Climate Warming Alters Nutrient Storage in Seasonally Dry Forests: Insights From a 2,300 m Elevation Gradient

• Published in: Global biogeochemical cycles Vol. 36; no. 11
• Main Authors: Yang, Yang, Berhe, Asmeret Asefaw, Barnes, Morgan E., Moreland, Kimber C., Tian, Zhiyuan, Kelly, Anne E., Bales, Roger C., O'Geen, Anthony T., Goulden, Michael L., Hartsough, Peter, Hart, Stephen C.
• Format: Journal Article
• Language: English
• Published: 01.11.2022
• ISSN: 0886-6236; 1944-9224
• DOI: 10.1029/2022GB007429

Understanding potential response of forest carbon (C) and nutrient storage to warming is important for climate mitigation policies. Unfortunately, those responses are difficult to predict in seasonally dry forests, in part, because ecosystem processes are highly sensitive to both changes in temperature and precipitation. We investigated how warming might alter stocks of C, nitrogen (N), and phosphorus (P) in vegetation and the entire regolith (soil + weathered bedrock or “saprock”) using a space‐for‐time substitution along a bioclimatic gradient in the Sierra Nevada, California. The pine‐oak and mixed‐conifer forests between 1,160–2,015 m elevation have more optimal climates (not too dry or hot) for ecosystem productivity, soil weathering, and cycling of essential elements than the oak savannah (405 m) and subalpine forest (2,700 m). We found decreases in overstory vegetation nutrient stocks with decreasing elevation because of enhanced water limitation and greater occurrence of disturbances. Stocks of C, N, and P in the entire regolith peaked at the pine‐oak and mixed‐conifer forests across the bioclimatic gradient, driven by thicker regolith profiles and greater nutrient input rates. These observations suggest long‐term warming will decrease ecosystem nutrient storage at the warmer, transitional pine‐oak zone, but will increase nutrient storage at the colder, subalpine zone. Assuming steady‐state conditions, we found the mean residence time of ecosystem C decreased with projected rising air temperatures and increased following a major drought event across the bioclimatic gradient. Our study emphasizes potentially elevation‐dependent changes in nutrient storage and C persistence with warming in seasonally dry forests. Plain Language Summary Carbon accumulation in forest ecosystems is a promising natural climate solution to offset rising temperatures. However, major knowledge gaps exist on warming impacts on carbon storage in seasonally dry forests, where ecosystem processes are highly sensitive to both changes in temperature and precipitation. We investigated nutrient pools in vegetation, soil, and weathered bedrock along a 2,300‐m bioclimatic gradient in the Sierra Nevada, California. We observed unimodal changes in ecosystem nutrient stocks with decreasing elevation, suggesting long‐term warming will decrease nutrient storage at relatively warm sites (e.g., transitional pine‐oak zone), but will increase nutrient storage at cold sites (e.g., subalpine zone). Deep soil and weathered bedrock respond to a changing climate similarly to surficial soils, demonstrating the importance of accounting for deep carbon to accurately assess carbon turnover rates. Estimated average time (assuming steady‐state) that a carbon atom resides in forests from initial photosynthetic fixation until respiration loss increases with projected rising air temperatures and decreases following a major drought event, which emphasizes the complex response of carbon storage to warmer and drier conditions. Our study provides a broader understanding of how ecosystem structure and function may respond to warming in Mediterranean‐climate regions, some of the most vulnerable to climate change. Key Points In Mediterranean‐climate regions, warming will decrease forest storage of C, N, and P at warmer sites but not at colder sites Climatic impacts on soil C storage can remain substantial even in deep soil and weathered bedrock Ecosystem C residence times will decrease with rising air temperatures but will increase with major droughts