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Longevity of Interrupted Fern Colonies
Andrea Faust and Raymond L. Petersen

Southeastern Naturalist, Volume 14, Special Issue 7 (2015): 203–209

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Southeastern Naturalist A. Faust and R.L. Petersen 2015 Vol. 14, Special Issue 7 203 Canaan Valley & Environs 2015 Southeastern Naturalist 14(Special Issue 7):203–209 Longevity of Interrupted Fern Colonies Andrea Faust1,2 and Raymond L. Petersen1,3 Abstract - Abundant data are available on the ages of perennial seed plants, but we know little about the longevity of ferns. In this study we estimated the ages of 30 colonies of Osmunda claytoniana (Interrupted Fern). Common in the Monongahela National Forest of northeastern West Virginia, Interrupted Ferns typically form convexly bulging, elliptically shaped colonies. We studied colonies larger than 6.6 ft (2 m) in average diameter. Each colony was comprised of dozens of ramets interconnected by a subsurface, dichotomously branching rhizome system. The fronds’ stipe bases persist through the colony’s life, endure along the entire length of the rhizome, and extend back to the origin of the colony’s founding ramet. We estimated the age of a fern colony by dividing the average radius of the colony by the growth rate of its rhizomes. We estimated rhizome growth rate by dividing the average number of stipe bases per length of rhizome by the average number of fronds generated by an individual ramet per year. Rhizome growth rates varied from 0.04 to 0.24 in (0.1–0.6 cm) per year, which represents a slow outward expansion by colonies. Our observational data and derived estimates indicated that these colonies were as old as 414 years. If our estimates are accurate, the Interrupted Fern may be one of the longest-lived organisms in the Appalachian Mountains. Introduction Ferns are a major component of the groundcover of central Appalachian forests (Strausburgh and Core 1977). Significant among these species is Osmunda claytoniana L. (Interrupted Fern), a member of the Royal Fern family, Osmundaceae. This relatively large plant, with fronds reaching more than 3.3 ft (1 m) long, is common throughout the hills of eastern North America (Gleason and Cronquist 1964). Interrupted Ferns prefer sunny, moist habitats within a forest setting (Bobrov 1967). Nearly all non-seed vascular plants reproduce by both asexual and sexual methods (Strausburgh and Core 1977). Here, we offer three definitions of terms used throughout the paper: (1) rhizome—a horizontally creeping underground stem that bears roots and leaves and usually persists from season to season; (2) ramet—an individual member of a clone, like a stem arising from a rhizome network; and (3) genet—a clonal colony or group of genetically identical individuals growing in a given location, each originating vegetatively from a single ancestor. An individual Interrupted Fern reproduces asexually by the repeated dichotomous branching of its subsurface rhizome system, which increases the number of ramets in its genet (Jones 1987). The continuing branching of its rhizome eventually produces an ellipse-shaped Interrupted Fern colony, as viewed from above. 1Department of Biology, Howard University, 415 College Street NW, Washington, DC. 20059.2Current address - 3020 14th Street NW, Washington, DC 20009. 3Current address - 317 Mill Street, Milton, DE 19968. Direct correspondence to: Ron Preston; rpreston23@ comcast.net. Southeastern Naturalist A. Faust and R.L. Petersen 2015 Vol. 14, Special Issue 7 204 Jones (1987) suggested that Interrupted Fern colonies are long lived. Further, colonies of several dozen ramets nearing 6.6 ft (2 m) in diameter have been found in West Virginia (Jones 1987). Interrupted Fern fronds emerge in early spring, and the plants produce an average of 7 to 8 fronds per ramet per year. Although fronds function aboveground through the growing season and then die back in the autumn, their stipe (i.e., stalk, stem) bases persist and remain attached along the length of the rhizome. In the springtime, a predetermined number of fronds are produced in a flush (Steeves and Sussex 1972). Taken together, the production of a predetermined number of fronds per ramet per year and the persistence of their stipe bases along the length of rhizome offer a record of past growth similar to, but lacking the precision of, the annual growth rings of trees. We estimated the ages of Interrupted Fern colonies by using the following features of the plant: (1) the well-defined elliptical shape of the colony, (2) the annual production of a single flush of fronds per ramet, and (3) the persistence of stipe bases along the length of rhizome. We calculated a colony’s growth rate by dividing the average number of fronds produced per ramet per year by the number of stipe bases per unit of rhizome length. We estimated colony age by dividing the average radius of the colony by that colony’s calculated growth rate. To our knowledge, our work is the first report of estimated growth rates and ages for Osmunda colonies. Estimating the Ages of Ferns: A Review Species in the Division Pteridophyta, the ferns and lycosphens, form a major group within Kingdom Plantae. Pteridophytes reproduce by spores. Botanists have used 2 methods to estimate the ages of pteridophytes. For colonial species, a colony’s area can be divided by the average incremental growth rate of the colony’s outer edge. Because of its invasiveness and toxicity, the ecology of the fern Pteridium aquilinum (L.) Kuhn (Bracken Fern) has been studied extensively. Watt (1954) obtained a 16-year average of 17.2 in (43 cm) per year for the advance of Bracken Fern colonies in the Lakenheath Warren in Brecks heathland (County Suffolk, UK). Fletcher and Kirkwood (1979) reported that Bracken Fern’s rhizomes grew up to 6.9 ft (2.1 m) per year. Using the annual incremental growth rate of peripheral rhizomes and colony size, Oinonen (1967) estimated that Bracken Fern colonies in Finland were at least 1000 years old. Using isozyme markers, Parks and Werth (1993) identified the individual genets of a population of colonial Bracken Ferns in the southern Appalachians of western Virginia. To estimate the ages of individual genets, they divided one-half the maximum distance between ramets by 17.2 in (43 cm), which is the average annual growth rate of the Bracken Fern rhizome (Watt 1954). Parks and Werth (1993) estimated that the largest genet they studied was at least 1180 years old. In contrast to the methods used for aging clonal ferns, the ages of non-clonal tree ferns have been estimated by dividing the total number of persistent leaf bases or leaf scars along the trunk of an individual plant by the number of fronds Southeastern Naturalist A. Faust and R.L. Petersen 2015 Vol. 14, Special Issue 7 205 produced seasonally. In southern Mexico and Central America, Seiler (1981) found that Cyathea salvinii (Hook.) Domin. (a tree fern) produced an average of 3 fronds per year and its trunk height increased by an average of 3.3 in (8.3 cm) per year. Based on these data, Seiler estimated that a 15.2-ft (4.6-m)-tall specimen of C. salvinii was about 60 years old. Tanner (1983) used the rate of frond production and number of leaf scars to estimate the pace of trunk growth by Cyathea pubescens Mett. ex Kuhn (a tree fern) in Jamaica. Based on a sample of 39 plants, he determined that the average growth-rate of the trunk was 2.7 in (6.7 cm) per year. This finding suggested that a 29.7-ft (9-m)-tall specimen was about 130 years old. Using a method similar to Tanner’s, Ash (1987) estimated that an 18.2-ft (5.5-m)-tall specimen of Cyathea hornei (Baker) Copel. (a tree fern) in Fiji was 80–105 years old. Leptopteris wilkesiana (Brack.) Christ (a tree fern) individuals produce a flush of 3–9 new leaves each wet season. Each growth episode is apparent as a distinct annual band of stipe scars on the trunk. In a primary forest in Fiji, Ash (1986) used these features to estimate that these tree ferns were 60–150 years old. On the Atlantic slope of the Cordillera de Tilaran in Costa Rica, Bittner and Breckle (1995) studied six species of tree ferns, including 4 species of Cyathea and two of Alsophila. They found that species in secondary forests grew three times faster than those in primary forests. Primary forest is a tree-covered area that has attained great age without significant disturbance; a secondary, or second- growth, forest is a wooded area that has regrown after a major disturbance such as fire, insect infestation, or timber harvest. The oldest of the 6 tree-fern species, Cyathea pinnula (Christ.) Domin., grew at 4.2 in (10.4 cm) per year and was estimated to be 48 years old. In a tropical rain forest in the Caribbean lowlands of northeastern Costa Rica, Sharpe (1993) determined that Danaea wendlandii Reichenb. (an herbaceous eusporangiate fern) produced an average of 1.6 fronds per plant per year. Based on growth rate and rhizome length, he estimated that the oldest individual of the population was 23 years old. These various reports allow us to summarize the growth rates and longevities of perennial ferns. Growth rates of rhizomes ranged from 2.7 in (6.7 cm) per year for the tree fern Cyathea pubescens to 6.9 ft (2.1 m) per year for Bracken Fern. Longevity ranged from 26 years for the eusporangiate fern Danaea wendlandii to more than 1000 years for Bracken Fern colonies. Methods Our study site, at 39o07"N, 79o35"W and 3152 ft (955 m) above sea level, was located in a hardwood forest mixed with conifers within Blackwater Falls State Park, Tucker County, in northeastern West Virginia. The park’s mean annual precipitation is 59 in (147 cm), mean annual temperature is 49 oF (9.5 oC), and frost-free season is about 145 days (Adams et al. 1994). Interrupted Fern is abundant in the park and grows in sunny, moist habitats. Our study site had a slope of 1–3%, an eastern aspect, sun to partial shade, and moist soil. Southeastern Naturalist A. Faust and R.L. Petersen 2015 Vol. 14, Special Issue 7 206 We conducted our field-work during mid-May in 1997 and 1998. We selected Interrupted Fern colonies that featured the elliptical shape typical of the genus and excluded colonies that were irregularly shaped, fragmented, or overlapped other colonies. For each of 30 colonies, we measured the length of the long and short axes and counted the numbers of ramets and fronds per ramet. We estimated the age of each Interrupted Fern colony by dividing its average colony radius by its annual rhizome-growth rate. To estimate the recent rhizome growth-rate, we randomly selected a ramet from each colony and collected a 10- cm (4-in)-long rhizome section 4 in (10 cm) below the ramet’s growing tip. We put each rhizome section in a labeled zip-lock plastic bag, and placed the sections in an ice chest for transport to the laboratory; the bags were refrigerated if we were unable to process the samples immediately after collection. We measured the length of each rhizome section, then removed and counted the number of stipe bases present. We calculated the average number of fronds per ramet per colony for each colony. We expressed stipe abundance as number of stipes per inch (stipes/2.54 cm) of rhizome. To estimate rhizome-growth rate for each colony, we divided the overall average number of fronds per ramet per year by the number of stipes per in (stipes/2.54 cm) of rhizome. To obtain each colony’s radius, we added the lengths of its 2 long and 2 short axes, then divided that sum by 4. We estimated 2 ages for each colony. One age estimate was based on the individual growth rate (in/year [cm/yr]) of each colony. We divided the average radius (inches [cm]) of the colony by the average growth rate to generate an age estimate in years. To obtain the second age estimate, we divided the average radius of each colony by the overall average growth rate for the 30 colonies. We used Excel 2003 to generate descriptive statistics. Results Our measurements of each Interrupted Fern colony, including long and short axes, number of ramets/colony, and number of stipes/cm of rhizome, and calculated estimates of average radius, average number of fronds/ramet/colony, rhizome-growth rate, and age are presented in Table 1. The Interrupted Fern colonies in our study had median and average fronds/ ramet of 11 and 11.1, within a range of 7 to 15, respectively (Table 1). Based on field observations, recently branched ramets sprouted seven fronds, and ramets that were about to branch had 14. Thus, the process of dichotomous-ramet branching halved the number of fronds per ramet. Because of this association between the number of fronds and ramet branching, we decided to use an overall average of 11 fronds/ramet for our calculations of growth and age. The largest colony (#26) had 39 ramets and a radius of 55.2 in (138 cm; Table 1). We estimated that the youngest colony (# 3) was 98 years old and the oldest (#4) was 414 years old (Table 1). On average, Interrupted Fern colonies Southeastern Naturalist A. Faust and R.L. Petersen 2015 Vol. 14, Special Issue 7 207 consisted of 19 ramets, spanned a radius of 31.6 in (79 cm), had a rhizomegrowth rate of 0.16 in (0.40 cm) per year, and were 197.4 years old (Table 1). When we graphed colony radius against number of years, we obtained a slightly better fit with a logarithmic curve (r2 = 0.675) than with a linear plot (r2 = 0.649). This result indicates that colonies expanded more slowly as they got older. Finally, the elliptical symmetry that is typical of Interrupted Fern colonies suggests that dichotomous branching occurred more-or-less synchronously. By Table 1. Observed measurements (length of axes 1 and 2, number of ramets) and calculated data for the series of variables used to estimate the ages of 30 Interrupted Fern colonies in Blackwater Falls State Park, Tucker County, WV. # stipes = number of stipes per cm of rhizome. Age 1 = age estimates based on the specific rhizome-growth rate calculated for each colony; age 2 = age estimates based on the overall average rhizome-growth rate for all colonies (0.40 ± 0.02 cm/yr, n = 30). Average Axis (cm) radius # fronds/ ramet # Rhizome-growth 1 2 (cm) # ramets (avg ± SE) stipes rate (cm/yr) Age 1 Age 2 1 217 193 105 19 12.8 ± 3.5 24.1 0.46 228 262 2 105 90 67 14 8.6 ± 1.8 45.6 0.24 275 168 3 141 126 44 13 11.3 ± 2 .4 24.6 0.45 98 110 4 174 168 86 39 9.8 ± 3.1 53.8 0.20 414 214 5 256 222 120 53 8.8 ± 1.5 33.6 0.33 363 300 6 192 151 86 28 9.6 ±1.8 30.5 0.36 236 215 7 141 126 67 23 12.9 ± 3.4 30.0 0.37 184 168 8 196 147 86 15 14.9 ± 3.7 27.9 0.39 216 215 9 180 152 83 15 12.0 ± 2.9 29.0 0.38 217 208 10 176 141 80 19 14.0 ± 3.1 35.7 0.31 257 200 11 183 157 60 29 13.4 ± 2.7 23.2 0.47 125 150 12 148 128 69 23 7.7 ± 0.8 19.7 0.56 122 172 13 185 173 90 13 7.0 ± 0.7 22.0 0.50 177 224 14 170 160 82 19 8.3 ± 1.4 22.3 0.49 166 206 15 133 121 64 13 9.0 ± 2.0 23.9 0.46 137 159 16 140 115 64 12 9.5 ± 2.3 24.3 0.45 140 160 17 138 125 66 5 14.4 ± 3.5 29.5 0.37 174 164 18 193 149 86 26 8.4 ± 1.1 42.1 0.26 324 214 19 173 138 78 12 10.5 ± 3.0 53.3 0.21 372 194 20 173 150 80 25 10.7 ± 2.3 20.1 0.55 146 203 21 195 160 89 16 12.0 ± 2.9 32.0 0.34 257 222 22 168 95 66 11 12.0 ± 3.2 19.5 0.56 116 165 23 178 145 81 13 13.0 ± 2.7 22.2 0.50 162 202 24 138 124 65 11 12.5 ± 2.6 30.6 0.36 179 162 25 195 188 96 23 10.0 ± 2.2 39.5 0.28 340 239 26 320 230 138 39 13.0 ± 3.3 23.4 0.47 291 345 27 116 115 58 12 14.4 ± 3.1 30.4 0.36 157 144 28 138 125 66 12 10.6 ± 2.5 26.7 0.41 158 164 29 200 180 95 19 11.2 ± 2.7 41.0 0.27 351 238 30 125 98 56 7 10.4 ± 1.9 28.3 0.39 142 139 Overall Mean 79 19 11.1 0.3 0.40 217.5 197.4 SE ± 3.6 ± 1.9 ± 0.4 ± 1.7 ± 0.02 ± 16.1 ± 8.9 Range (44–138) (5–53) (7.0–14.9) (19.5–53.8) (0.21–0.57) (97.5–414.0) (110–345) Southeastern Naturalist A. Faust and R.L. Petersen 2015 Vol. 14, Special Issue 7 208 associating synchronous dichotomous branching with average ramet number (19) and average colony age (197.4 years; Table 1), we estimated the time required for ramets to branch. According to our estimates, 4–5 successive dichotomousbranching events had occurred in an average Interrupted Fern colony with 19 ramets (Table 1). That is, colony branching progressed through an average of 5 divisions, as follows: from 1 founding ramet to 2 ramets, from 2 ramets to 4 ramets, then to 8, then to 16, and finally 16 ramets divided to form 32 ramets. By dividing the average estimated colony age of 197.4 years by 4, which is the minimum number of successive branching events experienced by a colony with 16 ramets, we obtained an average of almost 50 years between branching events. Discussion Our results suggest that Interrupted Fern grows at the slowest rate—an average of 0.16 in (0.4 cm) of rhizome length per year—of all fern species for which growth rates had been reported as of 2002. Our findings also suggest that growth slows as colonies age. Our age estimates, with one colony older than 400 years, place theses plants among Earth’s longest-lived ferns, exceeded only by the more than 1000-year-old colonies of Bracken Fern (Oinonen 1965, Parks and Werth 1993). It appears that the elliptical symmetry of an Interrupted Fern colony is an outcome of its slow, synchronous growth and the dichotomous branching of its subsurface rhizome. We acknowledge that our method of dividing the average colony radius by the annual growth rate of its rhizome yields only an approximate colony age. Because we used a constant growth rate to calculate colony age, but our results indicate that the actual growth rate varies over time, we may have overestimated colony age. Additional research to determine rhizome growth rate over time would be useful to assess the validity of our results. To improve age estimates, a number of rhizome sections from each colony should be sampled to gain a better understanding of the variation in rhizome growth. Also, the use of an average colony radius to represent the average length of the rhizome originating from the center of an idealized circular colony likely yields a conservative estimate of rhizome length because the rhizome begins at the point of origin of a colony’s founding ramet and continues outwards, passing through a series of dichotomous divisions to the periphery of the colony. If the actual rhizome length is significantly longer than the average radius of a colony, and if the average estimated growth rate of 0.16 inch (0.4 cm) per year represents a good estimate of rhizome growth, then the ages of some of our Interrupted Fern colonies may actually exceed 1000 years. Management Implications Because of their slow growth, age, and beautiful crown-like form, we recommend that Interrupted Fern colonies, common in the mountains of West Virginia, including Blackwater Falls State Park and Canaan Valley, be carefully protected. Further, the tissues of old Interrupted Fern colonies may harbor indicators of Southeastern Naturalist A. Faust and R.L. Petersen 2015 Vol. 14, Special Issue 7 209 long-term environmental conditions, a data source that could contribute to our understanding of climate change. Acknowledgments For funding, we thank the Northeastern area of the USDA Forest Service, Newtown, PA, and the Fund for Academic Excellence at Howard University, Washington, DC. We also thank the Tier Two Forest Biology Students of the University of the District of Columbia and Howard University. Literature Cited Adams, M.B., J.N. Kochenderfer, F. Wood, T.R. Angradi, and P. Edwards. 1994. Forty years of hydrometeorological data from the Fernow Experimental Forest, West Virginia. Northeastern Forest Experimental Station, USDA Forest Service, Radnor, PA. General Technical Report NE-184. 24 pp. Ash, J. 1986. Demography and production of Leptopteris wikesiana (Osmundaceae), a tropical tree fern from Fiji. Australian Journal of Botany 34:207–215. Ash, J. 1987. Demography of Cyathea hornei (Cyatheaceae), a tropical tree fern in Fiji. Australian Journal of Botany 35:331–342. Bittner, J., and S.W. Breckle. 1995. The growth rate and age of tree-fern trunks in relation to habitats. American Fern Journal 85:37–42. Bobrov, A.E. 1967. The family Osmundaceae (R. Br.) Kaulf: Its taxonomy and geography. Botanicheskii Zhurnal 52:1600–1610. Gleason, H.A., and A. Cronquist. 1964. The Natural Geography of Plants. Columbia University Press, New York, NY. 420 pp. Fletcher, W.W., and R.C. Kirkwood. 1979. The Bracken Fern (Pteridium aquilinum [L.] Kuhn): Its biology and control. Pp. 591–636, In A.F. Dyer (Ed.). The Experimental Biology of Ferns. Academic Press, London, UK. 657 pp. Jones, D.L. 1987. Encyclopaedia of Ferns: An Introduction to Ferns, Their Structure, Biology, Economic Importance, Cultivation, and Propagation. British Museum (Natural History), London, UK. 433 pp. Oinonen, E. 1967. Sporal regeneration of Bracken [Pteridium aquilinum (L.)] in Finland in the light of the dimensions and the age of its clones. Acta Forestalia Fennica 83:1–96. Parks, J.C., and C.R. Werth. 1993. A study of spatial features of clones in a population of Bracken Fern, Pteridium aquilinum (Dennstadtiaceae). American Journal of Botany 80:537–544. Seiler, R.L. 1981. Leaf-turnover rate and natural history of the Central American treefern Alsophola salvinii. American Fern Journal 71:75–81. Sharpe, J.M. 1993. Plant growth and demography of the neotropical herbaceous fern Danaea wendlandii (Marattiaceae) in a Costa Rican rain forest. Biotropica 25:85–94. Strausburgh, P.D., and E. Core. 1977. The Flora of West Virginia. Seneca Books, Parsons, WV. 1075 pp. Steeves, T.A., and I.M. Sussex. 1972. Patterns in Plant Development. Prentice Hall, Upper Saddle River, NJ. 302 pp. Tanner, E.V.J. 1983. Leaf demography and growth of the tree-fern Cyathea pubscens Mett. ex Kuhn in Jamaica. Botanical Journal of the Linnaean Society 87:213–227. Watt, A.S. 1954. Contribution to the ecology of Bracken (Pteridium aquilinum): VI. Frost and the advance and retreat of Bracken. New Phytologist 53:117–130.