Technical Comments

Response to Comment on “Global Convergence in the Temperature Sensitivity of Respiration at Ecosystem Level”

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Science  11 Mar 2011:
Vol. 331, Issue 6022, pp. 1265
DOI: 10.1126/science.1197033

Abstract

We estimated the sensitivity of terrestrial ecosystem respiration to air temperature across 60 FLUXNET sites by minimizing the effect of seasonally confounding factors. Graf et al. now offer a theoretical perspective for an extension of our methodology. However, their critique does not change our main findings and, given the currently available observational techniques, may even impede a comparison across ecosystems.

In a recent paper, we applied a methodology to estimate the temperature sensitivity of ecosystem respiration (the total CO2 release by plants and soils) from long-term monitoring data (1). One central finding was that the overall short-term temperature sensitivity of ecosystem respiration is relatively uniform across biomes and different climate zones. Our approach is based on the assumption that ecosystem-atmosphere interactions vary on multiple time scales, as shown in a series of site-level studies (2, 3). The conceptual advantage of the proposed scale-dependent parameter estimation (SCAPE) methodology compared with conventional estimates has now been confirmed by the synthetic experiments reported by Graf et al. (4): SCAPE approximates the short-term temperature sensitivity of ecosystem respiration in the presence of seasonally confounding effects quite well.

However, Graf et al. (4) correctly point out that the reported Q10;sc could be an underestimate if substantial fractions of the released CO2 are originating from deeper soil layers where temperature dynamics are delayed and dampened compared with air temperature. This valid point (5, 6) has already been anticipated in our original paper (1). Graf et al. (4) emphasize the difference between an intrinsic temperature sensitivity of respiration and the findings in (1). This objection illustrates a possible misunderstanding. Hence, we use this opportunity to further clarify a few conceptual aspects of our study.

First, although our study was motivated by the search for an intrinsic temperature sensitivity of ecosystem respiratory processes [in the sense of (7), referring to kinetic properties], we did recognize that such an intrinsic value does not exist at the ecosystem level and can only be approximated. As illustrated by Reich (8), a series of individual processes contribute to the overall release of CO2, via different mechanisms, and each of these processes may obey different metabolic relations (9). Therefore, we suggest using the term “SCAPE Q10” (denoted as Q10;sc) in forthcoming analyses. In fact, by definition no intrinsic Q10 exists at the ecosystem level (6).

Second, the main virtue of the presented Q10;sc values is that they are not influenced by the seasonal confounders (6, 10). We interpret Q10;sc as a parametric diagnostic of the short-term temperature sensitivity of respiration at ecosystem level. Such an integrated measure is essential for scrutinizing the validity of biosphere models. An ecosystem-level Q10;sc can form the basis for evaluating current and future coupled climate–carbon cycle simulations, similar to the climate sensitivity of the global carbon cycle, which is a diagnostic at Earth-system level (11).

Third, Graf et al. (4) conclude that we should have included temperature dampening effects to obtain more representative Q10;sc estimates. We agree with this idea and see its value for future applications. However, the prerequisite would be the availability of continuous soil temperature and flux data at different depths allowing us to pin-point the exact location of the sources of CO2 in the soil column. At present, however, such an approach is not feasible based on available FLUXNET data archives (12). It would also require taking delays in respiration responses into account, for instance, due to delayed belowground allocation and exudation rates (13).

Furthermore, fluctuations in soil water content may play an important role, acting as a direct constraint for soil respiration (14) but also by determining heat conduction (and storage) in the soil (15). This latter effect is itself dependent on the soil type, peat soils being far less conductive than sandy or clay soils, even with higher water content (15). Hence, the question of how to disentangle the temperature sensitivity of plant respiration remains unclear in this context. Besides the methodological advances, our reported Q10;sc values are practicable because they have been derived based on a minimum of a priori assumptions. Using air temperature instead of soil temperature profiles guarantees a comparability of the integrated response of soils and vegetation. The consistency of the analysis across so many sites is a key prerequisite for comparing different biomes and provides reference temperature sensitivities for terrestrial biosphere models that generally operate on very large spatial scales. The analysis by Graf et al. (4) would instead lead directly to the formulation of an ecosystem-specific empirical model, which was not the goal of our analysis.

As a final remark, we would like to highlight that the importance of our paper lies in the homogeneity of the temperature sensitivity observed across biomes and plant functional types. This emergent behavior at ecosystem level deserves more attention in future studies, where a model-data integration approach built on the ideas described by Graf et al. (4) may considerably advance our understanding of ecosystem-atmosphere exchange processes.

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