Endemic Jeffrey Pine Beetle Associates: Beetle/Mite Fungal Dissemination Strategies and Interactions That May Influence Beetle Population Levels

Fungal and mite associates may drive changes in bark beetle populations, and mechanisms constraining beetle irruptions may be hidden in endemic populations. We characterized common fungi of endemic-level Jeffrey pine beetle (JPB) in western USA and analyzed their dissemination by JPB (maxillae and fecal pellet) and fungivorous mites to identify if endogenous regulation drove the population. We hypothesized that: (1) as in near-endemic mountain pine beetle populations, JPB’s mutualistic fungus would either be less abundant in endemic than in non-endemic populations or that another fungus may be more prevalent; (2) JPB primarily transports its mutualistic fungus, while its fungivorous mites primarily transport another fungus, and (3) based on the prevalence of yeasts in bark beetle symbioses, that a mutualistic interaction with blue-stain fungi present in that system may exist. Grosmannia clavigera was the most frequent JPB symbiont; however, the new here reported antagonist, Ophiostoma minus, was second in frequency. As hypothesized, JPB mostly carried its mutualist fungus while another fungus (i.e., antagonistic) was mainly carried by mites, but no fungal transport was obligate. Furthermore, we found a novel mutualistic interaction between the yeast Kuraishia molischiana and G. clavigera which fostered a growth advantage at temperatures associated with beetle colonization.

Tree-ring climate response of Jeffrey pine in the Cascade Creek Watershed, Northern California

Understanding the forest response to ongoing climate change is crucial in forest management strategies under anticipated climate adversity. To understand the retrospective growth dynamics of Jeffrey pine (Pinus jeffreyi Grev. & Balf.), tree-ring chronology from the subalpine forest in the Lake Tahoe Basin, California was correlated with air temperature, precipitation, and Palmar Drought Severity Index (PDSI). The years 1757, 1782, 1886, 1859, 1876, 1920, 1929–30, 1977, 1988–89, 2001–02, 2008, and 2014 were some of the years with noticeable low growth. There was robust growth in 1747–49, 1792, 1828, 1866–68, 1913, 1969, 1984, 1998, and 2011. Ring width index (RWI) and basal area increment showed a recent growth increase. Climate-growth response analysis revealed the growth-inhibiting influence of the hot and dry summer. More pronouncedly, warm and wet winter was found to be conducive to tree growth in the following year. A significant growth correlation with the previous year climate (stronger with PDSI) and its absence in current spring may be suggestive of potential growth stimulation by predicted warmer and longer growing season in the future. However, since the RWI chronology consisted mostly of mature trees and because the old cambial age tends to have signal divergence, further studies incorporating younger trees and cohabitant species would provide deeper insights into the growth-climate response.

Do forest fuel reduction treatments confer resistance to beetle infestation and drought mortality?

Climate change is amplifying the frequency and severity of droughts and wildfires in many forests. In the western U.S., fuels reduction treatments, both mechanical and prescribed fire, are widely used to increase resilience to wildfire but their effect on resistance to drought and beetle mortality is not as well understood. We followed more than 10,000 mapped and tagged trees in a mixed-conifer forest following mechanical thinning and/or prescribed burning treatments in 2001 through the extreme 2012-2016 drought in California. Mortality varied by tree species from 3% of incense-cedar to 38% of red fir with proportionally higher mortality rates in the larger size classes for sugar pine, red fir and white fir. Treatment reductions in stem density were associated with increased diameter growth and rapidly growing trees had lower rates of mortality. However, the ultimate effects of treatment on drought-related mortality varied greatly by treatment type. All species had neutral to reduced mortality rates following mechanical thinning alone, but treatments that included prescribed burning increased beetle infestation rates and increased mortality of red fir and sugar pine. Fuels reduction treatments appear to benefit some species such as Jeffrey pine, but can reduce resistance to extreme drought and beetle outbreaks in other species when treatments include prescribed burning. In a non-analog future, fuels reduction treatments may require modification to provide resistance to beetle infestation and severe droughts.

Climate change and forest plagues: assessing current and future impacts of diprionid sawflies on the pine forests of north-western Mexico

The imminent threat of climate change lies in its potential to disrupt the balance of ecosystems, particularly vulnerable areas such as mountain-top remnant forests. An example of such a fragile ecosystem is the Sierra San Pedro Mártir (SSPM) National Park of Mexico’s Baja California state, where high levels of endemism can be found, and which is home to one of the country’s few populations of the emblematic Jeffrey pine (Pinus jeffreyi). Recent outbreaks of pine-feeding sawfly larvae in SSPM increase the vulnerability of this forest ecosystem, calling for immediate assessments of the severity of this threat. Here, we present a thorough study of the sawfly’s biology and distribution, carrying out molecular and morphology-based identification of the species and creating model-based predictions of the species distribution in the area. The sawfly was found to belong to an undescribed species of the genus Zadiprion (family Diprionidae) with a one-year life-cycle. The distribution of this species appears to be restricted to the SSPM national park and it will probably persist for at least another 50 years, even considering the effects of climate change.