Seasonal litterfall composition and carbon and nitrogen returns in New Zealand shrubland

Abstract

Mānuka–kānuka shrubland is an important carbon (C) sink in New Zealand, yet little is known about C cycling within these systems. The objective of our work was to assess seasonal litterfall rates, composition, and C and nitrogen (N) inputs in mixed mānuka (Leptospermum scoparium J.R. Forst & G. Forst.) and kānuka (Kunzea ericoides var. ericoides (A.Rich) J.Thompson) stands. Litterfall was collected for 2years at Tongariro National Park (NP) and Stoney Creek, Wairarapa (SC), separated into leaf, twig, bark, seed and ‘other’, and the C and N content of each component measured. Total litterfall was between 3557 and 4443kgha–1year–1, of which leaf material contributed 46–67%. Litterfall peaked during spring–summer months at both sites, and the overall litterfall rate was greater (P < 0.001) at SC than NP. Litterfall at SC contained greater (P < 0.001) amounts of ‘other’ due to higher undergrowth contributions, and also greater seed fall (P < 0.001), possibility due to the lower altitude at SC. The proportion of leaf material in litterfall also peaked during summer (P < 0.001). C inputs in the total litter were 1941–2448kgCha–1year–1 and N inputs ranged between 28 and 37kgNha–1year–1. There was little seasonal difference in C and N contents and the majority of both C and N inputs in litterfall were in the leaf material (P < 0.001). C inputs peaked during summer, but N inputs were closely aligned with total litterfall maximums during spring–summer. The leaf:wood ratio was 1.9 at both sites, indicating litter quality was consistent at both stands, regardless of differences in composition. Although the sites had similar rainfall and shrub ages, the rate of total litterfall differed, reflecting the potentially site-specific nature of litterfall in mānuka–kānuka shrubland. Further work is needed assessing litterfall and degradation rates across New Zealand to establish if mānuka–kānuka shrublands would remain carbon sinks under climate change.