to the canopy microhabitat (Longino and Nadkarni
1990).
This arboreal soil is often permeated by roots and
absorptive structures of vascular and nonvascular epi-
phytes and, in some cases, by the adventitious "can-
opy roots" of host trees (Nadkarni 1981). It functions
as a storage pool for organically bound nutrients and
as a "nutrient capaciter" for inorganic nutrients from
incoming atmospheric sources and from materials
mineralized in situ (Nadkarni 1984). This arboreal
substrate has never been classified in any standard soil
taxonomy system, and its characteristics are poorly
understood (Putz and Holbrook 1989, Vance and Nad-
karni 1990; Table 9.2; Clark and Nadkarni, "Micro-
climate Vanability," pp. 33-34). As does the forest
floor H horizon, epiphytic organic matter hosts an ac-
tive microbial community, although rates of nitrifica-
tion are suppressed in the canopy relative to that on
the forest floor (Vance and Nadkarni 1990; see Vance
and Nadkarni, "Microbial Biomass and Activity," p.
336). The invertebrate community within the arboreal
soil is similar in composition to the terrestrial com-
munity at the taxonomic level of order, but overall
density of invertebrates is much lower in the canopy,
which may lead to differences in processing of nu-
trients (Nadkarni and Longino 1990; see Nadkarni
and Longino, "Invertebrates," pp. 336-337).
Particularly important to the distribution of soil-
based organisms are soil moisture and temperature.
In cloud forests, where cloud cover and wind vary
seasonally, differences in the soil moisture and tem-
perature regimes on very small spatial scales may be
pronounced. Canopy soils may reflect the higher air
temperature (Chacon and Duval 1963), greater wind
speed (Lawton 1982), and lower humidity (Longman
and Jenik 1987) of the canopy relative to the forest
floor.
We measured soil temperature and moisture on the
forest floor and in the canopy at the leeward cloud
forest site for a 42-month period (Bohlman et al.
1995). Temperatures of the canopy material and for-
est floor soil fluctuated throughout the year (range
11.5-21.0°C), but remained within an average of 1°C
of each other for each measurement interval. Both
canopy material and forest floor soils were moist
throughout the wet and misty seasons (over 70%
water content). However, during the dry season,
canopy humus experienced periods of rapid and se-
vere dehydration (20-40% water content), whereas
forest floor soils remained at a consistently high water
content (60-70%; Fig. 9.9). The more extreme and
fluctuating moisture conditions of canopy organic
material may affect the distribution and activity of
epiphytic plants and associated canopy organisms.Forest structure. Stems in the 4-ha study area were
categorized as large (>30 cm DBH), medium (10-30
cm DBH), or small (2-10 cm DBH) and identified to
species by W. Haber and E. Bello (Nadkarni et al. 1995).
Based on a sample of 1850 stems, overall density of
live stems in all size classes was 2062 individuals per
hectare (large, medium, and small stems were 159,
396, and 1507, respectively). Mean tree DBH was 65.5
cm; canopy height was 18-25 m. A frequency dis-
tribution of size classes showed a reversed J-shape,
which is typical for a mature stand, with many small
stems and a few large ones. Over half of the total basal
area was represented by trees >50 cm DBH. The larg-
est trees (>90 cm DBH), which constituted only 1%
of the stems, accounted for 30% of the total basal area
(Nadkarni et al. 1995).Forest composition. Stems represented 47 families,
83 genera, and 114 species and varied among size
classes. The distribution of stems among taxa was
uneven; more than half of the individuals belonged
to members of the three most common families. The
stems of 26 plant families accounted for less than 1%Table 9.2. Comparison of epiphytic and forest floor
organic matter.Location
Epiphytic organic matter
Forest floor
0-10 cm below
10-20cm below%C
37.427.0
25.3%N
2.361.44
0.98C:N Ratio
17.918.7
25.8pH
3.74.6
5.4
Data are mean percentage C and N, C:N ratios, and pH of epiphytic organic
matter and organic matter taken from 0-10 cm below the forest floor and
10-20 cm below the forest floor at the study area in Monteverde, Costa Rica.
Samples were taken in May 1987. The epiphytic samples are composites of
three to five branches per tree from 13 trees located 1-3 m from the central
trunk and 16-23 m above the forest floor. The forest floor samples are from
20 samples taken at random locations and bulked into six composites for
each horizon (Vance and Nadkarni 1990).315 Ecosystem Ecology and Forest Dynamics