Trees Can’t Hide from the Heat

It has been quite the Summer!  The heat has been oppressive and thankfully with some breaks in the weather, things are feeling a little bit better, yet the high temperatures got me thinking about how trees handle these temperatures.  While humans can utilize air conditions, look for shade, or even jump in a river, trees are stuck and have very few ways to adequately deal with high temperatures.   Heat stress, a condition arising from excessive and prolonged exposure to high temperatures, poses a critical threat to tree health and survival. As global temperatures rise due to climate change, understanding the effects of heat stress on trees becomes increasingly important. Trees, integral components of both natural ecosystems and urban environments, provide numerous benefits, including carbon sequestration, habitat for wildlife, and cooling urban areas. However, heat stress can disrupt their physiological functions, stifle growth, and increase mortality rates, with significant ecological and environmental repercussions.

Heat stress occurs when ambient temperatures exceed the range that a tree species can tolerate, leading to physiological strain. Several factors contribute to heat stress, including high temperatures, prolonged heatwaves, and drought conditions. Persistent high temperatures can cause significant damage to trees, while extended periods of heat without relief exacerbate the stress. Unfortunately drought conditions often accompany heat stress events, compounding the problem by limiting the availability of water essential for tree survival and health.

Heat stress affects tree physiology in several ways. One major impact is on photosynthesis and respiration. High temperatures can reduce the efficiency of photosynthesis, limiting the tree’s ability to produce energy in the form of sugars. Concurrently, elevated respiration rates can lead to energy deficits as trees expend more energy to cool down. Additionally, heat stress affects water uptake and transpiration. Trees may close their stomata to conserve water, reducing transpiration and cooling but also limiting CO2 intake. This impairs water transport through the xylem, leading to reduced hydration and nutrient transport.

Heat stress significantly influences tree growth and development. Reduced growth rates are a common consequence, as trees prioritize survival over growth during stressful conditions. Leaves may exhibit scorching, browning, and premature drop, reducing the tree’s ability to photosynthesize effectively. Additionally, seed production can be impaired, resulting in lower seed viability and production, which affects the tree’s reproductive success.

The survival and mortality rates of trees are also impacted by extended periods of heat. Stressed trees are more susceptible to pests and diseases due to weakened defenses, increasing their vulnerability to infestations and infections. It is very common to see an uptick in Ambrosia Beetle infestations during heatwaves as these insects prefer to colonize stressed trees.  Prolonged exposure to extreme heat can lead to higher mortality rates, particularly in young or already weakened trees. This can have significant implications for forest ecosystems and biodiversity.

The broader ecological and environmental implications of elevated temperatures on trees are profound. Heat stress can alter forest biodiversity and composition by favoring heat-tolerant species and disadvantaging others. This shift can affect the overall health and stability of forest ecosystems. Additionally, reduced tree health and growth diminish the ability of forests to sequester carbon which is one of a tree’s most vital roles.

The effects of rising temperatures on trees also impacts human and urban environments. Trees play a crucial role in cooling urban areas through shade and transpiration. Heat-stressed trees are less effective in this role, contributing to urban heat islands and higher temperatures. This can have implications for public health and well-being, as reduced tree cover can lead to increased heat-related illnesses and discomfort.

To mitigate the effects of heat stress on trees, several strategies can be implemented. Planting heat-resistant species that are more tolerant of high temperatures can help reduce the impact of heat stress. Urban planning and green infrastructure, incorporating green spaces, tree canopies, and water features, can enhance cooling and reduce urban heat islands. Additionally, efficient irrigation systems and water conservation practices can ensure trees receive adequate hydration during periods of heat stress.