Climate Modification

Although most previous studies have emphasized the effect of climate on survival, population growth, and distribution of organisms (see Chapters 2, 6, and 9), communities of organisms also modify local and regional climatic conditions, perhaps influencing global climatic gradients (T. Chase et al. 1996, J. Foley et al. 2003, G. Parker 1995, Pielke and Vidale 1995). Climate modification largely reflects the capacity of vegetation to shade and protect the soil surface, abate airflow, and control water fluxes (Fig. 11.10). Isoprene emission by some plant species apparently increases leaf tolerance to high temperatures and also affects the oxidation potential of the atmosphere (Lerdau et al. 1997). Biomes and suc-cessional stages vary widely in ability to modify climate.

When vegetation development is limited or moisture is limited, as in deserts, the soil surface is exposed fully to sunlight and contains insufficient water to restrict temperature change (T. Lewis 1998). The reflectivity of the soil surface (albedo) determines absorption of solar energy and heat. Soils with high organic content have lower albedo (0.10) than does desert sand (0.30) (Monteith 1973). Albedo also declines with increasing soil water content. In the absence of vegetation cover, surface temperatures can reach 60-70°C during the day (e.g.,

Case 1 - Vegetated

More humidity and recycling of water - fueling high precipitation rates

High latent heat loss

Solar radiation

High evapotranspiration

Low sensible heat loss

Low surface albedo temperature

High evapotranspiration

Low surface albedo temperature

Case 2 - Deforested

Less humidity and recycling of water - reduced precipitation rates

Higher sensible heat loss

Lower latent heat loss

Higher surface

Low temperature evapotranspiration iL ^ .. ^ ,„ j. * « m. £ «

Solar radiation

| Diagrammatic representation of the effects of vegetation on climate and atmospheric variables. The capacity of vegetation to modify climate depends on vegetation density and vertical height and complexity. From J. Foley et al. (2003) with permission of the Ecological Society of America. Please see extended permission list pg 571.

Seastedt and Crossley 1981a) but fall rapidly at night as a result of long-wavelength (infrared) radiation from the surface. Exposure to high wind speeds dries soil and moves soil particles into the atmosphere. Soil desiccation reduces infiltration of precipitation, leading to greater runoff and erosion. These altered soil characteristics affect albedo and precipitation patterns.

Vegetation modifies local climate conditions in several ways. Even the thin (3 mm) biological crusts, composed of cyanobacteria, green algae, lichens, and mosses, on the surface of soils in arid and semiarid regions are capable of substantially modifying surface conditions and reducing erosion (Belnap and Gillette 1998). During the day, vegetation shades the surface, reducing temperature (T. Lewis 1998). The effect of vegetation on albedo depends on vegetation structure and soil condition. Vegetation absorbs solar radiation to drive evapotranspiration (G. Parker 1995). Albedo is inversely related to vegetation height and "roughness" (the degree of unevenness of canopy topography), declining from 0.25 for vegetation <1.0 m in height to 0.10 for vegetation >30 m height; albedo generally reaches lowest values in vegetation with an uneven canopy surface (e.g., tropical forest) and highest values in vegetation with a smooth canopy surface (e.g., agricultural crops) (Monteith 1973). Canopy roughness creates turbulence in air flow, thereby contributing to surface cooling by wind (sensible heat loss) and by evapotranspiration (latent heat loss) (J. Foley et al. 2003). At night, the canopy absorbs reradiated infrared energy from the ground, maintaining warmer nocturnal temperatures, compared to nonvegetated areas. Canopy cover intercepts precipitation and can reduce the impact of rain drops on the soil surface (Fig. 11.11, Meher-Homji 1991, Ruangpanit 1985), although this effect depends on rainfall volume and droplet size (Calder 2001). Vegetation impedes the downslope movement of water, thereby reducing erosion and loss of soil. Soil organic matter retains water, increasing soil moisture capacity and reducing temperature change. Exposure of individual organisms to damaging or lethal wind speeds is reduced as a result of buffering by surrounding individuals.

The degree of climate modification depends on vegetation density and vertical structure. Sparse vegetation has less capacity to modify temperature, water flow, and wind speed than does dense vegetation. Shorter vegetation traps less radiation between multiple layers of leaves and stems and modifies climatic conditions within a shorter column of air compared to taller vegetation. Tall, multicanopied forests have the greatest capacity to modify local and regional climate because the stratified layers of foliage and denser understory successively trap filtered sunlight, intercept precipitation and throughfall, contribute to evapotranspiration, and impede airflow in the deepest column of air. G. Parker (1995) demonstrated that rising temperatures during midday had the greatest effect in upper canopy levels in a temperate forest (Fig. 11.12). Temperature between 40 and 50 m height ranged from 16°C at night to 38°C during mid-afternoon (a diurnal fluctuation of 22°C); relative humidity in this canopy zone declined from >95% at night to 50% during mid-afternoon. Below 10 m, temperature fluctuation was only 10°C and relative humidity was constant at >95%. Windsor (1990) reported similar gradients in canopy environment in a lowland tropical forest.

in Q

20-30 40-50 50-60 60-70 70-80 80-90 Crown cover (%)

Effect of canopy cover on average runoff and soil erosion, based on 41 runoff-producing storms totaling 1128 mm in northern Thailand. Data from Ruangpanit (1985).

Fig. 11.11

Furthermore, evapotranspiration can affect local and regional precipitation. Salati (1987) reported that 30% of precipitation in tropical rainforests in the Amazon basin was generated locally by evapotranspiration. Local recycling of water may be most pronounced in montane areas, where steep vertical temperature gradients condense rising evapotranspired water.

Insects and other organisms (including humans) alter vegetation and soil structure (Fig. 11.13) and thereby affect biotic control of local and regional climate (see Chapters 12-14). Deforestation or desertification reduce evapo-transpirative cooling, offsetting the effect of increased albedo, thereby increasing surface temperatures and reducing precipitation and relative humidity (J. Foley et al. 2003,T. Lewis 1998, Salati 1987). Costa and Foley (2000) calculated a net warming of 1-2°C in tropical regions as a result of deforestation, an effect that would exacerbate the warming resulting from increased atmospheric CO2. Forest fragmentation increases wind fetch and penetration of air from surrounding crop or pasture zones into forest fragments (J. Chen et al. 1995). Belnap and Gillette (1998) found that trampling disturbance of the brittle biological crusts on desert soils greatly increased the effect of wind on soil loss. Increased levels of airborne particulates reduce the penetration of photosynthetically active radiation.

Photosynthetically Active Radiation

Height-time profiles of air temperature and relative humidity in a mixed-hardwood forest in Maryland. Temperature contours are 2°C; relative humidity contours are 10% units. Nocturnal temperature gradients are weak, but a hot spot develops in the upper canopy in mid-afternoon. Humidity declined in the upper canopy in mid-afternoon, coincident with peak temperatures, and was near saturation (>95%) outside the marked contours. From G. Parker (1995). Please see extended permission list pg 571.

Fig. 11.12

Height-time profiles of air temperature and relative humidity in a mixed-hardwood forest in Maryland. Temperature contours are 2°C; relative humidity contours are 10% units. Nocturnal temperature gradients are weak, but a hot spot develops in the upper canopy in mid-afternoon. Humidity declined in the upper canopy in mid-afternoon, coincident with peak temperatures, and was near saturation (>95%) outside the marked contours. From G. Parker (1995). Please see extended permission list pg 571.

Deforestation and desertification could initiate positive feedback between climate and vegetation change. Holocene warming led to northward advance of the boreal forest, which lowered albedo and contributed to continued warming of the ecotone (J. Foley et al. 1994). Schlesinger et al. (1990) reported that desertification results in a destabilizing positive feedback, whereby initial vegetation removal causes surface warming and drying that stresses and kills adjacent vegetation, leading to an advancing arc of desertified land. The effects of similar, large-scale vegetation changes resulting from insect outbreaks on climatic conditions have not been evaluated.

Tropical Rainforest Soil And Land
| Deforestation in Panamá. Removal of tropical rainforest cover has exposed soil to solar heating and severe erosion, leading to continued ecosystem deterioration and, potentially, to altered regional temperature and precipitation patterns.
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