Our research focuses on the population dynamics of plants and how they are influenced by impacts of natural disturbances and global environmental change. We are particularly interested in the interactive effects of fire, grazing and drought in grasslands and woodlands in southern Australia, and how climate change, fragmentation and shrub encroachment affect ecosystems.

Friday, 29 June 2012

Seeds, germination and climate change - optimums or extremes?

With changing climates, particularly drying and warming, how might plant species respond? If species are to persist in situ, or disperse to new habitats, then the fate of seeds becomes crucial to this outcome. The sensitivity to temperature at the germination phase will likely be one important predictor of a species' ability to respond to a rapidly changing climate.

Seeds come in all shapes and sizes. They are they starting point
for dispersal into new habitats, and the maintenance of plant
populations for many species. Understanding how seeds germinate in response to
climate change will be critical for interpreting their ecological resilience.
(Photo: John Morgan)

The underlying assumption here - about climate change and the temperature sensitivity of seed germination - is, in part, a question about niche breadth.
Species with "narrow" niches are thought to germinate over a narrow range of temperatures and this means that these species are the ones most likely to find a changed world difficult to deal with, particularly in the absence of dispersal. 

By contrast, species with "broader" niches have germination that occurs over a greater range (i.e. the mean has a large standard deviation around it). Hence, if we identify species which possess narrow temperature germination ranges, we may be able to identify those species more susceptible to rapid environmental change.

Seems simple - develop a screening protocol, use a temperature gradient plate to quickly assess germination niche breadth for a large number of species, and identify species with narrow versus broad temperature germination sensitivity.

Niches are, by definition, where R>0 (i.e. births>deaths, regardless of performance). Where R=0, the edge of the niche has been reached, and where R<0, we are outside of the niche. Hence, when assessing seed germination versus temperature, the "optimum" temperature for germination is not actually the most important component of the niche but rather, what happens at the temperature extremes is much more important - identifying whether these species have temperatures where germination is zero defines niche space better than optimums. This is rarely done in most germination studies, partly because scientists are limited in the number of germination cabinets they can use to assess sensivity to temperature at any one time. In my case, I have access to about five growth cabinets. Hence, the value of the temperature gradient plate to assess a large range of temperatures simultaneously!

Temperature Gradient Plate - capable of 192 temperature combinations!

At the edges of the temperature range for germination is where niche expansion is most likely. New sink populations are much more likely to develop from source populations from individuals at the extremes of the niche space (as defined by temperatures) than from optimums. Hence, whilst we might find that species germinate less well at higher (or lower) temperatures than some optimum, the interpretation about temperature extremes for germination should take on more than just statistical significance to us; germination at these extremes of the temperature range is the raw material for future niche expansions. Most authors tend to ignore this and still focus on identifying optimum germination temperatures.

Two papers recently caught my eye and serve as good examples of how we might study fundamental germination processes to help understand potential responses to climate change. The paper by Ooi et al (2009)
Climate change and bet-hedging: interactions between increased soil temperatures and seed bank persistence is one of the few I have seen that explicitly tests whether warmer soils will have implications for germination and the persistence of soil seed banks. It's worth reading if you're interested in how plant population processes underpin responses to climate change. You should also check out the excellent paper by Cochrane et al. (2011) on germination in restricted rocky outcrop species from Western Australia as a good example of narrow versus broad germination strategies, and what this might mean for responding to a warmer world.