By Alessio Collalti, Andreas Ibrom, Colin Prentice et al.
Forests are the largest carbon sink on land. Understanding how they will respond to climate change is key to predicting future climates.
Colin has a new paper out in Nature Communications, led by Dr Alessio Collalti (National Research Council of Italy) and Professor Andreas Ibrom (Technical University of Denmark). This paper “Forest production efficiency increases with growth temperature” challenges the assumption that the ability of plants to assimilate carbon into organic matter is constant, and demonstrates that a warming climate could increase the efficiency with which this conversion takes place.
A newly coined term, Forest production efficiency (FPE), describes the fraction of carbon assimilated by trees through photosynthesis (gross primary production, GPP) that is allocated to building their tissues (biomass production, BP) or – more generally – to the production of organic matter (net primary production, NPP), which includes non-structural components such as root exudates, which plants use to help acquire nutrients from the soil. FPE is important for both forest production and for the uptake of carbon dioxide from the atmosphere. It can be measured in two ways:
- As Carbon Use Efficiency (CUE), i.e. the NPP / GPP ratio.
- As Biomass Production Efficiency (BPE), i.e. the BP/ GPP ratio.
These two meaures have often been used interchangeably. They are not identical, but they are close. The amount of carbon allocated to building new tissues (BP), or to new tissues plus other non-structural components (NPP), are both highly dependent on how much organic carbon is broken down in the process of plant (autotrophic) respiration, which serves to provide the reducing power and energy needed both to create new tissues and to sustain the metabolism of living tissues. Two conflicting schools of thought have assumed either that FPE is constant (or very tightly constrained, around ca. 0.5) despite climate variations and forest ageing; or that FPE varies with climate, and specifically that it declines with increasing temperature – because respiration, as measured on short time-scales, increases with temperature more steeply than photosynthesis.
The authors of this paper challenged both assumptions. They postulated that trees across the world in different climates, and growing in stands of different age, are not all equally efficient at assimilating carbon into their various tissues and non-structural components. The team – led by Collalti – used an unprecedentedly large set of observations from forests across the world to examine the potential controls of stand age, climate and environment on FPE. They found that FPE:
- increases with precipitation
- increases with absolute latitude
- decreases with stand age
- increases with mean annual growth temperature,
Based on the limited information available from studies that distinguished BP from NPP, these statements also are true both for CUE and for BPE.
These results are not consistent with the assumption of constant FPE. They contrast with previously published works that were based on much smaller sample sizes. Importantly, they also differ sharply from what models predict. Models in the TRENDY v.7 project consistently, and incorrectly, show FPE declining with growth temperature. They do so because they all assume that the rate of respiration increases with growth temperature. They thus overlook a fundamentally important acclimation processes at the whole-plant level, which counteracts the short-term response of respiration to temperature by lowering the base rate. This process might be expected to yield FPE independent of growth temperature. A possible explanation for the observed increase in FPE with increasing growth temperature could be that in colder climates, there is a higher carbon cost to nutrient acquisition, and therefore less carbon can be assimilated into tissues.
Current models thus consistently predict that in a warming climate, forest production efficiency will decline; while results from this study indicate that the opposite is true, and we may be overestimating the amount of carbon loss to be expected in a warming climate. Put another way, we are currently underestimating the amount of carbon that forests can sequester from the atmosphere in a warming climate. This paper therefore gives us hope that as carbon dioxide increases and climate changes, forests may prove to have more mitigation potential than we thought.
Collalti, A., A. Ibrom., A. Stockmarr., A. Cescatti., R. Alkama., M. Fernandez-Martinez., G. Matteucci., S. Sitch., P. Friedlingstein., P. Ciais., D.S. Goll., J.E.M.S. Nabel., J. Pongratz., A. Arneth., V. Haverd. and I.C. Prentice. Forest production efficiency increases with growth temperature. Nat Commun 11, 5322 (2020). https://doi.org/10.1038/s41467-020-19187-w
Dr Alessio Collalti is a researcher from the the National Research Council of Italy, Institute for Mediterranean Agriculture and Forest Systems (CNR-ISAFOM, Perugia) and he is the Forest Modelling Laboratory Head (https://www.forest-modelling-lab.com/) in Italy. For more information about this paper, you can email Dr Collalti on alessio.collalti@.cnr.it