Monteverde : Ecology and Conservation of a Tropical Cloud Forest

MI.CROBIAL BIOMASS AND ACTIVIW IN CANOPY ORGANIC

MATTER AND THE FOREST FLOOR

Eric Vance & Nalini M Nadkarni

he environment of tropical cloud forests re-
sults in large accumulations of organic mat
ter and nutrients on the forest floor relative to
most lowland tropical forests due to slower rates of
decomposition (Grubb 1977). Although soils under
montane forests may have a high nutrient capital, the
availability of nutrients, especially nitrogen, is often
low and limiting to forest productivity (Vitousek and
Sanford 1986). Another characteristic of these cloud
forests is their high biomass of epiphytes (Nadkarni
1984). As epiphytes die and decompose, they form
mats of organic matter on tree branches that are per
meated by epiphyte and host tree roots (Jenik 1973,
Nadkarni 1981). This organic matter may be an im-
portant component of the nutrient cycling character-
istics of this forest by trapping and transforming nu-
trients that are deposited in rain, mist, and intercepted
litter. To understand the significance of epiphytic
organic matter (EM), we determined whether EM is
microbially active, and what role microorganisms
might play in conserving and transforming nitrogen
and carbon. Our study took place in the leeward cloud
forest of the MCFP.
Our study revealed that EM has high microbial
activity and that levels of biomass carbon and nitro-
gen were similar to those on the forest floor H (FF-H)
and A! (FF-A) horizons (Table 9.23). Carbon dioxide

evolution in the laboratory (an indicator of overall

microbial activity) and laboratory nitrogen mineral-

ization were also higher or similar for EM relative to

the forest floor. In contrast to the H and Al horizons

of the forest floor, however, nitrification was not de-

tectable or was only very low in EM during labora-

tory and field incubations. The potential for denitri-

fication was higher in the forest floor than in the

canopy (Vance and Nadkarni 1990). Because of their

high biomass and acivity, microorganisms in the

canopy should be considered when studying the regu-

lation of nitrogen availability in this forest type.

Table 9.23. Microbial biomass composition at

various forest levels.

Biomass

Data are microbial biomass concentrations (ug/g soil mean ± SD) in

epiphytic organic matter (EM), forest floor soil, H horizon (FF-H), and

forest floor A^ horizon (FF-A). Values designated by different letters

are significantly different at the P = .05 level (Vance and Nadkarni

1990).

INVERTEBRATES IN CANOPY AND GROUND ORGANIC MATTER

Nalini M. Nadkarni & John L Longino

n important functional aspect of dead organic
matter in forest ecosystems is the composition
and abundance of the detritivore fauna. These
invertebrates participate in the regulation of nutrient
transfer and contain a pool of nutrients and energy.
They are the main agents of litter fragmentation and
mixing of leaf litter with mineral soil, exposing a
greater surface area for microbial colonization (Sea-
stedt 1984). In many tropical cloud forests, consider-
able amounts of intercepted litter and crown humus
accumulate on branches and trunks of mature trees
(Nadkarni 1984). We compared the composition and
abundance of invertebrates that inhabit canopy and

forest floor litter and humus in a leeward cloud for-

est in Monteverde (Nadkarni and Longino 1990).

Field research was conducted in and around the

MCFP. Trees from which samples were taken included

those in the largest size class of the most common tree

species (Clusia alata, Didimopanax pittieri, Quercus

sp., Pouteria viride, and Ficus sp.). We climbed with

single-rope techniques (Perry 1978), which provided

access to the inner crowns of the middle canopy. Lit-

ter invertebrates were sampled by sifting bulk litter and

humus through a mesh sieve. Approximately 17 liters

of this bulk dead organic matter produced one liter of

sifted litter; each sample consisted of 2-8 liters of sifted

336 Ecosystem Ecology and Forest Dynamics

Sample

EM

FF-H

FF-A

Carbon

2650 a± 330

2670 a± 170

1950 a ± 30

Nitrogen

340 a ± 70

320 a± 20

220 a± 20

T

A