Reply to Adams et al.: Empirical versus process-based approaches to modeling temperature responses of leaf respiration

Abstract

Using an empirical approach, we report that the slope of the short-term log-transformed leaf respiration (R)-temperature (T) curves declines with increasing leaf T in a manner that is uniform across biomes (1); the results have utility for modeling carbon fluxes in terrestrial biosphere models (TBMs). The use of an empirical approach reflects the fact that, despite advances in understanding of factors regulating R (2-4) and its T-response (5), basic information on key determinants of R remains lacking, hindering development of a process-based model with utility for TBMs. Some, including Adams et al. (6), view Arrhenius theory as providing a way forward and argue that it is both predictive and mechanistic. As noted (6), this approach provides equivalent predictive power as the log-polynomial function (1), a finding that we do not dispute, and that was recently noted in a separate paper comparing several approaches to fitting shortterm T-function of R (7). We also agree that global convergence in the shape of R-T curves is an indication that respiratory regulation is likely to be common across plants (6). Where we differ, however, is whether the applied Arrhenius approach (6) is mechanistic. Arrhenius theory is applicable to reactions catalyzed by single enzymes that are substrate-saturated. For respiratory metabolism in plants, neither assumption holds, because the respiratory system is made up of numerous, highly-regulated reactions that are rarely substrate-saturated (8). Thus, although activation energy (E a ) values of R, including temperature-dependent ones (9, 10), provide estimates of the temperature coefficient of the overall respiratory system, they can be viewed as outputs of a statistical fit, because they do not necessarily provide insights into the individual mechanisms underpinning variation in R-T curves.In the temperature-modified Arrhenius approach (9), changes in the slope of log-transformed R-T curves are achieved via adding a factor (δ) to account for T-dependent changes in the activation energy (i.e., T-sensitivity) estimated at 0°C (E 0 ). Similarly, we (1) provide estimates of the T-sensitivity at 0°C, and how the T-sensitivity of R declines with increasing leaf T (i.e., b-and c-parameters in the polynomial). Thus, in general terms, b and E 0 describe the T-sensitivity at 0°C, with c and δ accounting for deceleration in R as leaves warm. Congruence in the two approaches therefore reflects their underlying operational similarities, raising the possibility that TBMs can indeed use either approach (1, 9).Looking forward, development of a process-based model to account for the complexity of taxa-and environment-driven variations in R (11) remains a high priority. Notable advances are clarifying individual and collective mechanistic controls of R through models and experiments (2-5). Ideally in the future, a truly mechanistic approach based on these advances will emerge that meets the TBM integration requirements of being parameter-sparse, scalable, and spatially robust; however, current knowled...

Implications of improved representations of plant respiration in a changing climate

“…However, availability of these remains a long way from routine usage, yet alone in large-scale climate models. This is an issue recently discussed in depth for the b,c instantaneous temperature response formulation 42 , 43 , and where that exchange in the literature has relevance to more general respiration modelling.…”

Variation in white spruce needle respiration at the species range limits: a potential impediment to northern expansion

“…After equilibrating the system to 5°C, the mesh bag holding the leaves was sealed inside. Once stability was reached the instrument was zeroed and the response curve was measured as described in Heskel et al (2016), O’Sullivan et al (2013), and Schmiege et al (2021). During measurements the flow rate through the cuvette was set to 500 ml min -1 and the CO 2 concentration to 400 ppm.…”

“…The respiration temperature response curves were analyzed as in Heskel et al (2016) by fitting a second-order polynomial model to the log transformed respiration rates between 10 and 45 °C. …”

“…Third we hypothesize that southern but not northern spruce respiration will be similar to the average for the Needle leaved Evergreen (NLEv) plant functional type to which they belong. To quantify white spruce respiratory temperature response and make these comparisons, we use the global polynomial model of Heskel et al (2016). Fourth, we hypothesize that trees from the southern edge of the species range will have a higher temperature tolerance than trees from the northern edge of the distribution.…”

Plasticity of Leaf Respiratory and Photosynthetic Traits in Eucalyptus grandis and E. regnans Grown Under Variable Light and Nitrogen Availability

“…In the past, responses of leaf respiration to short-term shifts in (measurement) temperature were generally described via temperature-insensitive Q 10 (i.e., often setting Q 10 = 2.0 in Earth System Models), or via classical Arrhenius kinetics where activation energy is constant (reviewed by Atkin et al, 2005). However, it has become increasingly apparent that overall activation energy of leaf respiration (or Q 10 ) is not constant across measurement temperatures (Atkin and Tjoelker, 2003;Noguchi et al, 2015;Heskel et al, 2016a;Adams et al, 2016;Atkin et al, 2017). In an extended, Arrhenius-type description, the temperature-dependency of activation energy is captured by the exponent parameter δ R (Kruse et al, 2016(Kruse et al, , 2017.…”