Intraspecific variation in functional wood anatomy of tropical trees caused by effects of forest edge

Abstract

Forest edge effects promote striking microclimatic shifts that result in greater light exposure, increased air temperature and evapotranspiration rates, and reduced soil moisture can alter stem hydraulic conductivity. These environmental conditions greatly increase the risk of emboli in xylem, which can disrupt or even cease water transport. We investigated tropical tree species in contrasting sites to understand the influence that edge effects have on wood functional anatomy. We compared wood anatomical traits between trees of forest edge and forest interior sites. The species exhibited two types of xylem arrangements based on the distribution of parenchyma cells: Pseudopiptadenia inaequalis and Copaifera lucens, with paratracheal axial parenchyma (PP), and Eugenia excelsa and Erythroxylum cuspidifolium, with predominant apotracheal axial parenchyma (AP). Tree species with PP xylem possessed shorter and wider vessel elements and lower ray and vessel frequencies, while species with AP xylem possessed longer and narrower vessel elements and higher ray and vessel frequencies. Regardless of the axial parenchyma arrangement, all four species had a greater percentage of axial parenchyma at the edge sites. The different xylem arrangements showed distinct responses to the edge effect in vessel frequency. The tropical tree species studied here shift to a greater abundance of axial parenchyma when subjected to effects of forest edge, probably to ensure safe water transport. This behavior, in the face of environmental stresses, is a plastic response, and likely due to functional versatility of parenchyma cells. The patterns of vessel frequency for PP and AP suggest that these xylem arrangements can be considered functional groups since the species in each group share similar traits and have similar strategies for conducting and maintaining water flow.