Integrating spread dynamics and economics of timber production to manage Chinese tallow invasions in southern U.S. forestlands

Aquatic Plant Invasion and Management in Riverine Reservoirs: Proactive Management via a Priori Simulation of Management Alternatives

Negative impacts from aquatic invasive plants in the United States include economic costs, loss of commercial and recreational use, and environmental damage. Simulation models are valuable tools for predicting the invasion potentials of species and for the management of existing infestations. We developed a spatially explicit, agent-based model representing the invasion, growth, and senescence of aquatic weeds as functions of day length, water temperature, water depth, and the response of aquatic weeds to biological control. As a case study to evaluate its potential utility, we parameterized the model to represent two historical invasions (1975–1983 and 2004–2007) of Hydrilla (Hydrilla verticillata (L. fil.) Royle) in Lake Conroe, Texas, USA, and their subsequent biological control using grass carp (Ctenopharyngodon idella). Results of several hypothetical alternative management schemes indicated that grass carp stocking densities needed to control Hydrilla infestation increased exponentially as the lag time between initial invasion and initial stocking increased, whereas stocking densities needed to control infestation decreased as the amount of time allowed to control the infestation increased. Predictions such as those produced by our model aid managers in developing proactive management plans for areas most likely to be invaded.

Understory upheaval: factors influencing Japanese stiltgrass invasion in forestlands of Tennessee, United States

BACKGROUND

Invasions by non-native plants contribute to loss of ecosystem biodiversity and productivity, modification of biogeochemical cycles, and inhibition of natural regeneration of native species. Japanese stiltgrass (Microstegium vimineum (Trin.) A. Campus) is one of the most prevalent invasive grasses in the forestlands of Tennessee, United States. We measured the extent of invasion, identified potential factors affecting invasion, and quantified the relative importance of each factor. We analyzed field data collected by the Forest Inventory and Analysis Program of the U.S. Forest Service to measure the extent of invasion from 2005 to 2011 and identified potential factors affecting invasion during this period using boosted regression trees.

RESULTS

Our results indicated that presence of Japanese stiltgrass on sampled plots increased 50% (from 269 to 404 plots) during the time period. The probability of invasion was correlated with one landscape condition (elevation) (20.5%) and five forest features (including tree species diversity, basal area, stand age, site productivity, and natural regeneration) (79.5%). Boosted regression trees identified the most influential (highly correlated) variables as tree species diversity (30.7%), basal area (22.9%), elevation (20.5%), and stand age (16.7%). Our results suggest that Japanese stiltgrass is likely to continue its invasion in Tennessee forests.

CONCLUSIONS

The present model, in addition to correlating the probability of Japanese stiltgrass invasions with current climatic conditions and landscape attributes, could aid in the on-going development of control strategies for confronting Japanese stiltgrass invasions by identifying vulnerable areas that might emerge as a result of likely changes in climatic conditions and land use patterns.

Range expansion of invasive shrubs: implication for crown fire risk in forestlands of the southern USA

Non-native plant invasions and changing management activities have dramatically altered the structure and composition of forests worldwide. Invasive shrubs and fire suppression have led to increased densification and biomass accumulation in forest ecosystems of the southeastern USA. Notably, Chinese and European privets are rapid growing, shade-tolerant shrubs which number among the most aggressive invasive species in these forests. Privet encroachment has caused losses of native diversity, alteration of ecosystem processes and changes in community structure. The latter has become manifest through decreases in fine herbaceous fuels concurrent with increases in coarse woody fuels in forest understoreys. These alterations in fuel structure will potentially lead to less frequent, but more severe forest fires, which threaten important forest resources during extreme weather conditions. Drawing on extensive data sets compiled by the US Forest Service, we integrated statistical forecasting and analytical techniques within a spatially explicit, agent-based, simulation framework to predict potential range expansion of Chinese and European privet (Ligustrum sinenseandL. vulgare) and the associated increase in crown fire risk over the next two decades in forestlands of Mississippi and Alabama. Our results indicate that probability of invasion is positively associated with elevation, adjacency (within 300 m) to water bodies, mean daily maximum temperature, site productivity and private land ownership, and is negatively associated with slope, stand age, artificial regeneration, distance to the nearest road and fire disturbance. Our projections suggest the total area invaded will increase from 1.36 to ≈31.39% of all forestlands in Mississippi and Alabama (≈7 million hectares) and the annual frequency of crown fires in these forestlands will approximately double within the next two decades. Such time series projections of annual range expansions and crown fire frequency should provide land managers and restoration practitioners with an invasion chronology upon which to base proactive management plans.

Conspecific plasticity and invasion: invasive populations of Chinese tallow (Triadica sebifera) have performance advantage over native populations only in low soil salinity

Global climate change may increase biological invasions in part because invasive species may have greater phenotypic plasticity than native species. This may be especially important for abiotic stresses such as salt inundation related to increased hurricane activity or sea level rise. If invasive species indeed have greater plasticity, this may reflect genetic differences between populations in the native and introduced ranges. Here, we examined plasticity of functional and fitness-related traits of Chinese tallow (Triadica sebifera) populations from the introduced and native ranges that were grown along a gradient of soil salinity (control: 0 ppt; Low: 5 ppt; Medium: 10 ppt; High: 15 ppt) in a greenhouse. We used both norm reaction and plasticity index (PI_v) to estimate the conspecific phenotypic plasticity variation between invasive and native populations. Overall, invasive populations had higher phenotypic plasticity of height growth rate (HGR), aboveground biomass, stem biomass and specific leaf area (SLA). The plasticity Index (PI_v) of height growth rate (HGR) and SLA each were higher for plants from invasive populations. Absolute performance was always comparable or greater for plants from invasive populations versus native populations with the greatest differences at low stress levels. Our results were consistent with the “Master-of-some” pattern for invasive plants in which the fitness of introduced populations was greater in more benign conditions. This suggests that the greater conspecific phenotypic plasticity of invasive populations compared to native populations may increase invasion success in benign conditions but would not provide a potential interspecific competitive advantage in higher salinity soils that may occur with global climate change in coastal areas.

Chinese tallow trees (Triadica sebifera) from the invasive range outperform those from the native range with an active soil community or phosphorus fertilization

Two mechanisms that have been proposed to explain success of invasive plants are unusual biotic interactions, such as enemy release or enhanced mutualisms, and increased resource availability. However, while these mechanisms are usually considered separately, both may be involved in successful invasions. Biotic interactions may be positive or negative and may interact with nutritional resources in determining invasion success. In addition, the effects of different nutrients on invasions may vary. Finally, genetic variation in traits between populations located in introduced versus native ranges may be important for biotic interactions and/or resource use. Here, we investigated the roles of soil biota, resource availability, and plant genetic variation using seedlings of Triadica sebifera in an experiment in the native range (China). We manipulated nitrogen (control or 4 g/m(2)), phosphorus (control or 0.5 g/m(2)), soil biota (untreated or sterilized field soil), and plant origin (4 populations from the invasive range, 4 populations from the native range) in a full factorial experiment. Phosphorus addition increased root, stem, and leaf masses. Leaf mass and height growth depended on population origin and soil sterilization. Invasive populations had higher leaf mass and growth rates than native populations did in fresh soil but they had lower, comparable leaf mass and growth rates in sterilized soil. Invasive populations had higher growth rates with phosphorus addition but native ones did not. Soil sterilization decreased specific leaf area in both native and exotic populations. Negative effects of soil sterilization suggest that soil pathogens may not be as important as soil mutualists for T. sebifera performance. Moreover, interactive effects of sterilization and origin suggest that invasive T. sebifera may have evolved more beneficial relationships with the soil biota. Overall, seedlings from the invasive range outperformed those from the native range, however, an absence of soil biota or low phosphorus removed this advantage.