Rainshadow plant communities: relictual or refugial?

Will dry-climate species spread from their current rainshadow niches, where they are a botanist’s curiosity item, to recolonise the surrounding, warming landscape? Are they waiting in these refuges for the opportunity, or merely relics on their way out?

Melbourne is forecast to get the climate of Dubbo this century if climate change continues without mitigation. That’s a 600km shift in climate zones, over 5 degrees of latitude, in less than a century. Many native species only spread their seed a couple of times the height of the tree dropping the seed, so they won’t keep up without assistance.

What’s the Dubbo area like? Based on my limited trips through the area, Dubbo is on a border between uplands and plains. Grassy box-ironbark woodlands are common on the slopes to the east, not unlike woodlands around Melton and Bacchus Marsh. Sandy rises and rocky hills standing out from the plains often have native cypress-pines, Callitris species, local populations of which I’ve blogged on already.

Grassy woodland of cypress and Eucalypts, on the plains southwest of Dubbo

There’s a number of semi-arid zone species with populations outside their main range, tucked into dryer topography (escarpments) and unusual climates (rainshadows). Even if we cut fossil fuel use drastically right now, the climate has warmed already, and will continue to do so for a some decades due to thermal “lag” in the global climate system. Will these dry-climate species spread from their current niche as a botanist’s curiosity item, and recolonise the surrounding, warming landscape? Are they just waiting in refuges for the opportunity, or merely relics on their way out?

There are many caveats to any answer to that question. The local rainshadow population may be a different genotype to the main population further north, and may not like an increasingly hot climate, any more than our southern native species. It is thought these outlier populations were left stranded here after the last ice age – when the climate was much drier, but cooler not warmer. If temperature, rather than water stress, is a key limiting factor for these species, then warmer climates may not help. That seems an unlikely proposition, though, for species commonly found in western NSW.

Second, tiny local populations of plants like Callitris, or Buloke (Allocasuarina luehmannii) may be on their way out due to inbreeding from having such a small population anyway – perhaps enhanced by our clearing of large parts of their populations over the last 180 years in Victoria. Even if they are not inbred, the fragmentation of the landscape may prevent their spread.

Some ecologists are asking whether we should start planting species from drier, hotter regions, rather than the historic baseline of what used to grow in an area, to adapt to the changing climate. Perhaps we don’t need to look far to the north for novel species. We have a lot of disjunct populations of appropriate species, already sitting in little refugia like the rainshadow areas. Maybe we should start planting them more widely.

A caveat is that these disjunct populations of northern, dry-country species may not be enough to reconstruct a stable, species-rich ecosystem over a larger area. Many cool-climate plants and the animals that depend on them may play vital roles in local ecosystems and may be irreplaceable if they become locally extinct. Climate change will be chaotic, upturning seasonal rainfall patterns, fire and storm frequencies, and more. Avoiding disastrous climate change is greatly preferable to accepting it! But as much as we fight for serious action to stop climate change, we can’t avoid dealing with some of the consequences which are already becoming apparent.

Goobang Ranges National Park, just south of Dubbo

Looking at this rainshadow area, there are species that live in northern, drier and hotter climates in a few categories. Some are simply generalists and live just about everywhere: Kangaroo grass Themeda triandra, for example. Others are locally rare and quite unusual in the region. The full range of some species includes large parts of Australia’s arid interior; for others, they extend no further than NSW western slopes, or Victoria’s “desert” mallee and heathland areas. Some are not only inland but also coastal, perhaps indicating a preferance for well drained soils as much as climatic factors.

Following is a partial list. These species are identified from distribution records in the online Atlas of Living Australia as being present around Bacchus Marsh, and from personal observation. Some of these may already be spreading, due to either planting, or natural vectors: Fragrant Saltbush, Rhagodia parabolica, has been suggested as an example. Is that a good or a bad thing? I don’t know, but it is worth thinking about.

Trees

Buloke Allocasuarina luehmannii
Cypress-pine Callitris columellaris/glaucophylla
White box Eucalyptus albens
Bull Mallee Eucalyptus behriana
Grey Box Eucalyptus microcarpa
Moonah Melaleuca lanceolata (also coastal)
Waterbush Myoporum montanum

Shrubs

Mallee Wattle Acacia montana
Turpentine Beyeria lechenaultii (also coastal)
Desert Goosefoot Chenopodium desertorum
Turkey-bush Eremophila deserti
Blue-bush Maireana (5 species)
Native hollyhock Malva weinmanniana
Fragrant Saltbush Rhagodia parabolica
Roly-poly Sclerolaena (2 species)
Desert Cassia Senna artemisioides
Heath Spyridium Spyridium eriocephalum

Herbs and forbs

Bindweed Convolvulus erubescens
Parakeelya Calandrinia eremaea
Austral tobacco Nicotiana suaveolens
Feather-heads Ptilotus macrocephalus

Grasses

Windmill Grass Chloris truncata
Silky Blue-grass Dichanthium sericeum
Irongrass Lomandra effusa
Native Millet Walwhalleya proluta