Mechanisms of water flux
As was observed after the first 3 yr of the Tapajós exclusion experiment (Belk et al., 2007), a one-dimensional hydrologic model using unsaturated flow accurately simulated VWC over the entire soil profile. In the Caxiuanã exclusion study, a similar physically based one-dimensional model also successfully simulated observed VWC (Fisher et al., 2007). In Caxiuanã, coefficients of determination between observed and predicted VWCs for the upper 3 m of the profile over 3 yr were 0.87 and 0.68 for control and treatment, respectively. At Tapajós, over the entire study, whole profile water contents were well predicted (Fig. 7), although the strong drought in 2003 clearly affected the goodness of fit of the model under the control. Nonetheless, under control conditions at Tapajós the model accurately simulated measured AET (Table 3) and the relative proportion of drainage to precipitation was well within the range of 18–63% previously reported for watershed studies in the Amazon (Bruijnzeel, 1990; Markewitz et al., 2006)
During these simulations, capillary rise (i.e. an upward hydraulic flux) was never induced across any of the lower layer boundaries even under the exclusion. Capillary rise was suggested by a deuterium tracer study within the Tapajós exclusion experiment and was estimated to account for as much as 32 cm of water moving up through the upper 240 cm of soil (Romero-Saltos et al., 2005). Capillary rise is possible within the hydrus® model architecture and can be induced, for example, if root uptake is restricted to the upper two soil layers, but then model fits with VWC through the profile are quite poor (data not shown). At Caxiuanã, capillary rise as a source of water to balance the water budget in the 5-m instrumented portion of the profile was not discussed (Fisher et al., 2006, 2007, 2008). Given the current model results it does not appear that large volumes of water are migrating upward through the profile as a result of capillary rise, although some mechanism for deuterium movement is occurring (e.g. gaseous phase movement or diffusion in biofilms).
The current simulation model did not include a mechanism for moving water through root redistribution. Although there is strong evidence for hydraulic redistribution in this location (Rocha et al., 2004; Oliveira et al., 2005) and others (Caldwell et al., 1998; Domec et al., 2010), the amount of water moved by these processes has rarely been quantified (Jackson et al., 2000; Ryel et al., 2002). The current results suggest that large volumes of hydraulic redistribution are not needed to accurately simulate VWC in either upper or lower layers. In the 0–40-cm layer, where relative errors in θ were greatest (i.e. 11%), the absolute water depth difference was < 1.5 cm and in the lower layers relative differences in θ of c. 5% were < 2 cm. Modeling an arid ecosystem with and without hydraulic lift, Ryel et al. (2002) found differences in θ ranging up to 22%, but for short time periods. The use of root uptake compensation in the present model, which allows for water uptake from deeper soil layers when the uptake restriction factor for a given layer (i.e. URF(z)) is positive, most accurately represented the water drawdown in deeper layers under the treatment conditions; a result consistent with the theoretical arguments of Šimůnek & Hopmans (2009). Some of the water removed from these lower layers may well have been interned in a shallower layer before transpiring through the canopy, as demonstrated in a temperate coniferous forest (Domec et al., 2010), but the timescale of VWC measures in this study does not capture these short-term processes.
Soil water limitations
How soil water limitations to gross or net primary productivity are simulated in these seasonally dry tropical rain forests has recently been a rich area of investigation (Lee et al., 2005; Fisher et al., 2007; Hutyra et al., 2007; Ichii et al., 2007; Saleska et al., 2007). Many early GCMs that used 1- or 3-m soil profiles presumed a seasonal water limitation to NPP. Currently, some empirical data (Hutyra et al., 2007) and modeling studies (Baker et al., 2008; Poulter et al., 2009) suggest that for many seasonally dry areas within the Amazon basin no such limitation exists in most years. The canopy water flux data (i.e. measured AET) generated in close proximity to this study from an eddy-flux tower demonstrated that, in contrast to dry season water limitation, increases in AET during the dry season were observed relative to the wet season (Hutyra et al., 2007). These AET data were used to drive soil water demand for the current simulations and, within the control plot, indicate a ready capacity for the soil profile through at least 5.5 m to provide sufficient water. On average the model estimate indicates that 58 ± 3, 73 ± 3, and 91 ± 1% of the water demand in the control plot was met by soil water from the upper 150, 250, and 550 cm of soil, respectively, which suggests that GCMs may have to incorporate a soil profile > 5 m to accurately assess water limitations. The model analyses of Ichii et al. (2007), simulating GPP in the Amazon basin, were quite consistent with this conclusion. In areas of the basin with longer dry seasons, deeper soil profiles (going from 3 to 10 m) were required to sustain GPP (Ichii et al., 2007). Utilization of hydraulic lift to supply sufficient soil water in GCMs, as done by Lee et al. (2005), is also possible but is still constrained by the availability of deep soil water for redistribution. Simply having deeply rooted soils with root water uptake compensation was sufficient in the present soil model to simulate VWC.
Of course, not all Amazonian soils are deep clay-rich Oxisols like those at the Tapajós exclusion experiment. Even in the Tapajós, Silver et al. (2000) found 0–10-cm clay contents to vary from 18 to 60%. In fact, it is estimated that Oxisols only cover 39% of the Amazon basin (Richter & Babbar, 1991) and soils within the Oxisol order may vary substantially. For example, at Caxiuanã soils are classified as an Oxisol (Latossolos amarelos in the Brazilian Classification) but possess only 9–20% clay in the upper 5 m (Ruvio et al., 2007) while, in contrast, the Oxisols at Tapajós (Latossolos vermelhos) possess > 70% clay in all layers above 5 m. The critical role of soil rooting depth was previously demonstrated in a fire sensitivity model (Nepstad et al., 2004). In this model a halving of soil rooting depth from 10 to 5 m doubled the area of Amazon forest that depleted soil to < 25% of the maximum plant available water, increasing its fire sensitivity. Clearly, conclusions about the role of deep root uptake will only be relevant were deep soil exists, but presently the ability to estimate these areas is limited.
Buffering by deep root uptake
Given the presence of deep soils, access to water reserves in these soils during drought may determine whether or not the tropical moist forests of Amazonia will be buffered from the deleterious effects of water deficits. The presence of roots at > 8 m in Amazonian Oxisols has been directly observed (Nepstad et al., 1994) and in moist tropical forests on average has been estimated to exceed 7 m (Canadell et al., 1996). The results from the Tapajós exclusion experiment are consistent with an ability of roots in these moist forests to extract water from soil depths up to 11.5 m (Figs 4, 5). During the first 3 yr of the exclusion (2000–2002) and particularly in 2001 and 2002 plants were able to sustain AET by increasing the absolute volume of water taken up from the deepest portions of the profile (750–1150 cm). The percentage of modeled root water uptake contributed by these depths under exclusion increased from 3% in 1999 and 2000 to 6 and 12% in 2001 and 2002, respectively (Table 4). In 2003 and 2004, however, virtually no additional water was utilized from these soil depths, and this is also when significant tree mortality began to be observed (Nepstad et al., 2007). During this study, it appeared that a contribution from deep soils of only c. 10% of water uptake was crucial for surviving dry periods.
In addition to increased mortality there was an observed decline in leaf area index of 21–26% in the exclusion plot from 2002 to 2005 and a decline in litterfall of 23% in 2003 and 10% in 2004 (Brando et al., 2008). Direct measures of sap flux for 27 trees in each plot also indicated declines in water utilization of up to 73% on average in the dry season of 2003 (Cardinot, 2008). These declines in leaf area or sap flux were associated with a modeled decline in AET of 3, 12, 7, and 5% in 2002–2005, respectively. Clearly, there was some reduction in plant transpiration but, given the soil physical basis of the model and the fact that in the simulations PET as well as root biomass were kept constant, if water was available it would have been transpired. Thus, in the model, lack of deep soil water uptake in 2003 and 2004 was driven by soil moisture conditions and a lack of soil moisture recharge in 2003 and 2004. Despite this lack of recharge, however, VWC at depth was still > 0.32 cm3 cm−3. One interpretation is that soil waters are held at high matric potentials at these depths which resist root uptake.
Knowledge of the hydraulic properties of soils and parameters representing those properties (i.e. θr, θs, α, and n) is critical for estimating plant available water (PAW) in deeper soil depths, which is typically estimated as
- ( Eqn 6)
Previous research at this site described the sensitivity of the present hydrologic model to these parameters (Belk et al., 2007). Sensitivity to θs was greatest with water contents increasing by 50% for a twofold increase in θs, while a twofold change in θr elicited a 25% change in water contents. These values differ, however, in that θs is a ‘physical’ parameter that is well constrained by the dry bulk density and particle density of the soil. By contrast, θr is not well constrained by measurement and is usually obtained through a fitting procedure (Hodnett & Tomasella, 2002). It remains unclear what is the lowest matric potential that plants can achieve for water extraction, as it depends upon a number of factors, including root density and osmotic potential, soil texture and the capacity of plants to resist xylem embolism (Sperry et al., 1998). A permanent wilting point at −1.5 MPa is commonly an assumed default value for this lower matric potential. Assuming the soil and leaves are in hydraulic equilibrium at pre-dawn, ‘average’ soil water potential may be inferred from pre-dawn leaf water potentials (Fisher et al., 2008). In the late dry season of 2002 and 2003, pre-dawn leaf water potential in the exclusion plot at Tapajôs were observed at −1.5 MPa (Nepstad et al., 2007).
Previously at this site calibrated θr values (c. 0.2 cm3 cm−3) were utilized that were lower than those determined analytically in the laboratory (c. 0.3 cm3 cm−3) or simulated in this study. This was done to achieve acceptable calibrations with VWC throughout the soil profile (Belk et al., 2007). Working in Paragominas, Brazil with a similar TDR network in deep soil pits, Jipp et al. (1998) also had trouble reconciling observed TDR values with laboratory-measured soil moisture retention curves, although in this location measured VWC was at times lower than VWC measured at −1.5 MPa in the laboratory. Fitted θr values exceeding 0.3 cm3 cm−3, which may indicate limitations to deep water uptake, were observed in 17% of 771 tropical soil profiles surveyed by Hodnett & Tomasella (2002).
High values of θr may be associated with limited deep soil water uptake, as suggested here, but the mechanism inducing this limitation is unclear. This physically based soil model emphasizes soil hydraulics rather than plant hydraulics, but both play a critical role (Sperry et al., 2002) and the presumption that high soil moisture potential (Ψsoil) exceeds suction potentials of plants (Ψplant) is only one possible mechanism available to limit deep soil water uptake. For example, very low unsaturated hydraulic conductivity with increasingly negative Ψsoil may result in loss of soil water conductivity in the rhizosphere between bulk soil and root surface, which could also limit water uptake (Newman, 1969). Similarly, cavitation in the root xylem may disrupt the cohesion–tension continuum and limit water uptake (Tyree & Sperry 1989), although some recent work suggests that the anatomical structure of deep roots may be well suited to minimize flow resistance and maximize deep water uptake (McElrone et al., 2004).
Research in the Caxiuanã exclusion experiment very specifically addressed the critical nature of the soil-to-root hydraulic resistance to water uptake (Fisher et al., 2007, 2008). The results obtained at Caxiuanã supported the idea that soil-to-root resistance exerted a strong control on transpiration relative to plant resistance (i.e. high-xylem resistance), particularly during the dry season. Under conditions of deep rooting as observed at Tapajôs, with a low density of fine-root biomass at depth and high θr, it is suggested that the soil-to-root resistance is critical in limiting deep root water uptake under extended drought.
Over the last 15 yr there has been a growing recognition that root water uptake in lowland tropical forests of the Amazon often extends to depths > 200 cm. Appropriately incorporating the mechanism of this root water uptake in models of forest function is important for predicting effects of forest management or climate change. In the current study, a 6-yr record of volumetric water contents from a throughfall exclusion experiment was simulated under control and treatment conditions with a one-dimensional vertically integrated version of the Richards mass balance equation. The simulation with root uptake compensation through an 11.5-m soil profile accurately simulated the seasonal, annual, and exclusion dynamics. The success of this model suggests that other processes of hydraulic flow such as capillary rise or hydraulic root distribution may not move large volumes of water in this system. Furthermore, contributions of deep root water uptake are crucial, with the 250 to 550-cm layer contributing c. 20% of water demand under control conditions, while the deepest layers (550–1150) contributed c. 10%. Under the exclusion, root water uptake was sustained for the first 2 yr but declined thereafter. In years 3 (2001) and 4 (2002) of the exclusion experiment (i.e. the second and third years of the throughfall exclusion), deep root water uptake increased on both an absolute and a relative basis. This increase was not sustained, however, and these deep layers contributed zero root water uptake in 2003 and 2004 despite high VWC (i.e. > 0.30 cm3 cm−3). It appears that the capacity for deep root uptake of water is limited by changing soil-to-root resistance under severe drought, which may in part result from the high matric potential of water retention in these high-clay soils. Hence, the deep rooting habit provides an important adaptation to seasonal drought, but its buffering capacity is limited for longer term reductions in soil moisture.