Historic Perspectives on Watershed and Landscape Consequences of Native Forest Insect Outbreaks
Constance Millar
. USDA Forest Service, Pacific Southwest Research Station, Albany, CA, USA
Bark beetle outbreak, Pinus albicaulis, Sierra Nevada
INTRODUCTION
Extensive insect‐mediated forest diebacks in recent decades have generated widespread research attention as well as alarm in the resource‐management community and general public. A generalization emerging is that these outbreaks are of unprecedented severity and extent, and driven primarily by anthropogenic land‐use changes and climate change. The forest insect literature is rich with historic and paleo‐historic analyses, and provides an important reference context against which to evaluate current epidemics and forest responses. Here I review long‐term features of several types of native insect outbreaks that have potential for widespread effects, and characterize consequences on forest and ecosystem health and condition. Two major categories of insects causing widespread effects on forests are defoliators and bark beetles. Species such as western spruce budworm, Douglas‐fir tussock moth, Pandora moth, and larch budmoth feed on conifer foliage (defoliators); mountain pine beetle, spruce beetle, and pinyon ips are examples of insects that feed on inner bark (phloem). Both life‐history strategies exemplify co‐evolved systems between host (conifer species) and insects. Defoliators often consume the entire live crown but rarely kill forests outright, although mortality (especially of understory trees) occurs in some cases. Bark beetle strategy includes killing large trees, and widespread stand and landscape‐scale mortality occurs during an outbreak. Highly complex endogenous (e.g., competitors, predators) and exogenous (e.g., climate) factors mediate outbreaks, and these are disentangled only with long‐term records. Infestation refers to insect attack at the stand level and outbreak implies widespread attack across landscapes.
TAKE HOME POINTS
1. As co‐evolved systems, native insects and their hosts have highly complex interactions and feedbacks that control infestations, outbreaks, and regional synchrony. Climate is not the only or key driver, and these systems have experienced significant climate change through their histories.
2. Outbreak characteristics and frequency depend on the insect and tree species involved. Many defoliators have high‐frequency cyclic or quasi‐cyclic outbreak patterns that may interact with high‐frequency climate (e.g., multi‐year drought or wet periods), but often appear not to be influenced by low‐frequency climate variability. Bark beetles have episodic outbreaks whose patterns are more difficult to evaluate. Native outbreaks of both types historically have ranged from small (stand level) to extensive (e.g., 1.5 million km2, hemlock decline in the Mid‐Holocene).
3. Due to the repeating nature of outbreaks, multi‐century to millennial‐long historic records are needed to discern trends and attribute causal factors.
4. Although usually viewed as forest‐health crises, native insect outbreaks can result in beneficial effects on trees and forests. These include: stand thinning, culling of suppressed trees, enhanced nutrient cycling, enhanced light to forest floor that benefits understory vegetation, growth release that can persist for decades, rapid genetic selection for adaptation to current climates and conditions, and increase in landscape‐scale forest mosaicism. Fig 1. 1200 years reconstruction of larch budmoth defoliation outbreaks and temperature in the European Alps showing regular frequency of minor and severe outbreaks. A) Outbreak reconstruction since 832 CE. B) Temperature model. From Esper et al. 2007.
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. Fig 2. Standardized tree‐ring width indices for old‐growth ponderosa pines showing periods of Pandora moth defoliation outbreak (shaded areas below lines) in western US. K is known outbreak, I is inferred outbreak. From Speer
et al. 2001.
Fig 3. Tree‐ring reconstructed defoliation outbreaks of western spruce budworm (WSBW) in Colorado. Top panel shows Palmer Drought Severity Indices (PDSI; negative is drier); bottom panel shows growth reduction in pines as a result of WSBW). From Ryerson et al. 2003.
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INSECTS REFERENCED
Western Spruce budworm: Choristoneura occidentalis
Pandora moth: Coloradia pandora
Douglas‐fir tussock moth: Orgyia pseudotsugata
Larch budmoth: Zeiraphera diniana
Mountain pine beetle: Dendroctonus ponderosae
Spruce beetle: Dendroctonus rufipennis
Pinyon ips: Ips confusus
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Fig 4. Reconstructed Mid‐Holocene hemlock decline based on pollen and lake‐recovered sediment deposits, NE USA. B) % sand indicates dry periods when high values. C) % hemlock pollen showing millennial decline in Mid Holocene. From Marsicek et al. 2013.
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Fig 6. Mortality in hectares for mixed conifer forests in western North America over the last 3 decades as a result of outbreaks in 3 species of bark beetles. A) Distribution of insects. B) Area of outbreak. From Raffa et al. 2008.
Fig 5. Tree‐ring growth of Pinus flexilis (A) and P. albicaulis (B) in Sierra Nevada, California showing depression of growth and mortality during late‐
20th/early 21st C outbreaks of mountain pine beetle. Dotted line indicates growth of trees that eventually died in the latest outbreak; solid line shows growth of trees that survived the outbreak. Trees that died grew better than survivors prior to ~1920 in both species, whereas the order of growth switched after 1925, suggested genetic differentiation for drought and warmth, and suggesting natural selection for increased fitness resulted from recent outbreaks in both species. From Millar et al. 2007, 2012.
Fig 7. 300 years of reconstructed major to minor growth‐release patterns (peaks in curves) in white spruce following outbreaks of spruce bark beetle in Alaska. During outbreaks, beetle kills effectively thin dense spruce forests and surviving trees grow more rapidly for several decades until canopies close again and growth is suppressed by recurring dense stand conditions. Asterisks indicate fire dates. From Berg et al. 2006.