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.

Sunday, 2 October 2011

Colder plants in a warmer world?

I'm keen on cross-country skiing. I guess it's an extension of one of my other great loves: bushwalking. I can think of almost nothing better than strapping on some skis, packing the tent and shuffling out across the high plains of the Australian alps to camp in the snow on some distant peak.

I particlarly love spring skiing - it's warm (so I get to ski in shorts!), the days are long, the snow is hard in the morning - which makes for some fast downhill runs, and there usually aren't as many people around. I was hoping to get one final ski in this season, but like many years (perhaps seven out of the last ten), it's been another disappointing year for snow in Australia. That doesn't stop the ski resorts here from charging top dollar, but that's another story.......

The SnowCams at Mt Buller don't lie! Even at one of Victoria's highest
peaks (>1800m), there ain't much white stuff and hasn't been all winter

What we are seeing now is probably a window into the future.

About a decade ago, Kevin Hennessey and collegues at the CSIRO modelled the potential distribution of snowcover at 2050 and showed that,  under 'high warming' scenarios, very few mountains in Australia would sustain snowcover for >90 days. I think we won't have to wait till 2050 for that to become a reality. This will likely have lots of consequences in the Australian Alps such as (a) there being less water released slowly in the spring thaw (with resultant greater stream surges with potential for downslope flooding) and (b) lower albido.

One of the biological consequences that hasn't really got much attention yet is how less snowcover will affect those plants (and animals) that rely on snow for protection from the cold. Generally speaking, snow is a magnificent insulator against the cold. In the sub-nivean space (the interface between snow and vegetation), it rarely drops below zero degrees. Hence, plants there are not exposed to extreme cold (frost) and can even continue to grow over winter in some cases. But with less snow, there will be less of an insulating blanket, and this will invariably melt earlier each year, exposing plants to extremes of cold they perhaps are incapable of resisting. Might alpine plants in a warmer world actually be more susceptible to the cold?

Susanna Venn, Janice Lord and I have been investigating the frost sensitivity of alpine plants with this question in mind. We've been assessing the early spring frost tolerance of a range of alpine plants to determine just how they might react to earlier snowmelt.

For those (mostly woody) species that occur on wind-exposed ridges where the snow rarely settles, it is clear they are very cold tolerant early in the season. This is because they are rarely insulated from winter cold. The Alpine Star Bush (Asterolasia trymaloides), for instance, has an LD50 of -17.6 deg C in the weeks after the spring thaw. Lethal temperatures were considered those at which 50% damage occurred to the photosynthetic apparatus in leaf samples. Pimelea axiflora is even more cold tolerant - it's LD50 is -18.7 deg C.

Brachyscome nivalis - less cold tolerant than many alpine shrubs
(Phto: John Morgan)
By contrast, species (mostly herbs) that are normally deep under snow for the entire winter and hence protected from the extremes of cold, and which don't melt out till mid-summer, exhibit much less cold tolerance early in the season. Brachyscome nivalis, for instance, has an LD50 of just -7.0 deg C when it was removed from snow early in the spring. This hints that it might be sensitive to early melting if that meltout coincides with very cold spring temperatures. To avoid frost damage, the species would likely need to remain in areas of late melting snow (as it currently does), areas which are predicted to decline in extent over the coming decades.

So, while there will be opportunities for extended growing seasons in the high mountains of Australia, it is likely that frost sensitivity may constrain some species. This needs to be ascertained for a far larger number of species than we have examined so far.

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