Article reviewed: Using light to predict fuels-reduction and group selection effects on succession in Sierran mixed-conifer forests

By S. Bigelow, M. North, and C. Salk. Published in Canadian Journal of Forest Research, vol. 41 pp 2051-2063

The plot line: The authors in this study looked at areas in the northern Sierra Nevada mixed conifer forest that had been thinned lightly (only small trees harvested), thinned moderately (small and some medium sized trees harvested), and thinned heavily (what they call a group selection with reserved large trees… what I would call a heavy thin that leaves only large trees). They tried to measure how the different thinning treatments have favored certain species by measuring the difference in competitive status for light between seedlings following the harvests (i.e. they predict which species will win or lose following the harvests). They found that ponderosa pine (the most light-demanding species) tended to be more competitive (i.e. predicted to win) on average (but with LOTS of variability) when the harvest was heavy enough to create light availability greater than about 40% of full sun. In this case, 40% of full sun was equivalent to about 58% canopy closure (i.e. when you look up, about 58% of the sky would be covered by tree canopy). The heavy thins (group selection with large trees retained) created conditions favorable to ponderosa pine across most of their harvested areas, while the light and moderately thinned areas provided much less space with adequate light for ponderosa pine (and would therefore favor more shade tolerant species such as white fir). They attempt to reconcile the epic conundrum of light-thinning fuels reduction treatments versus heavier regeneration treatments that every manager must ponder.

Relevant quote: “… regeneration opportunities for shade-intolerant species are severely limited in thinned stands.”

Relevance to landowners and stakeholders:

Forests throughout the west are out of whack in many respects. Of primary importance is the current high density of trees and surface fuels that lead to high-severity fires. The authors of this paper describe these high-density forests in the management context of restoration. That is, there is currently a societal objective to restore overly-dense forests to a density that existed before forests got out of whack following decades of fire suppression. This is a seemingly sensible approach given the increases in high severity fires that we have observed over the past several decades. But many folks who are concerned about forests are currently shifting their objectives from a restoration focus to one of resilience. The primary difference between these two management contexts in my mind is the focus on the future (resilience) instead of the past (restoration). I see this shift in thinking as a positive one, especially given climate change and the uncertainty we have in forests’ capacity to adapt to future conditions while sustaining the resources that we currently depend upon. In other words, a critique of restoration objectives would ask the question “why should we restore past conditions that may be irrelevant today?”

While this study is described in the context of backward-looking restoration, it has relevance for forward-looking resilience in that it measures short-term forest change following the primary thinning treatment options that we typically consider: thin lightly, moderately, or heavily. And, importantly, it provides examples of how different species might respond to different thinning intensities. Thinning intensity is probably the most controversial part fuel-reduction harvests. The desire for some to harvest only smaller trees and others to harvest larger trees have led (illogically, I argue [here]) to diameter cut limits. The limits have led to “thou shalt not cut” rules that ban cutting trees above certain sizes especially on federal lands.

Which brings me to the relevance of this study for landowners and stakeholders. It is simply that there is a tradeoff between thinning intensity and the future proportion of ponderosa pine trees that will exist. We cannot have it both ways. We cannot conduct light thinnings and also expect ponderosa pine to regenerate naturally. What this study provides is information on the levels of thinning intensity that might be necessary for pine regeneration, and how canopy gaps may be created in order to provide spatially distinct “sweet spots” of pine regeneration. At this point, however, there are considerable limitations because of the low precision of this information, as discussed below.

Relevance to managers:

The managers’ challenge is to reconcile this tradeoff between the desire of many to limit thinnings to small trees and the desire of others (usually fewer) to regenerate shade intolerant species by thinning more heavily or by creating sufficiently large canopy openings. This tradeoff will never be solved in a way that is pleasing to everyone, but there are some tools at hand for regenerating ponderosa pine in the context of treating forests to reduce fire hazard:

• Thin to what many would consider a low density (58% canopy closure is the density suggested in this study, although a lower density may be necessary for actual recruitment of pine as opposed to just early growth).
• Create canopy gaps and include the harvest of all or most large trees. Studies so far suggest that these gaps need only be ~ 0.3 acres if all trees are removed, and ~0.7 acres if maximum growth of ponderosa pine is desirable.
• Plant ponderosa pine seedlings following heavy thinnings or in those areas where the thinning was most heavy (planting seedlings skips the establishment phase, when most seedling mortality occurs).
• Plant landings that were created during thinning operations. This may only make sense if the plan is to follow up thinning treatments with prescribed fires over the long-term (if future mechanical thinnings are planned, then this would not make sense since the landings would be used again).
• Cut sub-merchantable trees surrounding gaps of pine regeneration to provide more light and water.
• Conduct vegetation control treatments in canopy gaps where ponderosa pine has been planted. Despite the dismissal of competition for water as an important growth limitation by the authors of this study, there are plenty of studies that show the importance of shrub competition in influencing seedling survival and growth.
• If leaving a high density of large trees is necessary because of legal requirements, illogical as that may be, prune up the large trees to provide more light and water. This is something that sounds crazy and is presently not feasible, but it is an engineering problem that someone out there might be able to solve someday.

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

The title implies, by using the term “succession,” that they projected forest change following these treatments. They did not, however, predict change but instead measured two-year growth and the current competitive position of trees. There are implications one can draw from this for how species might change in the short-term, but it falls short of predicting what most would call “succession.”

At this point, I consider the use of CPI (Crossover-Point Irradiance) to be of limited value in the dry forests of the Sierra Nevada. It relies on the assumption that light is the primary limitation of growth. I was confused about the authors’ citation of another study (Royce and Barbour 2001) to suggest that light is indeed the primary limiting resource in the Sierras, when I use the same exact citation as an example for why water is often just as important or more important as a limiting resource. The limited value of CPI was further demonstrated by the very high variability in the relationship found between growth and light. For the most part, there was little evidence that the relationships tested (i.e. the “candidate models”) were any better than no relationship at all. This was hard to confirm, however, because the AIC weights were not given (this provides the relative performance of the different models considered). Given the very low r2 values, I would expect fairly low AIC weights (also called “evidence ratios”). Ponderosa pine was probably the only species where this CPI could have been considered to have “worked.” The CPI did result, however, in 40% light availability as the threshold for ponderosa pine regeneration, which does indeed match with what others have previously suggested is needed for pine regeneration (as noted above, however, there is HUGE variability in this “threshold” which also suggests the importance of other co-limiting resources of water and nutrients- not just light).

The authors suggest that smaller canopy gaps than the 1.0 hectare sizes used in this study could be adequate for ponderosa pine regeneration. Smaller gaps would avoid some of the perceived problems (both social and ecological) that exist with gaps this large. They cite a study that found good ponderosa pine survival and growth in smaller gaps. They did not mention, however, that the gaps in that study did not retain any large trees. So in terms of light availability, those smaller gaps without large trees retained may have had more resource availability than the large gaps with large trees retained. We therefore don’t have the empirical evidence to suggest that small gaps can regenerate ponderosa pine if large trees are retained (although I think it is likely that they very well can if large tree density is low enough).