Posted: January 8, 2021

Seed banks are usually thought of as cryptic pools of buried seeds waiting for their “moment in the sun.” The scientific consensus on forest seed banks reveals a disappointing picture of an underground soil desert inhabited by weedy plant species that nobody cares about. This perspective is almost certainly false, and recent research reveals why we may have gotten it wrong … there’s far more to seed banks than we ever suspected.

The Pennsylvania wild pink or Pennsylvania catchfly (Silene caroliniana) is a cousin of the world's longest-lived seed, the Silene stenophylla. Photo courtesy of National Geographic/PNAS, Fritzflohrreynolds.

The Pennsylvania wild pink or Pennsylvania catchfly (Silene caroliniana) is a cousin of the world's longest-lived seed, the Silene stenophylla. Photo courtesy of National Geographic/PNAS, Fritzflohrreynolds.

In the Origin of Species, Charles Darwin recounted the results of an experiment involving a coffee cup filled with a bit of mud and muck taken from a pond near his home: “I kept it covered up in my study for six months, pulling up and counting each plant as it grew; the plants were of many kinds and were altogether 537 in number; and yet the viscid mud was all contained in a breakfast cup!" Darwin's little at-home experiment was likely the first study of seed banks and it kicked off research from the Arctic tundra to Amazonian rainforests. It raised the question, why allow your progeny to lie dormant in the soil seed bank, and risk decay or permanent burial? Early research suggested that plant species evolved seed dormancy to “hedge their bets." That is, they save a few seeds for the future in hope of more light and nutrients once they do germinate. In other words, seed banks increase the total number of offspring that survive and help ensure the continued survival of the species.

What do these long-lived seeds mean for ecosystems around the world, and specifically for our Pennsylvania forests? Seed banks can create spectacular, ephemeral seas of wildflowers, like the desert's version of the northern lights, where plant species long absent reappear en masse from buried seeds when environmental conditions become favorable. In Chile's Atacama desert, the driest place on Earth, and in the American Southwest, El Niño winters bring intense rainfall causing superblooms that stretch from horizon to horizon.

Do Pennsylvania forest seed banks have reservoirs of plant species, like Darwin's pond or the Chilean desert, biding their time until the right environmental cue tells them to emerge? The answer was thought to be no. Study after study concluded that forest seed banks were impoverished, or worse, contained primarily weedy and exotic species. Most of our beloved forest wildflowers, like Trillium, and trees like oak and ash, rarely occur in soil seed banks. Seeds of these species decay rapidly or are the favorite food of squirrels, rodents, and deer. One thing we do know is that forest soil seed banks have few species in common with the forest community growing above them. This makes it hard to understand how forest seed banks can be a reservoir of diversity or play a central role in conservation or restoration, like they do in other biomes. However, recent field studies are clarifying exactly how fundamental seed banks are to forest recovery from disturbance.

We now know that Pennsylvania forest seed banks have their own unique way of contributing to biodiversity. In our own work at the Powdermill Nature Reserve in southwestern Pennsylvania, we discovered that the seed bank was instrumental for forest recovery after a tornado swept through the reserve. The storm blew over trees in multiple large patches. As the forest grew back, we found more than 230 plant species within the blowdown compared to less than 100 plant species in the adjacent intact forest. Where did more than 130 plant species come from? Not dispersed as seeds from outside the site, because very few of the plant species in the blowdowns grew in the nearby forest. Moreover, working at the very same site, Dr. Tomás Carlo and colleagues at Penn State found that wind and birds dispersed only a few species into the blowdowns. Rather, hundreds of native plant species almost certainly emerged from the seed bank. We don't know the seed banking ability of every plant that emerged, but at least 70% of the plants we found can seed bank or have a close cousin which does. In summer 2021, we will work with Dr. John Wenzel from Powdermill and Dr. Alex Royo from the US Forest Service to determine exactly how much credit we should give to seed bank for regenerating the forest. We look forward to sharing our findings with you!

For now, we believe that it is premature to count the seed bank out. In fact, forest seed banks may be central to conservation and restoration because they actually do contain hundreds of plant species native to temperate forests across the world. Recent work by the Swedish ecologist Dr. Jan Plue taught us that we failed to recognize the critical role played by forest seed banks because we just did not sample them enough. Why? Likely because it was too much work, or it cost too much. To sample seed banks, we extract cores of soil, haul them back to the greenhouse, and then wait months or sometimes years to identify the hundreds or thousands of seedlings that germinate. Each core can be loaded with seeds of a few weedy species, or in some cases, non-native species, making it harder to find the rare forest plants we might hope to preserve or restore. It's no wonder ecologists haven't sampled forest seed banks over big areas or with lots of samples! The seeds that readily germinate during seed bank studies are usually disturbance-lovers like fire cherry (Prunus pensylvanica) and blackberry (Rubus), rather than rare or threatened forest herbs and trees. Seed bank research often dismisses these common plants, which provide important ecosystem services like nutrient capture, increased soil moisture, and food for wildlife (and us!). As important as these plants are for soft mast and disturbance recovery, it's still worth taking a closer look to see what other forest plants are present but are relatively rare or patchily distributed. While the seed bank seems monotonous at first glance, taking only a few samples could mean we are missing the majority of the species in the soil seed bank. These native plant species emerging from the seed bank are vital to forest recovery following large wind disturbances like tornadoes and hurricanes which remove huge swaths of mature forests.

So how long can seeds live buried in the soil?  Clearly, to emerge in our forest after a 70 to 100 years of closed-canopy, they must live for decades at the least! In 1879, Professor William James Beal buried bottles of seeds of 21 plant species at a secret location somewhere on the grounds of Michigan State University. To this day, only one scientist, Dr. Frank Telewski, knows the location of this site, and every 20 years, he digs up a bottle in the dead of night to see which seeds will germinate. The final bottle is scheduled to be excavated in 2100. In the most recent excavation in 2000, the seeds of three of these species were still alive more than 120 years after Dr. Beal buried the bottles!

However, 120 years is not even close to the record for longest-lived seed! Metheuselah the Date Palm germinated from a 2,000-year-old seed found in the fortress of Masada in Israel. The record for the longest-viable embryo, though, goes to a different species. That distinction belongs to Silene stenophylla, a small wildflower from Siberia. The seed tissue, while unable to germinate without human help, was still alive after being buried in fossilized squirrel burrows in the permafrost for nearly 32,000 years. In fact, our own Pennsylvania wild pink or Pennsylvania catchfly (Silene caroliniana) is a cousin of this longest-lived seed!

Let's not underestimate the seeds of hundreds of our native plant species that can live for many decades, or maybe centuries, in the soil seed bank. They provide us with a window to the past of our forests, and insight into their futures. Some say that we cannot see the forest because of the trees; we might say, to truly see all of the forest you need to look below your feet.   

To learn more about the 32,000year-old Silene stenophylla, Metheuselah and Dr. Beal's seed burial experiment, check out these articles!

  1. https://www.nationalgeographic.com/news/2012/2/120221-oldest-seeds-regenerated-plants-science/
  2. https://www.atlasobscura.com/articles/the-worlds-longestrunning-experiment-is-buried-in-a-secret-spot-in-michigan

Written by:
Castilleja Fallon Olmsted, PhD Student, University of Pittsburgh
Dr. Michelle Spicer, Postdoctoral Scholar, University of 
Puget Sound
Dr. Walter Carson, Assistant Professor, Plant Community Ecology, University of Pittsburgh

Center for Private Forests

Address

416 Forest Resources Building
University Park, PA 16802

Center for Private Forests

Address

416 Forest Resources Building
University Park, PA 16802