Coupling between carbon cycling and climate in a high-elevation, subalpine forest: A model-data fusion analysis

Fundamental questions exist about the effects of climate on terrestrial net ecosystem CO₂ exchange (NEE), despite a rapidly growing body of flux observations. One strategy to clarify ecosystem climate-carbon interactions is to partition NEE into its component fluxes, gross ecosystem CO₂ exchange (GEE) and ecosystem respiration (R_E), and evaluate the responses to climate of each component flux. We separated observed NEE into optimized estimates of GEE and R_E using an ecosystem process model combined with 6 years of continuous flux data from the Niwot Ridge AmeriFlux site. In order to gain further insight into the processes underlying NEE, we partitioned R_E into its components: heterotrophic (R_H) and autotrophic (R_A) respiration. We were successful in separating GEE and R_E, but less successful in accurately partitioning R_E into R_A and R_H. Our failure in the latter was due to a lack of adequate contrasts in the assimilated data set to distinguish between R_A and R_H. We performed most model runs at a twice-daily time step. Optimizing on daily-aggregated data severely degraded the model's ability to separate GEE and R_E. However, we gained little benefit from using a half-hourly time step. The model-data fusion showed that most of the interannual variability in NEE was due to variability in GEE, and not R _E. In contrast to several previous studies in other ecosystems, we found that longer growing seasons at Niwot Ridge were correlated with less net CO₂ uptake, due to a decrease of available snow-melt water during the late springtime photosynthetic period. Warmer springtime temperatures resulted in increased net CO₂ uptake only if adequate moisture was available; when warmer springtime conditions led into mid-summer drought, the annual net uptake declined.