Abstract
Uncertainty about the mechanisms driving biomass change at broad spatial scales limits our ability to predict the response of forest biomass storage to global change. Here we use a spatially representative network of 874 forest plots in New Zealand to examine whether commonly hypothesised drivers of forest biomass and biomass change (diversity, disturbance, nutrients and climate) differ between old-growth and secondary forests at a national scale. We calculate biomass stocks and net biomass change for live above-ground biomass, below-ground biomass, deadwood and litter pools. We combine these data with plot-level information on forest type, tree diversity, plant functional traits, climate and disturbance history, and use structural equation models to identify the major drivers of biomass change. Over the period 2002–2014, secondary forest biomass increased by 2.78 (1.68–3.89) Mg ha−1 y−1, whereas no significant change was detected in old-growth forests (+0.28; −0.72 to 1.29 Mg ha−1 y−1). The drivers of biomass and biomass change differed between secondary and old-growth forests. Plot-level biomass change of old-growth forest was driven by recent disturbance (large tree mortality within the last decade), whereas biomass change of secondary forest was determined by current biomass and past anthropogenic disturbance. Climate indirectly affected biomass change through its relationship with past anthropogenic disturbance. Our results highlight the importance of disturbance and disturbance history in determining broad-scale patterns of forest biomass change and suggest that explicitly modelling processes driving biomass change within secondary and old-growth forests is essential for predicting future changes in global forest biomass.
Highlights
Forests provide a significant reservoir of non-atmospheric carbon, equivalent to 861 ± 66 Pg, and this reservoir is thought to be growing at a rate of 2.4 ± 0.4 Pg per year (Pan and others 2011)
The ongoing capacity of forests to act as a net carbon sink depends on complex and often interacting forces of global change, including natural and anthropogenic disturbance (Kurz and others 2008; Reichstein and others 2013; Berenguer and others 2014), climate change (Phillips and others 2009; Reichstein and others 2013) and changes in community composition (Coomes and others 2014)
National-scale net biomass change was dominated by secondary forests, characterised by a large number of rapidly growing smaller trees, rather than old-growth forests (Figure 3)
Summary
Forests provide a significant reservoir of non-atmospheric carbon, equivalent to 861 ± 66 Pg, and this reservoir is thought to be growing at a rate of 2.4 ± 0.4 Pg per year (Pan and others 2011). Disturbance, environmental conditions and forest species composition and diversity affect all these processes, but the relative importance and magnitude of these effects is unresolved (for example, Fisher and others 2008; Coomes and others 2012, 2014; Fernandez-Martınez and others 2014; Duran and others 2015; Poorter and others 2015). The multiple drivers of forest biomass change can interact in complex ways across multiple spatial scales This introduces additional sources of uncertainty in future forecasting of biomass storage and can obscure the importance of these drivers at national or global scales (Fisher and others 2008; Chambers and others 2009; Erb and others 2013). Drivers that are important at local scales may become obscured or unimportant at larger spatial scales relevant to country-level UNFCCC reporting and international climate liabilities, or for assessing the relative impacts of drivers of forest change at a global scale. Efforts to disentangle the drivers of biomass change consider only a limited number of drivers in isolation (for example, Fisher and others 2008; Chisholm and others 2013; Coomes and others 2014; Fernandez-Martınez and others 2014) or are strongly reliant on extrapolations from few well-characterised systems
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