Direction and Height of Bryophytes on Four Species of Northern Trees

Abstract

Epiphytic bryophytes were sampled quantitatively at three heights and four exposures on 124 trees representing four host taxa (Acer saccharum, Betula lutea, Populus spp., and Quercus rubra) within the Dow Wilderness Area, Keweenaw County, Michigan. Several bryophytes were found to be significantly associated or dissociated with one or more host taxa. A marked vertical zonation of epiphytic bryophytes was found, with tree bases exhibiting highest mean bryophyte cover and largest mean number of species. Many bryophytes were significantly correlated with one of three vertical levels, which included basal, breastheight (1.2 m), and midway (0.6 m) locations, and several bryophyte associations were supported by cluster analysis. Significant differences in bryophyte cover due to direction of exposure were found on all host taxa, with north exposures exhibiting the most bryophyte cover at the base, south in the middle, and east at breast height. Within the range of diameters sampled, slight but significant increase in bryophyte cover as diameter increased was found on all host types but Betula lutea. A significant increase in number of species as diameter increased was observed only on Populus. The relationship of bark-inhabiting (corticolous) bryophytes to their host trees poses complex ecological questions. Previously, many who addressed these questions did so using primarily qualitative means, but recently quantitative analysis has permitted bryophyte ecologists to test the significance of their observations about ecological relationships. The present study uses quantitative data to examine host specificity, vertical distribution, direction of exposure, host diameter, and bryophyte associations. Slack (1976) has compared tree species by their epiphytes in wide ecological scope. Using her own data obtained from separate study areas in New York state and North Carolina along with that obtained by Brown (1948), Phillips (1951), Hale (1955), and Culberson (1955a), she verified that host specificity is localized phenomenon, and is not predictable over broad areas in the range of bryophyte species. Slack (1976) and Barkman (1958) have summarized the earlier studies, suggesting that bark factors such as water 1 Present address: 1511 Frederic St., Eau Claire, WI 54701. 2 To whom correspondence should be sent. 007-2745/82/281-300$2.10/0 This content downloaded from 207.46.13.102 on Sun, 31 Jul 2016 05:45:42 UTC All use subject to http://about.jstor.org/terms 282 THE BRYOLOGIST [Volume 85 capacity, rate of drying, and pH of host bark are important in host specificity and are dependent to some extent on geography and climate. Many authors who have investigated host specificity have also examined vertical distribution. Most have sampled discrete levels from base to breast height and the upper trunk (Billings & Drew 1938; Potzger 1939; Hale 1955; Culberson 1955a; Beals 1965; Pitkin 1975; Slack 1976; Hoffman & Boe 1977); Hoffman and Kazmierski (1969) and Stringer and Stringer (1974) used continuous transects. Others have sampled the upper limbs and crown as well as the trunk (Cain & Sharp 1938; Omura et al. 1955) or have taken advantage of cut or windthrown trees to sample the entire vertical range (Hale 1952, 1965; Iwatsuki 1960; Iwatsuki & Hattori 1966, 1970). Hale (1965) felt that a broad range of environmental conditions is compressed into the vertical height of tree. Japanese studies (Omura et al. 1965; Iwatsuki & Hattori 1966, 1970) have also described vertical zonation of epiphytic bryophytes with results paralleling those of most North American studies to the extent that many of the same bryophyte species are recorded, even though the host trees are different (Slack 1976). Culberson (1955b) reported differences in basal and breast-height frequencies of certain epiphytes between northern and southern latitudes in Wisconsin, suggesting that protective snow cover may be factor. Hosokawa and Odani (1957) and Hosokawa et al. (1964) suggest that basal species are adapted to low light intensities and cannot tolerate the level of light present on the upper trunk and crown. Similarly, they found species of the upper trunk and crown to be photophilous and uninhibited by high light intensities. Many investigators have chosen to eliminate the variable of direction of exposure, either through the use of cylindrical quadrats or by sampling one representative side of the tree, usually the north. Those who have included exposure in their consideration of epiphyte distribution often compared north with south (Young 1938; Billings & Drew 1938; Potzger 1939; Hoffman & Boe 1977), and few compared northeast with southwest exposures (Hoffman & Kazmierski 1969; Hoffman 1971). Both Young (1938) and Potzger (1939) found slightly greater moisture loss by evaporation on the south sides of trees, but neither felt that exposure had substantial effect on epiphyte distribution. Tagawa (1959), working in the beech forests of Japan, also came to this conclusion. Stringer and Stringer (1974), sampling north, east, south, and west exposures, found some bryophytes more sensitive than others to the effect of direction of exposure. Other authors acknowledge an effect on bryophyte distribution due to direction, but feel that habitat differences between stands constitute more important variable (Slack 1976). The age of the host tree has been correlated with epiphyte succession (Olsen 1917; Dudgeon 1924; Quaterman 1949; Phillips 1951; Barkman 1958). Mostly, age of the tree has been measured as relative variable rather than an absolute one, as diameter (diameter at breast height-DBH, 1.3 m) provides the simplest and quickest estimate of age. Studies that do not include very young and very old trees may not show changes in the epiphytic flora with tree age (Slack 1976), mainly because bark conditions exhibit the most dramatic changes during youth and old age (Olsen 1917; Barkman 1958). Bark properties also change with climate and geography, making it difficult to compare data from geographically different regions. Hosokawa and Omura (1959) have investigated the community dynamics of epiphytes by establishing permanent quadrats at various heights on host trees, then monitoring the species composition over period of several years. This method, although time-consuming, may provide the most accurate means of determining the relationship between phorophyte age and epiphyte composition. A recent prolific area of epiphyte study has been the response of corticolous bryophytes and lichens to air pollution. The present study was conducted in the Upper Peninsula of Michigan, far from any known major air This content downloaded from 207.46.13.102 on Sun, 31 Jul 2016 05:45:42 UTC All use subject to http://about.jstor.org/terms 1982] TRYNOSKI & GLIME: BRYOPHYTES OF TREES 283 pollution source and thus provides baseline data for the area on both species composition and ecological relationships. DESCRIPTION OF STUDY AREA The study was conducted within the Herbert H. Dow Wilderness Area, 300-acre tract of land donated to Michigan Technological University in 1973 for teaching and research purposes. The Dow Wilderness Area is located in central Keweenaw County, Michigan, and has been partially mapped with respect to arboreal vegetation by MTU researchers (Brown,3 unpubl. data). They mapped wooded area of approximately 2.3 hectares within the wilderness area with regard to tree species and DBH. This plot, the site of the present study, is about 350 m in elevation and is located at approximately 47'26'N latitude, 88005'W longitude. The sampling plot contains 2254 stems greater than 10 cm in diameter, or 980 stems per hectare, of both deciduous and coniferous species. Sugar maples (Acer saccharum Marsh.) contribute the most stems (nearly third); other important deciduous species include red maple (Acer rubrum L.), quaking aspen (Populus tremuloides Michx.), bigtooth aspen (P. grandidentata Michx.), red oak (Quercus rubra L.), paper birch (Betula papyrifera Marsh.), yellow birch (B. lutea Michx. f.), and ironwood (Ostrya virginiana (Mill.) K. Koch). Coniferous species include balsam fir (Abies balsamea (L.) Mill.), white spruce (Picea glauca (Moench) Voss), white cedar (Thuja occidentalis L.), and white pine (Pinus strobus L.). Of the stems 10 cm or greater in diameter, the mean DBH is 19 cm (Gereau,4 unpubl. data). Climatological data for the years 1940-1969 have been obtained from the Houghton County Airport, about 42 km to the southwest of the site. The data show the mean annual precipitation to be 88.3 cm with the month of March averaging the least precipitation and June the most. Mean monthly temperatures show July to be the warmest month and January the coldest (Mich. Dept. Agriculture et al. 1974). Snow cover was measured at several locations within the study area in early March 1977, the usual peak time for accumulations, and ranged from 1.0 to 1.2 m in depth.