Complex facilitation and competition in a temperate grassland: loss of plant diversity and elevated CO2 have divergent and opposite effects on oak establishment

Stress gradients and biodiversity: monoculture vulnerability drives stronger biodiversity effects during drought years

Climate change will increase the likelihood and severity of droughts into the future. Although diversity may buffer plant communities against the negative effects of drought, the mechanisms underlying this pattern remain unclear. Higher-diversity plant communities may have a higher likelihood of including more drought-resistant species that can compensate for drought-sensitive species ("insurance effects"). Alternatively, higher-diversity communities may alter environmental conditions and improve performance of even drought-sensitive species. Here we planted nonleguminous forbs and grasses into monocultures and four- and eight-species mixtures, and measured species and plot productivity every year from 2000 to 2010. We found that six of our eight species were suppressed when growing in monoculture during dry years. These same species were unaffected by drought when growing in higher-diversity mixtures. Because of this poor performance in monoculture (not insurance effects), the biodiversity productivity relationship was strongest during the driest years. If biodiversity ameliorates hot/dry conditions and therefore improves performance of drought-sensitive species during periods of low rainfall, this may mean biodiversity can be used as a tool to protect individual species from drought conditions.

Data-driven competitive facilitative tree interactions and their implications on nature-based solutions

Spatio-temporal data are more ubiquitous and richer than even before and the availability of such data poses great challenges in data analytics. Ecological facilitation, the positive effect of density of individuals on the individual's survival across a stress gradient, is a complex phenomenon. A large number of tree individuals coupled with soil moisture, temperature, and water stress data across a long temporal period were followed. Data-driven analysis in the absence of hypothesis was performed. Information theoretic analysis of multiple statistical models was employed in order to quantify the best data-driven index of vegetation density and spatial scale of interactions. Sequentially, tree survival was quantified as a function of the size of the individual, vegetation density, and time at the optimal spatial interaction scale. Land surface temperature and soil moisture were also statistically explained by tree size, density, and time. Results indicated that in space both facilitation and competition co-exist in the same ecosystem and the sign and magnitude of this depend on the spatial scale. Overall, within the optimal data-driven spatial scale, tree survival was best explained by the interaction between density and year, sifting overall from facilitation to competition through time. However, small sized trees were always facilitated by increased densities, while large sized trees had either negative or no density effects. Tree size was more important predictor than density in survival and this has implications for nature-based solutions: maintaining large tree individuals or planting species that can become large-sized can safeguard against tree-less areas by promoting survival at long time periods through harsh environmental conditions. Large trees had also a significant effect in moderating land surface temperature and this effect was higher than the one of vegetation density on temperature.

Effects of competition and herbivory over woody seedling growth in a temperate woodland trump the effects of elevated CO2

A trend of increasing woody plant density, or woody thickening, has been observed across grassland and woodland ecosystems globally. It has been proposed that increasing atmospheric [CO₂] is a major driver of broad scale woody thickening, though few field-based experiments have tested this hypothesis. Our study utilises a Free Air CO₂ Enrichment experiment to examine the effect of elevated [CO₂] (eCO₂) on three mechanisms that can cause woody thickening, namely (i) woody plant recruitment, (ii) seedling growth, and (iii) post-disturbance resprouting. The study took place in a eucalypt-dominated temperate grassy woodland. Annual assessments show that juvenile woody plant recruitment occurred over the first 3 years of CO₂ fumigation, though eCO₂ did not affect rates of recruitment. Manipulative experiments were established to examine the effect of eCO₂ on above-ground seedling growth using transplanted Eucalyptus tereticornis (Myrtaceae) and Hakea sericea (Proteaceae) seedlings. There was no positive effect of eCO₂ on biomass of either species following 12 months of exposure to treatments. Lignotubers (i.e., resprouting organs) of harvested E. tereticornis seedlings that were retained in situ for an additional year were used to examine resprouting response. The likelihood of resprouting and biomass of resprouts increased with lignotuber volume, which was not itself affected by eCO₂. The presence of herbaceous competitors and defoliation by invertebrates and pathogens were found to greatly reduce growth and/or resprouting response of seedlings. Our findings do not support the hypothesis that future increases in atmospheric [CO₂] will, by itself, promote woody plant recruitment in eucalypt-dominated temperate grassy woodlands.

Warming increases the sensitivity of seedling growth capacity to rainfall in six temperate deciduous tree species

Predicting the effects of climate change on tree species and communities is critical for understanding the future state of our forested ecosystems. We used a fully factorial precipitation (three levels; ambient, -50 % ambient, +50 % ambient) by warming (four levels; up to +4 °C) experiment in an old-field ecosystem in the northeastern USA to study the climatic sensitivity of seedlings of six native tree species. We measured whole plant-level responses: survival, total leaf area (TLA), seedling insect herbivory damage, as well as leaf-level responses: specific leaf area (SLA), leaf-level water content (LWC), foliar nitrogen (N) concentration, foliar carbon (C) concentration and C:N ratio of each of these deciduous species in each treatment across a single growing season. We found that canopy warming dramatically increased the sensitivity of plant growth (measured as TLA) to rainfall across all species. Warm, dry conditions consistently reduced TLA and also reduced leaf C:N in four species (Acer rubrum, Betula lenta, Prunus serotina, Ulmus americana), primarily as a result of reduced foliar C, not increased foliar N. Interestingly, these conditions also harmed the other two species in different ways, increasing either mortality (Populus grandidentata) or herbivory (Quercus rubra). Specific leaf area and LWC varied across species, but did not show strong treatment responses. Our results indicate that, in the northeastern USA, dry years in a future warmer environment could have damaging effects on the growth capacity of these early secondary successional forests, through species-specific effects on leaf production (total leaves and leaf C), herbivory and mortality.

Why are there no C4 forests?

C₄ photosynthesis is absent from the arborescent life form, with the exception of seven Hawaiian Euphorbia species and a few desert shrubs that become arborescent with age. As a consequence, wherever C₃ trees can establish, their height advantage enables them to outcompete low stature C₄ vegetation. Had C₄ photosynthesis been able to evolve in an arborescent life form, forest cover (by C₄ trees) could have been much more extensive than today, with significant consequences for the biosphere. Here, we address why there are so few C₄ trees. Physiological explanations associated with low light performance of C₄ photosynthesis are not supported, because C₄ shade-tolerant species exhibit similar performance as shade-tolerant C₃ species in terms of quantum yield, steady-state photosynthetic and use of sunflecks. Hence, hypothetical C₄ trees could occur in the regeneration niche of forests. Constraints associated with the evolutionary history of the C₄ lineages are more plausible. Most C₄ species are grasses and sedges, which lack meristems needed for arborescence, while most C₄ eudicots are highly specialized for harsh (arid, saline, hot) or disturbed habitats where arborescence may be maladapted. Most C₄ eudicot clades are also young, and have not had sufficient time to radiate beyond the extreme environments where C₄ evolution is favored. In the case of the Hawaiian Euphorbia species, they belong to one of the oldest and most diverse C₄ lineages, which primed this group to evolve arborescence in a low-competition environment that appeared on the remote Hawaiian Islands.

Adaptation to elevated CO2 in different biodiversity contexts

In the absence of migration, species persistence depends on adaption to a changing environment, but whether and how adaptation to global change is altered by community diversity is not understood. Community diversity may prevent, enhance or alter how species adapt to changing conditions by influencing population sizes, genetic diversity and/or the fitness landscape experienced by focal species. We tested the impact of community diversity on adaptation by performing a reciprocal transplant experiment on grasses that evolved for 14 years under ambient and elevated CO2, in communities of low or high species richness. Using biomass as a fitness proxy, we find evidence for local adaptation to elevated CO2, but only for plants assayed in a community of similar diversity to the one experienced during the period of selection. Our results indicate that the biological community shapes the very nature of the fitness landscape within which species evolve in response to elevated CO2.

Living close to your neighbors: the importance of both competition and facilitation in plant communities

Recent work has demonstrated that competition and facilitation likely operate jointly in plant communities, but teasing out the relative role of each has proven difficult. Here we address how competition and facilitation vary with seasonal fluctuations in environmental conditions, and how the effects of these fluctuations change with plant ontogeny. We planted three sizes of pine seedlings (Pinus strobus) into an herbaceous diversity experiment and measured pine growth every two weeks for two growing seasons. Both competition and facilitation occurred at different times of year between pines and their neighbors. Facilitation was important for the smallest pines when environmental conditions were severe. This effect decreased as pines got larger. Competition was stronger than facilitation overall and outweighed facilitative effects at annual time scales. Our data suggest that both competition and the counter-directional effects of facilitation may be more common and more intense than previously considered.

Plant succession as an integrator of contrasting ecological time scales

Ecologists have studied plant succession for over a hundred years, yet our understanding of the nature of this process is incomplete, particularly in relation to its response to new human perturbations and the need to manipulate it during ecological restoration. We demonstrate how plant succession can be understood better when it is placed in the broadest possible temporal context. We further show how plant succession can be central to the development of a framework that integrates a spectrum of ecological processes, which occur over time scales ranging from seconds to millions of years. This novel framework helps us understand the impacts of human perturbations on successional trajectories, ecosystem recovery, and global environmental change.