Seasonal drought stress in the Amazon: Reconciling models and observations

Abstract

[1] The Amazon Basin is crucial to global circulatory and carbon patterns due to the large areal extent and large flux magnitude. Biogeophysical models have had difficulty reproducing the annual cycle of net ecosystem exchange (NEE) of carbon in some regions of the Amazon, generally simulating uptake during the wet season and efflux during seasonal drought. In reality, the opposite occurs. Observational and modeling studies have identified several mechanisms that explain the observed annual cycle, including: (1) deep soil columns that can store large water amount, (2) the ability of deep roots to access moisture at depth when near-surface soil dries during annual drought, (3) movement of water in the soil via hydraulic redistribution, allowing for more efficient uptake of water during the wet season, and moistening of near-surface soil during the annual drought, and (4) photosynthetic response to elevated light levels as cloudiness decreases during the dry season. We incorporate these mechanisms into the third version of the Simple Biosphere model (SiB3) both singly and collectively, and confront the results with observations. For the forest to maintain function through seasonal drought, there must be sufficient water storage in the soil to sustain transpiration through the dry season in addition to the ability of the roots to access the stored water. We find that individually, none of these mechanisms by themselves produces a simulation of the annual cycle of NEE that matches the observed. When these mechanisms are combined into the model, NEE follows the general trend of the observations, showing efflux during the wet season and uptake during seasonal drought.

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

“…In recent years, attention has been almost singularly focused on fixing the supply side of the problem, implementing deep soil and/or deep roots (Ichii et al, 2007;Baker et al, 2008;Grant et al, 2009;Harper et al, 2010;Verbeeck et al, 2011), root hydraulic redistribution (Lee et al, 2005), unconfined aquifers (Oleson et al, 2008;Fan and Miguez-Macho, 2010;Miguez-Macho and Fan, 2012), or changes to the numerical solution of the Richards equation for soil water fluxes (Zeng and Decker, 2009) to improve seasonal patterns of soil moisture and/or the seasonality of ecosystem metabolism. Despite the attention given to these ecohydrological mechanisms, little is known as to the relative contribution of soil physical versus biological mechanisms mediating supply.…”

Earth System Model Needs for Including the Interactive Representation of Nitrogen Deposition and Drought Effects on Forested Ecosystems

“…Although models tend to have homogeneous soil horizontally (in a grid cell), the vertical structure of resources is dynamic. Allowing roots to proliferate in soil layers where resources are concentrated gives PFTs the chance to adapt to changes in environment and can further change the vertical distribution of C and N. Baker et al [63] improved the modeled Net Ecosystem Exchange cycle in the Simple Biosphere Model compared with observations in the Amazon by adding hydraulic redistribution and soil depth to 10 m. Other elements of root systems that should be included in models are root order and classification (which will differ in respiration, uptake, turnover, and storage capacity), root phenology and turnover, and resource uptake response to heterogeneity of resources [133]. Warren et al [121] provided additional suggestions for improving root representation in models, including scaling root function across temporal and spatial scales and including root traits that inform function and hydraulic redistribution.…”

“…There are many ecological traits that can make a system more or less tolerant to drought, thereby leading to improved water use efficiency, including control over stomatal conductance, allometric plasticity, hydraulic redistribution [63], or even long-term acclimation. To survive drought, plants may also reduce C demand, which can be achieved by leaf senescence and the down-regulation of respiration [64].…”

Representing the root water uptake process in the Common Land Model for better simulating the energy and water vapour fluxes in a Central Asian desert ecosystem

“…The amount of EC data is still climbing year by year. The increase in EC data obtained from various terrestrial land surfaces facilitates research into poorly represented or missing ecosystem processes in models, leading to improvements of the model's performance (Baldocchi et al, 2001;Baker et al, 2008;Stockli et al, 2008;Williams et al, 2009;Choi et al, 2010;Schwalm et al, 2010;Li et al, 2011b). Commonly used LSMs include SiB (Sellers et al, 1986), Common Land Model (CLM) (Dai et al, 2003), ORCHIDEE (Krinner et al, 2005), CABLE (Kowalczyk et al, 2006) and their updated versions (Sellers et al, 1996;Wang et al, 2010;Bonan et al, 2011).…”

“…For example, IBIS and TEM (Saleska et al, 2003), SiB3 (Baker et al, 2008), and CABLE (Li et al, 2012) all required modification before they could reproduce the observed latent heat flux and net ecosystem exchange in Amazon forest where rainfall varied seasonally and obvious wet and dry seasons appeared. Some ecophysiological or ecohydrological processes such as modified root water uptake function (RWUF) and hydraulic redistribution must be reformulated or incorporated into the model to improve the model's performance (Baker et al, 2008;Li et al, 2012). The essence is that the plants may have adopted to the seasonality of rainfall by means of morphological adjustment in developing rich and deep root systems for utilizing deep soil water during dry season (Davidson et al, 2011), and this is notoriously difficult to model.…”