Trees That Can Handle Heat and Drought

The original concept for this article was simply to identify the tree species that can handle the heat and drought in our future so that readers who are planting trees this fall could use the information to guide their purchases.  I soon discovered that this was not a simple task.

First of all, drought tolerance is not easily determined, and it can vary even among the same species based on local adaptations.   Perhaps this explains the conflicting information in lists of drought-tolerant plants.    Second, until recently, there appears to have been relatively little research on the subject of heat or thermal tolerance.  Finally, recent research suggests that trees that have been drought tolerant in the past may not be up to the challenges presented by the higher levels of heat and drought we can expect in the next decades — unless and until we reduce the greenhouse gases we are emitting.  In light of this research, I began exploring this subject and looking with a new perspective at all lists and labels for drought and heat tolerance.

The traditional method of identifying drought tolerance has focused on features like leaf size and crown characteristics.  Once expert summed it up this way: “An ideal tree for a drought-resistant landscape is a native, early to mid-successional, upland hardwood species with a multi-layered canopy, small and/or deeply lobed leaves, and a conical to cylindrical crown shape.”  University of Georgia School of Forestry (2012).   But newer approaches focus on climate adaptations and the hidden hydrology of tree functions.

Some scientists have used the “climate envelope” of a tree species to predict its resilience in a changed climate.  For example, a 2009 study compared the climate envelope of each tree species then growing in Philadelphia to the predicted future climate in Philadelphia. Basically, a tree’s climate envelope is the climate niche it occupies, based on “annual mean temperature, minimum temperature of the coldest month, maximum temperature in the warmest month, annual precipitation, precipitation in the warmest quarter, and precipitation in the coldest quarter.”  Also factored in was the predicted increase in diseases and pests due to climate change. The study concluded that the future climate of Philadelphia would become “less optimal” for 10 species, but would be more optimal for 2 species (American holly and sweetgum).

The climate envelope approach has been subject to criticism: “There is little evidence that the climate envelope of a plant species directly relates to the drought and thermal tolerance of that species, at least not at the resolution required to identify or rank species vulnerability.”  Hanley et al, Sci.Total Environ. 2021  In addition, some scientists have argued that while “the mechanisms of how trees die are becoming clear,  the environmental conditions under which trees can persist under very high temperatures associated with heat waves have not been documented,” Overwhelming Heat Waves: Climate Envelope Development for Pinus edulis Seedlings.

A flurry of research is underway on the mechanisms within a tree that make drought or heat tolerance possible.   One of the first things I learned is that trees do not sit by quietly through periods of drought; most alter their functioning in order to conserve water.   As I explored this phenomena, I began to slog through sentences like this one:

Leaf water potential at wilting or turgor loss point (πtip) is a determinant of the tolerance of leaves to drought stress and contributes to plant-level physiological drought tolerance. Recently, it has been demonstrated that leaf osmotic water potential at full hydration (πo) is tightly correlated with πtip.

British Ecological Society Journals/wiley.com

Having no idea what that meant, I decided I better learn a few basics about tree hydrology. Thankfully, I came upon an interview with a leading researcher in the field, Dr. Craig Brodersen, and soon I had better understanding of tree functioning.  You’ve probably heard the term xylem, which are the “hollow, pipe-like vessels that transport water and nutrients” from the roots up to the shoots and leaves. The xylem sap is under negative pressure, which means that basically “the evaporation of water out of the leaves pulls the water up the trunk.” One tactic that many plants employ during drought is to close their stomata, the tiny openings on the surface of leaves, thereby preventing the loss of water into the atmosphere.  Dr. Brodersen explains the process as follows:

We’re finding that as a tree enters a drought, the first thing to happen is that it closes its stomata in order to conserve water. But that puts the tree in a dilemma; if it closes its stomata to conserve water that means it can’t pull CO2 out of the atmosphere in order to do photosynthesis. It then must rely on its internal storage to get by during times of drought. Ultimately, the tree’s ability to survive drought is a function of how much carbon it has currently stored and available to keep itself alive.

In terms of a tree’s ability to recover, we are now finding that if a tree loses a certain percentage of its total conductivity — how much water it can transport — there’s a tipping point after which it can’t come back. It needs to be able to have enough water stored in its trunk, and then have enough of its vascular system functioning in order to grow more xylem the next year, in order to replace any of the wood that was lost to the hydraulic disfunction. When trees get pushed beyond that tipping point of not having enough water and not having enough stored carbohydrates, they become very susceptible to pests and disease because their defense systems are significantly reduced. And if the tree gets pushed too far there’s the possibility that they go beyond that tipping point, if the insects don’t get it first.

–Dr. Craig Brodersen, Yale School of the Environment (Interview, Yale.edu)

Drought tends to exacerbate the effect of heat stress.  Plants typically handle heat through the cooling of leaves and tissues via transpiration.  But since most trees’ response to drought stress is to close stomata to reduce water loss, the heat stress can be much more severe because transpirational cooling is reduced.  Teskey et al, Plant, Cell & Environment. In addition, the whole tree hydraulic system can be damaged when heat is accompanied by drought:

“During exceptionally warm conditions, if a particular tree’s soil becomes really dry, bubbles form in these tubes. When that happens to a particular xylem tube, it is unusable forever. If most or all of a tree’s xylem gets emptied out — or cavitated — the tree dies.” “How Plants Adapt to Climate Change,”news.harvard.edu.

Sadly, drought and heat have contributed to an increase in tree deaths worldwide.  I think of myself as a well-informed person, but this was news to me.  In our own country, the West has suffered the most, with California losing almost 150 million trees during the drought that began in 2011,  Smithsonian Magazine (July 2019), and in Colorado, tree mortality has increased dramatically.  University of Colorado Boulder News (July 2021).

Although the research was depressing and highlighted the number of questions that remain to be answered, I was unwilling to let it stand in the way of a tree list that might be both reasonably accurate and helpful.  Speaking of accurate, it was clear that the experts differ on the drought tolerance of a number of trees.  I started by compiling a list of trees that have been identified as drought tolerant by several authorities and then concentrated on trees that fit our local conditions and hardiness zone. The list is not exhaustive by any means, and it may very well be subject to change, but there seems to be a consensus to support it.

Drought Tolerant Trees (* denotes heat tolerant also)

*American Holly (Ilex opaca)

Black Gum (Nyssa sylvatica) (also known as black tupelo) (can adapt to both very wet and to relatively dry conditions)

Black Locust (Robinia pseudoacacia)

Catalpa (Catalpa speciosa and Catalpa bignonioides)

*Crape Myrtle (Lagostroemia indica)

Eastern Red Cedar (Juniperus virginiana)

∗Ginkgo (Ginkgo biloba)

*Golden Raintree (Koelreuteria paniculata)

∗Hackberry (Celtis occidentalis) and Dwarf Hackberry (Celtis tenuifolia)

*Honeylocust (Gleditsia triacanthos or var. inermis (thornless))

Hop Hornbeam (Ostrya virginiana) (somewhat drought tolerant but not flood-tolerant)

Japanese Pagoda tree (Styphnolobium japonicum, Sophora japonica) (not heat tolerant)

∗Kentucky Coffee tree (Gymnocladus dioicus)

Oaks (quercus):  White oak, Bur oak, Black oak, Northern red oak, Pin oak, Swamp White oak, Chinkapin oak, Willow oak (not all experts agree on Willow oak)

Persimmon (Diospyros virginiana)

Redbud (Cersis canadensis) (moderately drought tolerant)

Sassafras (Sassafras albidem) (could be threatened by new disease, laurel wilt)

Shagbark Hickory (Carya ovata)

Trident Maple (Acer buergeranum) (native to China and Japan)

*Also heat-tolerant

 

I’ve looked at quite a few lists of drought tolerant trees from states as far-flung as Minnesota and Alabama, and certain trees show up repeatedly.  These “drought winners” are hackberry, honeylocust, ginkgo, bur oak, Kentucky coffeetree, and golden raintree — and they are not all natives.  Nor are they even familiar specimens in local gardens. But I plan to learn more about the natives, especially the hackberries, the bur oak, and the Kentucky coffeetree.

The search for drought and heat tolerant trees could conflict with the goal of planting more natives.   Although several natives are among the “drought” winners, I was impressed by the fact that golden raintree — a native of Asia — seems to be a leading contender for a climate change resilience award!   Golden raintree has been described as “one of the most drought and heat tolerant trees and grows well all over the United States except where the winter temperature drops below -20°F.”  Urban Horticulture Institute/City of Ithaca. But it has also been “reported as becoming weedy in the eastern portion of Virginia,” according to the Va. Cooperative Ext.  You may have spotted this weediness yourself along the north side of Ivy Road in the vicinity of Ednam and Farmington.  Another non-native that scores well on drought and heat resistance is crape myrtle.

If at all possible, you’ll want to choose natives.  We gardeners will probably no longer be planting all of our old favorites — like beech and some maples —  at least not unless we are sure we can provide the type of site and amount of water that they will need to survive. Plants are not the only ones that are going to have to adapt!  Choosing a tree that is highly adaptable to a variety of situations is another way to give it a head start.  Trees that are adaptable are noted as such under the category of Growing Conditions on the Tree Stewards’ Right Tree/Right Place List,

Whatever trees we choose to plant, we gardeners can help to equip them for the challenges of climate change.  We can choose sites that provide plenty of room for root growth and avoid siting trees near heat-islands like sidewalks and tarmac. For excellent advice on site selection and its impact, see Site Assessment and Tree Selection for Stress Tolerance.  And we can be sure to water our new trees adequately and regularly for the critical first three years.  Only after it’s established does a tree begin to exhibit its tolerance for drought or heat.  For detailed guidance on watering new trees, review Watering newly planted trees and shrubs/Minn.Ext.

There’s some good news emanating from all the recent research.  As one scientist explained, “Genetic variation in the response of processes to heat has received limited study in trees, but variation exists within species and could be exploited to improve heat stress tolerance in economically important species.”  Teskey et al, Wiley Online Library   Breeding and genetic engineering for heat and drought tolerance is on the horizon.  Trees are not the only plants threatened by climate change; similar research is ongoing with respect to crops like corn and soybeans.  For example, see “Research aims to increase crop drought tolerance using biotechnology,” Nevada Today/University of Nevada, Reno (9/27/21).

SOURCES:

Featured Photo:  Kentucky coffee tree.  Photo courtesy of Missouri Botanical Garden PlantFinder

Piedmont Native Plants: A Guide for Landscapes and Gardens (Plant Northern Piedmont Natives Partnership), also available online at Plant Va. Natives.org

“Heat and Drought Tolerant Plants,” Penn State Ext (2017)

“Trees and Water,” Va.Coop.Ext. CNRE-34NP (2018)

“Plants That Tolerate Drought,” Clemson Coop.Ext. (2019)

“Water Wisely,” University of Minnesota Ext.

“Drought-Tolerant Landscapes for Alabama,” Alabama Ext (2019)

“Drought-Tolerant Trees,” University of Tennessee Ext

Recommended Urban Trees:  Site Assessment and Tree Selection for Stress Tolerance,”  Cornell University Dept. of Horticulture (2009)

“Drought and Landscape Trees: Effects, Signs, and Watering Guidelines,” Alabama Ext

USDA Plants Database, plants.usda.gov/home

“Are Northeastern U.S. forests vulnerable to extreme drought? DigitalCommons/University of Nebraska (2017)

“Celtis tenuifolia,” Wildflower.org/Plant Database

“Celtis occidentalis,” Wildflower.org/Plant Database

“Catalpa,” Univ. of Florida Environmental Horticulture Dept.

“Catalpa speciosa,” Missouri Botanical Garden Plant Finder

“Catalpa bignonioides,” Missouri Botanical Garden Plant Finder

“Sassafras albidum,”  Missouri Botanical Garden Plant Finder

“Acer buergerianum,” Missouri Botanical Garden Plant Finder

“Diospyros  virginiana,”  University of Florida Horticulture (Gilman & Watson 1993)

“Black Locust and Drought,” USDA Southern Research Station (2020) (Although black locust is relatively drought tolerant, drought can slow the rate of its symbiotic nitrogen fixing, impacting other plants that depend on the nitrogen it shares)

“Responses of tree species to heat waves and extreme heat events,” Plant, Cell & Environment. (Teskey et al, July 2014)

“Assessing the Impact of Climate Change on Urban Tree Species Selection: A Case Study in Philadelphia,” Journal of Forestry/ researchgate.net (2009)

“Drought and Tree Mortality: Science Reveals Harsh Future for World’s Forests,” Yale School of the Environment (News/7/19/2018)

Evolutionary relationships between drought-related traits and climate shape large hydraulic safety margins in western North American oaks, Proceedings of the National Academy of Sciences (Mar. 2021).

“Relating the climate envelopes of urban tree species to their drought and thermal tolerance,” Science of the Total Environment/PubMed.gov (Jan. 2021)

“Triggers of tree mortality under drought,” Nature (2018)

“Low Vulnerability to Xylem Embolism in Leaves and Stems of North American Oaks,” Plant Physiology 177 (July 2018), pubmed.ncbi.nim.nih.gov/29789436

“Discovery increases likelihood of growing food despite drought:  Genes to keep plants green,” UC Riverside News (Univ. of California 2021)

“Drought tolerance in the context of ongoing Eastern US oak decline: Are Quercus species less likely to die from drought than co-occurring non-oak species?” NASA Astrophysics Data System/Harvard.edu, American Geophysical Union, Fall Meeting 2020 (abstract #B064-0018, December 2020)

“Tree height and leaf drought tolerance traits shape growth responses across droughts in a temperate broadleaf forest,”  Wiley.com/New Phytologist (Oct. 2020)

“UChicago-led research could yield increased food production, boost drought tolerance,” University of Chicago News (July 2021)

“Northern forest tree populations are physiologically maladapted to drought,” Nature Communications/Nature.com (2018)

“The Dynamics of Embolism Repair in Xylem: In Vivo Visualizations Using High-Resolution Computed Tomography,” Plant Physiology, Vol.154, Issue 3, Nov. 2010.

Plant Physiology/pubmed.gov/2018.