Australasian Plant Conservation
Originally published in Australasian Plant Conservation 19(3) December 2010 - February 2011, p 6-8
Climate warming and the germination niche
Department of Environment and Conservation, Bentley, WA. Email: email@example.com
Germination is an important life history phase for obligate seeding species. Seeders need to germinate to persist in the environment after disturbance, with favourable environmental conditions required to ensure recruitment success. Failure to germinate after disturbance events may mean local population extinction.
Temperature is arguably one of the most important climatic variables influencing seeds since it synchronises germination to environmental conditions most suitable for seedling establishment. Seeds will germinate over a range of temperatures, with thresholds above and below which little or no germination will occur. Under climate warming scenarios temperatures are forecast to rise between 2–5°C. If species have specific temperature requirements for germination then climate warming could cause a mismatch between temperatures that seeds experience and temperatures over which germination is able to occur. Such a mismatch in the germination requirements of obligate seeding species could render them vulnerable to decline and extinction. Species that occupy specialised habitats and those inhabiting cooler and wetter climates are likely to be more susceptible to change.
Taking into account the current geographic distribution of species, bioclimatic modelling has been used to predict the future of plant diversity across a number of plant groups in Australia. Hughes et al. (1996) predict the displacement of temperature envelopes of Eucalyptus species; Pouliquen-Young and Newman (2000) envisage dramatic decreases in the geographic range of Western Australia’s eastern goldfields Acacia and Dryandra species, and Fitzpatrick et al. (2008) and Yates et al. (2010) forecast range contraction for Banksia species in the south-west of that state.
Models use spatial environmental data to infer species’ range limits and habitat suitability, but they do not take into account complex biological processes that drive species’ distributions and persistence, and are therefore subject to error. The collection of empirical data on biology, ecology and species’ interactions would increase the robustness of existing models.
The environmental space in which a species is located is termed its niche. Niche characteristics can be powerful indicators of species’ sensitivities to climate change. By measuring one dimension of the niche (e.g. germination) and studying the rate of change in that feature over time (vis a vis projected climate scenarios), insight can be gained into the effects of a warming climate on plant species persistence (Cochrane et al. in press).
This article describes the use of a temperature gradient system to determine whether such parameters as latitude, elevation, rarity or regeneration strategy influences the germination niche of a range of native species from south-west Western Australia with the aim of assisting in the prediction of species’ vulnerability to a warming climate.
A two-way temperature gradient plate (Model GRD1, Grant Instruments, Cambridge, UK) was used to profile the germination of more than 45 species across fluctuating and constant temperatures ranging from 5°C to 40°C. Seeds were sown on 1% water agar in 35 or 50 mm Petri dishes under a 12 hour photoperiod and monitored for 6–8 weeks depending on species, with seed germination scored every 2–3 days. Percentage germination and mean time to germination were calculated for each cell in which germination occurred. Contour plots were constructed from the resulting raw percentage germination data using Sigma Plot.
Results and discussion
Day and night temperatures are presented as the niche axis, with temperature profiles representing the two dimensional niche space for germination (figures 1 and 2). The plots show points of equal percentage germination connected by germination isopleths.The percentage germination shading calibration shows dark filling isopleths as high germination, light filling as low or no germination. Constant temperatures occur on the diagonal line from the bottom-left corner of the diagrams (lowest temperature approximately5°C) to the top-right corner (maximum temperature approximately 35–40°C). All points above and below the diagonal line represent alternating temperature regimes, with greatest amplitude at the top-left and bottom-right corners of each graph. Where high germination occurs over a large range of temperatures, the difference in suitability of the niche space is determined by the rate of germination (i.e. fitness contours overlaid on the germination niche) as presented in Figure 2.
Figure 1. Germination temperature profiles for (a) the rare Eucalyptus nutans, (b) the rare Sphenotoma drummondii, (c) the common Eucalyptus conferruminata, (d) the common Banksia dryandroides southern distribution, (e) Banksia speciosa with southern distribution, and (f) Banksia lanata with northern distribution.
Figure 2. Germination temperature profile for Banksia media with contours for mean time to germination (MTG) in days overlaid onto profile to indicate optimal temperatures for germination (i.e. highest level of germination in shortest possible time).
Over the range of species profiled, geographic range, rarity and reproductive mode did not appear to be good predictors of temperature limits in the germination niche. As a general rule, rare species did not necessarily differ in the width of their germination window compared to more commonly occurring species (see Figure 1a and 1c). Despite a general lack of relationship between geographic restriction and germination niche breadth, several species confined to the low mountains of the Stirling Range did display a narrow temperature window for germination (e.g. Sphenotoma drummondii; Figure 1b).
For other geographically restricted species from these upland areas, a wide thermal tolerance for germination suggests that temperature for germination would not be a constraining factor for population persistence or possible range extension under predicted climate warming (data not shown).
In the series of Eucalyptus and Banksia species screened, there was little difference in the germination niche between seeder and sprouter species (data not shown). There was a tendency for a greater number of Banksia species with southern distributions to have a narrow germination niche compared to those with northern distributions (e.g. Figure 1d and 1f), although some Banksia species limited to the south coast also showed broad niche tolerance (e.g. Figure 1e). The germination niche for three populations of the widespread Banksia baueri differed little across the species’ geographic distribution. This suggests for this species that population variation in the germination niche does not exist (i.e. for temperature) (data not shown) and that potentially a single population can be used to define temperature thresholds for germination for the species (i.e. no visible population variation within the germination niche of the species).
The available data do not provide any constructive trends to indicate seed germination traits (level or rate of germination) that may identify species’ vulnerability to a warming climate. Nevertheless, species with narrow germination windows may be restricted in their seasonality of germination, whereas species with wide temperature windows for germination may be less at risk of recruitment failure (i.e. reduced germination) under future climate scenarios. It is possible that species with a broad germination niche may be pre-adapted to warmer temperatures for germination and therefore less of a conservation concern than those with narrow temperature tolerances.
Short-term process studies like these laboratory incubation experiments using temperature gradient systems can give quick, basic insight to some questions regarding potential climate change impacts on seed germination requirements. For example, if a seed cannot germinate under elevated temperatures forecast under climate warming, then population extinction may occur. This novel seed-based predictive tool could become useful to highlight priorities for conservation actions and in particular, help in influencing choice of populations and sites for species reintroductions and restoration into novel environments.
I would like to thank colleagues Matthew Daws and Fiona Hay for assistance with this data and the Millennium Seed Bank RBG Kew for project support. The South Coast Natural Resource Management Inc. is gratefully acknowledged for the purchase of a temperature gradient plate.
Cochrane, A., Daws, M.I. and Hay, F.R. (in press). Seed-based approach for identifying flora at risk from climate warming. Austral Ecology.
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