High vapor pressure deficit constrains GPP and the light response of NEE at a Southern Hemisphere bog

We used eddy covariance measurements of net ecosystem CO2 exchange (NEE) in an undisturbed New Zealand bog to determine the major controls on gross primary production (GPP). In particular, we were interested in the relative importance of air temperature, saturation vapor pressure deficit (VPD), and diffuse fraction of incoming radiation (fdiff) in driving increases and restrictions to ecosystem photosynthesis. We found that NEE light response parameters for this peatland were remarkably similar year-round. We used four different light response functions to explore the influence of driving meteorological variables on daytime CO2 uptake. This revealed that including VPD as a model input reduced the bias associated with estimates of NEE relative to the standard rectangular hyperbola. An apparent temperature restriction of light-saturated photosynthetic capacity was relieved when high VPD data were excluded from light response curve fits. We thus, utilized the model that incorporated VPD to fill gaps in daytime NEE data and to estimate GPP. Increases in mid-day GPP were associated with increasing total photosynthetic radiation and air temperature, especially during winter and spring/autumn seasons. However, in summer, mid-day GPP was capped by high VPD, likely due to a stomatal response triggered to reduce transpiration water losses. Although GPP did not directly respond to fdiff, both temperature and VPD were coupled to sky conditions. Furthermore, the highest mid-day mean GPP for high light levels always occurred during the periods of highest fdiff. These results have important implications for the future C sink strength of New Zealand peatlands given a trend toward drier summers with clearer skies.