Managing Tropical Landscapes Flashcards
Forest Functions
Forests store 45% of terrestrial carbon- rainforests are carbon sinks
Sequester large numbers of carbon annually- contribute 50% of terrestrial net primary production
Greenhouse gas emission- deforestation releases 18% of CO2 in atmosphere
8% from tropical deforestation
Total potential mitigation from tropical forests 24-30%
Tropical forests now emitting more than they can absorb- more of a source than a store at present
Provide essential ecosystem services;
-catchment protection
-water flow regulation
-nutrient recycling
-rainfall generation
-disease regulation
Carbon Cycle Feedback
Plants respond to rising CO2 through photosynthetic enhancement
This CO2 fertilization is negative feedback to higher atmospheric CO2 concentration
Models suggest that land carbon storage incr. with higher atmospheric CO2
But long-term outcome unclear especially if consider nitrogen availability
CC reduces carbon storage from CO2 fertilization
Terrestrial carbon storage declines with increase Temp
Process of Albedo
Albedo = ratio of shortwave radiation reflected from a surface to total radiation falling on surface Impacts near surface energy balance, and thermal climate High Albedo (snow) -> lower T (more reflection of radiation) Low Albedo (dark) -> higher T (more absorption of radiation) Forests have low surface albedo (only ~8% of solar energy reflected) therefore absorb energy and warms air (sensible heat) -> carries moisture into atmosphere where it condenses as rain Forests can mask high albedo of snow, & contribute to global warming
Evaporation Process
Evapotranspiration
Higher transpiration ->
Increase latent (moisture) energy flux,
Decrease sensible (heat) energy fluxes -> lowers Temp
evaporative cooling (latent heat) -> Cools climate
Aerodynamic Roughness
Surface roughness
High roughness increases air turbulence which causes drag on air flow over them and reduces wind speed
Crops, grasses, ice have low aerodynamic roughness
Moisture content above forests become large causing convection, cloud formation and enhancement of rainfall
Cropland vs Tropical Rainforest
Cropland- higher albedo, lower evapotranspiration, cools the air, less humidity
Tropical forest- lower albedo, higher evapotranspiration, more upwelling- cools temp-more moisture-more rainfall
Tropical Rainforests
In comparison with pasture, tropical forests have:
Lower albedo -> warming
Higher rates of ETP (especially in dry season) -> cooling
Surface warming from low albedo of forests is offset by strong ETP cooling
-> Cooler
-> Incr. Precipitation
Deforestation leads to:
Higher albedo -> Less heat absorption -> less moisture in atmosphere - > cooling
Dec in ETP -> warming, less rainfall
Overall warmer, drier climate
Which will enhance CC through positive feedbacks that decrease Evaporative cooling, releases CO2 and initiates forest dieback.
Fire -> leads to particles which cause rain
Critical thresholds?
BUT
Complex interactions! Small scale deforestation may produce mesoscale circulation that enhance clouds and precipitation
Boreal Forests
In comparison with absence of trees, boreal forests:
Low surface albedo in snow season -> warms climate!
Conifers have low evaporation in summer (compared with broadleaf)
Deforestation leads to:
higher albedo -> cools climate, provides positive feedback for glaciation
May offset forcing from carbon emissions to cool climate
Has greatest bio-geophysical effect of all biomes on annual mean global temperature
Temperate Forests
Many temperate forests have been cleared for agriculture
In comparison to crops, trees
Have lower albedo -> Warm air (maintain warmer summer climate)
Deforestation – replacement with crops:
Watered crops = higher albedo, higher evaporative cooling -> cool/warm? RF?
Deforestation – replacement with grass:
Grass = higher albedo, lower aerodynamic conductance and evaporative cooling, higher surface radiative temperatures -> cool? Warm? RF?
Response also depends on soil moisture/drought
Net climate forcing of temperate forests is highly uncertain!
Higher albedo with forest loss could offset carbon emission so that net climatic effect of temperate deforestation is negligible
OR
Reduced ETP with loss of trees could amplify biogeochemical warming!
Do Forests cool or warm the planet?
Different mechanisms for each forest type
Boreal forests have low albedo, causing local warming
Temperate forests modify weather via albedo and ETP but exact impact uncertain
Tropical forests have high ETP, moisture causes clouds which reflect incoming solar radiation and cause further cooling
Generally forests have cooling impact on global climate due to uptake of CO2
But modelling study suggests global destruction of forests would cool Earth (Bala et al 2007)
Do Forests increase Rainfall?
Theory-
Height of trees increases orographic effect / turbulence
Slight inc in RF
Role of evapotranspiration (rainfall-recycling ratio)
Cooler climates?
Some changes on regional scale
See for example, Guyana/South American example later (Bovolo et al 2018)
Conclusion
Jury still out!
Need to know more about mechanisms, magnitude of effect and scale
Remote sensing observations show that air passing over extensive TROPICAL forests produce at least twice as much rain as rain passing over little vegetation (Spracklen et al, 2012)
Do forests reduce erosion?
Conventional Theory/Observation:
Natural forest : high infiltration rates / low soil erosion
Plantation forests : roads, logging, drainage ditches, windthrow, splash erosion
Not all forest canopies “protect” the soil from raindrop impacts. Potential for “splash induced” erosion
Importance of species in determining drop size and erosive impacts has not always been well understood
Conclusions
Low rates of erosion from natural forests
Competing processes can result in increased or reduced erosion from plantation forests
Effect likely to be site and species specific
Management activities paramount for plantation forests
For certain species, forest plantations may cause severe erosion.
NOT always true
Forest and water relationship
Do forests cool or warm local climate? – variable on biome
Do forests increase rainfall? - Probably small effect – more research needed to establish magnitude and spatial scales
Do forests reduce erosion? - Not always - management activities paramount
Do forests increase catchment water yield? - NO
Do forests reduce floods? – benefits for small events at small catchment size, little evidence of benefits for large events or large catchment size*
Do forests “sterilise” water supplies - improve water quality? – YES! But not in high pollution climates
Biotic Pump
NO SOLID EVIDENCE- a) Under full sunshine, forests maintain higher evaporation than oceans and thus draw in moist ocean air.
(b) In deserts, evaporation is low and air is drawn toward the oceans.
(c) In seasonal climates, solar energy may be insufficient to maintain forest evaporation at rates higher than those over the oceans during a winter dry season, and the oceans draw air from the land. However, in summer, high forest evaporation rates are re-established (as in panel a).
(d) With forest loss, the net evaporation over the land declines and may be insufficient to counterbalance that from the ocean: air will flow seaward and the land becomes arid and unable to sustain forests.
(e) In wet continents, continuous forest cover maintaining high evaporation allows large amounts of moist air to be drawn in from the coast.”
international Treaties
Kyoto Protocol 2008-2012- aims to reduce greenhouse gas concentrations, first commitment period 2008-2012, mainly developed countries, 37 industrialized countries, reduce GHG emissions to at least 5% of 1990’s levels, USA withdrew support in 2002 however they met as switched to natural gas, Canada withdrew 2011, Doha Amendment 2012-2020
Counter arguments
Most cuts at little or no effort due to collapse of GHG producing industries in E Europe & global economic crisis
World emissions have surged 50% since 1990 driven by economic growth in China, Asia, S America, Africa
In 1990 developed nations accounted for 2/3s of global emissions, now < 50%
Counter-counter arguments
First policy experiment with important lessons to take forward
Has flaws (e.g. split between developed/developing countries), but overall architecture is useful
Gave birth to carbon-trading
Methods developed for Reporting & verifying GHG and Land-use changes important for future treaties
