Monteverde : Ecology and Conservation of a Tropical Cloud Forest

local dominance by species that recover quickly from
injury (Knight 1975, Putz and Brokaw 1989). As pre-
dicted, the common canopy species in Monteverde
were those that sprout readily (Matelson et al. 1995).
In mature forests, gap-colonizing taxa have a higher
incidence of mortality than their proportion in the
forest. Pioneer species that grow fast and have weak
wood might be prone to breaking and therefore have
a propensity for resprouting (Putz and Brokaw 1989).
More long-term data are needed to record such phe-
nomena as standing broken stem regeneration and
attrition of dead standing stems, which operate on a
time scale of several years to several decades.

9.2. Forest Biogeochemistry
and Nutrient Cycling

In all ecosystems, materials circulate in characteris-
tic paths from the environment through living organ-
isms and back to the environment in biogeochemical
cycles. The movement of elements that are essential
to life is designated nutrient cycling. "Nutrient" ap-
plies to any substance taken into an organism that is
metabolized or becomes part of ionic balances (ex-
cluding toxins and substances used only as behavioral
signals). Most often, the emphasis in biogeochemical
studies is on inorganic elements and ions that are
present in water and soil and may be taken up to be-
come part of community function. The processes of
transfer and concentration of materials have increas-
ingly urgent significance to humans.
In contrast to the storage and movement of energy,
which cannot be recycled, chemical nutrients, the
building blocks of biomass, can change the form of
the molecule of which they are a part (e.g., organically
bound nitrogen can shift to ammonium nitrate). They
can be converted and recycled among different com-
ponents of the forest, and the process of locking some
up in living biomass reduces the supply remaining to
the community. The categories of supply and loss of
nutrients in terrestrial ecosystems are similar for all

forest ecosystems (Table 9.5), but the magnitude of

different components varies among forest types.

Some early studies of nutrient cycling in tropical

forests described productive forests rich in nutrients,

but others described tropical soils as acid, infertile

clays. In fact, patterns of nutrient cycling in tropical

forests are diverse (Vitousek and Sanford 1986). Mon-

tane tropical forests in general appear to be lower in

nitrogen and phosphorus than fertile lowland forests,

even though the former are located on what would be

classified as fertile soils (Grubb and Edwards 1982).

Decomposition rates in montane forests are lower than

in lowland forests (Proctor 1983), probably due to

lower temperatures.

Typically, the first step in building an ecosystem

nutrient budget is to estimate the amount of biomass

and nutrient capital in the standing stocks of vegeta-

tion. Ideally, subsamples of each component of inter-

est (e.g., foliage, stems, fine roots) are taken in a man-

ner and at a sampling intensity that encompasses the

natural variability of the material. Subsamples of each

component are then analyzed for nutrient content,

and this amount is used to extrapolate the total

amount of nutrients from the estimates of biomass of

that component.

We carried out a study of ecosystem nutrient cycling

from 1987 to 1997 in the leeward cloud forest study

area. Our objective was to quantify and characterize

epiphyte communities and their accompanying organic

matter and nutrients in an ecosystem context. Canopy

components form a forest subsystem thatons/has re-

ceived increasing attention from ecosystem ecologists

(Coxson and Nadkarni 1995). Drawing on botanists, soil

scientists, a microbial ecologist, an atmospheric chem-

ist, and an ecological modeller, we generated estimates

of the major pools and fluxes of terrestrially rooted and

canopy components in this forest.

Epiphytic organic matter (EM) is composed of roots

and shoots of vascular and nonvascular plants, ab-

scised leaves and stems of host trees and epiphytes

thatons/have been intercepted by branches, inverte-

brates, fungi, and microorganisms associated with

320 Ecosystem Ecology and Forest Dynamics

Table 9.5. Major routes of import, export, and circulation

Import

Precipitation (rain, mist)

Participate fallout

Biotic immigration

Fixation from the atmosphere

(e.g., nitrogen fixation)

Weathering of substrate

Fertilizer application

Pollution deposition

Export

Runoff and stream outflow

Particulate loss by wind

Biotic emigration

Release to the atmosphere

(e.g,. denitrification)

Loss by leaching

Human harvest

of nutrients in terrestrial communities.

Circulation

Litterfall

Crownwash (throughfall

Decomposition

Plant uptake

Defecation of animals

Plant retranslocation

and stemflow)