Regenerating the Forest: Seeds and the Mysterious Seed Bank

From where does the next forest come? For many of us, our experience growing plants comes from working with gardens. Even without a green thumb, we've likely all, at some point in our lives, placed a seed in a cup of dirt to see if we could get something to grow. While seeds play a role in starting the next forest, either re-occupying a site, or spreading beyond their current boundaries, trees have multiple strategies to ensure their continuity through offspring.

Most, if not all, of our hardwood trees have reproductive processes, in addition to seeds, which allow them continue to exist on a site in the woods. One such strategy is sprouting. While the ability can decrease as the tree ages, dormant buds at the root collar (where the stem connects to the roots) can sprout in response to damage or disease. We see a lot of stump sprouts after a tree has been harvested, and many of the trees in the current forest have sprouts as their start - red maple is rather successful at this strategy. Other trees can send sprouts up from their root networks - beech brush is an example of root sprouts. Another strategy used by some species is the ability to "layer." As a branch curves down to the ground, it may become covered by leaves and duff, eventually enabling that branch to grow roots and create a separate clone that will survive if something happens to the parent tree - some spruces have this ability and a few non-native shrubs as well. And then we've got a few plants that negatively impact our forests which can grow new plants from root fragments or rhizome fragments (rhizomes are subterranean stems that can send out both new roots and new stems - trees don't have rhizomes, but problematic plants like Japanese knotweed do). An important consideration for all of these "vegetative" reproduction strategies is that the new tree is a clone of the parent. If the parent is lost due to insect or disease, the sprouts will have the same susceptibility.

Seeds are a tree's potential offspring from sexual reproduction strategies. If you'll remember from your high school biology classes, genes are given to the seed from each parent. These genes will express differently as they combine, leading to different physical characteristics in an individual. While some tree species can self-pollinate, wind and insects are the prime distributors of tree pollen to the female flowers that will result in seed production.

Just as trees have different strategies to pollinate their flowers - wind and pollinators - so too do they have different strategies to disseminate the seed. Wind is an important seed disperser for light seeds with wings (maples, ash, yellow poplar, pines, black birch); gravity also does its part to move seeds. Overland water flow is responsible for movement of lighter seeds. Small mammals move larger seeds as they cache food for winter survival; however, many squirrels and chipmunks tend to take bites out of seeds, limiting their ability to start to grow. Blue jays are winners in distributing oak seeds - they have been recorded moving acorns several hundred to thousands of yards from parent trees. As humans, we also play a role in seed distribution - think about those seeds that attach to your clothing, or land on your car to blow off at random times.

Different seeds have different triggers that prompt a seed to germinate. Light and moisture are key for most species. Some require a period of exposure to cold, over-wintering, before they start to grow. Others germinate within days of falling to the ground. Some prefer bare mineral soil; others need to be covered in the duff of the forest floor to grow. There are also those that can hang out buried in the forest floor for years or decades before they respond to perfect conditions to grow - these seeds are part of the seed bank.

The seed bank has an air of mystery around it. It's an easy assumption to make that there are seeds on site that will respond to a harvest or natural event. But not all the trees that we want to continue to exist on a site have that ability.

Yellow poplar has one of the longer-lived seeds in the seed bank at eight years. White ash and black cherry can bank for up to three years. The real winners in the seed bank, with seeds remaining viable for 50 to 100 years, are pin cherry (also called fire cherry) and black raspberry. Knowing the history of a site will help you understand what may come back after disturbance. Unfortunately, there are a few bad actors with the ability to bank their seed. Paulownia tomentosa (princesstree) seed can survive for two to three years in the forest soil. Japanese stiltgrass seed can remain viable for three years. If you're struggling with these non-native invasive species, you may have to keep fighting for multiple years to exhaust the seed bank - an important consideration for your invasives treatment planning.

Native tree reproductive strategies

• Beech has limited seed dispersal. Blue jays are an important distributor. Most seeds germinate the first spring after seedfall, after over-wintering. Beech is a prolific root sprouter and can also stump sprout.
• White ash wind-blown seeds are viable for three to four years, and require exposure to cold before germination. Younger trees can resprout from the root collar, but the ability declines with age.
• Red maple seeds are wind-dispersed. Seeds germinate soon after they fall, with 95% of viable seed germinating within the first ten days. Red maple can stump sprout.
• Black cherry seeds must overwinter. Animals contribute to the seed dispersal. Black cherry can delay germination for up to three years, lying dormant in the forest soil until conditions are prime.
• Northern red oak acorns must overwinter before they germinate. The acorns fall in autumn and germinate in spring. Blue jays and mammals spread the seed. Oaks in the red oak group can stump sprout prolifically.
• White oak acorns germinate soon after they fall. Many white oak acorns are damaged by insects or seed eaters. The blue jay is a primary disperser. Oaks in the white oak group can stump sprout, but not quite as prolifically as those in the red oak group.
• Yellow poplar seed is wind-dispersed. Seed can remain viable in the forest floor for up to eight years.
• Sugar maple seed is wind-dispersed. In nature, few persist as viable seed for more than one year. In the northern part of its range, sugar maple can sprout prolifically; it sprouts less vigorously in the southern part of its range.
• Chestnut oak acorns, as part of the white oak group, germinate soon after they fall. Mammals help spread the seed. Chestnut oak stump sprouts more frequently as compared to other oaks.
• Hemlock seeds are gravity- or wind-dispersed. The seeds germinate the first spring after falling. Seed is the hemlock's only method for regeneration.
• Black birch seed is wind-dispersed. Seeds germinate the spring after dispersal. It is a less prolific stump sprouter.
• Black gum seed is dispersed by gravity and birds. The seed germinates the spring after seed fall; it can also stump sprout.
• White pine seed is dispersed by wind and animals. Seed germinates the first year after seed fall.
• Hickory seeds are dispersed by mammals. The nut must overwinter before germination. Hickories can stump sprout but that ability declines with age.

Non-native tree and plant reproductive strategies

• Tree of heaven has a tremendous ability to root and stem sprout. In fact sprouting is its most common reproductive method. Any threat or injury will cause tree of heaven to sprout prolifically. Its wind-dispersed seed must overwinter before germinating, but it does not persist in the seed bank.
• Paulownia seed is wind-dispersed and can persist in the seed bank two to three years.
• Asiatic bittersweet has tremendous sprouting ability - sprouting from the roots, root fragments, and the root collar. Any form of damage encourages sprouting. Its seeds are spread by birds, but are short-lived in the seed bank.
• Japanese knotweed also has tremendous ability to sprout, sprouting from the stem, rhizome, or rhizome fragments.
• Japanese stiltgrass can seed bank up to three years. Gravity, water movement, and human or animal carriers spread its seed.

As you plan for the future of the woods, a knowledge of forest history and the tree species that live in your woods, and an understanding of their strategies to continue to exist, will be key to understanding the potential of your forest and will allow your woods to attain the goals you've set.

If you're interested in learning more about other tree species, the website "Fire Effects Information System" managed by the USDA Forest Service contributed much to the background for this news release. Click on "Life Form" and the blue button labeled "Go" to get to the species list.