No abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecological Applications.Abstract. Water use by the introduced shrub Tamarix ramosissima and three co-occurring, native phreatophytes was measured in the lower Virgin River floodplain (southern Nevada) using the stem-heat-balance method. During the 1993 growing season, measurements were conducted on Tamarix in a closed, monospecific stand and in a mixed community with the native species Pluchea sericea, Prosopis pubescens, and Salix exigua. Our objectives were (1) to determine whether leaf-area-based water use of Tamarix is higher than that of co-occurring native riparian taxa, (2) to assess the role of Tamarix stand leaf area index (LAI, leaf area per unit ground area) on Tamarix water loss, and (3) to verify whether Tamarix is capable of using large amounts of water under the extreme evaporative demands that characterize arid environments. Leaf-area-based sap flow rates were comparable in the four species despite large differences in individual leaf area and total water loss. Daily water use of Tamarix (in grams per day) weighted by the daily potential evapotranspirati-on (PET) increased linearly with the total leaf area per plant, suggesting that water uptake was sufficient to compensate for water loss at the leaf level, even during times of extreme atmospheric water demand. Under high PET, maximum sap flow rates of Tamarix on a leafarea basis were significantly higher at locations where Tamarix LAI was lower, indicating that highly transpiring Tamarix stands may reduce leaf-level evaporative demand. However, daily estimates of transpiration of dense Tamarix stands (LAI up to 3.5 m2/m2) in our study site exceeded PET by a factor ranging from 1.6 to 2.0, which confirms that mature Tamarix stands can lose very high quantities of water due to the maintenance of high leaf area. Results from this and other studies demonstrate that, at least under moderate to high water tables, key variables controlling water use by riparian stands include structural characteristics such as LAI and density. Management practices aimed at conserving water should be geared towards avoiding the development of dense Tamarix thickets along arid water courses, particularly if it is confirmed that these tend to attain higher LAI than native riparian communities.

When assigning conservation priorities in endangered species, two common management strategies seek to protect remnant populations that (i) are the most genetically divergent or (ii) possess the highest diversity at neutral genetic markers. These two approaches assume that variation in molecular markers reflects variation in ecologically important traits and ignore the possibility of local adaptation among populations that show little divergence or variation at marker loci. Using common garden experiments, we demonstrate that populations of the rare endemic plant Arabis fecunda are physiologically adapted to the local microclimate. Local adaptation occurs despite (i) the absence of divergence at almost all marker loci and (ii) very small effective population sizes, as evidenced by extremely low levels of allozyme and DNA sequence polymorphism. Our results provide empirical evidence that setting conservation priorities based exclusively on molecular marker diversity may lead to the loss of locally adapted populations.

Tamarix ramosissima (Tamaricaceae) is a woody phreatophyte that has invaded thousands of hectares of floodplain habitat in the southwestern U.S. In this study, we examined the response of gas exchange and stem sap flow of Tamarix and three co-occurring native phreatophytes (Pluchea sericea (Asteraceae), Prosopis pubescens (Fabaceae) and Salix exigua (Salicaceae)) to drought conditions in an early successional floodplain community in the Mojave Desert of southern Nevada. In an analysis of a size/age series of each species across the whole floodplain (both mature and successional stands), stem growth rate was lowest for Tamarix. However, along the same successional chronosequence, Tamarix came to dominate the 50+ year old stands with dense thickets of high stem density. Xylem sap flow, when expressed on a sapwood area basis, was highest in Tamarix under early drought conditions, but comparable between the four species toward the end of the summer dry season. Multivariate analysis of the gas exchange data indicated that the four species differentiated based on water use under early drought conditions and separated based on plant water potential and leaf temperature (indices of drought effects) at the end of the summer dry season. This analysis suggests that the invasive Tamarix is the most drought tolerant of the four species, whereas Salix transpires the most water per unit leaf surface area and is the least tolerant of seasonal water stress. Therefore, Salix appears to be well adapted to early successional communities. However, as floodplains in this arid region become more desiccated with age, Tamarix assumes greater dominance due to its superior drought tolerance relative to native phreatophytes and its ability to produce high density stands and high leaf area.

On day 160 of 1996, a 28øC rise over a 9-hour period was correlated with increased wind speeds of greater than 4 m s -1. Subsequently, higher ET estimates were made in 1996 compared to 1994 (145 cm versus 75 cm). However, the energy balance was dominated by advection in 1996, with latent energy flux exceeding net radiation 65% of the measurement days compared to only 11% in 1994. We believe this advection was on a scale of the floodplain (hundreds of meters) as opposed to regional advection, since the majority of wind (90%) was in a N-S direction along the course of the river, and that a more open canopy allowed the horizontal transfer of energy into the Tamarix stand at the Bowen tower. Our results suggest that Tamarix has the potential to be both a low water user and a high water user, depending on moisture availability, canopy development, and atmospheric demand, and that advection can dominate energy balances and ET in aridland riparian zones such as the Virgin River.