Seasonal variation in net carbon exchange and evapotranspiration in a Brazilian rain forest: a modelling analysis

Abstract

Tropical rain forests account for a significant fraction of global net primary productivity, and are important latent energy (LE) sources, affecting extra-tropical atmospheric circulation. The influence of environmental factors on these fluxes has until recently been poorly understood, largely due to a paucity of data, but in recent years the amount of available data has been increased greatly by use of eddy covariance techniques. In this paper we examine the factors that control daily and seasonal carbon (C) and LE fluxes, by comparing a detailed model of the soil-plant-atmosphere continuum against a unique long-term data-set collected using eddy covariance at an undisturbed rain forest site north of Manaus, Brazil. Our initial application of the model was parametrized with simple measurements of canopy structure, and driven with local meteorological data. It made effective predictions of C and LE exchange during the wet season, but dry season predictions were overestimates in both cases. Sensitivity analyses indicated that the best explanation for this behaviour was a seasonal change in soil and root hydraulic resistances (R b ). An optimization routine was then used to estimate the increase in R b during the dry season that would be required to explain the reduced dry season fluxes. The local soil, a clay latosol, is typical of much of Amazônia, having very low available water and low hydraulic conductivity. We conclude that an increase in soil-root hydraulic resistance in the dry season introduces a significant seasonal cycle to carbon and water fluxes from this tropical forest. Furthermore, our model structure appears to be an effective tool for regional and temporal scaling of C and LE fluxes, with primary data requirements being regional and temporal information on meteorology, leaf area index (LAI), foliar N, critical leaf water potentials, and plant and soil hydraulic characteristics.Keywords: Amazon basin; biosphere -atmosphere interactions; eddy covariance; hydraulic resistance; photosynthesis; soil-plant-atmosphere model. INTRODUCTIONAmong the most important components of global biogeochemical cycling are the processes that mediate the fluxes of C, water and energy between biosphere and atmosphere. A major difficulty in improving our understanding of the functioning of the biosphere-atmosphere system lies in the problem of effectively scaling measurements of the key processes, such as photosynthesis and evapotranspiration, to generate regional estimates of these fluxes (Ehleringer & Field 1993). Generally, the ability to make measurements of the various components of these coupled systems is strictly limited in some dimension of space and time, according to particular financial and technical constraints. Often it is only possible to collect data at a limited number of sites and/or for restricted periods. Yet, to understand the regional and global issues of present concern, there is a requirement for detailed understanding of processes as they occur over vast areas and long time scales (including the ...

Effect of the revisit interval and temporal upscaling methods on the accuracy of remotely sensed evapotranspiration estimates

“…Moreover, the magnitude of the moisture flux can vary over a range of timescales in response to changes in the environmental conditions influencing ET. One example of this, which has been pointed out by Williams et al (2003), Scott et al (2014), and others, is the rapid and often persistent change in ET in response to a rain event.…”

Seasonal variation in the maximum rate of leaf gas exchange of canopy and understory tree species in an Amazonian semi-deciduous forest

“…Since the soils in the study site drain rapidly after rainfall, the forest can experience significant drought during the dry season , Williams et al 1998. With drought plants tend to close their stomata due to increasing evaporative demand at the surface of the leaf, which in turn decreases leaf photosynthetic rates (Meinzer et al 1993), and even slight decreases in leaf water potential (Ψ L ) can trigger stomatal closing (Wright et al 1992, Allen andPearcy 2000).…”

Can seasonal and interannual variation in landscape CO<sub>2</sub> fluxes be detected by atmospheric observations of CO<sub>2</sub> concentrations made at a tall tower?

“…SPA provides WRF with surface fluxes of heat, water and CO 2 exchange in response to meteorological drivers through a close coupling of its hydrological and carbon cycles, based on eco-physiological principles (Williams et al, 1996). Detailed descriptions of the major SPA developments can be found in Williams et al (1996Williams et al ( , 1998Williams et al ( , 2001Williams et al ( , 2005, Sus et al (2010) and Smallman et al (2013). WRF provides SPA with meteorological drivers, including air temperature, precipitation, vapour pressure deficit (VPD), wind speed, friction velocity, atmospheric CO 2 mixing ratios, air pressure, and short-and long-wave incoming radiation.…”