Jennifer L. Bufford

Woody exotic plant invasions and fire: reciprocal impacts and consequences for native ecosystems

Fire regimes influence and are influenced by the structure and composition of plant communities. This complex reciprocal relationship has implications for the success of plant invasions and the subsequent impact of invasive species on native biota. Although much attention has been given to the role of invasive grasses in transforming fire regimes and native plant communities, little is known about the relationship between woody invasive species and fire regime. Despite this, prescribed burning is frequently used for managing invasive woody species. In this study we review relationships between woody exotic plant invasions and fire in invaded ecosystems worldwide. Woody invaders may increase or decrease aspects of the fire regime, including fire frequency, intensity and extent. This is in contrast to grass invaders which almost uniformly increase fire frequency. Woody plant invasion can lead to escape from a grass-fire cycle, but the resulting reduction in fire frequency can sometimes lead to a cycle of rare but more intense fires. Prescribed fires may be a useful management tool for controlling woody exotic invaders in some systems, but they are rarely sufficient to eliminate an invasive species, and a dearth of controlled experiments hampers evaluation of their benefits. Nevertheless, because some woody invaders have fuel properties that differ substantially from native species, understanding and managing the impacts of woody invaders on fire regimes and on prescribed burns should become an important component of resource and biodiversity management.

Forest Invasion by the African Tulip Tree (Spathodea campanulata) in the Hawaiian Islands: Are Seedlings Shade-Tolerant?

Abstract: Native to West Africa, Spathodea campanulata (African tulip tree) is frequently viewed as a shade-intolerant invader. It commonly colonizes roadsides, human-disturbed forests, and abandoned agricultural land in tropical islands, where it can then become dominant in secondary forests. Some authors have suggested that the seedlings may be shade-tolerant and able to establish in closed-canopy forest, but the shade tolerance of seedlings has never been evaluated. We identified tolerated light environments of S. campanulata seedlings in wet forests in Hawai‘i by measuring photosynthetically active radiation (PAR) around naturally occurring seedlings (<30 cm height) in the field. We also measured photosynthetic responses of seedlings to light under field and laboratory conditions and determined seedling growth rates in sun and shade. Seedlings were found in shaded conditions in the field, and they consistently had positive net carbon gain at 50 µmol photons m-2.sec-1 PAR, with an estimated mean compensation point below 10 µmol photons m-2.sec-1, indicating high shade tolerance. The most frequent midday light environments of S. campanulata seedlings in the field were in the range of 50 to 200 µmol photons m-2.sec-1 PAR (i.e., 2.5% to 10% of full sunlight). Among seedlings found growing in shade, minimum saturating light (Ek), determined from chlorophyll fluorescence, averaged 260 µmol photons m-2.sec-1, suggesting that maximum seedling photosynthesis can occur at less than 13% of full sun. Growth rates of young seedlings in shade and sun were comparable. Widespread wind dispersal of seeds, seedling tolerance of low light, and our observations of some S. campanulata saplings establishing in rain forest without recent disturbance suggest that S. campanulata will be a persistent component of Hawaiian lowland rain forests.

Rapid Anthropocene Speciation Reveals Pull of the Recent: A Response to Thomas.

Evolutionary biologists have long recognized that evidence of increased species diversification rates in recent versus fossil records may simply reflect better sampling of contemporary biodiversity, a bias they have termed the ‘Pull of the Recent’ [1,2]. Estimates of speciation in geological time necessarily incorporate both speciation and extinction, as only species that persisted long enough to leave fossil or genetic traces will be included. By contrast, estimates of recent speciation rates do not incorporate extinction because of the lag between speciation and extinction, thus inflating contemporary estimates of speciation relative to geological estimates.

Import volumes and biosecurity interventions shape the arrival rate of fungal pathogens

Global trade and the movement of people accelerate biological invasions by spreading species worldwide. Biosecurity measures seek to allow trade and passenger movements while preventing incursions that could lead to the establishment of unwanted pests, pathogens, and weeds. However, few data exist to evaluate whether changes in trade volumes, passenger arrivals, and biosecurity measures have altered rates of establishment of nonnative species over time. This is particularly true for pathogens, which pose significant risks to animal and plant health and are consequently a major focus of biosecurity efforts but are difficult to detect. Here, we use a database of all known plant pathogen associations recorded in New Zealand to estimate the rate at which new fungal pathogens arrived and established on 131 economically important plant species over the last 133 years. We show that the annual arrival rate of new fungal pathogens increased from 1880 to about 1980 in parallel with increasing import trade volume but subsequently stabilised despite continued rapid growth in import trade and recent rapid increases in international passenger arrivals. Nevertheless, while pathogen arrival rates for crop and pasture species have declined in recent decades, arrival rates have increased for forestry and fruit tree species. These contrasting trends between production sectors reflect differences in biosecurity effort and suggest that targeted biosecurity can slow pathogen arrival and establishment despite increasing trade and international movement of people.

Detection of the entomopathogenic fungus Beauveria bassiana in the rhizosphere of wound-stressed Zea mays plants

Entomopathogenic fungi from the genus Beauveria (Vuillemin) play an important role in controlling insect populations and have been increasingly utilized for the biological control of insect pests. Various studies have reported that Beauveria bassiana (Bals.), Vuill. also has the ability to colonize a broad range of plant hosts as endophytes without causing disease but while still maintaining the capacity to infect insects. Beauveria is often applied as an inundative spore application, but little research has considered how plant colonization may alter the ability to persist in the environment. The aim of this study was to investigate potential interactions between B. bassiana and Zea mays L. (maize) in the rhizosphere following inoculation, in order to understand the factors that may affect environmental persistence of the fungi. The hypothesis was that different isolates of B. bassiana have the ability to colonize maize roots and/or rhizosphere soil, resulting in effects to the plant microbiome. To test this hypothesis, a two-step nested PCR protocol was developed to find and amplify Beauveria in planta or in soil; based on the translation elongation factor 1-alpha (ef1α) gene. The nested protocol was also designed to enable Beauveria species differentiation by sequence analysis. The impact of three selected B. bassiana isolates applied topically to roots on the rhizosphere soil community structure and function were consequently assessed using denaturing gradient gel electrophoresis (DGGE) and MicroRespTM techniques. The microbial community structure and function were not significantly affected by the presence of the isolates, however, retention of the inocula in the rhizosphere at 30 days after inoculation was enhanced when plants were subjected to intensive wounding of foliage to crudely simulate herbivory. The plant defense response likely changed under wound stress resulting in the apparent recruitment of Beauveria in the rhizosphere, which may be an indirect defensive strategy against herbivory and/or the result of induced systemic susceptibility in maize enabling plant colonization.

Strong fitness differences impede coexistence between an alien water fern ( Azolla pinnata R. Br.) and its native congener ( Azolla rubra R. Br.) in New Zealand

Despite considerable evidence that alien plants impact the fecundity, productivity and abundance of native plant species, support for alien plant species causing the widespread decline of native species is rare. Coexistence theory proposes that the outcome of competition between two species can be predicted through the invasion criterion, measured as a positive population-level growth rate of each competitor when that species is rare. Here we make use of coexistence theory to examine the likelihood of persistence of a native water fern (Azolla rubra) following invasion by an alien congener (Azolla pinnata) which has apparently displaced the native wherever their ranges overlap in New Zealand. We evaluate coexistence between the two water fern species using experimental measurements of population-level growth rates. We show that the alien A. pinnata has a higher fitness than A. rubra, which hinders coexistence between the two species. These experimental results match the rapid expansion of A. pinnata and the apparent decline of A. rubra observed in nature. Our study predicts that A. pinnata is capable of replacing its native congener, highlights the importance of fitness differences in invasion success, and demonstrates the value of experimental analyses of species coexistence for predicting longer-term invasion dynamics and impacts. Using experiments to test coexistence mechanisms between alien and native species is a valuable approach to predict invasion outcomes and one that can lead to insights on the long-term impacts of alien species, including extinction, on native species populations.