When extreme heat changes the role of plants: what science teaches us about global warming

In recent years, global warming is no longer a distant or theoretical concept, but a reality manifesting through increasingly intense and frequent heatwaves. High temperatures do not only affect our comfort or seasonal cycles: they profoundly interfere with the functioning of plants and their role in ecosystems. Recent scientific research shows that, beyond certain temperature thresholds, plants react in ways that can seriously alter the carbon balance and the global climate cycle.

Understanding how plants respond to extreme heat is essential not only for research, but also for those who grow, design, and maintain green spaces today, in a rapidly changing climate.

• Science confirms it: not only stomata
• Critical temperatures for plants
• A practical message for nursery workers and designers
• Resilient plants for a more difficult future

Science confirms it: not only stomata

For years, it was believed that water loss from leaves occurred almost exclusively through stomata, the tiny pores that allow gas exchange between the leaf and the environment. In conditions of intense heat or drought, stomata close to reduce transpiration: a defense mechanism similar to apnea in mammals.

However, a study published in New Phytologist Foundation, titled Temperature governs the relative contributions of cuticle and stomata to leaf minimum conductance, has highlighted another important aspect of plant behavior in the heat. The study shows that when the temperature increases, water loss through the leaf cuticle – the waxy layer covering the leaf epidermis – contributes much more significantly to water loss than previously thought.

Unlike stomata, the cuticle cannot close. When stomata close to limit losses, a "channel" always remains open through which the plant continues to lose water. This effect becomes increasingly relevant as the temperature rises and can represent a significant portion of leaf transpiration, especially in species with relatively thin cuticles.

Critical temperatures for plants

The same study analyzes how about 200 plant species react to high temperatures and identifies some critical thresholds. Already at 30°C, many plants begin to lose efficiency in the photosynthesis process and can even emit more CO2 than they absorb through cellular respiration. Beyond the threshold of 40°C, photosynthesis is drastically reduced and the ability to capture carbon decreases noticeably, while between 40 °C and 50 °C cellular respiration is strongly affected by heat stress.

This new understanding of plant behavior at high temperatures suggests that plants, in conditions of prolonged heat, no longer act as carbon sinks.

These findings are not just academic: they have direct implications for climate change prediction, ecosystem conservation, and the management of urban and rural green spaces. Traditional climate models may therefore underestimate the impact of high temperatures on global vegetation, precisely because in the past it was taken for granted that transpiration was almost exclusively mediated by stomata.

If plants become less efficient at fixing carbon with warming, the implications for forestry, agriculture, urban planning, and landscape design are significant: we must choose plant species better suited to the new environmental conditions and rethink our green management strategies.

A practical message for nursery workers and designers

This new knowledge reinforces a fundamental concept for those who work with plants: not all species are equally resistant to heat and drought. While some plants grow well in temperate climates with ample water availability, others have developed evolutionary strategies that allow them to cope with prolonged heat and dry conditions.

In particular, plants with thicker leaves, robust cuticles, deep root systems, and water conservation mechanisms are those most likely to withstand heatwaves and continue to play a positive role in the carbon cycle and ecosystem stability.

Therefore, in the nursery and landscape design, this scientific research goes in the same direction as a philosophy that is becoming increasingly necessary: the dry garden or low-water-use garden. It is not about eliminating water, but about using it better and choosing plants that, by nature, require less irrigation and withstand heat better.

Mediterranean plants, xerophytes, hardy species, and those adapted to difficult conditions not only reduce the need for irrigation, but are also less stressed by extreme heat, and therefore maintain their ecological function more actively even on the hottest days.

Resilient plants for a more difficult future

Science is showing us that global warming is not a problem that can be solved with more frequent irrigation or intensive care. The way plants "sweat" and lose water, especially through the cuticle, is not a detail in itself: it is a central element in understanding how greenery will react to an increasingly hot climate.

Choosing resistant plants, improving landscape design, and adopting conscious approaches such as dry gardening is not just a matter of aesthetics: it is a concrete and necessary response to the challenges of our time, to continue to make greenery, biodiversity, and sustainability coexist even in the most difficult conditions.