Herbaceous vegetation responses to experimental fire in savannas and forests depend on biome and climate.

Fire-vegetation feedbacks potentially maintain global savanna and forest distributions. Accordingly, vegetation in savanna and forest ecosystems should have differential responses to fire, but fire response data for herbaceous vegetation have yet to be synthesized across biomes. Here, we examined herbaceous vegetation responses to experimental fire at 30 sites spanning four continents. Across a variety of metrics, herbaceous vegetation increased in abundance where fire was applied, with larger responses to fire in wetter and in cooler and/or less seasonal systems. Compared to forests, savannas were associated with a 4.8 (±0.4) times larger difference in herbaceous vegetation abundance for burned versus unburned plots. In particular, grass cover decreased with fire exclusion in savannas, largely via decreases in C4 grass cover, whereas changes in fire frequency had a relatively weak effect on grass cover in forests. These differential responses underscore the importance of fire for maintaining the vegetation structure of savannas and forests.

Continent-wide synthesis of the long-term population dynamics of quaking aspen in the face of accelerating human impacts.

In recent decades, climate change has disrupted forest functioning by promoting large-scale mortality events, declines in productivity and reduced regeneration. Understanding the temporal dynamics and spatial extent of these changes is critical given the essential ecosystem services provided by forests. As the most widespread tree species in North America, quaking aspen (Populus tremuloides) is well suited for studying the dynamics of tree populations during a period of unprecedented climate change. Synthesizing continent-wide data, we show that mortality rates of mature aspen stems have increased over the past two-to-three decades, while relative gains in aspen basal area have decreased during the same period. Patterns were pervasive across multiple stand size classes and composition types in western North America biomes, suggesting that trends in demographic rates were not simply a reflection of stand development and succession. Our review of the literature revealed that increased aspen mortality and reduced growth rates were most often associated with hotter, drier conditions, whereas reduced recruitment was most often associated with herbivory. Furthermore, interactions between climate and competition, as well as climate and insect herbivory, had important, context-dependent effects on mortality and growth, respectively. Our analyses of aspen across its entire geographic range indicate that this important tree species is experiencing substantial increases in mortality and decreases in population growth rates across multiple biomes. If such trends are not accompanied by increased recruitment, we expect that the reduced dominance of aspen in forests will lead to major declines in the many essential ecosystem services it provides.

Decadal changes in fire frequencies shift tree communities and functional traits.

Global change has resulted in chronic shifts in fire regimes. Variability in the sensitivity of tree communities to multi-decadal changes in fire regimes is critical to anticipating shifts in ecosystem structure and function, yet remains poorly understood. Here, we address the overall effects of fire on tree communities and the factors controlling their sensitivity in 29 sites that experienced multi-decadal alterations in fire frequencies in savanna and forest ecosystems across tropical and temperate regions. Fire had a strong overall effect on tree communities, with an average fire frequency (one fire every three years) reducing stem density by 48% and basal area by 53% after 50 years, relative to unburned plots. The largest changes occurred in savanna ecosystems and in sites with strong wet seasons or strong dry seasons, pointing to fire characteristics and species composition as important. Analyses of functional traits highlighted the impact of fire-driven changes in soil nutrients because frequent burning favoured trees with low biomass nitrogen and phosphorus content, and with more efficient nitrogen acquisition through ectomycorrhizal symbioses. Taken together, the response of trees to altered fire frequencies depends both on climatic and vegetation determinants of fire behaviour and tree growth, and the coupling between fire-driven nutrient losses and plant traits.

Long-term and interactive effects of different mammalian consumers on growth, survival, and recruitment of dominant tree species.

Throughout the world, numerous tree species are reported to be in decline, either due to increased mortality of established trees or reduced recruitment. The situation appears especially acute for oaks, which are dominant features of many landscapes in the northern hemisphere. Although numerous factors have been hypothesized to explain reductions in tree performance, vertebrate herbivores and granivores may serve as important drivers of these changes. Here, using data from 8- and 14-year-old exclosure experiments, we evaluated the individual and interactive effects of large and small mammalian herbivores on the performance of three widespread oak species in California-coast live oak (Quercus agrifolia), California black oak (Q. kelloggii), and Oregon white oak (Q. garryana). Although impacts varied somewhat by species and experiment, herbivory by black-tailed deer (Odocoileus hemionus columbianus) reduced the height and survival of juvenile coast live oaks and altered their architecture, as well as reduced the abundance of black oak seedlings, the richness of woody species and the cover of nonoak woody species. Small mammals (Microtus californicus and Peromyscus maniculatus) had even more widespread effects, reducing the abundance of black oak seedlings and the height and cover of all three oak species. We also detected numerous interactions between small mammals and deer, with one herbivore having positive or negative effects on oak abundance and cover when the other herbivore was either present or absent. For example, deer often had negative effects on seedling abundance only when, or even more so when, small mammals were present. In summary, mammalian consumers play crucial roles in limiting oak recruitment by reducing seedling abundance, maintaining trees in stunted states, and preventing them from reaching sapling stages and becoming reproductive. Interactions between large and small mammals can also alter the intensity and direction of their effects on trees.