By Amy Grotta and Brad Withrow-Robinson, OSU Forestry & Natural Resources Extension

Our previous installment of this series introduced the key elements of forest diversity. Species diversity (a.k.a. compositional or biological diversity) is one of those elements.  So how do you achieve it, especially if what you are starting out with is a plantation forest with only a few species? The transition to an older forest with many plant species will not happen quickly. However, at every stage in a forest’s life cycle, there are opportunities and choices one can make to move the needle to a more species-rich forest.

The rest of this article and the accompanying infographic takes us through these choices, stage by stage.

Preparing to grow a new forest: have more than zero tolerance for shrubs

One of your best chances to grow a more diverse mature forest is on a just-logged site, even before you plant any new trees. This is (perhaps ironically) often one of the most diverse stages in forest development, when many sun-loving leafy plants show up.  They provide important food and cover for many birds and other animals. The trick is to balance this vegetation with the needs of growing tree seedlings.  You’ll still need to establish a stand of young trees successfully to meet Forest Practices rules. We’ll look at this stage again later in another article.

After logging, there are typically still native shrubs left behind. Sure, they may have taken a beating, but most will rebound if given the chance. So as you prepare the site for planting, whether with herbicides or machinery, you can intentionally save (choose to keep) some of these shrubs to meet your diversity objectives. Some specific strategies:

• Identify and mark the shrubs you want to preserve. Consider keeping them in scattered clumps and “sacrificing” the space they occupy, rather than planting trees there.
• Be selective in the species that you preserve, choosing those that won’t quickly outgrow planted trees. For example, Oregon-grape, cascara, and oceanspray grow slowly, while salmonberry and thimbleberry spread quickly. Bigleaf maple clumps are too competitive for most objectives/situations and should be controlled.
• Use spot sprays of herbicide instead of a broadcast spray.
• Evaluate what vegetation is growing on the site, and choose an herbicide strategy that targets the worst competitors while leaving desired vegetation. For example, you could select an herbicide that controls grasses, thistles and other non-woody weeds but is easier on shrubs. Consult product labels and the PNW Weed Management Handbook for guidance.
• Keep a close eye on things, and quickly take steps to correct a situation if things look like they are getting out of hand.

Establishing a new forest: species and spacing choices

Planting multiple species of trees may seem like an obvious strategy to increase species diversity. Yet it’s not as simple as mixing up the seedlings across your planting site if you want to keep that diversity long-term. Trees all have different growth habits and growth rates. When trees of different growth habits are closely mingled, the faster growing tree tends to win out.  Red alder and Douglas-fir are a good example (see illustration). Plant clumps of individual species rather than an intimate mixture to reduce competition, make subsequent tending of the stand easier, and keep diversity longer. If one of your motivations for having a diverse forest is to have a resilient forest, then keeping a mix of species in the canopy may be important.

Work with, not against, your site, and use microsites as a “palette” on which to paint your species choices. For example, plant wet areas to cedar, alder, or even ash.

The other planting decision you can use to enhance diversity is tree spacing. By planting trees further apart (say at 12-foot instead of 10-foot), you are giving more room and time for shrubs to re-establish. You can also resist the urge to come back in and replant spots where seedlings did not make it (as long as you’ve met Forest Practices Act reforestation requirements). Instead, allow hardwoods to fill in small gaps.

Tending a young forest: stay proactive

Your job of growing diversity is not done after planting.  Invasive weeds and crowding are two things to keep an eye on.  Some specific tips:

• Stay on top of invasive weeds before the canopy closes. Resist the temptation to leave blackberries or scotch broom to get shaded out once tree crowns touch. Unfortunately, shade is not a particularly selective form of weed control: by the time the weeds get shaded out, so will many of the understory plants you desire.
• Extend the life of your understory by thinning early and often.
• Thinning also is an opportunity to diversify the tree canopy, by choosing to leave behind “underrepresented” species. Thin around and release oaks, madrone, true firs, and any other minor species, as long as they are vigorous. (Don’t try to release suppressed trees that won’t respond to thinning.)

Harvest: leave a legacy

A mature forest can have many kinds of plants growing in the understory that you want to keep. When it comes time to do a timber sale, how can you carry that diversity into the next-generation forest? This is your opportunity to leave legacies.

• Retain some hardwood trees. Better to leave a single-stemmed bigleaf maple than to cut it and deal with the inevitable stump sprouts.
• Mark and protect clumps of native shrubs.

The bottom line with all of this is that growing a multi-species forest does not have to be a revolutionary form of forest management. As illustrated above, you can be opportunistic, make proactive decisions, and even small tweaks to your management practices to maintain and enhance species diversity according to your interests. This article provides some ideas, which you can tailor for your own forest. You might want to talk with a professional forester, including your local Extension agent, to design a site-specific strategy.

The next post in this series will look at structural diversity (the other major aspect of forest diversity), and ways to achieve it.