- Author: Rob York
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 (~1/10th 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.
York et al. 2011 Forestry
- Posted By: Rob York
- Written by: Zev Balsen, PhD candidate in the Stephens lab at UC Berkeley
Article reviewed: Post-1935 changes in forest vegetation of Grand canyon National Park, Arizona, USA: Part 1 – ponderosa pine forest
By Vankat, J.L. 2011. Post-1935 changes in forest vegetation of Grand canyon National Prak, Arizona, USA: Part 1 – ponderosa pine forest. Published in Forest Ecology and Management, Vol 262 pp 309-235.
The plot line: In 1935 the National Park Service sampled hundreds of vegetation plots in Grand Canyon National Park (GCNP). In 2004, the author of this paper relocated and remeasured many of these plots. Ninety-nine of the plots were in ponderosa pine forest (PPF); these PPF plots were analyzed in this article. The author looked for changes in tree density and basal area between 1935 and 2004. Comparisons were made based on size class, species composition, and PPF subtype (dry, mesic, and moist).
Because dozens of comparisons were made, there was a battery of findings, with some species and forest types showing significant increases, some decreases, and others showing no change. Perhaps the most surprising finding was that when all PPF forest types were analyzed together, there was no change in tree density and basal area actually decreased. These results were accompanied by changes in species composition: The number of quaking aspen decreased by over 80% and the number of white fir increased by a shocking 631%.
When the three PPF subtypes were analyzed separately, a different set of changes were seen. Dry PPF forest, which is found at lower elevations and is transitional with pinyon-juniper vegetation, showed no statistically significant changes. Mesic plots, on the other hand, showed a large decrease in basal area, mostly due to loss of ponderosa pine in the medium size class. Despite this decrease in basal area, tree density did not decrease. This can be explained by a significant increase in the density of small trees, including a staggering 2351% increase in small white fir. Finally, moist PPF showed a large decrease in total tree density, mostly due to loss of quaking aspen. In these moist forests, quaking aspen went from being the most common tree in 1935 to being third most common after ponderosa pine and white fir in 2004.
In the second part of this article, the author attempted to place his findings in the context of other studies on historical forest structure in GCNP. Estimates of tree density and basal area at different dates were plotted and trend lines were fit to the graphs. The results in this section of the paper were highly variable. They suggested an increase since the 19th century in total density of saplings and trees. However, for just trees--excluding saplings--the graphs showed no clear trend.
Relevant quote: “Previous studies...in the Southwest indicated that historical dynamics...have involved increases in forest densities... However, this study -- the first to examine multi-decadal changes across a never-harvested Southwestern PPF landscape using resampled historical plots -- documents that changes...also have included decreases since 1935."
Relevance to landowners and stakeholders:
Historical forest information is crucial to our understanding of current forest conditions, management goals, and appropriate management techniques. However, previous studies in GCNP (and elsewhere) have relied on estimates from extrapolated data, reconstructions based on tree rings, and sampling of contemporary reference sites thought to represent historical conditions. These methods all have their own particular pitfalls that may introduce large uncertainties about historical conditions. For example, extrapolation of data depends on assumptions about rates of change. Thus it may be dangerously circular to use these studies to form conclusions about how forests have changed during the 20th century. Tree ring studies may have inaccuracies because of loss of evidence from older trees. Reference sites present problems because the current and recent impacts of climate, fire, and diseases can never exactly duplicate historical conditions. Because the paper discussed here relied on direct measurement of contemporary and historical plots, landowners may feel more confident about its findings.
The lack of increase in density in any of the PPF types or in overall PPF was surprising. One explanation is that forest densities had already increased by 1935. Another is that increases in small trees were offset by losses of larger trees. The larger trees may have suffered from competition with the smaller, vigorous trees. In either case, this paper highlights the insight that forest changes are not monotonic: even in the absence of fire or other disturbance, forests density may increase, decrease, or remain constant. Different forest types and subtypes may change at different rates and in different directions.
Relevance to managers:
This paper highlights some of the uncertainties about past PPF structure. Yet there are even greater uncertainties regarding the future. Climate, invasive species, fire, and societal pressure will have unpredictable, synergistic effects on forests. The author of this paper suggests that, in light of the uncertainty about both past and future, managers need to shift their focus away from emulating the "historical range of variation" (HRV) and instead think in terms of avoiding "thresholds of potential concern" (TPC). For example, future climate conditions may make it impossible to achieve the HRV in certain places. The TPC approach provides a much broader range of acceptable conditions.
Critique (I always have one, no matter how good the article is):
The first section of the paper describing the remeasured plots was well done and full of novel findings. The second section, in which the remeasurements were combined with other data on historical forest conditions, was weaker and detracted from the robust findings in the first section. The regressions in the second section were presented without any significance values. Presumably that is because each fit was based on only a few scattered points and they would not have been statistically significant. If that is the case, these graphs would have been better presented as eyeballed trends, without the R2 values and exact regression parameters.
Finally, the unique management history of GCNP suggests that this paper's findings might not translate well to areas outside of the park. Since the study site has never been harvested and has experienced some prescribed fire since the 1980s, its developmental trajectory might be quite different from forests in which timber has been cut and all fire has been suppressed.