2,4-Dichlorophenoxyacetic acid and kinetin in anther culture of cultivated and wild oats and their interspecific crosses: plant regeneration from A. sativa L.

The effect of 2,4-dichlorophenoxyacetic acid (2,4-D) and kinetin was studied in anther culture of oat Avena sativa L., wild oat A. sterilis L. and progeny of crosses between them. A high 2,4-D concentration (5–6 mg l^–1) increased embryo production in genotypes of both species and promoted plant regeneration in anther cultures of A. sterilis and A. sativa×A. sterilis progeny, while kinetin caused severe browning. However, a low concentration of kinetin was essential for initiation of regenerable embryos from anther culture of A. sativa cv. Kolbu: one green and one albino plant were produced. In addition, medium containing W₁₄ salts gave higher regenerant recovery compared with medium containing Murashge and Skoog salts, when cross progeny were tested. Received: 6 March 1998 / Revised: 30 April 1998 / Accepted: 16 November 1998     

Refining the cheatgrass–fire cycle in the Great Basin: Precipitation timing and fine fuel composition predict wildfire trends

Larger, more frequent wildfires in arid and semi‐arid ecosystems have been associated with invasion by non‐native annual grasses, yet a complete understanding of fine fuel development and subsequent wildfire trends is lacking. We investigated the complex relationships among weather, fine fuels, and fire in the Great Basin, USA. We first modeled the annual and time‐lagged effects of precipitation and temperature on herbaceous vegetation cover and litter accumulation over a 26‐year period in the northern Great Basin. We then modeled how these fine fuels and weather patterns influence subsequent wildfires. We found that cheatgrass cover increased in years with higher precipitation and especially when one of the previous 3 years also was particularly wet. Cover of non‐native forbs and native herbs also increased in wet years, but only after several dry years. The area burned by wildfire in a given year was mostly associated with native herb and non‐native forb cover, whereas cheatgrass mainly influenced area burned in the form of litter derived from previous years’ growth. Consequently, multiyear weather patterns, including precipitation in the previous 1–3 years, was a strong predictor of wildfire in a given year because of the time needed to develop these fine fuel loads. The strong relationship between precipitation and wildfire allowed us to expand our inference to 10,162 wildfires across the entire Great Basin over a 35‐year period from 1980 to 2014. Our results suggest that the region's precipitation pattern of consecutive wet years followed by consecutive dry years results in a cycle of fuel accumulation followed by weather conditions that increase the probability of wildfire events in the year when the cycle transitions from wet to dry. These patterns varied regionally but were strong enough to allow us to model annual wildfire risk across the Great Basin based on precipitation alone.     

Effects of Nitrogen and Salinity on Growth and Competition Between a Native Grass and an Invasive Congener

Numerous studies show that an increase in the availability of limiting resources can increase invasion by non-native plants into natural communities. One possible explanation is that the ability of natives to compete with non-natives tends to decrease when resource availability is increased. We tested this hypothesis in a competition experiment using two closely matched plant species and two environmental factors related to limiting resources in a coastal grassland system on Bodega Head in northern California. We grew the native grass Bromus carinatus and the non-native grass B. diandrus together and apart at different levels of soil nitrogen crossed with different levels of soil salinity. Both species are abundant in the grassland and previous work suggested that the abundance of B. carinatus is lower and the abundance of B. diandrus is higher on soil that has been enriched with nitrogen. Salinity has been shown to be negatively associated with invasion by B. diandrus into another California grassland, and to vary significantly over short distances in the grassland at Bodega Head, where it could affect water availability, which strongly limits plant growth during the dry season. Contrary to our prediction that low resource availabilities would increase the relative competitive ability of the native, the ability of B. carinatus to compete with B. diandrus was not greater when nitrogen availability was lower or when soil salinity was higher. Instead, high salinity increased the relative competitive ability of the non-native, and low nitrogen had little effect on competition. This suggests that preventing resource enrichment will not suffice to control invasion by non-native plant species in this grassland.     

Regional analysis of the impacts of climate change on cheatgrass invasion shows potential risk and opportunity

Interactions between climate change and non-native invasive species may combine to increase invasion risk to native ecosystems. Changing climate creates risk as new terrain becomes climatically suitable for invasion. However, climate change may also create opportunities for ecosystem restoration on invaded lands that become climatically unsuitable for invasive species. Here, I develop a bioclimatic envelope model for cheatgrass ( Bromus tectorum ), a non-native invasive grass in the western US, based on its invaded distribution. The bioclimatic envelope model is based on the Mahalanobis distance using the climate variables that best constrain the species' distribution. Of the precipitation and temperature variables measured, the best predictors of cheatgrass are summer, annual, and spring precipitation, followed by winter temperature. I perform a sensitivity analysis on potential cheatgrass distributions using the projections of 10 commonly used atmosphere–ocean general circulation models (AOGCMs) for 2100. The AOGCM projections for precipitation vary considerably, increasing uncertainty in the assessment of invasion risk. Decreased precipitation, particularly in the summer, causes an expansion of suitable land area by up to 45%, elevating invasion risk in parts of Montana, Wyoming, Utah, and Colorado. Conversely, increased precipitation reduces habitat by as much as 70%, decreasing invasion risk. The strong influence of precipitation conditions on this species' distribution suggests that relying on temperature change alone to project future change in plant distributions may be inadequate. A sensitivity analysis provides a framework for identifying key climate variables that may limit invasion, and for assessing invasion risk and restoration opportunities with climate change.     

Contemporary evolution of an invasive grass in response to elevated atmospheric CO2 at a Mojave Desert FACE site

Elevated atmospheric CO₂ has been shown to rapidly alter plant physiology and ecosystem productivity, but contemporary evolutionary responses to increased CO₂ have yet to be demonstrated in the field. At a Mojave Desert FACE (free‐air CO₂ enrichment) facility, we tested whether an annual grass weed (Bromus madritensis ssp. rubens) has evolved in response to elevated atmospheric CO₂. Within 7 years, field populations exposed to elevated CO₂ evolved lower rates of leaf stomatal conductance; a physiological adaptation known to conserve water in other desert or water‐limited ecosystems. Evolution of lower conductance was accompanied by reduced plasticity in upregulating conductance when CO₂ was more limiting; this reduction in conductance plasticity suggests that genetic assimilation may be ongoing. Reproductive fitness costs associated with this reduction in phenotypic plasticity were demonstrated under ambient levels of CO₂. Our findings suggest that contemporary evolution may facilitate this invasive species' spread in this desert ecosystem.     

Plant Recruitment and Soil Microbial Characteristics of Rehabilitation Seedings Following Wildfire in Northern Utah

One goal of post‐fire native species seeding is to increase plant community resistance to exotic weed invasions, yet few studies address the impacts of seeding on exotic annual establishment and persistence. In 2010 and 2011, we investigated the influence of seedings on exotic annuals and the underlying microbial communities. The wildfire site in northern Utah was formerly dominated by Artemisia tridentata ssp. wyomingensis, but burned in September 2008. Experimental seeding treatments were installed in November 2008 to examine strategies for establishing native species using two drills, hand broadcasts and different timing of seed applications (resulting in 13 seeding treatments). We collected aboveground biomass of invasive annuals (Halogeton glomeratus, Salsola kali, and Bromus tectorum), other volunteer plants from the extant seed bank, and seeded species from all treatments in the second and third years after fire. We sampled soils within microsites beneath native perennial bunchgrass and exotic annuals to characterize underlying soil microbial communities. High precipitation following seeding led to strong seedling establishment and we found few differences between seeding treatments established with either drill. All seeded treatments reduced exotic biomass by at least 90% relative to unseeded controls. Soil microbial communities (phospholipid fatty acid analysis), beneath B. tectorum, Poa secunda, and Pseudoroegneria spicata microsites differed little 3 years after fire. However, microbial abundance beneath P. spicata increased from June to July, suggesting that microbial communities beneath successful seedings can vary greatly within a single growing season.     

The measurement of water relations of plant cell culture. I. Water release in response to centrifuge-induced water potentials

Water loss by cell suspensions during centrifugation is well defined by simple physical principles. The major factors affecting water release during centrifugation are: duration of centrifogation, depth of the cell mass, density of cells, relative centripetal acceleration and centripetal force. Water release during centrifugation was best described by an exponential decay process with a decay constant that increases with acceleration from 0.31 ± 0.01 to 0.66 ± 0.12 min^?1 (mean ± SE) between 4 825 and 19 300 m s^?2, respectively. The cell mass relative water content (RWC) at equilibrium was not a function of rate of water loss and was constant for each acceleration. A centripetal force was generated by the mass of the cells being accelerated away from the axis of rotation. This force generated a pressure that removed some of the cell wall and symplast water, by compression at contact points between the cells and by compression of the cytoplasm. Pressure induced by centripetal forces ranging from ?0.02 to ?0.23 MPa gave a linear relationship (r² > 0.99) between force and RWC. The slope (0.900 MPa) was proportional to the cell wall modulus of elasticity (±). and the intercept was interpreted to give the mass of the cells at full turgor without interstitial water (RWC=1). This interpretation is supported by the findings, of two independent experiments. Centrifuged cells suspended at 100% relative humidity for over 48 h reached the same water content as predicted by the intercept. Interstitial water was labelled with solutions of polyethylene glycol (PEG. M_r 8 000), the diameter of which was too large to enter the pores of plant cell walls. Centripetal accelerations greater than 10 900 m s^?2 removed PEG-labelled water to levels below 0.9% of cell water content. Removal of interstitial water and other loosely bound water provided a convenient method for determination of growth, RWC and ±. The centrifugal methods provide the foundation for new quantitative methods for cell culture water relations analyses.