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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.
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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
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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.
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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
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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)
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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
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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.
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