Gas exchange and leaf aging in an evergreen oak: causes and consequences for leaf carbon balance and canopy respiration

摘要

Leaves of Mediterranean evergreens experience large variations in gas exchange rates over their life span due to aging and seasonally changing environmental conditions. Accounting for the changing respiratory physiology of leaves over time will help improve estimations of leaf and whole-plant carbon balances. Here we examined seasonal variations in light-saturated net CO₂ assimilation (A_max), dark respiration (R_d) and the proportional change in R_d per 10 °C change in temperature (Q₁₀ of R_d) in previous-year (PY) and current-year (CY) leaves of the broadleaved evergreen tree Quercus ilex L. A_max and R_d were lower in PY than in CY leaves. Differences in nitrogen between cohorts only partly explained such differences, and rates of A_max and R_d expressed per unit of leaf nitrogen were still significantly different between cohorts. The decline in A_max in PY leaves did not result in the depletion of total non-structural carbohydrates, whose concentration was in fact higher in PY than CY leaves. Leaf-level carbon balance modeled from gas exchange data was positive at all ages. Q₁₀ of R_d did not differ significantly between leaf cohorts; however, failure to account for distinct R_d between cohorts misestimated canopy leaf respiration by 13% across dates when scaling up leaf measurements to the canopy. In conclusion, the decline in A_max in old leaves that are close to or exceed their mean life span does not limit the availability of carbohydrates, which are probably needed to sustain new growth, as well as R_d and nutrient resorption during senescence. Accounting for leaf age as a source of variation of R_d improves the estimation of foliar respiratory carbon release at the stand scale.