Physical and chemical characteristics of blue and Engelmann spruce relative to spruce beetle host selection and colonization

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Highlights

  • Resin flow was greater in blue spruce.

  • Bark was thicker and phloem was thinner in blue spruce.

  • Concentrations of within-phloem terpenes were more than double in blue spruce.

  • Engelmann spruce foliage emitted greater concentrations of volatile terpenes.

  • Spruce beetles landed and attempted colonization at higher rates on baited Engelmann spruce than baited blue spruce.

Abstract

Bark beetles are keystone species that can alter the structure and function of forested ecosystems, yet the mechanisms underlying host selection and successful colonization remain poorly understood for most species. Comparison of closely related tree species that vary in their susceptibility to bark beetles could provide insights into such mechanisms. Here, we compare physical and chemical characteristics of blue (Picea pungens Engelm.) and Engelmann (Picea engelmannii Parry ex Engelm.) spruce, species rarely (blue) and frequently (Engelmann) selected and colonized by the spruce beetle (Dendroctonus rufipennis Kirby) in the western U.S. At three sites (Utah, U.S.) where these species co-occur, 15 trees of each species were selected and traits important for bark beetle survival and population dynamics were measured and compared (bark and phloem thickness, resin flow, phloem and volatile chemistry, beetle landing and colonization success). There were significant differences in bark and phloem thickness and resin flow between species. Bark was thicker and phloem was thinner in blue spruce than Engelmann spruce whereas resin flow was highly variable but greater in blue spruce. Concentrations of within-phloem terpenes in blue spruce were more than double those for Engelmann spruce. Engelmann spruce foliage emitted greater concentrations of volatiles than blue spruce. Spruce beetles landed at higher rates on baited Engelmann spruce than baited blue spruce, and Engelmann spruce was more likely to be colonized. Collectively, these results suggest that blue spruce is a less suitable host for spruce beetle than Engelmann spruce due to a combination of factors including: thicker bark, thinner phloem, higher resin flow, lower concentrations of volatile terpenes, and higher concentrations of constitutive terpenes in phloem tissue, several which are known to be toxic to spruce beetles.

Introduction

The spruce beetle [(Dendroctonus rufipennis (Kirby)] is the primary cause of spruce tree mortality in North America (Massey and Wygant, 1954, Schmid and Frye, 1977, Maroja et al., 2007). Since the 1990s, spruce beetles have killed millions of Engelmann spruce (Picea engelmannii Parry ex Engelm.) across entire landscapes in the Rocky Mountains (Holsten et al., 1999, Ross et al., 2001, Maroja et al., 2007, Jenkins et al., 2014). However, spruce beetle colonization of blue spruce (Picea pungens Engelm.) in the Rocky Mountains has remained low (Jenkins et al., 2014). In the U.S., blue spruce is found primarily in Colorado and Utah, but its range also extends into parts of Idaho, Wyoming, Arizona, and New Mexico. The high elevational range of blue spruce overlaps the lower elevational range of Engelmann spruce in certain environments. Blue and Engelmann spruce are closely related species (Lockwood et al., 2013) that share many characteristics but differ in cone and needle morphology (Weng and Jackson, 2000) and habitat, with blue spruce more commonly found on mesic sites while Engelmann spruce is more commonly found on drier sites (Massey and Wygant, 1954). Hybridization has only been shown under laboratory conditions where Engelmann spruce is the female (Schaefer and Hanover, 1986, Ernst et al., 1990, Stine and Keathley, 1990, Ledig et al., 2006).

Climate change-induced warming and drying in blue spruce elevational zones and habitats will likely continue (Chmura et al., 2011, Stocker et al., 2013, Intergovernmental Panel on Climate Change, 2014, Anderegg et al., 2015). Therefore, blue spruce may experience increased stress as the climate changes. Rising temperatures are also likely to expand the geographic range of spruce beetle, and to increase outbreak population size in concert with an increase in voltinism (Price, 1997, Bentz et al., 2010, Hansen et al., 2011, Anderegg et al., 2015). Because tree condition and vigor have been shown to affect bark beetle colonization success (Moeck et al., 1981, Hebertson and Jenkins, 2008, Hart et al., 2013), these changes could result in increased spruce beetle pressure on blue spruce, further increasing spruce beetle range expansion and outbreak intensity and size (Bentz et al., 2010).

Tree defense, resistance, tolerance, and resilience to bark beetles have been studied extensively, including studies of spruce beetle in spruce-fir forests across North America. Some studies have linked changes in tree physiology, specifically tree response to drought, to bark beetle susceptibility (Hart et al., 2013, Gaylord et al., 2015, Kolb et al., 2016). Other studies have shown tree physical attributes and chemistry to influence bark beetle host landing (selection) and colonization (Massey and Wygant, 1954, Moeck et al., 1981, Raffa and Berryman, 1983, Byers, 1995, Wallin and Raffa, 1999, Wallin and Raffa, 2004, Safranyik and Carroll, 2006, Ott et al., 2011). In conifers, bark and phloem thickness (Amman, 1972), resin flow (Christiansen et al., 1999), and quantity and quality of phloem and volatile terpenes (Wallin and Raffa, 2004, Ott et al., 2011, Gray et al., 2015) have been shown to affect host selection and/or performance of bark beetles. While there are many factors that influence bark beetle host selection and colonization, mechanisms underlying low beetle colonization of blue spruce compared to Engelmann spruce remain unknown. Elucidating mechanisms of resistance to bark beetles is of fundamental interest and could help mitigate future spruce beetle impacts to forest resources.

The overall objective of this research was to identify tree characteristics that influence spruce beetle host selection and colonization of blue and Engelmann spruce. Specifically, the variables measured and compared were: (1) bark and phloem thickness; (2) resin flow following wounding; (3) the quantity and quality of the most abundant constitutive terpenes from volatile and phloem collections; and (4) beetle landing (selection) and colonization in response to synthetic pheromone baits.

Section snippets

Study sites and design

This study was conducted at three sites on the Uinta-Wasatch-Cache National Forest on the Salt Lake and Heber-Kamas Ranger Districts in Utah, U.S.: Shingle Creek (40.61095°N, −111.11794°), Silver Fork (40.63474°, −111.61826°), and Lost Mill (40.93021°, −110.75278°). Blue and Engelmann spruces were present at all sites. Across the three sites, selected study trees were found at elevations from 2300 to 2750 m. Subalpine fir [Abies lasiocarpa (Hook.) Nutt.] was the only other tree species present

Results

Mean DBH and height were 56.6 cm (SD 15.4) and 23.6 m (SD 3.4) for blue spruce, and 58.4 cm (SD 13.8) and 26.0 m (SD 4.3) for Engelmann spruce, respectively. There were no significant differences in DBH or height between species (P = 0.42).

Discussion

In this study, several tree traits related to host selection and colonization success of spruce beetle were compared in blue and Engelmann spruce. In areas where blue and Engelmann spruce are sympatric, Engelmann spruce experiences much higher rates of tree mortality attributed to spruce beetle than blue spruce (Schmid and Frye, 1977). Factors such as bark and phloem thickness may serve as a physical barrier to colonizing spruce beetle adults or available food resources for developing spruce

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank D. Blackford, V. DeBlander, R. Cruz, L. Dunning, A. Guinta, J. Guyon, D. Malesky, B. Meyerson, and J. Neumann (Forest Health Protection, USDA Forest Service), S. Seybold (Pacific Southwest Research Station, USDA Forest Service), F. Meinzer (Pacific Northwest Research Station, USDA Forest Service), K. Purser (Bridger-Teton National Forest) T. Upton (Oregon State University) and B. Pinand for technical assistance. We thank Oregon State University, College of Forestry Faculty and Staff

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    Present address: Department of Entomology, School of Natural Resource Sciences, College of Agriculture, Food Systems, and Natural Resources, North Dakota State University, Fargo, ND 58108, USA.

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