Ecosystem carbon and water cycling from a sky island montane forest

Authors: Knowles, John; Scott, Russell - Russ; MINOR, R.L. - University Of Arizona; BARRON-GAFFORD, G.A. - University Of Arizona

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/20/2019
Publication Date: 1/2/2020
Citation: Knowles, J.F., Scott, R.L., Minor, R., Barron-Gafford, G. 2020. Ecosystem carbon and water cycling from a sky island montane forest. Agricultural and Forest Meteorology. 281. https://doi.org/10.1016/j.agrformet.2019.107835.
DOI: https://doi.org/10.1016/j.agrformet.2019.107835

Interpretive Summary: Forests remove carbon dioxide (CO2) from the atmosphere and therefore represent an important buffer against rising temperatures due to natural and human CO2 sources. In the semiarid western United States (USA), forests are mainly located in the mountains where environmental conditions are favorable to forest growth; however, forecasted warming and changes in precipitation could affect mountain forest health with implications for their ability to buffer CO2 emissions. Since forests use significant water, changes in their seasonal growth or health also affects the amount of water in mountain-fed streams and aquifers, which provide the majority of water available to residential, industrial, and agricultural consumers in this region. As a result, this research continuously measured water and CO2 inputs and outputs from a mountain forest in Arizona, USA for nine years between 2009 and 2018. Because of its southerly location, the forest was relatively warmer than other mountain forests in the western USA, which allowed for increased forest growth during the winter. Although it was also drier than other forests regionally, it received more rain during the summer, and the combination of winter snow accumulation and summer precipitation promoted generally favorable moisture conditions for most of the year. Consequently, the forest removed more CO2 from the atmosphere annually than other monitored forests in the western USA. This study from a relatively warm and dry forest ecosystem provides information about the way that other forests in currently cooler and/or wetter locations may respond to climate change in the future. Our results suggest that the effect of rising temperatures on the future health of mountain forests in the western USA future may depend on the consistency of moisture availability throughout the year.

Technical Abstract: “Sky islands” are characteristic of sequential mountain-valley terrain where mountains form an island archipelago rising from the surrounding valley sea. At high elevations in the Madrean sky islands of southwestern USA and Mexico, mixed evergreen conifer forests occur near the latitudinal extent of their species distribution. This setting provides a unique opportunity to explore the ecosystem response to warmer and drier conditions that are forecasted to become more common throughout the region. Accordingly, this work used the eddy covariance method to quantify carbon and water cycling dynamics from a Madrean sky island forest ecosystem for nine years between 2009 and 2018. The forest functioned as net sink of carbon dioxide throughout the year, which resulted in more carbon sequestration than other monitored montane forests in the continental western USA. Sustained forest activity was made possible by the combination of mild winter temperatures and a bimodal precipitation regime that delivered moisture during both summer and winter. Seasonally, primary production was temperature limited in winter and could become moisture limited during the dry pre-monsoon period depending on antecedent snowmelt moisture. Otherwise, ecosystem respiration was more sensitive to moisture availability throughout the rest of the non-winter period. Forecasted warming could thus stimulate both forest carbon gains in winter and respiratory losses during the summer, whereas drying induced moisture limitation before the onset of monsoon rains, but preferentially suppressed respiration thereafter. Overall, a metric of snow aridity that included snow depth and potential evapotranspiration was the best predictor of the warm season carbon balance. The seasonally dissimilar impacts of warming and drying identified by this work inform current understanding of how climate change may affect forest water and carbon cycling dynamics throughout the montane forest biome.