Safety margins and adaptive capacity of vegetation to climate change

Vegetation is composed of many individual species whose climatic tolerances can be integrated into spatial analyses of climate change risk. Here, we quantify climate change risk to vegetation at a continental scale by calculating the safety margins for warming and drying (i.e., tolerance to projected change in temperature and precipitation respectively) across plants sharing 100km × 100km grid cells (locations). These safety margins measure how much warmer, or drier, a location could become before its ‘typical’ species exceeds its observed climatic limit. We also analyse the potential adaptive capacity of vegetation to temperature and precipitation change (i.e., likelihood of in situ persistence) using median precipitation and temperature breadth across all species in each location. 47% of vegetation across Australia is potentially at risk from increases in mean annual temperature (MAT) by 2070, with tropical regions most vulnerable. Vegetation at high risk from climate change often also exhibited low adaptive capacity. By contrast, 2% of the continent is at risk from reductions in annual precipitation by 2070. Risk from precipitation change was isolated to the southwest of Western Australia where both the safety margin for drier conditions in the typical species is low, and substantial reductions in MAP are projected

A survey of leaf phosphorus fractions and leaf economic traits among 12 co-occurring woody species on phosphorus-impoverished soils

Background and Aims: The leaf economic spectrum (LES) is related to dry mass and nutrient investments towards photosynthetic processes and leaf structures, and to the duration of returns on those investments (leaf lifespan, LL). Phosphorus (P) is a key limiting nutrient for plant growth, yet it is unclear how the allocation of leaf P among different functions is coordinated with the LES. We addressed this question among 12 evergreen woody species co-occurring on P-impoverished soils in south-eastern Australia. Methods: Leaf ‘economic’ traits, including LL, leaf mass per area (LMA), light-saturated net photosynthetic rate per mass (Amass), dark respiration rate, P concentration ([Ptotal]), nitrogen concentration, and P resorption, were measured for three pioneer and nine non-pioneer species. Leaf P was separated into five functional fractions: orthophosphate P (Pi), metabolite P (PM), nucleic acid P (PN), lipid P (PL), and residual P (PR; phosphorylated proteins and unidentified compounds that contain P). Results: LL was negatively correlated with Amass and positively correlated with LMA, representing the LES. Pioneers occurred towards the short-LL end of the spectrum and exhibited higher [Ptotal] than non-pioneer species, primarily associated with higher concentrations of Pi, PN and PL. There were no significant correlations between leaf P fractions and LL or LMA, while Amass was positively correlated with the concentration of PR. Conclusions: Allocation of leaf P to different fractions varied substantially among species. This variation was partially associated with the LES, which may provide a mechanism underlying co-occurrence of species with different ecological strategies under P limitation

Assessing the vulnerability of plant functional trait strategies to climate change

Aim: Our ability to understand how species may respond to changing climate conditions is hampered by a lack of high-quality data on the adaptive capacity of species. Plant functional traits are linked to many aspects of species life history and adaptation to environment, with different combinations of trait values reflecting alternate strategies for adapting to varied conditions. If the realized climate limits of species can be partially explained by plant functional trait combinations, then a new approach of using trait combinations to predict the expected climate limits of species trait combinations may offer considerable benefits. Location: Australia. Time period: Current and future. Methods: Using trait data for leaf size, seed mass and plant height for 6,747 Australian native species from 27 plant families, we model the expected climate limits of trait combinations and use future climate scenarios to estimate climate change impacts based on plant functional trait strategies. Results: Functional trait combinations were a significant predictor of species climate niche metrics with potentially meaningful relationships with two rainfall variables (R2 =.36 &.45) and three temperature variables (R2 =.21,.28,.30). Using this method, the proportion of species exposed to conditions across their range that are beyond the expected climate limits of their trait strategies will increase under climate change. Main conclusions: Our new approach, called trait strategy vulnerability, includes three new metrics. For example, the climate change vulnerability (CCV) metric identified a small but important proportion of species (4.3%) that will on average be exposed to conditions beyond their expected limits for summer temperature in the future. These potentially vulnerable species could be high priority targets for deeper assessment of adaptive capacity at the genomic or physiological level. Our methods can be applied to any suite of co-occurring plants globally

Parenchyma abundance in wood of evergreen trees varies independently of nutrients

The abundance of living cells in wood—mainly as interconnected axial and ray parenchyma networks—varies widely between species. However, the functional significance of this variation and its role in plant ecological strategies is poorly understood, as is the extent to which different parenchyma fractions are favored in relation to soil nutrients and hydraulic functions. We analyzed wood tissue fractions of 16 Australian angiosperm species sampled from two nearby areas with similar climate but very different soil nutrient profiles and investigated structure-function links with soil and tissue nutrient concentrations and other plant traits. We expected the variation in parenchyma fractions to influence nutrient concentrations in wood xylem, and to find species with lower parenchyma fractions and accordingly lower nutrient requirements on lower-nutrient soils. Surprisingly, both axial and ray parenchyma fractions were mostly unrelated to tissue and soil nutrient concentrations, except for nitrogen concentration in stem sapwood. Species from low nutrient soils showed higher fractional P translocation from both leaves and sapwood, but little patterning with respect to tissue nitrogen. While species from high and low nutrient soils clearly clustered along the soil-fertility axis, their tissue composition varied independently from plant functional traits related to construction costs and hydraulic anatomy. Our findings imply that there is considerable variation among species in the nutrient concentrations within different parenchyma tissues. The anatomical composition of wood tissue seems unrelated to plant nutrient requirements. Even though xylem parenchyma is involved in metabolic functions such as nutrient translocation and storage, parenchyma abundance on its own does not directly explain variation in these functions, even in co-occurring species. While parenchyma is highly abundant in wood of angiosperm trees, we are still lacking a convincing ecological interpretation of its variability and role in whole-tree nutrient budgets

Leaf trait adaptations of xylem-tapping mistletoes and their hosts in sites of contrasting aridity

Background and aims Xylem-tapping mistletoes may experience relaxed selective pressure to use water efficiently during photosynthesis because of lower per-unit costs for water acquisition than experienced by host plants. As a result, we hypothesised that mistletoes would exhibit parallel but dampened leaf-level adaptations and responses to aridity, compared to those seen in hosts. Methods Photosynthetic traits, leaf dark respiration, nutrient concentrations and specific leaf area (SLA) were measured on 42 mistletoe-host species-pairs sampled from five sites in Australia and Brazil that vary widely in aridity. Results Mistletoes exhibited similar trait-shifts to hosts in relation to site aridity. In both groups, arid-site species showed stronger control over stomatal water loss, larger drawdown of CO2 during photosynthesis (lower ci: ca), higher leaf N and P concentrations per unit leaf area, and lower SLA. Nevertheless, mistletoes were profligate water users compared to their hosts and showed substantially less efficient use of water during photosynthesis. On average, mistletoes showed twice higher leaf dark respiration rates at a given photosynthetic capacity, suggesting relatively higher leaf maintenance costs for these parasitic plants. Conclusions Despite fundamental differences in lifestyle and in photosynthetic traits, mistletoes exhibit trait responses and adaptations to site aridity in parallel and to approximately the same extent as their hosts

Disentangling direct and indirect effects of island area on plant functional trait distributions

Aim: Species diversity on islands generally increases with island area. This might arise either from direct effects of island area via neutral assembly processes or from indirect effects via habitat and structural differences between islands that scale positively with island area. Here, we tested whether community-weighted functional trait means of woody plants are directly or indirectly affected by island area to elucidate how functional traits mediate the assembly on differently sized islands. Location: Twenty-eight tropical islands (25 m2 – 12,000 m2) in the Raja Ampat archipelago, Indonesia. Taxon: Woody angiosperms. Methods: Studied islands had a shared geological history but differed in terms of area, habitat quality expressed by soil depth, forest structure expressed by tree basal area and degree of isolation. Traits studied were seed and fruit mass, tree height, wood density, leaf mass per area, leaf nitrogen concentration and chlorophyll content (estimated from chlorophyll-meter units) and summarised as community-weighted means (CWM) for each island. Using liner regression, we tested whether CWMs were correlated to island area and basal area and structural equation models (SEMs) to test on direct and indirect effects of island area, basal area, soil depth and isolation on trait distributions. Results: CWM of seed mass, tree height and chlorophyll content increased with both island area and basal area, whereas leaf nitrogen concentration decreased with increasing basal area. Fruit mass was not correlated to island area and basal area. SEMs revealed that the shifts in tree height, wood density, leaf nitrogen concentration and chlorophyll content were caused directly by basal area, which in turn was directly and positively affected by both island area and soil depth. Differences in seed mass among islands were explained by combined effects of basal area, island area and isolation, whereas fruit mass was only explained by isolation. Main conclusions: Trait values shifted systematically across islands of different sizes. Being small and having light seeds are prevailing trait combinations for establishing on small islands with simple forest structure. For establishment on larger islands with more complex forest structures, species are taller, have heavier seeds, higher chlorophyll content and lower leaf N concentrations. We conclude that mechanisms affecting CWM on islands directly link to ecological differences between islands like forest structure – and only indirectly to island area

A roadmap to plant functional island biogeography

Island biogeography is the study of the spatio-temporal distribution of species, communities, assemblages or ecosystems on islands and other isolated habitats. Island diversity is structured by five classes of process: dispersal, establishment, biotic interactions, extinction and evolution. Classical approaches in island biogeography focused on species richness as the deterministic outcome of these processes. This has proved fruitful, but species traits can potentially offer new biological insights into the processes by which island life assembles and why some species perform better at colonising and persisting on islands. Functional traits refer to morphological and phenological characteristics of an organism or species that can be linked to its ecological strategy and that scale up from individual plants to properties of communities and ecosystems. A baseline hypothesis is for traits and ecological strategies of island species to show similar patterns as a matched mainland environment. However, strong dispersal, environmental and biotic-interaction filters as well as stochasticity associated with insularity modify this baseline. Clades that do colonise often embark on distinct ecological and evolutionary pathways, some because of distinctive evolutionary forces on islands, and some because of the opportunities offered by freedom from competitors or herbivores or the absence of mutualists. Functional traits are expected to be shaped by these processes. Here, we review and discuss the potential for integrating functional traits into island biogeography. While we focus on plants, the general considerations and concepts may be extended to other groups of organisms. We evaluate how functional traits on islands relate to core principles of species dispersal, establishment, extinction, reproduction, biotic interactions, evolution and conservation. We formulate existing knowledge as 33 working hypotheses. Some of these are grounded on firm empirical evidence, others provide opportunities for future research. We organise our hypotheses under five overarching sections. Section A focuses on plant functional traits enabling species dispersal to islands. Section B discusses how traits help to predict species establishment, successional trajectories and natural extinctions on islands. Section C reviews how traits indicate species biotic interactions and reproduction strategies and which traits promote intra-island dispersal. Section D discusses how evolution on islands leads to predictable changes in trait values and which traits are most susceptible to change. Section E debates how functional ecology can be used to study multiple drivers of global change on islands and to formulate effective conservation measures. Islands have a justified reputation as research models. They illuminate the forces operating within mainland communities by showing what happens when those forces are released or changed. We believe that the lens of functional ecology can shed more light on these forces than research approaches that do not consider functional differences among species

Multispectral, aerial disease detection for myrtle rust (Austropuccinia psidii) on a lemon myrtle plantation

Disease management in agriculture often assumes that pathogens are spread homogeneously across crops. In practice, pathogens can manifest in patches. Currently, disease detection is predominantly carried out by human assessors, which can be slow and expensive. A remote sensing approach holds promise. Current satellite sensors are not suitable to spatially resolve individual plants or lack temporal resolution to monitor pathogenesis. Here, we used multispectral imaging and unmanned aerial systems (UAS) to explore whether myrtle rust (Austropuccinia psidii) could be detected on a lemon myrtle (Backhousia citriodora) plantation. Multispectral aerial imagery was collected from fungicide treated and untreated tree canopies, the fungicide being used to control myrtle rust. Spectral vegetation indices and single spectral bands were used to train a random forest classifier. Treated and untreated trees could be classified with high accuracy (95%). Important predictors for the classifier were the near-infrared (NIR) and red edge (RE) spectral band. Taking some limitations into account, that are discussed herein, our work suggests potential for mapping myrtle rust-related symptoms from aerial multispectral images. Similar studies could focus on pinpointing disease hotspots to adjust management strategies and to feed epidemiological models

Summer solstice marks a seasonal shift in temperature sensitivity of stem growth and nitrogen-use efficiency in cold-limited forests

In boreal forests and alpine treelines, it is debatable how the temperature sensitivity of tree-ring growth should vary with changes in climate over time and the extent to which seasonal stem increments are controlled by leaf physiology. We aim to test the hypothesis that, in cold-limited forests, maximizing stem growth rate around summer solstice is closely related to foliage turnover, which generally results in high sensitivity of stem growth and less sensitivity of nitrogen-use efficiency (NUE) to early-season temperatures. Our analysis was based on repeat-census observations of stem radial increment (2008–2013; made with dendrometers) and monthly litterfall (2007–2015) as well as the measurements of tree-ring width series (1960–2015; made with tree-ring cores) in two Tibetan treeline forests. NUE was estimated as the inverse of leaf-litter nitrogen concentration. We further examined a global dataset of tree-ring chronologies (1931–1990) from 139 sites across temperate and boreal coniferous forests in the northern high-latitude region. Weekly stem increments across species and years synchronously peaked around summer solstice, with more than half of annual increment produced in the first 28–35 days of the growing season when air and soil temperatures were still low. Monthly stem increments were positively related to previous-month litterfall, and higher litterfall generally resulted in higher NUE. NUE was insensitive or less sensitive to soil temperature in the early growing season. Among years, pre-peak increments were positively correlated with pre-solstice temperatures while post-peak increments varied little. The annual increment was dominated by and coherent with the pre-peak increment and well correlated with the ring-width measurements of monitored trees during 2008–2013. Variations in tree-ring width chronologies from the two Tibetan treelines and the global 139 forest sites mainly reflected the change of early summer temperatures. The findings suggest a day-length control on the linkage between seasonal stem growth and nitrogen cycling in a cold-limited forest ecosystem, and provide the basic for predicting responses of tree-ring growth and NUE to climatic warming

Developing a spectral disease index for myrtle rust ( Austropuccinia psidii

Since 2010 Australian ecosystems and managed landscapes have been severely threatened by the invasive fungal pathogen Austropuccinia psidii. Detecting and monitoring disease outbreaks is currently only possible by human assessors, which is slow and labour intensive. Over the last 25 years, spectral vegetation indices (SVIs) have been designed to assess variation in biochemical or biophysical traits of vegetation. However, diagnosis of individual diseases based on classical SVIs is currently not possible because they lack disease specificity. Here, a novel spectral disease index (SDI), the lemon myrtle–myrtle rust index (LMMR), has been developed. The index was designed from hyperspectral leaf-clip data collected at a lemon myrtle plantation in New South Wales, Australia. A total of 236 fungicide-treated (disease free) and 228 untreated (diseased) lemon myrtle leaves were sampled and a random forest classifier was used to show that the LMMR discriminates those classes with an overall accuracy of 90%. Compared to three classical SVIs (PRI, MCARI, NBNDVI), commonly applied for stress detection, the LMMR clearly improved classification accuracies (58%, 67%, 60%, respectively). If the LMMR can be validated on independent datasets from similar and different host species, it could enable land managers to reduce disease impact by earlier control. There might also be potential to collect useful data for epidemiology models. Calculating the LMMR based on hyperspectral data collected from aerial platforms (e.g. drones) would allow for rapid and high-capacity screening for disease outbreaks