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Zusammenfassung:
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 (Amax), dark respiration (Rd) and the proportional change in Rd per 10 °C change in temperature (Q10 of
Rd) in previous-year (PY) and current-year (CY) leaves of the broadleaved evergreen tree Quercus ilex L. Amax and Rd were
lower in PY than in CY leaves. Differences in nitrogen between cohorts only partly explained such differences, and rates of
Amax and Rd expressed per unit of leaf nitrogen were still significantly different between cohorts. The decline in Amax 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 Rd did not differ significantly
between leaf cohorts; however, failure to account for distinct Rd between cohorts misestimated canopy leaf respiration
by 13% across dates when scaling up leaf measurements to the canopy. In conclusion, the decline in Amax 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 Rd and nutrient resorption during senescence. Accounting for leaf age as a source of variation of Rd improves the estimation of foliar respiratory carbon release at the stand scale.