Montane forest productivity across a semi-arid climatic gradient

Authors: Knowles, John; Scott, Russell - Russ; Biederman, Joel; BLANKEN, P.D. - University Of Colorado; BURNS, S.P. - University Of Colorado; DORE - Northern Arizona University; KOLB, T.E. - Northern Arizona University; LITVIK, M.E. - University Of New Mexico; BARRON-GAFFORD, G.A. - University Of Arizona

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2020
Publication Date: 11/22/2020
Citation: Knowles, J.F., Scott, R.L., Biederman, J.A., Blanken, P., Burns, S., Dore, .S., Kolb, T., Litvik, M., Barron-Gafford, G. 2020. Montane forest productivity across a semi-arid climatic gradient. Global Change Biology. 26(12):6945-6958. https://doi.org/10.1111/gcb.15335.
DOI: https://doi.org/10.1111/gcb.15335

Interpretive Summary: Forests remove carbon dioxide from the atmosphere and act as an important buffer against climate change. Mountain forests are particularly important to carbon uptake in semi-arid climates where low elevations are dry and characterized by sparse vegetation. Here, we compare seasonal carbon uptake from six mountain forest sites in the southwestern United States that span broad gradients in elevation, air temperature, and precipitation. The main objective of this work was to investigate how mountain forest carbon uptake is likely to respond to regionally forecasted warming and drying. At higher and colder locations, forests were dormant with no carbon uptake during the winter, but spring snowmelt was accompanied by strong carbon gains that persisted throughout the summer. In contrast, carbon uptake was maintained at lower and warmer locations throughout the winter, but summer carbon uptake was lower than at the colder sites. Carbon uptake was also typically reduced at warmer locations after the spring snowmelt, but before the onset of summer rains, due to prevailing dry conditions. Overall, total annual carbon uptake was greatest at the warmest sites due to year-round vegetation activity. Across sites, forest vegetation density and moisture availability were the best predictors of carbon uptake, in part because the winter (positive) and summer (negative) effects of air temperature on carbon uptake canceled out. Accordingly, carbon uptake at lower elevations may be sensitive to reduced moisture availability; however, changes in future precipitation remain highly uncertain. As a result, we conclude that semi-arid mountain forest carbon uptake is likely to remain stable for the foreseeable future.

Technical Abstract: High-elevation montane forests are disproportionately important to carbon sequestration in semi-arid climates where low elevations are dry and characterized by low carbon density ecosystems. Here, we take advantage of newly available eddy covariance data from the semi-arid western United States to investigate drivers of ecosystem gross primary productivity (GPP) from six evergreen conifer forests that span broad gradients in air temperature, snow versus rain, and precipitation seasonality. We leveraged this analysis to extrapolate the future status of the western United States carbon sink within a framework of projected warming and drying. At colder locations, the seasonal evolution of GPP was characterized by a single broad peak during the warm season that corresponded to snowmelt moisture and a transition from winter dormancy to spring activity. Conversely, winter dormancy was variably transient at warmer locations, and GPP was responsive to both winter and summer precipitation resulting in two distinct GPP peaks separated by a period of foresummer drought. This scenario yielded tradeoffs between moisture and energy limitation that varied over space and time, but there was evidence of compensatory winter (positive) and summer (negative) GPP responses to air temperature at all sites. As a result, leaf area index and moisture availability principally contributed to multi-site GPP predictions at the annual scale. Overall, mean annual GPP was greatest at the warmest site due to persistent vegetation activity throughout the winter. Given the uncertainty of forecasted precipitation changes, and the potential for seasonally offsetting responses to warming identified by this work, we project that the semi-arid western United States montane forest carbon sink will remain relatively stable for the foreseeable future. However, regional carbon sequestration will be sensitive to reduced or delayed summer precipitation, especially if coupled to snow drought and earlier soil moisture recession.