Litterfall, carbon and nitrogen cycling in a southern hemisphere conifer forest dominated by kauri (Agathis australis) during drought

Abstract

Under future climates, droughts will be more frequent and severe in parts of New Zealand, but the impact of drought has not been studied in New Zealand forests. Litterfall is one of the major fluxes of forest carbon. We explored seasonal and annual patterns of litterfall, carbon and nitrogen cycling in wet (2012) and dry (2013) years at Huapai Scientific Reserve, west Auckland. During 2012, rainfall was close to average with a wet summer, while the summer rainfall in the period from January to April 2013 was only 1/3 of the volume that fell during the same period in 2012, causing significant reductions in soil moisture. For the wet year, total annual litterfall was 9.00 ± 0.44 t ha−1, while during the dry year, there was a 72 % increase to 15.46 ± 0.85 t ha−1. Kauri constituted 80 % of the basal area of the plot and also contributed 80 % of the litter. The majority additional litter in the dry year was kauri leaves and twigs. Drought impacted slightly more heavily on the C cycle (with an 85 % increase during the drought year) than the N cycle (with a 69 % increase during the drought year) because concentrations of N in kauri leaves were lower during the drought period, due to nutrient reabsorption. Drought clearly enhances litterfall in this forest, stimulating the carbon and nitrogen cycles. These results have implications for forest C and N budgets as well as fire management practices due to the build-up of dry litter during drought.