commercially important timber species. Seed predation was evaluated in areas containing three different bromeliad covers; high density (70-100% bromeliad), low density (0- 30% bromeliad) and no cover (0% bromeliad). Seeds of two important timber species were artificially dispersed and the number of removed seeds was followed over 30 days. The time of discovering seeds by seed removers did not vary significantly among bromeliad densities, while removal rates during the experiment and removal percentages at the end of the experiment significantly differed between areas with and without bromeliad cover. Though, not the density, but only the presence of bromeliad seemed to have an effect on seed predation. Herbivory, survival and growth of seedlings were evaluated using four treatments consisted of the four possible combinations between bromeliad and exclosure. Herbivory, survival and growth of seven important timber seedling species were measured once in November 2004 and second in January 2005, but showed no significant affect on the treatments. Survival could not be tested on significance, but seemed to react on the treatments. Seedlings protected from herbivores showed no effect on bromeliad cover, while unprotected seedlings in areas with removed bromeliad showed a greater number of survivors. Herbivory of seedlings reduced seedling survival and absolute shoot growth. Measuring several microhabitat characteristics gave more insight in the distribution of bromeliad through the forest. Bromeliad densities are greater in the higher parts of the forest which contain less soil moisture contents and experience higher seed removal rates. At these sites, the ground cover with herbs, the number of seedlings and seedling species is lower. This study supports the hypothesis that seed predation reduces the number of available seeds for germination and with this tree regeneration. However, the presence of bromeliad did not show to have any effect on the herbivory, survival and growth of seedlings.

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