Impacts of Vegetation Phenology on Surface-Energy Balance Dynamics: A Synthesis Integrating PhenoCam and AmeriFlux Data
Abstract
Vegetation phenology plays a key role in controlling ecosystem carbon and water balance, as well as seasonal cycles in the surface-energy balance (SEB). Currently, there is high uncertainty regarding the mechanisms that drive seasonal SEB changes during phenological transitions (e.g., aerodynamic resistance, surface roughness, albedo, surface temperature), as well as implications for predicting patterns in surface-energy fluxes. Here, we show how phenology and the underlying components of a one-layer SEB model covary on seasonal time scales, and quantify the impact of these seasonal changes on modeled SEB fluxes. For this analysis we used 30-minute eddy covariance data from AmeriFlux and daily measurements of canopy greenness from co-located PhenoCams. Our study area spans an aridity gradient from deciduous broadleaf ecosystems in the northeastern U.S. to semi-arid ecosystems in the southwest.
Our results focus on the following SEB components: aerodynamic resistance [ra] and TSURF - TAIR [ΔT]. For each of these components, we found strong seasonal cycles across the aridity gradient of our study area. From deciduous broadleaf to semi-arid ecosystems we found a consistent pattern with ΔT decreasing sharply in response to spring green-up, and then rising with fall senescence. The timing of ΔT spring decreases were well correlated to the timing of spring green-up in deciduous broadleaf (ρ = 0.72) and semi-arid (ρ = 0.74) ecosystems. One-layer SEB model predictions captured seasonal cycles of sensible heat flux. However, while there was good agreement (low bias) between observed and predicted sensible-heat fluxes during the growing season, there were large biases outside the growing season. For example, at an arid grassland site, mean bias was on average 150 W m-2 outside and 0 W m-2 during the growing season. We hypothesize these errors are due to parameterizations of ra not capturing complex land-atmosphere interactions and coupling during transition periods. Our results highlight consistent patterns between phenology and SEB components across a continental-scale aridity gradient, as well as significant implications for predicting seasonal surface-energy fluxes. More importantly, these findings may offer key information to improve model-based predictions of land-atmosphere interactions.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B41L2462Y
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 0426 Biosphere/atmosphere interactions;
- BIOGEOSCIENCES;
- 0428 Carbon cycling;
- BIOGEOSCIENCES;
- 0438 Diel;
- seasonal;
- and annual cycles;
- BIOGEOSCIENCES