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Opinion ARTICLE

Front. Ecol. Evol., 05 June 2018 | https://doi.org/10.3389/fevo.2018.00068

Increasing Humidity Threatens Tropical Rainforests

  • 1Department of Biology, University of Miami, Coral Gables, FL, United States
  • 2Fairchild Tropical Botanic Garden, Coral Gables, FL, United States

Li et al. (2018) discuss how global climate change is leading to faster increases of human-perceived temperatures (apparent temperature; TAP) than air temperatures (Tair), and that the predicted increases in TAP relative to Tair is fastest in the tropics. While Li et al. (2018) highlight how increasing TAP will lead to greater thermal discomfort and contribute to the substantially higher temperature-related mortality of humans, it is also important to consider how rapid increases in TAP will affect non-humans. In particular, global warming may lead to especially rapid increases in leaf temperatures (leaf temperature; Tl) potentially leading to decreases in growth rates and higher mortality in plants.

Leaf temperatures are a function of both the environment and the physiological characteristics of leaves that influence plant thermoregulation. Although several physiological traits act synergistically to influence leaf temperature, transpiration is a primary component of leaf thermoregulation that regulates heat dissipation (Lin et al., 2017). When transpiration rates are low, Tls can rise several degrees above ambient air temperature and beyond the optimal temperatures for photosynthesis (Doughty and Goulden, 2009; Leigh et al., 2012; Slot and Winter, 2017b). Extreme leaf temperatures can cause photosynthesis to cease, and in some cases, lead to permanent damage of photosynthetic machinery (Krause et al., 2010). Ultimately, the impairment of photosynthesis can diminish the capacity for carbon fixation, lead to decreased plant growth rates, and even death.

The ability of plants to transpire, and hence dissipate heat, is highly dependent on available soil moisture and atmospheric vapor pressure deficit (which is itself dependent on relative air humidity) (Damour et al., 2010). Under dry soil conditions, plants may close their stomata in order to limit water loss, and the resulting reduction in transpiration can lead to marked increases in leaf temperatures (Oren et al., 1999). Conversely, plants can maintain open stomata when soil moisture is not limited (Tibbitts, 1979). However, high relative humidity decreases the leaf-atmosphere vapor pressure gradients that reduce the efficacy of transpiration—again leading to higher Tl (Tibbitts, 1979). Soil and atmospheric moisture will both be affected by future global climate change, either directly through changes in precipitation patterns or indirectly though changes in temperatures. In other words, deleteriously high Tl may become more frequent in some species because of drought conditions or high humidity, causing carbon starvation or photosynthetic thermal damage, respectively.

If the greatest increases in TAP and humidity occur within tropical latitudes as Li et al. (2018) suggest, then tropical forests and their constituent plants are likely to face increasing thermoregulatory challenges. Tropical plant species tend to have large leaves with small boundary layer conductances and they rely heavily on transpiration to avoid lethal high leaf temperatures (Wright et al., 2017). By reducing the efficacy of transpiration, future increases in relative humidity may edge the Tl of many tropical plant species' upwards and beyond the thermal limits of photosynthesis. If Tl's are increasing as Li et al.'s (2018) findings suggest, then photosynthetic thermal stress may contribute to the decelerations in tree growth observed in some parts of the tropics. Several studies have documented decelerating tree growth in tropical forests. These decreases in growth are typically hypothesized to be caused by increased respiration due to elevated nighttime temperatures or the increased frequency and severity of droughts (Clark et al., 2003; Feeley et al., 2007; Brienen et al., 2015). Another potential explanation is that if tropical plants were already operating close to their thermal limits of photosynthesis (Doughty and Goulden, 2009; Krause et al., 2010), global warming may be causing thermal damage to photosynthetic machinery. This physiological mechanism for the deceleration tropical tree growth deserves further exploration.

Increased respiration as the hypothesized physiological mechanism for decelerating tree growth was developed from observational studies, but has received limited experimental support (e.g., Cheesman and Winter, 2013; Slot and Winter, 2017a). To reconcile the results from observational and experimental studies, additional mechanisms such as photosynthetic thermal stress should be evaluated for their effects on tree growth and fitness. Given the importance of the many ecosystem services provided by tropical ecosystems and plants (e.g., the provisioning of food, timber, and non-timber products, as well as carbon capture and sequestration) the dangers imposed by rapidly rising temperatures, and even more rapidly rising plant apparent temperatures, are cause for serious concern.

Author Contributions

TP and KF conceived of the study and wrote the manuscript.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The reviewer SBZ and handling Editor declared their shared affiliation.

Acknowledgments

The authors are supported by the US National Science Foundation 128 (DEB-1350125 to KF).

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Keywords: leaf temperature, humidity, plant apparent temperature, climate change, transpiration, stomatal conductance, tropical forests

Citation: Perez TM and Feeley KJ (2018) Increasing Humidity Threatens Tropical Rainforests. Front. Ecol. Evol. 6:68. doi: 10.3389/fevo.2018.00068

Received: 26 January 2018; Accepted: 02 May 2018;
Published: 05 June 2018.

Edited by:

Hang Sun, Kunming Institute of Botany (CAS), China

Reviewed by:

Shi-Bao Zhang, Kunming Institute of Botany (CAS), China
Zehao Shen, Peking University, China

Copyright © 2018 Perez and Feeley. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Timothy M. Perez, t.more.perez@gmail.com