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1985

Demographic Aspects of Coexistence in Engelmann Spruce and Subalpine Fir

Kathleen L. Shea

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Recommended Citation Shea, K. (1985). Demographic aspects of coexistence in Engelmann spruce and subalpine fir. Amer. J. Bot. 72(11): 1823-1833.

This Article is brought to you for free and open access by the Quinney Natural Resources Research Library, S.J. and Jessie E. at DigitalCommons@USU. It has been accepted for inclusion in The Bark Beetles, Fuels, and Fire Bibliography by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. Amer. J. Bot. 72(11): 1823-1833. 1985.

DEMOGRAPHIC ASPECTS OF COEXISTENCE IN ENGELMANN SPRUCE AND SUBALPINE FIR!

KATHLEEN L. SHEA 2 Department of Environmental, Population and Organismic Biology, University of Colorado, Boulder, Colorado 80309

ABSTRACT The mechanisms for the maintenance of coexistence of Engelmann spruce and subalpine fir in subalpine forests of the Colorado Front Range were examined by comparing age, size, and spatial distributions of spruce and fir in two adjacent, previously logged sites ofdiffering moisture availability. Adult tree ages were calculated from stem cores, while seedling ages were calculated from a multiple regression equation based on diameter, height, and number of branch whorls. Tree size was measured by height and diameter; spatial distributions were described by Morisita's index of dispersion. Cumulative age and size distributions were significantly different in the two species, with greater longevity and a larger overall size in spruce than fir. Both species showed a significant linear relationship between size and age, while fir showed a faster height growth rate than spruce. The linear relationship beween age and size was much closer in seedlings than in adults. Seedling spatial distribution was highly clumped in both species, but mature trees showed little or no clumping. Because both species are mainly wind dispersed, the greater clumping in spruce than in fir seedlings suggests that spruce have more specific establishment requirements than fir. Colonization patterns indicated that spruce seedlings were primarily found in forest gaps or associated with fir canopy trees, while fir seedlings were more commonly found in the forest, associated with either spruce or fir canopy trees. Tree density, growth rates, and mortality rates were higher in the wet site, with spruce showing the largest between site differences. These data suggest a new hypothesis for coexistence stating that Engelmann spruce and subalpine fir are maintained as codominants because the greater longevity and size of spruce is balanced by the faster height growth and more flexible seedling establishment requirements of fir.

THE QUESTION of what factors permit species they are relatively simple communities in which to coexist has long engaged both plant and spruce and fir coexist, often as the only two animal ecologists (Hutchinson, 1959; Grubb, tree species present (Marr, 1961), and because 1977; Werner, 1979; Brown, 1981). Accepted they comprise a large part of the forested area coexistence theory has been derived from in the central Rocky Mountains. According to Gause's (1934) principle of competitive exclu­ Alexander (1974), the subalpine forests are the sion; that is, species must show niche differ­ largest and most valuable timber resource in entiation in order to coexist. More recently, Colorado and Wyoming. Pickett (1980) suggested that coexistence of Although it is generally agreed that Engel­ plant species occurs either because species oc­ mann spruce and subalpine fir coexist and form cupy different niches or because disturbance the predominant climax community in the prevents competitive equilibrium, while Aars­ subalpine zone of the central and northern sen (1983) attributed coexistence of plant Rocky Mountains (Daubenmire, 1943; Sta­ species to niche differentiation or reciprocal helin, 1943; Marr, 1961; Romme and Knight, selection that maintains a balance of compet­ 1981), the mechanisms permitting their co­ itive abilities. In the present study, Engelmann existence are not clear. Four different hypoth­ spruce-subalpine fir forests were selected to ex­ eses have been presented specifically for En­ amine these questions of coexistence because gelmann spruce-subalpine fir communities: 1) Peet (1981) suggested that coexistence of spruce I Received for publication 1 October 1984; revision ac­ and fir is maintained by disturbance because, cepted 25 June 1985. !thank M. Farris, M. Grant, Y. Linhart, and T. Veblen in the absence of disturbance, the greater re­ for many helpful comments on this manuscript. N. Moss, productive success offir in the understory would D. McClearn, and L. Engkelking assisted in the field work. lead to a steady increase in the dominance of This work was supported by a J. S. Noyes Foundation fir and the virtual elimination of spruce after Fellowship and grants from the K. Lichty and G. Alexander Funds. about 1,000 years; 2) in Alberta, Day (1972) 2 Current address: Department of Biology, St. Olaf Col­ found that subalpine fir cannot compete with lege, Northfield, MN 55057. Engelmann spruce x white spruce hybrids 1823 1824 AMERICAN JOURNAL OF BOTANY [Vol. 72

(Picea engelmannii x Picea glauca) in the logical studies that include spruce-fir forests in overstory because of early predisposition to the central Rocky Mountains (0osting and disease and inability to withstand evaporative Reed, 1952; Marr, 1961; Peet, 1981), few stress, even though fir may be four times as workers have examined age structure (Miller, common as spruce in the understory (Day also 1970; Hanley, Schmidt and Blake, 1975; suggested that in the absence of fire, spruce WhippleandDix, 1979; ParkerandPeet, 1984) would decrease and fir would increase in dom­ or size structure (Peet, 1981), and only Knowles inance); 3) Oosting and Reed (1952) concluded and Grant (1983) have compared age and size that the shorter life span of subalpine fir to structures. No published studies of Engelmann Engelmann spruce (300 vs. 500 yr) results in spruce and subalpine fir have included com­ earlier death of fir canopy trees and that the parisons of age, size, and spatial distributions prolific nature of fir maintains it as a codom­ in seedlings and adults in different microhab­ inant; and 4) Fox (1977) suggested that differ­ itats. Both seedlings and adults are included ences in seedling establishment requirements because the probability of death for most plant result in alternation of species in the same lo­ species is highest in the seed and seedling stage cation and therefore coexistence. Peet's sug­ and often declines and becomes constant dur­ gestion is not likely ever to be testable because, ing the adult period, up to senescence (Harper, as Peet himself notes, 1,OOO-yr periods without 1977; Werner, 1979). disturbances are virtually unknown. Other re­ cent studies (Harper, 1977; White, 1979; Shu­ MATERIALS AND METHODS - The two study gart and West, 1981; Sousa, 1984) have shown sites were located on Niwot Ridge, elevation that disturbance at some level is nearly uni­ 3,200 m, in the Colorado Front Range, near versal and communities are rarely in compet­ the University of Colorado Mountain Re­ itive equilibrium. search Station, 10 km north of Nederland. The purpose of this study was to examine These particular stands of Engelmann spruce, Hypotheses 3) and 4) by comparing the de­ Picea engelmanii (Parry) Engelm., and subal­ mographic characteristics of age, size, and spa­ pine fir, Abies lasiocarpa (Hook.) Nutt., were tial distributions in all-aged spruce-fir stands selected because: 1) their proximity (200 m) typical of those currently found in the Colorado makes them replicates of each other for cli­ Front Range. Most of these stands have been matic conditions; 2) they have small, disturbed partially logged and the present study acknowl­ areas providing gaps for seedling establish­ edges this widespread situation and aims to ment; and 3) they represent examples of rel­ gain a better understanding of the demography atively wet and dry stands found on Niwot of such forest tree communities. More specif­ Ridge. As with most stands in the Rocky ically, the role of each tree species in the com­ Mountains, there is evidence of disturbance, munity was studied by 1) comparing age and both lumbering and fire; the area has been rel­ size distributions within and between popu­ atively undisturbed since 1900. The lumbering lations of spruce and fir, 2) comparing patterns occurred 90-110 yr ago (F. Schweingruber, un­ of spatial distribution between spruce and fir publ. data), and a count of stumps in the two and between seedlings and adults of each sites showed that lumbering reduced the total species, and 3) comparing age, size, and spatial sample size by about 20%. Only a few stems distributions of spruce and fir in two adjacent showed evidence of fire. sites of differing moisture availability. It is im­ Plant species diversity is low in mature En­ portant to examine both age and size distri­ gelmann spruce-subalpine fir forests. Spruce butions in plant populations because the plas­ and fir are the predominant tree species, with ticity and indeterminate growth form of plants an occasional lodgepole (Pinus contorta Dougl.) often makes size a more sensitive indicator of or limber pine (Pinus flexilis James) in dis­ individual interactions and microenviron­ turbed or drier areas. The understory is cov­ mental variation than age (Johnson and Bell, ered by herbaceous species, predominately 1975; Knowles and Grant, 1983). However, Vaccinium myrtillus L., Pedicularis racemosa gaps in the age structure provide evidence of Dougl., and Polemonium delicatum Rydb. past disturbance history and successional sta­ At each site, four 2 x 100-m transects were tus that may not be present in the size structure placed 10m apart, and all individuals in lOx (Whipple and Dix, 1979; Lorimer, 1980). In 2-m plots within these transects were num­ addition, the correspondence between age and bered. Division of each transect into smaller size distributions varies with different tree plots facilitated subsampling. Within these species (Knowles and Grant, 1983; Lorimer transects all adult trees (greater than 7.5 cm and Krug, 1983) and seral status (Despain, diam at breast height) and a random selection 1983). Although there are several general eco- of seedlings and saplings (30-40% of those November, 1985] SHEA-DEMOGRAPHY AND COEXISTENCE IN SPRUCE AND FIR 1825 originally marked) were chosen so that a total number of branch whorls on the same 30 spruce of 253 spruce and 257 fir were studied. Seed­ and 31 fir seedlings. If Y represents seedling lings were defined as individuals less than 0.5 age, Xl represents height (mm), X 2 represents m tall; saplings were individuals greater than diam (mm), and X3 represents number of 0.5 m tall and less than 7.5 cm diam. If two branch whorls, then the regression equation or more stems were physically connected, the for spruce is Y = 2.07 + 2.881nXI + 0.7181' group was considered to be one individual and InX2 + 0.588X3 (r2 = 0.82, P :s 0.001) and subsequent measurements were recorded for for fir is Y = -12.16 + 2.361nXI + 4.551· the largest stem. Diameter and height were InX2 + 0.219X3 (r2 = 0.81, p:s 0.001). The measured on each tree; heights greater than 2 correlation between predicted age and actual m were measured with a clinometer. Natural age (for the 30 spruce and 31 fir seedlings) is log transformations ofdiameter and height were r = 0.90 (P :S 0.001). Chi-square tests showed used successfully to normalize the data for both no evidence that the age estimates for either spruce and fir seedlings. The number of branch spruce or fir were biased; that is, nearly the whorls was also recorded for each seedling. same number of ages were over and underes­ Mortality estimates were made by recording timated. Further regression analyses indicated the number of missing or dead individuals in a significant linear relationship between age each plot for the two growing seasons after they and each independent variable alone, but the were labeled. As in most studies oflong-lived best age predictions were obtained by using all tree populations, data for the present study three variables: diameter, height, and number necessarily represent one point in time. of branch whorls. Number of branch whorls Soil moisture was measured gravimetrically in spruce and height in fir were the best single (Shea, 1985), at the 5-1O-cm depth, every 2 predictors of age, with correlation coefficients wk during the growing season from July through of r = 0.89 and r = 0.88, respectively, between September 1983. Moisture stress measure­ predicted and actual age. ments were made directly on the trees using a pressure chamber. Measurements were taken Spatial distribution. - The position of all in­ in the field, at dawn, and at the warmest part dividuals was mapped to the nearest decimeter of the day (noon to 2 p.m.) on the same days within each transect. Maps were drawn from that the soil moisture samples were collected. the X, Y coordinates by a Calcomp plotter for Pressure chamber measurements were made each species in each site. The number of in­ using small branch tips, 3-5 cm long, located dividuls per quadrat was determined from these 1.5 m above ground, from the same three to maps. six trees at each site for each sampling period. Morisita's (1959) index, J/j, was chosen as a Sampling procedures followed those of Ritchie measure of clumping. The use of this measure and Hinckley (1975) and Kaufmann and Thor has been described by Sakai and Oden (1983) (1982); equal numbers of spruce and fir trees and Goodall and West (1979). Morisita's index were measured. is given by

q Tree ages-Trees larger than 2.5 cm diam J/j = q ~ nj(nj - l)/N(N - 1), were aged using tree cores taken at 30 cm above j~l ground on the east-facing side ofeach tree. Ages were determined by counting rings on dried where q is the number of quadrats, nj is the and sanded cores with a dissecting microscope. number of individuals of the species in the ith Height and age measurements on 30 spruce quadrat, and N is the total number of individ­ and 31 fir seedlings were used to calculate uals in all q quadrats. The index J/j equals 1.0 regression equations for predicting age at a when the population is randomly dispersed, height of 30 cm. This predicted age, 23 yr for where random implies an independent distri­ spruce and 24 yr for fir, was added to each core bution ofindividuals in quadrats, with an equal measurement to estimate years lost by coring probability of each individal occurring in any trees at 30 cm instead of ground level. If Y one quadrat. If the individuals are clumped, represents seedling age and Xl represents height then J/j > 1.0, and if the individuals are evenly (mm), then the regression equation for spruce distributed or hyperdispersed, then J/j < 1.0. is Y = - 30.03 + 9.301nXI (r2 = 0.65, P :s Morisita's index was computed for seedlings 0.001) and for fir is Y = - 30.48 + 9.501nXI (individuals :s0.5 m tall) and adults (individ­ (r2 = 0.77, P :s 0.001). uals >0.5 m tall) using three quadrat sizes. Seedling ages were estimated nondestruc­ There were too few saplings to calculate a sep­ tively using multiple regression equations based arate index for them. The smallest quadrat size on measurements of diameter, height, and was selected by requiring the minimum mean 1826 AMERICAN JOURNAL OF BOT ANY [Vol. 72

.. j 1 ENGELMANN SPRUCE 2 SUBALPINE FIR AGE. AGE.

SIZEo SIZE 0

! .0 >- .0 >- () () z Zw w 5 15 5 15 W W ...a: ...a: 10 10

400 300 400 DIAM (em) 1 10 .0 30 40 DIAM(em) , '0 .0 30 40

Fig. 1, 2. Age and diameter frequency distributions. 1. Engelmann spruce (N = 245). 2. Subalpine fir (N = 251). of 0.6 individuals per quadrat necessary to de­ in the two species, fir had a larger proportion tect clumping (Sakai and Oden, 1983). For a of smaller and younger individuals than spruce. species in a given area the values of J/j calculated Cumulative age and size distribution curves for various quadrat sizes permit identification (Fig. 3, 4) further facilitate comparison be­ of the size at which clumping is greatest and tween species. The curves represent net sur­ suggest natural patch size for a given species vivorship, which is composed of each class's (Morisita, 1959). The patches must be large establishment minus mortality of all previous enough to accommodate more than one in­ classes. As tested by the Kolmogorov-Smirnov dividual so that clumping is a logical possi­ goodness-of-fit procedure, spruce and fir had bility. significantly different age class and size class distributions. Spruce had a higher proportion RESULTS-Although tree size was measured of older individuals than fir in each age class, by both diameter and height, the two variables and both species had a steady decline in pro­ had similar patterns, and in this discussion portion of individuals with increasing age. The diameter will represent size unless otherwise fir age distribution leveled off at 250 yr, while specified. The Pearson product-moment cor­ the spruce distribution showed a fairly steady relation between height and diameter, based decline through all age classes. Size distribution on all individuals studied, was highly signifi­ showed the same general pattern, with spruce cant in both species (P :::::; 0.001), r = 0.97 for having a higher proportion of larger individ­ spruce and r = 0.95 for fir. For clarity in pre­ uals in each size class than fir. When trees in sentation, measurements were grouped using the postlogging population, that is, trees less 10-yrage classes, 1-cmdiam classes, and 0.5-m than 110 yr old, were compared, age and size height classes. distributions were still significantly different The age and size frequency distributions (Fig. between species (P :::::; 0.01 for age and size), 1, 2) showed similar trends within each species with spruce having a higher proportion of older and similar patterns between spruce and fir. or larger individuals in each class. The largest percentage of individuals was in Regression analyses were used to examine the smallest size class in both species. Over the relationship between size and age (Fig. 5). 40% of the spruces and over 50% of the firs In both species, diameter and height showed were in the l-cm diam or smaller size class. In a significant linear relationship with age (P :::::; both spruce and fir the age class distribution 0.001). An a posteriori test showed that the showed a lower peak than the size class dis­ regression coefficients for height, but not di­ tribution, with nearly 30% of the spruce and ameter, were significantly different (P :::::; 0.01) nearly 50% of the fir individuals in the 20 yr in spruce and fir. The larger regression coeffi­ or younger age classes. Both species also had cient for height in fir suggested that fir may a small peak in the age class distribution around grow in height at a faster rate than spruce. The 100 yr, with nearly 15% of the individuals in proportion of variation explained by these the 90-110 yr age classes. This peak was not equations (r2, the coefficient of determination) evident in the size class distribution. While the varied from 0.61 to 0.72. When only trees ~2 range of age and size distributions was similar m tall were included in the analyses, there was November, 1985] SHEA - DEMOGRAPHY AND COEXISTENCE IN SPRUCE AND FIR 1827

1.000'..------,

.900 z o ENGELMANN SPRUCE o .BOO o ENGELMANN SPRUCE ...... • SUBALPINE FIR a: • SUBALPINE FIR a: o .700 \1 o... .700 ~ .600 'R ~ .600 ... Q. 10\ .500 \ 1U .500 ~ ~ > !( .400 \ ...J ~ ... 00 ::> ..J ::;: .300 ::> ::;: .300 ::> () .. ~:;:.::: ::> .200 () .200 .100 .100 °0L-~~~1~0-~1~5-720~::25~~30~~35~~.~0~J O~O------~1+00~----~~~==~~~~~4~0~O ONE-CM DIAMETER CLASSES TEN-YEAR AGE CLASSES Fig. 4. Cumulative size class distributions for Engel­ Fig. 3. Cumulative age class distributions for Engel­ mann spruce and subalpine fir. The two curves are sig­ mann spruce and subalpine fir. The two curves are sig­ nificantly different (P s 0.05) by the Kolmogorov-Smimov nificantly different (P s 0.001) by the Kolmogorov-Smir­ two-sample goodness-of-fit procedure. nov two-sample goodness-of-fit procedure. sites suggests that differences in plant water still a significant linear relationship of diam stress occurred during the hottest part of the and height on age (P ::5 0.001). However, the day, but not at night. In addition, the Pmin r2 for diam vs. age decreased to 0.18 (b = 0.06) and Pmax values of -0.6 to -1.5 MPa indi­ in fir and 0.43 (b = 0.07) in spruce; r2 for height cated that the trees were not under severe water vs. age was 0.08 (b = 0.02) in fir and 0.38 (b = stress, assumed to begin at xylem pressure po­ 0.03) in spruce. tential measurements higher than -2.0 MPa (Ritchie and Hinckley, 1975). Soil moisture Microenvironmental variation - Examina­ decreased significantly over the growing sea­ tion ofbetween-site variation showed that pop­ son; however, Pmax values varied depending ulation demographic characteristics can vary on rain and cloud cover. over short distances (::5 200 m). Moisture avail­ Total tree density, which includes both spruce ability was the most noticeable environmental and fir (Table 1), was significantly higher (1.26 difference between the two sites as evident in stems/m2) in the wet site than in the dry site both the soil and tree measurements. For 1970- (0.68 stems/m2). Part ofthe high density in the 1982 the mean total precipitation for the 9- wet site resulted from high-density seedling mo period, January through September, at the patches, but basal area was also greater in the C-l site on Niwot Ridge (elevation 3,000 m) wet site. Mature trees in the wet site occupied was 552 mm, as compared to 692 mm for January through September 1983 (Losleben, 1983). Although precipitation was above nor­ mal in 1983, percent soil moisture and max­ 40 imum plant water potential measurements av­ 16 eraged over the 1983 growing season were E significantly different between the two sites (soil 2 30 :z: moisture averaged 113.3 ± 9.0% wet site vs. a: 12 m w Ii) 86.5 ± 8.4% dry site, P ::5 0.05; Pmax aver­ I- W :z: -4 aged -1.27 ± 0.04 MPa wet site vs. -1.46 ± :::;: FIR DIAM < 20 b = 0.10 ~ 3 0.04 MPa dry site, P ::5 0.001; Pmin averaged c r2 =0.62 8 -0.68 0.03 wet site vs. -0.65 0.03 dry ± ± .;- site, not significantly different). Pmin and Pmax 10 ""-- SPRUCE D.AM refer to the mean predawn and midday ./ b = 0.09 4 xylem pressure potential measurements. Be­ r 2 =0.71 cause there were no significant differences in moisture stress between species, Pmin and 100 200 300 Pmax values for spruce and fir were combined AGE (yr) to examine between-site differences. The fact Fig. 5. Least squares regression curves of diameter that Pmin (minimum xylem pressure poten­ (dashed lines) and height (solid lines) on age for Engelmann tial) was not significantly different between the spruce (N = 245) and subalpine fir (N = 251). 1828 AMERICAN JOURNAL OF BOT ANY [Vol. 72

TABLE I. Mean and 95% confidence intervals (in parentheses) for sizes and ages of adult and seedling spruce and fir at two sites. Adults are trees 2:.2 m tall and 2:.7.5 cm diam; seedlings are ::;0.5 m tall. '" Denotes significant differences between sites, * P ::; 0.05, ** P ::; 0.01, *** P ::; 0.001

Measurement Wet site (1.26 stems/m') Dry site (0.68 stems/m') A. Adults Engelmann spruce N= 27 N= 54 Diameter (cm) 20.71 (16.48-24.94) 19.44 (17.65-21.22) Height (m) 10.64 (9.06-12.23) 9.74 (8.96-10.51) Age (yr)*** 144 (116-173) 233 (213-254) Subalpine fir N= 29 N= 37 Diameter (cm)** 19.93 (16.51-23.35) 14.49 (12.44-16.55) Height (m)** 10.99 (9.66-12.31) 7.92 (6.93-8.91) Age (yr)* 130 (104-157) 161 (143-179) B. Seedlings Engelmann spruce N= 69 N= 40 Diameter (cm) 0.42 (0.38-0.48) 0.50 (0.42-0.59) Height (m) 0.19 (0.17-0.21) 0.17 (0.14-0.20) Age (yr) 17 (16-18) 17 (15-18) No. branch whorls 13 (11-14) II (9-13) Subalpine fir N= 75 N= 68 Diameter (cm) 0.34 (0.30-0.40) 0.33 (0.28-0.39) Height (m) 0.11 (0.10-0.13) 0.12 (0.10-0.13) Age (yr) 14 (13-15) 14 (12-15) No. branch whorls 8 (6-9) 7 (6-9)

55.6 m 2 per hectare, as compared to 31.2 m 2 ofthe large age differences between sites, heights per hectare in the dry site. There were also and diameters were also compared using age differences in basal area occupied by each as a covariate. With age as the covariate, adult species in each site. Mature spruce trees oc­ spruce diams (24.52-cm wet site vs. 17.38-cm cupied 51.3% (48.7% for fir) of the basal area dry site) were significantly different between in the wet site, as compared to 71.2% (28.8% sites (P :::; 0.001), but heights (12.32 m wet site for fir) of the basal area in the dry site. vs. 9.2 m dry site) were not. In a similar analysis Mean sizes and ages in the two sites showed for fir, diams (20.81-cm wet site vs. 13.81-cm different patterns for adult spruce and fir (Table dry site) and heights (11.10-m wet site vs. 1). Both species had a significantly higher mean 7.73-m dry site) were significantly different be­ age in the dry site than in the wet site (P :::; tween sites (P :::; 0.001 for diam, P :::; 0.05 for 0.001 for spruce, P :::; 0.05 for fir). Spruce height). showed no significant size differences between The seedlings showed a pattern of size vari­ sites, while fir trees were significantly larger ation different from the adults (Table 1). For (P :::; 0.01) in diam and height in the wet site. both spruce and fir seedlings, univariate anal­ Because trees in the wet site were the same size ysis of variance tests for each character mea­ or larger than trees in the dry site, both species sured showed no significant differences be­ had faster growth rates in the wet site. Because tween the wet and dry sites. Since the size

TABLE 2. Morisita's index (I,j of dispersion for adult and seedling spruce and fir in wet and dry sites. Seedlings are individuals ::;0.5 m tall, adults are >0.5 m tall. Values ofI, = 1.0 represent a random pattern, values> 1.0 indicate clumping. .,. Values are significantly> 1.0 (P ::; 0.01) according to an F-test of Morisita (1959)

Spruce Quadrat size Fir (m') No. of quadrats Seedlings Adults Seedlings Adults A. Wet site (N = 69) (N= 60) (N = 75) (N = 48) 4 96 9.33* 0.81 3.18* 1.19 16 24 4.24* 0.98 1.88* 1.32 64 6 1.87* 0.95 1.30 1.19 B. Dry site (N = 40) (N= 82) (N = 68) (N = 59) 8 96 6.65* 1.44 1.85* 2.13* 16 48 4.12* 1.13 1.64* 1.60* 64 12 1.71 * 1.10 1.16* 1.02 November, 1985] SHEA - DEMOGRAPHY AND COEXISTENCE IN SPRUCE AND FIR 1829

TABLE 3. Density and mortality for seedlings (individuals TABLE 4. Proportion of seedlings in forest gaps and as­ sO.5 m tall) sociation with dominant canopy trees. Data were ob­ tainedfrom 10 x 2 m plots (N = 26) with six or more % Plots seedlings. For dominant tree plots, only plots in which (10 x 2 m) containing adults of one species occupied 70% or more of the seedlings basal area were included Density at least % Mortality Site (Nlm2) once 1981-83 Forest gaps Spruce Fir Wet site (no adult dominant dominant trees) canopy tree canopy tree Engelmann spruce 0.56 95 5 Site (5 plots) (12 plots) (9 plots) Subalpine fir 0.50 95 8 Wet site Dry site Engelmann spruce 0.63 0.03 0.34 Engelmann spruce 0.12 52 analysis of variance (manova) was used to de­ (N= 74) termine if the combined effects of diameter, Subalpine fir 0.01 0.50 0.49 height, and number of branch whorls produced (N= 262) differences between sites. Spruce seedlings were significantly different (Wilk's lambda = 0.788, P ::s 0.001) between sites, but fir seedlings were In addition, seedling density and mortality for not. A discriminant function analysis correctly both species were greater in the wet than in the classified 76% ofthe spruce seedlings as coming dry site. from the wet or dry site, and diameter was Examination of the locations of spruce and twice as important as either branch whorls or fir seedlings (Table 4) showed that spruce were height in determining group membership. more common in the forest gaps and fir were Spatial distribution, which is influenced more common in the forest. Dry site gaps had largely by competition and microhabitat vari­ a smaller proportion ofthe total seedlings, with ation, may be described by three general pat­ almost no fir seedlings. In both sites, spruce terns: 1) uniform, 2) random, and 3) clumped seedlings were more commonly found in plots (Hutchinson, 1953). Uniform patterns indicate with dominant fir rather than dominant spruce competition; clumped patterns suggest mu­ trees. Fir seedlings were more common under tualisms or microenvironmental variation. A dominant fir trees in the wet site and equally random pattern may result from the absence common under dominant fir and dominant of interactive forces between individuals in a spruce trees in the dry site. population or from disturbance factors, such as climate or predation (Gill, 1975). In both DISCUSSION- The overall patterns of the age sites, spruce and fir seedlings were significantly and size distributions examined here support clumped, while the adult trees showed a nearly Oosting and Reed's (1952) findings that spruce random distribution (Table 2). Spruce seed­ has greater longevity and a larger average size lings were consistently more clumped than fir than fir, and they also indicate a steady rate of seedlings, and seedlings of both species were replacement, as opposed to major bursts of more clumped in the wet than in the dry site. successful establishment, in both species over The patterns were reversed for adult trees; fir several centuries. Although instantaneous age adults showed a higher degree of clumping than data are not adequate for determination of the spruce adults, and adults of both species were exact proportions of recruitment and mortal­ more clumped in the dry than in the wet site. ity, I suggest that the age distributions in this The adult dry site fir were significantly clumped, study were predominately shaped by mortality showing as much clumping as the seedlings at of seeds and seedlings. While the large numbers the smaller quadrat sizes. of seeds produced (Alexander and Engelby, A comparison of presence or absence of seed­ 1983; Shea, unpubl. data) enable these species lings in lOx 2-m plots throughout the sites to respond to large disturbances with increased showed that at least one spruce or fir seedling seedling survival, the data showing a relatively was present in most plots in the wet site (Table constant number of survivors per age class sug­ 3). Fir seedlings in the dry site were more than gest that mortality is largely density dependent. three times as common as spruce seedlings and That is, as more seeds are shed, mortality rates were present in 90% of the plots, while spruce increase at the seed and early seedling stages, seedlings were present in only half the plots. resulting in approximately the same number 1830 AMERICAN JOURNAL OF BOTANY [Vol. 72 of individuals in each age class (Hett, 1971; dition, the similarity of the age and size dis­ Harper, 1977). In this study no single 10-yr tributions may be influenced by the age and age class over 30 yr had more than 10% of the size range of individuals sampled. Knowles and individuals sampled. I also examined the data Grant (1983) found larger differences between in 1-yr age classes to determine ifgrouping trees age and size frequency distributions in the co­ into age and size classes may have partially nifer species they studied, as did Linhart et al. obscured peaks of establishment. The number (1981) for ponderosa pine (Pinus ponderosa). of individuals in l-yr age classes (ungrouped However, these results are not directly com­ data) showed that cohorts over 30 yr of age parable to my study because Knowles and Grant usually had one to three individuals. The larg­ did not include seedlings, and in the Linhart est cohorts were 12- and 13-yr-olds in fir, each et al. study fewer than 10% of the individuals with 13 individuals (5%), and 14-yr-olds in were seedlings. The linear regression analyses spruce, with 10 individuals (4%). Over the age of size on age presented here showed a close span of the trees studied, no individuals rep­ relationship between the two variables, with resented 62% of the years for spruce and 68% age explaining 60-70% of the variation ob­ of the years for fir. Similar data were obtained served in size. The lower coefficients of deter­ by Knowles and Grant (1983), who also con­ mination (r2, see Results section) for mature cluded that mortality dominates the shape of trees alone are closer to values found by the age and size distributions in the conifer Knowles and Grant and indicate that the re­ species they studied, which included Engel­ lationship between age and size is closer in mann spruce. Parker and Peet (1984) believe seedlings than in adult trees. Also, it is im­ that cumulative age curves are shaped at least portant to note that outliers (in this case the as much by establishment as mortality in stands large number of seedlings) have a greater sta­ recovering from disturbance. tistical effect (leverage) in correlation and Although the cumulative age and size class regression than numbers in the middle of the distributions of spruce and fir differed statis­ distribution. tically between species, they were similar in shape. Their shape suggests that the stands were Microenvironmental variation - Between­ all-aged and had experienced decreasing mor­ site comparisons showed that species-specific tality with age. The negative power function differences persisted despite microenviron­ model, a mathematical description of decreas­ mental variation, but that site differences af­ ing rate of mortality with age, describes the age fected the relative frequencies of spruce and structure distribution of most shade tolerant fir, as well as the magnitude of the differences tree species (Hett and Loucks, 1976). Whipple in other demographic characteristics. Only the and Dix (1979) found similarly shaped, in­ dry site data (Tables 3 and 4) support the con­ verse-J, age distributions in spruce and fir stands clusions of Oosting and Reed (1952), Day in Colorado, although they also found bimodal (1972), and Peet (1981) that fir seedlings are and decreasing patterns. A bimodal age dis­ more common than spruce seedlings. How­ tribution for Engelmann spruce (Miller, 1970) ever, the higher frequency of spruce and lower in western Colorado was attributed to epidem­ frequency of fir seedlings in the wet site were ics of Engelmann spruce beetle, while in the largely the result ofa few highly clumped seed­ same population subalpine fir had an inverse-J ling patches in forest gaps with adequate light age distribution. Peet (1981) found bell-shaped and moisture. diameter distributions in younger, even-aged Between-site differences in age, size, and stands, and inverse-J diameter distributions in mortality show a faster growth rate and suggest all-size stands of spruce and fir in Colorado. a higher mortality rate for trees in the wet site. The information provided by age and size Because moisture was the only prominent en­ distributions is often similar (Lorimer, 1980), vironmental difference between the sites, I con­ a conclusion supported by the overall similar­ clude that moisture was an important factor ity of the age and size distributions in this contributing to the increased growth and mor­ study, but some information on stand history tality rates. Growth rate differences between may be lost by examining size distributions sites were not as readily apparent in the seed­ alone. For example, the small peaks in the fre­ lings, which suggests that it took many years quency age distributions of both spruce and fir of slightly lower growth rates in the dry site for at 90-110 yr were absent in the size distri­ the size differences to become measureable. butions. These peaks are likely the result of Diameter was more sensitive to microenvi­ seedling establishment after disturbance, such ronmental variation than height for mature as the lumbering, in the late 1800's (Knowles, trees of these species. Reasons for the adult age 1980; F. Schweingruber, unpubl. data). In ad- differences between sites may be found in dis- November, 1985] SHEA-DEMOGRAPHY AND COEXISTENCE IN SPRUCE AND FIR 1831 turbance and/or mortality patterns. The pres­ Ii shed in small open areas. The location of fir ence of more stumps in the wet than in the dry seedlings suggests that they have a wider range site suggests more logging damage in the wet of tolerances that enables them to become es­ site. The presence of recently downed mature tablished under both spruce and fir canopy trees, trees only in the wet site suggests a higher rate and in some cases, in forest gaps as well. Be­ of mortality for wet site mature trees, as well cause there was at least one seedling of each as seedlings (Table 3). Moisture in the wet site species in most plots (Table 3), the high degree may make the mature trees more susceptible of clumping was not the result of distribution to disease and decrease stability of root sys­ mechanisms, but rather results from the dis­ tems. The more common causes of mortality tribution of favorable habitat patches. in mature spruce and fir are windfall, insects, My height regressions, which show faster and disease (Alexander, Shearer and Shepperd, growth in fir than spruce, are supported by 1984; Alexander and Shepperd, 1984). For studies on saplings by McCaughey and Schmidt seedlings, the extra moisture may result in more (1982) and newly germinated seedlings by germination, higher seedling density, and, con­ Knapp and Smith (1982). Although other stud­ sequently, higher seedling mortality. ies have found faster growth rates in spruce Because spruce and fir are mainly wind dis­ (Schmid and Hinds, 1974; Peet, 1981), differ­ persed, the greater clumping in spruce than in ent habitat conditions and the age range of fir seedlings suggests that spruce seedlings have individuals sampled may determine which more specific establishment requirements than species grows faster. Spruce seedlings in this fir. These results support the findings of Noble study were generally larger than fir seedlings and Ronco (1978) that spruce is more exacting because of their greater average age and pro­ in its seedbed requirements than fir and that pensity to colonize areas with greater light seedbeds with exposed mineral soil are most availability. Colonization patterns may also favorable for spruce. A few squirrel caches were explain the size differences between sites found found that contained both spruce and fir seeds, in spruce, but not fir seedlings. Fir establish­ but their location and infrequent occurrence ment locations were similar in the two sites, suggest that they are a minor source of seedling while the extra moisture in the wet site gaps establishment. Because clumping in seedlings led to higher density seedling patches, with was greatest at the smallest quadrat sizes, nat­ taller and thinner spruce plants. ural patches are probably fairly small or less Because only two sites were examined in this than 20 m 2 • The higher degree of seedling study, the between-site differences observed in clumping in the wet site is likely the result of seedling frequency, seedling establishment pat­ more favorable patches, in terms oflight, mois­ terns, and adult sizes and ages need further ture, and soil in the wet site. The lack of sig­ study. Other possible explanations (besides nificant clumping in adults, except for the moisture) for the observed site differences are smaller quadrat sizes in fir in the dry site, sug­ sampling error or disturbance. While distur­ gests that most of the area studied was favor­ bances such as logging, fire, disease, herbivory, able for tree growth and that competitive in­ etc. undoubtedly had some cumulative effect teractions, density dependent mortality, and/ on the site age differences, the patterns of dif­ or disturbance decrease clumping as trees ma­ ferences in seedling frequency, seedling estab­ ture. Veblen, Schlegel and Escobar R. (1980) lishment, and adult sizes are more parsimo­ found that favorable patches were largely re­ niously explained in terms of the significant sponsible for significantly greater clumping environmental differences between the sites. (measured by Morisita's index) in seedlings than Although I suggest moisture as the likely en­ in adults in Nothofagus forests in Chile, while vironmental factor affecting size and estab­ in Costa Rica, Hubbell (1979) found various lishment patterns in these species, the point is clumping patterns in seedlings and adults, de­ that microenvironmental variation can cause pending on species, seed dispersal, predation, significant differences in demographic char­ competition, and microenvironment. acteristics and, in turn, community structure. The colonization patterns observed only In answer to the question of how spruce and partially support Fox's (1977) hypothesis, in fir coexist, the published hypotheses were only that my data indicate different establishment partially supported. The results of this study requirements for spruce and fir, but not that suggest a new hypothesis stating that coexis­ spruce and fir replace one another because tence is maintained by the greater longevity seedlings of one species are associated with and size of spruce, balanced by the faster height canopy trees of the opposite species. The lo­ growth rate and less-specific seedling estab­ cation of spruce seedlings suggests that they lishment requirements of fir. While spruce and will succeed fir canopy trees or become estab- fir no doubt compete for resources, their dif- 1832 AMERICAN JOURNAL OF BOTANY [Vol. 72 ferent demographic characteristics allow each GILL, D. E. 1975. Spatial patterning of pines and oaks to be at a selective advantage in different mi­ in the New Jersey pine barrens. J. EcoI. 63: 291-298. GOODALL, D. S., AND N. E. WEST. 1979. A comparison crohabitats. The niche differentiation observed of techniques for assessing dispersion patterns. 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The National Academy of Sciences (NAS) invites applications from American scientists who wish to make visits beginingduring the period 1 January 1987, through 31 December 1984, in the U.S.S.R., Bulgaria, Czechoslovakia, the German Demo­ cratic Republic, Hungary, Poland, Romania, and Yugoslavia. Long-term research visits of 5-12 months' duration are encouraged, particularly when contact with colleagues in the other country has already been established. The minimum length of visits is one month. Applicants must be U.S. citizens and have a doctoral degree or its equivalent by the time ofthe intended visit in mathematics; the physical, biological, or engineering sciences; social or behavioral sciences; or biomedical sciences. With regard to the social and behavioral sciences, emphasis is on empirical and quantitative analysis and on the analysis of individual and group behavior. Necessary expenses will be met by the NAS and the foreign academy, including reimbursement for long-term visitors for salary lost up to a predetermined maximum and expenses for accom­ panying family members for long-term visits. Requests for applications should reach the National Academy of Sciences not later than 14 February 1986. Deadline for receiving completed applications is 3 March 1986; applications must be postmarked by 28 February 1986. Address application requests to: National Academy of Sciences, Office of International Affairs, Soviet and East European Affairs, 2101 Constitution Avenue, N.W., Washington, D.C. 20418. For further information: (202) 334-2644.