Erik A. Lehnhoff

The Rationale for Monitoring Invasive Plant Populations as a Crucial Step for Management

Abstract Many land managers are faced with trying to optimize management of invasive plant species based on budget constraints and lack of knowledge of the true potential of the species. Generally, “early detection rapid response” (EDRR) is the assumed best management strategy and tends to drive management regardless of the invasion stage or possible variation in the invasion potential of the population. We created a simulation model to evaluate the optimal management strategy to reduce the rate of invasion of nonindigenous plant species. The strategies were specifically chosen to assess the value of information from monitoring populations. We compared four management strategies and a no-management control over a 20-yr period in the context of a management area: (1) managing a fixed number of populations at random each year (EDRR random), (2) managing an equivalent number of populations along a road each year (EDRR road), (3) managing half of the fixed populations that were determined by monitoring to be sources of new populations (monitoring every year), and (4) managing an equivalent set of source populations only on even years, leaving the odd years for monitoring (monitoring every other year). EDRR random location without regard to population invasion potential, and monitoring every year targeting management on populations determined to be invasive (sources for new populations), were the most successful strategies for reducing the increase in total number of populations. The model simulations suggest that managers could dedicate 50% of their management time to monitoring without risk of accelerating invasions or reducing the impact of their weed management program.

Non-indigenous species management using a population prioritization framework

Few agencies or land owners have sufficient resources to target every non-indigenous plant species (NIS) population once they have become established within a management area. Therefore, prioritization of NIS populations for management is a crucial component of the management process. Conceptually, effective management of NIS can be regarded as having four phases that revolve around the land management goals and how best to manage the NIS present in the area to achieve these goals. The key phases are determining the land management goals, inventory/survey, monitoring, evaluation and prioritization. Inventory/survey determines which species are present and their distribution within the landscape. These data can be used to develop probability of occurrence maps, which help in the nonbiased selection of populations for invasiveness and impact monitoring. Monitoring for invasiveness provides information on spatial and temporal changes within a population. Monitoring for impact assesses three types of impact: ...

Agricultural Weed Research: A Critique and Two Proposals

Abstract Two broad aims drive weed science research: improved management and improved understanding of weed biology and ecology. In recent years, agricultural weed research addressing these two aims has effectively split into separate subdisciplines despite repeated calls for greater integration. Although some excellent work is being done, agricultural weed research has developed a very high level of repetitiveness, a preponderance of purely descriptive studies, and has failed to clearly articulate novel hypotheses linked to established bodies of ecological and evolutionary theory. In contrast, invasive plant research attracts a diverse cadre of nonweed scientists using invasions to explore broader and more integrated biological questions grounded in theory. We propose that although studies focused on weed management remain vitally important, agricultural weed research would benefit from deeper theoretical justification, a broader vision, and increased collaboration across diverse disciplines. To initiate change in this direction, we call for more emphasis on interdisciplinary training for weed scientists, and for focused workshops and working groups to develop specific areas of research and promote interactions among weed scientists and with the wider scientific community.

Impact of Biotic and Abiotic Stresses on the Competitive Ability of Multiple Herbicide Resistant Wild Oat (Avena fatua)

Ecological theory predicts that fitness costs of herbicide resistance should lead to the reduced relative abundance of resistant populations upon the cessation of herbicide use. This greenhouse research investigated the potential fitness costs of two multiple herbicide resistant (MHR) wild oat (Avena fatua) populations, an economically important weed that affects cereal and pulse crop production in the Northern Great Plains of North America. We compared the competitive ability of two MHR and two herbicide susceptible (HS) A. fatua populations along a gradient of biotic and abiotic stresses The biotic stress was imposed by three levels of wheat (Triticum aestivum) competition (0, 4, and 8 individuals pot−1) and an abiotic stress by three nitrogen (N) fertilization rates (0, 50 and 100 kg N ha−1). Data were analyzed with linear mixed-effects models and results showed that the biomass of all A. fatua populations decreased with increasing T. aestivum competition at all N rates. Similarly, A. fatua relative growth rate (RGR) decreased with increasing T. aestivum competition at the medium and high N rates but there was no response with 0 N. There were no differences between the levels of biomass or RGR of HS and MHR populations in response to T. aestivum competition. Overall, the results indicate that MHR does not confer growth-related fitness costs in these A. fatua populations, and that their relative abundance will not be diminished with respect to HS populations in the absence of herbicide treatment.

Comparison of Transect-Based Standard and Adaptive Sampling Methods for Invasive Plant Species

Abstract Early detection of an invading nonindigenous plant species (NIS) may be critical for efficient and effective management. Adaptive survey sampling methods may provide unbiased sampling for best estimates of distribution of rare and spatially clustered populations of plants in the early stages of invasion. However, there are few examples of these methods being used for nonnative plant surveys in which travelling distances away from an initial or source patch, or away from a road or trail, can be time consuming due to the topography and vegetation. Nor is there guidance as to which of the many adaptive methods would be most appropriate as a basis for invasive plant mapping and subsequent management. Here we used an empirical complete census of four invader species in early to middle stages of invasion in a management area to assess the effectiveness and efficiency of three nonadaptive methods, four adaptive cluster methods, and four adaptive web sampling methods that all originated from transects. The adaptive methods generally sampled more NIS-occupied cells and patches than standard transect approaches. Sampling along roads only was time-efficient and effective, but only for species with restricted distribution along the roads. When populations were more patchy and dispersed over the landscape the adaptive cluster starting at the road generally proved to be the most time-efficient and effective NIS detection method. Management Implications: It is often not possible or cost-effective to conduct a complete inventory of potentially invasive plant species in large management areas, particularly at the early stages of invasion when populations may be infrequent and dispersed on the landscape. Detection at the early stages of invasion may be crucial for effective and cost-effective management. Thus managers must have survey methods that are effective and efficient for estimating the distribution of invading species. To accomplish different survey goals, which may include finding early invading populations, locating many different invasive plant species, finding the most populations of a single species, or collecting information to characterize species distributions, knowing which survey technique to use is critical. We tested three standard and eight adaptive survey methods on a virtual landscape populated with four empirically censused invasive plant species: Canada thistle, Dalmatian toadflax, smooth brome, and common St. Johnswort. The species exhibited somewhat different growth forms, reproductive patterns, and seed dispersal distances and were in different stages of invasion. Random transects with adaptive cluster sampling generally performed best when the survey goal was to find the largest number of populations in the shortest amount of time for species that were well established and occupied areas away from the road. If the species was in the early stages of invasion and only occupied roadside habitat, surveying along roads performed best. When the survey goal was to accurately assess the proportion of the landscape infested by each species, stratified random targeted transects without adaptive sampling performed best for all species. However, managers should be aware that adaptive sampling methods overestimate infested area. This study indicates that adaptive sampling methods can improve nonindigenous species patch detection for management, but regardless of the sampling method, detection remains relative low (maximum of 33% of patches) with typical management constraints and therefore seriously challenges the concept of early detection and rapid response.

Quantifying Invasiveness of Plants: A Test Case with Yellow Toadflax (Linaria vulgaris)

Abstract Land managers commonly assume that nonindigenous plant species (NIS) are rapidly increasing in population size in all environments in which they occur. In fact, these plant species have differing levels of invasiveness depending on environment. A method was developed that quantifies invasiveness of a plant population based on annual changes in plant density and area occupied, within a series of permanently placed 1 m2 (10.76 ft2) monitoring plots. An invasiveness index (I) was calculated from the change in proportion of cells occupied and the proportions of cells that had growth rates > 1 and < 1; the possible value is restricted from −4 to +4. The method was tested on populations of yellow toadflax over 6 yr on a total of six populations within three distinct environments (Ridge, Valley, and Forest). Invasiveness values were different between environments. The Ridge populations had the highest mean level of invasiveness (I  =  0.31), followed by the Valley (I  =  0.26), and then the Forest (I  =  −0.90). Invasiveness also varied by year. The highest annual value of invasiveness was at the Ridge (I  =  1.77) and the lowest was at the Forest (I  =  −1.90), both in 2005. Values of invasiveness were correlated (Pearsons correlation coefficient  =  0.82) with the traditional calculations of population growth rate, but our method provides an enhanced measure of invasiveness because it includes information on both change in population area and density. This research shows that populations of yellow toadflax are not equally invasive in different environments or through time, although consistent patterns can be observed. The method presented and tested was implemented in approximately three person-days per year at less than

Reduced Impacts or A Longer Lag Phase? Tamarix in the Northwestern U.S.A.

500 per year, and can be used to quantify the invasiveness of plant populations and thus allow land managers to prioritize the most invasive populations for management. Nomenclature: Yellow toadflax, Linaria vulgaris Mill. LINVU.

Tamarisk (Tamarix spp.) Establishment in its Most Northern Range

Tamarix spp. (tamarisk) have caused ecological impacts in the southwestern United States; however, such impacts have not been extensively studied in the Northwest where tamarisk is a relatively new invader. Here we present the results of soil, arbuscular mycorrhizal fungi, and vegetation studies from tamarisk-occupied and unoccupied areas on the dammed Bighorn River, Fort Peck Reservoir, and the free flowing Yellowstone River, in Montana. Soil sample results indicated that at Fort Peck Reservoir soil salinity was twice as high at occupied sites compared to unoccupied ones, and at the Bighorn River occupied sites nitrate, phosphorus and potassium were 2.2, 4, and 1.9 times higher, respectively, than at unoccupied sites. No soil differences were observed on the Yellowstone River. Mycorrhizal infectivity potential was high in both occupied and unoccupied soils, with a slight reduction (from 73% to 65% colonization) in tamarisk occupied soils. These impacts were statistically but not ecologically significant and did not extend to other metrics of impact such as richness, Simpson’s diversity or composition of plant communities. Our results indicate that either tamarisk has minimal impacts in the northwest, or it is still in a lag phase.

Effects of Soil Nitrogen and Atmospheric Carbon Dioxide on Wheat streak mosaic virus and Its Vector (Aceria tosichella Kiefer)

Abstract Tamarisk, a shrub or small tree native to Eurasia, was introduced to North America in the early 1800s and is now naturalized throughout many riparian areas of the southwestern United States, where extensive research has been conducted. It is a more recent invader to the northern Great Plains, and fewer studies have been conducted on tamarisk ecology and management in this area. The objectives of this research were to investigate the overwintering potential of tamarisk seeds in Montana and the relationship between hydrologic conditions and historic tamarisk establishment. Emergence of seedlings from seeds stored for different time periods at a range of temperatures was evaluated in a greenhouse study. Emergence rates declined after a 7-d storage period, but storage time had no effect on subsequent emergence rates, and seeds stored at −14 C and 5 C had greater emergence rates than those stored at 20 C and 35 C. Patterns in tamarisk establishment were assessed through age and hydrologic data collected from a reservoir (Fort Peck), a regulated river (Bighorn), and an unregulated river (Yellowstone) in Montana. These data indicated that tamarisk establishment at the reservoir was closely related to historic water levels, whereas establishment on rivers was not related to flow. However, data from the rivers indicated that recruitment differed between regulated and unregulated rivers, with the regulated river having less recruitment after the period of initial colonization than the unregulated river. Our results show that tamarisk seeds have the ability to overwinter in Montana and can establish under a range of flow conditions, indicating potential recolonization of sites after tamarisk removal. Nomenclature: Tamarisk, species in the genus Tamarix L., primarily Chinese tamarisk, Tamarix chinensis Lour., saltcedar, Tamarix ramosissima Ledeb., and their hybrids Interpretive Summary: Tamarisk is a shrub or small tree that has invaded riparian areas of the southwestern United States, and is becoming more of a concern in the northern Great Plains. Although considerable ecological research has been conducted on tamarisk in the Southwest, much less information exists to guide its management in its northern range. This research investigated tamarisk seed overwintering potential in Montana and the effects of reservoir (Fort Peck) level and river (Bighorn and Yellowstone) flows on tamarisk establishment. Results showed that tamarisk seeds have the ability to survive cold and cool temperatures (−14 and 5 C) for at least 6 mo, but that warm and hot temperatures (20 and 35 C) led to declines in seedling emergence, with no seeds surviving more than 90 d at 35 C. Results from rivers showed that tamarisk trees can establish under various flows, including very high and very low peak flows. Together, these results suggest that sites where tamarisk has been removed could be subjected to recolonization from a short lived seedbank or from other seed sources, regardless of flow conditions. Therefore, managers should implement a monitoring program to detect new colonization after treatment. Results from the reservoir showed that tamarisk established as the water level receded, suggesting that managers should monitor around the shoreline as water levels decline to enhance early detection of new tamarisk populations. Finally, tamarisk populations decline subsequent to submergence by rising water, indicating that they can be controlled by raising the water level in reservoirs.

Relative Canopy Height Influences Wild Oat (Avena fatua) Seed Viability, Dormancy, and Germination

Management of vector-borne plant viruses requires understanding how abiotic (e.g., resource availability) and biotic (e.g., virus-vector interactions) factors affect disease via effects on epidemiological parameters that drive disease spread. We conducted two complementary experiments using Wheat streak mosaic virus (WSMV): (i) a field study to determine the effects of nitrogen (N) fertilization on winter wheat (Triticum aestivum L.) susceptibility to WSMV infection and (ii) a growth chamber study to evaluate the effects of N and carbon dioxide (CO2) enrichment on population growth rates of the wheat curl mite (WCM), the vector of WSMV, and whether the effects of nutrient addition on WCM reproduction were modified by WSMV infection. The relationship between N fertilization and plant susceptibility to WSMV infection was nonlinear, with infection rates increasing rapidly as soil nitrate increased from 0 to 20 ppm and more gradually at higher nitrate concentrations. In the growth chamber study, N fertilization increased WCM population growth rates when the vectors transmitted WSMV but had the opposite effect on nonviruliferous mites. CO2 enrichment had no observable effects on WCM populations. These results suggest that, whereas the spread of WSMV is facilitated by N addition, increases in atmospheric CO2 may not directly alter WCM populations and WSMV spread.