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LC-14 Abstract

The Effects of Partial Throughfall Exclusion on Forest Flammability, Productivity, Metabolism and Nutrient Cycling in an Amazon Forest

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

Drought is

changing moist tropical forests in Amazonia and elsewhere. Drought severity has

increased in these regions in association with El Niño events and may increase

further through global warming and land use change. The consequences of this

trend include potentially large effects on greenhouse gas emissions, tree

mortality, reproductive biology, biogeochemistry, and forest flammability.

During the 1998 El Niño episode, for example, approximately one third of Amazon

forests experienced depleted soil moisture and may have become susceptible to

fire. An integrated understanding of the responses of moist tropical forests to

severe drought has eluded the scientific community in part because intensive

field studies of episodic natural drought “experiments” are extremely

difficult.





In this

research, we continue a large-scale throughfall reduction experiment in an

Amazon forest. Predicted responses to a total of 5 years of

experimentally-induced drought and 2 years of release from drought stress will

be tested.  The study exploits the research infrastructure previously

established with NSF funding, including one-hectare control and treatment study

plots with canopy towers (8), catwalks (200 m), 12-m soil shafts (10) and

perimeter trenches (800 m). The plots were compared during a one-year

calibration period prior to treatment initiation.  Measurements include

stem and root sapflow, photosynthesis, leaf water potential, xylem cavitation,

fine root dynamics, soil water, canopy phenology, stem growth, stem respiration,

nutrient fluxes through the forest canopy, floor and soil, and soil fluxes of

CO2, CH4, N2O and NO. Beginning in 2000, throughfall has been partially excluded

from the treatment plot with 5,660 plastic panels that are installed in the

understory during the rainy season and removed during the dry season.





Forests

growing on deep, clay soils throughout much of Amazonia are buffered against the

effects of moderate droughts by the large amount of water stored in the rooting

profile.  But even mild drought elicits important forest responses. 

After excluding 890 mm of throughfall in 2000 and 680 mm in 2001 from the

one-hectare treatment plot, several unanticipated forest responses were

documented that would have been very difficult to detect without a throughfall

manipulation of this magnitude. 





A reduction in soil methane

emissions relative to the control plot appeared within days of installation of

the exclusion panels. Surprisingly, a reduction in the growth of small tree

stems (<20 cm diameter) and a reduction in sap flow were the next major

treatment responses, appearing during the dry season of the first treatment

year.  This decline in stemwood production of nearly 1 Mg ha-1 yr-1 is

sufficient to offset the net carbon sink that has been reported for Amazon

forests.  The anticipated reduction of leaf area index (LAI) appeared only

during a second phase of drought in which deep soil moisture depletion began,

accompanied by declines in leaf water potential. Most of the LAI reduction was

caused by decreases in new leaf production instead of through increased leaf

shedding. Hence the stomatal closure provoked by mild drought stress was

manifested as slower stemwood growth; leaf production declined only as deep soil

moisture supplies were depleted. Estimates of NPP allocation to belowground

production are awaiting further analyses, but soil respiration measurements

indicate little total change.





During the next three years of

throughfall reduction, a third phase of drought responses is predicted in which

further soil moisture depletion provokes xylem embolism and the death of trees

and tree branches. Tree- and branchfall gaps will open the forest understory to

sunlight, but soil moisture deficits will limit the recovery processes that gap

formation usually triggers. Soil trace gas emissions will respond both to

changes in soil aeration and to changes in substrate availability caused by root

mortality. Movement of solutions through the soil profile will depend upon root

distributions, water content, and hydraulic conductivity.  Forest floor

accumulation combined with drought-induced reduction in leaf area index will

greatly increase forest flammability. After cessation of the throughfall

reduction treatment, persistent xylem damage in surviving large trees may

restrict the supply of water to canopy foliage and suppress photosynthesis for

years, while gaps are quickly filled by saplings and vegetative sprouts.





           

These complex forest responses to drought, and the cessation of drought, will be

tested and modeled. Field measurements will be used to test and refine canopy

metabolism and forest models, improving our ability to simulate the responses of

moist tropical forests to severe drought, and the cessation of drought. This

experiment is the focus of 6 PhD dissertations, 2 MS theses, 5 undergraduate

theses, and numerous field ecology courses, and has been widely disseminated in

the popular media.



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