Monteverde : Ecology and Conservation of a Tropical Cloud Forest

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The elfin forest pioneer, Didymopanax pittieri, has
a distinctive growth form (Leeuwenberg's architec-
ture; sensu Halle et al. 1978). Each twig sets a termi-
nal inflorescence bud and sprouts a whorl of twigs
below. This simplicity of growth form aids compari-
sons both within and among trees. In the windward
forest study watershed, D. pittieri growing on the
windy ridgecrests have thicker trunks for a given
height than those growing farther downslope (Lawton
1982). Similarly, twigs on trees of the ridgecrest are
thicker relative to their length than twigs on trees in
less wind-exposed spots.
Twigs might be stouter because they grew more in
diameter or because they elongated less. The latter is
the case with D. pittieri; twigs are thicker on ridgecrest
trees, but that is apparently because they start out
thicker, since they do not thicken more rapidly dur-
ing development (Lawton 1982). These mechanically
appropriate growth responses and physiognomic
trends are at least in part phenotypically plastic; twigs
on the windward side of ridgecrest crowns are stouter
than those on the lee sides of the same crowns. De-
creased stem elongation and increased allocation to
trunk thickening will both retard height growth, thus
contributing to the ontogeny of elfin stature. Slower
growth in height also slows exposure to the higher
winds farther above the ground and thus reduces the
increase in drag upon the crown.


Natural disturbance in Brillante. Trees fall, particu-
larly in "nortes" or "temporales," winter storms pro-
duced by the impact of polar air masses moving down
the Great Plains of North America into the trade wind
flows of the Gulf of Mexico and Caribbean (see
Chap. 2, Physical Environment). During temporales,
sustained winds across elfin forest canopies in Bril-
lante may exceed 100 km/hr^1. Gusts are even stron-
ger; trees creak and groan. The ground, laced and
matted with surface roots, rocks and heaves; the roar
of the wind is punctuated with the pop of breaking
wood and the crash of falling trees. In most years, 5
10 such storms occur. What impacts does this have
on the forest?
In 1982, all 72 existing recent treefall gaps on the
windward slope of the 12-ha study area were mapped
and measured. We used a modified version of Bro-
kaw's (1982) definition of a gap: an area opened by
fallen trees or fallen limbs, or under standing dead
trees, where the tallest living plants are less than 3 m
tall and less than 50% of the height of the surround-
ing canopy trees. This clearly delimits canopy disrup-
tion, even in the elfin forest thickets. In 1983, the
sample was extended to the rest of the watershed, and
the area sampled in 1982 was remeasured. In each
subsequent year until 1996, the entire watershed

study area has been searched for new gaps (Fig. 9.4).
Monthly censuses from June 1986 to June 1987 re-
vealed that 88% of the gaps were opened by treefalls
during the windy season temporales (December 1986
and January 1987). Although these storms are locally
of hurricane force, catastrophic damage such as that
associated with the impact of major hurricanes on the
Antilles (Lugo et al. 1983, Brokaw and Walker 1991,
Foster and Boose 1995) has not been sustained in the
Cordillera de Tilaran.
Demographic analyses of Didymopanax pittieri, a
facultative hemiepiphyte, suggest that the elfin for-
est is in quasi equilibrium and not the product of past
catastrophes (Lawton 1980). A subpopulation of this
elfin forest pioneer occupies the lee boundary ridge
of the Brillante study area. Each of the 503 D. pittieri
stems taller than 50 cm and growing within 3 m of
the ground within this subpopulation was mapped,
tagged, and measured. Remeasurement a year later
revealed size-specific growth and mortality rates.
Because establishment of vigorous D. pittieri saplings
occurs only in treefall gaps or similar canopy open-
ings, the number of saplings entering the study popu-
lation was calculated from the average number of
saplings per treefall gap and the number of new gaps
in the census year. If past catastrophic clearing had
occurred, D. pittieri populations would be dominated
by individuals thatons/had become established in
their aftermath and grown up together, with a lack of
subsequent sapling establishment. The population
should exhibit a demographically unstable size (or
age) class distribution.
There are, however, abundant saplings and juve-
niles in the existing population, which suggests no
failure of current regeneration. Population projections
using a matrix model with the between-size class tran-
sition probabilities measured in the census interval
yielded a calculated stable size class distribution not
significantly different from that observed. Because
D. pittieri, the dominant pioneer, appears demo-
graphically stable, the forest must be in quasi equi-
librium; the elfin forest is a stable feature of the veg-
etation of the Cordillera.
Although treefalls are balanced by regrowth to
maintain quasi equilibrium at the scale of small
watersheds, the rate of disturbance is both high and
variable. In the period between 1981 and 1990, be-
tween 13 and 93 gaps formed each year on the 10.6
ha of the Brillante study area (geometric mean = 33;
95% confidence interval [CI] = 23-48 gaps). These
opened between 0.6% and 3.8% of the monitored
area each year, a sixfold variation. The turnover time
(calculated from the geometric mean of the area
opened per year) was 77 years (95% CI = 51-116
years).

308 Ecosystem Ecology and Forest Dynamics
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