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

=== CCP7.3.2 Climate-Related Mortality and Regeneration in Tropical Forests ===

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Drought-related mortality of tropical trees shows complex patterns which could change forest community structure and composition with cascading effects on biodiversity (McDowell et al., 2020). During drought, the mortality rate is enhanced in larger-sized trees in tropical forests (as is the case with all forests globally), with significant impacts on forest structure, carbon storage and regional hydrology (Bennett et al., 2015). The mortality rate of neotropical moist forest trees appears to be consistently increasing since the 1980s (McDowell et al., 2020), with plant functional types such as softwood, pioneer and evergreen species suffering higher mortality during years of extreme drought (Aleixo et al., 2019). Large trees (&gt;30 cm diameter at breast height (dbh)) in tropical dry forests have much lower mortality rates than those reported for tropical moist forests (Suresh et al., 2010). Contrary to expectation, during prolonged droughts in these dry forests, deeper-rooted tree species are more ''likely'' to die than shallow-rooted ones, which are more adapted to changes in soil moisture content, because of water depletion in the deepest unsaturated zone (Chitra-Tarak et al., 2018).

Regeneration of tropical tree seedlings and their response to a changing climate is inadequately understood. Experimental work suggests that tropical moist forest tree seedlings and saplings can acclimate photosynthetically to moderate levels of warming and, unlike adults, may even exhibit increased growth rates ( [[#Cheesman--2013|Cheesman and Winter, 2013]] ; [[#Slot--2018|Slot and Winter, 2018]] ). Some moist forest seedlings also show plasticity to recurrent drought episodes by enhancing their growth rates when favourable moisture conditions return, while others fail to respond (O’Brien et al., 2017). The nature of response also seems to be mediated by neighbourhood diversity, with greater plasticity in more diverse communities (O’Brien et al., 2017). Seedlings in tropical dry forests subject to burning show enhanced growth rates post-fire and within two years attain similar height of seedlings in unburnt areas (Pulla et al., 2015), though the environmental drivers of seedling growth post-fire are not well understood (Bhadouria et al., 2017).

The net outcome of the population dynamics processes of growth, mortality and regeneration is change in species composition as a consequence of a changing climate. In the Amazon forests, dry habitat-affiliated genera have become more abundant among the newly recruited trees, while the mortality of moist habitat-affiliated genera has increased in places where the dry season has intensified most, thus driving a slow shift towards a drier forest type (Esquivel-Muelbert et al., 2019). A similar multi-decadal shift in West-African forest species composition towards more dry-affiliated species as a response to long-term drying has been recorded (Aguirre-Gutiérrez et al., 2020). While upward shifts in the tree line and in the range of individual tree species have been recorded at several temperate mountain regions, evidence from the tropics is rare. A large-scale study from 200 plot inventories of &gt;2000 tree species across a ~3000 m elevation gradient in the Andean tropics and sub-tropics has shown that the relative abundances of tree species from lower, warmer locations were increasing at these sites indicating that ‘thermophilisation of vegetation’ (increased domination of plant species from warmer locations) was indeed taking place as expected (Fadrique et al., 2018) [Section 2.5.4.2.1 in Chapter 2].

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[[File:c62eca070141c77f9b30de40b14d9f42 IPCC_AR6_WGII_Figure_CCP7_004.png]]

'''Figure CCP7.4 |''' '''Documented instances of tree mortality in tropical moist forests due to fire (1992–2016) and drought (1982–2005).''' These occurrences were associated with anomalies in precipitation and temperature over the study period. Adapted from Brando et al. (2019).

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