Drought can be good for trees, but only in cold environments

In an unexpected turn of events, new research reveals that certain trees can experience increased growth during record-breaking droughts. A study published in Global Change Biology led by Joan Dudney from UC Santa Barbara focused on the response of endangered whitebark pine to drought over the past century. Surprisingly, the study found that in cold and harsh environments, typically found at high altitudes and latitudes, drought can actually benefit the trees by extending their growing season. These findings shed light on the regions where extreme drought poses the greatest threats and how different species and ecosystems may respond to climate change.

Several factors can limit tree growth, including temperature, sunlight, water availability, and nutrient supply. The boundary between energy-limited and water-limited systems plays a crucial role. Trees attempting to grow in extremely cold temperatures, which are energy-limited systems, can face the risk of freezing to death. Conversely, insufficient water can also be detrimental, particularly in water-limited systems. Over time, various tree species have adapted to these extreme conditions, and their responses are generally similar. They often reduce growth-related activities, such as photosynthesis and nutrient uptake, to safeguard themselves until the weather improves.

Dudney, an assistant professor at the Bren School of Environmental Science & Management and the Environmental Studies Program, explained, “Interestingly, the shift from energy-limited to water-limited growth can lead to highly unexpected responses. In cold, energy-limited environments, extreme drought can actually enhance growth and productivity, even in regions like California.”

To investigate this phenomenon, Dudney and her team collected 800 tree cores from whitebark pine trees across the Sierra Nevada region. They compared the tree rings in these cores with historical climate records spanning from 1900 to 2018, which included three periods of extreme drought: 1959–1961, 1976–1977, and 2012–2015. By examining the relationship between tree growth and temperature, the researchers identified areas where growth responded positively or negatively to drought.

The study found a significant growth shift during drought periods when the average maximum temperature between October and May was approximately 8.4°C (47.1°F). Above this temperature threshold, extreme drought led to reduced growth and photosynthesis. However, below this temperature threshold, trees exhibited increased growth in response to drought.

These findings challenge conventional assumptions about the effects of drought on tree growth and emphasize the intricate relationship between environmental factors and species adaptation. Understanding how different trees and ecosystems respond to extreme weather events is crucial for predicting and managing the impacts of climate change.

Joan Dudney, the lead researcher from UC Santa Barbara, explained that the key factor influencing tree growth is the duration of the growing season. Colder winters and higher snowpack tend to result in shorter growing seasons, which limit tree growth. Surprisingly, even during extreme droughts, the trees in these harsh environments did not experience significant water stress. This finding was unexpected for the scientists, many of whom had witnessed unprecedented tree mortality at slightly lower elevations in the Sierra Nevada.

To investigate further, Dudney and her team wanted to determine if drought affected only the main trunk or the entire tree. They needed more data to understand if the observed trends were due to various processes responding differently to drought. Luckily, whitebark pine retains its needles for approximately eight years, providing additional data for analysis.

Shifting their focus from dendrology to chemistry, the researchers explored the isotopic composition of the tree’s tissues. Isotopes of the same element can have different weights, and the relative abundance of carbon-13 (heavy) and carbon-12 (light) can indicate the level of water stress a tree experienced during a drought. Analyzing needle growth and carbon and nitrogen isotopes revealed that the whole tree responded to the threshold between water-limited and energy-limited systems. Trunk growth, needle growth, photosynthesis, and nutrient cycling exhibited opposite responses to drought above and below this threshold.

The future of whitebark pine remains uncertain. The species, recently listed as threatened under the Endangered Species Act, faces multiple threats, including diseases, pine beetle infestations, and altered fire regimes. This research indicates that drought and warming will likely intensify these threats in water-limited regions, while warming may have a beneficial effect on growth in energy-limited environments.

Dudney emphasized the significance of these findings in developing targeted conservation strategies to restore the historically widespread whitebark pine. The tree’s range extends from California to British Columbia and east to Wyoming.

The implications of the research extend beyond whitebark pine. Approximately 21% of forests worldwide are considered energy-limited, and an even larger percentage can be classified as water-limited. Transitions between these climatic regimes likely occur globally and have an impact on nitrogen cycling. Trees in water-limited environments appeared to rely less on symbiotic fungi for nitrogen, which is vital for growth in harsh, energy-limited environments.

Dudney highlighted the global issue of widespread tree mortality caused by drought, which can contribute to accelerated global warming.

Understanding the diverse ways in which trees respond to drought will aid in predicting vulnerable ecosystems and developing targeted strategies to protect forests in the face of climate change.

Source: University of California - Santa Barbara

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