LIANA LOADS, MECHANICAL DESIGN AND SAFETY FACTOR OF TREE SPECIES IN FRAGMENTS OF THE TROPICAL SEMIDECIDUOUS FOREST

The vertical light gradient is one of the most important environmental heterogeneity axes in tropical rainforests. Most of the above incident sun radiation is intercepted by successive leaf layers along canopy forest floor route and, for this reason treelets, shrubs and saplings in the forest understorey have to cope with low light availability to be able to survive, and eventually, growth to maturity (Poorter et al., 2003). The light interception by trees is in large extent related to their architecture. Tree architecture refers to the overall plant shape and the spatial position of its components, such as crown depth, branch patterns, phyllotaxis and geometry of foliage. Thus, the architecture of a tree species may represent different strategies employed to get larger amount light (Aiba & Kohyama 1996). Investment in height growth allows trees either optimally to intercept light and competitively to shadow neighbors. Nevertheless, the investment to elevate leaves to forest canopy should be concomitant with a proportional increase in trunk diameter, in order to prevent the mechanical risk of breaking due to self weight and wind incidence. Hence, along tree ontogeny there is a trade off between growth in height and increase in trunk diameter (Niklas 1994). One way to assess this trade off is through investigating the allometric relationship between trunk diameter and height. The mechanical stability of trees can be evaluated by the stability safety factor (FS), which is the ratio between the buckling height and the observed height. It indicate the safety margin necessary to trees withstanding the stress of dynamic load such as wind, rain and falling debris. Coexisting tree species with similar DBH (trunk diameter at breast height) can greatly vary in height, and the ratio height:diameter is termed slenderness. Trees with a great slenderness have a high probability of snapping (Putz et al., 1983). Thus, slenderness can reflect tree architecture and the proportion of the investment in height versus trunk diameter growth.