Kimberley J. Simpson

Rising atmospheric CO ₂ concentrations are impacting the global terrestrial biosphere through indirect climate effects and direct effects on plant performance ^1–3 . In tropical forests, long-term monitoring indicates a substantial CO ₂ -driven carbon sink ⁴ . C ₄ -grass-dominated tropical and subtropical savannas contribute approximately 30% of terrestrial net primary production ⁵ , and yet equivalent long-term analyses of CO ₂ responses are lacking. Here we show a clear and consistent result across a meta-analysis of 70 CO ₂ -addition experiments and 32 years of in situ field observations from southern Africa: CO ₂ fertilization of wild C ₄ grasses is widespread in dry conditions. In experiments, grasses reduced stomatal conductance under higher levels of CO ₂ , limiting water loss while increasing carbon gain. In the field, improved water use efficiency translated into increased C ₄ grass biomass production across three decades of observations. Finally, simulations via the Community Land Model ⁶ suggest that CO ₂ fertilization of C ₄ grass aboveground productivity may continue to increase under future conditions. Together, these results challenge the view that C ₄ grasses are unresponsive to increasing levels of CO ₂ , demonstrating instead that annual aboveground production of grasses in the field in southern Africa has increased by 28% over three decades (a CO ₂ -driven increase of 75.1 g m ⁻² (95% confidence interval of 74.5–75.8 g m ⁻² ) or 0.37 tons C ha ⁻¹ of annual production). Although the fate of this carbon is uncertain (depending on feedbacks with fire, herbivory and woody vegetation), effects on the global carbon cycle may be profound.

Encroachment is a globally ubiquitous phenomenon, characterised by increasing indigenous tree densities in savanna and grassland. Encroachment has been attributed to rising atmospheric CO ₂ concentrations fertilising tree growth and shifting the competitive balance between trees and grasses. However, only a subset of savanna tree species are currently described as encroachers, raising the hypothesis that CO ₂ responsiveness differs among species. Within southern African savannas, encroachment is driven primarily by nitrogen (N ₂ )‐fixing species, implying the CO ₂ response may be mediated via traits that enhance plant‐available N. Using an open‐topped chamber system, we experimentally manipulated atmospheric CO ₂ concentrations and soil moisture for 12 savanna tree species (six encroachers and six non‐encroachers) under ambient (a)CO ₂ (~397.9 ppm) or elevated (e)CO ₂ (~545.1 ppm) treatments and water limited or well‐watered soil moisture treatments. We measured N‐dynamics traits including nodule mass fraction (NMF), leaf δ ¹⁵ N, stem δ ¹⁵ N, and percentage of N derived from fixation (%Ndfa). We found that encroachers and non‐encroachers differ in short‐term N‐dynamics but share similar long‐term N allocation strategies. Encroachers exhibited lower leaf δ ¹⁵ N, indicating greater utilisation of N ₂ fixation products to meet immediate short‐term protein synthesis. Long‐term N allocation strategies (NMF, stem δ ¹⁵ N, and %Ndfa) were similar between encroachers and non‐encroachers, with plants fixing more N ₂ under the eCO ₂ and well‐watered treatment. In encroachers, leaf and stem δ¹⁵N were unrelated, in contrast to the positive relationship in non‐encroachers, pointing to distinct tissue‐level N allocation, possibly reflecting differential N utilisation and retention. We demonstrate the significance of N ₂ fixation in mediating the CO ₂ responsiveness of encroaching savanna trees. N ₂ fixation increases plant‐available N, likely enabling encroachers to meet immediate N demands even under water limitation and increasing CO ₂ . The potential feedback loop, where eCO ₂ enhances photosynthesis, facilitating greater C allocation for N ₂ fixation, helps to explain the ecological success of the subset of species driving encroachment under increased atmospheric CO ₂ . Read the free Plain Language Summary for this article on the Journal blog.

Premise: Humans have used fire to manage landscapes for millennia, but this use of fire is declining in many ecosystems. Understanding how plants respond to these changes is key to predicting ecosystem resilience and impacts on services such as biodiversity and carbon sequestration. However, many ecosystems lack data on plant fire responses. A solution is to infer these responses through studying functional traits that offer fitness benefits under fire by enabling species to resist fire, or persist through fire, by resprouting or recruitment from stored seed banks or dispersed seed. Studying these traits helps predict species performance under fire. Methods: We used a trait‐based approach to create a database of fire‐relevant traits and fire responses for vascular and nonvascular plant species in UK heath and mire ecosystems managed by fire. By reviewing the available literature, we collected data on traits adaptive under frequent burns for 153 plant taxa that could then be used to make predictions about fire effects. Results: Of the 153 taxa assessed, 97% had fire‐adaptive traits; 149 taxa had resistance and/or persistence traits, enabling survival or regeneration after fire. Additionally, 20 taxa showed fire‐enhanced recruitment, while only four lacked traits aiding survival during or after fire. Conclusions: The database created here is designed to be a resource for those who study and manage heath and mire ecosystems. The use of fire in areas where heath is underlain by peat is much debated, and this database can inform how changing or removing the use of fire will impact plant communities.

Societal Impact Statement: Intervention strategies that involve supplementing crop‐lands with silicon have significant scope for carbon capture and drought mitigation, offering wide‐ranging societal impacts. These include contributing to decarbonisation goals, enhancing food security, providing economic benefits and reducing environmental damage associated with intensive agronomic practices. This article highlights emerging evidence that suggests elevated atmospheric CO ₂ and water limitation may impair silicon accumulation in plants. While this does not negate the outlined societal benefits, we argue that these limitations must be thoroughly quantified and incorporated into large‐scale implementation plans to ensure the reliability and effectiveness of silicon intervention strategies. Summary: Silicon accumulation in plants is increasingly recognised as playing an important functional role in alleviating environmental stresses. Most research to date has focussed on relieving agronomic stresses in crops, including pest and pathogen damage, soil salinity and drought. Recently, attention has turned to large‐scale silicon application to agricultural landscapes as a potential anthropogenic climate change mitigation strategy. This includes silicon fertilisation to enhance soil carbon storage through advanced weathering of silicates, or by incorporating carbon in phytoliths in plant tissues. While these geoengineering approaches have potential, they could also present significant challenges. This article explores the opportunities and limitations for silicon‐based interventions in mitigating the impacts of rising atmospheric carbon dioxide levels and increased incidences of drought. We argue that despite the promise of silicon supplementation in reducing plant stress under climate change, research paradoxically shows that these very climate conditions can significantly impede silicon accumulation in plants. We propose a framework to guide the development of silicon intervention strategies to mitigate climate change and the research questions that should be addressed to ensure their effectiveness under future environmental conditions.

Fire regimes are changing across the globe, with new wildfire behaviour phenomena and increasing impacts felt, especially in ecosystems without clear adaptations to wildfire. These trends pose significant challenges to the scientific community in understanding and communicating these changes and their implications, particularly where we lack underlying scientific evidence to inform decision-making. Here, we present a perspective on priority directions for wildfire science research—through the lens of academic and government wildfire scientists from a historically wildfire-prone (USA) and emerging wildfire-prone (UK) country. Key topic areas outlined during a series of workshops in 2023 were as follows: (A) understanding and predicting fire occurrence, fire behaviour and fire impacts; (B) increasing human and ecosystem resilience to fire; and (C) understanding the atmospheric and climate impacts of fire. Participants agreed on focused research questions that were seen as priority scientific research gaps. Fire behaviour was identified as a central connecting theme that would allow critical advances to be made across all topic areas. These findings provide one group of perspectives to feed into a more transdisciplinary outline of wildfire research priorities across the diversity of knowledge bases and perspectives that are critical in addressing wildfire research challenges under changing fire regimes. This article is part of the theme issue ‘Novel fire regimes under climate changes and human influences: impacts, ecosystem responses and feedbacks’.

Summary: Increasing genome size (GS) has been associated with slower rates of DNA replication and greater cellular nitrogen (N) and phosphorus demands. Despite most plant species having small genomes, the existence of larger GS species suggests that such costs may be negligible or represent benefits under certain conditions. Focussing on the widespread and diverse grass family (Poaceae), we used data on species' climatic niches and growth rates under different environmental conditions to test for growth costs or benefits associated with GS. The influence of photosynthetic pathway, life history and evolutionary history on grass GS was also explored. We found that evolutionary history, photosynthetic pathway and life history all influence the distribution of grass species' GS. Genomes were smaller in annual and C₄ species, the latter allowing for small cells necessary for C₄ leaf anatomy. We found larger GS were associated with high N availability and, for perennial species, low growth‐season temperature. Our findings reveal that GS is a globally important predictor of grass performance dependent on environmental conditions. The benefits for species with larger GS are likely due to associated larger cell sizes, allowing rapid biomass production where soil fertility meets N demands and/or when growth occurs via temperature‐independent cell expansion.

CO₂‐fertilisation is implicated in the widespread and significant woody encroachment of savannas due to CO₂‐stimulated increases in below‐ground reserves that enhance sapling regrowth after fire. However, the effect of CO₂ concentration ([CO₂]) on tree responses to the other major disturbance in savannas, herbivory, is poorly understood. Herbivory responses cannot be predicted from fire responses, as herbivore effects occur earlier during establishment and are moderated by plant palatability and defence rather than below‐ground carbon accumulation. The relationship between herbivory and [CO₂] is explored here using a widespread, strongly encroaching savanna tree, Vachellia karroo. Using greenhouse‐grown seedlings under past—through to predicted future—[CO₂] (180–1000 ppm) and field‐grown seedlings under ambient [CO₂], we assessed plant survival, growth, defence and palatability. Increasing [CO₂] improves the tolerance of greenhouse‐grown seedlings to herbivory by stimulating growth and allowing a critical size threshold associated with survival to be reached earlier, thereby decreasing the probability of fatal herbivory during the vulnerable recruitment phase. Elevated [CO₂] also decreases the time taken to reach a second size threshold linked to accelerated recovery of field‐grown seedlings following herbivory. Seedling growth responses to increasing [CO₂] are nonlinear, suggesting that historic growth and survival enhancements are smaller than those predicted for the future. Increasing [CO₂] is associated with greater resistance to herbivores (more branched shoot architecture) but not leaf palatability (C:N ratio) or defence (leaf tannins and spine density). Increasing V. karroo densities already constitute a major land management problem in southern African savannas. However, encroachment by this species, and likely other savanna tree species, may be greatly exacerbated under future [CO₂], as tolerance to herbivory at the recruitment stage is further enhanced. Read the free Plain Language Summary for this article on the Journal blog.

Summary: Plant plastic responses are critical to the adaptation and survival of species under climate change, but whether they are constrained by evolutionary history (phylogeny) is largely unclear. Plant leaf traits are key in determining plants’ performance in different environments, and if these traits and their variation are phylogenetically dependent, predictions could be made to identify species vulnerable to climate change. We compiled data on three leaf traits (photosynthetic rate, specific leaf area, and leaf nitrogen content) and their variation under four environmental change scenarios (warming, drought, elevated CO₂, or nitrogen addition) for 434 species, from 210 manipulation experiments. We found phylogenetic signal in the three traits but not in their variation under the four scenarios. This indicates that closely related species show similar traits but that their plastic responses could not be predicted from species relatedness under environmental change. Meanwhile, phylogeny weakened the slopes but did not change the directions of conventional pairwise trait relationships, suggesting that co‐evolved leaf trait pairs have consistent responses under contrasting environmental conditions. Phylogeny can identify lineages rich in species showing similar traits and predict their relationships under climate change, but the degree of plant phenotypic variation does not vary consistently across evolutionary clades.