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In alpine ecosystems, elevation broadly functions as a steep thermal gradient, with plant communities exposed to regular fluctuations in hot and cold temperatures. These conditions lead to selective filtering, potentially contributing to species-level variation in thermal tolerance and population-level genetic divergence. Few studies have explored the breadth of alpine plant thermal tolerances across a thermal gradient or the underlying genetic variation thereof. We measured photosystem heat (Tcrit-hot) and cold (Tcrit-cold) thresholds of ten Australian alpine species across elevation gradients and characterised their neutral genetic variation. To reveal the biogeographical drivers of present-day genetic signatures, we also reconstructed temporal changes in habitat suitability across potential distributional ranges. We found intraspecific variation in thermal thresholds, but this was not associated with elevation, nor underpinned by genetic differentiation on a local scale. Instead, regional population differentiation and considerable homozygosity within populations may, in part, be driven by distributional contractions, long-term persistence, and migrations following habitat suitability. Our habitat suitability models suggest that cool-climate-distributed alpine plants may be threatened by a warming climate. Yet, the observed wide thermal tolerances did not reflect this vulnerability. Conservation efforts should seek to understand variations in species-level thermal tolerance across alpine microclimates. [ABSTRACT FROM AUTHOR]

Most plant thermal tolerance studies focus on single critical thresholds, which limit the capacity to generalise across studies and predict heat stress under natural conditions. In animals and microbes, thermal tolerance landscapes describe the more realistic, cumulative effects of temperature. We tested this in plants by measuring the decline in leaf photosynthetic efficiency (FV/FM) following a combination of temperatures and exposure times and then modelled these physiological indices alongside recorded environmental temperatures. We demonstrate that a general relationship between stressful temperatures and exposure durations can be effectively employed to quantify and compare heat tolerance within and across plant species and over time. Importantly, we show how FV/FM curves translate to plants under natural conditions, suggesting that environmental temperatures often impair photosynthetic function. Our findings provide more robust descriptors of heat tolerance in plants and suggest that heat tolerance in disparate groups of organisms can be studied with a single predictive framework. [ABSTRACT FROM AUTHOR]

Given the rising frequency of thermal extremes (heatwaves and cold snaps) due to climate change, comprehending how a plant's origin affects its thermal tolerance breadth (TTB) becomes vital. We studied juvenile plants from three biomes: temperate coastal rainforest, desert and alpine. In controlled settings, plants underwent hot days and cold nights in a factorial design to examine thermal tolerance acclimation. We assessed thermal thresholds (T crit-hot and T crit-cold) and TTB. We hypothesized that (i) desert species would show the highest heat tolerance, alpine species the greatest cold tolerance and temperate species intermediate tolerance; (ii) all species would increase heat tolerance after hot days and cold tolerance after cold nights; (iii) combined exposure would broaden TTB more than individual conditions, especially in desert and alpine species. We found that biome responses were minor compared to the responses to the extreme temperature treatments. All plants increased thermal tolerance in response to hot 40°C days (T crit-hot increased by ~3.5°C), but there was minimal change in T crit-cold in response to the cold −2°C nights. In contrast, when exposed to both hot days and cold nights, on average, plants exhibited an antagonistic response in TTB, where cold tolerance decreased and heat tolerance was reduced, and so we did not see the bi-directional expansion we hypothesized. There was, however, considerable variation among species in these responses. As climate change intensifies, plant communities, especially in transitional seasons, will regularly face such temperature swings. Our results shed light on potential plant responses under these extremes, emphasizing the need for deeper species-specific thermal acclimation insights, ultimately guiding conservation efforts. [ABSTRACT FROM AUTHOR]

One application of plant physiological heat tolerance measurements is the assessment of vulnerability to increasing environmental temperatures under climatic change. A thermal safety margin (TSM), the difference between physiological tolerance and environmental temperature, is a common metric for the assessment of plant thermal vulnerability. However, there are biological and methodological aspects to consider when evaluating thermal vulnerability that have the potential to substantially alter the assessments. Two such aspects include the leaf to air temperature relationship and the scale at which air temperature data are collected.We grew plants of a desert species, Myoporum montanum, in situ under water‐stressed and well‐watered conditions, and measured their leaf temperatures and photosynthetic heat tolerance (T50 threshold) every third day over 12 days in summer. Thermal safety margins were calculated based on leaf temperatures and compared to those calculated with local and regional air temperatures.We found that heat tolerance and the thermal vulnerability assessment of a plant changed with water status. When water was readily available, plants maintained wide leaf temperature safety margins and displayed partial homeothermy. When cooling via transpiration was limited, increasing leaf temperature corresponded with occurrences of leaf poikilo‐ and megathermy, higher heat tolerance and narrower safety margins.Our study shows high physiological heat thresholds are not necessarily reflective of wide safety margins, but instead can indicate a greater vulnerability and increased risk of heat stress exposure. Calculating TSMs using air temperatures can also substantially alter margin widths. Where possible, the use of leaf temperatures in assessments of thermal vulnerability will lead to more meaningful vulnerability assessments. We recommend considering the source and temporal pairing of temperature measurements as well as plant water status, when measuring and interpreting plant TSMs. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

Close coordination between leaf gas exchange and maximal hydraulic supply has been reported across diverse plant life forms. However, it has also been suggested that this relationship may become weak or break down completely within the angiosperms. We examined coordination between hydraulic, leaf vein, and gas-exchange traits across a diverse group of 35 evergreen Australian angiosperms, spanning a large range in leaf structure and habitat. Leaf-specific conductance was calculated from petiole vessel anatomy and was also measured directly using the rehydration technique. Leaf vein density (thought to be a determinant of gas exchange rate), maximal stomatal conductance, and net CO2 assimilation rate were also measured for most species ( n = 19-35). Vein density was not correlated with leaf-specific conductance (either calculated or measured), stomatal conductance, nor maximal net CO2 assimilation, with r2 values ranging from 0.00 to 0.11, P values from 0.909 to 0.102, and n values from 19 to 35 in all cases. Leaf-specific conductance calculated from petiole anatomy was weakly correlated with maximal stomatal conductance ( r2 = 0.16; P = 0.022; n = 32), whereas the direct measurement of leaf-specific conductance was weakly correlated with net maximal CO2 assimilation ( r2 = 0.21; P = 0.005; n = 35). Calculated leaf-specific conductance, xylem ultrastructure, and leaf vein density do not appear to be reliable proxy traits for assessing differences in rates of gas exchange or growth across diverse sets of evergreen angiosperms. [ABSTRACT FROM AUTHOR]