Leaf and tree age-related changes in leaf ecophysiological traits, nutrient, and adaptive strategies of Alnus nepalensis in the central Himalaya

Leaf ecophysiological traits are known to change with leaf and tree age. In the present study, we measured the effect of leaf and tree age on leaf ecophysiological and morphological traits of nitrogen-fixing Alnus nepalensis (D. Don) which is a pioneer tree species in degraded lands. Three naturally occurring A. nepalensis forest stands, namely young (5-8 years old), mature (40-55 years old), and old (130-145 years old), were considered in this study. We also investigated the seasonal variations in leaf ecophysiological and morphological traits during leaf flushing, fully expanded, and leaf senescence phenological stages. The ecophysiological and morphological traits were compared between leaf and tree ages using a linear mixed-effect model (LMM) and Tukey's HSD test. Fully expanded leaves and young trees demonstrate ecophysiological traits consistent with acquisitive resource-use strategies. Our results revealed that net photosynthetic capacity (A_area and A_mass), leaf stomatal conductance (gsw_area and gsw_mass), transpiration rate (E_area and E_mass), specific leaf area (SLA), predawn and midday water potential (Ψ), leaf total chlorophyll concentration, photosynthetic N- and P-use efficiency (PNUE and PPUE) were higher in younger trees than mature and old trees. We found lower wateruse efficiency (WUE) and intrinsic water-use efficiency (WUE_i) in young trees than in mature and old ones. Mass-based net photosynthetic capacity (A_mass) was positively correlated with PNUE, PPUE, transpiration rate, stomatal conductance, SLA and chlorophyll concentrations but negatively correlated with WUE and WUE_i. However, mass-based leaf nitrogen (N) and phosphorus (P) concentrations were the highest in fully expanded leaves and did not vary with tree age despite N concentration being negatively correlated with SLA. Overall, this study provides valuable insights into the age-related changes in leaf ecophysiological traits of A. nepalensis. The findings underscore the importance of considering tree age when studying plant ecophysiology and highlight the acquisitive resource-use strategies employed by young trees for rapid growth and establishment.