Tree Crown Structure and Vascular Epiphyte Distribution in Sequoia sempervirens Rain Forest Canopies

22 Pages·1999·2.9 MB·

by  Sillett, Stephen

Selbyana 20(1): 76--97. 1999.
TREE CROWN STRUCTURE AND V ASCULAR EpIPHYTE
DISTRIBUTION IN SEQUOIA SEMPERVIRENS RAIN FOREST CANOPIES
STEPHEN C. SILLETT
Department of Biological Sciences, Humboldt State University, Arcata, CA 95521 USA.
E-mail: [email protected]
ABSTRACT. Crown structure and vascular epiphytes were studied in eight large (82-97 m tall, 3.3-7.2 m
dbh) redwood trees (Sequoia sempervirens) in old-growth temperate rain forests, using rope-based methods
of access. The trees had complex individualized crowns consisting of multiple (12-62 per tree) reiterated
trunks arising from other trunks and branches. Trunk-to-trunk and trunk-to-branch fusions were common,
and the diameter of a trunk above a fusion was often greater than below the fusion. Thirteen species of
vascular plants, including a spike-moss, three ferns, four shrubs, and five trees, grew as epiphytes. Many
of the species were accidental epiphytes whose primary habitat was the forest floor. They grew in deep
humus accumulations on large branches and in crotches formed by multiple trunks. Three species dominated
epiphyte assemblages. The deciduous fern Polypodium glycyrrhiza was abundant on two of the trees, where
it was always restricted to lower crowns. The evergreen fern Polypodium scouleri, the most abundant
vascular epiphyte, occurred in the upper and lower crowns of all eight trees. The ericaceous shrub Vaccinium
ovatum also occurred on all eight trees but was abundant on only four trees with large quantities of decaying
wood in their crowns. Polypodium ferns were more frequent on living branches, while ericaceous shrubs
were more frequent on trunks and dead branches. Complex crown structure clearly promoted humus ac
cumulation and vascular epiphyte abundance, but much of the tree-to-tree variation in epiphyte distribution
was attributed to differences in tree age, stand-level microclimate, tree health, and dispersal limitations.
Key words: epiphytes, redwoods, Sequoia sempervirens, temperate rain forest
INTRODUCTION a free-standing tree except for its location within
the crown of a larger tree. Few studies of crown
Tall conifers dominate temperate rain forests structure in tall-stature conifer forests are based
in western North America. Young conifers in on in situ sampling. Most canopy research in
these forests exhibit a characteristic architecture: these forests has focused on organisms inhabit
one large, orthotropic (Le., vertically oriented) ing tree crowns, especially epiphytes.
trunk supporting numerous smaller, more or less Rain forests of western North America harbor
plagiotropic (i.e., horizontally oriented) branch large quantities of epiphytes, whose biomass can
es. Trees growing in sheltered forests may retain exceed several metric tons per ha (Nadkarni
this simple crown structure well into old age. 1984, McCune 1993). Epiphytes of old-growth
Branches in the lower portion of conifer crowns forests dominated by Pseudotsuga menziesii
frequently are lost by self-pruning, especially in (Mirb.) Franco (Pinaceae) (hereafter Douglas-fir
shade-intolerant species. Thus, the lower trunks forests) have been studied for nearly three de
of tall trees in old-growth forests are typically cades (Sillett & Neitlich 1996). These forests
free of branches for 20 m or more (Franklin et support an abundance of epiphytic lichens and
al. 1981, Kuiper 1988, Stewart 1989). bryophytes (Pike et al. 1975). Only one species
Disturbances (e.g., treefalls) that increase of vascular plant, the fern Polypodium glycyr
light availability can lead to dramatic changes in rhiza D.C. Eaton (Polypodiaceae), regularly oc
tree crown structure (Halle et al. 1978, Oldeman curs epiphytically in the canopy. It is associated
1990). Epicormic branches, which appear as fan with thick bryophyte mats on branches (Sillett
shaped arrays (Pike et al. 1977), can sprout from 1995). Epiphyte assemblages in the oldest, wet
well-illuminated conifer trunks long after the test Douglas-fir forests are dominated by bryo
original branches have been lost. Main trunks of phytes and vascular plants, including a number
tall conifers that snap in severe storms often of accidental epiphytes whose primary habitat is
sprout new trunks. Even branches receiving terrestrial (Sillett & Neitlich 1996). Little is
abundant light can sprout orthotropic stems that known about canopy structure or epiphyte as
can be considered new trunks. These trunks are, semblages in western forests not dominated by
in effect, reiterations of the tree's architectural Douglas-fir.
model (Halle et al. 1978), with each one sup The tallest forests in North America are dom
porting its own system of plagiotropic branches. inated by Sequoia sempervirens (D. Don) Endl.
Thus, a reiterated trunk is indistinguishable from (Cupressaceae) (hereafter redwood). These for-
76
SILLETT: EPIPHYTES OF REDWOODS 77
TABLE 1. Locations and dimensions of the eight redwood trees in the study. Trees 1-4 were in Prairie Creek
Redwoods State Park, and Trees 5-8 in Iedediah Smith Redwoods State Park.
No. of Wood volume (m3)
Total reiterated
Tree height (m) DBH (m) trunks Main trunk All reiterations Largest reiteration
1 95.8 3.31 61 234.8 20.9 4.4
2 97.0 3.69 55 260.3 4.0 1.4
3 82.5 3.47 62 290.5 16.1 2.1
4 88.6 7.10 21 649.3 113.5 33.1
5 85.7 4.58 23 349.9 35.3 28.9
6 93.0 3.93 12 370.5 10.5 3.4
7 86.7 5.35 26 471.1 15.8 3.5
8 93.7 7.23 43 945.6 99.1 46.4
ests harbor the tallest known living trees (Carder sitchensis (Bong.) Carriere up to 90 m tall form
1995). Individual redwood trees can reach 112 ing most of the remainder. In the other two
m in height (Sawyer et aL 1999). These are the stands, redwood accounts for 90% of the basal
most massive forests on Earth. Individual forest area. All stands contain small amounts of Tsuga
stands can have a wood volume of more than heterophylla (Raf.) Sarg. (Pinaceae) as well as
10,000 m3 per ha and a biomass of more than Lithocarpus densifiorus (Hook. & Am.) Rehder
3000 metric tons per ha (Sawyer et aL 1999). (Fagaceae) and/or Umbellularia californica
The canopy structure of these impressive forests (Hook. & Am.) Nutt. (Lauraceae) in the lower
has been unexplored except for a single unpub canopy. Understory vegetation is dominated by
lished ground-based study (Mulder & de Waart Polystichum munitum (KauiO C. Presl (Aspidi
1984). There are no publications on epiphytes in aceae) and Vaccinium ovatum Pursh with small
redwood forests. er amounts of V. parvifolium Sm. (both Erica
I initiated the first in situ studies of redwood ceae), Rubus spectabilis Pursh (Rosaceae),
forest canopies in 1996 at Humboldt State Uni Rhamnus purshiana DC. (Rhamnaceae), and/or
versity. My rope-based research has focused on Acer circinatum Pursh (Aceraceae). I selected
canopy structure and epiphyte distribution in eight large redwoods from the four stands for
old-growth redwood forests. This study had two detailed study (TABLE 1). Trees 1 and 2 grow
specific objectives: 1) to describe crown struc side-by-side in the same stand along Godwood
ture of eight large redwoods and 2) to assess Creek in PCRSP. Trees 5, 6, 7, and 8 grow in
distribution and abundance of vascular epiphytes the same stand along Mill Creek in JSRSP.
in these tree crowns.
METHODS
STUDY AREA
To access tall tree crowns, I shot a rubber
This study focused on four old-growth forest tipped fiberglass arrow trailing 10 kg test
stands between 50 and 100 m elevation. Three strength Fireline® filament over sturdy branches
stands are located in Prairie Creek Redwoods with a powerful compound hunting bow (verti
State Park (PCRSP), Humboldt County, Califor cal range = 80 m) mounted to a spinning reel.
nia. Mean annual rainfall in PCRSP is 1.75 m; A 3 mm nylon cord, followed by 11 mm static
summer temperatures range from 4 to 24°C; and kemmantle rope, was then hauled over the
winter temperatures range from -1 to 13°C branches. I anchored one end of the rope at
(Redwood National and State Parks). One stand ground level and climbed the other using me
is located in ledediah Smith Redwoods State chanical ascenders. I used a 20 m long arborist's
Park (JSRSP), Del Norte County, California. rope lanyard fitted with a double-end, split-tail
Mean annual rainfall in JSRSP is 2.5 m; summer system (Sherrill Inc. 1997, see also Jepson 1998)
temperatures range from 7 to 29°C; and winter to access progressively higher branches and to
temperatures range from - 1 to 16°C (Redwood move laterally through tree crowns. A rescue
National and State Parks). Both Parks experi pulley was secured near the top of each tree with
ence a summer dry season ameliorated by per a tubular nylon webbing sling. The climbing
sistent fog. Stands have basal areas between 270 rope was lowered from the tree on nylon cord
and 370 m2 per ha. In two stands along God at the end of the day, and the pulley was used
wood Creek in PCRSp, redwood accounts for to haul the rope back into place for subsequent
75% of the basal area with co-dominant Picea ascents via single rope technique.
78 SELBYANA Volume 20(1) 1999
TABLE 2. Summary of product-moment correlation coefficients (r) between crown structure variables for reit
erated trunks on eight redwood trees. Broken reiterations (N = 47) were excluded from correlations for
total length. Statistically significant correlations (P < 0.01) are highlighted in bold. Samples sizes are
indicated in parentheses.
Height of origin Total length
All reiterated trunks (N = 298)
Total length -0.24
Basal diameter -0.20 0.91
Distance from Branch
Origin height Total length main trunk Basal diameter basal diameter
Reiterated trunks arising from branches (N = 149)
Total length -0.16
Distance from main trunk -0.36 0.06
Basal diameter -0.11 0.94 -0.01
Branch basal diameter -0.26 0.63 0.18 0.65
Branch diameter at reiteration -0.12 0.78 -0.07 0.82 0.75
Crown Mapping stems, 2) maximum stem basal diameter, 3) total
length, and 4) maximum crown width. Similarly,
I mapped the crown structure of each tree by
I measured trunk basal diameter, total length,
measuring heights, diameters, and distances be
and maximum crown width for epiphytic trees.
tween reiterated trunks. Numbered aluminum
In addition to variables describing epiphyte size,
tags were attached to major trunks at 5 m inter
I measured height above ground, distance and
vals with 2 cm paneling nails. These tags served
azimuth to main trunk, and dia.I1;leter of support
as benchmarks for height measurements of
ing branch beneath epiphyte (if present). I also
smaller trunks, branches, and epiphytes. Alu
recorded crown locations for individual epi
minum tags were also used to label individual
phytes by noting whether they occurred on
trunks for future reference. I recorded the fol
branches, on trunks, or in crotches at the bases
lowing data for each reiterated trunk: top height,
of multiple trunks.
height of origin, basal diameter, and diameter at
5 m intervals along the length of the trunk. For
reiterated trunks arising from branches, I also Data Analysis
recorded horizontal distance to main trunk,
branch height, branch basal diameter, and branch Wood volume of each tree was calculated by
diameter at reiteration. Trunks were referenced applying two equations to the trunk diameter
to each other by recording azimuths and dis data. I applied the equation for a parabolic frus
tances between them at 5 m height intervals. tum (volume = lengthl2*[AI + A2], where Al
Measurements were made with the aid of a com and A2 are the upper and lower trunk cross sec
pass, clinometer, and graduated fiberglass tape. tional areas) to reiterated trunks and upper sec
Since large trunks often gave rise to complex tions of the main trunk that tapered rapidly. I
arrays of smaller trunks, I sketched crown struc used the equation for a regular conic frustum
ture and noted physical connections between (volume = length*'lT/3*[lower diameter2 + (low
trunks and branches and whether trunks were er diameter)*(upper diameter) + upper diame
monopodial, sympodial, or otherwise broken. ter2]) for sections of the main trunk that tapered
The information was used to generate tree crown slowly (R. Van Pelt pers. comm.).
diagrams via DeltaGraph® and ClarisDraw® I used linear regression analysis to explore re
software for the Macintosh® computer. lationships between variables measured during
crown mapping and epiphyte sampling. Corre
Epiphyte Sampling lations between crown structur~ variables were
evaluated separately for all reiterated trunks and
I used nondestructive methods to measure siz for the subset of reiterated trunks arising from
es and locations of all vascular epiphytes occur branches. Correlations between epiphyte vari
ring in each tree crown. For each fern mat, I ables were evaluated separately for the three
measured five size variables: 1) number of living dominant epiphyte species (see RESULTS). I test
fronds, 2) maximum frond length, 3) maximum ed for associations between these species and
frond order (i.e., rows of pinnae per frond), 4) crown locations by using the R X C test of in
mat length, and 5) mat width. Four size variables dependence (Sokal & Rohlf 1995: 738), where
were measured for each shrub: 1) number of R = 3 epiphyte species, and C = 3 crown 10-
SILLETT: EPIPHYTES OF REDWOODS 79
50.0.--------,--------------.---..r--I
• from trunks
o from branches •
•
+ broken reiterations
+
10.0-
+
o o
o
•
+
1.0-
+
• o
-!(> + +
o o
0.44---~--~--~~~~-,r-------~---,--~--r-~~~~,--------4
2 10 100 200
basal diameter (em)
FIGURE 1. Relationship between total length and basal diameter of reiterated trunks in the crowns of eight
large redwood trees.
cations. I used G divided by William's correc Again, I used G divided by William's correction
tion factor as the test statistic. factor as the test statistic.
I divided the three dominant epiphyte species
into size classes using an unweighted ranking
RESULTS
procedure. For each species, individual epi
phytes were separately ranked in descending or Crown Structure
der according to each size variable. An average
rank was then calculated for each individual, The eight redwoods had between 12 and 62
which served as the best estimate of the plants' reiterated trunks per crown (TABLE 1). I mea
relative sizes. Ranked lists of epiphytes were sured a total of 298 reiterated trunks, 149 of
then divided into four size classes: first decile which arose from branches. Several significant
(hereafter huge), remainder of the upper third relationships were evident among crown struc
(hereafter large), middle third (hereafter medi ture variables (TABLE 2). Larger trunks tended to
um), and lower third (hereafter small). Associ occur lower in the crown; total length and basal
ations between epiphyte size classes and crown diameter of trunks were negatively correlated
locations were tested by using the R X C test of with height of origin. Reiterated trunks arising
independence (Sokal & Rohlf 1995: 738), where from outer branches were more prevalent in the
R = 4 size classes, and C = 3 crown locations. lower crown; distance from main trunk was neg-
80 SELBYANA Volume 20( 1) 1999
I
I
65
1
'"E"i ' 60.-1
'-' I
§ I I
e
55-1
C/) i I
~ I ))1
o 50'1
~ r-----------~~'.o If
:; 45l
~ I I
40'1
35,1 ~
I) I!
I I
/
301
I
25
20
15~
I
Wi
i
51
i
01
i
6 5 10
FIGURE 2, Crown structure and vascular epiphytes of redwood Tree 1, Yang, Prairie Creek Redwoods State
Park, California, All trunk diameters are drawn to the scale of the x -axis, which is expanded relative to the scale
of the y-axis. Circles correspond to the basal diameters of reiterated trunks, Circles with serrated edges indicate
broken trunks. Branches bearing reiterated trunks are depicted with single straight lines unless they are more
than 1.4 m in basal diameter (see right side of main trunk around 42 m). No other branches, including those
supporting "floating" epiphytes, are shown. Colored circles indicate dominant epiphyte species as follows:
orange = Polypodium glycyrrhiza, green = Polypodium scouleri, blue = Vaccinium ovatum. Circle sizes cor
respond to epiphyte size classes (see RESULTS).
SILLETT: EPIPHYTES OF REDWOODS 81
l.~r---------------------------------------------~
95
90
85
80
75
70
65
•
40 •
35
30
25
20
15
dominant epiphyte
size classes
10
• large
• medium
5 small
*o ther epiphytes
Picea sitchensis
0~3---f~-c---O~~I'-~2c-~3--~4--~5---6c-~7~-'8c-~9--~IO
distance from central axis (m)
FIGURE 3. Crown structure and vascular epiphytes of redwood Tree 2, Yin, Prairie Creek Redwoods State
Park, California (see FIGURE 2 caption).
82 SELBYANA Volume 20(1) 1999
85
80
75
70
65
~
60
.................•...............••...••.•.•
55
]:50
'g
i5
6:h45
Q.)
;>
~40
~
lhs
30
25
do•minan t epiphyte
20 size classes
huge
•
15 large
•
medium
small
10
*
Picea sitchensis
*
Tsuga heterophylla
5 ... Gaultheria sha/Ion
... Vaccinium parvijlorum
• Polystichum munitum
6 5 4 321 0 1 234 5 6
distance from central axis(m)
FIGURE 4. Crown structure and vascular epiphytes of redwood Tree 3, Floating Raft Tree, Prairie Creek
Redwoods State Park, California. Arrows with dotted lines indicate origins of reiterations that were graphically
displaced to the right for clarity (see FIGURE 2 caption).
SILLETT: EPIPHYTES OF REDWOODS 83
90
85
80
75
70
65
60
55
.-..
S 50
'-'
'g
5
45
;...
QI)
2Q)
40
CIS
.....
~35
•
] • •
30
25
20
dominant epiphyte
size classes
15
• huge
• large
10 • medium
small
5 other epiphytes
*
Rhamnus pUTshiana
"Ribes laxiflorum
0+---,--.--,--.---.--4--.~-r--,--,---,--,--,--~--,--+--,---,--,--4
10 9 8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 9 10
distance from central axis (m)
FIGURE 5. Crown structure and vascular epiphytes of redwood Tree 4, Atlas Tree, Prairie Creek Redwoods
State Park, California (see FIGURE 2 caption).
atively correlated with height of origin. Thicker thinner branches; branch basal diameter and
branches supporting trunks were more prevalent branch diameter at reiteration were positively
in the lower crown; branch basal diameter was correlated with total length and basal diameter
negatively correlated with height of origin. of trunks. Finally, trunk basal diameter was
Thicker branches supported larger trunks than strongly correlated with total length, especially
84 SELBYANA Volume 20(1) 1999
..
•
•
•
dominant epiphyte
size classes
huge
• large
• medium
.
small
other ..n 'inh,vtE~';:
5 Sela.g.i nella oregana
O+-~--~~~--~£'--'--r-'r-~-'--'--'--~"-'--'--'~
7 6 5 4 3 2 2 3 4 5 6 10 11
distance from central axis (m)
FIGURE 6. Crown structure and vascular epiphytes of redwood Tree 5, Aragom, Jedediah Smith Redwoods
State Park, California. Gray lines indicate dead trunks (see FIGURE 2 caption).
after removing broken trunks from the analysis (FIGURE 2), accounted for 8.2% of its total wood
(TABLE 2, FIGURE 1). Redwood crown structure volume (TABLE 1). Two massive trunks 0.8 and
differed dramatically from one tree to the next, 1.0 m in basal diameter arose from the right side
even if they grew side-by-side in the same stand. of the main trunk at 42-46 m and extended into
Tree 1. The 61 reiterated trunks, on Tree 1, the canopy gap above Godwood Creek. The
including five broken and two sympodial trunks larger of these trunks supported 20 trunks of its
Srr..,LETT: EPIPHYTES OF REDWOODS 85
90
.. e
•
•
dominant epiphyte
esiz e classes
huge
• large
• medium
small
8 7 6 5 432 1 0 1 2 3 4 5 6 7
distance from central axis (m)
FIGURE 7. Crown structure and vascular epiphytes of redwood Tree 6, Aldebaran, Jedediah Smith Redwoods
State Park, California (see FIGURE 2 caption).