Biology subjects

Tjoelker, M. G.

Publications and source records attributed to Tjoelker, M. G..

6 recordsLinked to original sources

bioRxiv2026

Intermediate, rather than Isohydric or Anisohydric, Species were More Resilient to an Extreme Heatwave in a Common Garden in Sydney

Urban forests are increasingly exposed to climate extremes, yet selecting resilient urban species remains difficult as it is unclear why some species fail and others succeed. We monitored growth and survival of 48 urban tree and shrub species, respectively, for about five years after planting, plus we quantified species' crown dieback after a natural extreme heatwave (three days with temperatures exceeding 40 degrees C) in a common garden in Sydney, Australia. For 30 species we measured 15 plant functional traits, including both hydraulic and morphological traits. Native species had less crown dieback and higher absolute growth but lower survival than exotic species. Plant functional traits linked to heat and drought tolerance explained 20-30% of the variation in observed crown dieback across species. Extensive dieback exceeding 40% after the extreme heatwave was found for seven species with large leaf size (and leaf width), high leaf relative water content, and low wood density. Nonlinear relationships between crown dieback and leaf water potential traits identified that both isohydric and anisohydric species were vulnerable, indicating that vulnerability did not align neatly with a single hydraulic strategy. Hydraulic failure likely occurred for some, but not all, vulnerable isohydric and anisohydric species, suggesting vulnerability of certain species was directly caused by cumulative thermal stress. Importantly, survival was not related to any single functional trait, indicating that crown dieback was a more sensitive and earlier indicator of cumulative thermal stress than mortality. These findings support a shift from climate of origin-based to trait-based species selection in urban forestry.

physiology↗

bioRxiv2026

Towards understanding stomatal decoupling under extreme heat: integrating stomatal kinetics, leaf light responses, and the role of stomatal versus cuticular conductance

{middle dot} Earth's forests are increasingly exposed to hotter drought events, causing unprecedented dieback of trees and forests. Evidence of stomatal decoupling at high leaf temperatures (Tleaf) is mounting, but the driving mechanisms and variability across species are unknown. {middle dot} We experimentally imposed heat and drought stress onto two rainforest and two semi-arid tree species in a glasshouse study to better understand accelerated water loss at high Tleaf by measuring plant thermal and hydraulic balance. {middle dot} Stomatal decoupling at high Tleaf occurred in two species, including the most anisohydric species, but did not occur in a vulnerable rainforest species. Accelerated leaf water loss occurred predominantly through open stomata, rather than the cuticle, but some species prioritised thermal acclimation over leaf thermoregulation during heat stress. This suggests decoupling occurs only in species with high capacity for transpirational cooling (>5{degrees}C), or alternatively, that full sunlight is required for stomatal decoupling. {middle dot} The maximum rate of light-induced stomatal opening was up to two times faster, while dark-induced stomatal closing was 14 minutes slower on average in leaves at 34{degrees}C, relative to 25{degrees}C. Stomatal kinetics are an overlooked trait that may help to better predict accelerated water loss under high Tleaf across species.

plant biology↗

bioRxiv2026

Water beneath the pavement: assessing the benefits of passive irrigation for urban Lophostemon confertus trees in western Sydney

Urban environments typically experience higher temperatures than surrounding natural landscapes, making urban vegetation crucial for cooling local areas and improving the health of city residents. Impervious urban surfaces limit the absorption and retention of precipitation, potentially limiting tree water access and threatening long-term survival. Here, we measured tree physiology and growth of Lophostemon confertus (Queensland brush box) trees to investigate how a passive irrigation system that stores stormwater affected the performance of young, establishing trees in a hot and dry suburb of western Sydney, Australia. During the 2024-2025 austral summer, three years after planting, the local climate was periodically hot and dry, with a total of 16 days above 35 C. Irrigated L. confertus trees had higher water availability (i.e., higher predawn leaf water potential, {Psi}pre), lower water stress (i.e., higher midday leaf water potential, {Psi}mid, more frequently above turgor loss point), greater stomatal conductance (gs) on hot and dry summer days, and reduced leaf temperatures (Tleaf), compared to control trees. No significant differences in growth rates were observed between irrigated and control trees during the first three establishment years, but irrigated trees had greater crown survival during the hot, dry summer. Our results suggest passive irrigation may mitigate periods of short-term heat and drought stress in urban trees by increasing water access to support transpiration that prevents leaves from overheating, improving tree health. Higher tree transpiration may lead to greater ecosystem services by increasing cooling benefits, contributing to mitigation of urban heat island effects.

plant biology↗

bioRxiv2021

Climate-change risk analysis for global urban forests

Urban forests (i.e. all vegetation present in urban areas), provide environmental and socioeconomic benefits1 to more than half of the global population2. Projected climate change threatens these benefits to society3-5. Here, we assess vulnerability to climate change of 16,006 plant species present in the urban forests of 1,010 cities within 93 countries, using three vulnerability metrics: exposure, safety margin and risk. Exposure expresses the magnitude of projected changes in climate in a given area, safety margin measures species sensitivity to climate change, and risk is the difference between exposure and safety margin6. We identified 9,676 (60.5%) and 8,344 (52.1%) species exceeding their current climatic tolerance (i.e. safety margin) for mean annual temperature (MAT) and annual precipitation (AP), respectively. By 2050, 13,479 (84.2%) and 9,960 (62.2%) species are predicted to be at risk from projected changes in MAT and AP, respectively, with risk increasing in cities at lower latitudes. Our results can aid evaluation of the impacts of climate change on urban forests and identify the species most at risk. Considering future climates when selecting species for urban plantings will enhance the long-term societal benefits provided by urban forests, including their contribution to mitigating the magnitude and impacts of climate change.

ecology↗

bioRxiv2020

Pastures and Climate Extremes: Impacts of warming and drought on the productivity and resilience of key pasture species in a field experiment

Shifts in the timing, intensity and/or frequency of climate extremes, such as severe drought and heatwaves, can generate sustained shifts in ecosystem function with important ecological and economic impacts for rangelands and managed pastures. The Pastures and Climate Extremes experiment (PACE) in Southeast Australia was designed to investigate the impacts of a severe winter/spring drought (60% rainfall reduction) and, for a subset of species, a factorial combination of drought and elevated temperature (ambient +3 {degrees}C) on pasture productivity. The experiment included nine common pasture and Australian rangeland species from three plant functional groups (C3 grasses, C4 grasses and legumes) planted in monoculture. Winter/spring drought resulted in productivity declines of 45% on average and up to 74% for the most affected species (Digitaria eriantha) during the 6-month treatment period, with eight of the nine species exhibiting significant yield reductions. Despite considerable variation in species sensitivity to drought, C4 grasses were more strongly affected by this treatment than C3 grasses or legumes. Warming also had negative effects on cool-season productivity, associated at least partially with exceedance of optimum growth temperatures in spring and indirect effects on soil water content. The combination of winter/spring drought and year-round warming resulted in the greatest yield reductions. We identified responses that were either additive such that there was only as significant warming effect under drought (Festuca), or less-than-additive, where there was no drought effect under warming (Medicago), compared to ambient plots. Results from this study highlight the sensitivity of diverse pasture species to increases in winter and spring drought severity similar to those predicted for this region, and that anticipated benefits of cool-season warming are unlikely to be realised. Overall, the substantial negative impacts on productivity suggest that future, warmer, drier climates will result in shortfalls in cool-season forage availability, with profound implications for the livestock industry and natural grazer communities.

ecology↗

bioRxiv2019

The fate of carbon in a mature forest under carbon dioxide enrichment

Atmospheric carbon dioxide enrichment (eCO2) can enhance plant carbon uptake and growth1,2,3,4,5, thereby providing an important negative feedback to climate change by slowing the rate of increase of the atmospheric CO2 concentration6. While evidence gathered from young aggrading forests has generally indicated a strong CO2 fertilization effect on biomass growth3,4,5, it is unclear whether mature forests respond to eCO2 in a similar way. In mature trees and forest stands7,8,9,10, photosynthetic uptake has been found to increase under eCO2 without any apparent accompanying growth response, leaving an open question about the fate of additional carbon fixed under eCO24, 5, 7,8,9,10,11. Here, using data from the first ecosystem-scale Free-Air CO2 Enrichment (FACE) experiment in a mature forest, we constructed a comprehensive ecosystem carbon budget to track the fate of carbon as the forest responds to four years of eCO2 exposure. We show that, although the eCO2 treatment of ambient +150 ppm (+38%) induced a 12% (+247 gCm-2yr-1) increase in carbon uptake through gross primary production, this additional carbon uptake did not lead to increased carbon sequestration at the ecosystem level. Instead, the majority of the extra carbon was emitted back into the atmosphere via several respiratory fluxes, with increased soil respiration alone contributing [~]50% of the total uptake surplus. Our results call into question the predominant thinking that the capacity of forests to act as carbon sinks will be generally enhanced under eCO2, and challenge the efficacy of climate mitigation strategies that rely on CO2 fertilization as a driver of increased carbon sinks in standing forests and afforestation projects.

ecology↗