Tropical forests are approaching critical temperature thresholds

来自 <Tropical forests are approaching critical temperature thresholds | Nature>

热带森林正在接近临界温度阈值

## Abstract:

• The critical temperature beyond which photosynthetic machinery in tropical trees begins to fail averages approximately 46.7 °C (Tcrit).

 However, it remains unclear whether leaf temperatures experienced by tropical vegetation approach this threshold or soon will under climate change.

## Intro:

• Tropical forest mean temperatures are high, and their diel and seasonal variations are relative small, thus even a small change in temperature could more greatly impact tropical plant species than a large temperature change in other global regions.

== Average temperatures have risen by 0.5 °C per decade in some tropical regions, and temperature extremes are becoming more pronounced.

== As temperatures in tropical forests are near or above the temperature optimum for photosynthesis, further increased temperatures may close stomata, reducing transpirational cooling and exposing leaves to damaging temperatures.

== Sachs (1864) first reported that leaves from different plant species could withstand temperatures of up to 50 °C, but would die at temperatures even slightly higher. In the era of climate change, this finding is still relevant. How close forests are to a high temperature threshold such as the one proposed by Sachs is a particularly important issue in tropical forests, which serve as critical stores and sinks of carbon, are host to most of the world’s biodiversity and may be more sensitive to increasing temperatures than other ecoregions.

• More recently, techniques to determine the ability for leaves to withstand high temperatures have advanced to focus on Tcrit, or the temperature at which irreversible damage to the photosynthetic machinery occurs.

== Over the past few years, Tcrit data have become increasingly available for tropical forests, specifically measured as the temperature at which the ratio of variable fluorescence yield to maximum fluorescence yield (Fv/Fm), reflecting photosystem II functioning, starts to decline. The decline in Fv/Fm is often followed by the development of necrosis and leaf death.

== Heat tolerance, measured by Tcrit, varies minimally among tropical species, mainly due to differences in growing environment and leaf traits. For instance, among 147 tropical tree species, the average Tcrit was found to be 46.7 °C (5–95th percentile: 43.5–49.7 °C).

==  Across the planet, heat tolerance generally increases with higher mean growing temperatures. For example, as average temperatures increase by approximately 20 °C from the Arctic to the Tropics, heat tolerance was 9 °C greater in tropical plants than in Arctic plants12. Similarly, as temperatures decrease by 17 °C along a tropical elevation gradient, heat tolerance decreases by approximately 2 °C.

== Heat tolerance also increases with increasing leaf mass area, suggesting that heat tolerance may be linked to construction costs of the leaves and their mean leaf lifetime.

•  Research contents:

== With a much-improved understanding of Tcrit across the Tropics, it is now important to know how close tropical leaves are to experiencing and surpassing these critical temperatures.

     In the past, tropical forest leaf and canopy temperatures were difficult and time consuming to measure, but new technologies such as drones and thermal cameras are making the process much easier.

1. used data from the new ECOSTRESS sensor to estimate peak pantropical forest canopy temperatures.
2. We began by ground truthing the satellite data with tower-based pyrgeometer data. We then used these data to determine what causes variation in peak temperatures at the canopy scale and show similar trends driving peak temperatures across all of the Tropics.

• Critically, we show that for a given canopy temperature, individual leaf temperatures display a ‘long tail’ of values in the distribution, in which the temperatures of a few individual leaves far exceed that of the overall canopy, and that this skewed distribution persists under leaf warming experiments of 2, 3 and 4 °C.

1. Finally, we developed a simple empirical model to explore the implications of observed leaf temperatures on the fate of tropical forests under future climate change.

## Results:

• Ground validation

Fig. 1: In situ and warming experiment leaf temperatures compared with canopy temperatures.

When we zoomed in on the long tail of each dataset (insets in Extended Data Figs. 4 and 5), the curve shows statistical regularity, which allowed us to estimate Tcrit as a percent of all canopy top leaves. For instance, when all data are aggregated across sites, we estimated that 0.01% (0.03% for more than 43.5 °C) of all leaves will surpass Tcrit at least once a season (Fig. 1c).

• Remote sensing data

We then estimated the highest temperatures during dry periods if temperature increased by 2 °C (to simulate climate change) and found that the percent of time above threshold temperatures would increase by an order of magnitude in all three regions. For example, the percent time that Amazon canopies spent at temperatures 38.0 °C or more would increase from 0.5 to 5% and the percent time of 40.0 °C or more would increase from 0.1 to 1% (Table 1).

Fig. 2: Remotely sensed peak canopy temperature across the tropics.

• Model results