What drives the seasonality of photosynthesis across the Amazon basin? A cross-site analysis of eddy flux tower measurements from the Brasil flux network

Abstract

a b s t r a c tWe 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 * Corresponding author. Tel.: +1 520 6261500; fax: +1 520 621 9190. 182-183 (2013) 128-144 129 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.

Mechanisms of water supply and vegetation demand govern the seasonality and magnitude of evapotranspiration in Amazonia and Cerrado

“…amount of time required for new leaf expansion. This corroborates recent work (Restrepo-Coupe et al, 2013) which demonstrates the importance of canopy leaf flush driving the seasonality of photosynthesis across the Amazon basin. In contrast to these observations, many models which captured the seasonality of E consistently underestimated seasonal variability in g s .…”

“…Some models, however, which simulated well the seasonal cycle of E t and its control by net radiation did so with near-constant canopy stomatal conductance throughout most of the year. We presented evidence at an equatorial site (Restrepo-Coupe et al, 2013) which suggests that the quantity and age distribution of leaves in the canopy plays just as significant a role in the seasonality of canopy stomatal conductance as does leaf-level stomatal control, implying that some of the data-model discrepancy is due to leaf phenology, a process poorly represented in vegetation models. Model biases in the magnitude of canopy stomatal conductance, in turn, could be related to light response (GPP) or the intrinsic water use efficiency of photosynthesis (iWUE), in addition to its effect on canopy temperatures.…”

“…The tower at the PDG Cerrado site is situated within a zone of cerrado sensu stricto (da Rocha et al, 2002(da Rocha et al, , 2009. For additional site characteristics and ecosystem behavior, see Restrepo-Coupe et al (2013) and references therein and da Rocha et al (2002Rocha et al ( , 2009. Table 1 also lists the temporal coverage and frequency of the climate measurements, eddy covariance data, and ancillary soil moisture data which are available at each site, in addition to the installation depths of soil moisture sensors.…”

“…Table 1 also lists the temporal coverage and frequency of the climate measurements, eddy covariance data, and ancillary soil moisture data which are available at each site, in addition to the installation depths of soil moisture sensors. All eddy covariance data have been processed according to a common protocol and are aggregated to an hourly timestep (Restrepo-Coupe et al, 2013). Soil moisture datasets were assimilated from various sources (see Table 1).…”

Land-use and climate change risks in the Amazon and the need of a novel sustainable development paradigm

“…In regions where the annual rainfall is above this value, water stored in the soil during the wet season seems to be able to supply ET and photosynthesis for the subsequent dry season. However, this normally is not the case for regions where the average annual rainfall is below this value, highlighting a clear spatial and seasonal pattern of ET and photosynthesis variability (66). The ET's controlling mechanisms also varied over this rainfall gradient, with climate demands (particularly radiation and vapor pressure deficit) controlling ET rates in wetter areas and soil moisture deficit controlling ET in the driest areas (64).…”