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Chapter 4 – The Terrestrial Environment Life on Land Imposes Unique Constraints Desiccation – the loss of water o Greatest constraint imposed by terrestrial environment (keeping water in the body) o Water evaporates form the cell + body surface via diffusion Organisms must maintain a water balance + minimize water loss o Ex. Waxy cuticle on plants prevent water loss Any water lost from the organism must be replaced to maintain water balance o Animals get water by eating + drinking o Vascular tissues transport water + nutrients through the bodies of plants Gravitational forces are greater for terrestrial organisms, which have to invest in structural materials to remain erect o Skeletons for animals o Cellulose for plants Terrestrial environments experience a high degree of variability o Temperature variation much greater o Timing + quantity of precipitation constrain availability of water Plant Cover Influences the Vertical Distribution of Light Vertical gradient + quality of light are determined by absorption + reflection of solar radiation by plants
Leaf area (of flat leaves) – surface area of one (or both) sides Leaf Area Index (LAI) – area of leaves per unit ground cover Ratio of red to far – red radiation decreases through the canopy o Light quality affects patterns of growth + allocations Orientation of leaves or angle the leaf is held on the plant influences attenuation of light through canopy Sun flecks can account for 70 – 80% of solar energy reaching the ground in forest environments Seasonal changes strongly influence leaf area o Deciduous plants shed leaves during winter months o Increased light availability during dry conditions in regions w/ wet + dry seasons Beer’s Law + Attenuation of Light The greater the surface area of leaves, the less light will penetrate the canopy + reach the ground The attenuation (vertical reduction) of light through a stand of plants is estimated using Beer’s Law o Beer’s Law : AIi = e-LAI x k o Tells you how much light can reach any position o Important because it tells you how much photosynthesis is happening Availability of light directly influences the levels of photosynthesis
b. During photosynthesis,, plants return some of the sun’s energy to the soil in the form of organic carbon c. Through decomposition, dead plants + animals become organic matter incorporated into the soil
- Climate affects physical + chemical breakdown of parent material a. Temperature, precipitation, winds b. Leaching – movement of solutes through the soil c. Temperature influences rates of biochemical reactions
- Topography affects erosion, deposition (where the soil ends up), + the influence of climate (gradient of slope)
- Time contributes to soil formation a. Considerable time is required for soil to form Soil Have Certain Distinguishing Characteristics o Soil color is an easily distinguished + useful characteristic of soil o Color has little influence on soil function o Soil texture is the proportion of different – sized soil particles o Gravel > 2.0mm (technically not part of soil) o Sand = 0.05 to 2.0mm o Silt = 0.05 to 0.002mm o Clay < 0.002mm
o Soil texture affects pore space + movement of air + water I + throughout the soil o Soil depth varies + depends on many factors o Slope o Weathering o Parent material o Vegetation o Shallow soils: forests, ridgetops, steep slopes o Deep soils: grasslands, bottom of slopes, + alluvial plains Soil Body has Horizontal Layers o Soil profile – a sequence of horizontal layering o Soil horizon – horizontal layers of soil material o General soil profile has 4 horizons o O horizon – dominated by organic matter o A horizon (topsoil) – mineral soil + organic matter leeched from above accumulation (plant roots grow here) E horizon – zone of maximum leeching (eluviation) sub – horizon of A horizon o B horizon (subsoil) – accumulates mineral particles + contains less organic matter then layers above Minerals leached from above are deposited in a process called illuviation Minerals go from E to I horizon
o Cation exchange capacity (CEC) – total number of negatively charged sites o CEC is a basic measure of soil quality o Soil – bound cations are in dynamic equilibrium w/ cations in the soil solution + are continuously being exchanged o The smaller + more positive an ion, the more tightly it is held to the soil particle o As H+^ ions are added to the soil, other cations are displaced + the soil becomes increasingly acidic o As soil acidity increased, the proportion of aluminum (Al+3) increase o High [Al+3] in the soil can be toxic to plants + damages the roots Basic Soil Formation Processes Produce Different Soils o The broadest level of soil classification is soil order o Twelve orders of soil: o Lateralization – process common to soils in humid environments in the tropical + subtropical regions Heavy leaching of nutrients (lots of rain, nutrients move through quickly) o Calcification – when evaporation + water uptake by plants exceed precipitation > deposition + buildup of alkaline salts (CaCO2) in subsoil o Salinization – occurs in very dry climates or costal regions as a result of salt spray
salt deposits near he soil surface
o Podzolization – occurs in cool, moist climates where coniferous vegetation (pine forests) dominates > acidic soil enhances leaching of cations, iron, + aluminum from topsoil o Gleization – occurs in areas w/ high rainfall or areas of poor drainage > organic matter is slowly decomposed + accumulates in upper layers of soil Ex. swamps
