Steven O. Link

Soil microbial biomass and activity of a disturbed and undisturbed shrub-steppe ecosystem

Abstract Disturbance of shrub-steppe soils and alterations in plant cover may affect the distribution, size and activity of soil microorganisms and their ability to biogeochemically cycle essential nutrients. Therefore, the soil microbial biomass and activity and selected soil enzyme activities were determined for two arid ecosystems, an undisturbed perennial shrub-steppe and annual grassland, which was initially shrub-steppe and has been an annual grassland since the disturbance caused by farming ceased in the 1940s. Soils were sampled at 0–5 and 5–15 cm depths beneath sagebrush ( Artemisia tridentata Mutt.), bluebunch wheatgrass [ Elytrigia spicata (Pursh) D.R. Dewey] and cryptogamic soil lichen crust at the perennial site and beneath downy brome ( Bromus tectorum L.) at the annual grassland site. Soils were analyzed for physical properties, inorganic N, microbial biomass C and N, respiration and several enzymes. The soil pH and bulk density usually increased, while inorganic N, total N and total C decreased as a function of soil depth. Soil microbial biomass C and N, soil respiration and soil dehydrogenase activity were 2–15 times higher in the top 5 cm of soil than at the 5–15 cm depth regardless of plant type. Loss of this surface soil would therefore be detrimental to microbially-mediated cycling of nutrients. Surface soil (0–5 cm depth) microbial biomass C and N and soil respiration, dehydrogenase and phosphatase activity were influenced by plant type and decreased in the order B. tectorum A. tridentata = E. spicata soil crust. Spatial distribution of plant species at the shrub-steppe site resulted in “islands” of enhanced microbial biomass and activity underneath the shrubs and grasses when compared to the interplant areas covered with soil crust. When plant cover was used to compute a landscape estimate of soil microbial biomass C and N for the perennial shrub-steppe and the annual grassland, similar values were obtained. This indicates that while the distribution of microorganisms may be more heterogeneous in the shrub-steppe, the average across the landscape is the same as the more homogeneous annual grassland.

Bromus tectorum cover mapping and fire risk

Fire risk in western North America has increased with increasing cover of Bromus tectorum, an invasive alien annual grass. The relationship between B. tectorum cover and fire risk was determined in a historically burned Artemisia tridentata-Poa secunda shrub–steppe community where B. tectorum cover ranged from 5 to 75%. Fire risk ranged from ~46% with an average of 12% B. tectorum cover to 100% when B. tectorum cover was greater than 45% based on prediction confidence limits. Reflectance of the green and red bands of aerial photographs were related to senescent B. tectorum cover to create fine resolution B. tectorum cover and fire risk maps. This assessment technique will allow land managers to prioritize lands for restoration to reduce fire risk in the shrub-steppe.

The effect of water stress on phenological and ecophysiological characteristics of cheatgrass and Sandberg's bluegrass.

Comparative field studies of cheatgrass (Bromus tectorum L.) with Sandberg’s bluegrass (PoasandbergiiVasey) were conducted to further our understanding of the plannt characteristics that contribute to success in habitats where water is a limiting factor. To evaluate the effect of soil water on phenological development, stomata1 conductance, and xylem pressure potential of these grasses, observations were made in the field for 2 growing seasons (1986 and 1987). Stomata1 conductance, transpiration, and xylem pressure potential data, gathered as soils dried during 1986, indicated that water stress developed earlier and to a greater degree in Sandberg’s bluegrass than in cheatgrass. Xylem pressure potential was lower in Sandberg’s bluegrass than in chedgrass, and the difference increased throughout the growing season. Stomata1 conductance and transpiration were greater for chedgrass than for Sandberg’s bluegrass. Maintenance of high soil water potentials by irrigating through the 1987 growing season retarded phenological development and delayed senescence by about 10 days for both species. Predawn xylem pressure potential for irrigated plants remained higher than for nonirrigated plants; however, as the plants senesced, xylem pressure potential also decreased in the nonstressed plants.

Responses of downy brome to nitrogen and water.

Downy brome (Bromus tectorum L.) is an alien grass that dominates disturbed ground in shrub-steppe ecosystems of the western United States. Responses of downy brome to added nitrogen and water were evaluated using intact soil cores obtained from an old field. Gas exchange data were gathered at the leaf and canopy scales. Stomatal conductance and net photosynthesis rates were greater at the leaf scale than at the canopy scale, decreased with time from germination, and were weakly affected by treatments. Water-use efficiency was weakly related to time from germination and treatments. Biomass was greater in the nitrogen-plus-water (7.4 g) treatment, compared with water (3.6 g), nitrogen (4.5 g), and control (3.3 g) treatments. The leaf-area index varied like biomass at the end of the experiment. Shoot nitrogen was the same in the nitrogen (2.5%) and nitrogen-plus-water (2.5%) treatments, nearly twice the level in the control (1.5%) and water (1.3%) treatments. Nitrogen-use efficiency was highest in the control (67) and water (80) treatments and lowest in the nitrogen (41) and nitrogen-plus-water (43) treatments. The most significant conclusion of this work is that gas exchange was strongly related to the time from germination and little affected by water and nitrogen while growth characters were strongly affected only when water and nitrogen were added together.

Lichen photosynthesis in relation to CO2 concentration

SummaryPhotosynthetic CO2 dependencies were measured for six lichen species, representing a variety of morphologies and collected from widely different habitats. All curves exhibited a linear increase in photosynthesis with increasing CO2 concentrations at the lower range of CO2 values, but little photosynthetic variation with increasing CO2 concentrations at the upper range of CO2 values. Half maximal CO2 concentration estimates varied from 147–440 μl CO2·l1, but had broadly overlapping confidence intervals. We conclude that lichen CO2 dependencies are basically similar to those reported for higher plants and discuss the reasons why widely varying results have previously been published.

Response of a shrub‐steppe ecosystem to fire: Soil water and vegetational change

Abstract A wildfire in August 1984 eliminated big sagebrush from a shrub‐steppe community in southeastern Washington. Bluebunch wheatgrass dominated the community in subsequent years. By comparing leaf area index and soil water storage on the burned site with an adjacent unburned site containing big sagebrush, we were able to test the hypotheses that plant community structure controls soil moisture dynamics and that wildfire can alter the water balance in these arid landscapes. Elimination of big sagebrush from the landscape greatly reduced the community leaf area index. Soil water storage computed to a depth of 125 cm was not affected by the change in community structure. However, storage computed to a depth of 275 cm was significantly greater on the burned site compared to the unburned site. This difference increased with time after the fire. Wildfire in shrub‐steppe communities dominated by big sagebrush effectively removes the deep rooted, woody component from these ecosystems. Consequently, the abili...

Photosynthetic Patterns of Sonoran Desert Lichens I. Environmental Considerations and Preliminary Field Measurements

Summary In comparison to the Negev Desert environment, where extensive desert lichen photosynthetic and productivity studies have been conducted, the environment of the interior Sonoran Desert at Phoenix, Arizona, is shown to be much less favorable for lichen photosynthetic activity. Lack of dewfall events is inferred to be the major difference, as the estimated dewfall frequency of 12 d per year is only 6 % of the number of days when dewfall occurs at Avdat, Israel. Favorable moisture periods for lichen photosynthesis are shown to be largely restricted to the winter period when temperatures are cool. The occurrence of dewfall and periods with low vapor pressure deficits is generally coupled with the occurrence of winter rainfall periods. Fog, a factor responsible for luxuriant lichen communities in some maritime, arid environments, is shown to be almost non-existent at Phoenix. Preliminary photosynthetic measurements over 28 d confirmed that days with photosynthetic activity are tightly coupled with precipitation events. Moisture conditions favorable for photosynthetic activity may persist for a day or two after precipitation events. Because of the marked differences in moisture conditions and in lichen biomass estimates between the Negev and the interior Sonoran Desert, it is inferred that lichen productivity must be much lower at the Sonoran Desert site.

Hanford prototype-barrier status report: FY 1997

An above-grade surface barrier consisting of a vegetated soil-cover, surrounded by gravel and rock side slopes, is being tested for the US Department of Energy (DOE). It is part of a treatability study at the 200-BP-1 Operable Unit in the 200 East Area of the Hanford Site, near Richland, Washington. The surface barrier, constructed in 1994, covers 2.5 ha (6.9 acre) of land surface and is situated over an inactive liquid-waste disposal crib. A set of under drains, built into the barrier using curbed asphalt, allows precise measurement of drainage from the soil cover and the side slopes. The treatability test includes measurements of water balance, wind and water erosion, subsidence, plant growth, and plant and animal intrusion. The test compares the performance of the barrier under ambient and simulated climate change (elevated precipitation) conditions. This report documents findings from the third year of testing.

The influence of water on CO2 exchange in the lichen Parmelia praesignis Nyl.

Net photosynthetic rates for the lichen Parmelia praesignis Nyl. were obtained as a function of 5 light levels, 5 temperature levels, and of water content as thalli dried from saturated conditions. Data are described as second order polynomials in the light, and as saturation curves in the dark. Rates in the light were depressed at high water contents reaching maximal rates between 110% and 180% water content and declining as thalli dried. Physiological parameters were derived from the drying curves to investigate temperature and light interactions. Dark respiration parameters are the maximal rate, the water content where the rate is half-maximal, the water content at which respiration is zero, and the maximal water efficiency. In the light, parameters are the maximal net photosynthetic rate, the water content at the maximal rate, the water compensation point, the maximal water efficiency, and the sensitivity of net photosynthesis to change in water content.Values of half-maximal rate water contents for respiration were found to increase as temperatures increased. The greatest maximal net photosynthetic rate occurred at higher temperatures as the light intensity increased. In the light, maximal water efficiency and the sensitivity to changes in water content were generally maximal at temperatures yielding the greatest maximal net photosynthetic rates.

CO2 Exchange in Lichens: Towards a Mechanistic Model

Photosynthesis, a process by which radiant energy is converted to chemical energy, is, of course, the essential foundation for most forms of life. To measure photosynthetic rate in atmospheric environments, one normally focuses on CO2 influx, which is partially balanced by respiratory CO2 efflux, and consequently the measurement of CO2 exchange in the light is called net photosynthesis (Pn). Ideally, to measure the absolute rate of CO2 incorporation, the enzyme activity of ribulose-1,5-biphosphate carboxylase, the enzyme controlling the incorporation of CO2 should be measuredin vivo. However, in practice it is much easier to measure CO2 exchange, particularly when dealing with a whole plant or intact portion thereof.