The rain forests of Malaysian Borneo are unusually diverse in tree species, and have particularly low rates of canopy gap formation. Often many related tree species coexist in the same forest, notably species of the Dipterocarpaceae, an economically important timber family. Knowledge of mechanisms allowing large numbers of tree species to coexist is essential for an understanding of forest ecology and management. Trees may have evolved life history strategies or patterns of biomass allocation that facilitate the long-term survival of seedlings in forests with low rates of canopy gap formation. Seedlings represent the early stages of tree populations, yet are rarely the subject of long-term investigation. In recent decades, droughts in this region have increased in frequency and intensity. The threat of increasingly severe droughts suggests a potential disruption of this relatively stable ecosystem.

This study examined seedlings from 13 species of shade-tolerant rain forest trees in two Bornean rain forests. Seedling demographics were examined for a two-year interval that included a drought (1996-1998) and for the previous ten-year (non-drought) interval (1986-1996). Demographic analyses were supplemented by analyses of above-ground biomass allocation, maps of seedlings and topography, and hemispherical photography.

Seedlings showed adaptations for long-term survival in the understory. Most seedlings were many years, if not decades, old. Growth rates were low and many seedlings lost height, but suppressed seedlings were able to return to fast growth. Fewer than 4% of seedlings reached sapling size after 10 years. Above-ground biomass turnover was also slow, with mean leaf lifespans of 4-11 years. Species within the shade-tolerant guild differed in their strategies of biomass allocation. Herbivory was minimal, indicating a high allocation of biomass to anti-herbivore defense. Older seedlings showed a decrease in biomass growth, but not in height growth or survival. While three species showed preferences for ridges, hillsides or valleys, demographic rates during the drought were unaffected by topography. Understory light levels increased as a result of drought, but seedling growth rates were unaffected. Drought caused increases in rates of height loss and mortality. In summary, these tree seedlings are adapted to persist through multiple and extended periods of light or water limitation.

For more information, contact the author at lisa.delissio@salemstate.edu.