EFFECTS OF BARK BEETLE ATTACKS ON SNOWPACK AND SNOW AVALANCHE HAZARD Michaela Teich 1,* , Martin Schneebeli 2 , Peter Bebi 2 , Andrew D. Giunta 1 , Curtis A. Gray 1 and Michael J. Jenkins 1 1 Department of Wildland Resources, Utah State University, Logan, UT, USA 2 WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland ABSTRACT: Unprecedented bark beetle outbreaks across western North America have resulted in the death of millions of trees, which profoundly affects snowpack in high elevation forests. Healthy, dense for- ests growing in avalanche terrain reduce the likelihood of avalanche release by inhibiting the formation of continuous weak layers, which are key for slab avalanche formation. Bark beetle outbreaks quickly change composition, structure, and functions of forest ecosystems and may alter the protective effect of forests against snow avalanches. We examined the snowpack under canopies of Engelmann spruce for- est stands in the central Rocky Mountains in Utah, USA, using the SnowMicroPen (SMP). Biweekly-re- peated SMP measurements along 20 m transects at 0.5 m intervals were recorded in winter 2015/16 in study plots beneath canopies of a recently infested trees (green), trees 3+-years after bark beetle infesta- tion (gray), a harvested forest stand, and a non-forested meadow. We describe the evolution of the snow- pack at our four study plots with two-dimensional snow density profiles as a measure of snow stratigraphy, which we derived from our high-resolution spatio-temporal SMP data. Our results indicate that at this relatively small spatial scale, differences in snow density layers between green and gray forest stands were not clearly observable. More homogeneous layering developed during periods of less to moderate snowfall where unloading or melting of intercepted snow from the canopy is reduced and snow metamorphism is the dominant process influencing snow stratigraphy. After harvesting, canopies of re- maining smaller diameter trees and woody debris had no significant impact on snow stratigraphy, which needs to be addressed when planning silvicultural measures in protection forests and ski resorts. Consid- ering changes in snowpack properties and local wind regimes following bark beetle attack is important for road and railroad safety, winter backcountry activities, avalanche forecasting, and protection forest and ski resort management. KEYWORDS: subcanopy snow stratigraphy, avalanche protection forest, spruce beetle, SnowMicroPen 1. INTRODUCTION In recent decades, billions of coniferous trees across millions of hectares in forests ranging from Mexico to Alaska have been killed by native bark beetles (Coleoptera: Curculionidae, Scolytinae; Bentz et al. 2010), profoundly affecting snowpack in high elevation forests (Pugh and Small 2012). Forests growing in avalanche terrain play an im- portant role in avalanche protection by reducing the formation of continuous weak layers that con- tribute to slab avalanches. Forests modify snow- pack properties through the interception of falling snow by tree crowns, the reduction of near-surface wind speeds, and changes to the energy balance beneath and around trees (Schneebeli and Bebi 2004). Together these processes lead to a highly variable snow stratigraphy (the characteristic mi- crostructural layering within seasonal snowpack), which inhibits avalanche formation (Fig. 1). Bark beetle-induced tree mortality alters forest cover creating potential avalanche release areas where the forest previously protected settlements, infra- structure, ski slopes and backcountry snow recre- ationists against avalanches. Fig. 1: Snow profile outside (A: homogenous snow stratigraphy) and beneath (B: heter- ogeneous snow stratigraphy) a spruce canopy. (Source: Schneebeli and Bebi 2004) Native bark beetles are key agents of disturbance and change in coniferous forests of western North * Corresponding author address: Michaela Teich, Utah State University, 5230 Old Main Hill, Logan, UT 84322-5230, USA; tel: 435-799-7742; email: michaela.teich@usu.edu Proceedings, International Snow Science Workshop, Breckenridge, Colorado, 2016 975