Influence of climate conditions on net primary production in a kauri forest

Abstract

Amidst escalating climate change impacts on forests, crucial research on forest carbon cycling estimates regional carbon budgets and promotes climate-responsible forest management. Forest net primary productivity (NPP) is a measure of a central forest carbon flux that indicates carbon uptake. However, the impact of climate change on NPP in kauri forests of Aotearoa New Zealand remains understudied. Examining kauri forest strategies to cope with extreme weather and climate change impacts on ecosystem productivity will enhance forest carbon quantification under future climates. We conducted an estimation of the annual and seasonal NPP of kauri forest at the University of Auckland Huapai Science Reserve using the data collected from 2012 to 2022. Our calculations used seasonal and annual live increment, and litterfall accumulation to estimate NPP for each season and year across the ten-year study period. To investigate the impact of climate factors on forest NPP, we explored the relationship between NPP values and meteorological data such as solar radiation, air temperature, precipitation, relative humidity, and New Zealand Drought Index (NZDI), examining their influence on both annual and seasonal scales. Between 2012 and 2022, kauri forests showed an annual mean NPP is 557.5 g m-2, with a range from 357.7 to 845.4 g m-2. The annual stand carbon increment ranged from 13.5 to 21.2 g m-2 and annual litterfall ranged from 344.23 to 826.35 g m-2. Litterfall made a larger contribution to NPP compared to stand carbon increment with approximately 97% of NPP associated with litterfall. The most influential factor for annual NPP is NZDI (r = 0.67). Seasonal NPP is mainly impacted by relative humidity in spring (r = -0.47) and NZDI in summer (r = 0.63), while autumn NPP is affected by NZDI (r = 0.56) and solar radiation (r = 0.49). In winter, NPP is primarily influenced by solar radiation (r = 0.83) and precipitation (r = -0.82). Additionally, we observed a decreasing trend in stand carbon increment with increasing precipitation and relative humidity. This study also suggested that the increased frequency and severity of prolonged drought events in New Zealand's future could negatively influence NPP.