Arid environments are characterized by limited and variable rainfall that supplies resources in pulses. Resource pulsing is a special form of environmental variation, and the general theory of coexistence in variable environments suggests specific mechanisms by which rainfall variability might contribute to the maintenance of high species diversity in arid ecosystems. In this review, we discuss physiological, morphological, and life-history traits that facilitate plant survival and growth in strongly water-limited variable environments, outlining how species differences in these traits may promote diversity. Our analysis emphasizes that the variability of pulsed environments does not reduce the importance of species interactions in structuring communities, but instead provides axes of ecological differentiation between species that facilitate their coexistence. Pulses of rainfall also influence higher trophic levels and entire food webs. Better understanding of how rainfall affects the diversity, species composition, and dynamics of arid environments can contribute to solving environmental problems stemming from land use and global climate change.

Changes in historical disturbance regimes have been shown to facilitate non-native plant invasions, but reinstatement of disturbance can be successful only if native colonizers are able to outcompete colonizing invasives. Reintroduction of flooding in the southwestern United States is being promoted as a means of reestablishing Populus deltoides subsp. wislizenii , but flooding can also promote establishment of an introduced, invasive species, Tamarix ramosissima . We investigated competition between Populus and Tamarix at the seedling stage to aid in characterizing the process by which Tamarix may invade and to determine the potential ability of Populus to establish itself with competitive pressure from Tamarix . We planted seedlings of Tamarix and Populus in five ratios at three densities for a total of 15 treatments. The growth response of each species was measured in terms of height, above-ground biomass, and tissue concentrations of nitrogen and phosphorous. These measurements across treatments were modeled as three-dimensional response surfaces. For both species, Populus density was more important than Tamarix density for determining growth response. Both species were negatively affected by increasing numbers of Populus seedlings. Due to the larger size of the native Populus , we predict that its superior competitive ability can lead to its dominance when conditions allow native establishment. Our results suggest that even in the presence of an invader that positively responds to disturbance, reestablishment of historical flooding regimes and post-flood hydrology can restore this ecosystem by promoting its dominant plant species.Competencia Entre la Especie Nativa Populus deltoides y la Invasiva Tamarix ramosissim a y las Implicaciones para Reintegrar Perturbaciones Causadas por Inundaciones Resumen: Los cambios en los regímenes históricos de perturbaciones han mostrado que facilitan invasiones de plantas no nativas; sin embargo, la reinstauración de la perturbación solo puede ser exitosa si los colonizadores nativos son capaces de competir y desplazar a las especies invasoras. La reintroducción de las inundaciones en el sudoeste de los Estados Unidos está siendo promovida como una forma de restablecer Populus deltoides subespecie wislizenii , pero las inundaciones pueden promover también el establecimiento de una especie invasora, Tamarix ramosissima . Investigamos la competencia entre Populus y Tamarix al estado de plántula para ayudar a caracterizar el proceso por el cual Tamarix puede invadir y para determinar la habilidad potencial de Populus para establecer presión competitiva contra Tamarix . Sembramos plántulas de Tamarix y Populus en 5 diferentes radios y densidades para un total de 15 tratamientos. La respuesta en crecimiento de cada especie fue medida en forma de altura, biomasa sobre el suelo y concentraciones de nitrógeno y fósforo en tejido. Estas mediciones en los tratamientos fueron modeladas en superficies de respuesta tridimensionales. La densidad de Populus fue más importante para la dete...

Despite growing concern and research interest in plant invasions, we know very little about ecological processes involved during invasion episodes. Understanding these dynamics is especially important for habitats subject to reestablishment of historical disturbance regimes; because many invasive species respond positively to disturbance, interactions between colonizing natives and nonnatives at the seedling stage may determine success of invasions. In the American Southwest, reduced flood disturbance in this century has been associated with the invasion of nonnative Tamarix ramosissima, with Tamarix appearing to displace native flood‐dependent species of Populus and Salix. We studied two riparian communities of these woody species that had established after over‐bank flooding was allowed to occur in two protected areas in New Mexico. Growth and mortality were monitored for each individual within 1‐m2 plots of seedling communities within these sites for 2–4 yr. Abiotic conditions measured for each plot included soil salinity, texture, and nitrogen and phosphorus concentration. Although Tamarix seedling densities were an order of magnitude greater than those of the native species at initial establishment, mortality of Tamarix was also much greater than for Populus or Salix and occurred primarily in plots where native species were the most dense. Both natives were taller and grew faster above ground than Tamarix in the first 4 yr, and growth of Tamarix was negatively correlated with neighbor densities. In contrast, Populus and Salix growth and survival were never correlated with Tamarix densities. For all species, none of the abiotic variables measured was as important as neighbor density for explaining growth and survival patterns. These results have positive implications for managers who wish to reestablish the native dominants of riparian forests through reinstatement of flooding. Although Tamarix can respond positively to flooding, a range of conditions clearly exists in which Populus and Salix establishment can be stimulated by flooding, even when Tamarix seedlings are present. Furthermore, the presence of such natives even at the seedling stage may help to reduce densities of invasive Tamarix seedlings.

Resource availability is often characterized by mean annual amounts, while ignoring the spatial variation within habitats and the temporal variation within a year. Yet, temporal and spatial variation may be especially important for identifying the source of stress in low productivity environments such as deserts where resources are often pulsed and resource renewal events are separated by long periods of low resource availability. Therefore, the degree of stress will be determined in part by the length of time between recharge events. Here, we investigated the effect of timing and total amount of water application on two congeneric pairs, each with a population from a low (desert) and a high (Mediterranean) productivity habitat. As expected, highest survival and greatest growth were found at low or intermediate recharge intervals, and the magnitude of response to increases in total seasonal amounts was greater for Mediterranean species than desert species. The species that had greater survival switched in the hierarchy under high total water depending on interval length. These results demonstrate that temporal variation in resource availability can be as important as annual total amounts for plant performance and that response to temporal dynamics can vary between species. This has implications for community-level processes, as competitive hierarchies may switch based on resource dynamics rather than only total availability.