Mind the gap: growing pine species in selectively harvested mixed conifer forests

Article reviewed: A gap-based approach for regenerating pine species and reducing surface fuels in multi-aged mixed conifer stands in the Sierra Nevada, California

By R. York, J. Battles, R. Wenk, and D. Saah, published in the journal, Forestry. Available for free download and attached below.

The plot line: Selectively harvesting trees from forests is one of the techniques that can be used to manage forests for objectives such as wildlife habitat, species composition, and timber revenue. Selective harvesting methods have many appeals, one of which is the romantic idea that they are “more natural.” Yet these methods also have their downsides, including the potential for high fire hazard and the inability for some tree species (especially pine) to survive and grow under such methods. This study tested a specialized approach of selective harvesting that aimed to encourage regeneration of pine species, while also reducing high severity wildfire hazard. The approach was to create small gaps (1/10 acre) and to then burn the resulting logging slash within the gaps. The burning of the slash had two benefits: it created an ash seedbed that led to slightly higher germination of pine seeds, and also got rid of some surface fuels which lowered fire hazard. The gap creation also led to patches in the forest where enough light penetrated the canopy so that pine species could likely survive. The authors suggest that forest management objectives can be met with selective harvesting (i.e. multi-aged management in Silviculture-speak) by altering treatment methods in ways that specifically address management objectives.

Relevant quote: “Many of the negative outcomes traditionally associated with multi-aged stands can be moderated or resolved by designing harvests and post-harvest treatments to specifically meet modern objectives.”

Relevance to landowners and stakeholders:

The benefits of selective harvesting (in this case referring to harvesting periodically to create multiple aged- and sized-trees in a forest) that folks often hear about include the following:

• Lower regeneration costs (i.e. you don’t have to plant)
• Continuous canopy coverage provides continuous soil protection
• Timber income can be spread out over time for small landowners
• There might be local regulations that make it the only option available

But there are also drawbacks that folks hear about as well:

• Damage to residual trees from harvesting
• Incompatibility with shade intolerant species (such as ponderosa pine in western forests)
• High fire hazards from perpetual surface or ladder fuels

These authors did a small study that experimentally explored how some of these drawbacks might be addressed, while still maintaining the essence of a multi-aged forest. They found that one may be able to regenerate ponderosa pine if gaps are created that are large enough (~1/10^th acre in high productivity forests). They also found that an ash substrate might lead to very small increases in ponderosa pine germination, but that the ash substrate did not make any difference for sugar pine germination. They also suggested two more treatments that were not tested, but could be used. These were planting seedlings in harvest-created gaps and using whole tree harvesting (having equipment harvest entire trees, rather than cutting trees and leaving their branches and tops in the forest).

Relevance to managers:

As was discussed in the most recent post, a relevant relationship in this case is that pine regeneration is inversely related to disturbance severity. If more large trees are harvested, then more shade intolerant trees such as ponderosa pine will regenerate as long as there is a seed source or there is planting. But this does not mean that clearcuts are the only viable way to grow ponderosa pine. This study suggests that selective harvests can indeed regenerate ponderosa pine as long as they harvest enough trees to create distinct canopy gaps that allow enough sunlight to reach the forest floor. Besides managing canopy density, many other treatments that managers are familiar with can be used to encourage pine regeneration and recruitment. Planting, control of competing vegetation, and thinning are all treatments commonly associated with even-aged management, but can also be used in uneven-aged stands. They will come at a cost, however, since efficiency is certainly lost when treating lots of small gaps as opposed to one large one. For example, I tend to see an efficiency loss of about 15% when I compare productivity in clearcuts versus small gaps.

Critique (I always have one, no matter how good the article is):

The researchers did not actually measure ponderosa pine seedling growth or survival, but instead measured light availability and predicted that there was enough light for ponderosa pine. A more thorough study design would have planted pine seedlings within the gaps and then tracked them over time.

The increased pine germination in ash substrates was very small. I am not sure that having an ash substrate (i.e. burning) versus a bare soil substrate would actually result in significantly more pine seedlings. It is interesting to consider this ash-germination relationship from an evolutionary perspective. It is likely that, prior to Euroamerican settlement, ponderosa pine colonized and established canopy gaps that were created by fires. So it would make sense if they had some adaptation toward germinating on ash substrates, where resource availability is in general higher because of the recent fire.