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Influence of Forest Fragmentation on Watershed Functions in Northern Vietnam
Forest Fragment Hydrology and Ecology Objective
 

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Project Information

 

 

 

 

 

 

 

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Forest Fragment Hydrology and Ecology Objective

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NSF Award: DEB-9613613

National Science Foundation--Environmental Protection Agency
Water and Watersheds Project

Start Date: January 1, 1997
Expires: December 31, 2000
Expected Total Amt.: $418,749


Investigators:

A. Terry Rambo and Jefferson M. Fox
East-West Center
Honolulu, Hawai`i
USA

Thomas Giambelluca
Department of Geography
University of Hawai`i at Manoa
Honolulu, Hawai`i 96822
USA

Collaborator:

Dr. Le Trong Cuc
Center for Natural Resources and Environmental Studies (CRES)
Vietnam National University
Hanoi
Vietnam

Abstract

Influence of forest fragmentation on watershed functions in Northern Vietnam Forest patches are rarely taken into consideration in rural development programs. This project aims to assess the management of forest fragments, as opposed to monolithic forest cover, as a valid new paradigm for sound watershed conservation. The investigators will test linked hypotheses relating to the consequences of forest fragmentation on watershed functions and socioeconomic forces and technological conditions that foster forest fragmentation. The project, to be undertaken in northern Vietnam, is expected to provide basic data for evaluating environmental changes and management actions, and develop some of the indicators and basic understanding necessary to manage ecosystems. Ecosystem experiments are the most direct method available for improving predictions of environmental response to management. This information is also urgently needed for the design of policies and programs that balance the need for watershed protection, forest conservation, and landscape restoration with the demands for socioeconomic development.

Study Objectives

  1. Building a spatial information database
  2. Understanding the socioeconomic forces driving and maintaining forest fragmentation
  3. Understanding forest fragment hydrology and ecology

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Forest Fragment Hydrology and Ecology

Research

We have conducted intensive field studies of the influences of landscape fragmentation on hydrologic processes. In particular, we studied effects of vegetation patchiness on regional evaporation and storm runoff. To study forest edge effect on evaporation, we measured meteorological, soil, and ecophysiological variables in and near a 12-ha forest patch in Ban Tat Hamlet, Hoa Binh Province, northern Vietnam. Preliminary results indicate transpiration is significantly higher for trees near the edge of the patch than those in the interior. This effect is more pronounced during the wet season. This result implies that reduction of regional evaporation due to deforestation is mitigated to some extent by enhanced evaporation near forest edges. For a given fractional forest cover, regional evaporation in highly fragmented landscapes is not reduced as much as in areas where cleared and forested patches are relatively large. Regarding storm runoff, we are investigating whether patches of vegetation on hillslopes can significantly reduce the delivery of storm runoff to streams. In general, clearing of forest reduces soil infiltration rates which can lead to overland flow during intense rainfall. This type of runoff is considered to have negative impacts due to associated flooding, soil erosion, and sedimentation of water resources. We believe landscape fragmentation may partially offset the generally negative effects of deforestation on overland flow by providing buffers zones were water flowing down slope can infiltrate the soil.

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Sapflow & Meteorological Data Analysis

Problem

It is well known that partitioning of incident solar radiation depends on the type of land cover. For example, net radiation, soil heat flux, sensible heat flux to the atmosphere, and latent heat flux are significantly affected by clearing forest. Estimates of energy and water fluxes for regions with heterogeneous land cover can take account of the relative proportions of the dominant land cover types relatively easily by computing area-weighted averages of the fluxes over each land cover type. However, patch-scale fluxes are not independent of the surroundings. Horizontal transfer of energy and water vapor in the atmosphere may significantly alter the fluxes within a patch and hence invalidate a strictly one-dimensional approach to estimating regional average fluxes. Such effects are greatest at the boundaries of dissimilar land covers. Near the upwind margin of a forest patch, processes are influenced by the advection of sensible energy generated in the clearing. Air entering the forest edge is relatively warm and dry, thus increasing evaporation potential. This edge effect diminishes with distance toward the patch interior, but remains significant for several tens of meters. In a highly fragmented landscape, edge effects, if not recognized, could contribute considerable uncertainty to regional energy and water flux estimates.

Objectives

1. Evaluate the spatial patterns of transpiration within a forest patch.

  1. determine the effect of proximity to upwind edge on transpiration of individual trees
  2. determine the dependence of transpiration on other location-dependent characteristics, e.g. crown exposure and degree of vine infestation.
  3. determine the dependence of transpiration on individual characteristics other than location, e.g. species, stem size, crown size, height, and leaf area
  4. evaluate areal average transpiration for near-edge and interior portions study patch

 

2. Evaluate changes in spatial patterns of transpiration with a forest patch as influenced by changes in:

  1. soil moisture
  2. foliage wetness
  3. time of day
  4. wind direction

 

3. Evaluate the use of the Klaassen (1992) model to simulate the spatial pattern of transpiration within a forest patch under different conditions

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Field Observations

Ban Tat Forest Patch Experiment

Ban Tat Hamlet, Tan Minh Village, Hoa Binh, Vietnam

Measurements taken June-July 1997 and March-June 1998

Contact: T. Giambelluca: thomas@hawaii.edu

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Four-station transect of meteorological stations

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Meteorological Observations:

Time series of Kd, Rnet, Tair, RH, and U
Wind roses
Soil moisture

301 Located in actively used swidden field located near western edge of forest patch

Observations: Rnet, Kd, Ku, Tir, G, Tsoil, Tair, RH, U, WD, SM0-30, SM50-80, SM120-150, RF
Interval: 10 min, except RF: 1 min
Period 1: June 29 to July 12, 1997
Period 2: March 24 to June 20, 1998

302 Located in forest near western edge of forest patch

Observations: Tir, Tair, RH, U, WD, SM0-30, SM50-80, SM120-150
Interval: 10 min
Period 1: July 3 to July 12, 1997
Period 2: March 25 to June 20, 1998
303 Located in interior of forest patch
Observations: Rnet, Ku, Tir, G, Tsoil, Tair, RH, U, WD, SM0-30, SM50-80, SM120-150
Interval: 10 min
Period 1: June 30 to July 12, 1997
Period 2: March 27 to June 20, 1998

304 Located in secondary vegetation near northeast edge of forest patch

Observations: Tir, Tair, RH, U, WD, SM0-30, SM50-80, SM120-150
Interval: 10 min
Period 1: July 2 to July 12, 1997
Period 2: March 28 to June 20, 1998
Two sapflow stations
Note: Rnet = net radiation, Kd = downward shortwave radiation, Ku= upward shortwave radiation, Tir = surface (canopy) temperature using infrared thermometer, G = soil heat conduction, Tsoil = soil temperature, Tair = air temperature, RH = relative humidity, U = wind speed, WD, SM0-30 = soil moisture (TDR) at 0 to 30 cm depth, SM50-80 = soil moisture (TDR) at 50-80 cm depth, SM120-150 = soil moisture (TDR) at 120 to 1500 cm depth, RF = rainfall

Sapflow Measurements

305 Located near western edge of forest patch; coincides with meteorological station 302

Observations: thermocouple measurements using Granier-type sapflow probes; temperature difference between heated and reference probes used to estimate sap velocity and transpiration
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Period 1: July 3 to 12, 1997
Sample: 12 individual trees, 2 probes each tree
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Period 2a: March 31 to April 24, 1998
Sample: 9 individual trees (3 replicates of 3 species), 2 probes each tree
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Period 2b: April 24 to June 5, 1998
Sample: 3 individual tress (1 replicate each of 3 species), 1 probe each tree
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Period 2c: June 5 to 19, 1998
Sample: 9 individual trees (3 replicates of 3 species), 2 probes each tree

306 Located in the interior of the forest patch; coincides with meteorological station 303

Observations: thermocouple measurements using Granier-type sapflow probes; temperature difference between heated and reference probes used to estimate sap velocity and transpiration
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Period 1: July 3 to 12, 1997
Sample: 5 individual trees, 2 probes each tree
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Period 2a: April 1 to 24, 1998
Sample: 9 individual trees (3 replicates of 3 species), 2 probes each tree
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Period 2b: April 24 to June 5, 1998
Sample: 3 individual tress (1 replicate each of 3 species), 1 probe each tree
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Period 2c: June 5 to 19, 1998
Sample: 9 individual trees (3 replicates of 3 species), 2 probes each tree

Transpiration Patterns

Publications and Presentations

Giambelluca, T.W., Ziegler, A.D., Nullet, M.A., Dao, T.M., and Tran, L.T. In review. Transpiration in a small tropical forest patch.

Ziegler, A.D., T.W. Giambelluca, D. Plondke. T.T. Vana, J. Fox, T.D. Vien. M.A. Nullet., S. Evett. In review. Near-surface hydrologic response in a fragmented landscape in northern Vietnam. [draft]

Giambelluca, T.W., Ziegler, A.D., Nullet, M.A., and Dao, T. 2001. Spatial Variation in Transpiration within a Small Forest Patch in Hoa Binh, Northern Vietnam. American Geophysical Union Fall Meeting, San Francisco, December, 2001.

Ziegler, A.D. and Giambelluca, T.W. 2001. Alteration of hydrological flow paths by fragmentation: Tat Hamlet, Da River Watershed, northern Vietnam. Symposium IV, Rio de Janiero, September 2001.

Giambelluca, T.W., Ziegler, A.D., Tran, L.T., Sutherland, R.A., and Nullet, M.A. 1999. How changing land-use is modifying hydrologic processes in mountainous areas of Southeast Asia, Environmental services and land use change: Bridging the gap between policy and research in Southeast Asia, Chiang Mai, Thailand, June 1999. (Invited).

Giambelluca, T.W. 1999. Influences of landscape fragmentation on hydrological processes, Hoa Binh, Vietnam, 95th Annual Meeting of the Association of American Geographers, Honolulu, March 1999. [Abstract]

Giambelluca, T.W., Modeling land-atmosphere interaction over tropical forests: lessons from the field, PILPS Strategy Forum, Honolulu, February 1999 (Invited).

Giambelluca, T.W. Reality check: Observing processes at small scales, First SEA/BASINS Partner Workshop, Chiang Rai, Thailand, November 1998 (Invited).

Giambelluca, T.W. Influence of forest fragmentation on watershed functions in northern Vietnam--Hydrological objective: Quantifying the influences of patch size on watershed hydrology, 1998 Water and Watersheds Program Review, EPA/NSF Partnership for Environmental Research, Corvallis, Oregon, January 1998 (Invited).

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T. Giambelluca
November 2001