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Tue Jun 4, 2024, 02:07 AM Jun 4

Novel temperatures are already widespread beneath the world's tropical forest canopies

Article | Open access | Published: 03 June 2024
Novel temperatures are already widespread beneath the world’s tropical forest canopies

Tropical forest biodiversity is potentially at high risk from climate change, but most species reside within or below the canopy, where they are buffered from extreme temperatures. Here, by modelling the hourly below-canopy climate conditions of 300,000 tropical forest locations globally between 1990 and 2019, we show that recent small increases in below-canopy temperature ( < 1 ° C ) have led to highly novel temperature regimes across most of the tropics. This is the case even within contiguous forest, suggesting that tropical forests are sensitive to climate change. However, across the globe, some forest areas have experienced relatively non-novel temperature regimes and thus serve as important climate refugia that require urgent protection and restoration. This pantropical analysis of changes in below-canopy climatic conditions challenges the prevailing notion that tropical forest canopies reduce the severity of climate change impacts.

Humid tropical forests are global hotspots of terrestrial biodiversity¹ ², playing critical roles in species conservation³, influencing climate regimes⁴ and terrestrial carbon cycles⁵. Yet the ecological integrity of global tropical forests is being diminished by clearing, selective logging and wildfires⁶, and by increasingly frequent extreme weather events, such as blowdowns and droughts, driven by climate change⁷. Moreover, novel climates—those with no recent historic analogues—are predicted to appear first in the tropics and subtropics⁸ ⁹ ¹⁰ ¹¹.

It is generally assumed that the impact of climate change on the forest subcanopy and understorey will be lower than elsewhere on Earth because temperature conditions below the canopy are buffered from temperature extremes, reducing the severity of warming impacts¹² ¹³ ¹⁴. Beneath forest canopies, direct sunlight is strongly reduced and evapotranspirative cooling is increased, dampening temperature fluctuations compared with open habitats and resulting in cooler below-canopy maximum temperatures, warmer minimum temperatures, and lower seasonal and interannual variability¹² ¹⁵. However, the relative stability in temperature regimes through evolutionary history means that tropical forest organisms evolved under a narrower range of climate conditions than extratropical biota and can therefore tolerate a smaller margin of warming above their thermal optima¹⁶ ¹⁷ ¹⁸. Key questions, therefore, are whether the range of below-canopy temperatures currently experienced by tropical forest biota are novel in relation to historic climate, and how novelty varies between structurally intact and degraded tropical forests.

There is little understanding of how microclimates beneath the forest canopy—the conditions actually experienced by tropical forest organisms—are changing pantropically. Recent efforts to monitor within-forest temperatures¹⁴ ¹⁹ have revealed that forests warm at a slower rate than non-forested areas, yet logging-induced canopy perforations increase understorey temperatures for up to 5 years relative to intact forest²⁰. Although these are an important first step to quantifying below-canopy climate novelty, they provide only limited temporal coverage from a relatively small number of locations. Accordingly, mapping forest microclimates at a global scale has been identified as an important, yet unexplored, future research avenue¹³. Integrating a recently developed mechanistic microclimate model²¹ with empirical temperature measurements and satellite-derived land-cover data, we quantify hourly below-canopy temperature (5 cm above the ground) at 5 km grid resolution between 1990 and 2019 across forests in the humid tropics, including tropical rainforest and tropical moist deciduous forest (hereafter tropical forests⁶ ). We tackle two key objectives: (1) quantifying recent temperature novelty of forests pantropically compared with a historic baseline, to map those most at risk from warming and those that are currently providing climate refugia; and (2) comparing how overall temperature change across the last 30 years affects the degree of below-canopy temperature novelty in (i) undisturbed forest within ecologically unfragmented areas (defined as wilderness areas²² ), (ii) undisturbed forest in more fragmented landscapes and (iii) degraded forest.

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