Patterns and Drivers of Recent Tree Mortality in Diverse Conifer Forests of the Klamath Mountains, California

A typical Shasta red fir (Abies magnifica var. shastensis) stand within the study area with substantial damage from dwarf mistletoe (Arceuthobium spp.), Cytospora, and fir engraver (Scolytus ventralis).

Thanks to the Society of American Foresters for posting a western round-up of papers in Forest Science and Journal of Forestry. I thought that this one was particularly interesting from several standpoints.Here’sa link.

1. Just because we leave trees alone doesn’t mean they will continue to do carbon storage. Maybe this works in the rain forests, but not so much where the cycle is “trees get established, get dense without fire, water can’t support so many trees and/or trees grow old, get bugs and diseases and die”.

2. It’s really hard or impossible to tease out how much of this mortality is due to changes in climate, past fire suppression, changes in bug and diseases, age of trees and so on, because through time they are all correlated. Back in the day when we wanted to tease out interactions, we would design experiments to do that. We could plant trees every 20 years, run fires through at different intervals, and wait 100 years (or 250) for our answer, except the climate would be changing all that time. So perhaps it’s time to simply say “we won’t know for the foreseeable future- if ever, but if it is climate we should do x, and if it’s age, we should do y, and if it’s fire suppression we should do z. Are x, y and z all that different? How sure are we that they will “work”? How much will they cost? What if we don’t do anything?”

3. From a pragmatic point of view, we need to consider a concept I’ll call “cats out of the baggery”.. one being climate change, and the other being non-native species. Even if we stopped climate change today, and kept all new non-native pests and pathogens from entering the country, managers would still have to deal with existing effects. Or not, in the case of this study, because it’s a Wilderness.

4. Philosophical question: the authors state.

Since this disease complex was first described in 1984, several management recommendations were made to increase the health and vigor of Shasta red fir stands, including selective removal of severely infested trees, and clearcutting and regenerating in heavily infested stands (Kliejunas and Wenz 1982). As of 2010, these silvicultural treatments were not yet prescribed and forest health continued to decline at this site (Angwin 2010), and in 2015, many of the severely infested stands burned in a wildfire (personal communication, Keli McElroy, USFS). Fire may be the best management strategy for slowing the spread of the disease complex, particularly in wilderness regions where silvicultural treatments are not feasible.

I think since it’s in Wilderness, management would mean simply mean no intervention, except maybe WFU?

5. It makes me wonder how much of what we think is conditioned by the most-studied forest areas. Note below how the Klamaths are different from other studied areas.
(CWD is climatic water deficit).

Several authors attribute recent die-off events to initial abiotic stress that predisposes trees to other agents of mortality, such as pests and pathogens. The region-wide dieback of piñon pine (Pinus edulis Engelm.) in the southwestern United States, for example, was attributed to the 2000–2003 drought accompanied by unusually high temperatures, which predisposed trees to attack by a secondary bark beetle species, Ips confusus LeConte (Breshears et al. 2005). Similarly, Millar et al. (2007) found that limber pine mortality in the Sierra Nevada was linked to increased temperature and decreased precipitation, followed by dwarf mistletoe, and finally by mountain pine beetle. We do not see surmounting evidence that recent tree mortality in our study area is linked to unusually high summer temperatures, as the rising temperature trend we observe is driven by minimum temperatures and is most pronounced in winter months (Figure S4, Figure 3). Our study is consistent with literature suggesting that fir engraver outbreaks are associated with increased evapotranspiration (Wright and Berryman 1978, Berryman and Ferrell 1988); however, similar to Rapacciuolo et al. 2014, we did not see an increase in CWD. This distinguishes our study from many other tree mortality studies in the Sierra Nevada, for example, where die-offs have been attributed to large increases in CWD in recent years (Millar et al. 2012, Mcintyre et al. 2015). Finally, we used a ten-year window (2004–2014) for calculating recent changes in climate parameters, but it is possible that a few years of extreme drought within this period could have accelerated mortality, as has been shown in other systems (Breshears et al. 2005). For example, the 2011–2014 regional drought almost certainly created physiological challenges for Shasta red fir trees that exacerbated drought stress from dwarf mistletoe, and increased susceptibility to fir engraver infestation, but further investigation is needed to tease apart the abiotic vs. biotic causal agents of mortality.

Changes in the historical disturbance regime of these forest types, particularly the effects of fire exclusion, may be partially responsible for the elevated levels of recent mortality in high density and/or more mature stands throughout our study area. Similar to other parts of California, forests in the Klamath region have notable signs of fire exclusion, including in-filling of small, shade-tolerant trees (typically white fir), and dense accumulations of litter and duff around older trees. Historically, this region is characterized by a mixed-severity fire regime, with a high frequency of fires prior to the twentieth century producing mostly low and moderate fire effects in most conifer vegetation types (Skinner et al. 2006). In the upper montane zone (where we observed the highest mortality), the estimated fire return interval based on studies done in nearby forests ranges from about 15 to 41 years (Taylor and Halpern 1991, Taylor 2000) According to Forest Service records, our study area has not burned in the past 100 years (Safford et al. 2011), and based on the aforementioned estimates, we determine that the upper montane zone in our study area has missed somewhere between two and six fire cycles. Our finding of higher mortality among smaller diameter trees is consistent with a growing body of literature suggesting that increased stand density and competition from fire exclusion are major contributing factors to the increasing mortality trend in western coniferous forests (Guarin and Taylor 2005, Maloney 2011, Millar et al. 2012). Furthermore, high stand density has been shown to amplify drought stress (Gleason et al. 2017) and increase the spread of pests and pathogens. While some of the mortality observed in this study is undoubtedly related to self-thinning in high density stands, the usually high numbers of recently dead and dying Shasta red fir trees (28%) coupled with the high levels of pathogens and forest insects is more indicative of a die-off event versus a slower change in community composition. In this case, the die-off was almost certainly precipitated by at least a decade of damage from dwarf mistletoe and Cytospora infestation.

4 thoughts on “Patterns and Drivers of Recent Tree Mortality in Diverse Conifer Forests of the Klamath Mountains, California”

  1. In my experience in northern California, I see these higher elevation trees often under stress. The red fir, jeffrey pine, western white pine and even the mountain hemlock seem susceptible to even small climatic changes. It’s hard to say what the pre-human ecology was, during similar climatic conditions. With all of these many variables flying around in a vortex, predictions don’t seem to have that much value.

  2. Sharon, could you repost the link to the article? I get an error message, but I’d like to review the article myself since I work in that geography. Thanks!

  3. In the early 90’s, I worked on the Eldorado NF, as a Harvest Inspector for a Helicopter Salvage project. This portion of the project was at the higher elevations, with red fir, western white pine and mountain hemlocks in the mix, along with the white fir and yellow pines. Almost 30 years later, there isn’t much evidence of logging here.,-120.0610098,1145m/data=!3m1!1e3?hl=en

    Despite the drought, these forests seem quite healthy. Zooming in, you don’t see much mortality, compared with the southern Sierra.,-120.0688365,286m/data=!3m1!1e3?hl=en

    This was a fun road to try and drive up with just a 2WD mini pickup.,-120.049766,1145m/data=!3m1!1e3?hl=en


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