Spreaders, igniters, and burning shrubs: plant flammability explains novel fire dynamics in grass-invaded deserts.

Novel fire regimes are an important cause and consequence of global environmental change that involve interactions among biotic, climatic, and human components of ecosystems. Plant flammability is key to these interactions, yet few studies directly measure flammability or consider how multiple species with different flammabilities interact to produce novel fire regimes. Deserts of the southwestern United States are an ideal system for exploring how novel fire regimes can emerge when fire-promoting species invade ecosystems comprised of species that did not evolve with fire. In these deserts, exotic annual grasses provide fuel continuity across landscapes that did not historically burn. These fires often ignite a keystone desert shrub, the fire-intolerant creosote bush, Larrea tridentata (DC.) Coville. Ignition of Larrea is likely catalyzed by fuels produced by native plants that grow beneath the shrubs. We hypothesize that invasive and native species exhibit distinct flammability characteristics that in combination determine spatial patterns of fire spread and intensity. We measured flammability metrics of Larrea, two invasive grasses, Schismus arabicus and Bromus madritensis, and two native plants, the sub-shrub Ambrosia dumosa and the annual herb Amsinckia menziesii. Results of laboratory experiments show that the grasses carry fire quickly (1.32 cm/s), but burn for short duration (0.5 min) at low temperatures. In contrast, native plants spread fire slowly (0.12 cm/s), but burn up to eight times longer (4 min) and produced hotter fires. Additional experiments on the ignition requirements of Larrea suggest that native plants burn with sufficient temperature and duration to ignite dead Larrea branches (time to ignition, 2 min; temperature at ignition 692°C). Once burning, these dead branches ignite living branches in the upper portions of the shrub. Our study provides support for a conceptual model in which exotic grasses are "spreaders" of fire and native plants growing beneath shrubs are "igniters" of dead Larrea branches. Once burning, flames produced by dead branches engulf the entire shrub, resulting in locally intense fires without historical precedent in this system. We suggest that fire models and conservation-focused management could be improved by incorporating the distinct flammability characteristics and spatial distributions of spreaders, igniters, and keystone shrubs.

Assessing the impact of fire on the spatial distribution of Larrea tridentata in the Sonoran Desert, USA.

In southwestern American deserts, fire has been historically uncommon because of insufficient continuity of fuel for spreading. However, deserts have been invaded by exotic species that now connect the empty space between shrubs to carry fire. We hypothesized that fire would change the spatial distribution of surviving Larrea tridentata shrubs. We established two study plots, one each in a burned and unburned area, and recorded location and living status of all shrubs. We performed univariate and bivariate point pattern analyses to characterize the impact of fire on the overall distribution of shrubs. Additionally, we used a simple wildfire model to determine how close we could come to reconstructing the observed spatial pattern of living and dead shrubs. We found a hyper-dispersed pattern of shrubs at finer scales and a random pattern at broader scales for both the unburned plot and for the living and dead shrubs combined in the burned plot, the latter providing an approximation of the pre-burn distribution of shrubs. After fire, living shrubs showed a clustered pattern at scales >2.5 m, whereas dead shrubs were randomly distributed, indicating that fire caused a change in the spatial pattern of the surviving shrubs. The fire model was able to partially reconstruct the spatial pattern of Larrea, but created a more clustered distribution for both living and dead shrubs. Our study reinforces the key role of fire in altering landscapes that had not been habituated to fire, and suggests the existence of potential cascading effects across the entire plant community.

Middle-Eastern plant communities tolerate 9 years of drought in a multi-site climate manipulation experiment.

For evaluating climate change impacts on biodiversity, extensive experiments are urgently needed to complement popular non-mechanistic models which map future ecosystem properties onto their current climatic niche. Here, we experimentally test the main prediction of these models by means of a novel multi-site approach. We implement rainfall manipulations--irrigation and drought--to dryland plant communities situated along a steep climatic gradient in a global biodiversity hotspot containing many wild progenitors of crops. Despite the large extent of our study, spanning nine plant generations and many species, very few differences between treatments were observed in the vegetation response variables: biomass, species composition, species richness and density. The lack of a clear drought effect challenges studies classifying dryland ecosystems as most vulnerable to global change. We attribute this resistance to the tremendous temporal and spatial heterogeneity under which the plants have evolved, concluding that this should be accounted for when predicting future biodiversity change.

Altered vegetative assemblage trajectories within an urban brownfield.

Recognizing the growing importance of both structure (maintenance of biodiversity) and function (fostering natural cycles) of urban ecologies, we examine coarse scale (herbaceous, shrub and forest) beta guild trajectory in an urban brownfield. The distribution of the pioneer forest assemblage dominated by Betula populifolia Marsh. and Populus spp. could be correlated positively with total soil metal load (arsenic, cadmium, chromium, copper, lead, zinc, lead and vanadium),whereas herbaceous and shrub guilds were negatively correlated. Distinct assemblage development trajectories above and below a critical soil metal threshold are demonstrated. In addition, we postulate that the translocation of metals into the plant tissue of several dominant species may provide a positive feedback loop, maintaining relatively high concentrations of metals in the litter and soil. Therefore assembly theory, which allows for the development of alternate stable states, may provide a better model for the establishment of restoration objectives on degraded urban sites.

Metal accumulation and performance of nestlings of passerine bird species at an urban brownfield site.

The use of passerine species as bioindicators of metal bioaccumulation is often underutilized when examining the wildlife habitat value of polluted sites. In this study we tested feathers of nestlings of two common bird species (house wren and American robin) for accumulation of Pb, Zn, As, Cr, Cu, Fe in comparison of a polluted, urban brownfield with a rural, unpolluted site. House wren nestlings at the study site accumulated significantly greater concentrations of all target metals except Zn. At the polluted site we found significant species differences of metal concentrations in feathers, with house wrens accumulating greater concentrations of Pb, Fe, and Zn but slightly lesser accumulations of Cr and Cu than American robins. Although house wren nestlings demonstrated significant accumulation of metals, these concentrations showed little effect on size metrics or fledge rates during the breeding season compared to nestlings from the control site.

Polyphenol oxidase activity in the roots of seedlings of Bromus (Poaceae) and other grass genera.

PREMISE OF THE STUDY: Phenolic compounds exuded by roots have been implicated in allelopathic interactions among plants. Root enzymes that destroy phenolics may protect plants against allelopathic inhibition and thus may aid in invasiveness. Phenolic-degrading enzymes are chiefly found in aboveground plant parts, but have also been previously reported in root tissues where the enzyme's function is unknown. We explored phenolic oxidase activity in emerging roots of grasses in a survey across different grass genera; in particular, we aimed to test whether grasses of the genus Bromus, known for their large invasion potential, differ in this respect from other grass taxa. • METHODS: We assayed a range of grass genera commonly found in the United States for root enzyme activity with spectrophotometric assays of phenol oxidase activity using l-DOPA as the main substrate. • KEY RESULTS: In the survey of a grass genera, we discovered that roots of the genus Bromus contain large amounts of polyphenol oxidase (PPO) activity, while all other tested grass genera, even ones closely related to Bromus, did not. PPO was found to be present at germination and remained active throughout the life of the plant. Compared to other PPOs, the enzyme present in Bromus appears to have a narrow substrate range. • CONCLUSIONS: The specific functions of the root PPO and the ecological ramifications of the special status of Bromus are not yet clear. The possibility that the enzyme plays a role in plant species interaction for bromes, a genus of grasses known to have high invasive potential, is raised.

Root cooperation in a clonal plant: connected strawberries segregate roots.

The ability to selectively avoid competition with members of the same clone should be highly advantageous but has not been demonstrated in plants. We found that physical connection between plants in a clone of the wild strawberry Fragaria chiloensis induced them to segregate their roots, significantly increasing clonal performance. Such increase in performance was not found when plants were grown in containers that artificially divided their rooting zones. There was no effect of connection in a different clone of F. chiloensis with a lower degree of carbon transport between connected plants, suggesting that the mechanism for root segregation depended upon transport of a signal through the strawberry runners. We suggest that clonal integration allows some clones to coordinate below-ground resource foraging with other clone members, thus exhibiting a type of root cooperation.

Coexistence and interference between a native perennial grass and non-native annual grasses in California.

Little is known about the potential for coexistence between native and non-native plants after large-scale biological invasions. Using the example of native perennial bunchgrasses and non-native annual grasses in California grasslands, we sought to determine the effects of interference from non-native grasses on the different life stages of the native perennial bunchgrass Nassella pulchra. Further, we asked whether N. pulchra interferes with non-native annual grasses, and whether competition for water is an important component of these interspecific interactions in this water-limited system. In a series of field and greenhouse experiments employing neighbor removals and additions of water, we found that seedling recruitment of N. pulchra was strongly seed-limited. In both field and greenhouse, natural recruitment of N. pulchra seedlings from grassland soil was extremely low. In field plots where we added seeds, addition of water to field plots increased density of N. pulchra seedlings by 88% and increased total aboveground N. pulchra seedling biomass by almost 90%, suggesting that water was the primary limiting resource. In the greenhouse, simulated drought early in the growing season had a greater negative effect on the biomass of annual seedlings than on the seedlings of N. pulchra. In the field, presence of annuals reduced growth and seed production of all sizes of N. pulchra, and these effects did not decrease as N. pulchra individuals increased in size. These negative effects appeared to be due to competition for water, because N. pulchra plants showed less negative pre-dawn leaf water potentials when annual neighbors were removed. Also, simply adding water caused the same increases in aboveground biomass and seed production of N. pulchra plants as removing all annual neighbors. We found no evidence that established N. pulchra plants were able to suppress non-native annual grasses. Removing large N. pulchra individuals did not affect peak biomass per unit area of annuals. We conclude that effects of interference from non native annuals are important through all life stages of the native perennial N. pulchra. Our results suggest that persistence of native bunchgrasses may be enhanced by greater mortality of annual than perennial seedlings during drought, and possibly by reduced competition for water in wet years because of increased resource availability.