Fairley J. Barnes

Tree die‐off in response to global change‐type drought: mortality insights from a decade of plant water potential measurements

Global climate change is projected to produce warmer, longer, and more frequent droughts, referred to here as “global change-type droughts”, which have the potential to trigger widespread tree die-off. However, drought-induced tree mortality cannot be predicted with confidence, because long-term field observations of plant water stress prior to, and culminating in, mortality are rare, precluding the development and testing of mechanisms. Here, we document plant water stress in two widely distributed, co-occurring species, pinon pine (Pinus edulis) and juniper (Juniperus monosperma), over more than a decade, leading up to regional-scale die-off of pinon pine trees in response to global change-related drought. Pinon leaf water potentials remained substantially below their zero carbon assimilation point for at least 10 months prior to dying, in contrast to those of juniper, which rarely dropped below their zero-assimilation point. These data suggest that pinon mortality was driven by protracted water stress,...

OVERSTORY‐IMPOSED HETEROGENEITY IN SOLAR RADIATION AND SOIL MOISTURE IN A SEMIARID WOODLAND

We thank Clif Meyer, Susan Johnson, Katherine Dayem, Laura Campbell, Robert Lucero, Teng-Chiu Lin, Diana A. Heisler, Kathy E. Lee, and Chris Heil for research assistance; John W. Nyhan for precipitation data; Mary Lu Breshears for editorial assistance; Shawki A. Ibrahim, William K. Lauenroth, and F. Ward Whicker for guidance; and Craig D. Allen, Christopher Field, Geoffrey M. Henebry, Bruce T. Milne, Bradford P.Wilcox, and an anonymous reviewer for their very helpful reviews. This work was supported by the Los Alamos National Environmental Research Park, Los Alamos National Laboratory Environmental Restoration Project, Kansas Applied Remote Sensing Program, the Kansas Biological Survey, and the University of Kansas Research Development and General Research Funds.

Scales of above-ground and below-ground competition in a semi-arid woodland detected from spatial pattern

Abstract. Semi-arid woodlands are two-phase mosaics of canopy and inter-canopy patches. We hypothesized that both aboveground competition (within canopy patches), and below-ground competition (between canopy patches), would be important structuring processes in these communities. We investigated the spatial pattern of trees in a Pinus edulis-Juniperus monosperma woodland in New Mexico using Ripleys K-function. We found strong aggregation of trees at scales of 2 to 4 m, which indicates the scale of canopy patches. Canopy patches were composed of individuals of both species. Crown centers of both species were always less aggregated than stem centers at scales less than canopy patch size, indicating morphological plasticity of competing crowns. In the smallest size classes of both species, aggregation was most intense, and occurred over a larger range of scales; aggregation decreased with increasing size as is consistent with density-dependent mortality from intraspecific competition. Within canopy patches, younger trees were associated with older trees of the other species. At scales larger than canopy patches, younger trees showed repulsion from older conspecifics, indicating below-ground competition. Hence, intraspecific competition was stronger than interspecific competition, probably because the species differ in rooting depth. Woodland dynamics depend on the scale and composition of canopy patches, aggregated seed deposition and facilitation, above- and below-ground competition, and temporal changes in the spatial scale of interactions. This woodland is intermediate in a grassland-forest continuum (a gradient of increasing woody canopy cover) and hence we expected, and were able to detect, the effects of both above- and below-ground competition.

The Boardman Regional Flux Experiment

Abstract A field campaign was carried out near Boardman, Oregon, to study the effects of subgrid-scale variability of sensible-and latent-heat fluxes on surface boundary-layer properties. The experiment involved three U.S. Department of Energy laboratories, one National Oceanic and Atmospheric Administration laboratory, and several universities. The experiment was conducted in a region of severe contrasts in adjacent surface types that accentuated the response of the atmosphere to variable surface forcing. Large values of sensible-heat flux and low values of latent-heat flux characterized a sagebrush steppe area; significantly smaller sen- sible-heat fluxes and much larger latent-heat fluxes were associated with extensive tracts of irrigated farmland to the north, east, and west of the steppe. Data were obtained from an array of surface flux stations, remote-sensing devices, an instrumented aircraft, and soil and vegetation measurements. The data will be used to address the problem of extrapolating from a...

DEVELOPMENT OF SPECIES DOMINANCE ALONG AN ELEVATIONAL GRADIENT: POPULATION DYNAMICS OF PINUS EDULIS AND JUNIPERUS MONOSPERMA

We evaluated species‐environment relationships within piñon‐juniper woodlands in northern New Mexico (United States) using canonical correspondence analysis (CCA). The first CCA axis was associated primarily with elevation. Our results showed separation between piñon and juniper along the elevation gradient, as expected: piñon is relatively more dominant at higher sites, whereas juniper is relatively more dominant at lower sites. To examine how this pattern of dominance might emerge with time, we plotted the position of centroids for three piñon and juniper size classes along the first CCA axis. We found that small piñons and junipers were distributed relatively uniformly across the gradient, whereas large piñons and junipers were strongly segregated along the gradient, with intermediate‐sized piñons and junipers intermediate on the CCA axis between small and large. This produced a pattern of increased divergence between the two species that increased with size. We suggest that this pattern emerges as a result of differential mortality between the species rather than as a result of differences in seedling establishment along the gradient. These differences between the species could result from differences in resource use (i.e., physiology) and resource acquisition (i.e., rooting patterns relative to plant available water). We present a conceptual model of how differences between the species in resource acquisition increase with size (age). We suggest that differences in resource acquisition between species, which increase as individuals mature, may play a greater role in determining species dominance along resource gradients than has been previously appreciated.

Lidar determination of horizontal and vertical variability in water vapor over cotton

Abstract A solar-blind lidar system is being developed for remote noninvasive measurement of atmospheric water vapor concentrations in three dimensions and over time. Based on a high-power KrF laser operating at 248 nm, the truck-mounted system has 1.5-m resolution of water vapor concentrations along the beam path from 100 m to 500 m from the truck location. Field tests in 1988 over an incomplete cotton canopy showed that the system (operating in a one-dimensional mode) was capable of detecting small changes in water mixing ratios close to the canopy. Both spatial and temporal trends agreed qualitatively with those measured with conventional point instrumentation. Vertical gradients measured by lidar exhibited a larger range than conventional hygrometric measurements, but the qualitative agreement suggests the potential for lidar to measure small spatial gradients. Further development of the system will allow us to observe the structural detail in turbulent processes over broad areas. Pattern analysis and correlation between time scans will permit the estimation of the movement rates of eddies and their contribution to the overall flux of water vapor.

A Water Balance Study of Four Landfill Cover Designs at Material Disposal Area B in Los Alamos, New Mexico

The goal of disposing of low-level radioactive and hazardous waste in shallow landfills is to reduce risk to human health and the environment by isolating contaminants until they no longer pose an unacceptable hazard. In order to achieve this, the Department of Energy Environmental Restoration Program is comparing the performance of several different surface covers at Material Disposal Area (MDA) B in Los Alamos. Two conventional landfill were compared with an improved cover designed to minimize plant and animal intrusion and to minimize water infiltration into the underlying wastes. The conventional covers varied in depth and both conventional and improved designs had different combinations of vegetation (grass verses shrub) and gravel mulch (no mulch verses mulch). These treatments were applied to each of 12 plots and water balance parameters were measured from March1987 through June 1995. Adding a gravel mulch significantly influenced the plant covered field plots receiving no gravel mulch averaged 21.2% shrub cover, while plots with gravel had a 20% larger percent cover of shrubs. However, the influence of gravel mulch on the grass cover was even larger than the influence on shrub cover, average grass cover on the plots with no gravel was 16.3%, compared with a 42% increase in grass cover due to gravel mulch. These cover relationships are important to reduce runoff on the landfill cover, as shown by a regression model that predicts that as ground cover is increased from 30 to 90%,annual runoff is reduced from 8.8 to 0.98 cm-a nine-fold increase. We also found that decreasing the slope of the landfill cover from 6 to 2% reduced runoff from the landfill cover by 2.7-fold. To minimize the risk of hazardous waste from landfills to humans, runoff and seepage need to be minimized and evapotranspiration maximized on the landfill cover. This has to be accomplished for dry and wet years at MDA B. Seepage consisted of 1.9% and 6.2% of the precipitation in the average and once in ten year events, respectively, whereas corresponding values for runoff were 13% and 16%; these changes were accompanied by corresponding decreases in evapotranspiration, which accounted for 86% and only 78% of the precipitation occurring on the average and once in ten year even~ respectively.

Pilotstudie Über Verfahren der Oberflächenstabilisierung bei Endlagern in Oberflächennahen Formationen im Südwesten der U.S.A.

in semiariden Klimaten hangt die langfristige Integritat einer Abfalldeponie auch von der Regelung des Wassergleichgewichts am Standort ab. Abflus und Erosion sind haufig als Folge der kurzen, aber hochintensiven Sommersturme besonders stark. Versuche zur Eindammung der Erosion konnen zu verstarkter Infiltration von Wasser fuhren und erhohen damit das Risiko, das Wasser in das Deponiegut eindringt und spater Schadstoffe aus der Deponie in das Grundwasser migrieren. Oberflachen-Stabilisierungsverfahren mit dem Ziel, die Schadstoffmigration aus der Unterflur und dem Mull zu minimieren, mussen sich darauf konzentrieren, den einander widersprechenden Anforderungen an die Kontrolle einerseits der Erosion und andererseits der tiefen Perkolation des Wassers unterhalb der Wurzelzone gerecht zu werden.

Pilot Study of Surface Stabilization Techniques for Shallow-Land Burial Sites in the South-Western U.S.A

In semi- arid climates, long- term integrity of a waste burial site depends in part on managing site water balance. Runoff and erosion are often particularly severe as a result of short- duration, high intensity summer storms. Attempts to mitigate erosion can cause enhanced infiltration of water, thus increasing the chances of seepage of water into the waste material and subsequent migration of contaminants from the site into the ground water. Surface stabilization techniques for the purpose of minimizing contaminant migration from subsurface soils and wastes must focus on resolving the conflicting demands of controlling erosion and the deep percolation of water below the plant rooting zone.