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.

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).

Article reviewed: A Large and Persistent Carbon Sink in the World’s Forests

By Y. Pan and many others. 2011. Published in Science. Vol. 333 pp. 988-983.

The plot line: This team of scientists gathered all of the data they could find from across the world to quantitatively describe the roll that forests have had in influencing the global carbon balance over the past two decades. They distinguished between temperate, tropical, and boreal forests in terms of which forests were sinks versus sources, and in terms of how recent management, land use, and climate factors have caused forests to be sources versus sinks. They found that, despite huge variation between and within the three different forest types, forests have been a net sink of carbon over the past two decades. They have been sequestering about one “Pg C yr^-1” (one “Petagram” of carbon is the equivalent of 1 billion metric tonnes). Of all types of vegetation on land (i.e. versus grasslands or shrublands), forests dominate the carbon balance equation. They found that tropical forests by far have the largest influence, both in terms of sinks and sources. They conclude that, while the carbon sequestered in forests can be at risk of loss from future climate change and human actions, forests nonetheless have and may continue to be important sinks of carbon originating from fossil fuel consumption.

Relevant quote: “Clearly, forests play a critical role in the Earth’s terrestrial C sinks, and exert strong control on the evolution of atmospheric CO2. Drivers and outlook of forest carbon sink.”

Relevance to landowners and stakeholders:

Forests are robust when it comes to sequestering carbon. They are usually carbon sinks (they remove CO2 from the atmosphere), and only become distinct sources when they are dramatically changed by actions that convert them from forests into something else. Converting a forest into agricultural or residential land, for example, takes away the carbon-sequestering power that a forest has. So the first step in conserving the beneficial roll of forests in sequestering carbon is to keep forests as forests. This does not mean forests shouldn’t be disturbed (by sustainable harvests or prescribed fires, for example). In fact, young forests can be significant carbon sinks if they are growing fast. In terms of carbon sequestration, effective conservation efforts are those that aim to limit long-term forest conversion from occurring. Because of the huge losses of carbon from tropical forests deforestation, the authors of this paper cite the REDD program (Reducing Emissions from Deforestation and Degradation) as having good potential to mitigate climate change impacts.

General public and cocktail-party audiences are often surprised when I tell them that the forest area in the United States has increased over the past half-century because of abandoned agricultural lands that are returning to forests. Now I have a reference to provide, as this paper cites this increase in forest area as being one reason why U.S. forests have been a net sink of carbon (other reasons being that many forests are young and fast-growing, and that there is likely some fertilization happening from CO2 enrichment and Nitrogen deposition (see this previous post about N deposition).

As briefly pointed out by the authors, forests are sequestering large amounts of carbon that can off-set some fossil fuel production of carbon, but forests are not a complete bail-out when it comes to climate change. When it comes to forests reducing impacts of climate change, they are like seat belts. They might save us from dying in minor crashes, but if a head-on collision occurs, we're still going to die.

Relevance to managers:

The primary relevance to managers that I interpret from this article is that it suggests that existing young forests, in particular, can be managed to increase their capacity to be carbon sinks. Cultural or commercial treatments that reduce stem density while allocating growing space to larger, fast-growing trees can have the carbon-related benefit of reducing the potential for massive carbon loss from wildfire or insect epidemics, while also contributing to long-term carbon storage by producing forest products.

The factors that influenced whether forests were sources or sinks over the past two decades mentioned in this article are:

• Wildfires – high severity fires in Russia and Western US were recent C sources
• Insect outbreaks – caused forests to be C sources in Western US and Canada
• Forest age – immature tropical forests recovering from disturbance were cited as C sinks
• Deforestation – long-term conversion of tropical forests have been huge C sources
• Afforestation – planting has been effective in creating C sinks in China forests
• Soil management – the draining of water-logged soils in Europe has caused forests to be a C source
• Fertilization- CO2 enrichment and N deposition in US may be increasing productivity and hence forests as a C sink.

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

It was often unclear what the authors meant when using the term “harvesting.” In some cases, they implied that harvesting was a carbon source (in European Russia), but they also present data that suggest that, globally, harvested wood products were a carbon sink. Similarly, it was unclear what they meant when referring to “managed” versus “unmanaged” forests.

Article Reviewed: Nitrogen critical loads and management alternatives for N-impacted ecosystems in California

M.E. Fenn , E.B. Allen, S.B. Weiss , S. Jovan, L.H. Geiser , G.S. Tonnesen, R.F. Johnson, L.E. Rao, B.S. Gimeno, F. Yuan, T. Meixner, and A. Bytnerowicz

Journal of Environmental Management 91 (2010) 2404-2423 doi:10.1016/j.jenvman.2010.07.034

The Plot Line

In this paper, Fenn and collaborators review the nitrogen input “critical loads” for a broad selection of California’s vegetation types and discuss management possibilities. Included are mixed conifer forest, oak woodlands, pinyon-juniper woodland, chaparral, desert scrub, coastal sage scrub, and annual grassland. This summary of the article focuses on the mixed conifer forest sections, but it is important to remember that vegetation types intermix and share nutrients throughout watersheds and ecosystems.

A critical load is “a quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge” (UBA 2004). Empirical critical loads are expressed as quantities per area over time, and are based on the responses of observed biological and chemical variables. High productivity long-lived systems like forests are slow to exhibit changes in species composition in response to nitrogen deposition, but may exhibit detectable changes in soil and stream water chemistry. Under extreme deposition rates, ponderosa pine trees show a measurable decrease in fine root biomass, as well as increased susceptibility to bark beetles and modified soil fungal communities. With even a minor addition to baseline N levels, acid-sensitive epiphytic lichens begin to disappear. These environmental thresholds were used to examine forest response to N loading. Lichens are the most sensitive, with shifts in tissue N concentration and species composition at 3.1 kg N ha^-1yr^-1; the most sensitive lichens are no longer present at 10.2 kgN ha^-1 yr^-1. Incipient N leaching to surface runoff and a 26% reduction root biomass reductions are expected to occur at 17 kgN ha^-1 yr^-1.

The authors used the Community Multiscale Air Quality model to create maps of N deposition statewide and predict where deposition is predicted to be in excess of the described critical load values. In some areas, nitrogen deposition was ground-truthed with canopy throughfall measurements. Deposition currently ranges from 15-20 kg ha^-1yr^-1 in the Central Valley and Sierra, and 30-70 kg ha^-1 yr^-1 in southern California. Natural background forest deposition is estimated to be much lower, around 1-4 kgN ha^-1 yr^-1. This means that an estimated 28.7% of California’s mixed conifer forest (30,596 km² of 106,663 km²) is already in exceedance, based on the extreme sensitivity of lichens. The potentially affected areas are primarily in the central Sierra at all elevations, and at decreasing elevations southward along their western flank. Portions of the San Bernardino and Santa Cruz mountains are also above the critical load for lichens. For nitrate leaching and root loss, the area is considerably lower, only 4.5% (4754 km²). This smaller area of elevated potential impact is primarily in the San Bernardino Mountains, low elevation central Sierra, and extreme southwest Sierra.

Relevant quote

“In the most polluted forests (e.g. estimated N deposition > 25-35 kgN ha^-1 yr^-1) N deposition in conjunction with ozone threatens forest sustainability by contributing to multiple stress complexes, thus increasing forest mortality and fire risk.”

Relevance to landowners and stakeholders

Many characteristics of ecosystem function, including plant species composition, are dependent upon the nutrients available. Systems are often limited by the amount of nitrogen available, and undergo major changes when released from this limitation. Impacts of excess N deposition to forests include: soil acidification and depletion of base cation pools, acidification or eutrophication of alpine lakes, leaching that makes nutrients inaccessible to plants, lower root: shoot ratios, decreased mycorrhizal diversity, and increased tree susceptibility to pests. Such impacts can lead to altered plant physiology, vegetation composition, and forest structure. Big enough impacts on their own, these effects can also act in concert with other pollutants and conditions to further alter forest function. This paper’s focus on critical loads for California adds to similar work from throughout the U.S. and Europe. Describing the ecosystem response to atmospheric deposition sheds light on this important ecological connection that can differ from region to region.

Nitrogen accumulation and flux may also be influenced by more severe droughts and increased extreme precipitation events as predicted by climate change scenarios. In general, a forest critical load would be expected to decrease somewhat due to litter accumulation over the course of repeated dry years, but would be accompanied by greater susceptibility to leaching in heavy rains. Lichen-based critical loads are also dependent upon climate: increases are expected in wetter regions where rainfall can leach accumulated N from lichen tissues, but critical loads may decrease in dry climates.

Relevance to managers

As described by the authors, in many cases there are no feasible management options available to amend the impacts of N deposition. Most likely, mitigation will only be applied in local high-value situations or locations where many interests combine to carry out a specific management practice (for example, a prescribed fire). That said, there are some ways to manipulate forests experiencing excess N input. One option is to relocate some of the N in mineral soil by the use of fire, which volatilizes N and deposits it elsewhere. Because mineral soil holds 65-80% of the N in a California mixed conifer forest, focusing on the subsurface N makes sense. However, fires typically release about 3% of total soil N, so they must be implemented repeatedly to effectively reduce N impacts; such a pattern would only be suitable in certain sites and forest types. Thinning or harvesting affected forests can also temporarily remove aboveground N accumulation. However, tree removal does little to decrease the primary, belowground, nutrient pool, which is largely inaccessible to managers.

Above all, the most direct route to avoid and reduce the impacts of N deposition to California ecosystems is to decrease atmospheric inputs. Nitrogen oxide emissions are currently declining somewhat, but ammonia emissions are on the rise and likely underestimated. Although statewide emissions reductions are far beyond the influence of the average forest manager, long term management planning should incorporate trends in air pollution and consider how N deposition and emissions policies could impact their stands in the future.

Critique

The model-based maps are very helpful in visualizing the areas where estimated N deposition is at or above the critical load. The authors clearly describe that the maps don’t necessarily indicate that ecosystem impacts have occurred in these locations, but one can’t help but wonder what such a map would look like. A display of the observations used to establish these critical loads might show the extent (or at the very least, location) of ecosystem effects thus far and illustrate some of the complexity behind the calculations.

The options for management section of the paper offers very few tools for N removal, focusing on biomass removal and fire. Even if the management options are indeed few, it would be useful to include a discussion of the merits of other approaches here. A broader discussion would provide some context for those seeking solutions to N deposition through management. Finally, there are two critical loads described for the mixed conifer type: one for the lichens, which are very N-sensitive, and one for streams and roots impacts, which are less so. It isn’t always clear which standard is an appropriate measure of ecosystem impacts for a given situation. Providing basic information on interpreting these disparate values (do I use 3.1 or 17 kgN ha^-1 yr^-1?) might help this interesting work reach a wider audience.

Other notes

A useful resource for understanding critical loads and how they are calculated:

http://nrs.fs.fed.us/clean_air_water/clean_water/critical_loads/faq/

UBA, (Ed.), 2004. Manual on Methodologies and Criteria for Modelling and Mapping Critical Loads and Levels, and Air Pollution Effects, Risks and Trends. German Federal Environmental Agency, Berlin, Germany, 190 pp. Available from: www.icpmapping.org.