Newswise — (Santa Barbara, Calif.) — Despite expectations, occasionally an unprecedented drought can amplify tree growth. The reasons behind and the locations where this phenomenon occurs are the focal points of a recent publication in Global Change Biology.
Under the leadership of Joan Dudney at UC Santa Barbara, a group of researchers analyzed the impact of drought on the endangered whitebark pine over the previous century. Their investigation revealed that in frigid, inhospitable surroundings, typically found at high altitudes and latitudes, drought can paradoxically promote tree growth by prolonging the growing period. This study offers valuable knowledge regarding the regions most vulnerable to severe drought and sheds light on the varied responses of diverse species and ecosystems to the effects of climate change.
Multiple variables influence tree growth, such as temperature, sunlight, water availability, and nutrient levels. Among these factors, the demarcation between energy-limited and water-limited systems holds notable importance. Trees attempting to thrive in extremely cold temperatures, which typically fall under energy-limited systems, face the risk of freezing to death. Conversely, insufficient water poses a significant threat to tree survival, particularly in water-limited systems. Throughout time, numerous tree species have evolved to adapt to these harsh conditions, showcasing comparable responses. They often curtail growth-related activities like photosynthesis and nutrient absorption to safeguard themselves until favorable weather conditions return.
Dudney, an assistant professor in the Bren School of Environmental Science & Management and the Environmental Studies Program, highlighted the intriguing nature of the shift from energy-limited to water-limited growth. "Remarkably, this transition can yield remarkably surprising outcomes," Dudney explained. "In cold environments constrained by energy, extreme drought can unexpectedly enhance growth and productivity, including in California."
Dudney and her team collected 800 tree cores from whitebark pine trees in the Sierra Nevada region. By examining the tree rings and cross-referencing them with historical climate records spanning from 1900 to 2018, they analyzed the relationship between tree growth and climate conditions. Specifically, they focused on three periods of extreme drought: 1959-1961, 1976-1977, and 2012-2015. The researchers documented instances where tree growth displayed a positive correlation with temperature and identified cases where the relationship was negative.
The study conducted by the authors revealed a significant change in tree growth patterns during drought periods when the average maximum temperature ranged around 8.4° Celsius (47.1° Fahrenheit) from October to May. When the temperature surpassed this threshold, extreme drought conditions led to a decline in growth and photosynthesis. Conversely, when the temperature remained below this threshold, trees exhibited increased growth as a response to drought.
Dudney explained that the duration of the growing season plays a crucial role, stating, "Essentially, it comes down to how long the growing season lasts." In regions characterized by colder winters and greater snowpack, the growing season tends to be shorter, which can limit tree growth. Surprisingly, even during periods of extreme drought, the trees in these harsh environments did not exhibit significant water stress. This finding came as a surprise to the team of scientists, as they had previously witnessed and documented unprecedented tree mortality at slightly lower elevations in the Sierra Nevada.
The researchers transitioned their focus from dendrology to chemistry in order to delve deeper into their investigation. They explored the isotopic composition of atoms within the same element, specifically carbon. Isotopes of carbon, such as heavy carbon-13 and light carbon-12, differ in their neutron count. Various metabolic processes within a plant can influence the relative abundance of these isotopes in tissues like leaves and needles. By analyzing these changes, the researchers were able to obtain a rough estimate of the water stress experienced by the trees during periods of drought. This proved to be advantageous for the team, as the isotopic data derived from the pine needles encompassed both drought and non-drought years, offering valuable insights for their analysis.
Through the analysis of needle growth and the examination of carbon and nitrogen isotopes, the researchers discovered that the impact of the threshold between water-limited and energy-limited systems extended throughout the entire tree. They observed that trunk growth, needle growth, photosynthesis, and nutrient cycling exhibited contrasting responses to drought, depending on whether the conditions were above or below the threshold between energy-limited and water-limited systems. This finding highlights the complex interplay between different physiological processes within the tree and their sensitivity to drought conditions.
The future of the whitebark pine species remains highly uncertain. With its recent classification as threatened under the Endangered Species Act, it faces multiple challenges including diseases, infestation by pine beetles, and disruptions caused by altered fire patterns. The research clearly indicates that drought and warming are likely to worsen these threats, particularly in water-limited areas. However, in energy-limited environments, warming may actually have a positive effect on growth. Dudney emphasized that this study can contribute to the development of targeted conservation strategies aimed at the restoration of this historically widespread tree species. The whitebark pine's range spans a diverse region, extending from California to British Columbia and reaching eastward to Wyoming.
The implications of the findings extend beyond the whitebark pine and have broader significance. Around the world, approximately 21% of forests are categorized as energy limited, and an even larger percentage can be classified as water limited. Therefore, transitions between these two climatic regimes likely occur on a global scale. Moreover, the transition between energy-limited and water-limited systems appears to influence nitrogen cycling as well. In water-limited environments, trees exhibited reduced reliance on symbiotic fungi for nitrogen, which is crucial for tree growth in challenging energy-limited environments. This insight provides valuable knowledge about the intricate dynamics of nutrient cycling in different climatic conditions and can contribute to a better understanding of forest ecosystems worldwide.
“Droughts are leading to widespread tree mortality across the globe,” Dudney said, “which can accelerate global warming.”
Understanding the diverse array of responses exhibited by trees in the face of drought is essential for predicting ecosystem vulnerabilities to climate change. By deciphering these responses, researchers can identify regions that are particularly susceptible to the impacts of drought and develop more precise and effective strategies to safeguard forest ecosystems. This knowledge will aid in the development of targeted approaches for protecting and preserving forests, ensuring their resilience in the face of ongoing climate challenges
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