Increasing humidity threatens tropical rainforests

Li et al. (2018) discuss how global climate change is leading to faster increases of human-perceived temperatures (apparent temperature; T AP ) than air temperatures (T air ), and that the predicted increases in T AP relative to T air is fastest in the tropics. While Li et al. (2018) highlight how increasing T AP 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 T AP will affect non-humans. In particular, global warming may lead to especially rapid increases in leaf temperatures (leaf temperature; T l ) 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, T l s 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 T l ( 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 T l 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 T AP 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 T l of many tropical plant species’ upwards and beyond the thermal limits of photosynthesis. If T l ‘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.


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


Brienen, R. J., Phillips, O. L., Feldpausch, T. R., Gloor, E., Baker, T. R., Lloyd, J., et al. (2015). Long-term decline of the Amazon carbon sink. Nature 519, 344–348. doi: 10. 1038/nature14283

Cheesman, A. W., and Winter, K. (2013). Growth response and acclimation of CO 2 exchange characteristics to elevated temperatures in tropical tree seedlings. J. Exp. Bot. 64, 3817–3828. doi: 10. 1093/jxb/ert211

Clark, D. A., Piper, S. C., Keeling, C. D., and Clark, D. B. (2003). Tropical rain forest tree growth and atmospheric carbon dynamics linked to interannual temperature variation during 1984-2000. Proc. Natl. Acad. Sci. U. S. A. 100, 5852–5857. doi: 10. 1073/pnas. 0935903100

Damour, G., Simonneau, T., Cochard, H., and Urban, L. (2010). An overview of models of stomatal conductance at the leaf level. Plant Cell Environ. 33, 1419–1438. doi: 10. 1111/j. 1365-3040. 2010. 02181. x

Doughty, C. E., and Goulden, M. L. (2009). Are tropical forests near a high temperature threshold? J. Geophys. Res. Biogeosci. 114, 1–12. doi: 10. 1029/2007JG000632

Feeley, K. J., Joseph Wright, S., Nur Supardi, M. N., Kassim, A. R., and Davies, S. J. (2007). Decelerating growth in tropical forest trees. Ecol. Lett. 10, 461–469. doi: 10. 1111/j. 1461-0248. 2007. 01033. x

Krause, G. H., Winter, K., Krause, B., Jahns, P., García, M., Aranda, J., et al. (2010). High-temperature tolerance of a tropical tree, Ficus insipida: methodological reassessment and climate change considerations. Funct. Plant Biol. 37, 890. doi: 10. 1071/FP10034

Leigh, A., Sevanto, S., Ball, M. C., Close, J. D., Ellsworth, D. S., Knight, C. A., et al. (2012). Do thick leaves avoid thermal damage in critically low wind speeds? New Phytol. 194, 477–487. doi: 10. 1111/j. 1469-8137. 2012. 04058. x

Li, J., Chen, Y. D., Gan, T. Y., and La, N.-C. (2018). Elevated increases in human-perceived temperature under climate warming. Nat. Clim. Change 8, 43–47. doi: 10. 1038/s41558-017-0036-2

Lin, H., Chen, Y., Zhang, H., Fu, P., and Fan, Z. (2017). Stronger cooling effects of transpiration and leaf physical traits of plants from a hot dry habitat than from a hot wet habitat. Funct. Ecol. 31, 2202–2211. doi: 10. 1111/1365-2435. 12923

Oren, R., Sperry, J. S., Katul, G. G., Pataki, D. E., Ewers, B. E., Phillips, N., et al. (1999). Survey and synthesis of intra- and interspecific variation in stomatal sensitivity to vapour pressure deficit. Plant Cell Environ. 22, 1515–1526. doi: 10. 1046/j. 1365-3040. 1999. 00513. x

Slot, M., and Winter, K. (2017a). High tolerance of tropical sapling growth and gas exchange to moderate warming. Funct. Ecol. 37, 1–13. doi: 10. 1111/1365-2435. 13001

Slot, M., and Winter, K. (2017b). In situ temperature response of photosynthesis of 42 tree and liana species in the canopy of two Panamanian lowland tropical forests with contrasting rainfall regimes. New Phytol. 214, 1103–1117. doi: 10. 1111/nph. 14469

Tibbitts, T. W. (1979). Humidity and plants. Bioscience 29, 358–363. doi: 10. 2307/1307692

Wright, I. J., Dong, N., Maire, V., Prentice, I. C., Westoby, M., Díaz, S., et al. (2017). Global climatic drivers of leaf size. Science 12, 917–921. doi: 10. 1126/science. aal4760