Editing IPCC:AR6/WGII/Cross-Chapter-Paper-7 (section)

=== CCP7.3.1 Tropical Tree Physiological Responses to Climate Change ===

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With rising temperatures and atmospheric carbon dioxide, possibly accompanied by greater variability in soil moisture availability, a key question is how tropical forest trees respond physiologically (especially photosynthesis and respiration which determine net growth rates) and how well they can acclimate (i.e., able to adapt) to climate change (Dusenge et al., 2019). Key climate factors influencing tree growth on pan-tropical forests are precipitation, solar radiation, temperature amplitude and relative soil moisture (Wagner et al., 2014).

The temperature response of photosynthetic carbon uptake in tropical trees seems remarkably similar across moist and dry forest types, as well as for light-demanding, fast-growing species compared with shade-tolerant, slow-growing species ( [[#Slot--2017|Slot and Winter, 2017]] ). It is generally agreed that photosynthesis in tropical species can acclimate to moderate levels of warming but beyond this there would be no net gain in carbon ( [[#Slot--2017|Slot and Winter, 2017]] ). The factor that limits photosynthesis in different tropical forests will depend on water availability. In water-limited dry forests, photosynthesis may decline largely due to stomatal closure, while in wet forests the decline may largely be driven by warming-related changes to leaf biochemistry ( [[#Slot--2017|Slot and Winter, 2017]] ). A recent modelling approach suggests that the limits of photosynthetic thermal acclimation may be an increase of about 2°C, in terms of maximum tolerated temperature, with enhanced tree mortality beyond this level of warming ( [[#Sterck--2016|Sterck et al., 2016]] ).

A critical concern for plant function has been that higher temperatures will enhance respiration rates, potentially resulting in tropical forests becoming net carbon sources (rather than photosynthesis-driven carbon sinks) ( [[#Gatti--2021|Gatti et al., 2021]] ). Some studies suggest that excessive respiration is less of a concern as respiration rates can acclimate to elevated temperatures over time ( [[#Lombardozzi--2015|Lombardozzi et al., 2015]] ; [[#Pau--2018|Pau et al., 2018]] ). Thermal acclimation of respiration has been shown in a seasonally dry neotropical forest ( [[#Slot--2014|Slot et al., 2014]] ), while models indicate that increases in plant respiration could halve by the end of the 21st century through acclimation, thereby partly ameliorating the potential release of carbon from tropical forests ( [[#Vanderwel--2015|Vanderwel et al., 2015]] ). A contrary view is that plant physiological processes, such as the photosynthesis in tropical canopy trees, are already functioning at levels close to or beyond their thermal optimum limits and that any further temperature increase would turn them from a sink into a carbon source ( [[#Mau--2018|Mau et al., 2018]] ). One of the most pressing questions regarding forest responses to increasing atmospheric CO 2 levels is whether trees experience enhanced growth rates as a result of the so-called CO 2 fertilisation effect [Box 2.3 in IPCC 2019b]. Observed changes in the terrestrial carbon sink and process-based vegetation models indicate that tropical vegetation response to CO 2 fertilisation ( [[#Schimel--2015|Schimel et al., 2015]] ) is combined with other factors such as nitrogen deposition and length of the growing season, while aerosol-induced cooling may also have played a role in enhancing the carbon sink [Box 2.3 in IPCC 2019b]. Contrastingly, evidence for CO 2 fertilisation of growth in individual tropical tree species is generally lacking or controversial ( [[#Silva--2013|Silva and Anand, 2013]] ), or not as substantial as expected ( [[#Sampaio--2021|Sampaio et al., 2021]] ). It is, however, widely agreed that the intrinsic water-use efficiency of a tree, that is, the amount of carbon assimilated as biomass per unit of water used, increases under elevated atmospheric CO 2 levels owing to the regulation of stomata (cells on the leaf surface which regulate the exchange of water and gases between the plant and the atmosphere) ( [[#Van%20Der%20Sleen--2015|Van Der Sleen et al., 2015]] ; [[#Bartlett--2016|Bartlett et al., 2016]] ; [[#Rahman--2016|Rahman and Alam, 2016]] ; [[#Keeling--2017|Keeling et al., 2017]] ). Tropical dry forests (ca. 1000 mm annual rainfall) exhibit changes in water-use efficiency (WUE), relative to CO 2 , at least twice as much as tropical moist forests (c. 4000 mm rainfall) ( [[#Adams--2019|Adams et al., 2019]] ).

Other key components in the forest system are plant–microbe–soil nutrient interactions, which play major roles in carbon cycling and plant photosynthetic response to increased atmospheric CO 2 and warming (Zhang et al., 2014; [[#Singh--2015|Singh and Singh, 2015]] ; Du et al., 2019). Phosphorus is generally a limiting factor in tropical forest soils, though this may be species-specific (Ellsworth et al., 2017; Turner et al., 2018). Mycorrhizal fungi (both arbuscular and ectomycorrhizal) play major roles in water acquisition of host plant and their responses to drought in dry tropical forest ( [[#Lehto--2011|Lehto and Zwiazek, 2011]] ) as well as in the capture and transfer of nutrients, especially nitrogen (which may otherwise become limiting), to host plants. Climate change factors can thus be expected to alter the nature of soil–plant interactions with consequences for the species composition and biodiversity of tropical ecosystems (Pugnaire et al., 2019; Terrer et al., 2019)

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