Canopy gaps facilitate establishment, growth, and reproduction of invasive Frangula alnus in a Tsuga canadensis dominated forest

Spatial Patterns of Frangula alnus (Rosales: Rhamnaceae): Implications for Invasive Plant Management

Glossy buckthorn (Frangula alnus) (Rosales: Rhamnaceae) is an invasive shrub from Europe that has been invading North America for over a century and threatening native vegetation in open and disturbed habitats. The treatment of F. alnus is currently restricted to the roadside, suggesting any individual F. alnus residing within the forest would be left unmanaged and would continue to spread in the area. This research was conducted to determine the spatial patterns and relationship of F. alnus with forest roads. The presence and density of F. alnus at 1412 sample points were recorded on four sites in the Allegheny National Forest, Pennsylvania, USA. Buffer analyses were conducted along roads to determine the relationship between F. alnus density and proximity to forest roads. Geostatistics and spatial analysis by distance indices (SADIE) were used to characterize the spatial pattern of F. alnus. Results of this study showed that F. alnus was spatially aggregated and resided beyond forest roads. Both the density and presence of F. alnus decreased as the distance from the forest road increased. These results imply the potential for precision management of F. alnus by locating and managing only where F. alnus presents.

Rodent-mediated plant seed dispersal: What happens to the seeds after entering the gaps with different sizes?

In general, it is accepted that gap formation significantly affects the placement of scatter-hoarded seeds by small rodents, but the effects of different forest gap sizes on the seed-eating and scatter-hoarding behaviors of small rodents remain unclear. Thus, we examined the effects of a closed-canopy forest, forest edge, and gaps with different sizes on the spatial dispersal of Quercus variabilis acorns and cache placement by small rodents using coded plastic tags in the Taihang Mountains, China. The seeds were removed rapidly, and there were significant differences in the seed-eating and caching strategies between the stand types. We found that Q. variabilis acorns were usually eaten after being removed from the closed-canopy forest and forest edges. By contrast, the Q. variabilis acorns in the forest gap stands were more likely to be scatter-hoarded. The dispersal distances of Q. variabilis acorns were significantly longer in the forest gap plots compared with the closed canopy and forest edge plots. However, the proportion of scatter-hoarded seeds did not increase significantly as the gap size increased. In small-scale oak reforestation projects or research, creating small gaps to promote rodent-mediated seed dispersal may effectively accelerate forest recovery and successional processes.

Extending coverage and thematic resolution of compositional land cover maps in a hierarchical Bayesian framework

Ecological models are constrained by the availability of high-quality data at biologically appropriate resolutions and extents. Modeling a species' affinity or aversion with a particular land cover class requires data detailing that class across the full study area. Data sets with detailed legends (i.e., high thematic resolution) and/or high accuracy often sacrifice geographic extent, while large-area data sets often compromise on the number of classes and local accuracy. Consequently, ecologists must often restrict their study extent to match that of the more precise data set, or ignore potentially key land cover associations to study a larger area. We introduce a hierarchical Bayesian model to capitalize on the thematic resolution and accuracy of a regional land cover data set, and on the geographic breadth of a large area land cover data set. For the full extent (i.e., beyond the regional data set), the model predicts systematic discrepancies of the large-area data set with the regional data set, and divides an aggregated class into two more specific classes detailed by the regional data set. We illustrate the application of our model for mapping eastern white pine (Pinus strobus) forests, an important timber species that also provides habitat for an invasive shrub in the northeastern United States. We use the National Land Cover Database (NLCD), which covers the full study area but includes only generalized forest classes, and the NH GRANIT land cover data set, which maps White Pine Forest and has high accuracy, but only exists within New Hampshire. We evaluate the model at coarse (20 km² ) and fine (2 km² ) resolutions, with and without spatial random effects. The hierarchical model produced improved maps of compositional land cover for the full extent, reducing inaccuracy relative to NLCD while partitioning a White Pine Forest class out of the Evergreen Forest class. Accuracy was higher with spatial random effects and at the coarse resolution. All models improved upon simply partitioning Evergreen Forest in NLCD based on the predicted distribution of white pine. This flexible statistical method helps ecologists leverage localized mapping efforts to expand models of species distributions, population dynamics, and management strategies beyond the political boundaries that frequently delineate land cover data sets.

Similar seed dispersal systems by local frugivorous birds in native and alien plant species in a coastal seawall forest

Frugivorous birds play an important role in seed dispersal. Alien plant species' seeds are dispersed by local birds in order to establish populations in new habitats. Alien plant species that produce fruits similar to that of native species have the potential to attract local birds, creating new mutualistic systems that are similar to the local ones. In autumn 2018 and 2019, we studied the seed dispersal systems of an alien plant species, Phytolacca americana, and a native species, Cayratia japonica, in a coastal seawall forest. Both plant species' fruit, frugivorous bird foraging behaviors, seed germination rates, and seedling microhabitats were examined to determine whether the alien species had a similar seed dispersal system to that of the native species. Our results showed that P. americana and C. japonica had similar fruit type, color, and ripening period. There was a positive correlation between the percentage rate of fruit ripening and the percentage rate of fruit missing for both plant species, indicating that local frugivorous birds have the potential to sufficiently disperse the alien seeds to enable its spread in the coastal seawall forest (simple linear regression, P. americana: β = 0.863 ± 0.017, R2 adj = 0.978, P < 0.01; C. japonica: β = 0.787 ± 0.034, R2 adj = 0.898, P < 0.01). Eleven bird species consumed the fruits of the alien species or native species during the study period. Similar results were shown across alien and native species in bird foraging behavior (feeding frequency, feeding duration and first stop distance) indicating that a similar seed dispersal relationship had been established between local frugivorous and both plant species. The alien plant had a higher number of fruits carried by birds, suggesting that P. americana had a slightly higher fruit consumption than that of C. japonica (t-test, P < 0.01). Alien plant seedlings grow more abundant in forest gap microhabitat (t-test, P < 0.01). Our results confirmed that bird digestion promotes seed germination success in both plant species. Our study suggests that in a narrow coastal seawall forest, alien plant species can successfully establish their populations by relying on similar seed dispersal systems as the local species.

Seed germination ecology of Ageratum houstonianum: A major invasive weed in Nepal

In recent years, spread of invasive alien plant species (IAPS) has been a major concern in Nepal. One such IAPS is Ageratum houstonianum, an Asteraceae, that is a prolific seed producer and difficult-to-control in farmland and various ecological regions causing crop yield and biodiversity losses. However, very little information is available on the germination biology and ecology of this species. Therefore, experiments were conducted to assess the effect of water stress, pH level, and light requirement on seed germination, and the effect of seed burial depth on seedling emergence. Water stress was simulated by polyethylene glycol solutions ranging from 0–5.56 MPa and pH solutions ranging from 4 to 9 were prepared using hydrochloric acid and sodium hydroxide. Germination tests were conducted in petri dishes lined with filter paper and placed in a controlled environment chamber set at 20° C. Light requirement comparisons were made by having petri dishes wrapped with aluminum foil or left unwrapped. Seedling emergence was evaluated by placing seeds at depths ranging from 0 to 20 mm in the soil. Results indicated that this species was moderately drought-tolerant because germination ceased beyond 0.51 MPa. Greater germination occurred at neutral to acidic than at alkaline pH levels. The seeds were positively photoblastic because no germination occurred under dark condition. No seedlings emerged from seeds placed more than 2 mm deep in the soil, indicating that this is a primarily surface germinating species. These findings will help predict future invasions and in development of management strategies for this IAPS.

Plant communities in harsh sites are less invaded: a summary of observations and proposed explanations

Plant communities in abiotically stressful, or 'harsh', habitats have been reported to be less invaded by non-native species than those in more moderate habitats. Here, we synthesize descriptive and experimental evidence for low levels of invasion in habitats characterized by a variety of environmental stressors: low nitrogen; low phosphorus; saline, sodic or alkaline soils; serpentine soils; low soil moisture; shallow/rocky soils; temporary inundation; high shade; high elevation; and high latitude. We then discuss major categories of hypotheses to explain this pattern: the propagule limitation mechanism suggests invasion of harsh sites is limited by relatively low arrival rates of propagules compared with more moderate habitats, while invasion resistance mechanisms suggest that harsh habitats are inherently less invasible due to stressful abiotic conditions and/or increased effects of biotic resistance from resident organisms. Both propagule limitation and invasion resistance may simultaneously contribute to low invadedness of harsh sites, but the management implications of these mechanisms differ. If propagule limitation is more important, managers should focus on reducing the likelihood of propagule introductions. If invasion resistance mechanisms are in play, managers should focus on restoring or maintaining harsh conditions at a site to reduce invasibility.

Effects of Invasive Winter Moth Defoliation on Tree Radial Growth in Eastern Massachusetts, USA

Winter moth, Operophtera brumata L. (Lepidoptera: Geometridae), has been defoliating hardwood trees in eastern Massachusetts since the 1990s. Native to Europe, winter moth has also been detected in Rhode Island, Connecticut, eastern Long Island (NY), New Hampshire, and Maine. Individual tree impacts of winter moth defoliation in New England are currently unknown. Using dendroecological techniques, this study related annual radial growth of individual host (Quercus spp. and Acer spp.) trees to detailed defoliation estimates. Winter moth defoliation was associated with up to a 47% reduction in annual radial growth of Quercus trees. Latewood production of Quercus was reduced by up to 67% in the same year as defoliation, while earlywood production was reduced by up to 24% in the year following defoliation. Winter moth defoliation was not a strong predictor of radial growth in Acer species. This study is the first to document impacts of novel invasions of winter moth into New England.

Modeling the impacts of life-history traits, canopy gaps, and establishment location on woodland shrub invasions

We used an individual-based model to identify how localized patterns of woodland invasions by exotic shrubs are likely influenced by (1) observed variation in age at first reproduction and fecundity, (2) hypothesized effects of canopy gaps on these life-history traits and dispersal, and (3) initial establishment location. Rates of spread accelerated nearly twofold as age at first reproduction decreased from eight to three years or fecundity increased from 3 to 20 offspring per year, illustrating the need to better understand the factors that influence these life-history traits. Canopy gaps facilitated spread by influencing these life-history traits, but not through their effects on dispersal. Invasions starting at the woodland center spread more rapidly than do those starting along the woodland edge. These findings suggest that managers should not only prioritize the removal of shrubs that reproduce the earliest or produce the most offspring, but they should also focus on the invasions in woodlands with high canopy openness and/or that are located in woodland interiors. Investigated factors also affected other invasion characteristics, often in surprising ways. For example, those changes in age at first reproduction and fecundity that increased the rate of spread produced nonparallel patterns of change in the proportions of invasion reproducing, whether or not invasions exhibited clumped or scattered spatial arrangements, and invasional lag. Additionally, canopy gaps influenced these characteristics by increasing fecundity, but not by decreasing age at first reproduction or altering dispersal, suggesting that canopy gaps affect local patterns of exotic-shrub invasions primarily through their positive effects on fruit production.

Internal habitat quality determines the effects of fragmentation on austral forest climbing and epiphytic angiosperms

Habitat fragmentation has become one of the major threats to biodiversity worldwide, particularly in the case of forests, which have suffered enormous losses during the past decades. We analyzed how changes in patch configuration and habitat quality derived from the fragmentation of austral temperate rainforests affect the distribution of six species of forest-dwelling climbing and epiphytic angiosperms. Epiphyte and vine abundance is primarily affected by the internal characteristics of patches (such as tree size, the presence of logging gaps or the proximity to patch edges) rather than patch and landscape features (such as patch size, shape or connectivity). These responses were intimately related to species-specific characteristics such as drought- or shade-tolerance. Our study therefore suggests that plant responses to fragmentation are contingent on both the species' ecology and the specific pathways through which the study area is being fragmented, (i.e. extensive logging that shaped the boundaries of current forest patches plus recent, unregulated logging that creates gaps within patches). Management practices in fragmented landscapes should therefore consider habitat quality within patches together with other spatial attributes at landscape or patch scales.

Monitoring and managing responses to climate change at the retreating range edge of forest trees

Rising temperatures and increasing drought severity linked to global climate change are negatively impacting forest growth and function at the equatorial range edge of species distributions. Rapid dieback and range retractions are predicted to occur in many areas as temperatures continue to rise. Despite widespread negative impacts at the ecosystem level, equatorial range edges are not well studied, and their responses to climate change are poorly understood. Effective monitoring of tree responses to climate in these regions is of critical importance in order to predict and manage threats to populations. Remote sensing of impacts on forests can be combined with ground-based assessment of environmental and ecological changes to identify populations most at risk. Modelling may be useful as a 'first-filter' to identify populations of concern but, together with many remote sensing methods, often lacks adequate resolution for application at the range edge. A multidisciplinary approach, combining remote observation with targeted ground-based monitoring of local susceptible and resistant populations, is therefore required. Once at-risk regions have been identified, management can be adapted to reduce immediate risks in priority populations, and promote long-term adaptation to change. However, management to protect forest ecosystem function may be preferable where the maintenance of historical species assemblages is no longer viable.