Editing IPCC:AR6/SRCCL/Chapter-2 (section)

=== 2.7.1 Temperature responses of plant and ecosystem production ===

<div id="section-2-7-1-temperature-responses-of-plant-and-ecosystem-production-block-1"></div>

Climate-change responses of net ecosystem production cannot be modelled by simple instantaneous response functions because of thermal acclimation responses of plants and soil microbes, as well as delayed responses arising from interactions between plants and the soil ( ''high confidence'' ) (Slot et al. 2014 <sup>[[#fn:r1948|1948]]</sup> ; Rogers et al. 2017 <sup>[[#fn:r1949|1949]]</sup> ; Tan et al. 2017 <sup>[[#fn:r1950|1950]]</sup> ; Tjoelker 2018 <sup>[[#fn:r1951|1951]]</sup> ). Photosynthesis and respiration of component plant species exhibit different functional shapes among species (Slot et al. 2014 <sup>[[#fn:r1952|1952]]</sup> ), and carbon balance at the stand level is influenced by respiration of ecosystem biomass other than plants. Large uncertainty remains for thermal responses of bacteria and other soil organisms (Section 2.7.5). Bayesian statistical estimates of global photosynthesis and total ecosystem respirations suggest that they exhibit different responses to thermal anomalies during the last 35 years (Li et al. 2018b <sup>[[#fn:r1953|1953]]</sup> ).

Thermal responses of plant respiration, which consumes approximately one half of GPP, have not been appropriately incorporated in most ESMs (Davidson et al., 2006 <sup>[[#fn:r1954|1954]]</sup> ; Tjoelker, 2018 <sup>[[#fn:r1955|1955]]</sup> ). Assumptions associated with respiration have been a major source of uncertainty for ESMs at the time of AR5. In most existing models, a simple assumption that respiration doubles with each 10°C increase of temperature (i.e., Q10 = 2) is adopted, ignoring acclimation. Even a small error stemming from this assumption can strongly influence estimated net carbon balance at large spatial scales of ecosystems and biomes over the time period of multiple decades (Smith and Dukes 2013 <sup>[[#fn:r1956|1956]]</sup> ; Smith et al. 2016b <sup>[[#fn:r1957|1957]]</sup> ). In order to estimate more appropriate thermal response curves of respiration, a global database including data from 899 plant species has been compiled (Atkin et al. 2015 <sup>[[#fn:r1958|1958]]</sup> ), and respiration data from 231 plants species across seven biomes have been analysed (Heskel et al. 2016 <sup>[[#fn:r1959|1959]]</sup> ). These empirical data on thermal responses of respiration demonstrate a globally convergent pattern (Huntingford et al. 2017 <sup>[[#fn:r1960|1960]]</sup> ). According to a sensitivity analysis of a relatively small number of ESMs, a newly derived function of instantaneous responses of plant respiration to temperature (instead of a traditional exponential function of Q10 = 2) makes a significant difference in estimated autotrophic respiration especially in cold biomes (Heskel et al. 2016 <sup>[[#fn:r1961|1961]]</sup> ).

Acclimation results in reduced sensitivity of plant respiration with rising temperature, in essence, down regulation of warming-related increase in respiratory carbon emission ( ''high confidence'' ) (Atkin et al. 2015 <sup>[[#fn:r1962|1962]]</sup> ; Slot and Kitajima 2015 <sup>[[#fn:r1963|1963]]</sup> ; Tjoelker 2018 <sup>[[#fn:r1964|1964]]</sup> ). For example, experimental data from a tropical forest canopy show that temperature acclimation ameliorates the negative effects of rising temperature to leaf and plant carbon balance (Slot et al. 2014 <sup>[[#fn:r1965|1965]]</sup> ). Analysis of CO <sub>2</sub> flux data to quantify optimal temperature of net primary production of tropical forests also suggest acclimation potential for many tropical forests (Tan et al. 2017 <sup>[[#fn:r1966|1966]]</sup> ). Comparisons of models with and without thermal acclimation of respiration show that acclimation can halve the increase of plant respiration with projected temperature increase by the end of 21st century (Vanderwel et al. 2015 <sup>[[#fn:r1967|1967]]</sup> ).

It is typical that acclimation response to warming results in increases of the optimum temperature for photosynthesis and growth (Slot and Winter 2017 <sup>[[#fn:r1968|1968]]</sup> ; Yamori et al. 2014 <sup>[[#fn:r1969|1969]]</sup> ; Rogers et al. 2017 <sup>[[#fn:r1970|1970]]</sup> ). Although such shift is a result of a complex interactions of biochemical, respiratory and stomatal regulation (Lloyd and Farquhar 2008 <sup>[[#fn:r1971|1971]]</sup> ), it can be approximated by a simple algorithm to address acclimation (Kattge et al. 2007 <sup>[[#fn:r1972|1972]]</sup> ). Mercado et al. (2018) <sup>[[#fn:r1973|1973]]</sup> , using this approach, found that inclusion of biogeographical variation in photosynthetic temperature response was critically important for estimating future land surface carbon uptake. In the tropics, CO <sub>2</sub> fertilisation effect (Box 2.3) is suggested to be more important for observed increases in carbon sink strength than increased leaf area index or a longer growing season (Zhu et al. 2016 <sup>[[#fn:r1974|1974]]</sup> ). Acclimation responses of photosynthesis and growth to simultaneous changes of temperature and CO <sub>2</sub> , as well as stress responses above the optimal temperature for photosynthesis, remain a major knowledge gap in modelling responses of plant productivity under future climate change (Rogers et al. 2017 <sup>[[#fn:r1975|1975]]</sup> ).

<span id="water-transport-through-soil-plant-atmosphere-continuum-and-drought-mortality"></span>