Testing shade tolerance as a mechanism of dynamics in three forests of Big Thicket National Preserve, southeast Texas
Harcombe, Paul A.
Doctor of Philosophy
I investigated the role of shade tolerance in the dynamics of three forests of Big Thicket National Preserve, southeast Texas. In a mature mesic forest, shade-intolerant species had higher high-light growth and lower low-light growth than tolerant species. Results suggested that there was a tradeoff between high-light growth and low-light growth across species. Moreover, low-light survival and high-light growth were negatively correlated across species. In contrast to northern hardwood forests where survival in low light may be achieved at the expense of growth, my results suggested that shade-tolerant species in this southern mixed forest can grow faster as well as survive better than shade-intolerant species in low light. I conclude that both juvenile growth and survival are important components of shade tolerance and their relationships may be system-specific. In a floodplain forest, growth responses to light were consistent with the expectation that shade-intolerant species grow faster than shade-tolerant species in high light and vice versa. But mortality risks of some shade-tolerant species were unexpectedly high. The increased flooding during one of the study time periods may be responsible for the high mortality risks of shade-tolerant species. The results further supported that the success of shade-tolerant species in this forest may be limited by flooding as previous studies suggested. Compared with the mesic site, common species showed little intraspecific differences in shade tolerance. In a sandy upland pine-oak forest, low-light growth responses of saplings corresponded to shade tolerance expectation. However, shade-intolerant species did not show faster growth than shade-tolerant species in high light possibly because these species are drought-tolerant. The correspondence between increasing stem density and increasing death rates of shade-intolerant xeric dominants suggested that recent increases in canopy shading may be responsible for high death rates of these species. Cross-site comparison showed that shade-tolerant species had better performance at the dry site than at the mesic site. Consistent with the facilitative effect found at dry sites, these species may benefit from the less competition from surrounding vegetation.