Leaf morphological traits show greater responses to changes in climate than leaf physiological traits and gas exchange variables

Adaptation to changing conditions is one of the strategies plants may use to survive in the face of climate change. We aimed to determine whether plants' leaf morphological and physiological traits/gas exchange variables have changed in response to recent, anthropogenic climate change. We grew seedlings from resurrected historic seeds from ex-situ seed banks and paired modern seeds in a common-garden experiment. Species pairs were collected from regions that had undergone differing levels of climate change using an emerging framework-Climate Contrast Resurrection Ecology, allowing us to hypothesise that regions with greater changes in climate (including temperature, precipitation, climate variability and climatic extremes) would be greater trait responses in leaf morphology and physiology over time. Our study found that in regions where there were greater changes in climate, there were greater changes in average leaf area, leaf margin complexity, leaf thickness and leaf intrinsic water use efficiency. Changes in leaf roundness, photosynthetic rate, stomatal density and the leaf economic strategy of our species were not correlated with changes in climate. Our results show that leaves do have the ability to respond to changes in climate, however, there are greater inherited responses in morphological leaf traits than in physiological traits/variables and greater responses to extreme measures of climate than gradual changes in climatic means. It is vital for accurate predictions of species' responses to impending climate change to ensure that future climate change ecology studies utilise knowledge about the difference in both leaf trait and gas exchange responses and the climate variables that they respond to.

Stepping up to the thermogradient plate: a data framework for predicting seed germination under climate change

BACKGROUND AND AIMS

Seed germination is strongly influenced by environmental temperatures. With global temperatures predicted to rise, the timing of germination for thousands of plant species could change, leading to potential decreases in fitness and ecosystem-wide impacts. The thermogradient plate (TGP) is a powerful but underutilized research tool that tests germination under a broad range of constant and alternating temperatures, giving researchers the ability to predict germination characteristics using current and future climates. Previously, limitations surrounding experimental design and data analysis methods have discouraged its use in seed biology research.

METHODS

Here, we have developed a freely available R script that uses TGP data to analyse seed germination responses to temperature. We illustrate this analysis framework using three example species: Wollemia nobilis, Callitris baileyi and Alectryon subdentatus. The script generates >40 germination indices including germination rates and final germination across each cell of the TGP. These indices are then used to populate generalized additive models and predict germination under current and future monthly maximum and minimum temperatures anywhere on the globe.

KEY RESULTS

In our study species, modelled data were highly correlated with observed data, allowing confident predictions of monthly germination patterns for current and future climates. Wollemia nobilis germinated across a broad range of temperatures and was relatively unaffected by predicted future temperatures. In contrast, C. baileyi and A. subdentatus showed strong seasonal temperature responses, and the timing for peak germination was predicted to shift seasonally under future temperatures.

CONCLUSIONS

Our experimental workflow is a leap forward in the analysis of TGP experiments, increasing its many potential benefits, thereby improving research predictions and providing substantial information to inform management and conservation of plant species globally.

Plant Diversity Conservation Challenges and Prospects-The Perspective of Botanic Gardens and the Millennium Seed Bank

There is a pressing need to conserve plant diversity to prevent extinctions and to enable sustainable use of plant material by current and future generations. Here, we review the contribution that living collections and seed banks based in botanic gardens around the world make to wild plant conservation and to tackling global challenges. We focus in particular on the work of Botanic Gardens Conservation International and the Millennium Seed Bank of the Royal Botanic Gardens, Kew, with its associated global Partnership. The advantages and limitations of conservation of plant diversity as both living material and seed collections are reviewed, and the need for additional research and conservation measures, such as cryopreservation, to enable the long-term conservation of 'exceptional species' is discussed. We highlight the importance of networks and sharing access to data and plant material. The skill sets found within botanic gardens and seed banks complement each other and enable the development of integrated conservation (linking in situ and ex situ efforts). Using a number of case studies we demonstrate how botanic gardens and seed banks support integrated conservation and research for agriculture and food security, restoration and reforestation, as well as supporting local livelihoods.

Time-traveling seeds reveal that plant regeneration and growth traits are responding to climate change

Studies assessing the biological impacts of climate change typically rely on long-term, historic data to measure trait responses to climate through time. Here, we overcame the problem of absent historical data by using resurrected seeds to capture historic plant-trait data for a number of plant regeneration and growth traits. We collected seed and seedling trait measurements from resurrected historic seeds and compared these with modern seed and seedling traits collected from the same species in the same geographic location. We found a total of 43 species from southeastern Australia for which modern/historic seed pairs could be located. These species were located in a range of regions that have undergone different amounts of climate change across a range of temperature, precipitation, and extreme measures of climate. There was a correlation between the amount of change in climate metrics, and the amount of change in plant traits. Using stepwise model selection, we found that for all regeneration and growth trait changes (except change in stem density), the most accurate model selected at least two measures of climate change. Changes in extreme measures of climate, such as heat-wave duration and changes in climate variability, were more strongly related to changes in regeneration and growth traits than changes in mean climate metrics. Across our species, for every 5% increase in temperature variability, there was a threefold increase in the probability of seed viability and seed germination success. An increase of 1 d in the maximum duration of dry spells through time led to a 1.5-fold decrease in seed viability and seeds became 30% flatter/thinner. Regions where the maximum heat-wave duration had increased by 10 d saw a 1.35-cm decrease in seedling height and a 1.04-g decrease in seedling biomass. Rapid responses in plant traits to changes in climate may be possible; however, it is not clear whether these changes will be fast enough for plants to keep pace with future climate change.

A research agenda for seed-trait functional ecology

Trait-based approaches have improved our understanding of plant evolution, community assembly and ecosystem functioning. A major challenge for the upcoming decades is to understand the functions and evolution of early life-history traits, across levels of organization and ecological strategies. Although a variety of seed traits are critical for dispersal, persistence, germination timing and seedling establishment, only seed mass has been considered systematically. Here we suggest broadening the range of morphological, physiological and biochemical seed traits to add new understanding on plant niches, population dynamics and community assembly. The diversity of seed traits and functions provides an important challenge that will require international collaboration in three areas of research. First, we present a conceptual framework for a seed ecological spectrum that builds upon current understanding of plant niches. We then lay the foundation for a seed-trait functional network, the establishment of which will underpin and facilitate trait-based inferences. Finally, we anticipate novel insights and challenges associated with incorporating diverse seed traits into predictive evolutionary ecology, community ecology and applied ecology. If the community invests in standardized seed-trait collection and the implementation of rigorous databases, major strides can be made at this exciting frontier of functional ecology.

Establishing a Wild, Ex Situ Population of a Critically Endangered Shade-Tolerant Rainforest Conifer: A Translocation Experiment

Translocation can reduce extinction risk by increasing population size and geographic range, and is increasingly being used in the management of rare and threatened plant species. A critical determinant of successful plant establishment is light environment. Wollemia nobilis (Wollemi pine) is a critically endangered conifer, with a wild population of 83 mature trees and a highly restricted distribution of less than 10 km2. We used under-planting to establish a population of W. nobilis in a new rainforest site. Because its optimal establishment conditions were unknown, we conducted an experimental translocation, planting in a range of different light conditions from deeply shaded to high light gaps. Two years after the experimental translocation, 85% of plants had survived. There were two distinct responses: very high survival (94%) but very low growth, and lower survival (69%) and higher growth, associated with initial plant condition. Overall survival of translocated W. nobilis was strongly increased in planting sites with higher light, in contrast to previous studies demonstrating long-term survival of wild W. nobilis juveniles in deep shade. Translocation by under-planting may be useful in establishing new populations of shade-tolerant plant species, not least by utilizing the range of light conditions that occur in forest understories.

Rising temperature may negate the stimulatory effect of rising CO2 on growth and physiology of Wollemi pine (Wollemia nobilis)

Rising atmospheric [CO2] is associated with increased air temperature, and this warming may drive many rare plant species to extinction. However, to date, studies on the interactive effects of rising [CO2] and warming have focussed on just a few widely distributed plant species. Wollemi pine (Wollemia nobilis W.G.Jones, K.D.Hill, & J.M.Allen), formerly widespread in Australia, was reduced to a remnant population of fewer than 100 genetically indistinguishable individuals. Here, we examined the interactive effects of three [CO2] (290, 400 and 650ppm) and two temperature (ambient, ambient+4°C) treatments on clonally-propagated Wollemi pine grown for 17 months in glasshouses under well-watered and fertilised conditions. In general, the effects of rising [CO2] and temperature on growth and physiology were not interactive. Rising [CO2] increased shoot growth, light-saturated net photosynthetic rates (Asat) and net carbon gain. Higher net carbon gain was due to increased maximum apparent quantum yield and reduced non-photorespiratory respiration in the light, which also reduced the light compensation point. In contrast, increasing temperature reduced stem growth and Asat. Compensatory changes in mesophyll conductance and stomatal regulation suggest a narrow functional range of optimal water and CO2 flux co-regulation. These results suggest Asat and growth of the surviving genotype of Wollemi pine may continue to increase with rising [CO2], but increasing temperatures may offset these effects, and challenges to physiological and morphological controls over water and carbon trade-offs may push the remnant wild population of Wollemi pine towards extinction.

Growing up or growing out? How soil pH and light affect seedling growth of a relictual rainforest tree

Seedling growth rates can have important long-term effects on forest dynamics. Environmental variables such as light availability and edaphic factors can exert a strong influence on seedling growth. In the wild, seedlings of Wollemi pine (Wollemia nobilis) grow on very acid soils (pH ∼4.3) in deeply shaded sites (∼3 % full sunlight). To examine the relative influences of these two factors on the growth of young W. nobilis seedlings, we conducted a glasshouse experiment growing seedlings at two soil pH levels (4.5 and 6.5) under three light levels: low (5 % full sun), medium (15 %) and high (50 %). Stem length and stem diameter were measured, stem number and branch number were counted, and chlorophyll and carotenoid content were analysed. In general, increased plant growth was associated with increased light, and with low pH irrespective of light treatment, and pigment content was higher at low pH. Maximum stem growth occurred in plants grown in the low pH/high light treatment combination. However, stem number was highest in low pH/medium light. We hypothesize that these differences in stem development of W. nobilis among light treatments were due to this species' different recruitment strategies in response to light: greater stem growth at high light and greater investment in multiple stem production at low light. The low light levels in the W. nobilis habitat may be a key limitation on stem growth and hence W. nobilis recruitment from seedling to adult. Light and soil pH are two key factors in the growth of this threatened relictual rainforest species.

Pushed to the limit: consequences of climate change for the Araucariaceae: a relictual rain forest family

BACKGROUND AND AIMS

Under predicted climate change scenarios, increased temperatures are likely to predispose trees to leaf and other tissue damage, resulting in plant death and contraction of already narrow distribution ranges in many relictual species. The effects of predicted upward temperatures may be further exacerbated by changes in rainfall patterns and damage caused by frosts on trees that have been insufficiently cold-hardened. The Araucariaceae is a relictual family and the seven species found in Australia have limited natural distributions characterized by low frost intensity and frequency, and warm summer temperatures. The temperature limits for these species were determined in order to help understand how such species will fare in a changing climate.

METHODS

Experiments were conducted using samples from representative trees of the Araucariaceae species occurring in Australia, Agathis (A. atropurpurea, A. microstachya and A. robusta), Arauacaria (A. bidwilli, A. cunninghamii and A. heterophylla) and Wollemia nobilis. Samples were collected from plants grown in a common garden environment. Lower and higher temperature limits were determined by subjecting detached winter-hardened leaves to temperatures from 0 to -17 °C and summer-exposed leaves to 25 to 63 °C, then measuring the efficiency of photosystem II (F(v)/F(m)) and visually rating leaf damage. The exotherm, a sharp rise in temperature indicating the point of ice nucleation within the cells of the leaf, was measured on detached leaves of winter-hardened and summer temperature-exposed leaves.

KEY RESULTS

Lower temperature limits (indicated by FT(50), the temperature at which PSII efficiency is 50 %, and LT(50) the temperature at which 50 % visual leaf damage occurred) were approx. -5·5 to -7·5 °C for A. atropurpurea, A. microstachya and A. heterophylla, approx. -7 to -9 °C for A. robusta, A. bidwillii and A. cunninghamii, and -10·5 to -11 °C for W. nobilis. High temperature damage began at 47·5 °C for W. nobilis, and occurred in the range 48·5-52 °C for A. bidwillii and A. cunninghamii, and in the range 50·5-53·5 °C for A. robusta, A. microstachya and A. heterophylla. Winter-hardened leaves had ice nucleation temperatures of -5·5 °C or lower, with W. nobilis the lowest at -6·8 °C. All species had significantly higher ice nucleation temperatures in summer, with A. atropurpurea and A. heterophylla forming ice in the leaf at temperatures >3 °C higher in summer than in winter. Wollemia nobilis had lower FT(50) and LT(50) values than its ice nucleation temperature, indicating that the species has a degree of ice tolerance.

CONCLUSIONS

While lower temperature limits in the Australian Araucariaceae are generally unlikely to affect their survival in wild populations during normal winters, unseasonal frosts may have devastating effects on tree survival. Extreme high temperatures are not common in the areas of natural occurrence, but upward temperature shifts, in combination with localized radiant heating, may increase the heat experienced within a canopy by at least 10 °C and impact on tree survival, and may contribute to range contraction. Heat stress may explain why many landscape plantings of W. nobilis have failed in hotter areas of Australia.

Axillary meristems and the development of epicormic buds in wollemi pine (Wollemia nobilis)

Intact trees of Wollemia nobilis Jones, Hill and Allen (Araucariaceae) routinely develop multiple coppice shoots as well as orthotropic epicormic shoots that become replacement or additional leaders. As these are unusual architectural features for the Araucariaceae, an investigation was made of the axillary meristems of the main stem and their role in the production of epicormic and possibly coppice shoots. Leaf axils, excised from the apex to the base of 2-m-high W. nobilis plants (seedling origin, ex situ grown), were examined anatomically. Small, endogenous, undifferentiated (no leaf primordia, no vascular or provascular connections) meristems were found in the axils from near the shoot apex. In the more proximal positions about half the meristems sampled did not differentiate further, but became tangentially elongated to compensate for increases in stem diameter. In the remaining axils the meristems slowly developed into bud primordia, although these buds usually developed few leaf primordia and their apical 'domes' were wide and flat. Associated vascular development was generally restricted to provascular dedifferentiation of the cortical parenchyma, with the procambium usually forming a 'closed loop' that did not extend back to the secondary vascular tissues. Development of the meristems was very uneven with adjacent axils often at widely differing stages of development into buds. The study shows that, unlike most conifers, W. nobilis possesses long-lived meristematic potential in most, if not all, leaf axils. Unlike other araucarias that have been investigated, many of the meristems in the orthotropic main stem will slowly develop into bud primordia beneath the bark in intact plants. It appears likely that this slow but continued development provides a ready source of additional or replacement leaders and thus new branches and leaves.