Science Friday: Reducing Tree Density in Dry Forests Can Help Drought and Insect Resistance- Bradford et al. Paper

If you click on this figure twice you can see it more clearly.

I have often observed that in Oregon, dry forest management tends to be seen through a wet forest filter due to the location of most scientists, interest groups, and politicians on the West Side. So I am in no way, shape or form, suggesting that this study is applicable to wetter forests than those studied (all east of Cascades and SW Or and NW Cal). The idea is that basically in a dry forest in drought, reducing BA and competition for water will keep trees healthier. Many of us have observed this effect in the field after thinning projects and some might say that this is old news, but the authors use large datasets to come to some interesting conclusions.

This seems to be  the opposite of the Williamette project in the previous post; lots of ecological reasons but no industry. In fact if you look at figure 3d, it appears that the areas that could use thinning the most (yellow) are least likely to have forest industry of any size.  Do the maps make sense in the areas you know?

Here are the conclusions of this study.

Interactions between competition and drought provide information for near-term forest management. In particular, the response of mortality to drought-density interactions reinforces the climate adaptation benefits of ongoing forest landscape restoration (Stoddard et al., 2021) that is increasingly widespread in ponderosa pine forests (Figure 1a). Increasing BA in the late 20th century over many forests in the western U.S. (Rautiainen et al., 2011), particularly in ponderosa pine forests (Covington & Moore, 1994; Reynolds et al., 2013), prompted restoration projects designed to reduce forest density, promote structural conditions consistent with the historical range of variability, and mitigate the risk of catastrophic wildfires that lead to rapid loss of forest cover and ecosystem carbon (McCauley et al., 2019). The interactions demonstrated here between competition and hot drought provide quantitative information about how density reduction enabled by these restoration projects, initially designed for other purposes, will also help buffer forests against heat- and drought-driven tree mortality that is increasing in forests around the world (Allen et al., 2010). In addition, these results identify areas where BA reduction may be most useful for enhancing dry forest sustainability. Geographical patterns in estimated benefits of reducing current BA (Figure 3c,d) and in overall climate-driven sensitivity of mortality to basal area (Figure 3e) may be useful for prioritizing future restoration projects. Our results suggest that substantial reduction in BA may be necessary to moderate drought-induced mortality (Figure S4c). These treatments would alter forest structure, and the impact of those changes need to be weighed against the benefits of imposing treatments. Although severe mortality events driven by hot droughts and insects would also reduce BA, restoration treatments may include benefits like selecting the trees to removed or retain, and avoiding rapid increases in fuel loads after mortality.

Although our focus was assessing how BA and drought combine to influence tree mortality, our data include the effects of insect activity. Tree mortality is often elevated by the combination of both drought and insects (Anderegg et al., 2015). Mortality events driven by these drought–insect combinations have been demonstrated in many areas, including in ponderosa pine forests within our study area and sampling period (Fettig et al., 2019; Stephenson et al., 2019). Including these recent insect outbreaks in the data we examined ensures that our results about how mortality responds to drought type and basal area are relevant even in the context of substantial insect activity. Specifically, the potential for reducing BA to decrease tree mortality encompasses the influence of both drought (whose effects may be exacerbated by high BA due to competition) and insect dynamics (whose effects may be exacerbated by BA due to insect population dynamics not directly related to tree competition). Unlike insects, we attempted to avoid including other mortality agents by excluding plots with wildfire or harvesting. As a result, our overall average mortality rate of ~0.8% per year (5th–95th percentile = 0.14% and 1.8% per year) is an estimate of background mortality and may be less than other studies of ponderosa pine mortality (Ganey & Vojta, 2011). Drought contributes to wildfire activity (Hicke et al., 2016), underscoring the need to untangle the interacting influences of these multiple mortality agents. In addition, our results may not fully account for the consequences of actual temporal changes in climate and/or forest because we utilized a space-for-time substitution, which has recognized limitations in modelling climate-induced changes in tree mortality with a single remeasurement of FIA plots (Dietze & Moorcroft, 2011).

Forest managers have relatively few proven strategies to enhance near-term drought resistance of intact dry forests to rising temperatures and more extreme droughts. Long-term forest management strategies for climate adaptation include harvesting and/or planting to shift composition towards tree species with higher drought tolerance (Paz-Kagan et al., 2017) or to promote forests with higher diversity in species composition or functional traits (Anderegg et al., 2016). Reducing BA in existing forests is a complementary and feasible strategy that our results suggest will have long-term benefits. The interactions identified here provide insight into the types of drought that most influence tree mortality, and how those drought conditions can be minimized by moderating competitive intensity. Specifically, BA reduction can enhance resistance to hot conditions and to multiyear drought events, whose frequency and severity are also expected to be increased as a result of elevated hydro-climatic variability (Swain et al., 2018). This elevated hydro-climate variability may create more multiyear wet periods that could enhance mortality in subsequent droughts by promoting structural overshoot (Jump et al., 2017), further highlighting the benefits of density reduction. Predictions of multiyear wet periods (Liu & Di Lorenzo, 2018) may represent important opportunities for intensive management (e.g. thinning) to promote forest structural conditions with high resilience to hot droughts (Bradford et al., 2018). Our findings that basal area interacts strongly with multiyear drought, and that 3-year wet periods partially mitigate ponderosa pine mortality, provide evidence that both the interactions and the occurrence of wet periods may be useful focal points for additional synthesis and analysis.

11 thoughts on “Science Friday: Reducing Tree Density in Dry Forests Can Help Drought and Insect Resistance- Bradford et al. Paper”

  1. I think that most people would have considered this forest to be ‘too thick’. Now that this ‘protected’ forest in the Giant Sequoia National Monument is this dead, its probable future is intense wildfire. With such minimal precipitation, all that dead wood would take a century to rot. It may take two wildfires, 20 years apart to finally eliminate all that fuel. It’s uncertain if forests would return within 30-50 years, without a seed source.,-118.5561779,149m/data=!3m1!1e3?hl=en

    Here’s an example of brush taking over after a wildfire. Most of this is the infamous whitethorn, which even the wildlife doesn’t like. (Whitethorn has thorns which can go through your jeans, into your skin, then break off. You have to dig out the broken tip.),-118.5610595,149m/data=!3m1!1e3?hl=en

    • Hello Dr. Anonymous: First, let me say what an honor it is to be discussing this topic with someone of your great professional renown and experience. When you are referring to such generic terms as “wet forest” and “dry forest” in western Oregon, is this generally analogous to saying Douglas fir/yellow pine forest? Or is it a rainfall thing, or what? As a long-time western Oregon resident, I’d be very interested in learning your expert opinion. Thank you.

  2. Tree health does not equal forest health. Tree mortality is doing the thinning for us and creating many valuable snags and avoiding many adverse trade-offs from commercial log removal. Why can’t we start seeing natural mortality events as part of the solution to be embraced, instead of part of the problem to be solved with logging?


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