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.
