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Summary Given that neither absolute measures nor direct model validations of global terrestrial net primary productivity (NPP) are feasible, intercomparison of global NPP models provides an effective tool to check model consistency. For this study, we tested the assumption that water availability is the primary limiting factor of NPP in global terrestrial biospheric models. We compared a water balance coefficient (WBC), calculated as the difference of mean annual precipitation and potential evapotranspiration to NPP for each grid cell (0.5° × 0.5° longitude/latitude) in each of 14 models. We also evaluated different approaches used for introducing water budget limitations on NPP: (1) direct physiological control on evapotranspiration through canopy conductance; (2) climatological computation of constraints from supply/demand for ecosystem productivity; and (3) water limitation inferred from satellite data alone. Plots of NPP vs. WBC showed comparable patterns for the models using the same method for water balance limitation on NPP. While correlation plots revealed similar patterns for most global models, other environmental controls on NPP introduced substantial variability. [ABSTRACT FROM AUTHOR]

Enough water is stored in the sapwood of large Douglas-fir to significantly contribute to transpiration. Sapwood water content falls through the season, causing the wood's conductivity to fall. This leads to low leaf-water potentials, stomatal closure, and reduced photosynthesis by the trees. The amount of water stored in the sapwood of Douglas-fir 50-60 m tall, growing in the Cascade Mountains of Oregon, was estimated periodically over two seasons from measurements of sapwood relative water content (Rs). The relationship between Rs and volume of water contained in the sapwood was determined in the laboratory, and an equation describing the variation of relative conductivity (K) with Rs was derived from the literature. Stomatal conductance (Ks) and leaf water potentials were measured in the field. The relative conductivity of the sapwood was calculated from estimates of the flow rate through the tree and differences in water potential between dawn and the time of comparison. Row rate was assumed to equal transpiration rate, calculated from the Penmam-Monteith equation using measured ks values. A sixfold decrease in K during the summer was attributed to changes in Rs. The maximum observed diurnal variation in K would require a change in Rs estimated at 25%. About 270 m3 ha-1 (27 mm) of water were stored in sapwood, and 75% of that was in the stemwood. Withdrawal from this store reached 1.7 mm day-1 on clear days after cloudy or rainy weather. Recharge could be almost as fast (up to 1.6 mm day-1) after rain, but was very slow if the foliage remained wet. [ABSTRACT FROM AUTHOR]

Regional scale changes to the hydrological cycle of the Murray-Darling Basin (MDB) in Australia have occurred as a result of European settlement 200 years ago. Replacement of deep-rooted perennial plants (trees) by shallower rooting plants (pastures and cropping) is of particular significance in altering water-tables and causing waterlogging and secondary salinization. The purpose was to locate the areas at risk of waterlogging and salinization as a result of tree clearing. To achieve this, present-day evaporation (ETI from 0.8% of the MDB (7750 km2) is compared with ET from a reconstruction of the pre-European condition. The spatial geographical database for the 155 × 50 km study area consisted of vegetation, soils, climate and topographic information at 1.6 × 1.6 km cell resolution (3072 individual cells for each data layer). Leaf area index (LAI) was used to define the amount of photosynthesizing and transpiring tissue. Present-day LAI was estimated for each cell using the Normalized Difference Vegetation Index calculated from NOAA-9 Advanced Very High Resolution Radiometer data. Pre-European LAI was obtained using the assumption that an equilibrium exists between LAI and climate—soil conditions. Daily climate was interpolated across the study area from Australian Bureau of Meteorology information and a microclimate simulator weather stations using topographic (MTCLIM). A water balance model previously described (Part II) was used to calculate past and present-day ET for each cell assuming that only the tree vegetation had changed. The simulation was confined to a 30-day period in autumn when a uniform senescent ground layer occurs. Maps of water use by trees for pre-European and present-day vegetation were produced. Differences in ET between pre-European and present-day vegetation were displayed spatially across the study area. These areas are interpreted as representing areas at risk from waterlogging and salinization. [ABSTRACT FROM AUTHOR]

Whole-tree potometers were used to estimate transpiration from two contrasting stands of 100-yr-old lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm.) forest growing on the same site in southeastern Wyoming. Although one stand had nearly seven times as any trees per hectare and 29% less biomass, both stands had nearly identical leaf area indices (7.3 and 7.1) and clear-day transpiration rates (3.3 and 3.4 mm/d). Individual tree basal area and maximum observed 24-h uptake were highly correlated, with the largest trees (20-26 cm dbh) transpiring 40-44 L on clear days in early summer. Maximum observed hourly uptake for the larger trees was 2.5-3.5 L, with total nighttime uptake being about 12% of 25-h uptake. On overcast days potometer uptake was reduced by 30-44%; during rainy periods uptake was reduced to nearly zero. The results are compared to data obtained with different methods by other investigators, with the conclusion that whole-tree potometers can be a useful tool for studies on tree water relations and for estimating short-term forest transpiration when leaf water potential is not limiting leaf conductance. [ABSTRACT FROM AUTHOR]

We have developed an approach which examines ecosystem function and the potential effects of climatic shifts. The Lake McDonald watershedof Glacier National Park was the focus for two linked research activities: acquisition of baseline data on hydrologic, chemical and aquatic organism attributes that characterize this pristine northern rockymountain watershed, and further developing the Regional Hydro-Ecosystem Simulation System (RHESSys), a collection of integrated models which collectively provide spatially explicit, mechanistically-derived outputs of ecosystem processes, including hydrologic outflow, soil moisture, and snowpack water equivalence. In this unique setting field validation of RHESSys, outputs demonstrated that reasonable estimatesof SWE and streamflow are being produced. RHESSys was used to predict annual stream discharge and temperature. The predictions, in conjunction with the field data, indicated that aquatic resources of the park may be significantly affected. Utilizing RHESSys to predict potential climate scenarios and response of other key ecosystem components can provide scientific insights as well as proactive guidelines for national park management. (KEY TERMS: wildland hydrology; ecological modeling; snow and ice hydrology; climate change; RHESSys model; watershed monitoring, aquatic ecosystems.) [ABSTRACT FROM AUTHOR]