Drastic Decline of Eastern Hemlocks After Infection by the Woolly Adelgid in Harriman State Park, New York

Alexander D. Koutavas1 and Marvin L. Cadornigara2*
Student1, Teacher2: New Explorations into Science, Technology and Math
111 Columbia Street, New York, NY 10002
*Correspondence: MCadornigara@schools.nyc.gov



Eastern hemlocks in Harriman State Park of New York, as in much of the eastern United States, are under attack by the hemlock woolly adelgid (HWA), an invasive pest that kills trees by desiccating and poisoning their needles. To place the HWA outbreak in the park in a broader context, we investigated historical growth variations in hemlocks using dendrochronology – the study of tree rings. Analysis of cores taken from hemlock trees revealed vigorous growth during AD 1875-1980, followed by a sharp decline in AD 1981-1982, swift recovery, and a second permanent crash in AD 1987. We attribute the temporary first decline to the widespread gypsy moth outbreak of 1981, and the second and more serious one to permanent infestation by the HWA. To firmly implicate HWA as the cause of this decline, the role of climate factors was also investigated by correlation of the tree ring data with historical climate records. It was found that hemlocks are sensitive to summer moisture availability, and can experience growth declines during episodes of drought. While early hemlock declines in the data correlate with historic droughts in AD 1909-1913 and AD 1962-1966, the more extreme declines after AD 1980 do not, pointing to the gypsy moth and woolly adelgid as the most probable causes. Although the studied trees have now survived more than 20 years after the arrival of HWA, they are not on the road to recovery. While on one hand wet climate conditions may be contributing to prolonged hemlock survival, on the other hand wintertime warming is favoring the reproduction and spread of the HWA, which is intolerant of cold temperatures. At the moment the HWA appears to be winning this battle and to be tightening its grip on infested trees. If winter warming continues in future years as predicted by climate models it will likely further tilt the odds in favor of the HWA, causing more widespread hemlock mortality.


Eastern hemlock (Tsuga canadensis) is a prolific and ecologically important tree in eastern North America notable for its longevity, extreme shade tolerance, and ability to form dense stands with a thick canopy that provide habitat for about 120 species of vertebrates and 90 species of birds1. Hemlock’s geographic range extends from Georgia in the south to Maine and Canada in the north, and Minnesota in the west. Since AD 1950, more than half of its geographic range in the eastern USA has been infested with the Hemlock Woolly Adelgid (HWA, Adelges tsugae), an invasive insect originating in southern Japan2, 3. HWA causes severe dehydration and poisoning of hemlock needles leading to death usually within 4 to 10 years.

Extensive mortality from HWA is now changing the structure of eastern forests. Hemlock dominated evergreen forests are being replaced by deciduous broad-leaved hardwoods such as birch and maple. The loss of hemlock habitat is affecting the abundance of bird species4, and increasing the spread of invasive plants benefiting from greater light reaching the forest floor5. The large scale on which these ecosystem changes are occurring makes it important to understand hemlock response to HWA under varying climatic and environmental conditions. Such knowledge may inform appropriate strategies for combating HWA infestation in various settings, or restoring affected ecosystems in the aftermath1.

This study aimed to investigate the effects of HWA in Harriman State Park in New York, a location with recognized presence of HWA and evidence of significant hemlock mortality assumed to be a consequence of the infection. HWA arrived in New York State in the 1980s at a time of significant climate changes characterized by warming temperatures but also increasing precipitation and amelioration of drought. A key question is whether the effects of HWA were aided or opposed by climate factors, and if so, what that might imply for the future. To address this question this study employed dendrochronologic methods6, 7 to establish the timing, severity and progression of HWA infection in living trees from an affected population in Harriman State Park.

The main conclusion is that the arrival of HWA in the 1980s reversed nearly a century of increasing hemlock growth related to a trend toward wetter climate. Despite prevalence of a favorable wet climate, the hemlocks experienced unprecedented declines in the last 30 years, first due to a short outbreak of gypsy moth in AD 1981-1982, and then to permanent infestation by the HWA beginning in AD 1987. These trees are currently under severe growth stress from a tight grip of the HWA, which is favored by increasing wintertime temperatures8. Judging from the high level of hemlock mortality in the park, the ill appearance of remaining living trees, and the precipitous growth decline evident in their tree rings, the odds of hemlock survival in this park appear grim.

Materials and Methods

Dendrochronologic methods6, 7 were employed in acquiring and studying tree ring samples from Harriman State Park, New York, approximately 40 miles north of New York City. A 5-mm Haglof increment borer was used to extract 21 cores from 11 living hemlock trees at a site located between lakes Stahahe and Kanawauke. The core samples were dried at room temperature, glued onto wooden mounts, and smoothed to a flat surface with sandpaper of increasingly fine grit, i.e., 120, 220, 320, 400. The finished samples were examined under a binocular microscope and their rings were assigned calendar years by counting backwards from the bark.

Crossdating, which is the matching of growth patterns from sample to sample, was possible due to distinctive growth years. Years 1935 and 1961 were observed to have unusually wide rings, while years 1944, 1950, 1965-66, and 1982 had narrow rings. Crossdating was not possible between 2000-2010, as many samples displayed erratic growth patterns that could not be matched. These were most likely related to infection by the HWA. After dating, the ring width was measured to a precision of 0.001 mm using a stereomicroscope, Velmex linear encoder system9, and Project J2X software10.

To develop a mean tree-ring chronology, i.e. an average sequence of tree growth over time from all samples, the data must first be standardized, because different trees grow at different rates and have biological growth trends that vary considerably from one tree to another. Standardization was accomplished by fitting a theoretical growth curve to the measured data, and taking the ratio of measured and theoretical value for each year7. The standardization and chronology calculations were made with the program ARSTAN, accessed through the National Oceanic and Atmospheric Administration (NOAA) Paleoclimatology Program website11.

To investigate the relationship of hemlock growth to climate, the tree-ring data were correlated with the monthly climate data from New York State Climate Division 5 (Hudson Valley Division, which includes Harriman State Park) from NOAA12. Correlations were expressed with the Pearson’s correlation coefficient (r) between the Harriman hemlock chronology and the monthly temperature, precipitation, and Palmer Drought Severity Index (PDSI) for 105 years of data (AD 1895-1999). Tree ring data from AD 2000-2010 were excluded because of the poor crossdating and erratic growth patterns attributed to HWA.


Typical examples of living and dead hemlock trees from the study site are shown in Figure 1, along with a core sample collected at the site. The measured ring-width data from all 21 cores collected are shown in Figure 2A. Raw tree-ring data are difficult to interpret because the variable length of individual samples, different growth rates, and biological and geometric effects can produce artifacts when calculating a simple arithmetic average. The solution to this problem is to standardize each data series by dividing raw measurements with a theoretical growth curve fitted to the raw data7. This procedure, known as standardization by ratios, was applied to each sample and the resulting standardized data are shown in Figure 2B. When averaged, these standardized data provide what is called a “standard tree-ring chronology” (red line in Figure 2B). The standard chronology for the Harriman hemlocks shows a long-term growth increase during AD 1875-1980 followed by precipitous declines during AD 1981-2010.

  • Fig1
  • Figure 1. Living (upper left) and dead (upper right) eastern hemlocks in Harriman State Park. Core sample (bottom) showing growth rings with time proceeding from right to left. The decadal years 1990 and 2010 are marked with a single black dot. The millennium year 2000 is marked with four black dots. Note the dramatic decrease in ring size over the 1990s and 2000s.



  • Fig2a
  • Figure 2. (A) Raw ring-width measurements (mm) from the 21 core samples of eastern hemlocks. The red line is the average of all the samples. (B) Standardized growth indices calculated as explained in the text. The red line is the average of all samples. Note the striking drop in AD 1982, followed by recovery, and more permanent decline starting in AD 1987.



Following standard dendrochronologic practice, the first step in understanding the origins and causes of the observed growth patterns is to evaluate the role of climate variability7. To this end, the dominant relationships between growth and various climate factors must be assessed by correlation of the growth chronology with contemporaneous records of climate. The presence of correlation is interpreted to indicate a causative effect on growth, which may be either positive (positive correlation) or negative (negative correlation). For this analysis, correlations were calculated with annual and monthly temperature, precipitation, and PDSI for each month starting with May of the previous year and ending in December of the growth year. The resulting Pearson’s correlation coefficients (r) are graphed in Figure 3.


  • Fig3
  • Figure 3. Correlations between the Harriman tree-ring chronology and monthly temperature (TMP), precipitation (PCP) and PDSI for the period 1895-1999. Correlation coefficients (R) outside the shaded region are statistically significant (p<0.05). The most important significant correlations are marked with black arrows. Months of the previous year are indicated with the letter “p” (pMay-pDec).



With respect to temperature, the results show no significant correlation, except for June of the previous year (pJun) (r=-0.23). This correlation is weak and not reinforced in June of the growth year so it is not considered meaningful. With respect to precipitation, significant positive correlations are observed in July of the growth year (r=0.33), July of the previous year (r=0.29), and total annual precipitation in the growth year (r=0.25). Although these correlations are low they are statistically significant and consistent, suggesting a real positive effect of precipitation on growth especially in July. This finding is further reinforced by correlations with PDSI. An index of soil dryness, PDSI takes into account not only moisture supply by precipitation but also loss to evapotranspiration which is temperature dependent13. Consequently, PDSI is a more relevant indicator of available moisture for plant growth. Negative PDSI values indicate droughts while positive values indicate wet spells. Significant positive correlations are found between growth and all monthly PDSI series except November and December of the growth year. The highest correlations are in July of the growth year (r=0.49) and July of the previous year (r=0.47), followed closely by June and August of both years, and by the annual average (r=0.39). These correlations show that hemlock growth is sensitive to soil moisture during summer, especially in July. They also show that this sensitivity is not restricted to the year of growth but lingers into the following year, so that humid conditions during a given summer have a positive effect on growth in the following summer as well. In summary, this analysis reveals that climate, especially summer precipitation and PDSI, has a significant effect on hemlock growth. This result is similar to what was found previously for hemlocks growing in Mohonk Mountain about 40 miles north of the study site, where a correlation of 0.495 was found between hemlock growth and July PDSI14.

In Figure 4, the hemlock chronology is compared to the variations in July PDSI during AD 1895-2010 to assess the possible role of drought on hemlock growth patterns. PDSI has a positive trend during this period indicating that the climate is becoming wetter. The hemlock chronology has a similar increasing trend from AD 1875 until about AD 1980, which can reasonably be attributed to the positive growth influence of the wetter climate. The positive trend in July PDSI was interrupted by two major multi-year droughts, first in AD 1909-1913 and again in AD 1962-1966. During each of these events the hemlocks experienced reductions in growth with recovery one to two years after the drought ended.  On the other hand single-year droughts, for example in AD 1949, did not cause large reductions in growth, and their effects were more likely to be felt in the following year. Relatively mild droughts, as for example in the 1950s also did not cause noticeable growth reductions. These observations indicate that hemlocks are resilient to short dry spells in isolated years, or to longer ones if the severity of drought is not too great (PDSI >-2). Significant adverse effects on growth result from multi-year droughts of relatively high severity (PDSI <-2).

Figure 4. Comparison of July PDSI variations in New York Climate Division 5 (red) with the hemlock tree-ring chronology from Harriman State Park (black). The error bars in the July PDSI are one standard deviation (±) of the full dataset.  The vertical blue bars highlight significant declines in hemlock growth. The declines in the early 1910s and mid-1960s were drought related. The sharp declines in the 1980s, marked with blue stars, are attributed to the gypsy moth in 1981-82, and the woolly adelgid in 1987.

  • Fig4


While declines in hemlock growth prior to AD 1980 appear to be causally linked to droughts, a very different pattern is present in the following 30 years. The marked decrease in growth seen in AD 1982 is not easily explained as a consequence of drought. Mild drought conditions were present during AD 1978-1981 but the PDSI did not drop below -2, and yet growth in AD 1982 was much lower than during the more severe drought of the mid-1960s. In fact, AD 1982 was associated with a micro-ring in many of the samples, meaning that growth was extremely limited and barely discernible. This is an important clue that something very unusual happened that year. It is tempting to think of this as the first instance of HWA infestation in these trees. However, the strong recovery in AD 1983 indicates that this was an acute but transient decline, whereas HWA infection is chronic and has lasting effects. It is proposed that the 1982 growth crash was related to a well-documented widespread outbreak of the gypsy moth caterpillar (Lymantria dispar), which peaked in 1981 and defoliated 12.9 million acres of forest in the northeast USA15. Young gypsy moth larvae are known to prefer hardwoods like oak and birch, but older larvae can feed on hemlock and cause significant harm. According to the U.S.D.A. Forest Service: “Although not preferred by the larvae, pines and hemlocks are subject to heavy defoliation during gypsy moth outbreaks and are more likely to be killed than hardwoods”15. The severe growth reduction of 1982 appears to have actually begun in 1981, consistent with the timing of the gypsy moth. Growth recovered in AD 1983-1986 although not to levels seen in the 1970s. Growth plummeted again in AD 1987-1988 in association with a mild two-year drought (PDSI >-2). Based on the prior history of growth we would not expect such PDSI values to have a serious impact on growth, yet the AD 1987-1988 decline was severe and irreversible. The first appearance of HWA in New York State has been estimated around AD 1985-199016, which corresponds well with the observed crash in growth in AD 1987-1988 and the sustained decline thereafter. Based on these tree ring data it can be inferred with a high level of confidence that by AD 1987 HWA was firmly established in Harriman State Park and had began to cause serious harm.


Our results are based on a small number of trees (N=11) and therefore caution is required when extrapolating to a broader scale. However the recorded signals in the tree rings are very distinct and allow some useful inferences to be drawn. HWA usually causes hemlock death within 4 to 10 years. Considering that the studied trees were most likely infected by AD 1987 they have now survived over 20 years – longer than expected. We attribute their prolonged survival to the beneficial role of the wet climate conditions that have prevailed in the 1990s and 2000s, which eliminated contributing stress from drought. On the other hand there is no sign that these trees are now recovering; their growth rate is the lowest it has been since AD 1875, indicating severe stress from the HWA. Their appearance was also consistent with ailing health. They retained a small crown on top but their lower and mid-section limbs were devoid of needles (Figure 1) revealing the characteristic pattern of progressive limb dieback upward from below. It is difficult to say whether these trees and many others like them in Harriman Park can overcome their plight and recover, but considering their evident ill health and the widespread hemlock mortality that surrounds them, it is suspected that they too will eventually succumb. In this sense the positive influence of recent humid climate may only be delaying the inevitable and protracting their death. Projections of climate change in the northeast USA are uncertain with respect to precipitation but more certain with respect to temperature, with warming expected in both summer and winter17. Cold winter temperatures can limit the reproduction and spread of the HWA8, and could help alleviate the infestation. Projected future warming instead is likely to further promote the infestation and reduce the odds of hemlock survival. It is feared that it is only a matter of time before Harriman State Park becomes entirely hemlock-free, a sad outcome for this majestic tree. Even sadder, this is only a small-scale example of a devastating process affecting hemlocks and their unique ecosystems throughout the eastern USA.


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Special acknowledgement goes to the following: New York State Office of Parks, for granting permission to obtain tree samples. Professor Athanasios Koutavas, College of Staten Island, City University of New York, for offering assistance and guidance with this research.