Disturbance can be good (PART 2)

A shorter entry this week as I’m catching up from a vacation I took last week to Washington State, which included hikes in Olympic National Park, kayaking around the San Juan Islands, and an amazing site visit to the Bullock Brothers Homestead, one of the oldest permaculture sites in the USA.

To pick up where the last post left off – disturbance – I mentioned wanting to expand more on the idea that we humans can design and induce disturbance in ecological design to propel a system to new levels of biodiversity, interconnection, etc.

A key element to this notion is that human created disturbance is NOT being proposed on the same scale or intensity we see in natural events. In fact since we are looking at disturbances in the context of systems we are managing for yields, we wouldn’t want to see change happen so drastically. The hundreds and thousands of downed trees up in the Adirondacks from the aftermath of hurricane Irene are going to be felt in the system for a long time to come. I’m not talking about this with our own interventions. 

So what do human-scale disturbances look like? A simple one is the creation of gaps or clearings in small forest stands, which stimulates understory growth, allows for the regeneration of sun loving species, and supports habit for a wide range of birds you won’t find in more mature stands.

A meaningful gap needs to be large enough to get sunlight to the floor – often a few trees won’t be worth the effort. I’ve read and seen the most effective gaps to be two to four times the height of the forest canopy. In other words, if you have an overstory that is 80 feet tall, a good gap would be somewhere between 160 and 320 feet wide.

Gaps could be circular, with a radius of the above figures, but it could also be a linear gap as well. I visited a forest in Delaware County, NY that demonstrated some interesting potential for small strip gaps in forests. The forest had been thinned in 1996 and all along the old logging roads now serving as trails were an amazing abundance of species that were shade tolerant but liked some light, including elderberries, currants, and many ribes species. I noted that the best growth was on the gaps that extended from East to West, giving the longest solar exposure during the warm summer months.

Nature plays out natural selection....slowly.

The size, shape, and location of a cleared gap depends on a lot of factors, including the size of the stand, the slope, soils, etc. One wouldn’t want to clear large areas around streams and in riparian corridors. Clearing nearby to genetically superior species that one wants to see seed regeneration from would increase the likelihood of success.

 Many folks might assume much of holistic forestry would be planting trees. While in some cases this may be the case, but a more effective (and realistic) approach is to create the conditions for regeneration, letting the forest determine the appropriate species composition based on good old natural selection. When I find a White Oak or a Tulip Popular around these parts I often see if a gap is feasible on the South side of the seed tree, to enhance to possibility for regeneration.

It IS appropriate to plant trees in cases where viable healthy specimens are scarce – such as large tracts of old farmland that don’t have excellent seed sources nearby. In this case we can employ another example of disturbance that mimics what nature does – overseeding.

Often when I come across tree planting guides, they emphasize that tress should be spaced to allow for maximum canopy development. So for example, Black Walnuts or Oaks should be give 50 to 60 feet of spacing, which they will eventually fill in.

The problem with this strategy is two fold. First is that likely a decent percentage will die off before maturity; especially when stock is grown from seed and the resulting genetics rather random in their growth characteristics. The other major problem is that a tree in an open field grows OUT as much as UP. I’ve seen several planted fields that, at a certain spacing, produce trees that are attractive but have little timber or wood value. Trees don’t grow without competition in the forest, and that struggle for sunlight is what makes the healthiest, strongest trees.

An important task we need to undertake as stewards of forests is collecting seed from local trees and growing it out, selecting out the inferior trees along the way. We could then re-grow a forest much like it would occur naturally – with thousands of trees per acre winnowing down over time to hundreds of mature specimens. Unless you are paying top dollar for high quality stock that someone else took the time to select out, the only way to do this is to get planting, observe, and make choices.

Close spacing makes these trees nice and straight.

During my trip to St Lawrence nursery Bill McKently showed me a Black Walnut plot he has started from seed he’d collected from some of the best trees he’d found on his property. Because of genetic diversity, he had not idea which of the seed trees would express the superior qualities of the parent and so planted them very close, at a ONE FOOT spacing. The stand was probably 20 years old when I saw it and the trees had gotten to be around 6 feet apart. This method not only allowed Bill to plant a lot of seed in a small space and do good genetic selection over decades, but the remaining trees were absolutely straight and gorgeous.

The only guaranteed way to get 100% superior trees growing is to graft species known to perform well onto rootstock. This method is one used by orchardists who want fruits that are true to the parent. While this method may work for those who have a direct commercial interest in a species, it becomes less practical on a large scale when our main goal is reforestation. In this case we might look for ways to set the system in motion, let nature play it out, and intervene as an agent of disturbance over a longer time scale.

So far we’ve discussed overall thinning, the creation of gaps, and overseeding as methods of human-induced disturbance.What are some examples you can think of?