Carolyn Y. Ivans

HYDRAULIC REDISTRIBUTION THROUGH THE ROOT SYSTEMS OF SENESCED PLANTS

Hydraulic redistribution, the movement of water from soil layers of higher water potential to layers of lower water potential through the root systems of plants, has been documented in many taxa worldwide. Hydraulic redistribution is influenced principally by physical properties of roots and soils, and it should occur whenever root systems span soil layers of different water potential. Therefore, hydraulic redistribution should occur through the root systems of plants with aboveground tissue removed or through the root systems of fully senesced plants as long as roots remain intact and hydrated. We examined our hypothesis in field and greenhouse studies with the annual grass Bromus tectorum. We used soil psychrometry to measure soil water potential and performed 2H-labeling exper- iments. In the field, following senescence of B. tectorum, we show substantial changes in soil water potential consistent with both upward and downward movement of water through roots. The amount of water redistributed represents a significant proportion of that which can be stored in the rooted zone. We also experimentally demonstrated upward movement of a 2H label by roots of senesced plants and by roots of plants without aboveground tissues. In the greenhouse, we further demonstrated redistribution by senesced individuals using a 2H label. Hydraulic redistribution through the roots of senesced plants should receive further attention because it may have important ecological consequences for soil water recharge, survival of plants through drought, and agricultural practices.

Water conservation in Artemisia tridentata through redistribution of precipitation

Water conservation is important for plants that maintain physiologically active foliage during prolonged periods of drought. A variety of mechanisms for water conservation exist including stomatal regulation, foliage loss, above- and below-ground allocation patterns, size of xylem vessels and leaf pubescence. Using the results of a field and simulation study with Artemisia tridentata in the Great Basin, USA, we propose an additional mechanism of water conservation that can be used by plants in arid and semi-arid environments following pulses of water availability. Precipitation redistributed more uniformly in the soil column by roots (hydraulic redistribution of water downward) slows the rate at which this water can subsequently be taken up by plants, thus prolonging water availability during periods of drought. By spreading out water more uniformly in the soil column at lower water potentials following precipitation events, water use is reduced due to lower soil conductivity. The greater remaining soil water and more uniform distribution result in higher plant predawn water potentials and transpiration rates later in the drought period. Simulation results indicate that plants can benefit during drought periods from water storage following both summer rain events (small summer pulses) and overwinter recharge (large spring pulse). This mechanism of water conservation may aid in sustaining active foliage, maintaining root-soil hydraulic connectivity, and increasing survival probability of plants which remain physiologically active during periods of drought.

Functional Differences in Soil Water Pools: a New Perspective on Plant Water Use in Water-Limited Ecosystems

Arid and semi-arid ecosystems cover roughly half of the earths surface. Significant changes in vegetation cover combined with climate change have increased concern over the future of these lands, which have considerable economic importance. Much research has focused on plant-soil water relations in these systems, yet many mechanisms and significance of water use patterns are not well under- stood. Here we describe a new conceptual model that considers two pools of soil water accessed by plants: a growth pool that is located in shallow soil layers, and a maintenance pool that is often in deeper soil layers. While they may be spatially and

Response of Water Vapor and CO2 Fluxes in Semiarid Lands to Seasonal and Intermittent Precipitation Pulses

Abstract Precipitation pulses are important in controlling ecological processes in semiarid ecosystems. The effects of seasonal and intermittent precipitation events on net water vapor and CO2 fluxes were determined for crested wheatgrass (Agropyron desertorum), juniper (Juniperus osteosperma), and sagebrush (Artemisia tridentata) ecosystems using eddy covariance measurements. The measurements were made at Rush Valley, Utah, in the northern Great Basin of the United States. Data were evaluated during the growing seasons of 2002 and 2003. Each of these communities responds to precipitation pulses in all seasons, but these responses vary among season and ecosystem, and differ for water vapor and CO2. The degree and direction of response (i.e., net uptake or efflux) depended upon the timing and amount of precipitation. In early spring, both evapotranspiration (ET) and CO2 fluxes responded only slightly to precipitation pulses because soils were already moist from snowmelt and spring rains. As soils dried lat...

Gas exchange and growth responses of the desert shrubs Artemisia tridentata and Chrysothamnus nauseosus to shallow- vs. deep-soil water in a glasshouse experiment

Abstract The aridland shrub species, Artemisia tridentata (big sagebrush) and Chrysothamnus nauseosus (rubber rabbitbrush), are distributed widely in the Intermountain region of western North America. Earlier research indicated that A. tridentata can utilize upper soil water from transient summer rain events while C. nauseosus apparently cannot, although both species have similar rooting depths. Thus, we hypothesized that C. nauseosus relies more on deep water than A. tridentata , while A. tridentata can take advantage of soil moisture in upper soil layers. We examined this hypothesis by growing A. tridentata and C. nauseosus in two-layer pots in which soil water content in the upper and lower layers was controlled independently. After plants were well established, they were subjected to one of the three water treatments: water applied both to upper and lower layers, water applied only to the upper layer, or water applied only to the lower layer. We measured above- and belowground biomass, leaf gas exchange and leaf carbon isotope composition (δ 13 C). Close to the end of the experiment, a deuterium- and 15 N-labeled solution was applied in a localized patch in either the upper or lower soil layer to quantify resource-pulse utilization. In general, our hypothesis was supported; δ 13 C and biomass indicated that A. tridentata performed better when water was available in the upper soil layer and gas exchange, and δ 13 C indicated that C. nauseosus performed better when water was available in the lower layer. There was, however, no significant variation among treatments for many of the variables examined. Roots of both species were involved in hydraulic redistribution of soil water between layers, which may have reduced the effectiveness of the water distribution treatments to some extent.