Converging patterns of vertical variability in leaf morphology and nitrogen across seven Eucalyptus plantations in Brazil and Hawaii, USA

Key message: Across sites in Brazil and Hawaii, LMA and N mass were strongly correlated with height and shade index, respectively, which may help simplify canopy function modeling of Eucalyptus plantations. Within tree canopies, leaf mass per area (LMA) and leaf nitrogen per unit area (N area) commonly increase with height. Previous research has suggested that these patterns occur as a strategy to optimize carbon gain by allocating available resources to upper canopy leaves that are exposed to greater light availability. We tested three hypotheses about the influences of height, shade index (a proxy for light), and stand age on LMA and leaf nitrogen for even-aged Eucalyptus saligna and Eucalyptus grandis × urophylla plantations in Brazil and Hawaii, USA, spanning most of the environmental conditions found across 19.6 million ha of Eucalyptus spp. plantations around the world. Shade index was developed by incorporating canopy depth (inner-crown shading) and a tree height ratio relative to neighbor trees (shading from other trees). Across all sites and ages, leaf height accounted for 45 % of the variation in LMA, whereas shade index accounted for only 6 %. A combination of both factors was slightly better in accounting for LMA variation than height alone. LMA-height relationships among sites were strongest under greater light availability and in older stands. Leaf nitrogen per unit mass (N mass) consistently decreased with shade index, whereas N area showed no consistent pattern with height or shade index. These relationships indicate that N mass is primarily driven by light, while height is the primary driver for LMA. The general relationships between LMA and leaf N mass across all sites may simplify canopy function modeling of E. saligna and E. grandis × urophylla plantations