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Community composition in semi-arid ecosystems has largely been explained by water availability; however, nitrogen is a common limiting nutrient, and may be an important control on plant function and carbon uptake. We investigated nitrogen relations and photosynthesis of several dominant species at shallow groundwater sites in Owens Valley, California. We measured soil nitrogen (N) availability, leaf N and isotopes, water isotopes, and gas exchange of dominant shrub species Atriplex torreyi and Ericameria nauseosa and grass species Distichlis spicata throughout the summer season in three sites that had similar watertable depths, but that varied in community composition and N availability. Surface soil inorganic N was greatest at the grassland site and declined from June to September at all sites. Leaf N declined throughout the season in all species, and was correlated with soil inorganic N. Photosynthesis of A. torreyi remained relatively constant throughout the season. In contrast, D. spicata and E. nauseosa experienced seasonal declines in photosynthesis at sites with greater inorganic N availability. Leaf N was significantly correlated with photosynthesis in D. spicata across all sites and measurement periods. Controls on N cycling are likely to be an important determinant of photosynthesis of D. spicata in this region.

Atmospheric CO2 enrichment may stimulate plant growth directly through (1) enhanced photosynthesis or indirectly, through (2) reduced plant water consumption and hence slower soil moisture depletion, or the combination of both. Herein we describe gas exchange, plant biomass and species responses of five native or semi-native temperate and Mediterranean grasslands and three semi-arid systems to CO2 enrichment, with an emphasis on water relations. Increasing CO2 led to decreased leaf conductance for water vapor, improved plant water status, altered seasonal evapotranspiration dynamics, and in most cases, periodic increases in soil water content. The extent, timing and duration of these responses varied among ecosystems, species and years. Across the grasslands of the Kansas tallgrass prairie, Colorado shortgrass steppe and Swiss calcareous grassland, increases in aboveground biomass from CO2 enrichment were relatively greater in dry years. In contrast, CO2-induced aboveground biomass increases in the Texas C3/C4 grassland and the New Zealand pasture seemed little or only marginally influenced by yearly variation in soil water, while plant growth in the Mojave Desert was stimulated by CO2 in a relatively wet year. Mediterranean grasslands sometimes failed to respond to CO2-related increased late-season water, whereas semiarid Negev grassland assemblages profited. Vegetative and reproductive responses to CO2 were highly varied among species and ecosystems, and did not generally follow any predictable pattern in regard to functional groups. Results suggest that the indirect effects of CO2 on plant and soil water relations may contribute substantially to experimentally induced CO2-effects, and also reflect local humidity conditions. For landscape scale predictions, this analysis calls for a clear distinction between biomass responses due to direct CO2 effects on photosynthesis and those indirect CO2 effects via soil moisture as documented here.

Populus fremontii (Fremont cottonwood) was once a dominant species in desert riparian forests but has been increasingly replaced by the exotic invasive Tamarix ramosissima (saltcedar). Interspecific competition, reduced flooding frequency, and increased salinity have been implicated in the widespread decline of P. fremontii. To elucidate some of the multiple and interacting mechanisms of this decline, we examined ecological processes in a control stand of P. fremontii along the Colorado River in Utah, USA, as well as a disturbed stand characterized by high groundwater salinity and invasion of T. ramosissima. Sap flux data showed that P. fremontii at the saline site experienced large reductions in afternoon canopy stomatal conductance relative to the control. Thus, average daily stand transpiration was 4.8 ± 0.1 mm/d at the saline site in comparison to 9.3 ± 0.2 mm/d at the control site over a two-month period. Light-saturated photosynthesis and apparent quantum yield were also reduced in saline P. fremontii. Stable isotope analysis indicated that trees at the saline site utilized evaporatively enriched groundwater that was likely derived from a nearby pond of irrigation runoff; this was also the probable source of high salinity. Interspecific competition for water at the saline site is unlikely, as T. ramosissima is still a minor species that is present only in the understory. However, reduced tissue N content in P. fremontii at the saline site suggested that physiological stress during salinity and halophyte invasion may be exacerbated by altered N relations.

Tree planting programs are being implemented in many US cities (most notably New York, Los Angeles, and Chicago) on the basis of the multiple environmental and health benefits they may provide. However, the magnitude and even the direction of the impacts of trees on specific urban environments have seldom been directly measured. In addition, there has been little research on the historical, cultural, political or institutional origins of such programs, or on their implementation process. Pending questions include the degree to which these programs are integrated in the existing frameworks of city government and infrastructure management, how they are paid for, and the kinds of collaborations between nonprofit organizations, the public, and public agencies at all levels they may require in order to succeed. This paper reports on an interdisciplinary research project examining the Million Tree Program of the City of Los Angeles.