Current Research
Current Research
At decreasing levels of tritium, the 5 spp. selected
(Larrea tridentata, Ephedra nevadensis, Krameria erecta, Pleuraphis rigida, and Bromus madritensis), will differ in their respective abilities to be used as sensitive indicators of radioactive contamination I the soil solution. A quantitative relationship will exist between tritium levels in the solution, tritium levels in the plant tissue, and response time.
Determine the ability of Mojave Desert plants to function s sol solution extraction samplers. Study will employ a randomized block design, with 5 spp of plants, 3 levels of tritium tracers, and 3 replications.
August 2000
November 1999 through November 2002
March 1999
April-October 1999
January 1999-September 2000
Seeds were collected and compared from parent plants of Bromus rubens L., an exotic Mojave Desert annual grass, grown in ambient (360 ppm) and elevated (700 ppm) CO2 to determine if parental CO2 growth conditions affected seed quality. Performance of seeds developed on the above plants was evaluated to determine the influence of parental CO2 growth conditions on germination, growth rate, and leaf production. Seeds of B. rubens developed on parents grown in elevated CO2 had greater pericarp surface area, higher C:N ratio, and less total mass than ambient developed seeds. Parental CO2 environment did not have an effect on germination percentage or mean germination time, as determined by radicle emergence. Seedlings from elevated CO2 developed seeds had a reduced relative growth rate and achieved smaller final mass over the same growth period. Elevated CO2 developed seeds had smaller seed reserves than ambient seeds, as determined by growing seedlings in sterile media and monitoring senescence. It appears that increased seed C:N ratios associated with plants grown under elevated CO2 may have a major effect on seed quality (morphology, nutrition) and seedling performance (growth rate, leaf production, etc.). Since the invasive success of B. rubens is primarily due to it’s ability to rapidly germinate, increase leaf area, and maintain a relatively high growth rate compared with native annuals and perennial grasses, reductions in seed quality and seedling performance in elevated CO2 may have significant impacts of future community composition in the Mojave Desert.
(Manuscript published in: Oecologia 1998 114(2):202-208.
Down-regulation of photosynthesis in response to elevated CO2 has been
frequently observed and may represent a key physiological process limiting
increases in productivity in a high-CO2 world. Yet, how other resources,
such as water availability, might control the occurrence or the degree
of down-regulation remains largely unknown. Understanding this issue is
critical to predicting how ecosystems that are limited by resources other
than carbon, or have highly variable patterns of nutrient or water availability,
will respond to rising CO2. In this study, we examined the photosynthetic
response of an evergreen Mojave Desert shrub, Larrea tridentata,
to elevated CO2. Since the Mojave Desert is seasonally water-limited and
temporally variable, it presents an opportunity to evaluate the interaction
of drought and increasing CO2 concentration. We hypothesized that the occurrence
of down-regulation would disappear under increasing water limited conditions,
and would therefore likely be a seasonally transient event in aridland
systems. To test this we measured photosynthetic, water relations and fluorescence
responses in two different treatment implementation techniques: (1) from
seedlings exposed to 3 CO2 levels (360, 550, and 700 ppm) in a glasshosue;
and (2) from intact adults exposed to 360 and 550 ppm CO2 treatments an
the Nevada Desert FACE (Free Air CO2 Enrichment) facility. FACE and glasshouse
well-watered Larrea significantly down-regulated photosynthesis
to increased CO2 environment, with reductions in maximum photosynthetic
rate, carboxylation efficiency, and Rubisco catalytic sites, whereas droughted
Larrea
showed a differing response depending on treatment technique. Amax and
CE were lower in all droughted Larrea compared with well-watered
plants, and elevated CO2 had no effect on already reduced rates, but Rubisco
catalytic sites were decreased in these plants in elevaed CO2. Our results
suggest drought can diminish the down-regulation response of Larrea
upon exposure to elevated CO2, possibly resulting in seasonally transient
patterns that may influence a number of important ecosystem processes.
Description: Gas exchange responses of three southwestern Yucca species to elevated CO2 and high temperature. Manuscript submitted to: Global Change Biology (March 31st 1998)
Abstract
The ability to tolerate temperature extremes is important in determining the distribution of perennial plants in the arid southwest U.S., and how elevated CO2 impacts the ability of plants to tolerate high temperature is relatively unknown. While effects of chronic high temperature (30-38° C) and elevated CO2 are well understood, little research has assessed plant performance in elevated CO2 during extreme (>45ºC) temperature events. We exposed three species of Yucca to 360 and 700 ?mol CO2 mol-1 for eight months, then nine days of high temperature (up to 53° C) to evaluate impacts of elevated CO2 on the potential for photosystem damage with increasing temperature. Seedlings of a coastal C3 species (Y. whipplei), a desert C3 species (Y. brevifolia), and a desert CAM species (Y. schidigera) that may use the C3 pathway as a seedling, were used to test for differences among functional groups. Before the initiation of high temperature, all Yucca species exposed to elevated CO2 showed decreases in carboxylation efficiency and the coastal Yucca showed significant reductions (33 %) in CO2 saturated maximum assimilation rate (Amax). Y. brevifolia and Y. schidigera showed no reductions in Amax. Stomatal conductance was lower in elevated CO2 as compared to ambient throughout the temperature event, however, there were species specific differences through time. Elevated CO2 enhanced photosynthesis in Y. whipplei at high temperatures for a short period, but not Y. brevifolia or Y. schidigera. Elevated CO2 offset photoinhibition in Y. whipplei as compared to ambient CO2, depending on exposure time to high temperature. This pattern is in contrast to current thought that suggests high temperatures in elevated CO2 may increase the potential for photoinhibition. Our results suggest that elevated CO2 might offset high temperature stress in coastal Yucca, but not in species native to drier systems. Therefore, elevated CO2 may allow plants to survive extreme temperature events, potentially enhancing Y. whipplei establishment in novel harsh habitats.
January 1997-November 1997
Abstract
Plant reproductive biology has been ignored within the climate change research community, even though, it has the potential to influence a large number of ecosystem processes. Changes in reproductive patterns are essential to understand in order to predict climate change impacts on terrestrial vegetation. Focusing on changes in trade-off schedules between allocation to growth, maintenance and reproduction may be the key to predicting reproductive patterns among different plant functional types. While both decreases and increases in reproductive biomass have been reported in the literature, few studies have adequately determined reproductive effort. Measuring the cost of reproduction on the vegetative characteristics of the flowering plant allows for interpretation of the impacts of elevated CO2 on whole plant performance and impacts on populations. The subsequent consequences of these changes in cost on fecundity in plants exposed to elevated CO2 are even more pronounced when interactions among other climate change scenarios (such as heating and drying) are considered. The life history strategy of terrestrial plants may be important in predicting patterns of change, but more important may be the relationship between life history schedules and environmental cues that invoke state changes in plants (cue reproduction). It appears that growth exposed to increasing levels of atmospheric CO2 decreases the ability of the plant to maintain the relationship between developmental cues (timing leaf senescence) and environmental (such as photoperiod) cues, resulting in altered patterns of allocation to growth and reproduction.
November 1996 -
The objective of this project is to assist with the development and implementation of the soils indicator for the FMH program. Three specific tasks will be undertaken. The first task is to provide support to the FMH Detection Monitoring program with the implementation of the soils indicator. The soils methods guide will be updated, a quality assurance plan will be prepared, regional trainers will be trained and certified, and soil samples will be prepared for laboratory analysis. The second task is to train FMH Indonesian scientists and field crews for the soils indicator. The third task is to prepare soil chapters for the report entitled "State of the Forests in the Mid-Atlantic region".
15 June 1997 - 30 September 1997
10 June 1997 - 30 July 1997
Seeds and seedlings will be collected from Halfway Wash site on Virgin River (36o40’N, 114o20’E, elevation 380m). Species of interest include: Tamarix ramosissima, Pluchea sericea, Salix exigua.
Plants will be grown alone or in combination with another species. Plants
grown alone will be grown in PVC pots of 7.62 cm radius; plants grown in
combination will be grown in PVC pots of 10.16 cm radius. All pots will
be 90 cm deep, and filled will coarse sand.
1 October - 18 October 1996.
16 October 1996 - 16 November 1996
Commencing in 2/96, we conducted a preliminary experiment in which we grew the primary dominants from the proposed NDFF site at 3 CO2 concentrations. The primary species being studied are: Bromus rubens (red brome, an exotic annual grass), Larrea tridentata (creosotebudh, an evergreen shrub), Camissonia claviformis ( a deert annual), and Stipa hymenoides ( Indian ricegrass, a C3 bunchgrass). Plants were grown in lg. (1 m tall x 10 cm diameter) pots to ensure adequate soil volume, and are instrumented with TDR soil moisture probes at 13 cm intervals to follow soil moisture depletion during dry down experiments and also to estimate soil moisture storage through time. Treatment concentrations of 550 and 700 micro l./ l CO2 were maintained in 2 glasshouse rooms while a control room was kept at ambient atmospheric levels of CO2. Air temperature was allowed to vary through a natural diurnal cycle representative of Mojave Desert conditions. Temperature and relative humidity were kept at similar levels across treatment rooms.
Preliminary results: The effects of CO2 on desert
vegetation are interesting both because of the known effect of CO2
on water use efficiency (WUE) and the potential that response might have
in alleviating water stress in desert plants. We are particularly interested
in comparative studies between different desert life forms that possess
contrasting adaptations to the arid environment. If the responses of desert
plants to elevated CO2 are dramatic, then there could be significant
shifts in the species composition and possible even in the physiognomy
of arid land vegetation. Preliminary results show a trend towards increasing
aboveground and below ground biomass for Bromus rubens with increased
CO2 concentration. Net CO2 assimilation of Stipa
hymenoides and Camissonia claviformis was significantly higher
in the 700 micro l/l treatment, with no treatment differences in stomatal
conductance. Larrea tridentata and Bromus rubens showed no
significant difference in Anet between the CO2 treatments; however,
a significant decrease was observed in stomatal conductance of both at
elevated CO2 concentrations. At elevated CO2 there
was a trend towards decreased flowering time in Bromus while time to flowering
in Camissonia tended to be increased. Cytokinins and chlorophyll
data for Bromus do not show significant trends from the initial
sampling period. Plant tissue concentrations of nitrogen and starch are
not yet fully analyzed. While the final biomass harvest will not occur
until June 1996, periodic samplings have shown large interplant variability
resulting in a lack of significant treatment effects at this time. This
may not be unusual in an invasive annual in which genetic variability likely
contributes to success in highly diverse environments.
11 September 1995 - 30 September 1996.
1 October 1995 - 1 October 1996
22 August 1994 - 22 November 1994
June 1994 - August 1995
15 June 1994 - 15 September 1994
November 1993 - September 1994
Species richness, density and abundance in Coleogyne ramosissima shrublands were compared on 15 elevational transects in the Spring and Sheep Mountain Ranges adjacent to Las Vegas valley. Coleogyne shrublands shared relatively broad upper and lower ecotones with Pinus-Juniperus and Larrea-Ambrosia vegetation. Lower Coleogyne ecotones generally had the highest species richness, and Pinus-Juniperus woodlands had the lowest species richness. DECORANA (Detrended correspondence analysis) suggested that elevation and soil depth were significantly associated with the distribution of stand and species groups identified from the 2-way indicator species analysis (TWINSPAN) in the Spring and Sheep Ranges. Precipitation and soil moisture were positively correlated with elevation, air and soil temperatures were negatively correlated with elevation. Coleogyne density was positively correlated with soil moisture, soil organic matter, Coleogyne water potential, Coleogyne leaf biomass, and Coleogyne stem and leaf phosphorus. Soil moisture and soil organic matter appeared to influence the distribution of Coleogyne at its lower elevational boundary in southern Nevada.
January 1993 - December 1993
November 1992 - August 1996.
9 April 1993 - 30 April 1993.
February 1993 - April 1994.
1 September 1992 - December 1992.
September 1992 -
