Rodent herbivory differentially affects mortality rates of 14 native plant species with contrasting life history and growth form traits

Growth Process and Mortality of Sasa borealis Seedlings over Six Years Following Mass Flowering and Factors Influencing Them

The sexual reproduction of Sasa borealis, a species of dwarf bamboo, occurred in central Japan from 2016 to 2017. S. borealis grows on the forest floor and serves as an important source of habitat and food for various animals. Sexual reproduction occurs in synchrony among individuals in a given area, leading to a decline in population and causing substantial disturbances to the forest ecosystem; however, the subsequent regeneration process remains unclear. In this study, we investigated S. borealis seedling regeneration over six years. Fixed plots were established in the forest in the year following the sexual reproductive event, and the growth of seedlings was monitored from seed emergence to seedling growth at the individual level. We considered biotic and abiotic factors to evaluate their influence on regeneration. We examined mammalian and arthropod foraging as biotic factors. Conversely, abiotic factors included temperature and humidity near the ground surface, solar radiation, soil conditions, and snow cover. Seedling growth was characterized by a slow rate and affected by morphological changes resulting from foraging and abiotic factors. The return of S. borealis to its presexual reproductive stage requires an extended duration. Our study provides precious information for future S. borealis conservation and management strategies.

Brandt's vole (Lasiopodomys brandtii) affects the dominant position of three gramineous species by altering defense traits and interspecific competition

Rodents can cause considerable changes in plant community composition. However, relationships between shifts in species dominance and plant functional traits caused by rodents have seldom been investigated, especially for belowground functional traits. In this study, a set of enclosures was constructed to analyze the effects of 10 years of Brandt's voles' activities on the defense strategies and dominant position changes of three gramineous plants (Leymus chinensis, Stipa krylovii, and Cleistogenes squarrosa) in Inner Mongolia. Here, we measured the dominance, biomass, and fourteen functional traits of three plants. The effects of Brandt's voles on dominance, biomass, and functional traits were analyzed, and then we explored the effect of functional traits on plant dominance by using the structural equation model. Results showed that long-term feeding by Brandt's voles resulted in a significant decrease in the dominance of L. chinensis and S. krylovii, whereas C. squarrosa was positively affected. The belowground biomass of L. chinensis and S. krylovii was higher in the vole treatment, which showed that they were increasing their escape characteristics. The leaf thickness of L. chinensis and the leaf C:N ratio of S. krylovii significantly increased, while the specific leaf area of C. squarrosa significantly decreased. All three gramineous showed increased resistance traits in response to Brandt's voles, which positively affected their dominance. Tolerance-related traits of S. krylovii significantly increased, with the increasing growth rate of root length contributing to enhancing its dominance. We highlight that selective feeding by rodents led to the selection of different defense strategies by three gramineous plants, and that changes in biomass allocation and functional traits in the different species affected plant dominance, driving changes in the plant communities.

Clair EA, St. Clair SB. Spatio-Temporal Patterns of Joshua Tree Stand Structure and Regeneration Following Mojave Desert Wildfires

Eastern Joshua tree (Yucca jaegeriana) plays a central role in the ecology of the Mojave Desert ecosystem. However, the emergence of invasive grass fire-cycles in the last several decades brings into question Joshua trees’ tolerance and resilience to changing fire regimes. This study’s objective was to examine the effects of wildfires on the structure and regeneration potential of Joshua trees forests. We examined the density, size class structure, and regeneration response of Joshua tree populations on a network of one kilometer transects along the boundaries of four independent wildfires and the interior of the largest fire that occurred 15 years ago (2005) in the northeast Mojave Desert. Burned edge and burned interior transects had 23- and 4.1-fold lower Joshua tree stand densities than unburned transects. The more robust recovery of stand density along burned interior transects compared to burned edge transects appears to be primarily driven by more prolific vegetative sprouting. Our data show that Joshua trees can sprout vegetatively following fire, but it is not a strong or consistent post-fire resprouter. Limiting the spread of invasive annual grasses and novel fire regimes will be critical to maintaining healthy Joshua tree populations into the future, particularly on the edge of its ecological range.

Enemy release from the effects of generalist granivores can facilitate Bromus tectorum invasion in the Great Basin Desert

The enemy release hypothesis (ERH) of plant invasion asserts that natural enemies limit populations of invasive plants more strongly in native ranges than in non-native ranges. Despite considerable empirical attention, few studies have directly tested this idea, especially with respect to generalist herbivores. This knowledge gap is important because escaping the effects of generalists is a critical aspect of the ERH that may help explain successful plant invasions. Here, we used consumer exclosures and seed addition experiments to contrast the effects of granivorous rodents (an important guild of generalists) on the establishment of cheatgrass (Bromus tectorum) in western Asia, where cheatgrass is native, versus the Great Basin Desert, USA, where cheatgrass is exotic and highly invasive. Consistent with the ERH, rodent foraging reduced cheatgrass establishment by nearly 60% in western Asia but had no effect in the Great Basin. This main result corresponded with a region-specific foraging pattern: rodents in the Great Basin but not western Asia generally avoided seeds from cheatgrass relative to seeds from native competitors. Our results suggest that enemy release from the effects of an important guild of generalists may contribute to the explosive success of cheatgrass in the Great Basin. These findings corroborate classic theory on enemy release and expand our understanding of how generalists can influence the trajectory of exotic plant invasions.

Clair SB. Invertebrate community response to fire and rodent activity in the Mojave and Great Basin Deserts

Recent increases in the frequency and size of desert wildfires bring into question the impacts of fire on desert invertebrate communities. Furthermore, consumer communities can strongly impact invertebrates through predation and top-down effects on plant community assembly. We experimentally applied burn and rodent exclusion treatments in a full factorial design at sites in both the Mojave and Great Basin deserts to examine the impact that fire and rodent consumers have on invertebrate communities. Pitfall traps were used to survey invertebrates from April through September 2016 to determine changes in abundance, richness, and diversity of invertebrate communities in response to fire and rodent treatments. Generally speaking, rodent exclusion had very little effect on invertebrate abundance or ant abundance, richness or diversity. The one exception was ant abundance, which was higher in rodent access plots than in rodent exclusion plots in June 2016, but only at the Great Basin site. Fire had little effect on the abundances of invertebrate groups at either desert site, with the exception of a negative effect on flying-forager abundance at our Great Basin site. However, fire reduced ant species richness and Shannon's diversity at both desert sites. Fire did appear to indirectly affect ant community composition by altering plant community composition. Structural equation models suggest that fire increased invasive plant cover, which negatively impacted ant species richness and Shannon's diversity, a pattern that was consistent at both desert sites. These results suggest that invertebrate communities demonstrate some resilience to fire and invasions but increasing fire and spread of invasive due to invasive grass fire cycles may put increasing pressure on the stability of invertebrate communities.