More Forest Thinning Science

1. You need Water for Ecohydrology

I previously wrote about the effects of thinning Southwestern ponderosa pine forests on forest hydrology.  AE Brown et al (Journal of Hydrology, 2005) summarized the last 50 years of hydrology research around the world on exactly this question.  Their conclusion was that yes, thinning increases water availability by decreasing evapotranspiration (ET).  However, at drier sites there is less of a difference.

In the figure above, the difference between the grass and forest curves represents the change in mean annual water yield for 100% conversion of one vegetation type to the other.  Partial conversion (i.e. thinning) was shown to have a proportional partial response.  The lack of difference between grass and forest in drier climates (below 500mm or 20 inches precipitation/year) indicates that most ET is actually just evaporation in these areas.  Therefore, because transpiration does not play a large role, reducing transpiration via thinning would not be expected to generate a large increase in water availability.

2. Don't Miss the Forest for the Trees

This classic forestry study found that thinning ponderosa forests increased growth of the remaining trees, but decreased total wood production.  In other words, the increase in vigor didn't compensate for the decrease in trees.  This even includes the decrease in disease (bark beetles) in thinned forests. So the question becomes, do you want a healthier forest or more wood?  

Data is from the The Level-of-Growing-Stock (LOGS) study on thinning ponderosa pine forests in the US West: A long-term collaborative experiment in density management.  A 2020 follow up provides a summary review of this study that started in 1962.  The The AZ portion of the study was conducted at Fort Valley experimental Forest just north of Flagstaff.   PDF with much more info is available from https://www.fs.usda.gov/rm/pubs/rmrs_p055.pdf.

Four Forests Restoration Project

The draft EIS for the first 500,000 acres has been released.

Thinning work is already ongoing, but is behind schedule:

"The company said it is now thinning about 30 acres a day, which works out to about 625 acres a month. That’s a significant increase in the pace of operations since January, but still far behind the schedule established for the project nearly four years ago.  Ultimately, the company’s 10-year contract with the Forest Service requires it to clear 40,000 acres annually. In the nearly two years the company has had the contract, it has cleared about 3,700 acres. That puts the company about 70,000 acres behind the original schedule."

In my experience, in the Jemez, the major time lags are for completing NEPA and EIS and waiting for good prescribed fire weather.  Cutting the trees is fast and easy, and if they skip the fire (unnecessary and possibly environmentally detrimental) nothing should slow them down.  One of my other main criticisms from the Jemez is that they're not thinning enough trees to reduce the basal area to the most beneficial levels.  I know the ideal density and pattern of trees has been argued about ad infinitum... it seems they are trying to avoid conflict by cutting less trees, which totally defeats the purpose of preventing catastrophic crown fire.

What stand density is optimal in Ponderosa pine forest restoration?

 In my humble opinion, there is really only one chart needed to answer the question of what density is best for Ponderosa pine trees:

Source: The role of stand density on growth efficiency, leaf area index, and resin flow in southwestern ponderosa pine forests.  McDowell, Adams, Bailey, and Kolb.  (link)

 The lower the basal area, the more trees grow.  Growth is especially important in drought-stressed trees because lower growth can weaken tree defenses.  The lowest treatment in this study reduced trees to 7 m2 ha-1, and trees in this treatment had the highest leaf area per tree, indicating that they were healthiest at the time of the study.

Leaf area of understory plants was also highest at the lowest stand densities:

Source: ibid.

However, in the interest of full disclosure, this chart also shows (in panel B) that total forest resource utilization, measured by total leaf area, is highest at medium tree densities.