Tye Morgan

Suppression of Bromus tectorum L. by Established Perennial Grasses: Potential Mechanisms—Part One

Bromus tectorum L. (cheatgrass) is an Eurasian annual grass that has invaded ecosystems throughout the Intermountain west of the United States. Our purpose was to examine mechanisms by which established perennial grasses suppress the growth of B. tectorum. Using rhizotrons, the experiment was conducted over 5 growth cycles: (1) B. tectorum planted between perennial grasses; (2) perennials clipped and B. tectorum planted; (3) perennials clipped and B. tectorum planted into soil mixed with activated carbon; (4) perennials clipped, B. tectorum planted, and top-dressed with fertilizer, and; (5) perennial grasses killed and B. tectorum planted. Water was not limiting in this study. Response variables measured at the end of each growth cycle included above-ground mass and tissue nutrient concentrations. Relative to controls (B. tectorum without competition), established perennial grasses significantly hindered the growth of B. tectorum. Overall, biomass of B. tectorum, grown between established perennials, increased considerably after fertilizer addition and dramatically upon death of the perennials. Potential mechanisms involved in the suppression of B. tectorum include reduced nitrogen (possibly phosphorus) availability and coopting of biological soil space by perennial roots. Our data cannot confirm or reject allelopathic suppression. Understanding the mechanisms involved with suppression may lead to novel control strategies against B. tectorum.

Suppression of annual Bromus tectorum by perennial Agropyron cristatum: roles of soil nitrogen availability and biological soil space

Sagebrush (Artemisia) rangelands of the western United States are becoming dominated by the exotic annual grass cheatgrass (Bromus tectorum). Rehabilitation of invaded rangelands is predicated on establishing healthy and dense perennial grass communities, which suppress cheatgrass. Our research investigated how established plants of the perennial grass, crested wheatgrass (Agropyron cristatum), suppress cheatgrass. Our data suggest that established crested wheatgrass reduces soil nitrogen availability and occupies biological soil space such that growth of cheatgrass is significantly reduced. Greater understanding of the role of biological soil space could be used to breed and select plant materials with traits that are more suppressive to invasive annual grasses.

Soil Engineering Facilitates Downy Brome (Bromus tectorum) Growth — A Case Study

Abstract Soil engineering by downy brome may be a facet of its competitiveness. Using rhizotrons in the greenhouse, we compared the growth and plant–soil relationships of downy brome grown in two field soil types: soil invaded for 12 yr by downy brome and a similar soil not yet invaded. For each soil type, downy brome was grown for two growth cycles. At harvest, root mass and soils were sampled at depths of 10, 40, and 80 cm (4, 16, and 32 in); aboveground biomass was also sampled. After the first growth cycle, downy brome grown in invaded soil had 250% greater aboveground biomass and nearly double the root mass per soil volume at 10 cm relative to downy brome grown in noninvaded soil; root mass per volume was similar at depths of 40 and 80 cm. For the second growth cycle, aboveground biomass declined, but was twice greater for downy brome grown in invaded soil; however, root mass per volume was similar between soil types for each depth. Soil attributes that positively related to aboveground biomass included bicarbonate-extractable P, DTPA (diethylentriamene pentaacetate)-extractable Mn, and solution-phase (80-cm depth). We conclude that the data support our hypothesis that downy brome has engineered the soil to increase its growth potential, but proof will require a more robust experimental design. Plant competition is affected by myriad interactions; however, a plant that can increase the availability of soil nutrients for itself and its growth potential, relative to competing plants, would appear to be at an advantage. The mechanistic underpinnings involved are inconclusive, but may involve increased availability of soil N, P, and Mn. Nomenclature: Downy brome, Bromus tectorum L Management Implications: In the ecosystem studied, our data suggest 12 yr of invasion by downy brome has engineered the soil to facilitate its growth. Mechanistic underpinnings of this positive feedback are possibly due to an increase in soil N, P, and Mn availability. The literature is clear: downy brome benefits and becomes more competitive when provided elevated sources of nutrients, particularly N and P. Given that this is a case study, spatial extrapolation is limited; however, by its ability to increase soil nutrient availability, one can expect downy brome to potentially invade communities thought somewhat resistant to invasion owing to inherently low soil nutrient availability.

Influence of livestock grazing, floodplain position, and time on soil nutrient pools in a Sierra-Nevada montane meadow.

Limited data exist on quantification of soil nutrient pools in montane meadow ecosystems. Along Big Grizzly Creek in the Plumas National Forest, CA (June 1999-September 2005), soil nutrient pools were quantified by livestock grazing treatment (grazed, ungrazed), floodplain location (stream edge, midfloodplain, forest edge), and season using resin capsules (15-cm depth) and bulk soil samples (0- to 25-cm depths). Resin capsules integrated nutrient availability for three periods: overwinter, plant growth, and senescence. Bulk soil samples were collected immediately after snowmelt (pregrowth), during plant growth, and during plant senescence. During the study period, pulses of N or P did not occur, suggesting strong coupling of mineralization with root/microbial uptake. Soil availability of most nutrients was affected by sampling time and floodplain location; however, differences were small. Soil samples from grazed areas had significantly greater K and Na on clay exchange sites than soil from excluded areas possibly because of supplementation with salt blocks. A seasonal reciprocal relationship occurred for the proportional content of K and Na on clay exchange sites and on resin capsules: Na highest during the cold season, and K highest in the plant senescent period. This relationship may be important in retaining K in soil during snowmelt. Surprisingly, the robust pools of extractable Ca, Mg, K, and Na, and their proportional content on the exchanger differed significantly with time. Overall, the data suggest that the present grazing management plan does not greatly impact nutrient availability.

Does a Trend in Declining Stem Density of Lepidium latifolium Indicate a Phosphorus Limitation? A Case Study

Abstract Lepidium latifolium (perennial pepperweed) is a weedy alien crucifer that has invaded wetlands throughout the western United States. We monitored L. latifolium invasion of an Elytrigia elongata (tall wheatgrass) community at the Honey Lake Wildlife Refuge in northeastern California. In 1993, a 40-m2 plot was delineated, at which time only two single plants of L. latifolium were present. Beginning in 1994, L. latifolium stem density was measured yearly until 2011. From 1994 through 2000, the density of L. latifolium increased to greater than 120 stems m−2. At its height of stem density and stature between 1998 and 2000, it appeared that E. elongata had been extirpated. From 2001 through 2006, stem density and plant stature of L. latifolium declined, but there were still areas of the plot where stem density exceeded 60 stems m−2. From 2007 through 2009, stem density decreased considerably and averaged less than 30 stems m−2 and a healthy recovery of E. elongata occurred. In the years 2010 and especially 2011, stem density increased, but individual plants were small in stature. Soil bicarbonate-extractable phosphorus data suggest that phosphorus availability may be crucial to the invasiveness of L. latifolium. Long-term biogeochemical cycling by L. latifolium may reduce soil phosphorus availability in deeper soil horizons and enrich availability in the soil surface, which alters the competitive relationship between L. latifolium and E. elongata. Nomenclature: Perennial pepperweed. Lepidium latifolium L., tall wheatgrass, Elytrigia elongata Host (Nevski). Management Implications: Over an 18-yr period, we monitored stem density of the exotic invasive crucifer Lepidium latifolium in an area at the Honey Lake Wildlife Refuge in northeastern California. Following a near record flood event in January 1997, in some areas density and stature of L. latifolium dramatically increased to form a near monoculture and appeared to have extirpated the perennial grass Elytrigia elongata planted in 1987 as nesting habitat. From its peak in 1998 to 2000, stem density and stature of L. latifolium has declined considerably and allowed E. elongata to once again assume dominance in many areas. The decline in stem density is not due to herbicide use or any management decisions. Instead, our evidence suggests that lowered precipitation and declining availability of soil phosphorous has tipped the competitive balance between L. latifolium and E. elongata. More research is needed to determine the range of soil phosphorus availability over soil depths that might constrain the invasiveness of L. latifolium. Our data, however, suggest that expensive control strategies for L. latifolium can be targeted to soils with high native levels of phosphorus availability; invasibility in areas with low phosphorus availability may naturally decline over time as phosphorus is bio-cycled by the plant.