Stem respiration is an important part of the autotrophic respiration in woody plants. Accounting for 14% ~ 48% of the whole plant respiration (Vose et al., 2002; Zeng et al., 2000; Xiao et al., 2005), stem respiration is a non-negligible carbon source.
Stem respiration is usually divided into growth respiration and maintenance respiration (Thornley and Cannell, 2000). Growth respiration provides energy for the synthesis of new tissues, which is produced in the phloem and cambium of the stem. Maintenance respiration is used to maintain the normal life activities of plant cells like protein turnover, maintaining cell ion concentrations and gradients, and ions uptake (Cannell and Thornley, 2000). And it is sensitive to the changes of environmental factors.
Stem respiration can be influenced by both internal and external factors. Internal factors include tree height, diameter, CO2 permeability of bark, stem growth rate, the non-structural carbohydrate content and nitrogen content in stems, etc. Environmental conditions influencing plant growth are usually considered as the external factors, such as temperature, atmospheric CO2 concentration, soil water content and soil nutrients. As a typical enzymatic reaction, temperature is the most important factor affecting stem respiration.
Note: rs25 represents the respiration rate at 25℃, rs represents the respiration rate at Ts ℃, r’s25 represents the respiration rate at 25 ℃ after thermal acclimation, and r’s represents the respiration rate at Ts ℃ after thermal acclimation.
On one hand, as an enzyme-regulated process, stem respiration can vary with temperature instantaneously (Westerband et al., 2022; Sheng et al., 2011; Acosta et al., 2008), following an exponential response:
Here, Q10 represents the dependence of stem respiration on temperature (also known as temperature sensitivity). It is usually estimated by the multiple of respiration rate increase for every 10℃ rise in temperature. The general value of temperature sensitivity (Q10) is 2 to 2.2 (Acosta et al., 2008).
On the other hand, at a longer time scale, stem respiration also shows a thermal acclimation similar as leaf respiration (Atkin et al., 2003; Heskel et al., 2016). Existing studies have shown that leaf respiration at standard temperature decreased with warming (Fig. 1). However, at present, the studies on stem respiration is not only limited, but also just focused on individual sites, showing inconsistent conclusions. For example, Gansert et al. (2002) found that the birch branches growing along the vertical transect had similar respiration rates, indicating that they adapted to the long-term low-temperature environment. Nicholas et al. (2019) conducted warming experiments for five species, and found that respiration at 25℃ decreased with the increasing of acclimated temperature. While Carey et al. (1997) did not observe the thermal acclimation of the stem in the comparative sites with different temperature.
In general, at present, people only pay attention to the instantaneous response of stem respiration to temperature, and do not fully confirm and consider the thermal acclimation, which affects the accurate evaluation of carbon budget globally. Hopefully, with the data of stem respiration gradually enriched, it is possible to conduct analysis at the global scale. Comprehensively considering the thermal response of stem respiration will help to more accurately capture the carbon source and carbon sink dynamics of the forest ecosystem after global warming in the future.
Thanks Wang Han for her detailed comments on improving this blog.
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