How will tropical forests respond to rising temperatures? Tropical trees are adapted to very narrow temperature ranges, which might make them vulnerable to even small amounts of warming. But how can we know for sure? It’s not as easy as just heating up a whole tropical forest and taking a look… Or is it?
Conveniently, while doing our PhDs, we had access to what is quite possibly the hottest tropical forest in the world—the Biosphere 2 tropical forest biome, a large-scale forest mesocosm located inside Biosphere 2 (B2, in Oracle, Arizona). Built 30 years ago to learn how to live on another planet, B2 is now being used to understand how this planet works, and how it is responding to climate change. Effectively a tropical forest in a greenhouse, air temperatures in the B2 forest get up to 40°C—that’s 6°C higher than the maximum temperatures at Amazon forest sites. This system offers a unique opportunity to test the sensitivity of tropical forest photosynthesis to temperatures not projected to occur in these regions until 2100.
Left: The tropical forest biome is a large-scale forest mesocosm located inside Biosphere 2 (B2), an Earth Science facility in Oracle, Arizona run by the University of Arizona. Right: instruments on a tower inside the B2 tropical forest measure the exchange of carbon dioxide between the forest and the atmosphere, used to calculate forest photosynthesis. Photos by Marielle Smith.
Left: The study's second author, Tyeen Taylor, collects leaf samples inside the B2 tropical forest for a different study of how tropical trees cope with high temperatures. Right: The metal “space frame” that holds up the glass allows access via ropes to any point in the biome, such as to maintain sensors on the observation towers as shown here, or for in situ measurements of tree leaves. Working inside the Biosphere 2 rainforest, encased in a huge greenhouse-like structure, is a truly unique experience. In the words of Neill Prohaska, a PhD student in our group who spent many mornings drenched in the humid tropical air in the middle of a desert, “It’s like being outside, inside”! The slippery mud underfoot and humid aromas emanating from a green wall of vegetation make even a seasoned tropical forest ecologist feel at home. Photos by Tyeen Taylor.
We compared the response of photosynthesis in the B2 forest to natural tropical forest sites in Mexico and the Brazilian Amazon. For our study, the key difference from natural forests was that in B2, the typical (strongly positive) relationship between temperature and air dryness could be altered. Rain systems, misters, soil evaporation, and leaf transpiration in B2 all accumulate moisture in the air to near saturating levels, even when very high temperatures caused by the literal “greenhouse effect” dramatically increase the vapour holding capacity of the air. We found that while photosynthesis at the natural forest sites declined at about 28°C, in the B2 forest—so long as high humidity was maintained—photosynthesis continued up to 38°C. Conversely, when different operating conditions reduced humidity in the forest enclosure, we found that the response of photosynthesis to air dryness (technically, “vapour pressure deficit”, VPD) in B2 closely matched the behaviour of natural forests. From these experimental observations, in addition to an analytical decoupling of temperature and VPD effects on photosynthesis, we show that VPD, not temperature directly, is the main driver of high-temperature declines in tropical forest photosynthesis. Identifying the dominant mechanism is important, because while finding direct susceptibility to temperature would have indicated that tropical forests will be highly vulnerable to future climate, sensitivity to VPD indicates that they may have some resilience.
Left: Marielle Smith on top of a canopy walkway at the K67 site in the Tapajós National Forest, one of the natural forest sites used in the study. Right: the 64 m tall eddy flux tower at the K67 site measures fluxes of carbon dioxide between the forest and the atmosphere, used to calculate photosynthesis. Photos by Marielle Smith.
The potential resilience mechanism is via an increase in water-use efficiency, hypothesised to occur under future elevated atmospheric CO2 levels. Basically, plants respond to dry air by closing the gas exchange pores (stomata) in their leaves to reduce water loss, which also reduces their intake of CO2 (and thereby photosynthesis). While future air may be hotter and therefore drier, having more CO2 in the air may effectively help plants to conserve water while sustaining photosynthesis, since stomata can open for a shorter period of time (losing less water) and receive the same amount of CO2. We did not test this mechanism in our study, but the balance of effects of higher CO2 and drier air on photosynthesis is currently a hot topic in plant research.
Our findings show that tropical forest photosynthesis can continue up to much higher temperatures than those currently experienced by natural tropical rainforests, indicating that tropical forests may be more resilient to future warming than previously thought. However, research is urgently needed to understand the combined effects of high temperatures and elevated CO2 on tropical forests. And critically, despite this potential resilience to warming, tropical forests face a myriad of threats from droughts, fires, deforestation, and degradation. Tropical forests are not “out of the woods” yet; we still must do our utmost to protect them.
The results of this study are published in: Smith MN, Taylor TC, van Haren J, Rosolem R, Restrepo-Coupe N, Adams J, Wu J, de Oliveira RC, da Silva R, de Araujo AC, de Camargo PB, Huxman TE, and Saleska SR. 2020. Empirical evidence for resilience of tropical forest photosynthesis in a warmer world. Nature Plants. 6(10): 1225-1230.
Credit for banner photo (at the top): John Adams, Biosphere 2