Abstract
Abstract. In Amazon forests, the relative contributions of climate, phenology, and disturbance to net ecosystem exchange of carbon (NEE) are not well understood. To partition influences across various timescales, we use a statistical model to represent eddy-covariance-derived NEE in an evergreen eastern Amazon forest as a constant response to changing meteorology and phenology throughout a decade. Our best fit model represented hourly NEE variations as changes due to sunlight, while seasonal variations arose from phenology influencing photosynthesis and from rainfall influencing ecosystem respiration, where phenology was asynchronous with dry-season onset. We compared annual model residuals with biometric forest surveys to estimate impacts of drought disturbance. We found that our simple model represented hourly and monthly variations in NEE well (R2=0.81 and 0.59, respectively). Modeled phenology explained 1 % of hourly and 26 % of monthly variations in observed NEE, whereas the remaining modeled variability was due to changes in meteorology. We did not find evidence to support the common assumption that the forest phenology was seasonally light- or water-triggered. Our model simulated annual NEE well, with the exception of 2002, the first year of our data record, which contained 1.2 MgC ha−1 of residual net emissions, because photosynthesis was anomalously low. Because a severe drought occurred in 1998, we hypothesized that this drought caused a persistent, multi-year depression of photosynthesis. Our results suggest drought can have lasting impacts on photosynthesis, possibly via partial damage to still-living trees.
Highlights
The Amazon’s tropical forests are pivotal to global climate, exchanging large, globally important quantities of energy and matter, including atmospheric carbon (Betts et al, 2004)
We examined measurements of rainfall, coarse woody debris (CWD), and aboveground biomass (AGB) for indications of drought or other disturbance during 2002–2011 to explain these patterns seen in annual net ecosystem exchange of carbon (NEE) totals
The model parameterization contained a seasonal decrease in respiration (a1) that was synchronous with the dry season, a timing that was consistent with other tropical forest sites but can exert the opposite influence depending on terrain, drainage, and inundation (Kiew et al, 2018)
Summary
The Amazon’s tropical forests are pivotal to global climate, exchanging large, globally important quantities of energy and matter, including atmospheric carbon (Betts et al, 2004). Amazon forests contain 10 %–20 % of Earth’s biomass carbon (Houghton et al, 2001). Increased emissions of the forest’s carbon can accelerate climate change, and attention is focused on how vulnerable this large reservoir of carbon will be to a potentially drier future climate (de Almeida Castanho et al, 2016; Farrior et al, 2015; Duffy et al, 2015; Longo et al, 2018; McDowell et al, 2018). Characterizing the response of present-day Amazon rain forest car-. Hayek et al.: Carbon exchange in an Amazon forest bon balance to climate and drought disturbance is a necessary step to improving predictions of future vulnerability
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