Abstract
We investigated the seasonal patterns of Amazonian forest photosynthetic activity, and the effects thereon of variations in climate and land-use, by integrating data from a network of ground-based eddy flux towers in Brazil established as part of the ‘Large-Scale Biosphere Atmosphere Experiment in Amazonia’ project. We found that degree of water limitation, as indicated by the seasonality of the ratio of sensible to latent heat flux (Bowen ratio) predicts seasonal patterns of photosynthesis. In equatorial Amazonian forests (5° N–5° S), water limitation is absent, and photosynthetic fluxes (or gross ecosystem productivity, GEP) exhibit high or increasing levels of photosynthetic activity as the dry season progresses, likely a consequence of allocation to growth of new leaves. In contrast, forests along the southern flank of the Amazon, pastures converted from forest, and mixed forest-grass savanna, exhibit dry-season declines in GEP, consistent with increasing degrees of water limitation. Although previous work showed tropical ecosystem evapotranspiration (ET) is driven by incoming radiation, GEP observations reported here surprisingly show no or negative relationships with photosynthetically active radiation (PAR). Instead, GEP fluxes largely followed the phenology of canopy photosynthetic capacity (Pc), with only deviations from this primary pattern driven by variations in PAR. Estimates of leaf flush at three non-water limited equatorial forest sites peak in the dry season, in correlation with high dry season light levels. The higher photosynthetic capacity that follows persists into the wet season, driving high GEP that is out of phase with sunlight, explaining the negative observed relationship with sunlight. Overall, these patterns suggest that at sites where water is not limiting, light interacts with adaptive mechanisms to determine photosynthetic capacity indirectly through leaf flush and litterfall seasonality. These mechanisms are poorly represented in ecosystem models, and represent an important challenge to efforts to predict tropical forest responses to climatic variations.
Highlights
The Amazon basin represents a major component of regional and global carbon and hydrological cycles
The dominant seasonal rhythm in many Amazonian forests is that of rainfall, with a distinct dry season, when precipitation inputs fall below 100 mm month−1, varying in length from 0 months to 5 months in central eastern equatorial Amazonia, and in southwestern edge (Sombroek, 2001) (Fig. 1)
After finding that photosynthetic seasonality is not well-explained by environmental variables alone, we advance the hypothesis that it arises instead from the interaction between biologically determined leaf phenology and environmental drivers. We explore this third hypothesis with a simple model of canopy phenology that assumes photosynthetic capacity arises from the balance between loss of capacity to litterfall and increase of capacity from new leaf-flush
Summary
The Amazon basin represents a major component of regional and global carbon and hydrological cycles. Significant variations in these cycles could be induced by climate change (Betts et al, 2004; Monteith, 1965), but predictions for the future of Amazon forests under climate change vary widely (Friedlingstein et al, 2006). The dominant seasonal rhythm in many Amazonian forests is that of rainfall, with a distinct dry season, when precipitation inputs fall below 100 mm month−1, varying in length from 0 months (no dry season in the ever-wet northwestern Amazon) to 5 months in central eastern equatorial Amazonia, and in southwestern edge (Sombroek, 2001) (Fig. 1). The dry season brings a decrease in cumulus cloud cover and, in areas close to the equator with low variation in top-of-atmosphere solar energy, an increase in solar radiation at the surface. The dry season brings increased smoke from biomass burning (Koren et al, 2004; Oliveira et al, 2007), shifting the composition of direct and diffuse radiation
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