Measuring Heat Stress on Forest Trees

• The impact of high temperatures directly and alone on trees is significant without the associated drought.
• Heat effects are recorded at cellular, leaf, and tree levels, inhibiting tree growth and productivity or even mortality.
• More research is necessary to fully understand the effects of heat stress, which must be supported by on-site, non-destructive data collection and analysis in real time.

Scientists are increasingly investigating the impact of climate change-driven extreme heat on plants, focusing on tree responses. It is now apparent that trees suffer from the direct effects of extreme heat. Understanding trees’ physiological, biochemical, and molecular responses is crucial for assessing how climate change will affect tree species. The knowledge gained will inform strategies for mitigating the impact of extreme heat on tree life.

The Effects of Heat Stress on Trees

Stress results from environmental pressures from living (biotic) and non-living (abiotic) factors. Trees respond to stress by making metabolic and physiological adjustments to prevent or repair damage caused by the stressor. These responses are specific to the type of stress and can include changes in gene expression, biochemical pathways, behavior, and physical traits. Genetics, prior exposure to stress, and overall health influence the tree’s ability to cope with stress.

Heat stress occurs when the temperature surpasses a critical threshold for a sustained period, resulting in irreversible damage to plant growth and development. 

The frequency and intensity of heat stress events/extreme heat have risen globally over the past two decades. Climatic models predict a continued increase throughout the 21st century. Heat waves, extreme heat over 35°C for more than two continuous days, are increasing in frequency and duration.

The heat stress effects usually discussed are those from prolonged heat stress and heat waves, which alter tree functioning at cellular, physiological, leaf, and tree levels and take time to appear.

Some scientists also report acute effects observed within days called the heat domes.

Heat Waves

Abnormal and prolonged heat stress due to heat waves and extreme heat affects various aspects of tree biology, ranging from cellular to whole-tree scales.

Cellular Level: This includes impairment of photosystem II photochemistry, electron transport, thylakoid and cell membrane fluidity, and the activity of the enzyme ribulose bisphosphate carboxylase–oxygenase (RuBisCO). Heat stress increases dark respiration rates and the synthesis of reactive oxygen species (ROS), such as glutathione peroxidases, Cys-rich receptor-like kinases, and serine/threonine protein kinases. Elevated ROS levels can harm plants, causing photooxidation of cellular membranes.

Leaf Level Effects: At the leaf level, photosynthesis is reduced, leading to decreased productivity. Stomatal conductance increases due to heat stress to cool leaves, but growth and reproduction are all negatively affected by heat stress. Heat can result in premature leaf abscission, yellowing of leaves, and necrosis. The development of leaves decreases, affecting the tree’s ability to capture sunlight for energy.

Whole Tree Level Effects:  The impact extends to the entire tree, decreasing overall growth and productivity. Biomass allocation between roots, shoots, leaves, and branches is altered, potentially affecting the tree’s structure and function. Heat stress affects plants ‘ different vegetative and reproductive stages to varying extents. Ovules, for example, are mentioned to be less heat-sensitive than pollen. The severity of these effects can lead to tree death, depending on the duration, frequency, and severity of the heat stress episodes.

The detrimental effects are significantly enhanced when heat stress is combined with drought. Without irrigation, these combined stressors can rapidly lead to tree death. However, there is a distinction between droughts and heat waves since heat waves can occur independently of drought conditions.

Effects of Heat Domes

During a heat dome, the primary cause of damage is direct exposure to heat and solar radiation, rather than indirect damage from drought caused by extreme heat. The stress induced by hot droughts can lead to hydraulic failure, causing the collapse of water-carrying tissues inside trees. However, heat dome effects are seen even in wet conditions.

In a recent publication, Still et al., 2023, say that the 2021 heat dome that killed hundreds of people in the Pacific Northwest also resulted in widespread foliage damage within days. Green leaves and needles turned orange, brown, and red.  Research suggests the leaves didn’t dry; they were scorched.

The “foliage scorch” happened primarily on the southern and western sides of trees and forests, corresponding to the sun’s track across the summer sky, while the other unexposed sides of the same trees were unharmed. The scientists observed the scorching pattern resembled a sunburn across the entire forest.

Extreme heat, as experienced during the Heat Dome, should be considered a crucial factor in explaining the observed damage to trees, as the damage occurred in days and differed from heatwave effects. Leaf temperature can be 5–15 °C warmer than air temperature in these conditions. Even short exposure to this extreme heat can be lethal to leaf tissues and is not connected to soil drought or hydraulic failure.

Figure 1: “From right to left of the figure is a summary of known effects of high temperatures on trees at the cellular, leaf, and whole plant scales. An increase, decrease, or no change in a process in response to high temperature, is indicated by +, −, and 0, respectively. More than one symbol associated with a process indicates between- or within-species variation. Y notes reported acclimation in response to exposure to high temperature; N indicates no acclimation has been reported; and “?” indicates acclimation may exist, but the evidence is limited. Known genotypic variation in the response of a process that may be useful for genetic selection for increased heat tolerance is indicated by (x).” Teskey et al. 2014. (Image credits: https://onlinelibrary.wiley.com/doi/full/10.1111/pce.12417)

However, some trees and species tolerate heat stress better than others. These trees minimize stress effects on chloroplasts and photosynthesis and reduce dark respiration. Though there have been only a few studies on the topics, there is clear genetic variation in heat tolerance within species. Scientists are studying these mechanisms to understand heat tolerance for application in forestry.

In all these emerging venues of research in heat stress in trees, measuring and monitoring stress and responses will be crucial.

How to Monitor Heat Stress

Heat stress can be measured by monitoring its various impacts on the plant functions and the damage caused. While large-scale remote techniques exist, this section focuses on field methods. These methods commonly measure reduction in photosynthesis, chlorophyll fluorescence measurements, loss in pigmentation, foliage loss, and increase in stomatal conductance.

Foliage loss

Loss of leaves is measured in many ways. It can be through crown defoliation/canopy cover and Leaf Area Index (LAI) at the tree level.

The CI-110 Plant Canopy Imager, by CID Bio-Science Inc., is a suitable field instrument for non-destructive rapid canopy cover and LAI estimation. The instrument uses 150° fisheye to capture hemispherical images of the canopy to calculate the LAI. The Gap Fraction method gives the amount of canopy cover.

Leaf Area

A simple parameter, the leaf size, which reflects growth effects, can be estimated non-destructively by leaf meters. For example, the CI-202 Portable Laser Leaf Area Meter and CI-203 Handheld Laser Leaf Area Meter can scan leaves in real time to estimate leaf length, width, perimeter, and area.

Spectroscopy

Heat stress results in loss of greenness by affecting chloroplast by altering thylakoid membrane fluidity. Leaf spectroscopy, which depends to a great extent on pigments, can measure changes in greenness to estimate heat stress.

The CI-710s SpectraVue Leaf Spectrometer is a non-destructive leaf spectrometer that can be used to estimate light’s absorbance, reflectance, and transmittance in the 360-1100 nm range, visible and near-infrared light bands. It can record stress by collecting and analyzing data in real time on the field through many preloaded plant indices.

Leaf gas exchange

Changes in stomatal conductance and transpiration can be measured through leaf gas exchange systems.

The CI-340 Handheld Photosynthesis System can measure these gas exchanges on-site for immediate results. Customized leaf chambers of various sizes make it suitable for many species, including needle leaves.

Chlorophyll Fluorescence

Chlorophyll fluorescence can also measure changes in chloroplast thylakoid membrane fluidity and photosynthetic efficiency. It is also a good measure of all types of plant stresses. This parameter helps estimate real-time heat tolerance variations between and within species in various sites.

The heat tolerance threshold is the critical temperature at which the minimum value of chlorophyll fluorescence (F0) increases rapidly. Or where response relative to the maximum quantum efficiency of PSII (Fv/Fm) starts to decline rapidly.

In trees, damage to PSII by temperatures below 40 °C is reversible. The temperature above 40 °C, which causes harm, is specific to tree species and exposure duration.

The CI-340 Handheld Photosynthesis System has a module to measure chlorophyll fluorescence simultaneously with photosynthesis.

Other methods include estimating leaf temperature by using meteorological and leaf parameter readings.

Developing Strategies

The current findings underscore the vulnerability of trees and ecosystems to climate change-induced stressors, with potential cascading effects on biodiversity and ecosystems. As more effects of climate change on trees and plants are uncovered, more research will be necessary to develop strategies.  What we know so far indicates that heat effects depend on species and location, so studies will be needed in all ecosystems to mitigate and adapt to these changes to sustain the health of ecosystems and the services they provide. This may involve strategies for forestry, conservation, and reforestation.

Sources

Buchner, O., Karadar, M., Bauer, I. et al. (2013). A novel system for in situ determination of heat tolerance of plants: first results on alpine dwarf shrubs. Plant Methods 9, 7. https://doi.org/10.1186/1746-4811-9-7

Extreme heat. MIT Climate Portal. (n.d.). https://climate.mit.edu/explainers/extreme-heat

Húdoková, H., Petrik, P., Petek-Petrik, A. et al. (2022). Heat-stress response of photosystem II in five ecologically important tree species of European temperate forests. Biologia 77, 671–680. https://doi.org/10.1007/s11756-021-00958-9

Percival, G. C. (2023). Heat tolerance of urban trees − a review. Urban Forestry & Urban Greening, 86, 128021. https://doi.org/10.1016/j.ufug.2023.128021

Ruehr, N. K., Gast, A., Weber, C., Daub, B., & Arneth, A. (2015). Water availability as dominant control of heat stress responses in two contrasting tree species. Tree Physiology. https://doi.org/10.1093/treephys/tpv102

Still, C. J., Sibley, A., DePinte, D., Busby, P. E., Harrington, C. A., Schulze, M., Shaw, D. R., Woodruff, D., Rupp, D. E., Daly, C., Hammond, W. M., & Page, G. F. (2023). Causes of widespread foliar damage from the June 2021 Pacific Northwest Heat Dome: More heat than drought. Tree Physiology, 43(2), 203–209. https://doi.org/10.1093/treephys/tpac143

Tarvainen, L., Wittemann, M., Mujawamariya, M., Manishimwe, A., Zibera, E., Ntirugulirwa, B., Ract, C., Manzi, O. J., Andersson, M. X., Spetea, C., Nsabimana, D., Wallin, G., & Uddling, J. (2021). Handling the heat – photosynthetic thermal stress in tropical trees. New Phytologist, 233(1), 236–250. https://doi.org/10.1111/nph.17809

Teskey, R., Wertin, T., Bauweraerts, I., Ameye, M., McGuire, M. A., & Steppe, K. (2014). Responses of tree species to heat waves and extreme heat events. Plant, Cell & Environment, 38(9), 1699–1712. https://doi.org/10.1111/pce.12417

Zubler, A. V., & Yoon, J. Y. (2020). Proximal Methods for Plant Stress Detection Using Optical Sensors and Machine Learning. Biosensors, 10(12), 193. https://doi.org/10.3390/bios10120193