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Patterns of Reproduction in Wright's Sycamore1
Jane H. Bock and Carl E. Bock
2
Abstract.--In southeastern Arizona this tree
produces large numbers of viable seeds that fall in
a compact fruit shadow around parent individuals.
Sexual reproduction usually fails due to drought
or flash-flooding.
Large numbers of seedlings and
saplings grew in one site with permanent water and
little flooding.
Young trees grew in clumps, usually
of similar-sized individuals, and away from mature
tree canopy but always in the stream channel.
INTRODUCTION
Platanus wrightii Wats.(Platanaceae)
occurs in southern Arizona, southwestern
New Mexico, and northern Mexico (Elias
1980).
Glinsky (1977) reported on problems
with natural regeneration in southeastern
Arizona.
We (Bock and Bock 1985) found
causes of reproductive failure to be desiccation and death of embryos prior to germination, and loss of seedlings during flash
floods.
In the present study we measured
patterns of seed fall, size classes of
seedlings and saplings, and seedling distribution in relation to streambed characteristics and tree canopy, in three canyons
in southeastern Arizona.
METHODS
Field work was conducted between 1982
and 1984 in Lyle and Corral Canyons (Santa
Cruz Co.), and in Carr Canyon (Cochise Co.).
All sites are within the Coronado National
Forest.
Sexually mature trees (> 10cm
basal stem diameter) were tagged and mapped in each study area.
1
Paper presented at the North American
Riparian Conference, University of Arizona,
Tucson, April 16-18, 1985.
There were 52 fruit-producing trees in
Lyle and Corral Canyons which were sufficiently isolated that seedfall patterns
could be constructed for them.
We placed
a meter tape up and downstream from each,
and counted the numbers of fruits within
one meter on each side of the tape, by 2-m
intervals.
This was done in January and
March, 1983 and 1984, when seed fall and
viability are at their maxima (Bock and
Bock 1985).
No seedlings survived in Lyle or
Corral Canyons during our study, apparently due to desiccation and flash flooding.
However, there were large numbers of young
trees in one 580-m section of Carr Canyon,
along with 43 adults, where a spring kept
the streambed moist year-round, and where
flood scouring was limited by the small
size of the watershed.
Here, we counted
and measured basal stem diameters of all
trees in July, 1982.
We examined the dispersion patterns of the young individuals
by measuring nearest neighbor distances
and comparing these with expected (random)
values calculated from the overall density
of individuals (Vandermeer 1981).
We determined whether each young tree was under
a canopy and compared these data with
canopy frequency under 193 points at 3-m
intervals along the streambed.
At 47
places along the stream channel we constructed cross sectional profiles and
plotted the sizes and locations of young
sycamores in them.
2
Jane H. Bock and Carl E. Bock are
Professors of Biology, University of Colorado, Boulder, Colo. 80309, and Co-Directors, Appleton-Whittell Research Ranch
Sanctuary, National Audubon Society, P.O.
Box 44, Elgin, Ariz. 85611.
RESULTS AND DISCUSSION
Sycamore fruits were dispersed in a
clearly defined pattern around parent
trees (fig. 1), with 90% falling between
10 m upstream and 16 m downstream.
More
fruits were spread downstream (2,674 vs.
1,381 for all 52 trees; Chi-square = 412.3,
493
100
(P..c.. 0.001).
Fruits contained an average
oY 667 seeds (n = 30). The 52 trees shed
an average of 78 fruits in 2x58-m strips
running up and downstream (fig. 1).
Therefore, an averagE tree shed 449 seeds/m2
in its seed shadow in the stream channel.
Because seeds are up to 90% viable (Bock
and Bock 1985), we conclude that plentiful
supplies of sexual propagules are produced
by Wright's sycamores in southeastern
Arizona.
The smallest seedlings in Carr Canyon
were about 100 times more common than saplings up to 10-cm basal stem diameter
(fig. 2).
Size and age were highly correlated (r = 0.87, P<:0.01, n = 20), with
the largest saplings being about 10 years
old.
If this represents a stable age/size
distribution, then under ideal circumstances
it appears that establishment, growth rate,
and survivorship of Wright's sycamore
seedlings to maturity can be quite high.
YOUNG SYCAMORES
80
CAAR CANYON
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BASAL
Young sycamores in Carr Canyon never
occurred outside the stream channel (n =
315), and they had a clumped dispersion
within it.
Expected (random) mean nearest
neighbor distance was 2.04 m, while actual
distance was 1.07 m.
The basal stem diameter of each seedling and sapling was
positively correlated with that of its
nearest neighbor (r = 0.43, P~ 0.01).
We
ruled out position-in the stream cross
section as a cause of this clumping, be-
4
STEM
6
DIAMETER
8
(em)
Figure 2. Size class distribution of young
sycamores in Carr Canyon.
cause there were no clear patterns of seedling or sapling distribution in relation
to stream depth or distance from the shoreline.
However, young trees were more
common away from mature tree canopy (actual
canopy = 50%; 219 seedlings and saplings
unshaded, 77 shaded; Chi-square = 68.1, P
L 0. 00 1).
Clumping may also be due to
historic patterns of fruit fall.
Acknowledgments.--This project was
supported by Joseph and Helen Taylor, and
by Earthwatch and the Center for Field
Research.
We thank K. Bell, M. O'SheaStone, and Earthwatch volunteers for
assistance.
10
SEED
TREE
8
LITERATURE CITED
Bock, J.H., and C.E. Bock.
1985.
Reproductive ecology of Platanus wrightii
in southeastern Arizona.
Proc. 1983
Chihuahuan Desert Symposium, in press.
Elias, T.
1980.
Trees of North America.
948 p. Van Nostrand Reinhold Co., New
York, N.Y.
Glinsky, R.L.
1977.
Regeneration and distribution of sycamore and cottonwood
trees along Sonoita Creek, Santa Cruz
County, Arizona.
p. 124-127.
In Importance, preservation, and management
of riparian habitats: a symposium. USDA
Forest Service General Technical Report
RM-43.
217 p. Rocky Mountain Forest
and Range Experiment Station, Fort
Collins, Colo.
Vandermeer, J.
1981.
Elementary mathematical ecology.
294 p. John Wiley
and Sons, New York, N.Y.
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UPSTREAM(m}
10
20
DOWNSTREAM (m)
Figure 1.
Average numbers of sycamore
fruits shed various distances up and
downstream from 52 mature trees.
494