Coarse woody debris decay rates for seven indigenous tree species in the central North Island of New Zealand
Introduction
Coarse woody debris (CWD) can comprise a significant proportion of the carbon stock in mature forest ecosystems in New Zealand and elsewhere (Beets, 1980, Levett et al., 1985, Benecke and Nordmeyer, 1982, Schonenberger, 1984, Harmon et al., 1995, Allen et al., 1997, Eaton and Lawrence, 2006). Large quantities of woody debris are deposited periodically on the forest floor during severe storms (see for example Keller et al., 2004, Ryall et al., 2006, Woodall and Nagel, 2007). The effect of severe storms on forests was noted in an investigation initiated following cyclone Bernie, in April 1982, when it was concluded that they have had a major influence on stand structure and pattern in indigenous forests throughout New Zealand (Shaw, 1983). Carbon losses due to CWD decay are therefore an important aspect of the terrestrial carbon balance.
New Zealand has determined under the 1992 United Nations Framework Convention on Climate Change to quantify the levels of carbon exchange occurring within its portion of the terrestrial biosphere, and research has commenced to quantify the organic carbon balance in indigenous forest and shrubland, including changes over time. Initial research to establish the 1990 baseline (Hall et al., 2001) has identified a number of sampling and carbon estimation issues. These are being addressed by implementing a national inventory, which involves mapping the area of indigenous forest and shrubland, establishing a network of permanent vegetation plots (Coomes et al., 2002) on an 8 km grid placed over the mapped area, measuring the dimensions of the trees, shrubs, and coarse woody debris in all plots over a five year period following standard protocols (Payton et al., 2004), and concurrently measuring fine litter, humus and mineral soil carbon to 30 cm depth on about one third of the plots (Davis et al., 2004).
In accordance with IPCC Good Practice Guidance for Land Use, Land-Use Change and Forestry (2003), carbon stocks in New Zealand's natural forest and shrubland are being estimated in five pools: (1) above ground biomass, (2) below ground biomass, (3) dead wood, (4) litter, and (5) mineral soil. A national estimate of above ground biomass carbon in New Zealand's natural forest estate is reported in Hall et al. (2001) and total carbon within forest and shrubland in a 60 km wide transect across the South Island of New Zealand are reported in Coomes et al. (2002). A few site-specific studies in New Zealand have provided data suitable for developing regression relationships for predicting biomass from tree and log dimensions (Beets, 1980) and it is acknowledged that the improvement of these functions is highly desirable.
Estimates of CWD carbon stocks are currently based on measurements of the debris volume over bark, which are calculated from lengths and small and large end diameters, and an assumed density, with modifiers used to account for reductions in CWD density in subjectively assessed decay classes (Coomes et al., 2002, Harmon et al., 1995, Waddell, 2002, Eaton and Lawrence, 2006). Carbon is then estimated from the CWD mass using a conversion factor of 0.50 (Allen et al., 1997). Changes in CWD carbon stocks over time can therefore be estimated from repeated measurements of CWD volume by decay class. Remeasurement of residual debris volume allows for fragmentation and decay of existing CWD and for the addition of new CWD due to tree mortality (Stone et al., 1998). A measurement based approach for estimating CWD carbon stock changes over time entails ongoing remeasurement costs and can be inaccurate, for example, following major windthrow events considerable quantities of material are added to the forest floor and can be difficult to locate and remeasure consistently over time. Furthermore, CWD decay classes are a subjective assessment of external features, and may not accurately depict CWD density (Creed et al., 2004).
An alternative modelling approach for estimating CWD stocks and changes in plots that are remeasured over time involves documenting when tree mortality events occur, estimating the initial volume, whole stem density, and resultant mass of fallen trees using appropriate biomass equations developed for live trees, and then applying decay functions to estimate the loss in mass over time. Studies undertaken elsewhere have shown that decay rates differ among tree species, and that factors including the diameter of the debris, and climate (temperature) are important considerations when developing decay functions, for example, Frangi et al. (1997), Chambers et al. (2000), Mackensen et al. (2003), Busing and Fujimori (2005), Wilson and McComb (2005), and Eaton and Lawrence (2006). Decay functions therefore need to be applied to plots taking into account the tree species, diameter at breast height, CWD age, and mean annual temperature.
CWD decay rates for New Zealand tree species have not been determined under forest conditions. A large number of species are involved, and our intention was to measure decay rates for species that comprise a large proportion of the carbon stock in New Zealand's natural forest, and estimate decay rates for less common species based on their expected natural durability ratings.
The durability of heartwood of a wide range of New Zealand's timber trees has been determined using a universal system of testing involving 50 mm × 50 mm heartwood stakes placed in the ground in test areas referred to as graveyards (Clifton, 1994), following a classification scheme used routinely in Australia (Page et al., 1997). Based on this system, New Zealand indigenous tree species have been broadly classified into five durability classes ranging from perishable (<5 years) to very durable (>25 years), which may provide an initial basis for grouping species and allocating decay constants to these classes, as attempted in Australia (Mackensen et al., 2003). It is currently not known how closely species durability ratings based on graveyard studies compare with decay rates obtained under forest conditions.
Variation in CWD decay rates owing to the particular fungal species involved has been reported previously, although few studies have been undertaken on this topic (Worrall et al., 1997, Edman et al., 2006, Buchanan et al., 2001). As part of this study we therefore quantified the influence of basidiomycete fungal species populating the CWD on decay rates, following assessment methods published previously (Hood et al., 2004).
Research has shown that accurate estimation of CWD decay rates requires the use of methods that take account of losses owing to fragmentation of stems and branches while allowing for any decay that occurred in damaged trees prior to tree mortality (Means et al., 1985, Harmon and Sexton, 1996, Stone et al., 1998, Creed et al., 2004). Practical guidelines for measuring CWD, and for determining unbiased decay rates using chronosequence (space for time substitution) and age series approaches are given in Harmon and Sexton (1996). Decay rates can be estimated accurately provided that wood density is measured consistently by taking into account voids associated with stem fluting and hollowing (Palace et al., 2007) using mensurational methods that apply both to live trees and CWD. The volume of samples immediately prior to mortality should be used to calculate the density of CWD. Furthermore, the precision of the decay constant estimates can be improved by allowing for internal log decay that may have occurred prior to tree mortality. The methods described in this paper address these issues, and also incorporate double sampling which facilitates the development of more precise decay constant estimates than otherwise possible when the sample size is relatively small, as in this study.
The objectives of this paper are to provide new CWD decay data and models for estimating species-specific decay rates for major indigenous tree species growing in the North Island of New Zealand, and to identify the main fungal agencies involved, in order to facilitate national predictions of carbon loss following tree mortality using carbon models.
Section snippets
Study sites
Sites where cyclone Bernie resulted in substantial areas of tree mortality in 1982 were identified in Whirinaki and Kaimanawa forests, each dominated by different tree species.
Densities of live trees and CWD
The number of sample trees, and their mean dimensions are summarised in Table 1. Note that the predominant podocarps at Whirinaki were generally larger in height and diameter than the beech species at Kaimanawa. Tree height and diameter data for standing live trees are shown in Fig. 1. The live tree survey data showed that breast height outer wood basic density varied both between and within species. Within a species density varied by about ±50 to ±75 kg m−3, with a weak trend of density reducing
Discussion
Carbon sequestration by forest trees can be calculated from the gains in carbon due to tree recruitment and growth minus losses due to mortality and decay. Because decay rates for indigenous tree species were not known, a widely used stock change approach was adopted in New Zealand (Coomes et al., 2002), where carbon in CWD is estimated from its volume, which is then converted to carbon by classifying debris into decay classes, and applying decay class-specific density modifiers (Harmon and
Conclusions
The mean exponential decay rate constant for CWD in indigenous forests located in the central North Island of New Zealand was 0.023 and the predicted time for CWD to lose 50% mass was 30 years. Decay constants were found to differ among species, with species decay times varying by up to ±50% from the overall mean. However, because of variability in decay rates between trees of the same species, and limited sample sizes for some species, these differences were mostly not statistically
Acknowledgements
This work was funded by the New Zealand Government through the Foundation for Research Science and Technology. Access to trial sites was facilitated by Paul Cashmore, Deirdre Ewart, Dave Hunt, Nicholas Singers, and John Sutton of the Department of Conservation. Former and present staff of the New Zealand Forest Research Institute who established and monitored the Whirinaki forest trial or who previously conducted research in Kaimanawa forest assisted in discussion during field visits, including
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