Abstract
Big Leaf Magnolia (Magnolia
macrophylla Michaux) has the largest size simple leaf of any North
American tree. Its geographic range includes the southeastern U.S. In
North Carolina it is predominantly found in Gaston County, located in
the western Piedmont region of the state. André Michaux first described
this species from a population site in Gaston County in 1789. This is
a preliminary study of a larger research project to better understand
the ecology of M. macrophylla. This preliminary study includes
data from six population sites sampled during the last two growing seasons.
Initial data show a significant correlation of M. macrophylla with
mesic habitats. There is a significant correlation between the presence
of M. macrophylla and Fagus grandiflora at these sites.
There is also evidence of recent disturbance at most of the study sites.
It appears that disturbance may be an important factor in the success
and/or establishment of M. macrophylla populations.
Introduction
Magnolia macrophylla Michaux (Big-Leaf Magnolia) has the largest
sized leaf diameter of any simple leaf tree species in North America.
Its range extends from North Carolina westward to southeast Kentucky and
south through Tennessee, Alabama and Louisiana. Approximately 95% of the
populations in N.C. are in Gaston County, with only a few widely scattered
populations in other sections of the state (NC Natural Heritage Program
1998). In fact, Michaux first described it from an extant population in
Gaston County, NC in 1789 (Williams 1999). The historical distribution
of M. macrophylla and factors that have contributed to its localized
distribution pattern, are not well understood. In addition, only one known
previous study (Doyle unpubl. Thesis 1989) on the ecology of this species
has been conducted. Magnolia macrophylla has generally been described
as inhabiting sheltered coves or valleys protected from strong winds.
Its leaves are often 50-76 cm long and 23-26 cm broad. The flowers
are large as well, measuring 32-46 cm in diameter and bloom from April
to early June (Thein 1974).
To better understand the ecology and distribution pattern of Magnolia
macrophylla, a study was initiated in the spring of 2000 to collect
data from several large populations sites within Gaston County, NC. The
objective of this study focuses on providing baseline information on the
species ecology of M. macrophylla and its current status in Gaston
County, NC.
Methods
Six populations were sampled using a total of 205 5m2 contiguous
quadrats. Within each quadrat data were collected for each of the following:
total number of Magnolia macrophylla individuals and a corresponding
diameter at breast height measurement (dbh) for each, a list of all associate
woody species and a designation of whether they were tree or sapling size,
and a total cover class value for all vegetation. DBH values were assigned
as follows:
1 = dbh>5 in.,
2 = dbh>2 in.<5 in.,
3 = dbh<2 in.
Cover class values were as follows:
50 – 100% = 5,
25– 49% = 4,
11 – 24% = 3,
1 – 10% = 2,
<1% = 1.
The presence or absence of flowers/fruits
on individuals of M. macrophylla was also recorded. In addition,
aspect and soil series data for each population were ascertained. Chi-Square
analyses were performed on the data.
Results
Magnolia macrophylla and Associate Species
Among associates within sampled quadrats with Magnolia macrophylla,
Fagus grandifolia is the most significant associate species (P
= .012). Of the 177 quadrats sampled with M. macrophylla, 81 (46%)
contained at least one individual of F. grandiflora. Liriodendron
tulipera is also significantly correlated with M. macrophylla
within the sampled quadrats (P = .048). Of the 177 quadrats with M.
macrophylla, 66 ( 37%) contained at least one individual of L.
tulipfera (Fig. 1).
Figure 1. Species occurrence
Presence of Fruit/Flowers and DBH Values of M. macrophylla
Of the Magnolia macrophylla individuals with a dbh value of 1,
41% possessed flower/fruit
structures. Only 8% of individuals with a dbh value of 2 possessed flowers/fruits.
No flower/fruit
structures were observed on individuals with a dbh value of 3 (Fig. 2).
Slope/Aspect Features
The six populations were located on moderate to severe north facing
slopes. Soils series from each study site were of similar taxonomic groupings.
Discussion
Of the associate species found within quadrats with Magnolia macrophylla,
only Fagus grandifolia and Liriodendron tulipfera
are significantly correlated. Doyle’s (1989) study documented F. grandifolia
as an important associate species in Mill Creek Forest in Pearl River
County, Mississippi. Also, the slope and aspect positions for these population
sites are consistent with historical descriptions of M. macrophylla
populations (Raynal 1938, Sargent 1947). The population sites meet the
critera for classification as Mesic Mixed Hardwood Forest (Piedmont Subtype).
These sites are often along slopes and ravines, which may be north facing
and steep. The soils are deep, well drained and often acidic. They are
characterized as often having uneven-aged members and reproduction which
occurs in canopy gaps (Schafale and Weakly 1990). Doyle (1989) describes
a “clumping ” in the landscape of M. macrophylla seedlings
as a result of apparent vegetative reproduction from surrounding root
systems. This was also my observation among my study sites. Initial observations
suggest increased asexual propagation from root systems of M. macrophylla
in both canopy gaps and areas of less intense competition from other species.
Much of the county, including portions of these sites, experienced the
creation of canopy gaps due to tree loss during Hurricane Hugo in 1989.
This might suggest the necessity of periodic disturbance mechanisms within
a M. macrophylla population site to optimally sustain it.
The author hopes that further research will yield a better understanding
of the ecology of this species.
Literature Cited
Doyle, A.M. 1989. Unpubl. Honor Thesis. Magnolia Macrophylla:
Population structure, reproductive strategies and the disturbance regime
of Mill Creek Forest. Tulane University.
N.C. Natural Heritage program records for Magnolia macrophylla
sites. 1998. North Carolina Dept. of Environment and Natural Resources.
Raynal, C.E. 1938. Bigleaf magnolia. American Forests. 44:204-240.
Sargent, C.S. 1947. The silva of North America. Vol. I. Peter Smith.
New York.
Schafale, M.P. and A.S. Weakly. 1990. Classification of the natural communities
of N.C. Third Approximation.
Thein, L.B. 1974. Floral biology of magnolia. Amer. J. Bot. 61 (10):
1037-1045.
Williams, C. 1999. Andre Michaux and the discovery of Magnolia macrophylla
in North Carolina. Castanea 64(1):
Acknowledgement for Grant Support
Blumenthal Foundation of Charlotte, NC
Fidelity National Bank of Atlanta
Berlin America Co.
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