If you could choose a Super Power what would you choose? Incredible speed, rubber arms, hulking strength or maybe X-ray vision?
If I had the chance, I’d want to produce seeds. And not just any seeds. I’d want seeds that persist in the soil for ages. As every gardener knows, that’s the way to world domination. And if I could make a wish for our endangered grassland plants, I’d wish that they too could form persistent seed banks. If they did, then we wouldn’t have to worry so much about them.
The awesome power of Super Seeds was demonstrated in a recent study that got lots of media attention across the world. A group of Russian ecologists managed to grow plants from seeds that were over 30,000 years old. Now that’s persistence for you. The original plants flowered beneath mammoths and woolly rhinoceroses, and their cultivated offspring flowered in a temperature-controlled glasshouse. That’s quite a generation gap.
The seeds belonged to a small perennial herb, Silene stenophylla, which grows in the harsh Siberian tundra. We have a few exotic Silene species in Australia, including the widespread Silene gallica (French Catchfly), but no native species.
The super Silene seeds were buried by arctic squirrels, and were then preserved in the frozen permafrost for 30,000 years until the researchers dug them up. It wrecks the story a wee bit, but none of the seeds actually germinated and grew in the laboratory. Instead the researchers used tissue culture techniques to grow plants from frozen immature fruits. Nevertheless, it’s amazing that any tissues remained ‘alive’ for this long.
Compared to these Super Seeds, most seeds have the powers of ordinary mortals. They germinate, get eaten, or die from various causes, and it’s all over pretty quickly. But seeds of different species do behave in different ways, so ecologists categorise plants based on how long their seeds last in the soil. Species with transient seed banks don’t last long in the soil; most seeds germinate within a year. Persistent seed banks remain viable in the soil for longer. Short-term persistent seeds remain viable for 3-5 years, and long-term persistent seeds can stay viable for decades or more.
One of the reasons why our native grasslands and grassy woodlands have suffered such big declines is that few native species have long-term, persistent seed banks.
It’s interesting to reflect back on the big ideas that fundamentally changed how we think about, and how we manage, remnant native grasslands in southern Australia. I reckon there have been three big, game-changing ideas in the last 40 years. You might think of some more perhaps.
The first game-changer was the recognition that frequent fires maintain plant diversity in small, burnt remnants. John Stuwe, Bob Parsons and Neville Scarlett pioneered this back in the 1970s. We take this for granted nowadays, but it wasn’t widely accepted initially. Fires were generally seen as being ‘bad’, and many experienced naturalists decried the suggestion that frequent fires could be ‘good’ for grasslands. We view the world very differently to people 50 years ago.
The most recent game-changer was the recognition that changes to nutrient levels are equally, if not more, important than fires and other disturbances. Suzanne Prober, Josh Dorrough and others showed that high nutrient levels (especially nitrate and phosphorus) promote annual exotics that control nutrient cycles, out-compete natives and prevent restoration, regardless of disturbance regimes. Again, this seems sensible in retrospect, but before it became the norm, ‘disturbance management’ was really the only idea. Now we have to think about how disturbances affect plants and nutrients, and how both processes (acting together) influence plant composition. (You can read more about nutrients and weeds in this blog).
The third game-changer arrived in the 1990s, when John Morgan showed that most native plants in Themeda-dominated grasslands in southern Victoria had transient or short-term seed banks. Many native plants, including showy lilies and daisies, produce relatively few seeds, and these seeds germinate quickly or die. Consequently, grassland seed banks are often dominated by exotic annual plants, even in high quality remnants.
When I first worked in grasslands in the early 1980s, the general idea (perhaps it was just a ‘hope’) was that grasslands would function like heathlands. After a fire, existing plants would resprout from buds, and lots more new plants would emerge from buried seeds. We’d see new native species that we hadn’t seen before, and species that were rare before the fire would become much more abundant. Unfortunately, this didn’t happen. Few native plants produced lots of new seedlings, simply because there weren’t many seeds in the soil.
The 1990s seed bank studies demonstrated that soil seed banks can’t help us to restore degraded grasslands. They also showed us how important it is to manage the vegetation well, so that existing plants grow vigorously, flower abundantly and drop lots of seeds, from which new plants can grow.
The seed bank caveat
The seed bank game-changer came with one big caveat. The work was done in Themeda grasslands in high rainfall regions. So we never knew how well the findings would apply to grasslands in drier areas. Do native plants form large, persistent seed banks in Austrodanthonia (Wallaby-grass) and Austrostipa (Spear-grass) grasslands in northern Victoria or the NSW Riverina, for example? There’s a good reason why they might.
Most native plants in Themeda grasslands are perennials, and Themeda grasslands contain few native annuals. By contrast, Austrodanthonia grasslands contain many native annuals, such as the Sunray daisies, Hyalosperma glutinosum and Rhodanthe corymbiflora. Like many short-lived organisms, annual plants usually make lots of seeds, many of which persist for more than a year. This provides a good insurance strategy to prevent populations from going extinct.
So, we might expect to find more persistent seed banks in dry Austrodanthonia grasslands than in Themeda grasslands. At last, 20 years after the Themeda game changer studies, we finally have some data from the northern plains.
In a fantastic new paper, Andrew Scott and John Morgan provide the first detailed look at grassland seed banks in northern Victoria. Overall, their results are similar to those from Themeda grasslands. Few native species formed large, persistent seed banks. Indeed, the species that did form large persistent seed banks were rarely or never seen in the vegetation, such as small sedges (Isolepis) and rushes (Juncus species). These species are notorious for forming huge seed banks in many ecosystems across the world.
As expected, native annuals like the Sunray Hyalosperma glutinosum did carry some seed over across years, but most seeds germinated (or disappeared for other reasons) within each year. So, suprisingly, even the native annuals don’t form large persistent seed banks. And, as has been found in other regions, many native species weren’t found in the seed bank at all. These included species that were common in the vegetation, like the lilies Athropodium minus, Hypoxis glabella and Wurmbea dioica, and the bluebushes, Maireana species.
This great paper contains far more information than I’ve described here, so grassland aficionados should definitely download it and pore through the results.
This new study extends the seed bank game-changer from the western plains to the northern plains. Once again, if we want to restore degraded grasslands, then we can’t rely on long-lived seed banks to give us much help. We’ll need to re-introduce seeds if we want to rapidly improve the quality of grasslands that have been degraded in the past.
All of this is a far cry from the Super Silene Seeds.
It’s a pity that we don’t want to create novel ecosystems dominated by Isolepis, Juncus, Cyperus and other rushes and sedges. That’d be a breeze, as these species form huge, persistent ‘super seed banks’. But most of our grassland wildflowers are mere mortals. Their seeds don’t last long, so populations rely on good grassland management for their persistence. When the plants disappear, the seeds soon follow.
In the absence of Super Seeds, our grasslands need super management.
The Microseris and Arthropodium photos are from two fantastic collections of photos of Victorian Flora by Colleen Miller, which are well worth viewing (follow links to web site 1 and 2) . I’m grateful to Colleen for allowing me to use the photos in this blog. The seed photo in the header image is from wikipedia.
Morgan JW (1998) Composition and seasonal flux of the soil seed bank of species-rich Themeda triandra grasslands in relation to burning history. Journal of Vegetation Science 9(2), 145-156.
Scott AJ & Morgan JW (2012) Resilience, persistence and relationship to standing vegetation in soil seed banks of semi-arid Australian old fields. Applied Vegetation Science 15(1), 48-61.
Yashina S, Gubin S, Maksimovich S, Yashina A, Gakhova E & Gilichinsky D (2012) Regeneration of whole fertile plants from 30,000-y-old fruit tissue buried in Siberian permafrost. Proceedings of the National Academy of Sciences.