We measured fine litter deposition and decompo-
sition within the upper tree canopy and on the forest
floor with standard litterfall collectors and litterbags
(Nadkarni and Matelson 1991). A comparable amount
of fine litter passed through the canopy (752g/m^2 /yr)
as arrived on the forest floor (820 g/m^2 /yr), but less
than 1% of the biomass and nutrients of this gross
litterfall was retained within the canopy. Canopy
standing litter (ca. 170 g/m^2 branch surface area, 8.8
g/m^2 ground area) is equivalent to only 1% of stand-
ing crop of litter on the forest floor. Measurements of
marked leaves documented that 70% of leaves depos-
ited on branches are lost within a 2-week period, and
nearly all are gone in 16 weeks. Branch characteris-
tics such as branch angle and number of epiphyte
stems and clumps appear to affect the amount of lit-
ter retained at particular microsites. Leaf litter decom-
position of tethered leaves within the canopy over a
12-month period was half that of leaves on the forest
floor (canopy turnover time = 2.8 yr; Nadkarni and
Matelson 1991).
Assuming that litter accumulation within the
canopy is at steady state, the biomass of fine litter
retained and decomposed within the canopy was cal-
culated as only 2.0 g/m^2 /yr and less than 0.02 g/m^2 /
yr for all nutrients (Fig. 9.16). Nutrient replenishment
of epiphyte communities appears to be decoupled
from the litterfall pathway, as input from litterfall
retained within the canopy is small relative to epi-
phyte productivity and nutrient requirements re-
ported in other studies.
In most terrestrial systems, the major portion of the
nutrients required for net primary production is re-
plenished via the litterfall pathway, especially for
nitrogen, phosphorus, and calcium. Within mature
tree crowns in tropical cloud forests, where vascular
plants, bryophytes, root mats, and humus occur, nutri-
ents are bound in organic matter as they are in the
forest floor below. However, the amount of inter-
cepted tree litterfall on inner branches is extremely
small (<1% in this study) relative to gross litterfall.
The reasons for low litter interception and high litter
attrition include disturbances such as wind, falling
branches and rain and animal activities such as mon-
keys and rodents moving along branches and birds
foraging in leaf litter. These disturbances cause sub-
stantial leaf litter displacement before the nutrients
contained in the intercepted litter can be mineralized
and thus made available for epiphyte uptake. The few
leaves that do remain in the canopy decompose very
slowly, which may at least be due in part to dry envi-
ronmental conditions and low densities of canopy
macroinvertebrates (Nadkarni and Longino 1990).
Because the replenishment of nutrients to many
cloud forest epiphytes appears to be only partially
derived from litterfall decomposition, the balance
must be derived from the other sources outlined in
Table 9.24. Two sources, foliar leachate (autochtho-
nous) and atmospheric deposition (allochthonous),
seem the most likely candidates for most of the bal-
ance. Nutrients derived from foliar leachates (espe-
cially mobile nutrients K+ and NO 3 ~) may be impor-
tant, as throughfall concentrations collected at the
forest floor are occasionally lower than in bulk pre-
cipitation (Clark 1994), indicating that at least some
of the canopy components are "scavenging" nutrients
from bulk precipitation. The majority of host tree fo-
liage, however, is sclerophyllous, with waxy cuticles,
and many cloud forest trees retranslocate high pro-
portions of nitrogen and phosphorus, which presum-
ably minimizes water and nutrient transfer through
foliage (Grubb 1977, Tanner 1977).
Atmospheric deposition is an allochthonous source
that is likely to contribute nutrients to all epiphytes.
On outer branches, poikilohydric epiphytes such as
bryophytes and filmy ferns capture atmospheric nu-
trients and incorporate them into their biomass. When
they die, their detritus contributes to the development
of humus buildup, which no doubt increases nutri-
ent retention. The epiphytes that occupy inner branch
areas and lack morphological adaptations for direct
atmospheric uptake must acquire nutrients by root
uptake from nutrients sequestered in the accumulated
mats. These plants may obtain at least some of their
nutrients by physically intercepting precipitation (es-
pecially wind-blown mist) with their shoots and chan-
neling it to the humus mats that are permeated with
their root systems. Understanding the ultimate nutri-
ent sources of canopy solutions and sinks of nutri-
ents by quantifying the uptake and release of specific
canopy components is needed to differentiate these
two sources.342 Ecosystem Ecology and Forest Dynamics