Landscape Response to Hemlock Woolly Adelgid in Southern New England 1997-2011

The hemlock woolly adelgid (Adelges tsugae Annand; HWA), a small, aphid-like
insect native to Japan, is currently migrating northward through eastern North
America and threatens to eliminate eastern hemlock (Tsuga canadensis (L.) Carriere),
one of the most abundant, long-lived shade tolerant species, across its range. In
addition, a second invasive pest, the elongate hemlock scale (Fiorinia externa; EHS),
often co-occurs with HWA on hemlock with unknown long-term consequences. The major
objectives of this study were: (1) To characterize the pre-HWA distribution,
composition, and structure of hemlock stands; (2) to characterize the spatial
patterns of damage generated by HWA across southern New England since the time of its
arrival in 1985; (3) to examine environmental and stand factors that are associated
with declines in crown vigor and mortality of hemlock; (4) examine the dynamics of
HWA and EHS in hemlock stands over time; and (5) assess whether there is a difference
in the response of these insects to abiotic conditions (winter temperature, summer
temperature, summer precipitation), and whether the distribution and abundance of
each insect species is dependent on biotic interactions with the co-occurring insect
species. Connecticut Stands (originally sampled in 1997 and 1998) Aerial photographs, photographic overlays, and extensive field study were used to
map and develop GIS overlays of almost 6000 hemlock stands in a 4900 km2 study region
extending from Long Island Sound northward to the Massachusetts border and including
the Connecticut River Valley in Connecticut, USA. Intensive sampling of a random
selection of 114 hemlock stands across the study area was used to document patterns
of hemlock infestation, vigor, and mortality in relation to stand and site
characteristics. Mantel tests were utilized to assess the relative importance of
environmental and stand variables in controlling the intensity of HWA infestation and
damage. Most stands were located along ridge tops, steep hillsides, and narrow
valleys. Hemlock importance values ranged from 22 to 96% and stand densities varied
from 300 to 1450 stems ha-1. Adelgid presence and adelgid-induced hemlock mortality
were found in 88% and 74% of the sampled forests, respectively. Approximately 25% of
stands were logged recently, ranging in intensity from partial hemlock cutting to
large clearcuts. A geographical trend in reduced HWA infestation intensity and tree
mortality and enhanced crown vigor of overstory and understory hemlock occurs from
south to north, coincident with the temporal colonization pattern of HWA. Mantel
analyses indicated that patterns of HWA infestation, hemlock mortality, and crown
vigor were most strongly correlated with latitude. Mortality was also weakly related
to aspect and stand size. Average mortality was highest on western aspects but
exceeded 20% on most slopes. Remaining trees averaged over 50% foliar loss, with no
significant difference among aspects. Results suggest that as HWA becomes abundant,
stands on xeric aspects succumb rapidly, but that stand and landscape variables such
as overstory composition and structure, slope, and elevation, exert little control
over susceptibility or eventual mortality. Ultimately, duration of infestation
controls the intensity of hemlock decline and mortality. Over 4,290 ha of hemlock
forest were eliminated by logging or HWA just within the southern part of our
transect since the mid-1980s, and we predict continued HWA infestation will lead to
unprecedented hemlock loss throughout the northeastern U.S., regardless of site
conditions or location. Massachusetts Stands (originally sampled in 2002-2004) Using aerial photographs, we documented hemlock distribution throughout central
Massachusetts and subsampled 123 stands to examine the spatial pattern of HWA and its
impact on tree vigor and mortality since its arrival in 1989. In the study region,
over 86,000 ha of hemlock forest were mapped in 5,127 stands. White pine (Pinus
strobus), red oak (Quercus rubra), red maple (Acer rubrum), and black birch (Betula
lenta) were common overstory associates. Hemlock abundance increased from south to
north, commonly on western and northwestern slopes. Average stand size was 55 ha,
overstory basal area ranged from 23 to 55m2 ha-1 and overstory stem densities
averaged 993 ha-1. By 2004, 40% of sampled stands were infested, but most stands
remained in good health overall; only 8 stands contained high HWA densities and only
two had lost >50% overstory hemlock. Out of fifteen stand and landscape predictor
variables examined, only latitude and winter climate variables were related to HWA
density. Cold temperatures appear to be slowing the spread and impact of HWA at its
northern extent as HWA infestation intensity and hemlock mortality and vigor were
significantly correlated with average minimum winter temperature. Contrary to
predictions, there was no regional increase in hemlock harvesting. The results
suggest that regional HWA-hemlock dynamics are currently being shaped more by climate
than by a combination of landscape and social factors. The persistence and migration
of HWA continues to pose a significant threat regionally, especially in the northern
portion of the study area, where hemlock dominates many forests. Insect dynamics over time We created a multivariate spatio-temporal model of the joint abundance of HWA and
EHS, which share a host plant. Data on abundance of both insects were examined from
the time the stand was initially sampled and again in 2005, 2007, 2009, and 2011. The
model revealed differences in the way each species responded to weather extremes, and
indicated a commensalism in which high abundance of HWA increased the probability of
subsequent infestation by EHS– but not vice versa. This study provides evidence that
incorporating biotic interactions into a species distribution model indicates whether
a species’ distribution and abundance is dependent on other species in the community,
and importantly, improves estimation of the abiotic niche for species whose abundance
depends on interactions with other species in the community.