High-elevation montane forests are disproportionately important to carbon sequestration
in semiarid climates where low elevations are dry and characterized by low
carbon density ecosystems. However, these ecosystems are increasingly threatened
by climate change with seasonal implications for photosynthesis and forest growth.
As a result, we leveraged eddy covariance data from six evergreen conifer forest
sites in the semiarid western United States to extrapolate the status of carbon sequestration
within a framework of projected warming and drying. At colder locations,
the seasonal evolution of gross primary productivity (GPP) was characterized by a
single broad maximum during the summer that corresponded to snow melt-derived
moisture and a transition from winter dormancy to spring activity. Conversely, winter
dormancy was transient at warmer locations, and GPP was responsive to both winter
and summer precipitation such that two distinct GPP maxima were separated by a
period of foresummer drought. This resulted in a predictable sequence of primary
limiting factors to GPP beginning with air temperature in winter and proceeding to
moisture and leaf area during the summer. Due to counteracting winter (positive) and
summer (negative) GPP responses to warming, leaf area index and moisture availability
were the best predictors of annual GPP differences across sites. Overall, mean
annual GPP was greatest at the warmest site due to persistent vegetation photosynthetic
activity throughout the winter. These results indicate that the trajectory of this
region’s carbon sequestration will be sensitive to reduced or delayed summer precipitation,
especially if coupled to snow drought and earlier soil moisture recession,
but summer precipitation changes remain highly uncertain. Given the demonstrated
potential for seasonally offsetting responses to warming, we project that decadal
semiarid montane forest carbon sequestration will remain relatively stable in the absence
of severe disturbance.
