Beech Bark Disease Impacts Information

Contents

1.0 Introduction 2.0 Damage to host species 3.0 Ecosystem change 4.0 Reduction in native biodiversity 5.0 Nutrient cycling 6.0 Economic 7.0 Human health 8.0 Other

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1.0 Introduction bark disease (BBD) is caused by the combined impacts of beech scale ( fagisuga) and several species of ascomycete fungi in the genus Neonectria. BBD affects American () and European beech (F. sylvatica). The basic model of BBD is: beech trees + beech scale + Neonectria sp. = BBD (Ehrlich, 1934 in Houston, 1994a). Two principal species of Neonectria fungi are associated with BBD in North America. The probably introduced Neonectria faginata only infects F. grandifolia and is the main species involved with the disease. Native N. ditissima (N. galligena) affects a range of tree species, including beech (Houston, 1994a). In many cases N. faginata spreads to stands infected with N. ditissima and replaces this species as the dominant pathogen (Houston, 1994b; Kasson et al., 2009). A third species N. ochroleuca (now named Bionectria ochroleuca) has been found in association with BBD in some regions of the United States (Houston, 2005). In Europe the fungi associated with BBD are N. ditissima and N. coccinea (Twery & Patterson, 1984; Castlebury et al., 2006).

2.0 Damage to host species The beech feeds on host parenchyma cells which collapse and die, resulting in small fissures on the bark that allow Neonectria to enter the tree. The most important factor determining the course of infection of beech by Neonectria species is the infestation of beech scale prior to infection. Heavy infestations of scale allow Neonectria to spread rapidly within the bark (Houston, 1994a). The arrival of Neonectria follows the arrival of the scale insect by up to a decade (Dukes et al., 2009), but usually occurs 3 to 6 years after scales infest an area (Shigo, 1963 in Fernandez & Boyer, 1988). As the fungal mycelia grow, large areas of tissues become weakened and die, sometimes causing cankers on the trunk and branches. Sometimes red-brown liquid oozes from the bark tissues killed by the fungi, and the foliage of severely affected trees may become sparse and turn yellow (LeGuerrier et al., 2003). If enough tissue is killed the tree will be girdled and die (Koch et al., 2010). The course of the disease may take as little as two years, but other trees may linger for several years. Sometimes the secondary scale insect Xylococcus betulae may also infest diseased trees. This often contributes to stem deformity or further infection by Neonectria spp. (Teale et al., 2009). Ambrosia beetles may also bore into dead sapwood, and

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other fungi such as Hypoxylon and Armillaria may secondarily invade which further weakens the tree (McCullough et al., 2003). The pattern of development of BBD has been divided into three phases. 1) the area of establishing scale insect populations forms the “advancing front”. Ecosystem change is minimal in this phase, but damage caused by the insect predisposes trees to infection by Neonectria spp; 2) high insect and high fungal populations and high mortality form the “killing front”. This often results in 50-85% mortality of large trees within 10 years; and 3) the regions where BBD is endemic constitute the “aftermath zone”. This consists mostly of small trees of root sprout origin. In aftermath forests mortality of trees tends to be low, but surviving small diameter trees are often distorted and less vigorous (Shingo, 1972 in Gavin & Peart, 1993; Loo, 2009; Twery & Patterson, 1984; Houston, 1994a; Morin et al., 2007).

3.0 Ecosystem change Much research has suggested that BBD mainly affects large, older trees, and may cause up to 80% mortality of beech within a stand (Houston, 1994a). Death of older trees leads to gradual gaps in the canopy. This gives the opportunity for other tree species to take over, sometimes leading to drastic changes in the composition and structure of stands (Twery & Patterson, 1984; Runkle, 1990; Wiggins et al., 2004). Particularly in stands dominated by BBD-tolerant species such as eastern hemlock (Tsugacanadensis) and sugar maple (Acer saccharum); these species dominate and American beech is expected to become a minor component of the stand (Twery & Patterson, 1984; Runkle, 1990). Disease damaged trees become prone to snapping in high wind, a condition called “beech snap” (McCullough et al., 2003; Papaik et al., 2005; Koch et al., 2010). This increases the amount of coarse woody debris (CWD) two to four times in stands affected by BBD. Higher CWD, which is an important seed bed substrate for trees such as hemlock and yellow birch, and higher light levels due to canopy thinning of BBD could further increase these species and reduce American beech abundance (Papaik et al., 2005).

However the course of vegetation change after death of large overstorey beech trees appears to vary considerably between forest stands, and can depend on forest composition, age, slope, susceptibility of beech trees and dispersal of the scale insect (Latty et al., 2003; Lovett et al., 2006; Munck & Manion, 2006). In most forest stands BBD favours the development of dense beech thickets that interfere with the regeneration of other trees (Houston, 1975; Houston,

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1994a; Garnas et al., 2011), due to beech’s propensity to reproduce vegetatively via adventitious root sprouts, especially from damaged root sprouts (Garnas et al., 2011). Thus in many forests there is actually an increase in beech volume accumulation, particularly 10-20 years after BBD invasion (Morin et al., 2007). In fact BBD represents the first known case where a specialist plant pathogen actually increases the density of its host (Garnas et al., 2011). The main effect of many forests infected with BBD is a strong shift in the size of beech trees, with small trees between11-50 cm increasing (Houston, 1994a). A large scale analysis of data from over 125,000 plots found that the magnitude of this effect increased approximately linearly with the duration of BBD infection in a region. Thus, in forests infected for > 50 years with BBD, an average square kilometer will generally not contain a single beech > 90 cm DBH. This compares to disease free forests that have an average density of c. 70 beech per square kilometer with > 90 cm DBH (Garnas et al., 2011). As small trees grow they too become infected with BBD, and thus are often deformed and less vigorous. These trees do not usually attain dominance in the canopy and are easily broken (Loo, 2009). Increased competition for light and space from increased density of small beach limits the abundance and diversity of many plant species that are adapted to the understorey of mature forests (Schemske et al., 1978 in Garnas et al,. 2011). Sugar maple seedlings may also be suppressed by dense thickets of beech saplings due to shading. Thus BBD indirectly influences the regeneration of other species. Regeneration failure in sugar maples may have major implications for future forest composition in northern hardwood forests as it currently makes up around 30% of forest biomass (Hane, 2003). “Long-term effects of on northeastern North American forest ecosystems are likely to be substantial, but the full implications are yet to be understood as the disease is still in progress. For example, it is unknown whether aftermath-zone forests will be stable over time, or if subsequent outbreaks will occur, resulting in periodic elevated mortality. Some of the diseased but relatively vigorous trees in the aftermath-zone may attain large size and canopy dominance, or they may be destroyed in windstorms because of a greater tendency to stem breakage. Continued evaluation of ecosystem change and beech development in aftermath-zone forests is thus important” (Loo, 2009). LeGuerrier et al. (2003) used a modeling approach in an attempt to assess the long-term impact of BBD. They predict the disappearance of large trees and an increase in 11-50 cm DBH trees in the first 50 years, but a progressive reestablishment of the species population structure after 300 years (LeGuerrier et al., 2003).

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4.0 Reduction in native biodiversity American beech provides important food and habitat for many species of , birds and mammals (Storer et al., 2005; McCullough et al., 2003). The masting aspect of beech biology (beech usually produce beechnut every second year) is particularly important to some species of birds and mammals that rely on this food source, including, gray squirrels, chipmunks, turkeys and deer (McCullough et al., 2003), and particularly black bears (Ursus americanus) (Jakubas et al., 2005). Branching characteristics of beech trees make them attractive to raptors for perches, and several species of hawks prefer to nest in beech trees. Standing beech trees that have some decay are used for nesting, roosting or insect foraging by a variety of birds, ranging from pileated woodpeckers to nuthatches. Pine martens, fishers and other mammals use cavities in beech trees for shelter or dens (McCullough et al., 2003).

Trees begin to produce substantial nut crops at about age 40; as BBD mainly affects older trees, this disease may significantly reduce this important food source. Thus changes to abundance and class structure of this tree can negatively impact the that use American beech (Wiggins et al., 2004), and such changes are likely to ramify through direct and indirect linkages throughout the food web (Lovett et al., 2006). Effects on wildlife will depend on density and size of beech trees in a stand and availability of alternative mast producers (McCullough et al., 2003).

The abundance of wood-inhabiting of beech may also be affected by BBD (Beachy & Storer, 2005).

5.0 Nutrient cycling In stands where American beech dominance is affected, nutrient cycling dynamics may be altered. Beech leaves have higher lignin concentration and break down more slowly than sugar maple and yellow birch leaves (Lovett et al., 2006). Thus replacement of beech by maple results in reduced forest-floor mass, increased nitrification, increased leaching of nitrate into stream water, and decreased retention of atmospherically deposited nitrogen (Lovett et al., 2006). In other cases there is a gradual shift to a greater proportion of hemlock and yellow birch in the canopy. A shift to hemlock may result in the opposite effect to that of increase maple; with increased forest-floor mass, and less nitrogen loss (Lovett et al., 2006).

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6.0 Economic Beech bark disease may have economic consequences through the loss of large trees (Twery & Patterson, 1984). For example, in 1977 the loss of merchantable timber volume caused by BBD in Vermont was estimated at 300 million board feet (Miller-Weeks, 1983 in Houston, 1994a).

However, in general, American beech has never been highly valued for lumber or veneer. Indeed BBD may actually provide an incentive for harvesting of young trees for pulp (Garnas et al., 2011).

7.0 Human health Beech snap poses a hazard to people and personal property where large trees occur in campgrounds, recreation areas or near homes (McCullough et al., 2003; Heyd, 2005).

8.0 Other Dense stands of small stemmed beech are often formed after infection by BBD. Such stands may tend to become candidates for conversion to land for agriculture or housing, as they are less aesthetically valued than old large beech trees (Peattie, 1991 in Garnas et al., 2011).

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