Variability and grouping of leaf traits in multi-species reforestation (Leyte, Philippines)

Abstract

Conventional reforestation in the tropics often results in stands with low tree species and functional diversities. A different approach to reforestation, the so-called rainforestation, has been developed in the Philippines. It emphasizes mixed stands and the preferential use of native species supplemented by fruit trees. In such stands, we studied several functional leaf traits (stomatal conductance for water vapour, leaf morphology and chemistry) with the objectives (1) of assessing the species-specific variation of leaf traits and in particular that of maximal leaf stomatal conductance ( g smax), (2) of determining relationships between g smax and other tree variables, and (3) of assessing whether leaf traits group the species studied. Sixteen broad-leaved species were studied, using five individual trees per species and ten fully expanded sunlit leaves per individual tree. Species-specific g smax differed fivefold (165–772 mmol m −2 s −1). Among studied leaf traits, only the carbon isotope ratio δ 13C exhibited a simple linear correlation with g smax. A separate analysis for dipterocarp species indicated a strong negative relationship between g smax and specific leaf area (SLA) ( r 2 = 0.96, P < 0.001, n = 5). For all 16 species, a multiple linear regression with the combinations leaf size/tree height and leaf size/canopy projection area also resulted in significant relationships, which partly explained the variability in g smax. A multivariate approach (principal component analysis) combining the leaf traits provided an explanation of 75% of the variability along the first two axes. All native dipterocarps species, a native Guttiferae and the durian tree ( Durio zibethinus) were associated with more depleted δ 13C, small leaves and a low leaf width to length ratio. Two exotic species frequently used for reforestation ( Gmelina arborea and Swietenia macrophylla) and the native early successional Terminalia microcarpa were differentiated by their high SLA and high leaf nitrogen content per leaf area (N area). Both species of Artocarpus ( A. blancoi and A. odoratissima) were also differentiated and had large leaves with low SLA and low N area. These associations of species with leaf traits as variables indicate that species have different leaf investment strategies, which may imply that there are differences in whole plant performance. We conclude that rainforestation creates substantial variation in leaf traits, which is based on the combination of species with different leaf trait groupings. This can be seen as an important step towards – partly – restoring the functional diversity which characterizes many natural tropical rainforests.