Eric E. Small

Convergence across biomes to a common rain-use efficiency.

Water availability limits plant growth and production in almost all terrestrial ecosystems. However, biomes differ substantially in sensitivity of aboveground net primary production (ANPP) to between-year variation in precipitation. Average rain-use efficiency (RUE; ANPP/precipitation) also varies between biomes, supposedly because of differences in vegetation structure and/or biogeochemical constraints. Here we show that RUE decreases across biomes as mean annual precipitation increases. However, during the driest years at each site, there is convergence to a common maximum RUE (RUEmax) that is typical of arid ecosystems. RUEmax was also identified by experimentally altering the degree of limitation by water and other resources. Thus, in years when water is most limiting, deserts, grasslands and forests all exhibit the same rate of biomass production per unit rainfall, despite differences in physiognomy and site-level RUE. Global climate models predict increased between-year variability in precipitation, more frequent extreme drought events, and changes in temperature. Forecasts of future ecosystem behaviour should take into account this convergent feature of terrestrial biomes.

Assessing the Response of Terrestrial Ecosystems to Potential Changes in Precipitation

Abstract Changes in Earths surface temperatures caused by anthropogenic emissions of greenhouse gases are expected to affect global and regional precipitation regimes. Interactions between changing precipitation regimes and other aspects of global change are likely to affect natural and managed terrestrial ecosystems as well as human society. Although much recent research has focused on assessing the responses of terrestrial ecosystems to rising carbon dioxide or temperature, relatively little research has focused on understanding how ecosystems respond to changes in precipitation regimes. Here we review predicted changes in global and regional precipitation regimes, outline the consequences of precipitation change for natural ecosystems and human activities, and discuss approaches to improving understanding of ecosystem responses to changing precipitation. Further, we introduce the Precipitation and Ecosystem Change Research Network (PrecipNet), a new interdisciplinary research network assembled to encourage and foster communication and collaboration across research groups with common interests in the impacts of global change on precipitation regimes, ecosystem structure and function, and the human enterprise.

ECOHYDROLOGICAL IMPLICATIONS OF WOODY PLANT ENCROACHMENT

Increases in the abundance or density of woody plants in historically semiarid and arid grassland ecosystems have important ecological, hydrological, and socioeconomic implications. Using a simplified water-balance model, we propose a framework for con- ceptualizing how woody plant encroachment is likely to affect components of the water cycle within these ecosystems. We focus in particular on streamflow and the partitioning of evapotranspiration into evaporation and transpiration. On the basis of this framework, we suggest that streamflow and evaporation processes are affected by woody plant en- croachment in different ways, depending on the degree and seasonality of aridity and the availability of subsurface water. Differences in landscape physiography, climate, and runoff mechanisms mediate the influence of woody plants on hydrological processes. Streamflow is expected to decline as a result of woody plant encroachment in landscapes dominated by subsurface flow regimes. Similarly, encroachment of woody plants can be expected to produce an increase in the fractional contribution of bare soil evaporation to evapotrans- piration in semiarid ecosystems, whereas such shifts may be small or negligible in both subhumid and arid ecosystems. This framework for considering the effects of woody plant encroachment highlights important ecological and hydrological interactions that serve as a basis for predicting other ecological aspects of vegetation change—such as potential changes in carbon cycling within an ecosystem. In locations where woody plant encroach- ment results in increased plant transpiration and concurrently the availability of soil water is reduced, increased accumulation of carbon in soils emerges as one prediction. Thus, explicitly considering the ecohydrological linkages associated with vegetation change pro- vides needed information on the consequences of woody plant encroachment on water yield, carbon cycling, and other processes.

Simulation of regional-scale water and energy budgets: Representation of subgrid cloud and precipitation processes within RegCM

A new large-scale cloud and precipitation scheme, which accounts for the sub- grid-scale variability of clouds, is coupled to NCARs Regional Climate Model (RegCM). This scheme partitions each grid cell into a cloudy and noncloudy fraction related to the average grid cell relative humidity. Precipitation occurs, according to a specified autoconversion rate, when a cloud water threshold is exceeded. The specification of this threshold is based on empirical in-cloud observations of cloud liquid water amounts. Included in the scheme are simple formulations for raindrop accretion and evaporation. The results from RegCM using the new scheme, tested over North America, show significant improvements when compared to the old version. The outgoing longwave radiation, albedo, cloud water path, incident surface shortwave radiation, net surface radiation, and surface temperature fields display reasonable agreement with the observations from satellite and surface station data. Furthermore, the new model is able to better represent extreme precipitation events such as the Midwest flooding observed in the summer of 1993. Overall, RegCM with the new scheme provides for a more accurate representation of atmospheric and surface energy and water balances, including both the mean conditions and the variability at daily to interannual scales. The latter suggests that the new scheme improves the models sensitivity, which is critical for both climate change and process studies.

Dynamics of evapotranspiration in semiarid grassland and shrubland ecosystems during the summer monsoon season, central New Mexico

higher at the grassland than at the shrubland by 20% or 70 W m � 2 because of differences in net radiation (Rn) and soil heat flux (G). At both sites, midday evaporative fraction and daily ET are strongly correlated with surface soil moisture (q0–5cm) but poorly correlated with water content at greater depths or averaged throughout the entire root zone. The sensitivity of EF to q0–5cm is 30% lower at the grassland site. The differences in Qa and EF cancel, yielding similar time series of ET at the two sites. Decreases in q0–5cm, ET, and EF following rainfall events are rapid: exponential time constants are less than 3 days. With the exception of the largest storms, infiltration following rainfall events only wets the top 10 cm of soil. Therefore the surface soil layer is the primary reservoir for water storage and source for ET during the monsoon season, suggesting that direct evaporation is a large component of ET. Given these results, predicting ET based on root zone–averaged soil moisture is inappropriate in the semiarid environments studied here. INDEX TERMS: 1818 Hydrology: Evapotranspiration; 1833 Hydrology: Hydroclimatology; 1866 Hydrology: Soil moisture; 1878 Hydrology: Water/energy interactions; KEYWORDS: Bouteloua eriopoda, Bowen ratio, evapotranspiration, grassland, Larrea tridentata, shrubland Citation: Kurc, S. A., and E. E. Small (2004), Dynamics of evapotranspiration in semiarid grassland and shrubland ecosystems during the summer monsoon season, central New Mexico, Water Resour. Res., 40, W09305, doi:10.1029/2004WR003068.

Association between Plant Canopies and the Spatial Patterns of Infiltration in Shrubland and Grassland of the Chihuahuan Desert, New Mexico

AbstractShrubs have invaded extensive areas of grassland in the southwestern United States. The zones of nutrient-rich soil found beneath plant canopies, referred to as “islands of fertility,” are more intense and spaced farther apart in shrubland than in grassland. This difference in the spatial pattern of soil nutrients may reinforce shrub invasion. Changes in water availability in the soil could also influence shrub invasion. Here we compare the spatial patterns of infiltration, defined as the total equivalent water depth entering the soil following individual rainfall events or summed over many events, at adjacent grass- and shrub-dominated sites in the Sevilleta National Wildlife Refuge. We use two infiltration data sets. First, following four rainfall events, we measured soil moisture and wetting front depth at 10-cm intervals along 24-m transects. We estimate infiltration from these data. Second, we use vertical arrays of soil moisture probes to compare infiltration between adjacent canopies and interspaces following 31 storms. In both the grassland and shrubland, infiltration is typically greater beneath plant canopies than beneath interspaces. Canopies are oases where soil moisture is higher than in the surrounding areas. However, infiltration is not greater beneath canopies when surface runoff is limited. In the shrubland, the canopy–interspace infiltration ratio increases as storm size, and therefore runoff, increases. This relationship also exists in the grassland, but it is not as strong or clear. The magnitude of spatial variability of infiltration is similar in shrubland and grassland. In addition, the distance over which infiltration is correlated is approximately 50 cm in both environments. Most of the spatial variability exists between the stem and canopy margin in the shrubland and straddling the canopy margin in the grassland. The most notable difference is that subcanopy oases are spread farther apart in the shrubland because canopies are separated by larger interspaces in this environment.

Estimates of the rate of regolith production using 10Be and 26Al from an alpine hillslope

Abstract The production of regolith is a fundamental geomorphic process because most surface processes transport only unconsolidated material. We use concentrations of the cosmogenic radionuclides (CRNs) 10 Be and 26 Al in regolith and bedrock to deduce the rate of production of regolith on an alpine hillslope in the Wind River Range, WY. These calculations are based on a theoretical model which we develop here. This model shows that it is important to consider dissolution of regolith in regolith production and in basin-averaged erosion rate studies. Rates of production of regolith are uniform along the hillslope and the mean rates for the entire hillslope deduced from 10 Be and 26 Al are 14.3±4.0 and 13.0±4.0 m Ma−1, respectively. Rates of production of regolith deduced from 10 Be concentrations in regolith-mantled bedrock support the rates deduced from regolith concentrations. In the alpine environment examined here, the rate of production of regolith beneath ∼90 cm of regolith is nearly twice as fast as the average rate of production of regolith on bare rock surfaces, which Small et al. [Small, E.E., Anderson, R.S., Repka, J.L., Finkel, R., 1997. Erosion rates of alpine bedrock summit surfaces deduced from in situ 10 Be and 26 Al . Earth and Planetary Science Letters 150, 413–425] previously documented. Rock-mantled with regolith probably weathers more rapidly than bare rock because the water required for frost weathering is limited on bare rock surfaces. Because the hillslope examined here is convex with constant curvature and regolith production and thickness are uniform down the slope, the regolith volume flux must be proportional to the local slope of the hillside. Therefore, our results are consistent with Gilberts [Gilbert, G.K., 1909. The convexity of hilltops. Journal of Geology 17, 344–350] steady state hillslope hypothesis. If tor height and the difference between rates of weathering on bare and regolith-mantled rock provide a fair estimate of the age of summit flats, steady-state hillslope conditions have been attained in less than several million years.

EROSION RATES OF ALPINE BEDROCK SUMMIT SURFACES DEDUCED FROM IN SITU 10BE AND 26AL

We have measured the concentration of in situ produced cosmogenic 10Be and 26Al from bare bedrock surfaces on summit flats in four western U.S. mountain ranges. The maximum mean bare-bedrock erosion rate from these alpine environments is 7.6 ± 3.9 m My−1. Individual measurements vary between 2 and 19 m My−1. These erosion rates are similar to previous cosmogenic radionuclide (CRN) erosion rates measured in other environments, except for those from extremely arid regions. This indicates that bare bedrock is not weathered into transportable material more rapidly in alpine environments than in other environments, even though frost weathering should be intense in these areas. Our CRN-deduced point measurements of bedrock erosion are slower than typical basin-averaged denudation rates (∼ 50 m My−1). If our measured CRN erosion rates are accurate indicators of the rate at which summit flats are lowered by erosion, then relief in the mountain ranges examined here is probably increasing. We develop a model of outcrop erosion to investigate the magnitude of errors associated with applying the steady-state erosion model to episodically eroding outcrops. Our simulations show that interpreting measurements with the steady-state erosion model can yield erosion rates which are either greater or less than the actual long-term mean erosion rate. While errors resulting from episodic erosion are potentially greater than both measurement and production rate errors for single samples, the mean value of many steady-state erosion rate measurements provides a much better estimate of the long-term erosion rate.

Pleistocene relief production in Laramide mountain ranges, western United States

Gently sloped summits and ridges (collectively referred to as summit flats) are abundant in many Laramide ranges in the western United States. The erosion rate of summit flats is ~10 m/m.y., on the basis of the concentrations of cosmogenic radionuclides. Because erosion rates in valleys between summit flats are an order of magnitude faster, relief within these ranges is currently increasing by about 100 m/m.y. If summit-flat erosion is slower than rock uplift driven by the isostatic response to valley erosion, then this relief production could result in increased summit elevations. The mean depth of material eroded from a smooth surface fit to existing summit flats varies from 280 to 340 m in four Laramide ranges, based on geographic information system (GIS) analyses of digital elevation models. This erosion would result in a maximum of 250‐300 m of rock uplift, assuming Airy isostasy. However, because the Laramide ranges examined here are narrow relative to the flexural wavelength of the lithosphere, erosionally driven rock uplift is limited to ~ 50‐100 m. Over the past several million years, summit erosion would approximately offset this rock uplift. Therefore, we conclude that summit elevations have remained essentially constant even though several hundred meters of relief has been produced. On the basis of valley and summit erosion rates and the average depth of erosion, we estimate that relief production in Laramide ranges began at ca. 3 Ma. We hypothesize that this relief production was climatically driven and was associated with the onset or enhancement of alpine glaciation in these ranges.

Geomorphically Driven Late Cenozoic Rock Uplift in the Sierra Nevada, California

Geologists have long accepted that the Sierra Nevada, California, experienced significant late Cenozoic tectonically induced uplift. A flexural-isostatic model presented here shows, however, that a large fraction of the primary evidence for uplift could be generated by the lithospheric response to coupled erosion of the Sierra Nevada and deposition in the adjacent Central Valley and therefore requires less tectonic forcing than previously believed. The sum of range-wide erosion and the resultant isostatic rock uplift would have lowered Sierra mean elevation by 200 to 1000 meters since 10 million years ago and could also have increased summit elevations during the current period of relief production.