How Drought Affects Tree Physiology and Ecosystem Services

Droughts are often seen as a threat to trees, especially in regions where water is scarce. However, not all trees respond to drought in the same way.

In fact, some trees may even benefit from drought conditions, depending on what factors have been limiting their growth.

The threshold between energy-limited and water-limited systems

(Photo : DSD/Pexels)

Trees need water, sunlight, nutrients, and temperature to grow and survive. However, these factors are not always available in optimal amounts.

Sometimes, trees face trade-offs between different factors, such as water and temperature, as per Phys.org.

The threshold between energy-limited and water-limited systems is a key concept to understand how trees cope with drought. Energy-limited systems are those where the temperature is too low for trees to grow well. Water-limited systems are those where the water availability is too low for trees to grow well.

In energy-limited systems, such as cold, high-altitude, or high-latitude environments, drought can actually extend the growing season for trees by reducing snowpack and increasing soil temperature. This can allow trees to photosynthesize more and increase their growth.

In water-limited systems, such as warm, low-altitude, or low-latitude environments, drought can reduce the growing season for trees by increasing water stress and decreasing soil moisture. This can limit the photosynthesis and growth of trees.

The case of whitebark pine

Whitebark pine is an endangered tree species that grows in the mountains of western North America. It faces multiple threats, such as climate change, fire suppression, invasive species, and disease, as per UMassAmherst.

A team of scientists led by Joan Dudney at UC Santa Barbara examined the drought response of whitebark pine over the past century.

They used tree rings to measure the growth of 800 whitebark pines across the Sierra Nevada and compared them with historical records of climate conditions.

They found that whitebark pine growth varied depending on the temperature and precipitation regimes of their locations.

In colder and wetter sites, whitebark pines grew more during droughts than during normal years. In warmer and drier sites, whitebark pines grew less during droughts than during normal years.

The researchers identified a threshold temperature of about 8.4°C (47.1°F) between October and May that separated energy-limited from water-limited systems.

Above this threshold, extreme drought reduced whitebark pine growth and photosynthesis. Below this threshold, extreme drought increased whitebark pine growth and photosynthesis.

The researchers also analyzed needle growth, carbon and nitrogen isotopes to reveal how the whole tree was affected by drought.

They found that trunk growth, needle growth, photosynthesis and nutrient cycling responded in opposite directions to drought above and below the threshold.

Also Read: Boreal Forests and Climate: 3 Trillion Trees in World

Implications for Conservation and Management

This study shows that drought effects on trees are not uniform across different environments. It also highlights the importance of considering multiple factors that influence tree growth, such as temperature, water availability, and nutrient cycling, as per NoozHAwk.

Understanding how different tree species and ecosystems respond to drought can help inform conservation and management strategies in the face of climate change.

For example, whitebark pine may be more resilient to drought in colder regions than in warmer regions.

Therefore, protecting and restoring whitebark pine populations in colder regions may be more effective than in warmer regions.

However, the researchers caution that their findings do not imply that drought is beneficial for all trees in cold environments.

Other factors, such as fire regime, insect outbreaks, and disease may also affect tree survival and growth.

Moreover, future climate change may alter the threshold between energy-limited and water-limited systems or push some regions beyond their adaptive capacity.

Therefore, more research is needed to understand how different tree species and ecosystems will cope with the increasing frequency and intensity of droughts in a changing world.

Related article: Can Trees 'Talk' to Humans? Experts Unlock Their Communication Secrets