Composition and Elevation of Spruce Forests Affect Susceptibility to Bark Beetle Attacks: Implications for Forest Management

Massimo Faccoli,Iris Bernardinelli

doi:10.3390/f5010088

Massimo Faccoli, Iris Bernardinelli

Open Access

https://doi.org/10.3390/f5010088

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Abstract

The spruce bark beetle, Ips typographus (L.) (Coleoptera: Curculionidae, Scolytinae), is one of the most destructive insects infesting spruce forests in Europe. Data concerning infestations of I. typographus occurring over the last 19 years (1994–2012) on the Southern Alps were analyzed in seven spruce forest types: (1) pure spruce plantations; (2) pure spruce reforestations; (3) pure spruce mountain forests; (4) pure spruce alpine forests; (5) spruce-conifer mixed forests; (6) spruce-broadleaf mixed forests; and (7) spruce-conifer-broadleaf mixed forests. The collected data included the amount of I. typographus damage and the location and composition of the infested forests. The results indicate that different forest types are differently susceptible to I. typographus. Plantations, reforestations and mountain spruce forests show mean damage and mean number of infestations higher than other forest types. Within pure spruce forests, alpine forests growing at high elevations (>1300 m) suffer low damage. Furthermore, the mean number of infestation spots recorded annually in the different spruce forest types is negatively correlated with a Naturality Index value. The results suggest that forest composition and elevation are the main factors driving the risk of I. typographus damage. A new management strategy for some spruce forest types is needed, with a progressive reduction of pure spruce forests at low altitude and an increase of broadleaf composition.

Highlights

Forest composition and temperature are among the main environmental factors affecting tree suitability to herbivorous insects and, management decisions concerning the control of forest pests [1]
Different forest types were differently susceptible to I. typographus infestations, suffering damage of different intensity (ANOVA, df = 6; 119, F = 3.84, p < 0.001)
alpine forests (ALP), Mix B and Mix C-B were less prone to I. typographus infestations, with a mean damage about 10 times lower than the previous types

Summary

Introduction

Forest composition and temperature are among the main environmental factors affecting tree suitability to herbivorous insects and, management decisions concerning the control of forest pests [1]. According to the theory of predisposition and trigger [2], insect outbreaks rely on a spatial and temporal coincidence of susceptible (i.e., predisposed) forests with disturbance factors affecting tree survival and the production of ecosystem goods and services [3]. The poor health conditions of forests growing in sites characterized by environmental traits unsuitable for a particular tree species, such as frequent climatic anomalies, elevation, soil characteristics and forest composition, are expected to trigger insect infestations, with potentially serious consequences on forest survival [5,6]. Many studies on the population dynamics of bark beetles suggest that climatic anomalies, such as drought and heat waves, may trigger large insect outbreaks [8,9,10], acting both directly on the insect populations and indirectly on the health of the potential host trees. In the last few years, the spruce bark beetle, Ips typographus (L.), one of the most destructive insects infesting spruce forests (Picea abies Karsten), caused serious damage almost everywhere in Europe [3,11,12,13]

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