GEOG 402 Forests and Clearings
Microclimate
DEFORESTATION What difference does it make when forest is cleared? Forests differ from cleared land in two hydrologically-significant ways. Forests promote: High Evapotranspiration (ET) Rapid Infiltration
FORESTS High Evapotranspiration High R net : Low albedo & low daytime T sfc Low r a : High aerodynamic roughness Low r c : High leaf area High RF interception: High leaf area and roughness High soil water capacity: Deep roots
Evapotranspiration Penman-Monteith Equation λe = Δ(R n G) + ρc p r a (e s e) Δ + γ ( r a + r c r a )
Net Radiation R net = ( ) ( 4 ) 1 α K + ε L σtsfc ALBEDO: fraction of sunlight reflected by the surface
Aerodynamic Resistance Brutsaert and Stricker (1979): r a = ln z a z 0 d k 2 ln u z a z d 0h ROUGHNESS LENGTH: a function of the vegetation height and roughness
Canopy Resistance r c = r s LAI Mean stomatal resistance Leaf Area Index
Global Deforestation Hotspots Central and South America Southeast Asia Congo Basin In these regions, cleared land is often abandoned after a few years. Secondary vegetation becomes the dominant land cover in many deforested areas.
Secondary Vegetation Broad land-cover category with characteristics that change rapidly with time: Albedo initially high Rnet initially low ET initially low Ks initially low
Albedo of Secondary Veg Albedo 0.19 0.18 0.17 0.16 0.15 0.14 0.13 0.12 0.11 0.10 Based on field measurements in eastern Amazon Basin and northern Thailand Typical forest albedo Albedo 0 5 10 15 20 25 30 Years since abandonment
R net of Secondary Veg Rnet/Kd 0.75 0.70 0.65 0.60 0.55 0.50 0.45 0.40 Amazonian rainforest value Radiation Efficiency Based on field measurements in eastern Amazon Basin and northern Thailand 0 5 10 15 20 25 30 Years since abandonment
ET of Secondary Veg λe/rnet 1.40 1.30 1.20 1.10 1.00 0.90 0.80 0.70 0.60 0.50 0.40 Amazonian rainforest value Evaporative Fraction Based on field measurements in eastern Amazon Basin and northern Thailand 0 5 10 15 20 25 30 Years since abandonment
Importance of the Scale of Heterogeneity In deforested tropical landscapes, land cover is distributed in patches. Through time, these patches are becoming smaller. Does patch size have an influence on hydrological processes?
Landscape Fragmentation
Field Site
Ban Tat Hamlet
Effects of Fragmentation Transpiration edge effect Overland flow buffering
Does patch size influence regional latent energy flux?
Edge Effect: Higher rate of evapotranspiration for trees near the forest edge due to: advection of warm dry air from clearing (horizontal transfer of sensible energy) greater turbulent mixing at the boundary between short and tall vegetation
Flux = Velocity x Sapwood Area Transpiration = Flux/Crown Area
Granier Probes: Sample ΔT Time Series Δ T (K) 9 8 7 6 5 4 10 9 ΔT (K) 8 7 6 5 4
Soil Moisture Clearing Area Forest Clearing Area SAP FLOW STATION SAP FLOW STATION CLEARING STATION EDGE STATION FOREST STATION EDGE STATION 0 301 302 303 304 0.00 0.00 0.00-10 -10.00-10.00-10.00 0.42-20 -20.00-20.00-20.00 0.40 0.38-30 -30.00-30.00-30.00 0.36-40 -40.00-40.00-40.00 0.34 Depth (m) -50-60 -50.00-60.00-50.00-60.00-50.00-60.00 0.32 0.30 0.28 0.26-70 -70.00-70.00-70.00 0.24-80 -80.00-80.00-80.00 0.22 0.20-90 -90.00-90.00-90.00-100 -100.00 100 150-100.00 100 150 100 150-100.00 100 150 Day Day Day Day
TRANSPIRATION BASED ON SAPFLOW BAN TAT FOREST PATCH EXPERIMENT HOA BINH, VIETNAM 400 300 Edge = 1.126Interior r 2 = 0.9473 10-min Transpiration (W m2) (1:1) Edge 200 100 0 0 100 200 300 400 Interior
Day-to-Day Variability
Make Sense of the Spatial Patterns What factors are associated with strong edge effect patterns? Expectation 1: Edge effect stronger when conditions produce dry, hot surface in surrounding clearings Expectation 2: Edge effect stronger when wind blows into the edge Is the transpiration pattern responsive to rapid variations in external conditions? Expectation 3: Patterns may be more evident for short interval data
Humidity
Net Radiation
Southwest Wind
Does transpiration in the stand vary during the day in response to rapid fluctuations in external conditions? Can the edge effect pattern be seen more clearly during short intervals when conditions are favorable for enhancing transpiration along the forest edge?
Let s take a look at the change pattern of transpiration during selected days.
Effects on Transpiration Edge effect was observed within the study area and influenced the ET pattern to a distance of at least 100 m from the edge of the patch Edge effect more apparent when dry, hot conditions prevailed in adjacent cleared area Transpiration pattern was responsive to wind direction Edge effect can significantly increase patch ET Greater fragmentation could result in less deforestation-related reduction in regional ET
Effect of Clearing Forest Higher albedo Reduced evapotranspiration Reduced aerodynamic roughness Higher daytime temperature If land cover is highly fragmented, horizontal energy exchange may reduce the regional impacts of deforestation on ET and energy balance