The relationship between site factors and white ash (Fraxinus americana L.) decline in Massachusetts

Forest Ecology and Management, 60 ( 1993 ) 2 7 1 - 2 9 0

271

Elsevier Science Publishers B.V., A m s t e r d a m

The relationship between site factors and white ash (Fraxinus americana L. ) decline in Massachusetts H. Woodcocld, W.A. Patterson III*'a, K.M. Davies, Jr.b aDepartment of Forestry and Wildlife Management, Universityof Massachusetts, Amherst, MA 01003, USA bConsuhing Forester, Box 601, Northampton, MA 01061, USA (Accepted 1 March 1993)

Abstract Forest inventory records incorporating individual tree data are an important source of information about the extent and severity of past rates of forest decline, thus providing a temporal perspective for contemporary observations. We demonstrate the potential of this approach by using continuous forest inventory (CFI) data to reconstruct the extent, severity, pattern of development and site-factor associations for white ash decline in Massachusetts. White ash (Fraxinus americana L.) increased in basal area on Massachusetts CFI plots at about 1.5% per year from 1962 to 1979. Ash decline is, however, locally severe. Plot decline status was estimated, based on vigor rating, and growth and mortality rates of white ash. Plots with a mean change in basal area per year of ~<- 0.5% a n d / o r with a mean vigor rating of ~<2.0 (on a scale of 0 - 4 ) were classified as 'decline'. Using these criteria, 20% of the 82 CFI plots with ~< 10% total basal area of white ash in 1962 were classified as 'decline' in both 1979 and 1991. These results suggest that statewide there has been no net loss of ash basal area since 1962. The total forest area affected by ash decline has not increased in the past decade. Analyses of the CFI data indicate that decline was most prevalent on mesic sites, high on the landscape a n d / o r on steep slopes. Such sites are potentially subject to large fluctuations in soil moisture availability during drought periods. Relationships identified between the prevalence of ash decline and site factors were further evaluated within a 2 ha intensive study site in the center o f a 13 ha white ash stand affected by ash decline in 1990. Decline was heterogeneously distributed within the stand, with declining trees most frequent on the 'decline-prone' site-type identified through CFI plot analyses.

Introduction Decline of forest trees, defined here as slow growth, low vigor, dieback, and above-average mortality on a stand-wide basis, is a complex phenomenon reflecting the interaction of multiple factors (Houston, 1981; Manion, 1991 ). Since 1980, forest decline in both Europe and North America appears to have increased in both severity and extent (Chevone and Linzon, 1988; Krahl* C o r r e s p o n d i n g author.

© 1993 Elsevier Science Publishers B.V. All rights reserved 0 3 7 8 - 1 1 2 7 / 9 3 / $ 0 6 . 0 0

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Urban et al., 1988 ). Early studies of the phenomenon frequently implicated climate aberrations as causal factors, but in the last decade the role of anthropogenic stress (e.g. air pollution, forest management practices) has been a focus of research. The positive identification of causal factors has proved elusive (see Smith, 1985). White ash (Fraxinus americana L.) decline in the northeastern United States has been reported since 1925 (e.g. Marshall, 1930; Pomerleau, 1944; Brandt, 1961; Tegetoff and Brandt, 1964; Hibben and Silverborg, 1978; Castello et al., 1985; Sinclair et al., 1990). White ash is sensitive to air pollution, especially ozone (Sinclair et al., 1987 ), but pollution has not been implicated in ash decline in forests (Millers et al., 1989). Many researchers have associated ash decline with drought (Marshall, 1930; Ross, 1964; Tobiessen and Buchsbaum, 1976; Hibben and Silverborg, 1978 ) and, in recent years, with infection by mycoplasma-like organisms (MLOs) associated with the disease 'ash yellows' (Hibben and Silverborg, 1978; Matteoni and Sinclair, 1985; Castello et al., 1985; Sinclair et al., 1990). Recently, Han et al. ( 1991 ) reported evidence for a synergistic effect of MLO infection and drought in reduced growth rates of white ash. Not all cases of ash decline can be attributed to ash yellows, however (Sinclair et al., 1990). Despite widespread reports of ash decline in northeastern North America, the regional growth curve for white ash in New England showed an upward trend between 1965 and 1980 (Hornbeck et al., 1988 ), and in the period 19721985 the growing stock volume in Massachusetts almost doubled (Dickson and McAfee, 1988 ). Nevertheless, declines of red oak, (Quercus rubra L. ), sugar maple (Acer saccharum Marsh. ) and white ash are among the top research priorities cited by practicing foresters in the Northeast (Broderick et al., 1991 ). The perception that ash decline is a serious problem coupled with an actual increase in both growth rate and volume of this species could be considered either contradictory or indicate that decline is patchy and locally severe at the 2-5 ha scale at which forest management decisions are made in the Northeast (Leak, 1980). To understand forest declines in general and ash decline in particular we need to consider present observations of decline within a temporal framework and ask whether observed decline might not be part of natural ecosystem processes (Manion, 1991 ). An understanding of past trends and their causes would enable us better to evaluate present observations and predict the impact of future declines. Given the difficulty of demonstrating cause and effect relationships in phenomena as complex as forest decline, an alternative approach, the identification of associations between the distribution of decline and regional factors (e.g. Millers et al., 1989), can be useful for interpreting large-scale patterns. Studies of the role of factors operating locally at a site are needed to complement regional studies (Woodcock and Patterson,

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1990). A first step could be the identification of associations between ash decline and local site factors. We have addressed four questions in our study: ( 1 ) what proportion of ash stands in Massachusetts were affected by ash decline in 1991? (2) is there evidence for an increase in past decades in the proportion of plots affected? (3) is there an association between ash decline and site factors and can 'decline-prone' site-type (s) be described? and (4) at what scale might such sitetype (s) occur at the stand level? To address the first two questions, we have used continuous forest inventory (CFI) data to estimate the proportion of state forest land affected by ash decline in 1979 and 1991. We have also examined the relationship between site factors and decline symptoms on these plots to identify a 'decline-prone' site type. We have evaluated our findings with a case study of a single stand affected by ash decline to address the third and fourth questions. The study area

CFI plots Continuous forest inventory (CFI) plots were established on state forest land throughout Massachusetts at intersection points on a state-wide 0.8-km grid. Prior to our study most of the plots had been sampled three times, in the early 1960s, the late 1960s and the late 1970s. Sampling years varied slightly from plot to plot. Tree data collected included diameter at breast height (dbh) and status (live or dead) for trees of >i 12.7 cm dbh. White ash represented approximately 2% of the total basal area (BA) on all CFI plots combined in 1979. It was most important in the Berkshire hills (6.1% BA in 1962 increasing to 6.3% in 1979) in the western part of the state and was infrequent or absent in the east. We used growth and mortality rates to estimate past plot decline status (see Methods). With less than five trees on a plot, the death of a single tree would result in a mortality rate of > 25% in a sampling period and could lead to misclassification of plot decline status. We therefore restricted our analyses to plots with more than four live ash at the time of plot establishment. Eightytwo (6.4%) of the total of 1279 plots satisfied this criterion (Fig. 1 ). Plot age in 1979 ranged from 41 to 117 years. The state has yet to resample the CFI plots in the 1990s, so in 1991 we resampled the plots that had more than 14 white ash trees (>125% white ash by density or BA) at any time during the period 1960-1979 ( 18 plots). These 18 plots supported 42% of all ash trees represented on the 82 plots. All 18 were located west of the Connecticut River (Fig. 1 ). Western Massachusetts has a north temperate, continental climate. Average winter temperature is --4 °C and average summer temperature is 20 °C at

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.,)/-.

i

Scale 20 km

.

,

.

.,..

•

t

Fig. 1. Map of Massachusetts showing the location of state forest land, and the location of all continuous forest inventory plots with more than four white ash and the intensively studied ash stand and study site. Open circles are 'non-decline' plots; closed circles are 'decline' plots (see text for definition of decline); double circles are plots resampled in 1991; triangle indicates the location of the intensive study site in Conway.

Amherst (Fig. 1 ). Precipitation averages 110 cm year- 1, and is evenly distributed through the year (Bradley et al., 1987 ). Soils on the CFI plots are mainly derived from medium-textured glacial tills overlying granite, gneiss, or schist bedrock. A fragipan is often present. Elevation ranges from 31 m to 1074 m. Several major forest types occur on state forest land in western Massachusetts. These include transition hardwoods at lower elevations, northern hardwoods at midelevations west of the Connecticut River and spruce-fir northern hardwoods at the highest elevations (Westfeld et al., 1956 ).

The study site To evaluate our conclusions from analyses of the CFI plot data at the stand level, we selected a 2-ha site for intensive study. The site is within a 13-ha ash stand on watershed land abutting Conway State Forest owned by the City of Northampton in the town of Conway (Figs. 1 and 2 ). This stand was selected

H. Woodcock et al. /Forest Ecology and Management 60 (1993) 271-290

> ZU%

~-~

275

mortaht}

~ 20% mortality permanent stream

-- -~- ephemeral stream study site boundary

Fig. 2. Map of the ash stand showing topographic and hydrological features, the distribution of white ash ( >/25% total BA), the distribution of high ash mortality ( > 20% of standing trees in 1991 ) and the location of the 2-ha study site.

because: ( 1 ) it lies within the same general geographic area as the CFI plots identified for analysis (Fig. 1 ); (2) ash decline occurs within the stand; (3) the stand contains a range of site-types; (4) ash BA is similar to that on CFI plots sampled in 1991; and (5) trees are 50-100 years old, which is within the range of tree ages represented on the CFI plots. The stand is located on a northeast-facing slope that extends about 500 m from a brook upslope to the watershed boundary and 900 m along the contour. Elevations range from 230 m at the brook to 300 m at the top of the slope (Fig. 2 ). The soil is a coarse-textured till derived from mica schist and impure limestone over schist bedrock (Colrain soil series; fine sandy loam; well-drained entic haplorthod) (Mott and Fuller (1967), confirmed by examination of a profile). Depth of soil to bedrock averages > 1 m. Site types within the stand, range from relatively dry on and near the ridge top, to wet near the brook. Moist sites associated with ephemeral streams and seeps are also present on the slope (Fig. 2). The area was pasture or grazed woodlot at one time, as evidenced by the presence of stone walls and old wire

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fences. The largest trees were 90-100 years old in 1990, and the stand had had no management since the salvage of dead American chestnut (Castanea dentata (Marsh.) Borkh) in the 1920s. Forest cover is northern hardwoods (SAF cover type 25; Eyre 1980). Sugar maple (Acer saccharum Marsh. ), red maple (Acer rubrum L. ), black birch (Betula lenta L. ), white birch (Betula papyrifera Marsh. ), yellow birch (Betula alleghaniensis Britton), northern red oak (Quercus rubra L. ), white ash (Fraxinus americana), beech (Fagus sylvatica Ehrh. ) and eastern hemlock ( Tsuga canadensis L. ) are the most important tree species present. Methods

Evaluation of decline status of sampling units Total CFI plot BA at each sampling time was calculated. These data were used to estimate percent change in BA per year including (ztBAI) and excluding (zlBA) ingrowth in each sampling period. Tree health (and hence decline status) is usually assessed on the basis of crown condition (Brooks et al., 1991 ). These data were available for CFI plots sampled in 1991 and for the study site quadrats but had not been recorded during the past CFI plot sampling. For this reason we used two separate methods to define CFI plot decline status in 199 l: crown condition, and growth and mortality rates. Growth and mortality rates alone were then used to define past plot decline status and to compare sampling years. To classify decline status on the basis of crown condition, each white ash >t 12.7 cm dbh in the CFI plots sampled in 1991 was assigned a vigor rating (VR) based on percentage crown dieback (defined as twig mortality in the canopy) where 0 is dead, 1 is >50%, 2 is 21-50%, 3 is 6-20%, 4 is 0-5%. These categories correspond to those used in assessments of tree health in the northeast (Brooks et al., 1991 ). Other symptoms of decline (e.g. foliage transparency) were present to a degree that was reflected by crown dieback. Trees of VR ~<2.0 were judged to be of deafly low vigor on the basis of crown symptoms. We rated a plot 'decline" if the mean VR for white ash was ~ 2.0. To establish whether growth and mortality rate data could give comparable decline ratings, regression analysis (SAS Institute, 1988) was used to assess the strength of the relationship between mean plot VR in 1991 and growth and mortality rates excluding ingrowth of ash (zlBA) for the period 19791991. A value for ABA corresponding to a VR of 2.0 was obtained by solving the regression equation for the model ztBA = constant + VR. A plot was classified as decline in 1979, if ABA was equal to or less than this value for the period 1967-1979.

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27 7

Site factors analyzed In order to determine whether slow growth and high mortality rates (i.e. decline symptoms) of ash were related to site factors, the relationships among ABA and aspect (ASP), soil moisture (MOIS), soil depth to impermeable layer (SOIL), site index (SI), percent slope (SLOPE), and landscape position (POS) were examined. The variable plot age (AGE) was also included in the models. Methods for estimating each factor are summarized in Table 1. All factors except SLOPE and POS were recorded at the time of plot establishment. SOIL (estimated from four cores per plot, cross-checked with soil map data), ASP and MOIS (from landscape position and soil texture data) were each recorded in three classes. SI and AGE were estimated from cores from three or four dominant trees on or near the plots (for details see Mawson, 1977). If the SI species was not white ash, SI was adjusted to white ash SI using curves developed by Carmean ( 1979 ). Percent slope (SLOPE) at the plot center was estimated from USGS topographic maps (scale 1:24 000, 3.08 m contour) (for plots not resampled in 1991 ) or with a clinometer for plots and quadrats sampled during our study. Landscape position is a site factor that influences topographically controlled spatial variation in soil moisture (Burt and Butcher, 1985). We defined a factor, POS, which integrates distance to the top of the watershed in meters (a) and slope angle at the sampling point (0) and is expressed as POS = a/tanO (adapted from Burt and Butcher, 1985 ). Distance from the plot center to the top of the watershed and slope at the plot center, 0, were estimated from topographic maps (USGS, scale 1:24 000, 3.08 m contours). Values for POS Table 1 Summary of factors tested in regression models and methods of estimation Data set

Factor

Method of estimation a

1 and 1 and 1 and 1 and 1 2 and 1 2 and 1 and

SI AGE SOIL ASP MOIS MOIS SLOPE SLOPE POS

Site index for three or four dominant or co-dominant trees (see Methods) Plot age estimated from SI cores Soil depth to impermeable layer, three classes ( < 30 cm, 30-60 cm, > 60 cm ) Aspect in three classes; NE, NW-SE, SW Moisture in three classes from soil texture and topography Weighted average wetlands index (see Methods ) % slope from USGS topographic maps (scale 1 : 24 000 ) Measured with clinometer at plot center Estimated from USGS maps (see Methods)

2 2 2 2 3 3 2

aAll factors used in analyses were recorded at the time of plot establishment except SLOPE, POS and MOIS (data set 2). Data set l: 82 CFI plots with more than four white ash in 1962. Data set 2." 18 CFI plots with > 25% white ash (by BA or frequency) in 1962, a subset of data set 1. Data set 3:96 study site quadrats.

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were log transformed (LOGPOS) so that the scale for this factor would be linear. A high value for LOGPOS indicates a site that is located on a low landscape position a n d / o r has a shallow slope; conversely, a low value for LOGPOS indicates that the site is located high on the watershed and/or is steeply sloping. Slope shape (concavity or convexity) was not estimated. At the time of plot establishment, MOIS was classified on the basis of topography and soil texture (Table 1 ) rather than on-site measurements. Ash is generally associated with moist sites (Leak, 1980) and 82% of the 82 ash plots identified for analysis were classified as 'moist' so that the original use of three classes to classify site moistness was too coarse for our purposes. It has also been questioned whether estimates of 'soil moistness' on the basis of topography and soil texture in fact reflect moisture available to plants, especially in rocky soils (Fralish et al., 1978 ). Thus, in order to compare moisture availability among the CFI plots and study site we used vegetation analysis to estimate relative soil available water capacity (Fralish et al., 1978 ) for the 18 resampled plots and for the study site quadrats. Using data for the frequency of occurrence of species in wetlands, Allen et al. (1989 ) obtained good correlations between weighted wetlands index average (WAWI) and soil drainage class for soils in transition zones of red maple swamps in Rhode Island. Soils in the Allen et al. (1989) study included 'moderately well-drained' soils. Since most of the ash sites in our study had originally been classified as 'moist', and 87.6% of the plants identified to the species level on the plots were listed in Reed ( 1988 ), we used wetland index (WI) data to estimate soil moisture availability on our sample sites. Our species list was similar to the list published by Allen et al. (1989) for Rhode Island red maple swamps, and this source was used to classify a further 7.4% of the species as either obligate upland (a category not included by Reed, 1988) or 'not available' (na) (insufficient information to classify species' wetland status). By combining the two sources it was possible to classify the WI status of 95% of the species identified on our sample plots. We defined MOIS as the weighted average wetland index (WAWI) (Wentworth and Johnson, 1986) estimated as follows. Each species occurring in the herbaceous and shrub layer of a plot was assigned a wetlands indicator status (Reed, 1988 ) with numerical indices (WI = wetland indicator value) from 1 to 5 assigned according to Wentworth and Johnson ( 1986 ) and Allen et al. (1989). Of the species occurring on the plots, 2.3% were classified as obligate wetland ( W I = 1 ), 15.3% as facultative wetland ( W I = 2 ) , 30% as facultative ( W I = 3 ), 39.3% as facultative upland (WI = 4), 2.4% as upland ( W I = 5 ), for 5% not available, and for the remaining 5% of species the wetlands indicator status was unknown. Unclassified species were excluded from the analyses. The WAWI for each plot was calculated following Wentworth and Johnson's formula (Wentworth and Johnson, 1986 )

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WAWI = 27(WI × IV ) / N where WI is the wetland index, IV is the species importance value from vascular plant cover/abundance estimates (Clark and Patterson, 1985), and N is the number of species (Wentworth and Johnson, 1986). A low value for WAWI indicates a wet site.

Disease assessment Because ash yellows has been implicated in many cases of ash decline, we estimated the prevalence of the disease on the 18 plots resampled in 1991 and on the study site. All ash trees on and near each CFI plot and on the 13-ha ash stand were examined for ash yellows specific symptoms (witches' brooms). Root tissue from 20 trees on the study site ( 10 o f V R ~<2.0), from three lowvigor trees on each of the CFI plots classified as decline and from two randomly selected CFI plots with healthy ash were assayed for the presence of MLOs in the sieve tubes according to the method of Sinclair et al. (1989).

The relationship between ash decline and site factors within an ash stand A preliminary survey included an estimate of the distribution of live and dead standing ash within the 13-ha stand. BA was estimated with a 5-factor Cruz-all (ft 2 acre- ~) at approximately 45-m intervals along four transects established at 50-m intervals along contours. The ratio of dead to living trees at each sampling point was calculated and the mortality rate of white ash in two categories ( ~ 20% or > 20%) was mapped for the portion of the stand on which white ash was >t 25% of the BA (comparable with the stocking of the resampled CFI plots) (Fig. 2 ). The relationship between ash decline and site factors identified from CFI plot analyses (see Results) was further investigated within an approximately 2 ha area (the study site) that included areas of both healthy and declining trees (Fig. 2). The study site was divided into 96 contiguous 15 m × 15 m quadrats to classify principal site types on the basis of vegetation analysis and to map tree condition. Vegetation cover was sampled using the releve method (Mueller-Dombois and Ellenburg, 1974) with an importance value (Clark and Patterson, 1985 ) calculated for each species. Site types were classified on the basis of vegetation analysis.

Data analysis An exploratory analysis to identify factors associated with slow growth and high mortality rates (low values for ABA) of ash on the CFI plots carried out using stepwise regression analysis (SAS Institute, 1988 ) (see Table 1 for fac-

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tors used in models) with a significance level of a = 0.15 for inclusion in the model. The analysis was repeated with the mortality rate of dominant and codominant trees as the dependent variable for the 18 plots resampled in 1991. Results of the analyses were used to identify factors or combinations of factors associated with slow growth and high mortality of white ash. To complement results from the regression analyses and to determine whether 'decline-prone' site-type (s) could be identified, the 18 resampled CFI plots were classified by site factors into site type groups using detrended correspondence analysis (DCA) (DECORANA, Hill, 1979 ). Tukey's test (SAS Institute, 1988 ) was used to compare mean ABA for the groups. Cluster analysis of plant species importance values was used to classify the study-site quadrats into site types using an agglomerative clustering routine (AGGLOM) patterned after Orloci (1967). Chi-square analysis (SAS Institute, 1988 ) was used to test for differences between the proportion of decline (VR ~<2.0) to healthy (VR >2.0) ash on each site type. Results

CFI plot analysis On a majority of CFI plots, white ash increased in importance statewide between 1962 and 1979, particularly in the western counties. ABAI (i.e. including ingrowth) of white ash on the subset of 82 plots with more than four ash per plot increased at a rate of approximately 1.5% year-1 between 1962 and 1979, comparable with estimates of Dickson and McAfee ( 1988 ) for volume change of ash in Massachusetts between 1972 and 1985. There is no evidence that, on a statewide basis, decline of white ash has led to a net loss of ash basal area since 1962. Change in BA, however, varied considerably among the 82 plots with >t 10% white ash. Values for ABA (i.e. excluding ingrowth) ranged from 6% year- ~to no live trees remaining on a plot, suggesting that decline, defined as slow growth and high mortality of trees alive in 1962, is locally severe (Fig. 3 ). Sampling a subset of 18 CFI plots in 1991 enabled us to evaluate plot decline status in 1991, identify changes in plot decline status between 1979 and 1991 and repeat the analyses with data for a longer time period and with improved soil moisture estimates. Values for ABA on the 18 resampled plots in each sampling period and mean plot VR in 1991 are shown in Table 2. ABA (1979-1991) and mean VR (1991) were significantly correlated (R2=0.6814, P <0.0001 ). Plots with a mean VR of 2.0 or less in 1991 were classified as declining. To classify past plot decline status (i.e. for periods for which no VR data were available) the corresponding value for ABA was found by substituting V R = 2 . 0 in the regression equation (Table 2). Plots were classified as declining ifABA was - 0.5% year- ~or less (Table 2 ). Using this

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# OF PLOTS 14 [ t

,2 i lO

:! 6

DECLINE PLOTS

6

4

3

2

1

0

- 1

-2

-3

-4

-5

-6

% CHANGE 1N BA/YR

Fig. 3. Histogram to show the frequency of percent change in ash BA per year excluding ingrowth from 1962 to 1979 (ABA)for 82 CFI plots in 12 classes. Plots with ABA ~ - 0.5% year- t are classified 'decline' (see text). Note: a plot on which no live ash trees remained in 1979 was assigned a value for ABAof - 6% year- '. criterion, 16 o f the 82 plots (20%) were classified as decline in 1979 (Fig. 3 ). Three o f the 18 resampled plots ( 17% ) were classified as decline in both 1979 and 1991 (Table 2 ). The decline status for two plots changed in this period. One showed sufficient i m p r o v e m e n t to be reclassified as 'healthy' (Plot 464) and one was reclassified as 'decline' (Plot 445 ) (Table 2). There was thus: ( 1 ) no evidence that decline was m o r e prevalent on plots with a high basal area o f ash; (2) no evidence o f an increase in the proportion o f plots affected by decline in the 1980s; and (3) no change in decline status o f the majority o f resampled plots between 1979 and 1991. For the 82 plots with more than four live white ash in 1962, stepwise regression analysis identified a single factor, landscape position ( L O G P O S ) significantly associated with ABA (excluding ingrowth) between 1962 and 1979 (R 2= 0.2215, P = 0.0001 ). The analysis was repeated for the 18 plots for the period 1962-1991 with new estimates for the variable MOIS (Table 1 ). For this data set, LOGPOS, MOIS and SI were all significant in the model (R 2= 0.5578, P = 0.0081 ). Substituting mortality rates o f d o m i n a n t and cod o m i n a n t trees as the d e p e n d e n t variable gave similar results but the relationship was stronger (R 2 = 0.6926, P = 0.0007 ). Plot age ( A G E ) was not significant, possibly because o f the fairly narrow range of values for tree age represented in Massachusetts stands. Results o f the regression analyses indicate that ash decline on the CFI plots is associated with a high landscape position a n d / o r steep slopes, and with moist sites. This conclusion is supported by results o f DCA. Three principal site type groupings were identified (Table 3 ) and a comparison o f m e a n VR

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Table 2 Percent change in BA year - t (ztBA) for CFI plots with > 14 ash trees per plot for sampling periods 1962-1967, 1968-1979, 1980-1991, and mean plot vigor rating ( V R ) for white ash in 1991 Plot No.

185 197 212 327 362 428 444 445 464 47 i 553

600 605 636 654 950 1524 b 1776 b

ABA

VR 1991

1962-1967

1968-1979

1980-1991

1.90 0.71 1.16 - 1.03" 0.38 2.86 0.50 1.68 !.72 3.66 2.72 2.32 1.49 6.87 2.24 1.68 NA NA

2.32 0.65 2.75 2.90 2.50 - 3.86 a -5.10 a 0.06 - 1.88 a 3.50 2.78 2.42 -0.21 8.31 3.97 1.00 NA NA

0.84 - 0.47 2.52 0.82 1.78 - 2.62" -2.78 a -4.63" 1.11 3.10 3.15 2.25 1.50 4.14 1.03 0.37 2.16 2.29

2.47 2.09 2.52 2.35 3.15 1.13 1.19 1.42 2.08 2.93 3.22 2.45 3.33 3.50 3.64 2.01 3.84 3.48

The relationship between ABA and VR can be expressed as J B A = - 4.9001 + 2 . 1 9 8 6 ( V R ) ( R 2 = 0 . 6 8 1 4 , P < 0 . 0 0 0 1 ).

Plots with z/BA ~< - 0 . 5 are classified as decline. "Plots classified as decline. bPlots not sampled until 1979.

for the three site types indicates that the three plots classified as decline in 1991 were all located on site type 'C' (high landscape position (low LOGPOS), moist (medium WAWI ) ) (Table 3 ). Two of the three site type C plots are located near ephemeral streams. There was no obvious source of the relatively low values for WAWI (i.e. high soil moisture) for the third site type C plot. Among the three site types, percent change in BA year-~ (JBA) did not differ significantly between 1962 and 1967 ( P > 0.05 ) but differed in the second and third sampling periods ( P < 0.05 ) because of significantly different values for mean ABA for plots in site type C (Tukey's test; SAS Institute, 1988 ) (Table 4). The lack of significant differences between JBA on the three site types prior to 1967 suggests that decline of ash on site type C plots could have been triggered by an event or events that occurred in the 1960s. Possible causes include a severe drought that occurred in the mid-1960s in the northeast (Bradley et al., 1987 ) and/or the spread of ash yellows. Witches' brooms were not observed on ash trees on or near any of the plots,

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Table 3 Mean and standard deviation for site factors associated with ash decline (identified through analysis of CFI plot data) for three principal site types (identified by DCA) for the 18 resampled CFI plots and for the study site quadrats (identified by cluster analysis) Site factor

Site type A

CFI Plots (0. 08 ha) N (no. of plots) LOGPOS a MOIS VR Study site quadrats (0.02 ha) N (no of quadrats) LOGPOS MOIS VR

B

C

6 3.46 + 0.17 3.30+_0.06 2.44+-0.43

9 3.92 +_0.76 3.01 +0.17 3.16+-0.58

3 2.90 + 0.26 3.13_+0.11 1.25+0.15

33 3.01 + 0.23 3.27+0.14 3.83+0.60

12 3.49 + 0.23 2.89+_0.14 2.59+_ 1.52

21 2.99 + 0.20 3.12+_0.13 1.84+_ 1.09

RLOGPOS =landscape position. MOIS = soil moisture (weighted average wetlands index). A low mean vigor rating (VR) for white ash was most prevalent on site type C. Table 4 Mean and standard deviation for percent change in basal area year- ~ excluding ingrowth (ztBA) for the CFI plots 1962-1967, 1968-1979, 1980-1991 for site types A, B, and C Period

1962-1967 1968-1979 1980-1991

Site type A

B

C

(n=6)

(n=9)

(n=3)

1.19+ 1.28 2.07 + 0.99 0.89+ 1.21

2.67+2.10 2.66 + 3.25 2.15+0.99

1.68+ 1.18 - 2.97 + 2.69 - 3 . 3 4 + 1.12

ABA for plots on site-type C differs significantly from ,~BA on site types A and B (Tukey's test, P < 0.05 ) in 1968-1979 and 1980-1991.

but MLOs were detected in root tissue from low vigor trees on two of the three 'decline' plots. MLOs were not detected in root tissue from trees on the two randomly selected healthy plots (results not shown ).

Stand analysis The small size of the CFI plots (0.08 ha) and their 0.8 km spacing made this data set unsuitable for evaluating either the extent or distribution of ash decline at the stand level. We therefore determined the distribution of decline as well as the site factors identified as associated with decline through analysis

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of CFI plot data within a single stand. We also estimated the area of 'declineprone' site types within the stand. Figure 2 shows the area of the stand where mortality of white ash (measured as the percent of dead standing trees) exceeded 20% in 1989. High mortality to the north and west of the stand is consistent with results of the CFI plot data analysis, which would predict that higher rates of white ash decline (and hence high mortality) would be associated with locations closer to the top of the watershed (low LOGPOS sites). Results of the analyses of resampled CFI plots predict that white ash on moist, rather than dry, sites, are more likely to show symptoms of decline. Mean values for landscape position (LOGPOS) and soil moisture (MOIS) for the three principal site types identified on the basis of cluster analysis are shown in Table 3. Values for LOGPOS and MOIS for the site type C on the study site were similar to those for CFI plot site type C (decline sites ) (Table 3 ). For the three site types, X2 analysis showed significant differences between the proportion of healthy to declining (VR ~<2.0) trees on the study site quadrats. This was due to a high ratio of decline: healthy trees on site type C ( 18: 14) compared with site types A (4:47) and B (2: 34) (X2 35.462, df2, P <0.0001 ). Site type C (high landscape position, moderately moist) was thus identified both by CFI plot and study site analyses as 'decline prone'. Declining trees were distributed throughout individual CFI plots (0.08 ha ) and study site quadrats (0.02 ha) (H. Woodcock, personal observation, 1991 ). At the scale of the study site (2 ha ), however, both declining trees and site types were heterogeneously distributed and occupied an area of 0.7 ha to the north and west of the intensive study site (Fig. 2 ). Ash yellows was not positively identified in either healthy or declining trees on the intensive study site. Tan-colored autofluorescence was, however, noted in sieve tubes in root tissue from three of the ten low-vigor trees. This symptom is considered by Sinclair et al. ( 1989 ) and C.R. Hibben (personal communication, 1989 ) to be indicative of, but not diagnostic for, ash yellows, so the site was classified as tentatively positive for ash yellows. Discussion

Our estimates of the frequency of ash decline on CFI plots indicate that approximately 20% of ash stands in Massachusetts could have been affected by ash decline in 1991. We found no evidence that the proportion of affected stands had changed since 1979. The current incidence of decline is apparently insufficient to result in a net loss of ash BA statewide, but decline can be locally severe, for example in the case of our study within a single ash stand at a scale of one to several hectares. The distribution of mature white ash on the landscape is related to soil moisture and nutrient status (Wright, 1959; Leak, 1980). Soils in Massachu-

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setts west of the Connecticut River (Fig. 1 ) are often characterized by fine texture and hardpans and have a higher available water capacity than soils in the eastern counties which tend to be more sandy. Average annual precipitation in western Massachusetts is about 9% higher than in the east. Ash density and frequency on CFI plots decreases from west to east following broad edaphic and climate gradients. In north-~-entral Massachusetts ash is found chiefly on lower slope positions and coves (Whitney, 1991 ), but Damman and Kirschner ( 1977 ) report ash occurring on a range of landscape positions in western Connecticut. We found a similar range of landscape positions represented among the CFI plots supporting white ash in western and central Massachusetts. The relationship we find between ABA and landscape position indicates that ash decline is most prevalent on mid to upper and/or steep slopes (low LOGPOS ). D a m m a n and Kirschner (1977 ) report that ash on higher slope positions is often associated with seeps and ephemeral streams, an association we also noted. The relationship we find between ash decline and sites that are moist for a given landscape position raises the question of the role that soil moisture and nutrient availability might play in the syndrome. If ash is drought-susceptible, as has been suggested (Marshall, 1930; Ross, 1964; Tobiessen and Buchsbaum, 1976; Hibben and Silverborg, 1978), one explanation for the relationship between landscape position, soil moisture and ash decline could be fluctuations in soil moisture supply. This factor would be especially severe on high landscape positions near ephemeral water courses and seeps. Such sites tend to have thinner soils with greater leaching, although Ross (1964) found no significant differences in nutritional status or pH for sites in New York exhibiting varying degrees of ash decline. A relationship between topographic control of moisture availability and the distribution and severity of decline and mortality of white ash would not be unique. Yeakley et al. (1990) reported high mortality rates of oak associated with drought on high slope positions near streams in North Carolina oak/ hickory sites. They suggest that trees growing on these sites will be subject to a relatively large moisture gradient shift in dry years and are thus more likely to become drought stressed. The effects of regional factors such as drought could therefore be highly site dependent. Similar arguments could, of course, apply to the effects of air pollutants such as acid deposition. White ash could be more at risk from drought stress in Massachusetts than in many other parts of its range. Although well within the geographical range limits for white ash (Little, 1971 ), Massachusetts, with an average growing season rainfall at Amherst (Fig. 1 ) of 49 cm (USDA, 1941 ), is at the lower limit for growing season rainfall for ash (51 cm; Wright, 1959 ). From 1962 to 1964, growing season rainfall was well below normal for western Massachusetts, with only 24 cm in 1964 (Bradley et al., 1987). Significant differences between the decline status of CFI plots on different site types were only

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observed after 1967 (Table 4), consistent with the hypothesis that the 1960s dry spell could have triggered the current decline. Several reports have described fine root damage preceding expression of crown symptoms in northern hardwood species (see Manion, 1991 ). Fine root development in black walnut is closely related to soil moisture as affected by position on the slope, with more extensive fine root development in drier soils (Pham et al., 1978). White ash root development is variable, with a deep root system on porous soils and a shallow, spreading one on rocky soils (Kim, 1988 ). Root distribution is thus likely to vary with landscape position and moisture supply. White ash may develop shallow root systems on sloping sites, especially in western Massachusetts where soils on such sites tend to be shallow and rocky, and root development may be less extensive where moisture is generally plentiful, such as areas near ephemeral streams and seeps. On such sites, trees could be not only potentially drought-sensitive but also located where the soil moisture supply is potentially variable. It has been proposed that trees infected with MLOs may be more droughtsusceptible than uninfected trees (Hibben and Silverborg, 1978; Castello et al., 1985; Hart et al., 1991 ). The single study of which we are aware that characterized 'ash yellows' sites on an ecological basis found the disease to be associated with exposed (i.e. potentially droughty) sites (Smallidge et al., 199 l a). These authors propose that an MLO infection that is tolerated on favorable sites may result in decline and mortality on less favorable sites during periods of severe stress (Smallidge et al., 1991 a,b). Our findings that ash decline is site-related and that trees on some decline sites were infected with MLOs do not contradict this hypothesis. Since we did not detect MLOs in trees on sites classified as healthy, we were unable to test the hypothesis further. With generally adequate soil moisture, white ash is apparently able to establish on topographically unsuitable sites. At present we lack a good understanding of age-dependent responses of long-lived species to environmental stress, but if the severity and distribution of ash decline is in part related to soil moisture variability we can postulate that a period free from severe dry spells could have led to the persistence of potentially off-site white ash populations prior to the 1960s. The uniformity (spatial and temporal) of disturbance associated with the European settlement of the 'New World' probably set up a number of tree species, especially those we classify as 'disturbance species' such as white ash, for the kinds of forest decline that are being manifested in the latter part of the 20th century. Massachusetts forests are relatively even-aged as a result of anthropogenic disturbance in the 19th and early 20th century, and it seems likely that a severe, regionwide stress following a relatively stress-free period, coupled with local site differences, could lead to episodes of temporally synchronous but spatially patchy decline, such as we have observed. If this hypothesis is correct, the 'boom and bust' syndrome

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could repeat itself for white ash in Massachusetts if future disturbances make sites available for ash establishment. The heterogeneous distribution of ash decline within the 13-ha stand and the 2-ha study site, highlights the need for a better understanding of the interaction between regional and local factors in declines of forest trees, and for an appreciation of the scales at which ecosystem processes operate. Recently established forest-health-monitoring surveys will be helpful in describing regional tree-health trends, but the grid size is large (Burkman and Hertel, 1992), with only 32 plots in Massachusetts (G.C. Smith, personal communication, 1992 ). Thus the data from these surveys will not be appropriate for evaluating the role of local factors. There are, however, many long-term forest monitoring projects, like the Massachusetts CFI project, established on smaller grids. These data sets not only have good potential for evaluating past trends in forest health but, if individual trees are sampled, have potential for identifying relationships between individual tree health and local factors within a region. The identification of such relationships can point the way to further studies to elucidate the role of the multiple causal factors implicated in episodes of forest decline. It can also provide information of more immediate value to practicing foresters in a field in which clear causal relationships have proved difficult to demonstrate.

Acknowledgements We would like to thank the City of Northampton for permission to conduct part of this study on their watershed property. We would also like to thank J.C. Mawson and C.A. Thompson for providing CFI plot data, W. Rivers for assistance in locating CFI plots and C.P. Laing and M.M. Wallace for field assistance. We thank W.Y. Hsiung, W.B. Leak, G.B. Sweet, M.S. Twery, and an anonymous reviewer for reviewing earlier versions of the manuscript, and M.A. Aizen, M.J. Kelty and G.C. Smith for helpful discussions and criticism. We also thank C.R. Hibben for advice on interpreting results of the MLO assay. This work was supported by a MacIntire-Stennis Cooperative Forestry Research Programs grant (MS-60) to WAP, a Grant-in-Aid from Sigma Xi to HW and a Dissertation Fellowship from the American Association of University Women to HW.

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