September 21, 2012

Forest scientists bet on the trifecta SWEEP in the Sierra Nevada

Article reviewed: Forests and water in the Sierra Nevada: Sierra Nevada Watershed Ecosystem Enhancement Project (SWEEP)

By R.C. Bales, J.J. Battles, Y. Chen, M.H. Conklin, E. Holst, K.L. O’Hara, P. Saksa, and W. Stewart

The plot line: [Note that this is a “white paper” (self-published), so I am straying from my typical format of reviewing only peer reviewed articles. Given the relevance for management and the quality of this particular paper, it seems worth making an exception]. This group of forest scientists quite aggressively makes the argument that forests in the Sierra Nevada can be managed for improving both the quantity and quality of water to benefit the commonwealth of California, and that there should be monetary incentives for the landowners who do such management. Their case is built upon the notion that water is of very high value and that several studies done in other similar forests clearly document that lower density forests (i.e. recently harvested) do increase water yield and potentially increase snow pack persistence. They make the case for large-scale studies that can be used in the future to help foresters and landowners meet the triad objectives of water, fire severity reduction, and species restoration (the trifecta SWEEP).

Relevant quote: “The perspective that forest management for water supply is not worth the trouble is ingrained in both upstream and downstream resource managers. The SWEEP team contends that forest management for water supply is worth the trouble…”

Relevance to landowners and stakeholders:

Forest landowners pay attention any time a scientist or economist suggests that they should be paid more for the “ecosystem services” that they provide to society. Forests support wildlife, clean air, and natural beauty that people from the city enjoy. So why shouldn’t the folks that own these forests get paid for it? There is of course a way in which landowners can be paid for protecting their forests. That is, through a conservation easement. But what these scientists are suggesting is something quite different than a conservation easement. Instead of a forest landowner getting paid to do nothing with an easement, they are suggesting that they get paid to do something! Doesn’t that sound more feasible as an economic model?

We are of course a ways away from this actually happening, but this team of scientists is trying to conduct research that will help such a system to develop. Rigorous experiments will have to be done in order to measure with accuracy how much more water can actually come from a forest managed for water quantity and quality (when I say quality, I am referring mainly to the timing of snow melt- if snow melts later in the spring/summer, then it is of higher quality in terms of value).

You can find the arguments for why forest management could be managed for water in any forest ecology text book. A simplified version of it is this:

1. All plants have leaves.
2. Leaves do photosynthesis, which pulls water from the soil and transpires some of it into the air
3. Leaves intercept snow and rain, some of which evaporates directly back into the atmosphere
4. The fewer leaves that are present, the less water will be sent into the air, and the more water will leave the site and go into reservoirs or hydro-electric facilities.

Relevance to managers:

I think the relevant quote above says it all for managers. These authors are right- water can no longer be ignored. I have personally heard other scientists and mangers state that forest management simply cannot make a significant difference when it comes to water yield or the timing of runoff. But the large amount of evidence presented in this paper suggests the contrary. And it is no secret that water is becoming a more valuable resource every year, so even small increases in yield can be meaningful. It is only a matter of time before markets force us managers to more explicitly manage forests with water as the objective. If the research these and other scientists propose comes to fruition, then we’ll be more ready for the challenge.

The UC Center for Forestry has been managing for what we call a “water efficient forest” for the past decade. It is at a slightly lower elevation than what these authors say will be optimal for increasing water yield and runoff timing, but they also provide some logic in this paper that suggests these lower elevations could increase yield as well. The easy part was thinning the forest down to a level where one could reasonably expect an increase in throughfall and runoff. In this particular case, we have harvested to a density at about 50% of the maximum that we observe on nearby stands. The density then fluctuates between about 50 and 75% of maximum over time in between harvests. Based on the estimates from this paper, this level of density reduction might result in somewhere between a 9 and 18% increase in water yield and it should mean snow persisting for a little while longer (although for low elevation forests, it is likely more about water yield than the timing of snow melt).

In my experience, the easy part in managing for water has been conducting the commercial thins. After all, it is a productive forest so we can generate revenue from the thins by harvesting commercial sized trees. We have been able to, concurrently or immediately following harvests, reduce the small tree cover and surface fuels to make the forest resistant to high severity fires. According to this paper, this action has resulted in what should be a structure that yields more water (somewhere between 9 and 18% increase). The challenge over time has been in managing the understory vegetation in order to prevent it from developing a significant amount of leaf area that would defeat the purpose of increasing water yield. It is challenging because this means conducting treatments that are not paid for with a commercial harvest. Theoretically, one could save the revenue from the commercial thin and apply it to understory treatments in between thins. This is indeed what we have done in this particular case (in the form of mastication and broadcast burn treatments), but without a financial incentive to do these treatments, I can see the situation occurring where these follow-up treatments simply aren’t done. So the authors make a good point in this paper that a water efficient forest needs to be maintained over time. It is not a one-and-done situation.

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

They say that many of the upper watershed forests are zoned as wilderness areas, the implication being that these areas cannot be managed for increased water yield. I would argue that these areas can also be managed for water quality with fire being the mechanism for maintaining low density. Without fire in these areas, they will burn with higher severity fire that could input massive amounts of sediment into downstream watercourses, thus countering any positive effect of water quantity and quality treatments that are done in non-wilderness areas.

They make an excellent point that, if runoff is delayed because of forest management activity, then hydro-electric energy production can occur later in the summer, when demands are high. I think they missed out on a point to make about the further potential for these treatments to benefit energy production during the summer. If the treatments are done in the summer and involves a biomass harvest of small trees and tops/limbs, then this would also potentially result in energy production during a time when it is most needed. Perhaps this is too speculative, but it is interesting to think about the potential for biomass harvests to by synergistic with water yield treatments from an energy production perspective.

They focus on forests between 5,000 and 12,000 feet elevation as having the most potential for increasing water yield and runoff timing, because they are productive and warm (above freezing). 12,000 feet… really? Any time I’ve been at 12,000 feet in the Sierra Nevada, I have not noticed many trees. At 12,000 feet, I’m catching my breath and enjoying the view because there aren’t many trees, if any at all. And lots of the winter period is cold at this elevation. Given their logic, it seems like this elevation should be shifted downward, perhaps between 4000 and 9000 feet. 4000-5000 foot elevation forests may not be dominated by snow, so the potential to delay runoff timing is less. But based on their logic and points scattered throughout the paper, forests in this elevation could increase yield substantially. The paper could use some clarity in reconciling all of the different factors of water yield and runoff timing in order to justify the 5 to 12,000 foot elevation target.

They report an average basal area in one of their targeted study areas of 400ft2/acre, with an average canopy cover of only 51% and an average canopy height of only 60 feet in a forest dominated by 100 year old trees. These numbers are not adding up in my head. That basal area seems very high for a forest that does not appear to be highly productive (trees growing 60 feet in 100 years). On the other hand, the LAI they report is also exceptionally high. A high LAI is the only way that I can visualize a forest like this having such a high basal area, so perhaps the numbers are good. But their statement about this forest being typical of much of the northern Sierra Nevada is a stretch- especially considering the 5000 to 12000 foot elevation range that they are talking about.