Background:
Tropical montane cloud-forests occur at high elevation in equatorial and
subtropical regions, where conditions are persistently moist and cooler
than in nearby lowland forests (Vance and Nadkarni, 1990; Hartschorn,
1983). Precipitation is generally heavier, fog interception provides an
added input between events and consistent high humidity levels prevent
evaporation (Cavalier et al., 1997; Bruijnzeel and Proctor, 1995).
These wet conditions support a great deal of epiphytic vegetation on the
trunks, branches and even the leaves of TMCF trees.
TMCF trees tend to be shorter in stature than their lowland counterparts, with a higher density of small tree stems and, curiously, "xeromorphic" leaves (Bruijnzeel and Veneklaas, 1998). Several factors have been suggested to account for the short stature of these forests, but no dominant control is known. Photosynthesis is reduced relative to the lowland forests because persistent cloudiness reduces solar radiation input and frequent precipitation inhibits gas transport due to leaf wetness (eg., Grubb and Whitmore, 1966). Exposure to winds on steep slopes could reduce tree height through increased treefall frequency, but winds are generally light in equatorial regions (Proctor et al., 1988; Hafkensheidt, 1994). Landslides could also play a role in this regard, though these effects would be local in nature. Dry season water shortages in shallow or stony soils are unlikely to be a significant factor, since TMCF trees rarely experience severe soil-water deficits (Kapos and Tanner, 1985). On the other hand, consistently near-saturated soils could inhibit root respiration (Bruijnzeel and Veneklaas, 1998). Leaf and soil nutrient concentrations generally decrease with altitude, which might require more sequestered carbon to be allocated to rooting systems. Ultraviolet radiation can be particularly intense during clear periods. This could reduce tree height by requiring an investment of carbon into xeromorphic leaves. Finally, epiphytes thrive in this environment, occupying the trunks, branches and leaves of most terrestrially-rooted plants.
A critical need exists for an understanding of the interactions between the various factors controlling productivity in TMCFs (Wade et al., 1988). In this study, the single and multiple factor controls on productivity will be examined through the collection of extensive and varied field measurements, modelling with GIS software such as PCRaster, and laboratory studies of TMCF plant growth under controlled conditions.
Field measurements:
The field measurements required in this study can be broken down into
three types. Firstly, continuous meteorological and hydrological
measurements are taken at three stations in the Tambito and Palo Verde
catchments. With continuous point measurements in primary, secondary
and cleared sites at different altitudes, weather conditions can be
modelled throughout the catchment (see contact below for further
information). Factors measured at these sites include temperature,
humidity, solar radiation, ultraviolet radiation,
photosynthetically-active radiation (PAR), rainfall, soil moisture,
throughfall, fog interception, and leaf wetness.
Secondly, carbon uptake and gas transport are measured using infrared gas-analysers and porometers. Simultaneous readings of temperature and photosynthetically-active radiation give an indication of how C uptake varies with the environmental conditions encountered throughout the catchment. These measurements are taken in both primary and secondary forest at different altitudes and on specific plant functional types. Periodically, stem length, stem girth, leaf size and density are determined to estimate the carbon uptake rates and to help determine how sequestered carbon is allocated in the plant.
These data are supplemented by measurements of litterfall (to estimate productivity), river depth, throughfall, stemflow, overland flow and throughflow (to determine the hydological balance), leaf drying rates (to determine limitations to gas transport), root, trunk, branch and leaf biomass (for the carbon allocation model), erosion, soil type and possibly, nutrient content.
These three types of data will yield important information on plant structure and function, permit modelling of photosynthesis across the catchment and provide critical information regarding carbon allocation.
Laboratory study:
The laboratory component of this project will involve the use of a
'cloud chamber.' This is an enclosed area in which temperature, light,
humidity and soil conditions can be controlled to examine the effect of
these variables on TMCF plants. Small, easily obtainable plants of
different functional groups will be grown within the cloud chamber under
specific conditions. Carbon uptake and gas transport will be measured
under prescribed conditions using the infrared gas-analyser and
porometer. The meteorological variables will be changed individually
and then together, to examine both the individual and interactive
controls on photosynthetic rates.
Expected results:
Upon completion, a three-dimentional PCRaster model will simulate
photosynthetic rates in the Tambito and Palo Verde catchments at daily
time-steps. The project will lead to a greater understanding of the
controls on plant structure and function in the tropical montane
cloud-forest environment. Finally, a C allocation model will examine
the distibution of sequestered carbon within TMCF plants of differing
functional types.