Extreme changes in climate, especially in temperature, could have implications for herbal plants in various world regions. Medicinal plants often produce a wide variety of natural phytochemicals to enhance their defence and survival mechanisms against harsh environmental conditions, and when these mechanisms fail, plants consequently die. We investigated the impact of high temperatures coupled with the specific duration of exposure on the yield and composition of essential oils and trichomes in leaves of Lippia javanica. Plants were exposed to increasing temperatures (25 °C to 47 °C) for different durations (48 h to 144 h). Response surface methodology was applied to assess the interaction between temperature and length of exposure on the essential oil yield, trichome length, and trichome diameter. Essential oils were recovered from the control and treated leaf samples using hydrodistillation and volatile compounds were identified through gas chromatography–mass spectrometry (GC-MS). Multivariate analysis modelling allowed different clustering patterns to be detected. That is, increasing temperatures raised the oil yield, trichome length, and diameter from 1.007 mg/100 g to 3.58 mg/100 g, 50 µm to 160 µm, and 25 µm to 60 µm, respectively. Significant chemical differences between the essential oils were confirmed by the principal component and orthogonal projections to latent structures, which identified separate clusters for the control and treated samples. The current findings indicate that L. javanica has coping mechanisms against high temperatures.Significance: High temperatures significantly alter the trichome morphology and secretion of essential oils in L. javanica, which adversely affects the shrub’s medicinal properties. Regardless of climate change, this finding could have major implications for indigenous people who continue to use the shrub for therapeutic purposes. L. javanica showed coping mechanisms against high temperatures for a maximum of six days; however, a prolonged exposure would be more detrimental. As a result, climate change will negatively influence the plant’s developmental and defence mechanisms.

Woody plant expansion into historically grass‐dominated savannas is reshaping ecosystem structure and function, with consequences for biodiversity, nutrient cycling and land use. This shift is thought to be driven by the physiological responses of certain C ₃ tree species to elevated atmospheric CO ₂ concentrations (eCO ₂ ), modulated by water availability, among other constraints. However, the species‐specific mechanisms regulating photosynthetic acclimation under these conditions remain unclear. Using an Open‐Top Chamber system, we examined the responses of five southern African C ₃ tree seedlings to ambient (400 ppm) and elevated (550 ppm) CO ₂ concentrations under contrasting soil water availability. Our results revealed that Vachellia karroo , V. tortilis and V. sieberiana exhibited clear photosynthetic upregulation under eCO ₂ , with increased photosynthetic capacity (carboxylation capacity and/or electron transport rate), particularly under well‐watered conditions when stomatal conductance was highest. These responses diminished under water limitation but remained higher than in V. robusta and Senegalia burkei , which downregulated under eCO ₂ at high water availability and showed only modest improvements under dry conditions. Importantly, photosynthetic upregulation did not consistently translate into above‐ground growth gains, with significant enhancement observed only in V. sieberiana under high water supply. These divergent acclimation strategies highlight why some species emerge as dominant encroachers under rising CO ₂ , while others remain constrained by stronger physiological limitations. Read the free Plain Language Summary for this article on the Journal blog.

Rainfall pulses create brief but critical opportunities for carbon uptake in seasonally dry forests; however, how tree seedlings recover photosynthetic function during the establishment phase following these short-lived rewetting events under field conditions remains poorly understood. In particular, the coordination and timing of hydraulic, stomatal and biochemical recovery processes during natural rainfall pulse–dry-down cycles are not well quantified, despite their importance for carbon-water coupling and drought resilience. Here, we investigated short-term physiological responses of establishing Eucalyptus seedlings during naturally occurring rainfall pulse–dry-down cycles. We measured leaf water potential (Ψleaf), gas exchange and photosynthetic capacity (Vcmax, Jmax) before and after rainfall to assess recovery dynamics of diffusional and biochemical processes under contrasting atmospheric demand. Across species, Ψleaf and stomatal conductance improved rapidly following rainfall, reflecting transient hydraulic relief, while net photosynthesis increased by 40–60% within 1–3 days. In contrast, biochemical capacity responded more gradually: Vcmax declined by up to ~ 15% and Jmax by 20–40% during dry-down and showed limited or partial recovery after rewetting. Limitation partitioning revealed asynchronous recovery, with stomatal limitation relaxing rapidly after rainfall under low vapour pressure deficit (VPD), whereas under high VPD, biochemical recovery preceded full stomatal reopening. The xeric-origin Eucalyptus cladocalyx sustained assimilation at more negative Ψleaf and exhibited greater biochemical stability, whereas intermediate and mesic species (E. grandis, E. urophylla, E. cloeziana) showed rapid but short-lived post-rain responses. Together, these results demonstrate that photosynthetic recovery during the seedling phase is asynchronous and strongly modulated by atmospheric demand, shaping short-term carbon-water coupling under increasingly pulsed hydroclimates.

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.