Assessing the effects of forest canopy cover on understory vegetation composition and biomass along an experimental hydrological gradient

Background/Question/Methods

The establishment and development of plant communities is controlled by both the availability of abiotic resources and by biotic interactions including their effects on environmental resources.  In forest ecosystems, where understory vegetation accounts for a substantial fraction of plant diversity and plays an important role in ecosystem dynamics, biotic interactions among different height strata are generally marked by a strong influence of overstory structure on understory vegetation composition and production. Nonetheless, the nature and strength of such effects vary in space and time, resulting in distinct understory vegetation assemblages. With a broader goal of understanding structural and functional dynamics in developing forests along a hydrologic gradient, we initiated a long-term vegetation study in the Loxahatchee Impoundment Landscape Assessment (LILA) experimental site in Palm Beach County, Florida. The objective of this study was to examine whether understory plant species show a tradeoff in spatial position in response to variation in the availability of light resources along a hydrologic gradient from drought to flooding. Our study has important implications for maintenance and restoration of ecologically imperiled Everglades tree islands that are highly biodiverse forest patches embedded in a wetland matrix of herbaceous graminoid vegetation.

Results/Conclusions

We studied the influence of hydrology and canopy cover on understory vegetation on eight constructed tree islands at LILA. Our results showed water level was the strongest determinant of community differentiation within an island. The light regime associated with the developing forest canopy further partitioned the available habitat and determined species occurrence and abundance, especially on the drier portions of the islands. The observed species distribution pattern in the developing forests at LILA agrees with the adaptive tradeoff hypothesis i.e. no species can be dominant over the entire range of available resources (water and light), and therefore must specialize in a limited range of resource combinations. Because these resources vary across space in the LILA islands, so does the distribution of species. In summary, as in other successional studies, our results support the influence the developing woody canopy exerts on understory vegetation. However, since many opportunistic species are ephemerally present in the early stage of forest development, understory composition may become much more organized over time in relation to interacting effects of available water and light resources influenced by changes in the hydrologic regime and the developing canopy structure.