Recent media headlines have highlighted the importance of heat waves on human health. Sadly, climatic modeling predicts that this trend is likely to continue throughout the 21st century.
While the importance of planting trees with tolerance to de-icing salts, drought, soil compaction and waterlogging within urban landscapes is recognized, surprisingly limited information exists regarding the tolerance of trees to heat. This article aims to summarize what we currently know regarding the impact of heat waves on trees growing in our towns and cities.
Introduction
Extreme temperatures associated with prolonged heat waves, i.e., lasting for several weeks, now impact approximately 10% of land surfaces. Increases in the frequency and intensity of heat waves worldwide are expected to continue through the 21st century. For example, Della-Marta et al. (2007) reported that the length of summer heat waves in Western Europe has doubled and the frequency of hot days has tripled since 1880. During 2021, all-time maximum temperatures were recorded in seven U.S. states (California, Arizona, New Mexico, Utah, Colorado, Wyoming and Montana), while in Phoenix, Arizona, temperatures ranged between 115 and 120 degrees F (46 and 49 degrees C).
Similar unprecedented heat waves also were recorded in Canada, where temperatures exceeded 116 F (47 C). Within the United Kingdom, the Meteorological (Met) Office issued its first-ever extreme-heat warning for parts of the U.K.
In summary, all climatic predictions state that global warming will be represented by more frequent and intense high-temperature events. Consequently, as extreme-heat events increase in frequency, it is important to select and plant trees that will be able to cope. Such a selection process is even more important within urban landscapes due to the urban-heat-island effect (UHI effect), when the air temperature can be 3 to 4 degrees F (2 to 3 degrees C) higher than that of the surrounding rural area. (Photo 1)
Heat stress and influences on trees
Heat stress is defined as the rise in temperature beyond a threshold level for a period of time sufficient to cause damage to plant growth and development that cannot be reversed, even when conditions become favorable to plant growth. Heat stress impacts plant function from the cellular to whole-plant level. Photosynthesis and growth are reduced, which can lead to premature leaf drop, visible leaf damage and eventually tree death. At temperatures above 113 F (45 C), severe injury and death can occur within minutes. (Photos 2, 3 and 4)
In Europe, forest mortality due to drought and heat stress has occurred across many Mediterranean regions throughout the 1990s and 2000s. Symptoms across forest stands include foliage yellowing and browning, branch dieback, premature leaf loss and tree death. oak, fir, spruce, beech and pine have been severely affected, raising concern about the consequences for forest health. Similar scales of tree mortality have been recorded in the United States, Asia, Australasia and South and Central America following heat- and drought-stress episodes. These raise concern about how prolonged heat waves have the potential to alter forest structure, composition and subsequent ecosystem services such as carbon sequestration.
Temperature tolerance
The temperature optimum for most deciduous and coniferous trees ranges between 68 and 86 F (20 and 30 C). However, some species can tolerate temperatures up to and between 95 and 104 F (35 and 40 C) without any major effects on their biology. A number of tropical tree species have adapted to thrive in high-temperature climates, i.e., Dipteryx oleifera (eboe, choibá or almendro) and Zygia longifolia (barbasquillo) can survive and grow at temperatures greater than or equal to 113 F (45 C).
This raises the question, “What temperature should professionals involved in urban tree care use as a guide above which tree dieback and death will be expected to occur?” Research using forest trees has shown that temperatures below 104 F (40 C) are generally reversible for damage, irrespective of tree species. However, prolonged temperatures of 104 F (40 C) or higher tend to cause irreversible damage (Yordanov 1992).
Experiments using seedlings of seven common U.K. tree species (Acer pseudoplatanus [sycamore], Betula pendula [silver or white birch], Fagus sylvatica [European beech], Fraxinus excelsior [ash], Juglans regia [walnut], Quercus petraea [oak] and Quercus robur [English oak]) grown under nursery conditions revealed that the critical temperature at which heat damage occurred was around 116 F (47 C), with little difference among species (Dreyer et al. 2001).
However, in further work evaluating the heat tolerance of temperate and boreal trees (Quercus muehlenbergii [chinquapin or chinkapin oak], Q. macrocarpa [burr oak], Q. suber [cork oak], Q. canariensis [Algerian oak], Q. robur [English oak], Q. petraea [oak], Picea glauca [white spruce], Populus deltoids [cottonwood], Ficus insipida [Ojé Rosado], Acer pseudoplatanus [sycamore], Betula verrucosa [European white birch], Fagus sylvatica [European beech], Fraxinus excelsior [ash] and Juglans regia [walnut]), a value of 107 F (42 C) or higher was proposed as a critical temperature from which trees suffer irreversible damage, rather than 116 F (47 C).
Besides direct damage to trees caused by heat waves, Allen et al. (2010) importantly emphasizes the fact that many trees die indirectly by insect and disease ingress caused by repeated years of drought and heat stress weakening trees.
Within the U.K., for example, changes in drought/heat intensity and frequency have potentially altered the impacts and severity of tree-damaging insect and fungal pathogens. These include Armillaria (honey fungus) and Heterobasidion root disease; cankers such as Septoria musiva, Diplodia sapinea (pine tip blight), Hymenoscyphus fraxineus (ash dieback), Cryphonectria parasitica (chestnut blight) and Massaria (Splanchnonema platani, London plane canker); and bark-boring beetles such as Agrillus (jewel beetles) or Scolytus (Dutch elm disease). (Percival and Banks, 2019).
Tree selection: Heat and/or drought?
One of the great challenges for urban-landscape managers will be to select heat-tolerant trees for future, hotter climatic conditions. As heat and drought are often concurrent stress factors in nature, then, by default, selection for drought resistance also is associated with selection for heat tolerance.
However, such an assumption may be flawed, as both heat and drought exert very different effects on plants. The primary target of heat is reductions in leaf photosynthetic processes caused by direct damage to the chlorophyll molecule. Drought, on the other hand, does not directly reduce photosynthesis, but prevents carbon dioxide (CO2) entry into the chloroplasts as the stomata close to reduce water loss. See Correia et al., 2018, for more information.
Indeed, little if any research exists showing a clear association between heat and drought stress in trees, i.e., that a selection of drought-tolerant trees equates to heat-tolerant trees. With respect to crops, where more research exists, results are conflicting. A clear relationship between drought and heat tolerance has been shown in chickpea and sorghum. However, with respect to rice and peanut, drought tolerance does not necessarily confer heat tolerance. Indeed, many drought-tolerant crop cultivars have been shown to be highly sensitive to heat stress (Wahid et al., 2007).
Preliminary studies
Prior to larger whole-tree studies, research at the Bartlett Tree Research and Diagnostic Laboratory subjected leaves of three species of Acer (maple) to heat stress under laboratory conditions. Half of the leaves were kept hydrated during the heat stress (heat only) and half were dehydrated to simulate both heat and drought. At the end of the experiment, damage to the leaf photosynthetic system was recorded.
Results (Graph 1) clearly show that leaves that were kept hydrated throughout the heat stress had 50-90% less damage to the leaf photosynthetic system, emphasizing the importance of irrigation throughout a heat wave. In addition, results also showed a marked difference in heat tolerance between species, with A. campestre ‘Louisa Red Shine’ and A. platanoides (Norway maple) having a higher heat tolerance than A. ‘Princeton Gold.’ This information is important, as it offers opportunities to identify heat-tolerant trees for future urban plantings.
Tree selection for heat tolerance based on scientific studies
Although the European Union and the United States have developed guides for tree selection for urban landscapes that identify drought-, waterlogging- and shade-tolerant species, no criteria exist for heat. (https://citree.de/about.php?language=en; https://www.arborday.org/trees/index-choosing.cfm)
Likewise, little study with respect to heat adaptation and/or subsequent tolerance of urban trees has occurred. Of that available, Pinus taeda (loblolly pine) and Quercus rubra (red oak) seedlings exposed to repeated moderate or extreme heat waves under greenhouse conditions resulted in significant growth reductions at the extreme but not moderate heat. Cedrus brevifolia (Cyprus cedar) has been shown to be more heat tolerant than C. libani (cedar of Lebanon) and C. atlantica (Atlas cedar) when seedlings of the three species were exposed to temperatures above 113 F (45 C) for five hours (Ladjal et al 2000). Populations of Pesudotsuga menziesii (Douglas fir) also were shown to be more heat tolerant than populations of Pinus ponderosa (Ponderosa pine) by Marias (2017).
Urban landscapers, planners and arborists are faced with thousands of ornamental tree species to select for urban landscape plantings, yet the bulk of scientific research into quantification of heat tolerance has focused on a limited number of genera (Quercus, Pinus, Eucylaptus, Acer, Picea), and within that genera, only a limited number of species that in turn are predominately planted for forestry purposes.
Recent research at the Bartlett Tree Research and Diagnostic Laboratory has identified variation in the heat-stress response of three Acer (maple) species. (Graph 2) In this experiment, leaves of A. campestre, A. ‘Princeton Gold’ and A. spaethii (Spaeth’s alder) were subjected to five hours at 111 F (44 C). Reductions in the efficiency of the leaf photosynthetic system were then recorded. Results clearly show a species response with heat tolerance of A.spaethii being greater than A.campestre, and A.campestre greater than that of A. ‘Princeton Gold’. Further studies are ongoing.
Tree selection based on experience
A number of heat-tolerant urban tree species have been identified and/or recommended by organizations such as the Royal Horticultural Society, U.K. (https://www.rhs.org.uk/advice/profile?PID=712) and U.S. university-based extension services (https://extension.psu.edu/heat-and-drought-tolerant-plants). These selection processes are based on, or restricted to, either post-event heat analysis and/or observation of species that grow well locally in hot “site locations” – i.e., close to large masses of asphalt and concrete, close to and adjacent to buildings, near underground utilities and/or in containers/raised beds that were recorded hotter than in-ground planting areas – rather than actual scientific-based methodology.
For example, Table 1 is amended from Appleton et al., 2015, recommending heat-tolerant tree species for urban landscape plantings.
Tree selection based on provenance
Provenance, or place-of-origin, selection also offers an abundance of largely untapped genetic resources to select for heat-tolerant trees. Robakowski et al. (2012) investigated the temperature responses of A. rubrum (red maple) and Q. rubra (red oak). The provenances of A. rubrum and Q. rubra that originated from southern sites with a higher ambient temperature could photosynthesize at higher temperatures compared to A. rubrum and Q. rubra that originated from northern provenances.
Weston and Bauerle 2007 demonstrated that variation in the response of Acer rubrum to excessive heat exists. Acer rubrum trees collected from Florida (yearly average temperature of 72.7 F, or 22.6 C) maintained close to a two-fold increase in leaf photosynthesis at temperatures of 91.4 to 107.6 F (33 to 42 C) compared to trees from Minnesota (yearly average temperature of 37.4 F (3 C). Research introducing Acer saccharum (sugar maple) of three Canadian provenances with mean annual temperatures of 37.4 F (3 C), in Manitoba; 39.5 F (4.2 C), in Quebec; and 49 F (9.4 C), in Ontario; to an introduced site in subtropical China (60.4 F, or 15.8 C) monitored survival, growth and summer photosynthetic rates under field conditions.
The Ontario provenance had the highest rate of survival and growth, followed by the Quebec provenance, while the Manitoba provenance had the lowest. Likewise, the Ontario provenance had a higher photosynthesis rate than those of Quebec and Manitoba. Overall, the Ontario provenance had the best physiological adjustment for self-protection from heat stress, followed by the Quebec and then the Manitoba provenances (Zhu et al. 2019).
Further work supporting the potential of provenance selection for heat tolerance in Eucalyptus globulus and E. pauciflora is provided by Pita et al. (2005). For future research purposes, identification of species from the warmest portion of their range may be critical in selection of heat-tolerant trees.
How trees protect themselves from heat stress – lessons for tree selection
Although the response of trees to heat stress has received limited study, from the information available, it has been shown that genetic variation exists within trees that could be exploited to select for improved heat tolerance. One important response to heat waves shown by some tree species is to minimize heat absorption and maximize heat loss through adjusting their stomata. Namely, under heat stress, stomata stay open, providing a protective leaf-cooling mechanism via transpiration. It is not known how many tree species have the capacity to utilize transpirational cooling to avoid extreme heat stress, since it has been the subject of only a few studies. According to the available information, Pinus taeda, Quercus rubra and Acer Rubrum have displayed this leaf-cooling tactic.
Another form of damage to plants under prolonged heat stress is death of proteins. To reduce this form of damage, a common plant response is the production of heat-shock proteins (HSPs). Research with crops has shown that HSP synthesis is an important criteria associated with heat tolerance. HSP expression has been shown to be important in tolerance to heat stress in Pinus banksiana (jack pine), P. taeda (loblolly pine), Picea mariana (black spruce), P. glauca (white spruce), Populus nigra (black poplar), Prunus persica (peach) cv. Loring; Malus domestica (apple) cv. golden delicious; and Rubus sp. (thornless blackberry) cv. Chester.
HSP expression has been shown to be important in tolerance to heat stress in hybrid poplar (Populus nigra); weeping willow (Salix babylonica); flowering dogwood (Cornus florida); sassafras (Sassafras albidum) and black locust (Robinia pseudo-acacia).
Developments in molecular DNA technologies offer a means of identifying heat-tolerant tree genotypes based on HSP synthesis. Further research is warranted in this area.
Many tree species can synthesize volatile organic compounds (VOC) in response to high-temperature exposure. In response to heat stress of 96.6 to 102.2 F (36 to 39 C), Populus tremuloides (quaking aspen), a VOC-emitting species, had less damage to the leaf photosynthetic process than Betula papyrifera (paper birch), a non-VOC-emitting species. In addition, a Populus tremuloides clone (No 271) that produced the most VOC was able to tolerate higher temperatures than a lower VOC-emitting P. tremuloides clone (No 42E). Consequently, the choice of an emitting or non-emitting tree species could be important in determining the heat tolerance of trees for urban planting. Steinbrecher et al. (2009) and Oderbolz et al. (2013) provide detailed information regarding the “top” emitters of VOC by trees that may prove helpful for tree-selection purposes.
Conclusion
Selection of heat-tolerant trees will become of greater importance, as the warming effects of climate change will increase the intensity of the UHI within urban landscapes. This, in turn, will increase levels of indoor and outdoor thermal discomfort, potentially inducing adverse health effects such as sunburn, skin cancer and cataracts.
Presently, little information exists regarding the heat tolerance of urban trees. Of that available, it is recognized that trees possess physiological and biochemical mechanisms to allow them to survive extreme heat events. These present opportunities for research that will identify superior heat-tolerant trees, as well as increase our knowledge of how urban trees acclimate and adapt to heat stress.
Understanding the mechanisms of tree responses to extreme-heat-temperature events also will be critically important for understanding how tree species will be affected by climate change.
References:
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Dr. Glynn Percival is the senior arboricultural researcher at the Bartlett Tree Research Laboratory based in Charlotte, North Carolina.
Christopher Percival is a research technician at the U.K. Bartlett Tree Research Laboratory in Reading, England. He graduated from the University of Wolverhampton, U.K., with a BSc degree in biological sciences. He also is Dr. Percival’s son.
