2 -- ENERGY FLOW AND IMPORTANT CYCLES IN THE BIOSPHERE

Relevant to this series is a description of the flow of energy through Earth systems, particularly the biosphere. Why is energy flow important? It enables and sustains life, influences the atmosphere and other realms, and affects weather, climate, hydrologic cycles and erosion. Within the biosphere, life-giving energy transfers and the cycling of matter have evolved over time. Vegetation plays critical roles in these dynamic processes.

2.1 Energy Flow

Energy produced by photosynthesis forms the base of most ecosystem food chains, and moves "up" through trophic (food) levels by consumption. The primary producers, largely comprised of plants, form the foundation of the food chain and use solar radiation to convert carbon dioxide and water into carbohydrates via photosynthesis. Organisms at the next level in the food chain eat the products of photosynthesis, and they are collectively called the "primary consumers." Primary consumers in turn are eaten by "secondary consumers," and so on (Figure 3.01). Each graphic layer in Figure 3.01 is proportional to its value's cubic root for volumetric biomass and to its value's square root for cross-sectional energy flow.

The food chain does not usually go beyond a fourth level of consumers, because there are significant energy costs at each level such that decreasing amounts of energy are available at each "higher" level. For example, the energy stored in a plant is only a fraction of the energy it takes to produce the plant material. At the next level, if an insect eats a leaf, only a fraction of the energy available in the leaf remains in the insect, because most of the consumed energy is used to maintain the insect's bodily functions (i.e., growth, metabolism, reproduction, digestion, searching for food, etc.) and dissipated as heat to the environment. Generally consumers at each successive level are larger organisms, however there are usually fewer of them in number (relative to "lower" levels).

2.2 Hydrologic Cycle

Water is a critical component of all ecosystems. Without water, there would be no life in its present form. A localized hydrologic cycle is illustrated in Figure 3.02. Solar energy (and its derivative wind energy) evaporate water, and the resulting water vapor rises into the atmosphere. Once in the atmosphere, water vapor condenses to form clouds and falls back to Earth's surface as rain or snow. From there it may run off the surface or get absorbed by the ground, where it can reevaporate, move through groundwater flows or be stored underground. Uptake and evapotranspiration of water by global vegetation is a significant part of the hydrologic cycle.

Although the hydrologic cycle maintains a balance (incoming equals outgoing), humans have modified Earth's surface (for example, by the clearcutting of forests) to the extent that water's movement on surfaces has been altered. As a result, water runoff (water that is quickly and surficially drained away) has increased, which in turn has decreased the amount of water absorbed in ecosystems. This change has a number of undesirable effects including aquifer (underground water reservoir) depletion, increased erosion, and habitat degradation, as well as increasing the frequency and severity of floods and droughts.

2.3 Biogeochemical Cycles

Biogeochemical cycles involve biological, geological, and chemical processes that move various elements around, in and between ecosystems, biomes, and the various Earth realms or spheres. Like the hydrologic cycle, they are vital for the survival of living organisms. These cycles have terrestrial, marine/aquatic and atmospheric components, and biogeochemical reserves are stored in the atmosphere, standing biomass, soil, sediments and rocks. As with the transport of energy through ecosystems, biogeochemical cycles significantly affect plant communities (primary producers), and consequently they also influence various levels of consumers, including humans. Much research has been done on the transport, use and cycling of many chemical elements and compounds.

2.3.1 Carbon -- The cycling of carbon has great significance to the biosphere. Carbon is the most abundant element in the nonwater portions of plants and animals. Important components in the atmosphere enveloping Earth, the carbon-containing gases carbon dioxide (CO2) and methane (CH4) moderate diurnal temperature changes. Carbon transfer between the biosphere and the atmosphere (via photosynthesis and respiration) has significant effects on atmospheric CO2 concentration.

Rapid increases in atmospheric CO2 and CH4 (resulting from the burning of fossil fuels and deforestation) can alter Earth's climate by absorbing more of the Sun's energy and releasing it as heat (global warming). In contrast, a process that removes carbon from the atmosphere and stores it in the biosphere is reforestation, since plant tissues are composed of approximately 50% carbon (on average). Carbon exchange between various sources (releasing carbon) and sinks (storing carbon) is illustrated in Figure 3.03, which emphasizes terrestrial components. Oceans also store carbon, however that is not examined here. Figure 3.03 presents a relatively local view, however the rate values given represent global carbon flow.

The amount of carbon stored in plants can be estimated by measuring vegetation biomass for given areas. Improved, more accurate global estimations of vegetation biomass and NPP (see Section 1.2 above in this chapter for definition of net primary production) are made possible by the use of satellite remote sensing measurements. These estimations are important to the accurate assessment of the global carbon budget and the distribution of carbon throughout ecosystems, biomes and Earth realms. Photosynthesis and primary production take CO2 (and hence carbon) out of the atmosphere, while respiration releases it back into the air. NPP provides a measure of the rate at which carbon is extracted from the atmosphere in excess of plant respiration. If plant production and respiration do not balance in a specified area (for example, an ecosystem or a biome), that area is either a carbon source (adds carbon to the atmosphere) or a carbon "sink" (removes carbon from the atmosphere) depending on the relative magnitudes of the production and respiration fluxes.

The distribution of carbon storage (total carbon) and net primary production (NPP) in global terrestrial vegetation is shown in Table 1. Vegetation types down the left of Table 1 are ranked in order of kilograms of carbon per square meter. The rankings for the top ten values of Total Carbon and NPP are also shown. For example, tropical humid broadleaf forest ranks first in Total Carbon and NPP, yet when its Total Carbon is divided by the areal extent for this vegetation type, it ranks second to temperate and tropical conifer forest in kilograms of carbon per square meter.

	CARBON kg/m2	AREA 105km2	AREA %	TOTAL CARBON 1012 kg	TOTAL CARBON %	TOTAL CARBON rank	ANNUAL NPP 1012kg/yr	ANNUAL NPP %	ANNUAL NPP rank
FORESTS
Temperate and tropical conifers	16.8	3.5	2.4	59.0	10.6	3	2.1	3.5
Tropical humid broadleaf	15.0	10.4	7.0	156.0	27.9	1	8.3	13.8	1
Temperate broadleaf	10.0	1.5	1.0	15.0	2.7		0.9	1.5
Temperate mixed	10.0	3.5	2.4	35.0	6.3	5	2.1	3.5
Main and southern boreal	8.7	7.2	4.9	62.0	11.1	2	3.0	5.0	9
Tropical dry	7.0	4.7	3.2	33.0	5.9	6	2.7	4.5	10
DISCONTINU- OUS WOODS
Northern and maritime boreal	5.0	4.4	3.0	22.0	3.9	9	1.3	2.2
Forests with fields	5.0	5.2	3.5	26.0	4.7	3	3.1	5.1	8
Tropical montane	5.0	1.2	0.8	6.0	1.1		0.8	1.3
Dry woods mosaic	4.2	8.5	5.7	35.4	6.3	4	3.3	5.5	6
Fields and woods	3.5	4.0	2.7	14.0	2.5		2.0	3.3
Coastal woods	3.0	0.4	0.3	1.0	0.2		0.2	0.3
Savannah	3.0	6.7	4.5	20.2	3.6	10	3.3	5.5	6
Wetlands	2.6	2.5	1.7	6.6	1.2		3.6	6.0	5
NONWOODS
Irrigated cropland and urban	2.6	3.6	2.4	9.2	1.6		5.0	8.3	4
Very cold grass, shrublands	1.7	2.6	1.8	4.2	0.8		0.4	0.7
Warm/cool grass, shrublands	1.2	21.4	14.5	26.8	4.8	7	8.2	13.6	2
Other cropland and urban	1.0	12.3	8.3	12.3	2.2		7.1	11.8	3
Tundra	0.8	11.0	7.4	9.0	1.6		1.4	2.3
Cool desert	0.6	2.0	1.4	1.2	0.2		0.4	0.7
Warm desert	0.4	11.0	7.4	4.4	0.8		0.5	0.8
Sandy desert	0.1	5.2	3.5	0.3	0.1		0.5	0.8
Ice and snow	0.0	15.2	10.3	0.0	0.0		0.0	0.0
TOTALS		148.0		558.6			60.2
from Olson et al., 1983

It is possible to reconstruct past climates (paleo climates) in part by measuring the relative concentration of different types of carbon and oxygen isotopes in various materials (fossil plants and oceanic microfossils for example). Stable (nonradioactive) isotope compositions, which have shown variations over time, are influenced by biological and physical processes and factors (such as temperature and photosynthesis). They can be useful as nonradioactive tracers. Not all of the CO2 produced by biomass burning and fossil fuel consumption is accumulating in the atmosphere -- much of it is also stored in the biosphere, pedosphere and hydrosphere. This storage to some extent slows the rate of increase in atmospheric CO2.

Because processes in soil transform detritus (dead organic material) into humus which contains carbon, soils are generally major carbon sinks. With proper management and conservation, soils have the capacity to store large amounts of carbon. On the other hand, disturbance and depletion of soils (for example, by tillage, removal of surface organic materials, or increased erosion) release carbon into the atmosphere and hydrosphere. The amount of carbon that soils store depends on a number of biogeochemical and physical factors including vegetation type, soil texture (grain size and shape distribution), soil composition, soil moisture and temperature, and climate.

The importance of NPP in the global carbon cycle, changes in NPP associated with climate, and methods to estimate NPP with satellite observations are explored further in later chapters and associated exercises in the remote sensing tutorial.

2.3.2 Additional elements -- Besides carbon, there are additional elements present in much smaller amounts that have a significant effect on plant growth. Table 2 lists fifteen elements that must be present in sufficient quantities for plants to grow. Other elements are not required for plant growth but do occur in plants and are essential for animals (for example, iodine and selenium).

ELEMENT	APPROXIMATE CONCENTRATION		ELEMENT	APPROXIMATE CONCENTRATION
nitrogen	1-3%		copper	2-75 ppm
potassium	0.3-6.0%		manganese	5-1500 ppm
calcium	0.1-3.5%		zinc	3-150 ppm
phosphorus	0.05-1.00%		molybdenum	0.1-20.0 ppm
magnesium	0.05-0.70%		boron	2-75 ppm
sulfur	0.05-1.50%		cobalt	trace
iron	10-1,000 ppm		sodium	trace
chlorine	100-10,000 ppm
% refers to the relative amount of the element in the total composition of the plant ppm = parts per million trace = amounts so small they are difficult to quantify
from Strahler, A. A., and A. H. Strahler. Modern Physical Geography. John Wiley & Sons, 1978, p. 190, fig. 12.2.

Processes involving nitrogen in a lodgepole pine ecosystem are depicted in Figure 3.04. Nitrogen is used in enzymatic reactions during photosynthesis. Plants take in nutrients via their roots and use them for various metabolic processes. Nutrients not entirely consumed by metabolism can be stored in plant biomass.

Nitrogen and other nutrients are eventually released back to the environment as a result of animal excretion, decomposition of dead plants and animals, and rainfall leaching of these products into soils, to be held for reuptake by plants through microbial activity (fixation and mineralization). Depending on the soil, nutrients can exist in a form available to plants, be transformed to an unavailable form, or leached into groundwater. Substantial amounts of these nutrients are also lost to the atmosphere and by runoff. The concentration of assorted nutrients relative to carbon material in plants varies. The compositional mix of the resulting plant litter affects decomposition rates, which in turn are related to vegetation growth rates. In general, areas where plants grow quickly have high decomposition rates, thus nutrients are cycled rapidly. In contrast, nutrient cycling is usually slow in areas where plants have slow growth rates.

Human activities have modified the cycling of nutrients, particularly nitrogen, through the production and application of fertilizers, cultivation of nitrogen fixing plants, combustion of fossil fuels, and increased erosion and runoff which rapidly deplete and remove soils.