Germination temperature sensitivity differs between co-occurring tree species and climate origins resulting in contrasting vulnerability to global warming

Climate change is shifting temperatures from historical patterns, globally impacting forest composition and resilience. Seed germination is temperature-sensitive, making the persistence of populations and colonization of available habitats vulnerable to warming. This study assessed germination response to temperature in foundation trees in south-western Australia's Mediterranean-type climate forests (Eucalyptus marginata (jarrah) and Corymbia calophylla (marri)) to estimate the thermal niche and vulnerability among populations. Seeds from the species' entire distribution were collected from 12 co-occurring populations. Germination thermal niche was investigated using a thermal gradient plate (5–40°C). Five constant temperatures between 9 and 33°C were used to test how the germination niche (1) differs between species, (2) varies among populations, and (3) relates to the climate of origin. Germination response differed among species; jarrah had a lower optimal temperature and thermal limit than marri (To 15.3°C, 21.2°C; ED50 23.4°C, 31°C, respectively). The thermal limit for germination differed among populations within both species, yet only marri showed evidence for adaptation to thermal origins. While marri has the capacity for germination at higher thermal temperatures, jarrah is more vulnerable to global warming exceeding safety margins. This discrepancy is predicted to alter species distributions and forest composition in the future

Signatures of natural selection in a foundation tree along Mediterranean climatic gradients

Temperature and precipitation regimes are rapidly changing, resulting in forest dieback and extinction events, particularly in Mediterranean-type climates (MTC). Forest management that enhance forests’ resilience is urgently required, however adaptation to climates in heterogeneous landscapes with multiple selection pressures is complex. For widespread trees in MTC we hypothesized that: patterns of local adaptation are associated with climate; precipitation is a stronger factor of adaptation than temperature; functionally related genes show similar signatures of adaptation; and adaptive variants are independently sorting across the landscape. We sampled 28 populations across the geographic distribution of Eucalyptus marginata (jarrah), in South-west Western Australia, and obtained 13,534 independent single nucleotide polymorphic (SNP) markers across the genome. Three genotype-association analyses that employ different ways of correcting population structure were used to identify putatively adapted SNPs associated with independent climate variables. While overall levels of population differentiation were low (FST = 0.04), environmental association analyses found a total of 2336 unique SNPs associated with temperature and precipitation variables, with 1440 SNPs annotated to genic regions. Considerable allelic turnover was identified for SNPs associated with temperature seasonality and mean precipitation of the warmest quarter, suggesting that both temperature and precipitation are important factors in adaptation. SNPs with similar gene functions had analogous allelic turnover along climate gradients, while SNPs among temperature and precipitation variables had uncorrelated patterns of adaptation. These contrasting patterns provide evidence that there may be standing genomic variation adapted to current climate gradients, providing the basis for adaptive management strategies to bolster forest resilience in the future

The roles of divergent and parallel molecular evolution contributing to thermal adaptive strategies in trees

Local adaptation is a driver of biological diversity, and species may develop analogous (parallel evolution) or alternative (divergent evolution) solutions to similar ecological challenges. We expect these adaptive solutions would culminate in both phenotypic and genotypic signals. Using two Eucalyptus species (Eucalyptus grandis and Eucalyptus tereticornis) with overlapping distributions grown under contrasting ‘local’ temperature conditions to investigate the independent contribution of adaptation and plasticity at molecular, physiological and morphological levels. The link between gene expression and traits markedly differed between species. Divergent evolution was the dominant pattern driving adaptation (91% of all significant genes); but overlapping gene (homologous) responses were dependent on the determining factor (plastic, adaptive or genotype by environment interaction). Ninety-eight percent of the plastic homologs were similarly regulated, while 50% of the adaptive homologs and 100% of the interaction homologs were antagonistical. Parallel evolution for the adaptive effect in homologous genes was greater than expected but not in favour of divergent evolution. Heat shock proteins for E. grandis were almost entirely driven by adaptation, and plasticity in E. tereticornis. These results suggest divergent molecular evolutionary solutions dominated the adaptive mechanisms among species, even in similar ecological circumstances. Suggesting that tree species with overlapping distributions are unlikely to equally persist in the future

Tree traits and microclimatic conditions determine cooling benefits of urban trees

Trees play a key role in mitigating urban heat by cooling the local environment. This study evaluated the extent to which street trees can reduce sub-canopy air temperature relative to ambient conditions (DT), and how DT relates to tree traits and microclimatic variables. Air temperature under the canopies of 10 species was recorded within residential areas in Western Sydney, Australia, during summer 2019–2020. Tree and canopy traits, namely tree height, specific leaf area, leaf dry matter content, leaf area index, crown width and the Huber value (the ratio of sapwood area to leaf area) were then measured for all species. Species differed significantly in their DT values, with peak cooling (maximum DT 3.9 C) observed between 9–10 am and sub-canopy warming (i.e., positive DT values) typically occurring during afternoon and overnight. Trees with high LAI and wider canopies were associated with the greatest daytime cooling benefits and lower levels of nighttime warming. DT was also negatively related to windspeed and vapor pressure deficit, and positively to solar irradiance. This study provides valuable information on how tree characteristics and microclimate influence potential cooling benefits that may aid planning decisions on the use of trees to mitigate heat in urban landscapes

Plant functional traits differ in adaptability and are predicted to be differentially affected by climate change

1. Climate change is testing the resilience of forests worldwide pushing physiological tolerance to climatic extremes. Plant functional traits have been shown to be adapted to climate and have evolved patterns of trait correlations (similar patterns of distribution) and coordinations (mechanistic trade-off). We predicted that traits would differentiate between populations associated with climatic gradients, suggestive of adaptive variation, and correlated traits would adapt to future climate scenarios in similar ways. 2. We measured genetically determined trait variation and described patterns of correlation for seven traits: photochemical reflectance index (PRI), normalized difference vegetation index (NDVI), leaf size (LS), specific leaf area (SLA), δ13C (integrated water-use efficiency, WUE), nitrogen concentration (NCONC), and wood density (WD). All measures were conducted in an experimental plantation on 960 trees sourced from 12 populations of a key forest canopy species in southwestern Australia. 3. Significant differences were found between populations for all traits. Narrow sense heritability was significant for five traits (0.15–0.21), indicating that natural selection can drive differentiation; however, SLA (0.08) and PRI (0.11) were not significantly heritable. Generalized additive models predicted trait values across the landscape for current and future climatic conditions (>90% variance). The percent change differed markedly among traits between current and future predictions (differing as little as 1.5% (δ13C) or as much as 30% (PRI)). Some trait correlations were predicted to break down in the future (SLA:NCONC, δ13C:PRI, and NCONC:WD). 4. Synthesis: Our results suggest that traits have contrasting genotypic patterns and will be subjected to different climate selection pressures, which may lower the working optimum for functional traits. Further, traits are independently associated with different climate factors, indicating that some trait correlations may be disrupted in the future. Genetic constraints and trait correlations may limit the ability for functional traits to adapt to climate change

Aridity drives clinal patterns in leaf traits and responsiveness to precipitation in a broadly distributed Australian tree species

Aridity shapes species distributions and plant growth and function worldwide. Yet, plant traits often show complex relationships with aridity, challenging our understanding of aridity as a driver of evolutionary adaptation. We grew nine genotypes of Eucalyptus camaldulensis subsp. camaldulensis sourced from an aridity gradient together in the field for ~650 days under low and high precipitation treatments. Eucalyptus camaldulesis is considered a phreatophyte (deep-rooted species that utilizes groundwater), so we hypothesized that genotypes from more arid environments would show lower aboveground productivity, higher leaf gas-exchange rates, and greater tolerance/avoidance of dry surface soils (indicated by lower responsiveness) than genotypes from less arid environments. Aridity predicted genotype responses to precipitation, with more arid genotypes showing lower responsiveness to reduced precipitation and dry surface conditions than less arid genotypes. Under low precipitation, genotype net photosynthesis and stomatal conductance increased with home-climate aridity. Across treatments, genotype intrinsic water-use efficiency and osmotic potential declined with increasing aridity while photosynthetic capacity (Rubisco carboxylation and RuBP regeneration) increased with aridity. The observed clinal patterns indicate that E. camaldulensis genotypes from extremely arid environments possess a unique strategy defined by lower responsiveness to dry surface soils, low water-use efficiency, and high photosynthetic capacity. This strategy could be underpinned by deep rooting and could be adaptive under arid conditions where heat avoidance is critical and water demand is high

Remarkable resilience of forest structure and biodiversity following fire in the peri-urban bushland of Sydney, Australia

In rapidly urbanizing areas, natural vegetation becomes fragmented, making conservation planning challenging, particularly as climate change accelerates fire risk. We studied urban forest fragments in two threatened eucalypt‐dominated (scribbly gum woodland, SGW, and ironbark forest, IF) communities across ~2000 ha near Sydney, Australia, to evaluate effects of fire frequency (0– 4 in last 25 years) and time since fire (0.5 to >25 years) on canopy structure, habitat quality and biodiversity (e.g., species richness). Airborne lidar was used to assess canopy height and density, and ground‐based surveys of 148 (400 m2) plots measured leaf area index (LAI), plant species com‐ position and habitat metrics such as litter cover and hollow‐bearing trees. LAI, canopy density, litter, and microbiotic soil crust increased with time since fire in both communities, while tree and mistletoe cover increased in IF. Unexpectedly, plant species richness increased with fire frequency, owing to increased shrub richness which offset decreased tree richness in both communities. These findings indicate biodiversity and canopy structure are generally resilient to a range of times since fire and fire frequencies across this study area. Nevertheless, reduced arboreal habitat quality and subtle shifts in community composition of resprouters and obligate seeders signal early concern for a scenario of increasing fire frequency under climate change. Ongoing assessment of fire responses is needed to ensure that biodiversity, canopy structure and ecosystem function are maintained in the remaining fragments of urban forests under future climate change which will likely drive hotter and more frequent fires

Applications of molecular markers in plant conservation

Plant conservation geneticists aim to use molecular markers to assess the short- and long-term viability of populations and species with small effective population sizes, found in altered landscapes, and/or experiencing genetic exchange from divergent gene pools. Critically, conservation managers need to determine the likely consequences of these threats. Is there any loss of plant fitness from inbreeding or outbreeding? What are the historical and contemporary barriers to genetic exchange? Is adaptive potential reduced in small or interbreeding populations? Population genetic research quantifies molecular variation and how it is distributed among individuals and populations. Understanding these measures and being able to relate the factors and threats likely to cause temporal changes in population dynamics and connectivity represents the background for the development of sound conservation and management strategies

Isolation and characterization of microsatellite markers for the neotropical tree, Pachira quinata (Malvaceae)

Twelve microsatellite loci were developed from a genomic library enriched for SSRs of Pachira quniata, a canopy tree species of dry and some humid forests in the neotropics (Central and northern South America). Genotyping of 30 trees from one population on the Pacific coast of Nicaragua was successfully performed. The loci were across amplified in related species. These microsatellites will be used to compare regional population structuring and mating patterns in fragmented landscapes