Characteristics of photosynthesis and stomatal conductance in the shrubland species manuka (Leptospermum scoparium) and kanuka (Kunzea ericoides) for the estimation of annual canopy carbon uptake

Responses of photosynthesis to carbon dioxide (CO2) partial pressure and irradiance were measured on leaves of 39-year-old trees of manuka (Leptospermum scoparium J. R. Forst. & G. Forst.) and kanuka (Kunzea ericoides var. ericoides (A. Rich.) J. Thompson) at a field site, and on leaves of young trees grown at three nitrogen supply rates in a nursery, to determine values for parameters in a model to estimate annual net carbon uptake. These secondary successional species belong to the same family and commonly co-occur. Mean (± standard error) values of the maximum rate of carboxylation (hemi-surface area basis) (Vcmax) and the maximum rate of electron transport (Jmax) at the field site were 47.3 ± 1.9 micromol m-2 s-1 and 94.2 ± 3.7 micromol m-2 s-1, respectively, with no significant differences between species. Both Vcmax and Jmax were positively related to leaf nitrogen concentration on a unit leaf area basis, and the slopes of these relationships did not differ significantly between species or between the trees in the field and young trees grown in the nursery. Mean values of Jmax/Vcmax measured at 20 °C were significantly lower (P < 0.01) for trees in the field (2.00 ± 0.05) than for young trees in the nursery with similar leaf nitrogen concentrations (2.32 ± 0.08). Stomatal conductance decreased sharply with increasing air saturation deficit, but the sensitivity of the response did not differ between species. These data were used to derive parameters for a coupled photosynthesis¡stomatal conductance model to scale estimates of photosynthesis from leaves to the canopy, incorporating leaf respiration at night, site energy and water balances, to estimate net canopy carbon uptake. Over the course of a year, 76% of incident irradiance (400-700 nm) was absorbed by the canopy, annual net photosynthesis per unit ground area was 164.5 mol m-2 (equivalent to 1.97 kg C m-2) and respiration loss from leaves at night was 37.5 mol m-2 (equivalent to 0.45 kg m-2), or 23% of net carbon uptake. When modeled annual net carbon uptake for the trees was combined with annual respiration from the soil surface, estimated net primary productivity for the ecosystem (0.30 kg C m-2) was reasonably close to the annual estimate obtained from independent mensurational and biomass measurements made at the site (0.22 ± 0.03 kg C m-2). The mean annual value for light-use efficiency calculated from the ratio of net carbon uptake (net photosynthesis minus respiration of leaves at night) and absorbed irradiance was 13.0 mmol C mol-1 (equivalent to 0.72 kg C GJ-1). This is low compared with values reported for other temperate forests, but is consistent with limitations to photosynthesis in the canopy attributable mainly to low nitrogen availability and associated low leaf area index.