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.

Recent improvements to native grassland conservation

Native grasslands are one of the most endangered ecosystems in Australia. More than a century of agricultural use, coupled with increasing urbanization, has reduced these once abundant ecosystems on fertile soils to distinct rarities. In some areas, much less than 1% of the original system persists.


But this isn't the case everywhere across the range of native grasslands in southern Australia. In the more xeric areas (300-400 mm rainfall), native grasslands are still a feature of the landscape, e.g. the Riverine Plains grasslands of northern Victoria. In part, they survive because agricultural use has been of lower intensity, i.e. sheep grazing has been rather conservative and cropping largely unsuccessful. Additionally, perennial exotic grasses are absent (it's too dry) meaning that the integrity (if not composition) of the native grasslands remains largely intact.


For many years, however, the extent of xeric grasslands in southern Australia was overlooked. The conservation imperative first focused on the mesic C4 grasslands on volcanic soils near Melbourne. Xeric grasslands protection was so poor that  by the early 1990s, none of the ecosystem was under conservation management.


Thankfully, this has changed - reservation has increased the area of xeric native grasslands in the National Reserve System from zero hectares in 1995 to an estate now in excess of 10 000 ha. This is a remarkable achievement. This increase has been driven, to a large extent, by government land purchase coupled with private conservation agreements. I, for one, have much greater hope that grasslands in these areas can now be conserved in a meaningful way. The challenge now will be to manage them for their biodiversity and ecosystem processes in the face of a changing climate. Understanding the factors that affect the resilience of these systems to change is a pressing research need.


I recently undertook a tour of some of the new conservation reserves protecting grasslands in northern Victoria and I thought I'd share some of these sites with you. While it's mid-winter here (and not the best time to see the diversity and colour of grasslands), I was impressed by the scale of grassland protection being achieved.

Chenopod grassland at Boundary Bend, north of Nyah.
Rainfall here is about 320 mm per annum. The grasslands
are dominated by widely-spaced C4  grasses (Sporobolus is common)
as well as scattered chenopod shrubs. Soil crusts - dominated by crustose and foliose 
lichens - are well developed. Black Box can be seen in the background.

Wanderer's Plain, west of Kerang. This is a large grassland (2000 ha)
acquired and managed by the Trust for Nature. Annual rainfall is
approx. 375 mm. Dominant grasses are C4 type, including Sporobolus and
Enteropogon. The grassland plains are often fringed by Buloke and Black Box.

Korrak Korrak grassland, west of Kerang. Another Trust for Nature
grassland, in the 350-375 mm rainfall zone. C4 grasses co-dominate
with C3 grasses (Austrostipa), along with scattered chenopods.
Exotic species are non-existent while native annual forbs are ubiquitous.

In some areas, it is obvious that overgrazing in the past has
led to dramatic soil loss. Large, bare scalds result and there is almost
no recolonization by native species from the surrounding grassland.

Travelling Stock Routes, like this one on the Cobb Hwy near Echuca,
have had minimal disturbance from grazing relative to grazed paddocks. They support many
endangered species that have been grazed out of the broader landscape. Their linear nature, 
however, makes them vulnerable to edge effects

Fabians grassland at Terrick Terrick National Park, an example
of C3 grassland with an intertussock flora dominated by lillies, orchids and
daisies. Rainfall is approx. 400 mm per annum.H ere, the first disturbance
manipulation experiments were conducted in the mid-1990s (the Foreman Plots)
to determine how xeric grasslands respond to fire, grazing exclusion, and soil disturbance.