Factors that reduce tamarack vigor and increase the risk of eastern larch beetle attack on individual trees or localized groups of tamaracks are well known. At small scales, many eastern larch beetle infestations are less than a quarter hectare in size and appear to begin as a result of a localized stress impacting tree physiology such as insect defoliation, flooding, drought, fire, mechanical damage, natural senescence, snow damage, or wind-throw (Albers, 2010; Furniss & Carolin, 1977; Langor & Raske, 1989a, 1989b; Seybold et al., 2002).
The most common form of physiological stress thought to predispose tamaracks to eastern larch beetle attack is defoliation. The species of defoliator most commonly associated with eastern larch beetle activity varies regionally (Langor & Raske, 1989b). Defoliators include the larch sawfly (Pristiphora erichsonii (Hartig)), spruce budworm (Choristoneura fumiferana (Clemens)), the larch casebearer (Coleophora laricella (Hübner)), and larch budmoth (Zeiraphera spp.) (Langor & Raske, 1989b; Seybold et al., 2002; Ward & Aukema, 2019a, 2019b; Werner, 1986). Defoliation typically occurs within 1–3 years preceding eastern larch beetle activity (Seybold et al., 2002). For example, repeated defoliation of tamaracks in Newfoundland, Canada by spruce budworm during the mid 1970s caused little direct tamarack mortality but did reduce tree growth and vigor. Trees with reduced growth were subsequently attacked by eastern larch beetles causing a large outbreak to occur (Langor & Raske, 1989a, 1989b). Similarly, an eastern larch beetle outbreak that began in Alaska in 1974 rapidly intensified across 240,000 hectares after beetles began infesting tamaracks that had been defoliated for two successive years by the larch budmoth (Werner, 1986). Tamaracks killed during the Alaskan outbreak from 1974 to 1977 had reduced radial growth for the 3 years before the attack compared to trees that were not killed (Werner, 1986). In Minnesota, the geographic overlap between severe defoliation and consequential bark beetle activity in tamarack has been less than 10%, but the pattern is still unmistakable after accounting for other drivers in insect activity (Ward & Aukema, 2019a). Similar patterns of multi-trophic interactions among species have also been observed in other bark beetle-host-defoliator systems. For example, severe defoliation of ponderosa pine (Pinus ponderosa Douglas ex. C. Lawson) by the pine looper (Phaeoura mexicanaria (Grote)) created a surplus of suitable breeding substrate for Ips pini (Say) and I. calligraphus (Germar), permitting Ips populations to increase beyond normal densities (Dewey, Ciesla, & Meyer, 1974). In some instances, tamaracks weakened by pinewood nematode (Bursaphelenchus xylophilus) (Steiner and Buhrer) Nickle have also been linked to localized eastern larch beetle outbreaks (Langor & Raske, 1989b).
Localized flooding events caused by road construction or beaver dams are a common cause of tamarack stress and mortality. Trees killed by either agent can provide substantial amounts of substrate for beetle breeding and reproduction (Seybold et al., 2002). Tamaracks killed in the current beetle outbreak in Minnesota have been found within a variety of growing conditions that range from upland to lowland sites and within dry and saturated soils, making it difficult to associate patterns of site-specific growing conditions with observed tree mortality (Albers, 2010).
At landscape-level scales, factors that are responsible for stressing tamaracks physiologically and enhancing the potential for outbreaks of eastern larch beetles also include the age and composition of tamarack stands, geographic location, and long-term climatic patterns. Langor and Raske (1989b) note that stands of over-mature tamaracks commonly support outbreaks of eastern larch beetles, possibly as a result of mature trees experiencing decreased vigor and defensive capacities relative to younger trees. The extent of the eastern larch beetle outbreaks that occurred in northeastern North America and Alaska during the 1970s may have been exacerbated by the increased maturity of tamaracks within the affected regions (Langor & Raske, 1989a, 1989b). Moreover, although tamaracks in both mixed and pure stands are susceptible to attack by eastern larch beetles, stands that exhibit an even-age distribution and/or low species diversity may be at an increased risk of beetle attack (Seybold et al., 2002).
The shallow root system of tamaracks renders the trees susceptible to physiological stress in saturated as well as dry soils (Johnston, 1990). In northern areas of North America where permafrost or semi-permafrost is present year-round, tamaracks may be stressed by cold, nutrient-poor soils with poor drainage through the frost layer within the soil. Cold and wet soils may be especially detrimental to larger trees that have more extensively developed root systems. These roots remain in permanent contact with frozen soils and water that accumulates above the permafrost layer (Werner, 1986). Conversely, the shallow root systems of tamaracks have difficulty absorbing soil moisture when drought conditions lower the subterranean water table. In Minnesota, drought conditions and fluctuating water levels over the last 17–19 years have been suggested to have reduced the fitness and defensive abilities of tamaracks across large tracts of land (Jones et al., 2011). Albers (2010) notes that although drought may facilitate the continuation of outbreaks, its role in triggering outbreaks is uncertain (Crocker, Liknes, McKee, Albers, & Aukema, 2016).
Given the wetter conditions on which eastern larch is commonly observed, one of the major disturbance agents for eastern larch is windthrow. The combination of the moist conditions and types of soils in peatlands result in eastern larch having relatively shallow roots; black spruce, a common associate of eastern larch, also has shallow roots (Johnston, 1990). Individual or small gaps created by wind disturbance can influence successional dynamics based on the timing and the presence or absence of advanced regeneration (Hanson & Hargrave, 1996; Minnesota DNR, 2021). If advanced regeneration of more shade-tolerant conifer species is present such as black spruce, eastern white cedar, or a balsam fir, the creation of gaps can advance successional dynamics and potentially result in increased structural complexity. However, if the gap is larger (multiple trees) without advance regeneration, eastern larch may have the competitive advantage over other slower-growing species, allowing for its continued dominance.
Fire also influences eastern larch forest communities and can predispose trees to colonization by eastern larch beetles. While there is no documented fire return intervals for eastern larch forests within the USDA Fire Effects Information System (https://www.feis-crs.org/feis/faces/ReviewResults.xhtml), fire return intervals where eastern larch is a component of the system range from a minimum of 16 years in midwestern shrub and herbaceous wetlands (U.S. Department of Agriculture, Forest Service, Missoula Fire Sciences Laboratory, 2012a) to 1000 years in northern white-cedar swamps in the Great Lakes and the Northeast (USDA FS Missoula Fire Sciences Laboratory, 2012b). Eastern larch as a tree species has thin bark resulting in a high level of mortality from fires including surface fires (Uchytil, 1991). However, given its common location on wetter microsites, fires may not always reach eastern larch trees. As a living reserve tree, eastern larch can successfully regenerate in postburned stands (Johnston, 1973; Parminter, 1983; Payette & Gagnon, 1979). However, the regeneration response can be mixed with reduced eastern larch regeneration reported in some studies (Duncan, 1954). In addition, the severity of the fire may also reduce eastern larch regeneration and result in dominance by herbaceous or shrub species (Rowe, D'Amato, Palik, & Almendinger, 2017).
As ectothermic organisms, much of the development and biology of insects are tightly regulated by ambient environmental conditions (Bale et al., 2002). Therefore, short- and long-term climatic patterns may be the most important aspect governing eastern larch beetle population dynamics across broad geographic scales. Changes in disturbance intensity and frequency may cause more outbreaks of eastern larch beetle in the future (McDowell et al., 2020).