118. Black, B. A. and M. D. Abrams. 2005. Dendroecological history and climatic responses of an ...

13 Pages·2014·0.31 MB·English

Disturbance history and climate response in an old-growth hemlock-white pine
forest, central Pennsylvania
Author(s): Bryan A. Black and Marc D. Abrams
Source: The Journal of the Torrey Botanical Society, 132(1):103-114. 2005.
Published By: Torrey Botanical Society
DOI: http://dx.doi.org/10.3159/1095-5674(2005)132[103:DHACRI]2.0.CO;2
URL: http://www.bioone.org/doi/full/10.3159/1095-5674%282005%29132%5B103%3ADHACRI
%5D2.0.CO%3B2
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Journalof theTorreyBotanicalSociety132(1),2005,pp. 103–114
Disturbance history and climate response in an old-growth
hemlock-white pine forest, central Pennsylvania
Bryan A. Black1,2 and Marc D. Abrams
203 ForestResourcesLab,PennStateUniversity,UniversityPark,PA16802
BRYAN A. BLACK AND MARC D. ABRAMS (203 Forest Resources Laboratory, University Park, PA 16802).
Disturbancehistoryandclimateresponseinanoldgrowthhemlockforest,centralPennsylvania.J.TorreyBot.
Soc. 132: 103–114. 2005.—Radial growth patterns are examined in relation to the historical development,
disturbance history, and climate responses of an old-growth hemlock forest located in central Pennsylvania.
Hemlock recruited continuously from the mid 1700s through 1890 with a sharp pulse of regenerationbetween
1860and1870,whilewhitepineformedaneven-agedcohortinanarrowintervalbetween1865and1870.No
recruitmentofanyspeciesoccurredinthe20thcentury,likelybecauseofdeerbrowsing.Isolatedpulsesinradial
growth occurred in almost every decade of the chronology, indicating a high frequency of small-scale distur-
bances. Major stand-wide pulses in hemlock radial growth occurred around 1810 and 1850. The 1850 event
correspondswiththedateoftwointensewindstormsandwastheonlydisturbanceeventlargeenoughtorecruit
whitepine.Long-termeffectsofclimatewereapparentinthe20thcenturyinwhichperiodsoflowradialgrowth
occurred in the cool, dry periods of 1910–1930 and 1965–1970. Correlations and response function analysis
revealed that hemlock radial growth was limited by drought in winter and early fall, and by low March tem-
peratures.
Key words: tree-ring,standdynamics,disturbance,easternhemlock,old growth.
Dendroecological analysis of old growth for- European settlement forest dynamics as well as
ests represents an important tool by which to a means to characterize and explain vegetation
evaluate long-term stand dynamics and devel- changefollowingEuropeansettlement(Fosteret
opment (Nowacki and Abrams 1994, Ruffner al. 1992, Orwig and Abrams 1994).
and Abrams 1998, Orwig and Abrams 1999). Throughout eastern North America, old
Couplingstandagestructurewithtreeringchro- growth is rare and represents only a small per-
nologies provides insight on speciesrecruitment centage of total forested area (Davis 1996). In-
patterns, the periodicity and intensity of distur- deed, in central Pennsylvania very few forests
bance (such as wind, fire, and insectoutbreaks), haveescapedagriculturalclearingorloggingfor
communitystructure,andsuccessionaldynamics timber and charcoal production (Nowacki and
(Foster 1988, Lorimer and Frelich 1988, Fritts Abrams 1992, 1994, Abrams et al. 2001). Yet
and Swetnam 1989, Frelich and Graumlich due to its low economic value and tendency to
1994, Abrams and Orwig 1996). When inter- occupy inaccessible sites, old growth stands of
preted in the context ofland-usehistory,chang- eastern hemlock (Tsuga canadensis (L.) Carr.)
esinforestcompositionandstructurealsoreveal persist in Pennsylvania and elsewhere through-
the effects of anthropogenic activities such as out its range from southeastern Canada through
agriculture, logging, fire suppression, or the in-
the southern Appalachian Mountains. With lon-
troductionofexoticpathogensandinsects(Ruff-
gevity of more than 500 years, eastern hemlock
ner and Abrams 1998, Abrams et al. 2001,
provides unique opportunities to describe forest
Shumway et al. 2001, Rozas 2003). Thus the
dynamics spanning the pre- and post-European
historical perspective afforded by old growth
settlementeras(NowackiandAbrams1994,Or-
forests provides valuable baseline data on pre-
wig and Abrams 1999, Abrams et al. 2000,
2001).Old-growthhemlockisalsousefulforde-
terminingtheeffectsofclimateonradialgrowth
1The authors wish to thank the 2000 Pennsylvania
Governor’s School for Agricultural Sciences for their and quantifying patterns of long-term climate
assistancewithvegetationdata,andMattFromm,Aar- variability. Historically, dendrochronologists
on Stottlemeyer, and Casey Townsend for their assis-
working in closed-canopy forests have focused
tancewithcorecollection.Wewouldalsoliketothank
on reconstructing disturbance regimesandstand
twoanonymousreviewersandJimColbertforreview-
ing earlierdraftsofthismanuscript. dynamics, assuming the effects of climate were
2Address for correspondence: Hatfield Marine Sci- minimal in comparison to those of competition
ence Center, Oregon State University, Newport, OR
(Fritts 1976). Relationships between radial
97365. E-mail:[email protected]
growth and climate were pursued in more arid
Received for publication December 10, 2004, and
in revisedformNovember22,2004. environments where climate is more limiting
103
104 JOURNALOFTHETORREYBOTANICALSOCIETY [VOL.132
and competitive effects are low (Fritts 1976). CollegeinCentreCounty,Pennsylvania.There-
However, increasingly more studies are docu- gion lies within the Ridge and Valley physio-
menting climate signals in species occurring in graphic province, characterized by long, level,
closed-canopy forests (Graumlich 1993, Larsen parallel ridges of sandstone and shale rising ap-
and MacDonald 1995, Abrams et al. 2001, Tar- proximately500feetabovebroad,limestone-de-
dif etal. 2001).Foreasternhemlock,significant rived valleys (Braker 1981). Schall’s Gap is a
climatic impacts on radial growth have been narrow valley along the north slope of Tussey
identified throughout the northeastern United Ridge with an elevation of 450 to 490 m asl.
States and southeastern Canada (Cook and Cole The stand is surrounded on both sides by steep
1991, Abrams et al. 2000, Tardif et al. 2001). slopes of sandstone talus. Hemlocks are most
These climatic-related analysesprovide datanot abundant in the narrow (30–50 m wide), gently
onlyforclimatereconstructions,butalsoforun- sloping floor of the gap, with some extending
derstanding which variables limit eastern hem- into the lower slopes of the talus fields. Soilsof
lock growth and how the stand responds to cli- the gap floor are Andover, a deep, poorly
maticvariability.Thepotentiallyprofoundeffect drained loam of low permeability derived from
ofclimateonradialgrowthalsounderscoresthe sandstone and shale colluvium (Braker 1981).
needtosimultaneouslyconsiderclimateanddis- The forest exhibits several old-growth charac-
turbance. teristics including diverse structure, large
In this study, we report on the development, amounts of coarse woody debris, and dead-
disturbance history, and effects of climate on a standing trees. Fire scars are evident on a few
previously undocumented old-growth eastern hemlocks growing on the upper slopes of the
hemlockstand,locatedinSchall’sGapincentral gap, but there is no indication of fire on thegap
Pennsylvania. Documenting remaining old- floor. Deer droppings are common and intense
growth hemlock forests has become especially browsing occurs on hemlock seedlings and sap-
important in recent years, given the expansion lings throughout the stand.
of the hemlock wooly adelgid (Adelges tsugae The climate of Centre County is mainly dry
Annand). This small, aphid-like insect, intro- continental with some humid, maritime influ-
duced from Japan, is spreading northwestward ence. Winters are typically cold and dry with
across eastern hemlock’s native range, causing averagemonthlyminimumtemperaturesranging
widespread hemlock mortality (Orwig et al. from25 to278C (December–March). Summers
2002).Thoughwehavenotseenevidenceofthe are warm and humid with average maximum
hemlock wooly adelgid in Schall’s Gap, it is temperatures ranging from 26 to 288C (June–
now present in central Pennsylvania and given August).Annualprecipitationaverages93.4cm,
its progression, will soon threaten hemlock in and the average frost-free season is 170 days
this stand. Prior to the arrival of the hemlock (April 27–October 14) (Braker 1981).
wooly adelgid, our objectives were to (i) quan-
tify the present compositionandstructureofthe Methods. In the summer of 2000, a total of
old-growthhemlockforest(ii)describethelong- 20 fixed-area plots were established at 20 m in-
termvariationsinhemlockradialgrowthandes- tervals along two parallel transects separatedby
timatetheperiodicityofdisturbanceandrecruit- approximately 30 m. Plot centers were located
ment (iii) quantify the response of hemlock ra- on the gap floor between the stream and talus
dialgrowthtoannualandmonthlylevelsofpre- slopes. The species, diameter at breast height
cipitation and temperature and (iv) test the (dbh), and crown class were recorded for all
consistency of hemlock’s response to climate trees . 8.0 cm in diameter within 0.02 ha cir-
over the course of the 20th Century by compar- cular plots at each plot center. Crowns were as-
ing the response of climate in the first half of signed one of four classes (dominant, codomi-
the 20th century with responses in the latter half nant, intermediate, overtopped) based on the
ofthecentury.Anintegrationofdisturbancehis- amount and direction of incident light (Smith
tory and climate responses provides a detailed 1986).Eachspecies’relativedensity(numberof
interpretationoftheecologyanddevelopmentof trees), relative frequency (presence or absence
the hemlock forest in Schall’s Gap. in plots), and relative dominance (basal area)
was calculated and averaged into an importance
StudyArea. Thisstudywasconductedwith- value for the stand. At each plot, two to four
ina1.5haold-growthhemlockstandinSchall’s randomly selected trees were cored at 0.5 m, or
Gap, approximately ten miles southeast of State higher (e.g., 1.5 m) if rot or buttressing wasen-
2005] BLACKANDABRAMS:OLDGROWTHHEMLOCKFOREST 105
countered.Thesampleincludedavarietyofspe- nique of Nowacki and Abrams (1997) in which
cies, size classes, and some recently dead indi- percent growth change for a year is equal to
viduals. Additional cores were taken off-plot (M—M)/M whereM equalsaveragegrowth
2 1 1 1
from trees that appeared to be exceptionally old over the prior 10 years and M equals average
2
(large size and/or deeply furrowed bark) to ex- growth over the subsequent 10 years. Next, av-
tend the chronology. At each plot center seed- erage growth over the past ten years is calculat-
lings and saplings were tallied in 5 m2 and9m2 ed for each ring width in each tree-ring series.
circular plots, respectively. Seedlings were de- For example, the prior growth calculation for a
fined as all trees , 1.5 m in height while sap- tree-ring increment in 1990 would be the aver-
lings were identified as all individuals . 1.5 m ageradialgrowthfrom1979to1989.Whenper-
in height, and less than 8 cm in diameter at cent growth change is plotted against prior
breast height. growthforallgrowthincrementsofalltree-ring
Cores were dried, mounted, and sanded with series, percent growth-change values form a
increasinglyfinesandpapertorevealthecellular clear upper boundary is quantified by the func-
structure (Phipps 1985). Any cores with rotten tiony5974.54e21.1202x.Thusmaximumpercent
ormissingsegmentswerediscardedfromfurther growth change, or release response, is relative
analysis. Hemlock (N 5 47), white pine (N 5 to prior growth rate and diminishes at a steep,
5), and birch (N 5 7) cores (black birch(Betula negative exponential rate as prior growth in-
lentaL.)andyellowbirch(Betulaalleghaniensis creases (Black and Abrams 2003, 2004). Fur-
Britton) were combined) were then cross-dated thermore,priorgrowthappearstobeamorefun-
using the signature-year technique to identify damental predictor of release response than
any partial, missing, or false rings (Yamaguchi crown class, age, or size of a tree in eastern
1991).Eachhemlockcorewasthenmeasuredto hemlock.
the nearest 0.002 mm using a Unislide ‘‘TA’’ Weusethisupperboundaryofpercentgrowth
tree-ring measuring system (Velmex, Inc., change as an indicator of maximum release re-
Bloomfield, NY). Once the hemlock cores were sponseforagivenlevelofpriorgrowth.Growth
measured, cross-dating was verified using the pulsesthatapproachtheboundarylinerepresent
program COFECHA available on the Interna- large releasesin which tree growthwasinduced
tional Tree-Ring Data Bank Program Library to nearly maximum growth rates, as predicted
(ITRDBL)version2.1(Cooketal.1997).Hem- by rate of prior growth. To scale releases to the
lockmeasurementserieswerefitwithanegative boundary line, pulses in percent growth change
linear regression or negative exponential curve. that exceed ten percent were identified in each
Each measurement series was then standardized tree-ring series. These pulses represent potential
by dividing the observed ring width values by releases, and the year of maximum percent
the expected values, given the exponential or growth change isretainedforeachpulse.Pulses
linear function. Standardization transforms all fallingwithin50–100%oftheboundarylineare
ring width measurements into dimensionless in- classified as major releases and those falling
dex values, removing the effects of age and mi- within 20–49.9% of the boundary line are clas-
crosite and equally weighting all series. Stan- sified as moderate releases (Black and Abrams
dardized ring width indices were then averaged 2003). All pulses below 20% of the boundary
using a biweight robust mean to form the stan- line are eliminated. Effects of short and long-
dard chronology. All chronology development termclimateeventsaremitigatedbythepercent
procedures were conducted using the program growth change calculation. Growth pulses in-
ARSTAN, available through the ITRDBL ver- ducedbyaseveredroughtoveramoderatetime
sion 2.1 (Cook et al. 1997). scale (approximately 5 years) fall below 20%
To analyze the frequency and intensity of the value of the boundary line (Black and
smaller-scalevariationsinadditiontolargerdis- Abrams 2003).
turbance events, releases were calculated for For climate analysis, the 47 hemlock mea-
each of the 47 hemlock measurement series. A surementserieswerestandardizedusingaspline
new technique is applied that takes intoaccount function with a 50% frequency response of 50
the effects of a tree’s growth history on release years (Cook and Peters 1981). Using this meth-
response (Black and Abrams 2003, 2004). The od,highfrequencyvariationsareretained,while
procedure can be summarized in a few gener- lower-frequencytrends,suchasthoseduetodis-
alized steps. First, percent growth change of turbance, are reduced. Autoregressive modeling
each core is calculated according to the tech- was also performed to remove any remaining
106 JOURNALOFTHETORREYBOTANICALSOCIETY [VOL.132
FIG.1. DiameterdistributionoftreespeciessampledinSchall’sGapoldgrowthforest.
temporal autocorrelation, and ensure indepen- crown classes (Figs. 1, 2). Importance value of
dence in the data set. After autocorrelation had hemlockwas72.2%,muchgreaterthantheother
been removed, all the residual series were av- species occurring in the stand (Table 1). Black
eragedusingabiweightrobustmeantoformthe birch, the second most important species in the
residual chronology. Detrending, autoregressive stand, was most abundant in the smaller diam-
modeling, and chronology development proce- eter classes, although some individuals attained
dures were performed using the program AR- diameters in excess of 60 centimeters (Table 1,
STAN (Cook et al. 1997). Monthly temperature Fig. 1). The majority of black birch occurred in
averages and precipitation totals were then ob- the codominant crown class (Fig. 2). Similar to
tained from the National Oceanic Atmospheric black birch, yellow birch was most common in
Administration Climate visualization website the small to medium diameter classes and prev-
(NOAA 2003). These data span the periodfrom
alent as codominant and intermediate individu-
1895 to 1999 and are averaged across climate
als(Figs.1,2).Finally,whitepinewasverynar-
Region 7 of northcentral Pennsylvania (NOAA
rowly distributed across diameter and crown
2003). We entered the residual chronology and
classes, occurring only as dominant individuals
monthly temperature and precipitation data into
with diameters between 40 and 70 cm (Figs. 1,
the program PRECON32 (DendroPower, Tuc-
2). In the understory of Schall’s Gap, we ob-
son, AZ) for correlation and response function
served that regeneration was sparse and oc-
analysis.Inresponsefunctionanalysis,principal
curred in dense patches under medium to large
components regression is used to evaluate the
gaps. Across the twenty plots, 82 hemlock, 32
significance of climatic variables on radial
black birch, 7 red maple, and 5 yellow birch
growth. A bootstrap method is then applied to
seedlings were tallied. Only eight hemlock sap-
estimate the standard deviations of the regres-
lings were recorded along with one black birch
sion coefficients. Mean regression coefficients
(data not shown).
were considered significant if they were at least
Hemlockrecruitedcontinuouslyfromthemid
twice as large as the standard deviation follow-
1700s through 1890 with a sharp pulse of re-
ing 1000 bootstrap iterations.Totesttheconsis-
generationbetween1860and1870(Fig.3).Yel-
tency of the climate relationships over time, we
also performed correlation and response func- low birch also recruited continuously, but at a
tion analysis separately on the first andlastfifty lower, more constant rate, while white pine
years of the 20th Century. formedaneven-agedcohortinanarrowinterval
between 1865 and 1870 (Fig. 3).Peakhemlock,
Results and Discussion. DENDROECOLOGY. blackbirch,andwhitepinerecruitmentfollowed
Hemlock dominated all diameter and crown a major stand-wide disturbance that occurred
classes, especially the largest and smallest di- around 1850 as indicated by an increase in the
ameterclassesandtheovertoppedanddominant ring width index between 1845 and 1855 just
2005] BLACKANDABRAMS:OLDGROWTHHEMLOCKFOREST 107
FIG.2. Crownclassdistributionoftreespeciessampledin Schall’sGapold growthforest.
prior to a much steeper acceleration in growth indexfrom1965to1970.Thenhemlockshowed
between 1855 and 1860 (Fig. 3). In contrast, a the most consistent above-average growth dur-
more clearly defined disturbance occurred ing the remainder of the century (Fig. 3). The
around1810,producingaresponseinringwidth apparently infrequent disturbances if the 20th
indexofgreatermagnitudethanthatofthe1850 Century still allowed the establishment of sev-
disturbance. But this disturbance did not initiate eral seedlings and saplings, but hemlock indi-
anunusuallyhighlevelofrecruitment,basedon viduals failed to recruit into the 8 cm diameter
current age structure (Johnson et al. 1994). Fur- class.
thermore, the high value of this growth pulsein Calculation of major and moderate releases
the standard chronology could be an artifact of for each of the 47 hemlock trees revealed the
smallsamplesize.Agrowthpulseoccurred,but combined effectsoflocalandstand-widedistur-
there may be error in defining its exact magni- bances. In conjunction with pulses in the hem-
tude. lock master chronology, a relatively large pro-
Although stand-wide releases over the past portionoftreesexhibitedreleasesinthetwode-
hundred years were muted in comparison to cades following the 1810 disturbance and the
thoseofthe19thcentury,considerablevariability decade after the 1850 disturbance (Figs. 3, 4).
stilloccurredinthechronology(Fig.3).Overall, The proportion of trees released in the 1850s
the growth trends of the 20th century can be waslargerthanthatofthe1810disturbance,and
summarized in four major periods of growth some of the releases qualified as major (Fig.4).
(Fig. 3). The first of these was a period of de- Additional widespread and major releases oc-
cliningradialgrowthfromapproximately1910– curred in the 1910s and 1930s, following sharp
1935. This trend reversed between 1935 and stand-wide increases in radial growth (Figs. 3,
1965, resulting in primarily above average 4). Overall, the abundance of releases reflects
growth, followed by another drop in ring width the high frequency of local disturbances in the
Table1. Frequency,density,anddominanceinformationfortreesinSchall’sGapforest.
Frequency Density Dominance Relative Relative Relative Importance
Species (#plots) (stems/ha) (m2/ha) frequency density dominance value
Betulaalleghaniensis 6 28 1.3 15.4 6.3 2.5 8.0
Betulalenta 12 55 4 30.8 12.5 8.0 17.1
Pinusstrobus 1 8 1.9 2.6 1.7 3.8 2.7
Tsugacanadensis 20 350 43.3 51.3 79.5 85.7 72.2
Totals 39 441 50.5 100.0 100.0 100.0 100.0
108 JOURNALOFTHETORREYBOTANICALSOCIETY [VOL.132
able plasticity of hemlock’s response to distur-
bance and the insignificance of age as a predic-
torofreleaseresponse(Fig.5).Inthistree,slow
radial growth characterized the first three hun-
dred years of growth, followed by a gradual in-
crease until 1850. During that 300-year period,
three moderate releases occurred with an aver-
age frequency of approximately one hundred
years (Fig. 5). These releases caused only tran-
sient pulses in growth, and it wasn’t until the
1850 disturbance that the tree was significantly
released from competition. Despite the tree’s
age, this disturbance event increased growth by
more than three hundred percent and induced a
sustained effect that lasted through the early
1900s (Fig. 5). After the impacts of the 1850
disturbance had diminished, no additional re-
leases occurred (Fig. 5).
As suggested by the standard chronology, re-
lease calculations, and the 550-year measure-
ment chronology, disturbances of the 19th cen-
turywerelikelyofgreatermagnitudethanthose
of the 20th century (Figs. 3, 5). Indeed, the 19th
FIG.3. Age-diameterrelationshipsforcoredtrees, century was a period of intense forest clearing
and the standard chronology for Tsuga canadensis in
and exploitation (Linn 1883). Fertile valley
Schall’s Gap. Sample size in the standard chronology
isalsoshown. floors in Centre County were cleared for agri-
culture while the remaining upland forests were
periodically harvested for lumber and the fuel
forest. Many of these releases were moderatein for the charcoal-iron industry (Linn 1883).
size, but isolated major releases also occurred Schall’s Gap is located in an upland site not
with high regularity (Fig. 4). morethanamilefromthenearestfurnace.How-
A unique feature of this stand was the excep- ever, human influences were likely low in
tionally old 550-year chronology of the oldest Schall’sGapgivenitssomewhatinaccessiblelo-
coredhemlock,whichdemonstratedtheremark- cation, infertile soils, and the poor quality of
FIG.4. Decadaldistributionofhemlockreleasesin Schall’sGap.
2005] BLACKANDABRAMS:OLDGROWTHHEMLOCKFOREST 109
FIG. 5. Radial growth of a 550-year-old hemlock in Schall’s Gap. An * indicates moderate release, andan
arrowindicatesamajorreleaseaccordingtothecriteriaofBlackandAbrams(2003).Thedottedlinerepresents
0.0 in theplotofpercentgrowthchange curve.
hemlock as a lumber or charcoal species. Al- al.1994).Althoughthemagnitudeofthegrowth
thoughfirescarsoccuronsomehemlocksatthe pulse in the master chronology was greater in
borderofthestand,nofirescarsorsoilcharcoal the 1810 disturbance, its effects only lasted ap-
were found within the stand. Furthermore,there proximately ten years while the effects of the
is no evidence of stumps, barbed wire, or log- 1850 disturbance were apparent for more than
ging trails. Instead, windstorms were probably 30years.Thisisconsistentwiththeideathatthe
responsible for the large pulses in radial growth level of white pine recruitment is proportional
in the nineteenth century. Indeed, two major to the size of the release following disturbance
windstorms that came through central Pennsyl- (Abrams 2001). Hemlock, which is able to cap-
vania in quick succession appear to be the most ture smaller gaps, also benefited from the mas-
likelyexplanationforthemajorreleasesthatoc- sive 1850s disturbance, showing a large amount
curred around 1850 (Ludlum 1963). The firstof of plasticity in its recruitment patterns.
these, the Great Hurricane of 1846, corresponds The less extreme and more localized distur-
with a sharp decrease followed by a gradual in- bancesofthe20thcenturydidnotfacilitatewhite
creaseinradialgrowthinthemasterchronology pine regeneration in the hemlock understory.
(Fig. 3). Additionalreleasefromcompetitionby However, this does not explain the low number
the second windstorm, the July Storm of 1850, ofhemlockandbirchseedlingsandsaplingsand
likely contributed to the sustained growth pulse the complete lack of tree recruitment that oc-
that lasted through approximately 1885 (Fig.3). curred after 1890. Hemlock is an extremely
The impacts of these events have been previ- shade tolerant species capable of enduring pro-
ously reportedinoakonnearbyridgesites(No- longed periods of suppression, yet is capable of
wacki and Abrams 1997). dramatic release when gaps occur in thecanopy
The 1850s disturbance was the most influen- (Burns and Honkala 1990, Orwig and Abrams
tialtohaveoccurredinthepast550years,based 1999, Abrams et al. 2001). Before 1890 in
on current age structure, and was the only dis- Schall’s Gap, hemlock recruited under a variety
turbance that recruited white pine and a large of disturbance levels, including periods of low
number of hemlocks (Figs. 3, 4, 5) (Johnson et variability inthemasterchronology(Figs.3,4).
110 JOURNALOFTHETORREYBOTANICALSOCIETY [VOL.132
FIG. 7. Correlation coefficients between hemlock
residualchronologyandprecipitationandtemperature
inSchall’sGap.Dottedlinesindicatethresholdforsig-
nificant correlation coefficients (a 5 0.05). An * in-
dicatesa significantresponsefunction(a50.05).
FIG. 6. Hemlock standard chronology with mean
1970 corresponded to periods of drought while
annual temperature and precipitation at Schall’s Gap,
centralPennsylvaniafortheperiod1895–1999.Alow- above-average growth from 1935–1965 and
pass filter is shown to emphasize low-frequencyvari- 1970–2000 corresponded with high levels of
ability.A brokenlineindicatesmeanvalue. precipitation(Figs.6).Thedroughtthatoccurred
in the mid 1960s has been reported as one of
themostseveretoaffectthenortheasternUnited
In other hemlock-dominated stands, hemlock
Statesthiscentury(CookandJacoby1977),and
continued to recruit into the 8 cm diameter
it clearly affected hemlock radial growth in
through atleast 1950(AbramsandOrwig1996,
Schall’s Gap.
Abrams et al. 2000, Abrams et al. 2001). One
Giventheverylowautocorrelation,theresid-
factor that could delay or prevent regeneration
ual chronology generated by spline detrending
is an overpopulationofwhite-taileddeer.Heavy
and autoregressive modeling was used to per-
browse wasevidentonmanyhemlockseedlings
form more detailed analyses of therelationships
and saplings throughout thestandandcouldpo-
between climateandradialgrowth.Inthischro-
tentially reduce regeneration of hemlock, white
nology, 33.8% of variance is explained in the
pine, or birch (Abrams et al. 2000, Abrams
first eignevenctor of principal components anal-
2001).
ysis, and subsample signal strength of 0.85 is
CLIMATE RESPONSES. In addition to distur- reached with 10 radii.Also,intertreecorrelation
bance, climate affected hemlock radial growth is 0.315 and agreement with the population
in Schall’s Gap. The influences of long-term chronologyis0.954,indicatingacommonsignal
fluctuations in precipitation are evident in the in the chronology. Indeed, correlations and re-
standardchronology,generatedbystandardizing sponse function analysis indicated that monthly
with negative exponential or linear regression. climatevariablesalsosignificantlyinfluencedra-
This chronology preserved the most low fre- dial growth (Fig. 7). Temperature and precipi-
quency variance, including the effects of long- tation accounted for 36% of the variance in the
term climate shifts. Indeed, four major growth hemlock residual chronology. Of the climatic
periods closely followed long-termprecipitation variables, precipitation had the greatest number
trendsoverthe20thCentury(Fig.6).Lowradial of significant relationships with radial growth.
growth from 1910–1935 and again from 1965– GrowthwaspositivelyassociatedwithpriorAu-
2005] BLACKANDABRAMS:OLDGROWTHHEMLOCKFOREST 111
gust,priorSeptember,currentFebruary,andcur- year when frozen precipitation is most likely to
rent October precipitation, and negatively asso- fall and accumulate in central Pennsylvania
ciated with current January precipitation (Fig. (Braker 1981, NOAA 2001).
7).Overall,moistureinlatewinterandearlyfall A positive correlation with current March
appears to limit growth in this stand. Tempera- temperatures is widely reported for hemlock
turewaspositivelyassociatedwithradialgrowth (Gove and Fairweather 1987, Cook and Cole
in current March and current October while it 1991, Abrams et al. 2001, Tardif et al. 2001).
wasnegativelyassociatedwithcurrentMaytem- AboveaverageMarchtemperatureswouldwarm
peratures. soil and air temperatures, melt any remaining
In an analysis of 42 hemlock chronologies snow cover, and allow early resumption of pho-
fromsitesacrosseasternhemlock’srange,Cook tosynthesis, thereby extending the growing sea-
andCole(1991)foundthatonwell-drainedsites son (Cook and Cole 1991). However, above av-
hemlock growth is consistently correlated with erage temperatures in May are associated with
precipitationduringthesummer,especiallyJune slow growth in Schall’s Gap (Fig. 7). May tem-
andJuly.Severalotherstudieshavereportedthis peratures were also limiting farther north in
relationship (Gove and Fairweather 1987, southwestern Quebec (Tardif et al. 2001). Typi-
Abrams et al. 2000, Tardif et al. 2001). How- cally, high temperatures in the summer (espe-
ever, in poorly drained sites where moisture is ciallyJuneandJuly)limithemlockgrowth,even
less limiting, summer precipitation was not in stands located elsewhere in Pennsylvania
found to be asimportanttoradialgrowth(Cook (Gove and Fairweather 1987, Cook and Cole
and Cole 1991). For example, in a bog forest 1991). While not significant, in Schall’s Gap,
near Schall’s Gap no significant correlations higher June and July temperature is positively
weredetectedbetweensummerprecipitationand correlated with growth (Fig. 7).
radial growth (Abrams et al. 2001). Although Over the course of the 20th Century, the syn-
there was a slightly positive correlation in chrony of hemlock growth, as indicated by the
Schall’s Gap between summer precipitation and standard deviation of growth among all trees,
hemlock radial growth, the relationship was not correlated with the standard chronology (r 5
significant.Instead,moisturebecomesmorelim- 0.67, P , 0.0001). The standard deviation of
iting to growth in the early fall (Aug–Oct). Au- growth and the standard chronology also corre-
gust, September, and October typically receive lated with precipitation (r 5 0.32, P , 0.001
less rainfall than June and July in central Penn- and0.33,P,0.001respectively).Thissuggests
sylvania(NOAA2003).Thuscloseproximityto thatduringperiodswhenclimatelimitedgrowth,
astreamandprotectedtopographicpositionmay growth of trees throughout the stand became
delay the effects of drought until later in the more synchronized, resulting in a low stand-
growing season. wide standard deviation. When climate wasless
Low rainfall in the late winter months also limiting, more variability in radial growth oc-
limited hemlock growth in Schall’s Gap, as in- curred as competitive factors were most impor-
dicated by a positive association between radial tant. Individuals in a favorable competitive en-
growth and February precipitation (Fig. 7). A vironment would have been free to grow more
positive association between hemlock and Feb- rapidly, increasing the standard deviation of
ruary PDSI was documented in a nearby old growth across the stand. Not only did the mag-
growth forest (Abrams et al. 2001). Sufficient nitudeofclimateresponsefluctuate,butalsothe
precipitation in the late winter months charges correlations between radial growth and the
thegroundwaterandprovidesadequatemoisture monthly climatic variables. The only climatic
through the beginning of the growing season. variable that was significantly related to radial
But what complicates this seemingly consistent growth in both the early and late 20th Century
pattern between winter moisture and growth in was current March temperature (Fig. 8). Other
Schall’s Gap is hemlock’s negative correlation climate relationships are comparable between
with January precipitation (Fig. 7). However, the early and late portions of the century, with
this negative correlation may be due to an as- theexceptionofcurrentAugust,September,and
sociation between precipitation and damaging October temperature. These three variables ex-
snowfallsoricestorms.InsoutheasternQuebec, hibitacleardecreaseoverthecourseofthecen-
hemlock growth was negatively correlated with tury, especially in September and October with
snowfall between December and March (Tardif decreasesof0.66and0.68degrees,respectively.
et al. 2001). January is the coldest month ofthe Cooler temperatures likely became more limit-