Decomposition of Litter from a Pinus radiata Plantation Forest, Considered as a Model 'Two Component' Conifer‐Broadleaf Forest Ecosystem

Abstract

This study compares the rate and pattern of decay and nutrient release in leaf litter from Pinus radiata trees and the broadleaved shrubs Coprosma robusta, Coriaria arborea and Buddleja davidii, and investigates whether rates of decomposition differ in mixtures of conifer and mesophyllous broadleaved leaf litter types compared to unmixed litter. Mixtures of gymnosperms and broadleaved angiosperms are a particular feature of New Zealand forests, the occurrence of which has fostered interest in the study of demography and competitive relationships between these two groups. Furthermore, the perception that broadleaved litter enhances the decomposition of conifer needles has contributed to the premise that the establishment of broadleaved species amongst conifer plantations improves nutrient cycling and soil condition. P. radiata plantation forest with a well established understorey of broadleaved shrubs provides an opportunity to examine a much simplified 'model' conifer - broadleaf forest ecosystem, as plantation forests generally lack the structural and habitat complexity of old-growth indigenous forests. The specific objectives of this study were to synthesise currently available literature on litter decomposition with respect to interactions between conifer and angiosperm litter types, and experimentally test the hypothesis that mixtures of coniferbroadleaved litter decompose more rapidly than unmixed litter. Experimental work comprised two field trials undertaken across five sites, and one glasshouse experiment. Four of the five field sites chosen comprised P. radiata forest stands of varying ages and extent of understorey development, while one site was located in indigenous podocarp/ broadleaf forest. Estimates of decomposition rate were gained from direct measurements of confined litter from component species using the 'litterbag method'. Plant material is enclosed in mesh bags which are placed in the environment and harvested at intervals to determine dry biomass losses. A portion of samples were also analysed for nutrient contents. Taking into account ephemeral synergistic effects, experimental trials and a synthesis of data from the wider literature both identified a tendency for slightly enhanced decomposition of some mesophyll litter species in mixtures with conifer litter, and slightly suppressed decomposition of conifer litter in mixtures with some mesophyll litter species. However, such interactions tend to be short-lived, small in magnitude, and may be at least partly explained by physical litter characteristics. As such, synergistic litter decomposition interactions are likely to contribute to small-scale heterogeneity in the forest environment, but are unlikely to be ecologically significant in terms of their influence on the composition of 'two-component forests' containing both conifers and angiosperms.