temporal variability in climate response of eastern hemlock in …storage.googleapis.com ›...

southeastern geographer, 55(2) 2015: pp. 143–163

estudios de anillos de árboles. Las posibles causas

de los cambios en esa relación son numerosas y

los factores a menudo covarían en el espacio y el

tiempo, complicando el desarrollo de las recon-

strucciones climáticas y la conservación de espe-

cies forestales fundacionales. Aquí se presentan

los resultados de un estudio que examina la

respuesta climática del abeto canadiense (Tsuga

canadensis (L.) Carrière) en la latitud media de

la región central de Appalachia. Se recogieron

muestras de núcleo de árbol dentro del parque

estatal Pipestem Resort, situado en el sur de West

Virginia. Para esta área de estudio se desarrolló

una cronología de anchura de anillo que abarca

145 años desde 1868 hasta 2012, y se analizó la

relación entre el clima y el crecimiento para el

período entre 1896 y 2012. Nuestros resultados

indican que la respuesta de crecimiento del abeto

canadiense al clima ha variado con el tiempo, par-

ticularmente en respuesta a un cambio abrupto a

temperaturas más bajas del promedio durante la

mitad del siglo XX y como resultado de una plaga

de pulgón de la tsuga (Adelges tsugae Annand).

key words: dendrochronology, climate, tree

growth, hemlock

palabras clave: Dendrocronología; clima;

crecimiento arbóreo; abeto canadiense

Shifting climate-growth relationships in high-

latitude northern hemisphere forests have been

demonstrated by numerous tree-ring studies. The

possible causes of these shifting climate-growth

relationships are numerous and factors often

co-vary across space and time, complicating the

development of climate reconstructions and the

conservation of foundational forest species. Pre-

sented here are the results of a study examining

the climate response of eastern hemlock (Tsuga

canadensis (L.) Carrière) in the mid-latitude

Central Appalachian region. Tree core samples

were collected within Pipestem Resort State Park,

located in southern West Virginia. A composite

ring-width chronology was developed for the study

area, spanning 145 years from 1868 to 2012 and

climate-growth relationships were analyzed for

the period 1896–2012. Our results indicate that

hemlock growth response to climate has varied

over time, particularly in response to an abrupt

shift to below average temperatures during the

mid-20th century and as a result of hemlock woolly

adelgid (Adelges tsugae Annand) infestation.

El cambio en la relación entre el clima y el creci-

miento en los bosques de latitudes altas del hem-

isferio norte se han demostrado en numerosos

Temporal Variability in Climate Response of Eastern Hemlock in the

Central Appalachian Region

THOMAS SALAdYGAConcord University

R. STOCKTON MAXWELLRadford University

144 saladyga and maxwell

introduction

Abrupt, decadal scale shifts in regional climate patterns impact forest ecosys-tems by increasing physiological stress, which can result in decreased resilience to forest pest and pathogen outbreaks (Walther et al. 2002; Dukes et al. 2009) or widespread drought-induced mortality (Adams et al. 2009; van Mantgem et al. 2009; Allen et al. 2010; Williams et al. 2012). Sensitivity of tree species to abrupt climate change is typically greatest at geo-graphic range margins, or ecotones, where a species is already at its physiolgical and ecological limits (Fritts 1976; Peteet 2000; Hampe and Peteet 2005; Murphy et al. 2006). Shallow rooting and the presence of steep slopes and unique edaphic condi-tions such as thin, well-drained, nutrient poor soils are also site indicators of in-creased climate sensitivity in tree species (Fritts 1976). More recently, however, drought sensitivity has been demonstrated at non-traditional sites in the decidous forests of eastern North America (Maxwell et al. 2011; Pederson et al. 2012). Further understanding of climate-growth relation-ships in the context of recent rapid climate warming and environmental changes (Foster and Rahmstorf 2011; IPCC 2013) will be particularly useful when utiliz-ing tree rings to reconstruct past climate changes.

A number of tree-ring studies have demonstrated shifting climate-growth relationships in northern forests when comparing early to late 20th century tem-perature conditions. Some studies high-light decreased temperature sensitivity (or “divergence”) in high-latitude north-ern forests to late 20th century warming (Jacoby and D’Arrigo 1995; Briffa et al.

1998; Büntgen et al. 2006; Pisaric et al. 2007). Others have demonstrated a switch from a positive to negative temperature- growth relationship ( D’Arrigo et al. 2004; Wilmking et al. 2004), while seasonal shifts in temperature sensitivity during the late 20th century have occurred in other northern forest sites (Wilmking et al. 2008; De Grandpré et al. 2011; Weber et al. 2013). Possible causes to these shifting climate-growth relation-ships include increasing winter tempera-tures and the lengthening of the growing season in northern latitudes (De Grandpré et al. 2011), shifts in synopic-scale climate occillations (e.g., Maxwell et al. 2012), local industrial emmissions (Visser and Molenaar 1992; Wilson and Elling 2004), and ‘global dimming,’ a recent decrease in the amount of solar radiation availa-ble for photosynthesis, having a negative impact on tree growth at high northern latitudes (D’Arrigo et al. 2006; Mercado et al. 2009; Wild 2009). Regardless of the cause, these changing climate-growth re-lationships have generally been limited to high latitude northern forests. How-ever, recent studies have demonstrated similar changes in mid-latitude Asia (Zang et al. 2009; Fang et al. 2010; Zang and Wilmking 2010; Zang et al. 2013). Advancing our understanding of dynamic climate-growth relationships is of par-ticular importance in the northern mid- latitudes where forest ecosystem resilence can and will potentially be compromised by current and predicted rapid changes in climate conditons (IPCC 2013). This study examines the growth response of eastern hemlock (Tsuga canadensis (L.) Carrière) to 20th century changes in temperature and precipitation conditions in the Central Appalachian region.

Climate Response of Eastern Hemlock 145

The geographic range of eastern hemlock (subsequently referred to as ‘hemlock’) extends from Nova Scotia west to northern Wisconsin and Minne-sota and south through New England to the southern Appalachian Mountains. Isolated stands are found in Ohio, In-diana, and Kentucky along its western margins and in northern Alabama and Georgia in the south (Godman and Lan-caster 1990). Hemlock is valued for its longevity and late-successional status (Onken 1995), growing best throughout its central and southern range on north- and east-facing slopes where soils remain moist throughout the year and tempera-tures remain relatively cool. The species can be found on a variety of substrates including well-drained rocky, acid soils and loams and moist benches and flats (Hough 1960). Regardless of substrate, hemlock roots are situated near the soil surface making the species sensitive to extreme drought or high moisture con-ditions (Graham 1943). Paleoecological evidence (i.e., lake sediment, pollen, and macrofossil records) indicates that drought (Hass and McAndrews 2000; Foster et al. 2006) and the hemlock looper (Lambdina fiscellaria Guenée), a defoliating insect native to North Amer-ica (Bhiry and Filion 1996), were the major drivers of rapid hemlock decline during the mid-Holocene. More recently, however, hemlock forests are experienc-ing rapid decline due to the hemlock woolly adelgid (HWA; Adelges tsugae An-nand), an invasive forest pest that feeds on the sap of hemlock twigs.

Introduced from East Asia to central Virginia in the early 1950s (Souto et al. 1996), HWA has caused widespread hemlock mortality from north Georgia to

southeastern New Hampshire. Alterations in forest composition, structure, and func-tion have been documented as a result of HWA-induced hemlock mortality. Spe-cifically, these changes include increased presence of invasive species (Eschtruth et al. 2006), a conversion to hardwood forest types (Spaulding and Rieske 2010), an accumulation of coarse woody debris (Orwig and Foster 1998), and alterations to the carbon cycle due to increased inputs of organic material (Nuckolls et al. 2009). The decline of this foundational forest spe-cies will undoubtedly have lasting impacts on associated wildlife habitats and regional forest dynamics (Orwig et al. 2012; Martin and Goebel 2013; Orwig et al. 2013). While the radial growth of hemlock has been shown to decline following HWA introduc-tion across the species range (Rentch et al. 2009; Walker 2012), previous work has not considered the effect of HWA on the climate-growth response of the species. Alternatively, the climate-growth response of hemlock across the species range has been investigated without reference to the more recent HWA infestation (Cook and Cole 1991; D’Arrigo et al. 2001; Hart et al. 2010). Neither approach investigated the potential change in radial growth response to climate over time or sought to differenti-ate the effects of climate regime shifts from HWA infestation.

In this study, we investigate the tempo-ral variability in climate response of hem-lock at one site in the Central Appalachian region in the context of past climatic change and more recent HWA infestation. The general objectives of this study were to identify unique patterns in the instru-mental temperature and precipitation records in southern West Virginia and ex-amine the influence of climate on annual


hemlock growth. Specific objectives were to 1) develop a tree-ring growth chronol-ogy in an area in West Virginia lacking tree-ring records; 2) analyze relationships between climate and the annual growth of hemlock and 3) assess temporal changes in climate-growth relationships in the con-text of HWA infestation. The use of hem-lock in this study provided a unique oppor-tunity because the regional dendroclimatic record is diminishing rapidly due to HWA infestation (Hessl and Pederson 2013). Furthermore, results will facilitate under-standing of hemlock decline and mortality in the context of rapid climate changes.

methods

Study AreaTree core samples were collected

within Pipestem Resort State Park (PIPE), located in southern West Virginia in Mercer and Summers Counties (Figure 1), where HWA was detected in 2000 (WVDA 2007). The meandering Bluestone River Gorge is the dominant physical feature of the approximately 1720 ha park, with steep slopes rising up from the river and hemlock-dominated stands covering north-facing slopes. Minimum and max-imum elevations within the park are 453 m and 903 m, respectively. We established eight monitoring plots in June 2013 for the purpose of assessing long-term im-pacts of HWA on forest structure and com-position (Table 1, Figure 1). While other forest pests and pathogens, such as elon-gate hemlock scale (Fiorinia externa Fer-ris), are known to impact hemlock health, these have not been documented within the study area.

Hemlock health was assessed at each plot using a qualitative vigor scale based on

canopy defoliation (NPS 2010). Of the 148 hemlock observed, only 2 percent were considered healthy, while 66 percent were experiencing light to severe decline, and the remaining 32 percent were function-ally dead. Other canopy species encoun-tered in the plots included red oak (Quercus rubra L.), American beech (Fagus grandi-folia Ehrh.), sugar maple (Acer saccharum Marsh.), chestnut oak (Quercus montana Willd.), and tulip poplar (Liriodendron tu-lipifera L.). Dominant understory species included red maple (Acer rubrum L.), sour-wood (Oxydendrum arboretum L.), white ash (Fraxinus americana L.), and striped maple (Acer pensylvanicum L.).

Pipestem Resort State Park is situated within West Virginia Climate Division 5, which is characterized as a humid conti-nental climate with an average January temperature of 0.4°C and an average July temperature of 22.3°C (NOAA 2013). Pre-cipitation peaks in July with an average total rainfall of 11.3 cm, while October is the driest month with an average total rainfall of 6.4 cm (NOAA 2013). Trends in the divisional climate record (1895–2012) indicate a sharp decrease in aver-age annual temperature beginning in the late 1950s followed by a rapid increase through 2012 (Figure 2a), while total an-nual precipitation (Figure 2b) and Palmer Drought Severity Index (PDSI; Figure 2c) have been increasing since the early 1970s.

Tree-Ring ChronologyTwo increment cores were removed

from approximately 10 cm above the tree buttress, at a position perpendicular to slope aspect, from at least ten living and/or standing dead hemlock at each for-est monitoring plot. Forest canopy dom-inant and co-dominant hemlock were

Table 1. Long-term monitoring plot information including topographic variables

(i.e., slope, aspect, and elevation), number of live and standing dead hemlock (≥ 10 cm DBH),

and number of trees and series used in the hemlock growth chronology.

Plot Slope

(%)

Aspect

(°)

Elevation

(m)

Hemlock

- Live

Hemlock

- Dead

CRN Trees CRN Series

PIPE01 45 332 672 11 2 7 10

PIPE02 12 334 683 22 10 0 0

PIPE03 29 310 663 7 10 0 0

PIPE04 82 24 555 8 3 5 7

PIPE05 54 358 550 8 3 4 7

PIPE06 16 286 716 9 1 1 1

PIPE07 72 310 665 12 4 5 7PIPE08 66 14 705 24 14 0 0

Figure 1. Location of Pipestem Resort State Park (PIPE) in the Central Appalachian region (inset)

and long-term monitoring plots (■) within PIPE. Elevation contours are in meters.

Figure 2. a) Mean annual temperature b) total annual precipitation and c) mean

annual Palmer Drought Severity Index (PDSI) for West Virginia Climate Division 5 with

35-year smoothing splines (1895–2012). The dotted horizontal lines in a) and

b) indicate mean values for the full length of the climate record.


prioritized for sampling. If, however, stand structure was even-sized, then the first ten hemlock encountered were sam-pled. Core samples were taken up to 1 m above ground if rot was encountered at the tree base. Samples were dried, mounted, and surfaced with progressively finer grit sandpaper until cells were clearly visible under magnification according to established tree-ring analysis procedures (e.g., Stokes and Smiley 1968; Orvis and Grissino-Mayer 2002).

Digital images of mounted and sanded core samples were captured using a high resolution (3200 dpi) flatbed scanner. Annual rings were measured to the near-est 0.01 mm using the image analysis soft-ware CooRecorder 7.6 (Larsson 2003). Individual series were crossdated using CDendro 7.6 (Larsson 2003), a ring-width correlation analysis software program used to assign precise years to annual tree growth (e.g., Bijak 2010; Koprowski 2012). Additional checks on crossdating were performed using the program COFECHA (Holmes 1983; Grissino-Mayer 2001). COECHA creates a master ring-width chronology by compositing the weights of all series, and then individually tests the relationship between each series and the master chronology. This process results in a correlation value for each individual series. The interseries correlation is the average correlation value for all dated series. All dating was validated by examining a sam-ple under a stereomicroscope if very nar-row, missing, or false rings were suspected.

Raw ring-width measurements from the PIPE collection were processed into a site composite chronology using the pro-gram ARSTAN (Cook 1985). Criteria for inclusion of individual series in the collec-tion included: 1) 75-year minimum series

length; 2) absence of heartwood rot; and 3) a correlation with the master chronol-ogy exceeding 0.32. Series younger than 75 years were excluded to avoid bias in climate-growth relationships caused by juvenile growth (Fritts 1976). The cross-dated raw ring widths were standardized using a 30-year smoothing spline, remov-ing age-related growth trends and most releases and suppressions related to stand dynamics while maintaining the necessary climate signal (Cook and Peters 1981; Hart et al. 2010). The standardized chronology was developed by first dividing raw ring-width measurements by the predicted growth curve values for each series. Raw ring-width measurements may have sig-nificant temporal autocorrelation asso-ciated with the influence of the previous year’s growth on the current year growth (Fritts 1976). We chose not to remove the autocorrelation of the individual series to better understand the effect of the previ-ous growing season climate on the current year growth. The resulting ring width in-dices were then averaged together to gen-erate a site-wide standard master chronol-ogy (Cook 1985). Finally, we calculated the expressed population signal (EPS) to assess the common variance in tree growth over the length of the chronology (Wigley et al. 1984). The EPS typically declines as sample size decreases and an EPS below 0.85 suggests that a chronology may not fully represent the common signal at a site.

Instrumental Climate DataClimate-growth relationships were

analyzed using instrumental monthly mean temperature and monthly total pre-cipitation values. Instrumental climate records were obtained from the National Climatic Data Center (NCDC) for West


Virginia Climate Division 5 (NOAA 2013). Divisional or regionally averaged climate data shows a stronger relationship to tree growth than single climate stations (Blas-ing et al. 1981) and has been used in a re-cent study analyzing climate-growth rela-tionships in hemlock at its southern limit in northern Alabama (Hart et al. 2010).

Regime shift detection (Rodionov 2005) was used to identify rapid shifts in the instrumental mean annual tem-perature, total annual precipitation, and annual PDSI records (1895–2012). This method has been applied to other climate datasets (D’Arrigo and Wilson 2006; Tian et al. 2013), fisheries management and marine ecosystem studies (Daskalov and Mamedov 2007; Green 2013; Tomczak et al. 2013), and the analysis of tree-ring data (Elliott 2012; Knaap et al. 2013; Stella et al. 2013). Regime shift detection is an exploratory tool that uses a sequen-tial t-test to identify rapid reorganizations (i.e., shifts) in time series data (e.g., mean annual temperature). The t-test evaluates whether a new observation in the time se-ries is a statistically significant deviation from the mean value of the current re-gime. The user sets a “cutoff length” that determines the minimum regime length with all regimes longer than the user-set cutoff length being detected. For this study, a cutoff length of 60 years was used (α = 0.10). A 60-year cutoff length was selected as it represents approximately half the length of the climate record (118 years) and it approximated the timing of the warming trend in the late 20th century.

Climate–Tree Growth RelationshipsCorrelation and response function anal-

ysis were run in the program Dendroclim

2002 (Biondi and Waikul 2004) to ana-lyze relationships between the hemlock chronology and the divisional tempera-ture and precipitation records from 1896 to 1999 and 1896 to 2012 (Fritts 1976). The latter time period includes the years following HWA detection in Mercer and Summers Counties. Although climate data begins in 1895, all analysis starts with the year 1896 due to the inclusion of previ-ous growing season monthly climate var-iables. Correlation coefficients are based on univariate Pearson product moment correlations between climate variables and the tree growth chronology. The re-sponse function coefficients are multivar-iate estimations from a principle compo-nents model ( Biondi and Waikul 2004). Dendroclim2002 computes partial re-gression coefficients based on 1000 boot-strapped estimates obtained by random extraction with replacement from the ini-tial dataset. The analysis utilized the hem-lock chronology and 38 monthly climate variables: 19 months of mean temperature and total precipitation beginning in the previous April and ending in October of the current growing season (α = 0.05).

Temporal trends in climate-tree growth relationships were analyzed using a 35-year moving window that was shifted for-ward each year beginning with 1896–1930 and ending with 1978–2012 (n = 83). The earliest monthly climate variable exhibit-ing a statistically significant relationship to tree growth as determined by correlation and response function analysis was used as the starting point for moving window analysis. The moving window response functions were computed separately for temperature and precipitation due to a lack of degrees of freedom corresponding to such a short period (i.e., 35 years).


Figure 3. Composite ring width index spanning the period 1868–2012 with

mean growth standardized to 1.0 (solid line). All core samples have a minimum series length of

75 years and a mean series length of 103 years (series = 32; trees = 22) with an interseries correlation

of 0.63. Hemlock core samples collected at PIPE 01, 04, 05, 06, and 07 were used to develop

the composite ring width chronology. Sample size is indicated by the dashed line.

Therefore, changes in climate–growth re-lationships throughout time were assessed using the bootstrapped Pearson correla-tion coefficients (Andreu et al. 2007).

results and discussion

Tree-Ring ChronologyA composite site chronology was de-

veloped using 32 cores samples (22 trees) from five of eight forest plots (Table 1). These samples met all three criteia de-scribed in the Methods section. Trees sampled at plots with the highest hemlock mortality rates (i.e., PIPE 02, 03, and 08) did not provide core samples for the chro-nology. The hemlock chronology spans 145 years from 1868 to 2012 and has an interseries correlation of 0.63 (p < 0.01) with an average mean sensitivity of 0.25 (Figure 3). Of the 131 50-year segments analyzed by the program COFECHA, only 1 segment was flagged as not statistically

significant (α = 0.05). The flagged seg-ment was reexamined and accurate crossdating was confirmed. There is an increasing growth trend in the standard chronology following the 1860s and below average growth at the turn of the 20th cen-tury and during the 1960s and 1980s. Also of note is the recent growth decline during the first decade of the 21st century. The chronology EPS never fell below 0.85 for the entire period of growth suggesting that any changes in growth response to cli-mate variation is not attributed to changes in sample size.

Instrumental Climate DataA significant shift in average annual

temperature was determined to have oc-curred in 1958 using the decribed regime shift detection parameters (p < 0.001; Figure 4). The early regime was warmer with a 0.007°C annual temperature increase during the 63-year period. Conversely,


temperatures were cooler during the 55 years of the recent regime, but the rate of annual temperature increase (0.021°C/year ) was three times greater than that of the early regime. Severe winter tempera-tures, such as in 1958 and 1963, have not been recorded in southern West Virginia since the middle of the 20th century. No statistically signifcant shift in total annual precipitation was detected. However, a significant shift in annual PDSI to above average moisture conditions was con-firmed in 1971 (p < 0.001), though this trend may be partly driven by the increase in temperature during the same period. Both trends in temperature (Hansen et al. 2001) and precipitation (Karl and Knight 1998) have been reported in the US.

Climate-Tree Growth RelationshipsPrior to HWA detection at Pipestem,

hemlock growth was positively correlated (p < 0.05) with summer precipitation in

the current growing season, previous June precipitation, and late winter tempera-tures (Figure 5). The positive relationship to growing season precipitation has been reported in previous hemlock research in the northern (Cook and Jacoby 1977; D’Arrigo et al. 2001; Abrams et al. 2000; Pederson et al. 2013) and southern por-tions of its range (Hart et al. 2010). The positive relationship with late winter tem-peratures is also well-replicated (Cook and Cole 1991), but less well understood. Warm February and March temperatures in southern West Virginia likely melt snow and allow for an earlier onset of photosyn-thesis (Cook and Cole 1991; Hart et al. 2010). Both correlation and response function results identify the same months of climate sensitivity of radial growth in hemlock. It is assumed that monthly cli-mate variables are significant because they act on growing season moisture avail-ability and evapotranspiration demand (Cook and Pederson 2011).

Figure 4. Results of the temperature regime shift analysis, which identified 1958 as

the start of a new temperature regime (p < 0.01). Temperature increased at a rate of 0.021°C per

year during the period 1958–2012, three times the rate of 1895–1957.


Figure 5. Results of the correlation and response function analysis comparing

annual hemlock growth to climate prior to and including the years after hemlock woolly adelgid

(HWA) detection. Analyses included the previous year April through current year October

climate variables. a) Growth-temperature relationships for the period 1896–1999. b) Growth-

precipitation relationships for the period 1896–1999. c) Growth-temperature relationships

for the period 1896–2012. d) Growth-precipitation relationships for the period 1896–2012.

The sensitivity to variation in all sig-nificant climate variables decreased or became non-significant when years of growth following HWA detection were included in the analysis (Figure 5). This result demonstrates that static correlation and response function analysis might ob-scure climate-growth relationships if HWA or other known disturbances are not taken into account when choosing the period of analysis. Growth became negatively corre-lated with previous June temperature and positively correlated to previous August precipitation following infestation. The

increased sensitivity to previous growing season climate suggests that HWA caused the trees to draw more from previous year photosynthate reserves to maintain growth in the current year.

The relationship between climate and hemlock ring width has varied over time at Pipestem (Figure 6). The positive rela-tionship between May and June precipita-tion and growth was most consistent over time when using a 35-year moving analy-sis window. Climate-growth relationships changed at the time of HWA detection, further confirming the static analysis

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decrease (Figure 5) in positive correlation and response function coefficients. These relationships are noteable even though our time series includes only 13 years of post-HWA detection data used in the mov-ing window analysis. Specifically, hemlock showed a decreased positive response to summer precipitation (May–July), though the timing of the response differs between months. For example, the lag in the May precipitation response can be attributed to the influence of the strong positive corre-lation prior to HWA detection. This indi-cates that the climate-growth relationship is weakening, but may not become insig-nifcant until years after HWA is detected. The greater the initial correlation, the longer the lag between HWA detection and changes in climate-growth response. The moderate correlation with February temperature diminished abruptly follow-ing HWA detection, while there was a lag in the development of newly signifcant negative correlations with current year June and July and previous June, July, and September temperatures. The decrease in response indicates that hemlock is becom-ing less active during the historical growth period for the species with the additional environmental stress caused by HWA. The increased sensitivity to summer tempera-tures acts to reduce stomatal openess dur-ing the growing season and helps explain the decreased response to summer precip-itation. The decreased sensitivity to win-ter temperatures indicates that hemlock is not able to take advantage of the earlier growing season during years of earlier snowmelt. Warm late winter temperatures likely stimulate earlier activity by HWA further limiting the tree-ring response to beneficial climate conditions (Parket et al. 1999; Paradis et al. 2008).

Additional shifts in the climate-growth response occurred in the mid-20th century that were not related to HWA infestation (Figures 6 and 7). The most important changes for temperature were: 1) growth became negatively correlated with April temperature from the late 1960s to the late 1980s; 2) the positive correlation with March temperature became non- significant after the late 1960s and shifted to a positive correlation with February temperature; 3) the negative correlation between growth and September tempera-ture became non-significant in the 1960s; and 4) growth showed a positive corre-lation with previous September temper-ature from 1970 to 1990 and a negative correlation after HWA detection. Further investigation of the temperature correla-tions over time showed a period of tran-sition from 1958–1969 for each of these variables (Figure 7). A visual analysis of the annual temperature anomaly revealed that temperatures abruptly shifted to below average during this time period.

The mid-century temperature shift (i.e., 1958) was confirmed using regime shift detection (60-year cut-off length; α = 0.10). The mechanism causing this shift is not confirmed; however, the North Atlantic Oscillation (NAO) is known to in-fluence winter temperatures in the eastern US (Hurrell et al. 2003). The NAO shifted to a predominately negative phase from the late 1950s through the late 1970s. During the negative phase of the NAO, cli-mate conditions are typically cooler than average and precipitation is likely to be frozen. Cooler winter temperatures would inhibit hemlock growth via the control on moisture avalability, and perhaps, cause a switch in sensitivity to temperature in some winter months. Also, the shift in the


Figure 7. a) Results of the 35-year moving window analysis showing trends

in the correlation between hemlock growth and current year February, March, April,

and September temperature. b) Departures in average annual temperature from the 1895–2012

mean value. The transparent vertical bar highlights the years 1958–1969, a period marked by an

abrupt shift to below average annual temperatures. c) Results of the 35-year moving window

analysis showing trends in the relationship between hemlock growth and previous year June and

current year May, June, and July precipitation. d) Departures in total annual precipitation from the

1895–2012 mean value. The transparent vertical bar highlights the years 1972–1979, a period mark-

ing the transition to increasingly above average precipitation.

climate-growth relationship from current September to previous September temper-ature appears to be related to the climate shift in the mid-20th century (Figure 6).

For precipitation, the relationship be-tween current year May, June, and July, and previous year June remained signifi-cant through the mid-20th century but the strength of the correlations decreased in the 1970s (Figure 7). The 1970s marked a period

of transition to an increase rate of change in annual precipitation despite not showing a shift in the regime (Figure 2). Further inves-tigation of monthly precipitation revealed a shift in May precipitation in 1971 and a shift in June precipiation in 1954. Both regime shifts coincide with a decrease in the cor-relation coefficients. The shift in moisture availability is further evidenced by a signif-icant regime shift for PDSI in 1971.


For both precipitation and tempera-ture, a steady decrease in correlation coef-ficients was found following regime shifts in climate during the mid-20th century (Figure 7). This decrease occurred prior to the abrupt change in climate-growth cor-relations following HWA detection. In cen-tral Appalachia, the shift to a warmer and wetter climate may weaken hemlock re-sponse to variation in climate because cli-mate might no longer be a strong limiting factor or the climate-growth relationship might be shifting to different months of importance. The decrease in the strength of a limiting factor might signal that a tem-perature or precipitation threshold has been exceeded. For example, the warming trend in temperatures might have induced complacency in the growth response to winter months (Maxwell et al. 2012). This complacency means that winter tempera-tures in the past few decades were not cold enough to cause reductions in tree growth. Also, we note that the decrease in growth response to winter temperatures between the 1970s to the present is also linked to a strong positive phase of NAO, further suggesting that hemlock growth might be teleconnected to oscillations in the North Atlantic Ocean. We caution that these hy-potheses need to be tested with additional analysis of hemlock and other species sen-sitive to variation in winter temperature.

conclusions

The results of our analyses provide valuable insights into understanding climate-growth relationships in eastern hemlock, a foundational forest species in the Central Appalachian region. The temporal variability in hemlock climate-growth relationships shown here

further highlights the need to investigate the stability of these relationships prior to performing tree-ring based climate re-constructions in eastern North America. Previous hemlock work has either focused on static climate-growth relationships or growth response to HWA, without ref-erence to the other. We have shown that hemlock growth has varied over time in response to historical changes in climate and recent infestation by HWA. We link the change in past climate-growth corre-lations to mid-20th century regime shifts in temperature and precipitation which may be controlled in part by teleconnections to the North Atlantic Ocean. Current and projected climate change in eastern North America (i.e., increased warming and moisture availability) might exacerbate HWA impacts, resulting in difficult condi-tions for hemlock conservation efforts.

acknowledgmentsThis research was supported in kind by the

West Virginia Division of Natural Resources. We

would like to thank Joshua Gelinas, Ben Lusk,

and Sam Spencer for assistance in the field.

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dr. thomas saladyga is an Assistant

Professor of Geography at Concord University

in Athens, West Virginia, 24712. Email:

[email protected]. His research is

focused on understanding forest ecosystem

stability and resilience in the context

of changing climate, land use and/or

management strategies.

dr. r. stockton maxwell is an

Assistant Professor of Geospatial Science at

Radford University in Radford, Virginia, 24142.

Email: [email protected]. His research

interests include paleoclimatology, carbon

dynamics, and forest disturbance history.

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