Sensitivity of Forest Productivity to Trends in Snowmelt at Niwot Ridge,
Colorado
Abstract
Anthropogenic global warming caused by increased atmospheric carbon
forcing is expected to cause a decrease in peak snow water equivalent
(SWE), shift the timing of snowmelt to earlier in the year, and lead to
slower melt rates in the mountains of the Western United States.
High-elevation forests in mountainous terrain represent a critical
carbon sink. Understanding the ecohydrology of subalpine forests is
crucial for assessing the health of these sinks. The Niwot Ridge Long
Term Ecological Research station, located at 3000 m amsl in the southern
Rocky Mountains of Colorado, receives just over 1 m of annual
precipitation mostly as snow, supporting a persistent seasonal snowpack
in alpine and subalpine ecosystems. Previous studies show that longer
growing season length is correlated with shallower snowpack, earlier
spring onset and reduced net CO2 uptake. Co-located sensors provide over
20 years of continuous SWE and eddy covariance (EC) data, allowing for
robust direct comparison of snow and carbon phenomena in a
high-elevation catchment. Linear regression and time series analysis was
performed on snowmelt, meteorological, phenological and ecosystem
productivity variables. Peak productivity is correlated with peak SWE
(R2=0.54) and further correlated with snowmelt disappearance (R2=0.38)
and the timing of spring growth onset (R2=0.30). Timing of both peak
productivity and spring growth onset are correlated with snowmelt and
meteorological variables. A multivariable regression of meteorological
variables, timing of spring growth onset, a temporal trend, and snowmelt
rate and explains 94% of interannual variability in the timing of peak
forest productivity. These results develop support and introduce new
evidence for the existing studies of Niwot Ridge ecohydrology. Future
work will investigate the meteorological and hydrological record
extending back to 1979 and the long-term trends in snowmelt and forest
productivity.