Michael S. Peek

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

Non-destructive estimation of lateral root distribution in an aridland perennial

Estimation of root distributions in natural systems remains challenging due to the difficulties in excavation and easy breakage of fine roots. Identifying lateral fine root distribution is necessary to determine the potential exploitation of spatially and temporally variable nutrient supplies that characterize most arid ecosystems. We estimated this potential by taking field measurements of lateral root distribution of the small herbaceous perennial Cryptantha flava (A. Nels.) Payson using 15N-enriched nutrient solutions wicked into the soil at various distances from study plants. Leaves were subsequently harvested from these plants and analyzed for N isotopic ratios. C. flava plants were capable of N uptake at distances of greater than 1.0 m from the outer edge of their aboveground canopy. The considerable lateral root neighborhood area of C. flava increases the amount of spatially variable N that is exploitable in these low-N soils. The ability to acquire spatially variable N and rapidly translate N uptake into photosynthetic carbon gain are traits that aid C. flava in maintaining its position as a successful subordinate competitor in a community dominated by larger, woody perennials.

POTENTIAL CONTRIBUTION OF RESPIRATION BY ANABRUS SIMPLEX (MORMON CRICKETS) TO NET CO2 EXCHANGE IN THREE GREAT BASIN ECOSYSTEMS

Abstract Disturbance events can significantly influence net CO2 exchange (NCE) in ecosystems. High densities of Anabrus simplex (Mormon crickets) periodically afflict large areas of the western USA; their sheer numbers could make them a significant source of CO2. We modeled cricket respiration at the ecosystem level using air and body temperatures and insect gas exchange measurements. Cricket CO2 efflux values were compared to ecosystem CO2 flux from eddy covariance measurements in 3 Great Basin ecosystems: a juniper woodland, a sagebrush shrubland, and a crested wheatgrass pasture. Mean respiration from Mormon crickets was 0.96 g CO2 · m−2d−1. Since Mormon crickets are present when NCE is otherwise near 0, they can potentially alter NCE between 20% (juniper woodland) and 60% (crested wheatgrass pasture). Transient pests such as Mormon crickets can be an important component of NCE.