Constraints on physiological function associated with branch architecture and wood density in tropical forest trees

Abstract

This study examined how leaf and stem functional traits related to gas exchange and water balance scale with two potential proxies for tree hydraulic architecture: the leaf area:sapwood area ratio (AL:AS) and wood density (?w). We studied the upper crowns of individuals of 15 tropical forest tree species at two sites in Panama with contrasting moisture regimes and forest types. Transpiration and maximum photosynthetic electron transport rate (ETRmax) per unit leaf area declined sharply with increasing AL:AS, as did the ratio of ETRmax to leaf N content, an index of photosynthetic nitrogen-use efficiency. Midday leaf water potential, bulk leaf osmotic potential at zero turgor, branch xylem specific conductivity, leaf-specific conductivity and stem and leaf capacitance all declined with increasing ?w. At the branch scale, AL:AS and total leaf N content per unit sapwood area increased with ?w, resulting in a 30% increase in ETRmax per unit sapwood area with a doubling of ?w. These compensatory adjustments in AL:AS, N allocation and potential photosynthetic capacity at the branch level were insufficient to completely offset the increased carbon costs of producing denser wood, and exacerbated the negative impact of increasing ?w on branch hydraulics and leaf water status. The suite of tree functional and architectural traits studied appeared to be constrained by the hydraulic and mechanical consequences of variation in ?w.