Simulated carbon uptake for a canopy of two broadleaved tree species with contrasting leaf habit

Abstract

Summary We used an environmentally driven, process‐based model to investigate the relative benefits of deciduous and evergreen leaves, in terms of net annual canopy carbon uptake, for a canopy of two tree species with contrasting leaf habit: deciduous Fuchsia [Fuchsia excorticata (J. R. Forst. and G. Forst) L. f.]; and annual‐evergreen Wineberry (Aristotelia serrata J. R. Forst. and G. Forst, W. R. B. Oliv.). Parameters for the model were derived from a year of measurements at a field site with a maximum leaf area index (L) of 5·4 m2 m−2, of which Wineberry and Fuchsia comprised 4·9 and 0·5 m2 m−2, respectively. Net annual C uptake (ANET; daily photosynthesis minus respiration at night) was 930 g C m−2, of which Wineberry and Fuchsia contributed 850 and 80 g C m−2, respectively. Canopy respiration at night (RN) accounted for 29 and 18% of daily uptake for Wineberry and Fuchsia, respectively. The model was used to simulate the effects of an evergreen phenology for both species. This increased ANET relative to that estimated using the observed phenology by only 1% for Wineberry, and 4% for Fuchsia. This was attributable to increased net C uptake during winter with the evergreen phenology being offset by increased RN, and reduced ANET during summer. This comparison was repeated for sites spanning the natural range of Wineberry and Fuchsia across New Zealand. In terms of net C balance, the relative advantage of an evergreen phenology decreased with average winter temperature for both species, to the point where a deciduous phenology resulted in a higher value for ANET for Fuchsia at sites where frosts limited winter uptake. Our simulations suggest that there is no clear advantage, in terms of annual C balance, for a deciduous or evergreen phenology at sites with relatively mild winters. In contrast, at sites where frost‐induced photoinhibition limits photosynthesis for overwintering leaves, a deciduous phenology results in a higher value of ANET for species, such as Fuchsia, with leaves that are very active photosynthetically during spring and summer. The lack of deciduousness in the New Zealand flora may be due to the low number of tree species with such photosynthetic characteristics, and this may be attributable to the generally poor nutrient availability in forest soils.