Deadwood in New Zealand's indigenous forests

We present the results of a systematic, unbiased national survey of deadwood volume and biomass in New Zealand's remaining indigenous forests based on an 8-km grid of 894 permanent plots. New Zealand's old growth evergreen temperate forests are largely comprised of long-lived, slow-growing tree species typically growing in cool, humid conditions; collectively these conditions are thought to promote accumulation of high deadwood stocks. We estimated deadwood biomass and volume in New Zealand's forests and compared these stocks with published values from other broadleaved evergreen temperate forests. Mean deadwood biomass in New Zealand was 54 Mg ha⁻¹ but ranged across plots from 0 to 550 Mg ha⁻¹. Mean deadwood volume was 158 m³ ha⁻¹ and ranged across plots from 0 to 1890 m³ ha⁻¹. Fallen logs accounted for 63% of total deadwood volume and 65% of total deadwood biomass, with standing dead trees being the remainder. Each piece of deadwood was classified into one of three broad decay classes and >40% of deadwood was fallen logs of the intermediate decay class. Deadwood biomass and volume varied 1.8- and 1.9-fold, respectively, among forest types and was greatest in broadleaved forests, dominated by Weinmannia racemosa (Cunoniaceae), Metrosideros umbellata (Myrtaceae) and Metrosideros robusta, and broadleaf-Nothofagus (Nothofagaceae) forests supporting the large tree species Nothofagus fusca. Deadwood biomass and volume were least in broadleaf-conifer admixtures. We used structural equation models to determine whether deadwood biomass could be predicted from climate and environment (vapor pressure deficit, elevation and slope), live tree biomass, forest composition (captured by two ordination axes), wood density of live trees, and tree size (a proxy for stand age). The model that best fit the data retained only vapor pressure deficit, live tree biomass and the first ordination axis as predictors of deadwood biomass. However, this model predicted just 2.4% of the variation in deadwood biomass, suggesting that additional factors not captured by this dataset, such as disturbance dynamics, may control deadwood abundance. Comparisons with other temperate and tropical forests did not support the hypothesis that New Zealand's cool temperate rainforests support higher than expected biomass or volume of deadwood.