Forest Ecology and Management, 13 (1985) 205--222 205 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

B I O M A S S D I S T R I B U T I O N O F U N M A N A G E D U P L A N D F O R E S T S IN M I N N E S O T A

LEWIS F. OHMANN and DAVID F. GRIGAL ~

USDA Forest Service, North Central Forest Exper iment Station, St. Paul, MN 55108 (U.S.A.) ' Departments o f Soil Science and Forest Resources, University o f Minnesota, St. Paul, MN 55108 (U.S.A.)

(Accepted 6 June 1985)

ABSTRACT

Ohmann, L.F. and Grigal, D.F., 1985. Biomass distribution of unmanaged upland forests in Minnesota. For. Ecol. Manage., 13: 205--222.

A floristic analysis of natural and logged upland forest communities previously identified 12 forest and one non-forest community-types within the Boundary Waters Canoe Area Wilderness of Minnesota. Data collected for the floristic analysis were applied to biomass estimation equations to produce estimates by species for stands within each community- type. Total above-ground biomass was 4 t ha -1 in the non-forest community-type, and ranged from 121 t ha -1 in the jack pine--oak type to 268 t ha -~ in the red pine type. Highest biomass occurred in community-types with long-lived tree species; however, all community-types have similar mean annual biomass increments. Biomass distribution by vegetative strata within and among the community-types was examined. Distribution of biomass among undergrowth strata varied significantly with community-type. Total undergrowth biomass, however, had a narrow range from 1.9 to 4.4 t ha- ' because the biomass for the different strata summed in a compensatory manner. Biomaas among vegetative strata were related; an increase in biomass of the tree stratum was related to reduced biomass of lower strata. Such relationships were not sufficient to fully explain variation in biomass of those strata among community-types. Significant differences in biomass among community-types, and the lack of differences among random assignments of the same communities into 12 groups, suggest that the original floristic classification provided a valid basis to compare ecoystem properties.

INTRODUCTION

T o t a l f o r e s t b i o m a s s a n d t h e c o n t r i b u t i o n o f v a r i o u s s t r a t a o f f o r e s t c o m - m u n i t i e s to t h a t t o t a l are o f w i d e i n t e r e s t ( S a t o o a n d M a d g w i c k , 1 9 8 2 ) .

W h i t t a k e r a n d L i k e n s ( 1 9 7 3 ) d e m o n s t r a t e d a ge ne r a l r e l a t i o n s h i p b e t w e e n a b o v e - g r o u n d b i o m a s s o f f o r e s t s a n d a n n u a l n e t p r i m a r y p r o d u c t i v i t y , a n d b i o m a s s d a t a have b e e n u s e d to c o m p a r e p r o d u c t i v i t y o f p l a n t c o m m u n i t i e s ( S t a n e k a n d S t a t e , 1 9 7 8 ) . K n o w l e d g e o f v a r i a t i o n in p r o d u c t i v i t y o f b o t h

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managed and unmanaged forests is important in determining the role of management in meeting the increasing demand for fiber and energy while maintaining long-term product ivi ty of the land. Biomass information may help us understand that variation. Forest communit ies probably do not have equal productive potential, and cannot equally meet increased demands for biomass. Data from unmanaged forest communit ies may provide insight into the capabilities of similar communit ies under management.

Differences in carbon distribution within (i.e., structural) and among (i.e., compositional) community- types are also impor tant as they affect processes and pathways of ecosystem development. If these differences can be ascer- tained from other ecological information, such as floristic data, then a much broader data base is available for extrapolation. Biomass estimates in the literature are either based on detailed sampling o f all strata within one or a few forest stands (Cole and Rapp, 1981) or are broad estimates of tree bio L mass over wide geographic areas (Raile and Jakes, 1982; Yarie and Mead~ 1982). Estimates for all strata from a number of forest stands are generally lacking.

An earlier floristic analysis o f 121 natural and previously logged upland forest stands within the Boundary Waters Canoe Area Wilderness (BWCAW) identified 12 forest communi ty- types and one non-forest communi ty- type (Grigal and Ohmann, 1975). We have applied the data collected during that original floristic inventory to biomass est imation equations. These equa- tions, based either on our own work or from the literature, have become available since the initial inventory. In this paper we use the resulting esti- mates of biomass (oven<lry basis) to (1) provide summary data for un- managed upland forest communi ty- types of the BWCAW, including relation- ships concerning the distribution of biomass among vegetative strata within community- types , and to (2) test the earlier floristic classification of the BWCAW vegetation by comparing biomass among the identified community- types.

METHODS

Site

The BWCAW (Fig. 1), centered in northeastern Minnesota at about lat. 48°N, long. 91°W, is part o f the Superior Physiographic Region. Glacial erosion has left a rugged topography with elevation ranging from 350 m to 700 m and up to 150 m of local relief. The area is part o f the Laurentian Peneplain and is underlain exclusively by Precambrian rocks. The configura- tion of the bedrock determines the l andfo rm; thus long, narrow, steep ridges are typical on slates and low, irregular, round- topped hills prevail on granites. Upland soils of the BWCAW are formed in sandy and gravelly loam glacial deposits. They are generally shallow, with rock outcrops common along the ridgetops and along lake and stream shores. Boulders are common. The soils

207

COOK CO.

LAKE CO.

ST. LOUIS CO.

Fig. 1. Location of Boundary Waters Canoe Area Wilderness in Minnesota.

are classified as Inceptisols and Entisols, with a few Spodosols (Cummins and Grigal, 1981).

The climate of the BWCAW is typical ly mid-continental. Winters are long and cold with a moderately heavy snowfall, and summers are warm and moist. The average annual precipitation is about 68 cm, including an average snowfall of about 150 cm. Temperatures vary be tween - 4 6 ° C and 38°C. The average monthly mean temperatures range from - l l ° C in January to 17°C in July. The growing season is about 100 days.

Field

We sampled two groups of upland plant communit ies (fully-stocked forest stands) within the BWCAW. The first group were stands that have, so far as can be ascertained, never been disturbed b y modern man -- they represent "virgin fores t" communities. They are referred to in this paper as natural stands and range in age from about 60 to 350 years. Natural wildfire was the most common disturbance before the stands were established (Heinselman, 1973). A second group contains stands that were disturbed by logging, usually followed by slash fires, from 1890 to 1930. They are referred to as logged stands. The two groups of stands were classified into community- types, based on their floristic composit ion, via a series of numerical tech- niques (Grigal and Ohmann, 1975).

The sampling design and vegetation descriptions have been reported in detail (Ohmann and Ream, 1971a, 1971b; Grigal and Ohmann, 1975). Brief- ly, we randomly located stands within the BWCAW by using a grid overlay

208

on standard topographic maps. Only upland plant communities, defined as those on which surface water never accumulates, were sampled. The minimum size of a sample area was about 2 ha in the case of communities with trees and about 100 m 2 in communit ies wi thout trees (i.e., bedrock outcrops). Only uniform sites, with no apparent discontinuities in either vegetation or topography, were selected. We used 20 points within each stand to sample the vegetation. The point-centered quarter method (Cottam and Curtis, 1956) was used for trees, circular plots (4.0 m 2) were used for tree seedlings and tall shrubs, and rectangular plots (0.2 m 2) were used for other vegeta- tion.

Numerical

Trees Above , round total biomass was estimated separately for each tree species

and summarized for each communi ty- type. Biomass estimation equations for trees were based on the abundant recent literature. For uniformity and con- sistency, we used the same funct ional relationship for all tree biomass estimation equations. Because of its wide use, we selected the allometric rela- tionship

y = aD b (1)

where y is mass of a tree component, D is diameter at breast height (dbh), and a and b are regression coefficients. When available, general equations from many sites were used (e.g. Schmitt and Grigal, 1981; Crow, 1983). In some cases our search only indicated one set of equations for a species (e.g., black ash (Fraxinus nigra Marsh.), Parker and Schneider, 1975). In other cases, several equations, over a wide geographic range, were available for a species. Regression coefficients were generally similar for a species, indepen- dent of location. We pooled the coefficients for those equations that were similar to arrive at a more generalized expression (Table 1). The approach was similar to that used by Pastor et al. (1984).

The usual method of computa t ion for data collected by the point<tuarter method uses stem diameter and distance from a number o f points (usually 20 or more). The data are then aggregated by summation over all points, and tree density and basal area are calculated on the basis of the aggregated data (Cottam and Curtis, 1956). Our approach was to perform the appropriate computations for data from each point. Summary statistics, including mean and standard error (SE), could then be developed over all points. The total biomass for a stratum was the sum of the biomass for each species in the stratum.

Tall shrubs Biomass estimation equations for tall shrubs, also of the allometric form,

were primarily based on three studies from northern Minnesota: Grigal and

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TABLE 1

Source of and coefficients for biomass estimation equations used for tree and sapling strata. Equations are of the form y = aD b where Y is oven-dry mass in kg and D is diam- eter at breast height in cm

Species a b Source

Abies balsamea 0.0911 2.3530

Acer rubrum 0.1802 2.3340

Betula papyrifera 0.0882 2.5620

Fraxinus nigra 0.0941 2.3200

Larix laricina 0.1265 2.2450

Picea glauca 0.0980 2.3650

Picea mariana 0.1760 2.1550

Pinus banks&no 0.1054 2.3810

Pinus resinosa 0.0917 2.3730

Pinus strobus 0.1042 2.4240

Populus balsamifera 0.0950 2.3630

Populus grandidentata 0.0614 2.5940

Populus tremuloides 0.1000 2.4300

Quercus rubra 0.1702 2.3420

Sorbus americana 0.1560 2.1950

Tilia americana 0.0614 2.5940

Thuja oceidentalis 0.2302 1.9270

Baskerville (1965) Ker (1980) Young et al. (1980)

Crow (1983)

Schmitt and Grigal (1981)

Parker and Schneider (1975)

Young et al. (1980)

Harding (1982) Ker (1980) Young et al. (1980)

Barney et al. (1978) Grigal and Kernik (1984) Ker (1980)

Green and Grigal (1978)

Alban and Laidly (1982) Ker (1980) Young et al. (1980)

Kinerson and Bartholomew (1977) Monteith (1979) Young et al. (1980)

Bella and De Franceschi (1980)

Koerper and Richardson (1980)

Ker (1980) Pastor and Bockheim (1981) Pollard (1972) Young et al. (1980)

Monteith (1979) Whittaker and Woodwell (1968 )

Young et al. (1980)

Koerper and Richardson (1980)

Young et al. (1980)

Ohmann (1977), Roussopoulos and Loomis (1979) and Balogh (1983). Equations from Telfer (1969) were used for a few less common species. For some common species, all three primary sources contained estimation equations. In those cases, the three relationships were plotted and the inter- mediate equation was selected. Because this was often an equation from Balogh (1983), equations for less well-represented species were also obtained

210

from that source. All equations were based on shrub stem diameter at 15 cm above ground.

The floristic inventory o f plant communit ies in the BWCAW recorded tall shrubs in 1-cm diameter size classes at 15 cm (Ohmann and Ream, 1971a). However, other data (Balogh, 1983) indicate that the majori ty o f shrubs in smaller size classes are near the lower end of those classes. Use of the allom- etric function with 1-cm diameter classes would therefore overestimate shrub biomass. We determined diameter--densi ty distributions for each shrub spe- cies in the BWCAW inventory. The data were fit to a linearized power func- tion,

in(De) = a + b ln(Di) (2)

where ln(Di) is the natural log of stem diameter and ln(De) is the natural log of proport ion of total stems o f that diameter. The slope of this equat ion (b) for each species is the change in propor t ion o f individuals with diameter.

Shrub data from a sampling point, in the 1-cm classes, and the slope (b) of the above relationship were used to compu te the expected number of in- dividuals by 0.25~cm diameter classes. The estimation equations were then applied to those classes. Biomass was calculated for each tall shrub species on a plot. The mean of those plot values were calculated by species and summed for total biomass o f the shrub stratum.

Seedlings The floristic inventory of the BWCAW (Ohmann and Ream, 1971a) re-

corded cover and density of seedlings. Most published biomass estimation equations for seedlings are based on either diameter (Telfer, 1969; Rousso- poulos and Loomis, 1979), or on height (Young et al., 1980). We modif ied the published equations to determine biomass for seedlings from our data.

Our independent variable was cover per individual seedling. We collected samples to determine minimum and maximum biomass and cover for repre- sentative deciduous species (northern red oak, Quercus rubra L.; red maple, Acer rubrum L.; quaking aspen, Populus tremuloides Michx.; and paper birch, Betula papyrifera Marsh.); coniferous species (balsam fir, Abies balsa- mea (L.) (Mill.); black spruce, Picea mariana (Mill.) B.S.P.; jack pine, Pinus banksiana Lamb.; and white pine, Pinus strobus L.) that were most c o m m o n as seedlings in the data set. Relationships from bo th Roussopoulos and Loomis (1979) and Young et al. (1980) were used to determine change in mass with size. Estimation equations of the allometric form were then developed for total mass, using cover, per individual as the independent variable. Seedling biomass for a stand was calculated similarly to that for tall shrubs.

Herbs and low shrubs A series of biomass equations using cover as the independent variable

have been developed for herbs, ferns, and low shrubs sampled on 0.2-m 2

211

plots (Ohmann et al., 1981). Those equations were used for the BWCAW data (Ohmann and Ream, 1971a). For species for which no equation had been specifically developed, we used equations for similar species (e.g. Fragaria spp. equation for Walsteinia fragaroides (Michx.) Tratt.). A miscella- neous herb and fern equation (unpublished) was used for species with no obvious morphological match. For each species, stand biomass was simply the mean of the estimates for each plot (usually 20). Stand totals over all species were calculated as with the other strata.

Mosses and lichens Cover estimates for bo th mosses and lichens were also collected on 0.2-m 2

plots in the BWCAW inventory (Ohmann and Ream, 1971a). Based on sampling within northeastern Minnesota, we determined mass per unit area for various moss and lichen species and species groups (Table 2). These data were then converted to linear relationships be tween cover and mass on 0.2-m 2 plots. Means and errors were calculated as for the herb stratum. The values obtained are within the range of those reported for jack pine stands on shallow soils in the area (Green and Grigal, 1978).

T A B L E 2

Mass per un i t area o f c o m m o n mosses and l ichens in n o r t h e a s t e r n Minneso ta

Species x SE n (g m "2) (g m -2)

Polytrichum commune Hedw. 630 120 5 Callier&onella schreberi (Willd., Br. & Sch.) G r o u t 560 60 7 Dicranum group a 280 30 5 Sphagnum spp. L. 380 20 20 Other mosses 500 50 17 Cladon~a spp. (L.) Hof fm. 1060 70 20 Pelti&era spp. L. Willd. 390 40 4 Other foliose lichens 260 20 5

a Inc ludes Dicranum spp. Hedw. , Hypnum crista-castrensis L., and Hylocomium splendens (Hedw.) Bry. Eur .

Analysis

Estimates of above-ground biomass for each stratum and for all strata were subjected to analysis o f variance (ANOVA), testing for differences among community- types . A sequence of analyses of covariance was con- ducted to determine the influence o f the biomass of upper strata on lower strata within communi ty- types . Basically, linear regressions relating over- s tory biomass to biomass o f various undergrowth layers were developed for each communi ty- type , and slopes and intercepts o f these regressions were compared among communi ty- types . The assumption of analysis of co-

212

variance concerning homogeneity of variance in the model was tested by Bartlett's test (Snedecor and Cochran, 1967). To test adjusted means, slopes of the dependent variable with the covariate must be equal to each other among groups and must be different from zero (Dixon and Brown, 1979). These assumptions were tested by comparison of mean squares (Dixon and Brown, 1979). The results of these analyses were used to develop equations to express the relationship of biomass among vegetative strata within com- munities.

Stand age {based on date of most recent disturbance) and tree stand densi- ty were also used as covariates to determine their effect on biomass. We also used a simple Monte Carlo simulation to determine the validity of the established community-types for recognizing differences in biomass. We only used data from the stands in the forested types, and created 12 "artificial community-types" by randomly aggregating those data. We used ANOVA to determine if significant differences in biomass existed among those artificial types. We performed these rearrangements and tests 100 times.

RESULTS AND DISCUSSION

Thirteen community-types were recognized in the initial floristic classifi- cation (Grigal and Ohmann, 1975). The lichen community-type (L) consists of lichen-covered bedrock outcrops on ridgetops and upper slopes. Although overstory trees are absent, the type contains a few woody seedlings and tall shrubs, and some low shrubs and herbs. The remaining 12 types are forested. The jack pine--oak community-type (JPO) occurs in similar physiographic situations as the lichen type, except that sufficient soil is present to support a forest community. The community is dominated by jack pine, with some northern red oak and red maple. Where these stands have been severely disturbed by logging and fire, the coniferous component has been markedly reduced, giving rise to the maple--oak community-type (MO). Red maple and northern red oak dominate the overstory in that type, but the under- story is similar to that in the jack p ine -oak type.

Two other community-types are dominated by jack pine. The jack pine--fir type (JPF) is on deep soils, and has an overstory of jack pine with balsam fir, paper birch and black spruce saplings and seedlings. A tall shrub layer is common, and Acer spicatum Lam. and Lonicera canadensis Marsh. are im- portant. Aster macrophyllus L. dominates the herb layer, but a variety of other herbs occur in the type. The jack pine--black spruce type (JPBS), as the name implies, contains an overstory of jack pine with black spruce trees, Saplings, and seedlings. Tall shrubs are" rare, and moss species rather than herbaceous species dominate the forest floor. In the black spruce--feather moss community.type (BSFM), black spruce dominates the overstory al- though some jack pine are present. Shrubs and herbs are less common than in the previous type, and the moss layer covers the entire forest floor.

Four types are partially to completely dominated by quaking aspen and paper birch. The aspen--birch community-type (AB) is dominated by those

213

two species, although balsam fir seedlings are abundant . Tall shrubs are plentiful, dominated by Corylus cornuta Marsh. Low shrubs, especially Diervilla lonicera Mill., are also abundant . Aster rnacrophyllus is an impor- tant herb species, along wi th Maianthemum canadense Desf., Aralia nudi- caulis L., and Cornus canadensis L. Mosses are unimportant . The aspen-- birch--white pine communi ty- type (ABWP) is similar to the aspen--birch type, but contains a componen t of large white pine and a greater abundance of balsam fir in the overstory than does the aspen--birch communi ty- type . The maple--aspen--birch communi ty - type (MAB) contains red maple, especially as a seedling, in addit ion to quaking aspen and paper birch. Tall shrubs include Ainu8 crispa (Ait.) Pursh. and Acer spicaturn in addit ion to Corylus cornuta. The type also contains balsam fir, bu t that species is more important in the maple--aspen--birch--fir communi ty - type (MABF). The type is similar to the maple--aspen--birch type, bu t Acer spicatum is more dominant as a tall shrub, low shrubs are less frequent, and Maianthemum canadense is the most common herb.

Both the conifer and the broadleaf types previously ment ioned appear to occur in a successional sequence to the fir--birch communi ty - type (FB). Balsam fir is the most c o m m o n tree, with bo th paper birch and white cedar (Thuja occidentalis L.) as associates. Acer spicatum is the dominant tall shrub, and low shrubs are sparse. The herb layer is much reduced compared to the broadleaf types, wi th Cornus canadensis the most abundant herb. With the decline of the herb strata, mosses are more abundant . The white cedar communi ty- type (WC) appears to be even further along the succes- sional sequence than the fir--birch type, and the stands in this t ype are somewhat older than in the previously ment ioned types. The t ype is similar to the fir--birch type, bu t white cedar is more important and the herb strata is more mixed. MiteUa nuda L. is a common herb.

The final type, the red pine (Pinus resinosa Ait.) (RP) communi ty- type , does not fit well in sequence with the other types. It consists o f old-growth red pine with a poorly developed understory resulting in open, nearly park- like stands. White pine is a common associated species. Low ericaceous shrubs are common, as are mosses and lichens.

The validity of our results is predicated in part on the generality of bio- mass estimation equations. Such generality across sites and even across regions has been examined for trees, and relationships be tween diameter and total above-ground biomass are very consistent for a number of species included in our data set. These species include red maple (Crow, 1983), paper birch (Schmitt and Grigal, 1981), jack pine (Green and Grigal, 1978), black spruce (Grigal and Kernik, 1984), red pine (Alban and Laidly, 1982), and quaking aspen, white pine, and northern red oak (Pastor et al., 1984). These results lend credence to our approach. Extensive tests of generality of equations have not been conducted for most species common to lower forest strata, bu t in this s tudy most of those equations were derived from data collected in or near the BWCAW.

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and tends to increase with tree density and site quality (Satoo and Madg- wick, 1982). Mean age (Table 3) of all stands among the 12 forested com- munity-types was significantly different (P < 0.01, df = 11,103), due to the presence of long-lived red pine, white pine, and white cedar in the red pine and white cedar community-types (Ohmann and Ream, 1971b). Mean tree density (Table 3) was also significantly different among the 12 community- types (P < 0.01, df = 11,103). However, age and density, either alone or to- gether, could not be used as covariates in testing differences among com- munity-types in biomass of either an individual stratum or of all strata. Al- though the assumption of homogeneity of variance was met, age and density did not have equivalent effects among community-types, invalidating analysis of covariance. High variability in age, density, and biomass within each community-type obscured expected age--density--biomass relationships.

Community-types of wide floristic variety have similar estimated total above-ground biomass (Table 3), but there are some significant differences among the types (P < 0.05, df = 11,103). These differences are more ex- treme when the non-forest {lichen) community-type is also considered (P < 0.01, df = 12,108). In forested types, extremes of biomass were found in the red pine and white cedar community-types at the high end and in the jack pine--oak community-type at the low end (Table 3). Mean annual biomass increments for those extreme types, however, were nearly equal (Table 3). The range of that increment is narrow over all types. The simple computa- tion of the increment based on mean age and mean biomass of each type, however, neglects the wide variation in age and biomass within each type. This variation was the reason that covariate analysis could not be used to detect a significant effect of age on biomass.

Total biomass in the community-types is within the range reported for other studies in temperate forests (Whittaker et al., 1974; Cannell, 1982), and is also similar to values reported elsewhere in this region. Crow (1978) suggests that from 100 to 200 t ha -1 (above-ground dry weight) would in- clude the range of most second growth forests in the Lake States region, and that from 60 to 600 t ha -1 might represent the upper and lower bounds for mature forest communities.

Three broadleaf forest community-types similar to those of this study but located in Wisconsin had somewhat lower estimated biomass: aspen 95, aspen--maple--birch 96, and maple--birch--aspen 119 t ha -1 (Crow, 1978). Biomass estimates of 124 t ha -1 for a mature upland oak forest (Reiners, 1972) and 164 t ha -1 for an upland oak woodland (Ovington, 1963), both from southern Minnesota, were more similar to those for the BWCAW broadleaf community-types. Biomass in natural jack pine communities over 50 years old near the BWCAW ranged from 92 to 169 t ha -~ (Green and Grigal, 1979). Tappeiner and John (1973) reported that biomass in 50 to 90 year-old jack pine stands further south in Minnesota ranged from 102 to 136 t ha -1. These estimates are well within those ranges of the various BWCAW jack pine community-types (Table 3).

216

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217

Because total above-ground biomass in forested communities is dominated by biomass of the tree layer, it is not surprising that tree layer biomass varies little among types (Fig. 2). There was a significant difference in tree layer biomass among the 12 forested community-types when data from both natural and logged stands were combined (P <~ 0.05, df = 11,103), but not when the 68 natural stands (P > 0.05, df = 10,57) or the 46 logged stands (P > 0.05, df = 9,36) were tested separately. Bayes least significant differ- ence (BLSD) (Smith, 1978) suggests that the significant differences were again related to the greater tree layer biomass in the red pine and white cedar community-types because of the large, long-lived red pine, white pine, and white cedar in those types, and the lesser tree layer biomass in the xeric, rock outcrop, jack pine--oak community-type (Ohmann and Ream, 1971b; Grigal and Ohmann, 1975).

Differences in undergrowth biomass within the various strata among community-types were more pronounced than were overstory biomass differences (probability values for each undergrowth stratum were < 0.05, df = 11,103). These differences were confirmed by BLSD values (Fig. 3A, B, C). Comparisons of undergrowth biomass among different forests are generally considered ambiguous because of disturbance factors (Satoo and Madgwick, 1982), but because we focused on mature stands the results may be both more comparable and better related to environmental factors. For example, although the sequence of community-types from left to right in Figs. 2 and 3 is somewhat arbitrary, it does represent gradients from xeric to more mesic conifer types, and from post-disturbance hardwoods to more mesic conifer types (Grigal and Ohmann, 1975).

In the context of environmental gradients, it is interesting that tall shrub and herb biomass generally dominate the undergrowth of the more mesic coniferous types, and low shrub, lichen, and moss biomass dominate the more xeric types. This difference in above-ground carbon distribution among types is analogous to the recent reports of differential below~round bio- mass distribution with site quality (Keyes and Grier, 1981). The difference in abundance of undergrowth layers is often readily discernible when one passes through forests of changing environments.

The differences in biomass distribution among the undergrowth layers in different community-types (Fig. 3A, B, C) are moderated in summing the values for all strata to produce an estimate of total undergrowth biomass. Like that of the tree layer, this estimate does not vary greatly among com- munity-types (Table 3). Estimated undergrowth biomass ranged from 1.9 t ha -] for the aspen--birch--white pine community-type to 4.4 t ha -] for the aspen--birch community-type, with only two community-types containing less than 2.5 t ha -~ (Table 3).

Biomass relationships between overstory and understory within com- munity-types were evaluated by analyses of covariance. In these cases, the assumptions of homogeneity of variance, equality of slopes, and differences of slopes from zero were met for all data sets. The results indicated signifi-

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cant decreases in undergrowth biomass with increases in overstory biomass. There was no significant difference in the slope o f the relationships among community-types, although the intercepts varied. The relationships shown in Fig. 4 use the pooled slopes and the mean intercepts over all types. Biomass of the tree layer showed the greatest influence on biomass o f lower layers (Fig. 4A, B, C). Addition o f biornass of the tall shrub layer to that of the overstory showed less effect (Fig. 4D, E), but there was still some effect o f biomass of all higher strata on biomass o f the lowest stratum (Fig. 4F). When biomass of the tree layer was used as a covariate, total undergrowth biomass was not significantly different among community-types (P/> 0.05, df = 11,102), but the biomass o f the vascular layer (all undergrowth strata except the moss--lichen layer) and the biomass o f the tall shrub layer (tall shrubs and tree seedlings) were significantly different (P < 0.01, d f = 11,102, P < 0.01, df = 11,102 respectively). When biomass o f the tree plus the tall

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Fig. 4. The influence of increased tree layer biomass on (A) all biomass growing below, on (B) all undergrowth except the ground layer (mosses and lichens); and on (C) the tall shrub layer (tree seedlings and tall shrubs); the influence of the tree and tall shrub layers on (D) the herb layer, and on (E) the herb plus ground layers; and (F) the influence of all upper b!omass layers on the ground layer based on data from 121 forest stands within the Boundary Waters Canoe Area Wilderness, Minnesota.

220

shrub layer was used as a covariate, biomass of the herb layer (low shrubs, herbs, and ferns) and of the herb plus ground layer (mosses and lichens) differed significantly (both P ,~ 0.01, df = 11,102) among community-types. When biomass of all upper layers was used as a covariate, biomass of the ground layer was significantly different (P < 0.01, df = 11,102) among community-types. It is clear from these analyses that although the tree layer affects undergrowth biomass, that effect differs among community-types. Knowledge of the floristic composition of the types, which imply structure, are important in understanding the relationships.

The efficacy of the original floristic classification for analysis of biomass differences may be questioned. Other arrangements of the stands into groups might be expected to show biomass differences, especially if a few stands are especially dominant in the data set. However, the results of 100 random assignments of all the forested stands into 12 groups (artificial community- types) indicated few significant differences in biomass among the groups. Whereas total biomass and biomass of every vegetative stratum differed significantly among community-types as already reported, in 100 random assignments no significant difference occurred in total biomass, tree bio- mass, or tall shrub and seedling biomass among the 12 groups. Only four significant differences occurred in the low shrub, herb, and fern layer, and only one difference in the moss and lichen layer (P ,~ 0.05, df = 11,102). This suggests that the small biomass differences among the floristically- derived community-types are real. The biomass of mature forest communi- ties within the BWCAW results from their floristic composition and stand structure, which are in turn related to the time since last major disturbance, the intensity of that disturbance, and the floristic composition of the com- munity at the time of the disturbance (Grigal and Ohmann, 1975). The addi- tional influence of environmental factors (site) is more obvious in extreme situations such as rock-outcrop areas. Site differences also result in biomass being accumulated in different strata within the various community-types in compensatory ways. The results of the random assignments also suggest that the original floristic classification, probably because of its emphasis on all strata, provides a valid basis to compare ecosystem properties.

Our study may represent a general case for unmanaged mature communi- ties within small geographic areas. The biomass estimates determined for the BWCAW community-types are within the range of those observed for north- eastern North America (Whittaker et al., 1974). One would expect more disparity in biomass among communities of widely varying age or among communities under different levels of management intensity or disturbance (CanneU, 1982; Satoo and Madgwick, 1982).

ACKNOWLEDGEMENTS

This article was written and prepared by United States Government em- ployees on official time; it is therefore in the public domain. Published as

221

paper No. 13953 of the scientific journal series of the Minnesota Agricultural Experiment Station on research conducted under project 25-54.

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