Abstract
The mid-elevation forest of California's Sierra Nevada poses a bioclimatic paradox. Mid-elevation trees experience a montane Mediterranean climate, with near-freezing winter days and rain-free summers. The asynchrony between warmth and water input suggests low primary production, limited by photosynthetic dormancy in winter cold, and again in summer and early autumn with drought, yet this forest is characterized by tall trees and high biomass. We used eddy covariance in a mid-elevation Sierra stand to understand how winter cold and summer drought limit canopy photosynthesis and production. The trees exhibited canopy photosynthesis year-round. Trees avoided winter dormancy, and daytime CO2uptake continued despite a deep snowpack and near-freezing temperatures. Photosynthesis on sunny days continued at half of maximum rates when air temperature was 0 °C. Likewise, the vegetation avoided summer drought dormancy, and high rates of daytime CO2uptake and transpiration continued despite a 5-month period with only negligible water input. We attribute this drought avoidance to deep rooting and availability of deep soil water. Year-round photosynthesis helps explain the large biomass observed in the Sierra Nevada, and implies adaptive strategies that may contribute to the resiliency or vulnerability of Sierran vegetation to climate change.
Highlights
The mechanisms that allow Sierran mixed-conifer forest to support a biomass that is comparable to tropical rainforest despite a seemingly harsh environment remain unresolved
Precipitation typically arrived in multiday winter storms; ∼70% fell as snow during water year (WY) 2008 and 2009 and a snowpack persisted through most of the winter (Bales et al 2011b)
The effects were relatively weak, and autumn gross ecosystem CO2 exchange (GEE) rates remained high under the conditions observed (Figure 6). These results show that winter photosynthesis and late summer drought avoidance are important to the annual fluxes of carbon and water in Southern Sierra mixed-conifer forest
Summary
The mechanisms that allow Sierran mixed-conifer forest to support a biomass that is comparable to tropical rainforest despite a seemingly harsh environment remain unresolved. The 1000– 2500 m elevation belt in California’s Sierra Nevada experiences a montane Mediterranean climate, with near-freezing days and a mix of rain and snow from about November to May, and a long, snow- and rain-free period from about June to October. The long period from late fall to spring with below-freezing nights, combined with the long period from late spring to fall without appreciable precipitation, suggests that high rates of canopy gas exchange could be confined to a short window from about April to June (J ohnson 1982, Urban et al 2000, O RNL D AAC 2008, Berlekamp et al 2015). The apparent paradox of a high-biomass forest despite a cold winter and dry summer has received only brief attention in the literature
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