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LC-14 Abstract

Effects of Rainfall Exclusion on an Amazon Forest

Paulo Moutinho — IPAM - Instituto de Pesquisa Ambiental da Amazônia (SA-PI)
Daniel Nepstad — Woods Hole Research Center (US-PI)

Objectives





Moist

tropical forests in Amazonia, Southeast Asia and Mexico are exposed to

increasingly severe and frequent droughts associated with El Niño episodes. The

consequences of these droughts include potentially large effects on forest

metabolism, reproductive biology, tree mortality, biogeochemistry, and forest

flammability. Forests growing on deeply-weathered clay soils are buffered

against the effects of mild droughts by the large amount of water stored in the

rooting profile. Severe droughts deplete deep soil moisture reserves, however,

causing tree water stress that may inhibit photosynthesis, provoke reproductive

failure, and stimulate root growth and leaf turnover at the cost of stem growth.

The drought-stressed forest may therefore be less productive, have lower rates

of evapotranspiration, and, yet, have higher soil nutrient concentrations and

trace gas fluxes resulting from drought-induced root mortality. Drought-provoked

shedding of leaves, and reductions in populations of litter decomposer

organisms, may increase forest susceptibility to fire. With continued drought,

the structural integrity of the forest deteriorates as water-demanding,

sun-exposed trees, and obligate evergreen trees, succumb to the effects of acute

drought stress and die.





We

have begun to test these and other hypotheses through a rainfall exclusion

experiment initiated in eastern Amazonia, Brazil. Approximately half of annual

rainfall is excluded from a one-hectare, trenched plot using a system of 6100

plastic panels and gutters in the forest understory. A similar control plot is

trenched, but receives natural rainfall. The rainfall exclusion treatment began

in February of 2000, following a one-year pre-treatment intercalibration period.

We measure canopy photosynthesis, leaf water potential, and phenology from

wooden canopy towers and catwalks. Tree growth, tree mortality, sapflow,

litterfall, LAI, forest floor decomposition, soil respiration, trace gas

emissions, water and nutrient fluxes, and forest floor flammability are measured

in the forest understory. Volumetric soil water and fine root dynamics are

monitored in 12-m-deep soil shafts. The remote sensing of forest drought stress

will be analyzed through comparisons of leaf chemistry and leaf spectral

properties with Hyperion high-resolution satellite imagery. Soil water movement

is studied using deuterium pulses. Models of water flux, ecosystem productivity,

biogeochemical processes, and forest fire prediction will be parameterized for

Amazonia through this experiment. This project will allow us to conduct an

integrated assessment of the effect of a three-year, severe drought on the

structure and function of an Amazon forest, increasing our ability to predict

the ecological consequences of increasingly frequent El Niño episodes.





 






Project Goals & Hypotheses:





We

will test the following predictions about the initial (first 1-2 years) effects

of rainfall exclusion upon an Amazon forest:









H1:

a decrease in photosynthetic capacity, leaf water potential, sapflow, specific

leaf  

area, leaf chlorophyll, leaf N, P and lignin;

 



H2:

increased leaf shedding and replacement; lower LAI





H3:

a decrease in rates of stemwood and root growth and soil respiration, but an

increase in litterfall;





H4:

a decrease in total nitrogen oxide emissions, and an increase in the ratio of

NO:N2O emissions;





H5:

a decrease in litter fauna and decomposition rates; and





H6:

higher flammability because of greater forest floor mass and drier forest

microclimate





We

predict the following responses to the long-term (yrs 2-4) effects of rainfall

exclusion, when plant-available water is depleted:


H7:

increased reproductive failure through fruit abortion;





H8:

the forest becomes open and patchy as shade-intolerant, fast-growing canopy

trees, and shade-tolerant, slow-growing understory trees die; slow-growing,

shade-tolerant canopy species will best withstand severe drought;





H9:

further decline in NPP, as the forest shifts to drought-tolerant foliage, and

as trees die;





H10:

fine root production continues at low rates, but the decomposition of roots

killed by drought increases soil respiration;





H11:

net N mineralization, net nitrification, N oxide emissions, and solutions of

nutrients increase as fine root biomass with low C: nutrient ratios

decomposes;





H12:

a build-up of coarse woody debris on the forest floor, increasing the

flammability of the forest;





H13:

at very advanced stages when mortality causes large inputs of woody tissues

with high C: nutrient ratios, the flow of nutrients into the mineral soil

surface will decline as nutrients are immobilized in an accumulating forest

floor that is both impoverished in decomposer fauna and contains high lignin

content, and as fungi translocate nutrients into decomposing woody debris; a

decrease in N oxide emissions will accompany net N immobilization.










Field

Site: 









Floresta

Nacional do Tapajós is the field site of this project.  A comparative, pilot study is also being conducted in the IBGE Reserve of

Brasília, in cerrado savanna vegetation.



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