Height-related variations of leaf traits reflect strategies for maintaining photosynthetic and hydraulic homeostasis in mature and old Pinus densiflora trees.

ABSTRACT

Because tree size and age co-vary, it is difficult to separate their effects on growth and physiological function. To infer causes for age-related height-growth decline, we compared various leaf traits between mature (ca. 100 years) and old (ca. 300 years) trees of Pinus densiflora, having similar heights (ca. 30 m) and growing in the same stand. For many leaf traits, mature and old trees showed similar height-related trends reflecting acclimation to height-related hydraulic limitation for maintaining photosynthetic and hydraulic homeostasis. Photosynthetic capacity was constant within crowns of both age-classes, though 4.9-5.4 µmol CO2 m-2 s-1 lower for old than for mature trees. Biochemical acclimation of photosynthesis, allocating more nitrogen to treetop leaves, was observed only for mature trees. Leaf turgor loss point was also constant within crowns of both age-classes with no significant effect of age on leaf hydraulic traits. In mature trees, leaf capacitance increased, while bulk tissue elastic modulus decreased with height, whereas opposite height-related trends were observed for old trees. For both age-classes, leaf mass per area (LMA), transfusion-tissue area, and xylem area all increased with height, but LMA was ca. 30 g m-2 greater for old than for mature trees. In old trees, mesophyll area decreased with height, suggesting anatomical acclimation to height may negatively affect photosynthetic capacity. We inferred that old trees rely more on morphological than biochemical acclimation and that such post-maturational shift in resource allocation could underlie height-growth decline of P. densiflora after reproductive maturity.