H54C-01 16:00h
Horizontal and vertical variability of soil moisture in savanna ecosystems
Soil moisture is a key hydrological variable that mediates the interactions between climate, soil, and vegetation dynamics in water-limited ecosystems. Because of the importance of water limitation in savannas, a number of theoretical models of tree-grass coexistence have been developed which differ in their underlying assumptions about the ways in which trees and grasses access and use soil moisture. However, clarification of the mechanisms that allow savanna vegetation to persist as a mixture of grasses and trees remains a vexing problem in both hydrological and vegetation science. A particular challenge is the fact that the spatial pattern of vegetation is both a cause and effect of variation in water availability in semiarid ecosystems. At landscape to regional scales, climatic and geologic constraints on soil moisture availability are primary determinants of vegetation structural pattern. However, at local to landscape scales the patchy vegetation structural mosaic serves to redistribute the availability of soil moisture in ways that have important consequences for structural dynamics and community composition. In this regard, the emerging field of ecohydrology is well suited to investigate questions concerning couplings between the patchy structural mosaic of savanna vegetation and the kinds self-organizing dynamics known to exist in other light and nutrient-limited vegetation systems. Here we address the role of patchy vegetation structure through the use of a lumped model of soil moisture dynamics that accounts for the effect of tree canopy on the lateral and vertical distribution of soil moisture. The model includes mechanisms for the drying of the ground surface due to soil evaporation in the sites with no tree cover, and for the lateral water uptake due to root invading areas with no canopy cover located in the proximity of trees. The model, when applied to a series of sites along a rainfall gradient in southern Africa, is able to explain the cover fractions observed in this region.
H54C-02 16:15h
Toward characterizing soil moisture and evaporative demand controls on photosynthesis using a coupled ecohydrological model
Vegetation modulates the effects of climate variability through soil- vegetation-atmosphere (SVAT) interactions. A quantitative understanding of such interactions requires the proper integration of the water cycle and photosynthesis. While biochemical models have been widely used to estimate primary production, the effects of water stress on transpiration and carbon assimilation rates, and its feedbacks into the water cycle are not generally represented. The objective of this study is to investigate the limiting effects of soil moisture and evaporative demand on photosynthesis, and to understand its interactions with other hydrological processes. Our approach consists of integrating a physically based land-surface hydrological model with a biochemical model for leaf photosynthesis and a substrate-structure separation model for respiration, including parameterizations of the diurnal cycle of Rubisco concentration and species-specific stomatal conductance (resistance). Exploratory simulations to evaluate the model against results from previous studies indicated that the model captures basic processes of canopy physiology well. Sensitivity analysis shows that water stress at sub-daily time-scales is an important limiting factor of photosynthesis, thus constraining carbon assimilation. On the other hand, the results further suggest that the biological control of transpiration via stomatal sensitivity is only significant under soil water stress conditions. Overall, the integrated model is capable of estimating not only carbon assimilation, but also the length of the growing season, as well as feedbacks between vegetation and soil hydrology processes.
http://www.duke.edu/cee/~barros
H54C-03 16:30h
Large Eddy Simulation of Canopy Flows Using Lagrangian Dynamic Model and Comparison with PIV Field Experimental data
The exchange between plants and the atmosphere is one of the main themes in ecohydrology. The realistic simulation of the turbulent flow around plants remains a major challenge. Canopy turbulence is characterized by momentum transfer through aerodynamic drag of foliage throughout the whole depth of the canopy, which leads to an unstable inflected mean velocity profile and enhances dissipation of turbulent kinetic energy by means of wake-scale eddies. We present results on the turbulence structures within and above a canopy at scales appropriate to a corn canopy, using large eddy simulation. The LES employs a dynamic Lagrangian subgrid-scale model. The corn canopy is simulated by two numerical approaches: global-scale and local-scale models. The former treats the canopy as a porous body of horizontally uniform area density. The latter accounts for the specific arrangement of corn plants, taking into account the heterogeneity of the corn canopy from a local-scale view. The computational results are extensively compared with our recent PIV measurements and two previous field experiments by Shaw et al. (1974) and Wilson et al. (1982). The numerical predictions of turbulence statistics and energy spectra are in good agreement with the experimental data. A quadrant analysis shows that sweep events dominate the momentum flux within the canopy while ejection events dominate above the canopy.
H54C-04 16:45h
Aspects of Radiation Budget, Subsurface Lateral Moisture Exchange, and Vegetation Function in Areas of Complex Topography.
There is evidence that topography strongly affects the state, function, and distribution of vegetation by controlling incoming solar radiation and lateral redistribution of soil moisture. However, numerical experiments studying the effects that a topography can have on vegetation have oversimplified the treatment of topography and/or the representation of vegetation. We investigate the control of topography on vegetation state and stress via detailed modeling of radiation and soil moisture budgets across the varied terrain of a watershed. A detailed vegetation-hydrology model parameterizes the processes of canopy radiative transfer and rainfall interception and couples the processes of infiltration and evapotranspiration to photosynthesis via moisture uptake through a root systems with varied profiles. The model is applied on a continuous basis to synthetic watersheds of topography dominated by either convex or concave hillslopes. The numerical analysis is carried out for several plant functional types and soils. Inferences from the spatially-distributed dynamics are used to examine topographic niches favorable to vegetation.
H54C-05 17:00h
Using Stable Isotopes to Trace Orographic Precipitation in a Tropical Montane Cloud Forest, Monteverde, Costa Rica
The ecology of the tropical montane cloud forests (TMCFs) in Monteverde, Costa Rica is influenced by sharp differences in precipitation patterns that vary with season, which is controlled by the migration of the Intertropical Convergence Zone (ITCZ) over Central America. The majority of precipitation to the region occurs during the wet season when the ITCZ brings convective rainfall. During the transitional and dry seasons, mist and fog are generated by orographic uplift of moisture carried by trade winds from the Atlantic. This hydrologic flux may be most important in sustaining canopy species during the dry season, yet it may also be most sensitive to changes in climate. To assess the viability of using stable isotopes to trace orographic precipitation through the environment, we sampled open precipitation and throughfall from June 2003-June 2004 in secondary and primary forests located on the leeward slope of the Cordillera Tilaran. The wet season yielded a wide range of isotopic values, from -13.9 to -3.1\permil (\delta$^{18}$O) and -103 to -13\permil (\delta$^{2}$H), and the lightest samples were collected when the ITCZ was positioned over Costa Rica. Rain and mist collected during the transitional and dry seasons are much heavier, ranging from -3.9 to -1.4\permil (\delta$^{18}$O) and -14 to +7\permil (\delta$^{2}$H), and are similar to cloud water compositions reported elsewhere. This observed variation is consistent with storms having different travel histories. Heavy compositions result from orographic events and compositionally light precipitation is produced from air masses with longer storm tracks. From the wet to the transitional season, d-excess values increase from +15 to +19\permil, and the highest d-excess values occur during the transitional and dry seasons when orographic precipitation dominates, indicating a recycled water contribution to the regional hydrologic cycle. The presence of a recycled water indicates that lowland deforestation, and an associated reduction in transpiration, could reduce precipitation in Monteverde, and this impact would be most severe in the dry season.
H54C-06 17:15h
Field observations of oxygen isotopes in two forest ecosystems - linking the water and carbon cycles at the ecosystem scale
Oxygen isotopes are valuable tools for studying the coupling of water and carbon cycles at the ecosystem scale. Our study focuses on plant foliage, where water and carbon are concurrently exchanged through the stomatal openings. We report photosynthetic $^{18}$O discrimination from branch bag experiments and $\delta$$^{18}$O signatures of plant water measured during field campaigns in a spruce plantation in Scotland and a beech forest in central Germany. The contrasting micro-climate and species characteristics at the two sites enabled us to investigate the response of plant gas exchange to fluctuating environmental conditions through their effects on foliage water $\delta$$^{18}$O signatures and photosynthetic $^{18}$O discrimination. The two sites show pronounced differences in magnitude and variability of $^{18}$O discimination. At both sites, the $\delta$$^{18}$O signatures of evaporating site foliage water played an important role in determining photosynthetic $^{18}$O discrimination. The extent of the $^{18}$O enrichment of evaporating site water, in turn, depended largely on foliage transpiration rates and their response to changes in environmental conditions. For example, the non steady state effects of transpiration limited foliage water turnover on evaporating site $\delta$$^{18}$O signatures were more apparent at the spruce site with smaller transpiration rates compared to the beech site. This resulted in 2 - 3 permil higher $^{18}$O discrimination on average for the spruce, but 1 - 2 permil lower for the beech branches. The results of our field measurements combined with a simple model of well mixed canopy air emphasize the importance of foliage transpiration in determining the $\delta$$^{18}$O signatures of water and CO$_{2}$ exchanged between ecosystems and the atmosphere.
H54C-07 17:30h
Cloud Water Interception at two Tropical Montane Cloud Forest Sites in Hawai'i
A preliminary analysis of fog gauge measurements and canopy water balance estimates is presented, based on field measurements at dry (Auwahi) and wet (Waikamoi) cloud forest sites on the Island of Maui. Estimates of cloud water flux are based on fog gauge observations, with corrections for the effects of windblown rainfall and varying wind direction. We estimate event totals of cloud water interception by the vegetation at each site based on throughfall measurements, incident rainfall estimates and calculated evaporation. Measured throughfall was about 65% of incident rainfall at Auwahi, and 119% of incident rainfall at Waikamoi. Throughfall is dominated by rainfall at both sites, and is significantly influenced by fog only at Waikamoi. Fog contributed to water input events at an average frequency of once every two days at Auwahi and about twice in three days at Waikamoi. Cloud water interception was equivalent to 268 mm yr$^{-1}$ at Auwahi and 1073 mm yr$^{-1}$ at Waikamoi. At Auwahi, however, the majority of intercepted water is evaporated from the wet vegetation, never reaching the ground. Cloud water interception is related to fog screen catch and cloud water flux at Waikamoi, but not at Auwahi.
http://water.usgs.gov/nrp/proj.bib/hawaii/maui_fog.htm
H54C-08 17:45h
Wet-Season Throughfall in Primary and Secondary Tropical Montane Cloud Forests, Monteverde, Costa Rica
From June 11 through July 23, 2004 throughfall was recorded four times per week at two sites in Monteverde, Costa Rica on the leeward side of the Cordillera de Tilaran. Each site comprised a regular grid of collectors that were not moved during the collection period and a set of roving collectors that were moved weekly. At one site, twenty-two collectors were spread over 144 m$^2$ in a primary tropical montane cloud forest. At the second site, thirty-two collectors were spaced over 192 m$^2$ in a secondary forest. Summed for the period of collection, open rainfall is 302 mm, mean throughfall at the primary forest site is 240 mm (80% of gross precipitation), and mean throughfall at the secondary forest site is 191 mm (63% of gross precipitation). Standard deviations of total throughfall among the regular grids of collectors are 114 mm for the primary forest (CV = 48%) and 57 mm for the secondary forest (CV = 30%). Histograms of throughfall at each site show positively skewed distributions with a few collectors receiving high volumes of water. Collectors that received high volumes of throughfall for one event, tended to receive high volumes for all events. This persistence indicates canopy and vegetation control of the spatial distribution of throughfall. Throughfall depths show weak correlation to percent canopy and understory cover, distance to nearest tree bole, and diameter of nearest tree, however. Variograms constructed for weekly throughfall totals indicate very short correlation lengths. The short correlation scale and lack of correlation between throughfall and tree location indicates that a random placement of gauges is appropriate for estimating throughfall in these environments.