Quantifying the temperature independent controls of nocturnal plant respiration

Abstract

Autotrophic respiration is a critical determinant of plant, ecosystem and global carbon exchange, constituting a major control on the evolution of the contemporary carbon cycle with the potential to modulate the magnitude of future climate change. Due to an incomplete understanding of plant respiration and its underlying mechanisms, the process remains an important yet poorly quantified component of the global carbon cycle and currently dominates uncertainties in carbon cycle modelling. Plant respiration is currently represented by a fixed exponential temperature function in vegetation and earth system models. This rather simplistic description is inadequate to describe the co-regulation of respiration by endogenous mechanisms over longer timescales, such as the control exerted by substrate supply, product demand and the circadian clock. This study compiles the first comprehensive dataset of nocturnal leaf respiration to explore and quantify the temperature-independent control of leaf respiratory metabolism at night. A down-regulation in nocturnal respiration was observed to occur under constant temperature conditions which decreased the basal rate of respiration by ~40% of the initial rate at the onset of darkness, indicating the base rate of respiration cannot be considered constant as generally assumed in all modern field studies and models. An empirically derived term representing the non-temperature dependent component of leaf respiration at night was applied to the land surface component of an earth system model to describe nocturnal variation in endogenous metabolism in addition to the temperature dependency of respiration. Accounting for the non-temperature dependency of nocturnal respiration reduced annual rates of modelled plant respiration by up to 10% and increased annual net primary productivity by up to 16% across all tropical and temperate forest sites, suggesting that previous models have overestimated global respiration and underestimated net primary productivity, particularly in the tropics. The significant impact of the novel term presents important implications for land-atmosphere studies and estimates of global terrestrial carbon balance and storage. This study provides the foundation from which to advance research on endogenous rhythms in plant metabolism to develop a more comprehensive understanding and description of plant respiration for modelling frameworks, ultimately to increase the realism of vegetation models for greater confidence in simulations of the current and future terrestrial and global carbon cycle.