Chapters 1 and 2

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Chapter 1

Why study ecosystems?

Ecosystem definitions; has a focus on the interactions between organisms and environment. Also has a focus on the processes that cause biological systems to function and interact with abiotic systems.

Gives its own definition; The study of terrestrial ecosystems is the study of the metabolism and physiology of units of the landscape, concentrating on energy, chemical and water balances.

Discusses the importance of ecosystem studies as ecosystems provide us with services. The studies can apply to wild ecosystems or highly managed, domesticated ones.

Understanding global environmental issues relies on ecosystem knowledge as ecosystems are the smallest, most basic block of studying global systems.

Studying ecosystems involves many different disciplines.

Development of Ecosystem Concepts

Can be studied 'top down', looking at large scales systems and function and figuring out how they work OR 'bottom up', looking at components and seeing how the fit together.

The term was coined by Tansley in 1935, mainly in regards to plant ecology.

How do you classify what is part of an ecosystem, what do all the components have in common?

Lindeman, 1942, decided it was energy and that studying ecosystems was studying the storage and movement of energy. Energy forms the universal currency between different trophic levels. Lindeman also mentioned but did not develop the currency of nutrients.

JD Ovington (50s and 60s) developed on these ideas and studied terrestrial ecosystems in terms of inputs and outputs. He had a particular focus on woodlands. He studied energy, nutrient and water flows.

Delimiting the Ecosystem

How do you define the limits of an ecosystem? In some ways it depends on the study and questions being asked.

It can be argued that the whole world is the limit of the ecosystem - look at the effect of DDT worldwide, this pesticide was even found in penguins.

You can find complete ecosystems in much smaller microcosms though.

In terrestrial systems it is often defined as the watershed or the stand.

The watershed is an area in which all the falling precipitation can be collected in a single stream. They can vary enormously in size. They are used because water has such a large part to play in ecosystems. This obviously doesn't work in some topographies, and deserts.

A stand is an area of sufficient homogeneity in some regard, biotic or abiotic, to classify it as a unit. These tend to be trickier to classify as homogeneity is rare in natural systems and there are grey areas but they tend to be smaller than watersheds so allow more targeted study

Components of Terrestrial Ecosystems

What do different ecosystems have in common and how do they differ?

They tend to have components at different levels - autotrophs (primary producers), heterotrophs (consumers), saprotrophs (decomposers). Generally saprotrophs are a larger component of energy transfer than heterotrophs.

These three levels interact with the atmosphere and soils.

Autotrophs can be broken down into trees, shrubs and grasses and then further into leaves, stems and roots.

Saprotrophs make up a huge assemblage of microorganisms and animals.

Heterotrophs can be herbivores or carnivores

Carbon, hydrogen and oxygen make up to 98% of plant tissues, but inorganic nutrients such as nitrogen, phosphorus and calcium also reside in the system and are constantly recycled. Plants can take up the simple forms of these nutrients whereas decomposers break down the more complex forms found in the structures of plants and animals.

Chapter 2

Distribution and Characteristics of Major Ecosystem Types

Climate plays the largest part in determining the structure of vegetation and soils in natural ecosystems.

Temperature and the balance of evaporation and precipitation are important due to their effects on biological and chemical reactions.

Water availability has a huge effect on plant growth and soil structure and nutrient content.

Vegetation Type, Plant Structure and Major Processes

Major ecosystem types can be characterised by total height, distribution of roots, total above ground biomass or the dry weight of live plants.

You can compare ecosystems by annual net primary production (P) which is all the dry weight produced in one year by plants.

OR you could compare annual nitrogen uptake/use by above ground vegetation. This is figured out by determining the nitrogen content of different types of plant structure and then multiplying it by P.

OR you can compare actual evapotranspiration (AET) which is the sum of all the water lost through evaporation from the soil and transpiration from the ground. This is compared to PET - potential evapotranspiration if there were unlimited water supplies.

Climate data can be used to predict the productivity of an area. Productivity is often restrained to a particular growing season or limited by drought.

The vegetation type that can achieve the highest leaf mass is predicted to dominate a suitable location.

These models can be used to predict responses to climate change scenarios.

Soil Processes and Distribution of Soil Types

There are 5 major soil horizons:

O - organic horizon. This is the surface accumulation of partially decomposed litter and organic matter, unmixed with the mineral soil.

A - horizon. This is the upper layer of the mineral soil, interacting with decomposed matter from the O horizon and the root network of plants. There is also some chemical weathering in this layer as water runs through.

E - horizon. This is the layer of mineral soil mostly influenced by chemical weathering (as opposed to decomposition in the A horizon). Many soils have either an A or an E depending on the relative importances of these two influences.

B - horizon. This is the layer that has been deposited by the processes in the above layers.

C- horizon. This is the mineral layer which is not affected by the above processes

The kind of chemical processes which occur in the E - horizon are one of 4 basic processes:

Podzolization - the leaching of iron, aluminium and organic matter producing a high silicon horizon of low fertility. It occurs under cool, moist, acidic conditions. A large O horizon is often found above which by the addition of organic acids, increases the process further.

Laterization - the opposite of podzolization. This occurs in warm, moist conditions where silicon is leached more readily than iron, aluminium and clays. This creates red clay soils. The silicon is not deposited into a B horizon but instead is leached away into the groundwater.

Melanization - occurs in temperate areas with fine, textured soils. The A and B horizons are well mixed and difficult to distinguish due to the activity of soil animals. The soil profiles tend to be dark due to the mixing of decomposed organic matter. There is no visible E horizon.

Calcification/salinization - in arid habitats with little water, salts and carbonates remain throughout the soil layers instead of being percolated down with water.

Major Vegetation and Soil Types of the Earth

Tropical rain forests are highly productive due to constant high precipitation and temperature. Evapotranspiration is high but there is usually more precipitation than evaporation so water percolates through the soil in large volumes. They tend to have very deep E horizons which is a sign of soil age. The leading process in the E horizon is laterization so the soils have high iron, aluminium and clay content. Conditions are very good for decomposition so there is only a thin O horizon despite the high biomass production. These soils are called Oxisols and are actually nutrient poor. This is because most of the nutrient is held up in the biomass, not the soil, as growing conditions are so good. This is also represented in the concentration of fine roots through the soil, sucking up all nutrients as soon as they are available.

Tropical season forests have similar climate to above but go through spells of droughts. This lowers plant growth, litter fall, nitrogen cycling and results in lower evapotranspiration. The main process is again laterization and creates a soil similar to Oxisoils called Ultisoils.

High temperature Grassland, Savannah's, Shrub or Thorn Forests occur in even drier climates. Which one of these may be determined by disturbance regimes such as fire and grazing. There is low AET but high PET. Roots may reach deep into the ground to find water. Leaching rates are low so soils form slowly. Heavy storms can cause considerable erosion, particularly as there is reduced root volume. Grasslands differ from shrublands in that the extensive shallow roots increase percolation and reduce erosion. Grassland soils are called mollisols and are typified by melanization where shrubland soils are called aridisols and are typified by calicification.

Temperate rainforests are fairly rare and in many ways similar to tropical rainforests in terms of processes. Their vegetation differs however as, in North America at least, they are dominated by conifers rather than broadleaves. Production is lower and nitrogen cycling is lower as the conifer evergreens do not produce anywhere near as much leaf litter. Due to the cooler conditions and acid O horizon, podzolization is the dominant force with iron and aluminium leached down to the B horizon, leaving a distinct E horizon. This is called spodosol. Roots are found in the O horizon and nutrient enriched B horizon but not the E horizon.

Temperate seasonal forests can be broadleaved or coniferous. This tends to be decided by the timing of precipitation and the extremes of temperatured. Broadleaves are found in situations with wet summers and cold winters. Conifers are found in dry summers and warmer winters. Conifers also have lower water and nutrient demands. Leaf litter differs significantly between the two. Broadleaf decomposes faster and supports more animal life. This leads to a strong A horizon. Clays are leached leading to an enriched B horizon. The soils are called Alfisols. AET and PET are similar meaning that drought stress is low so productivity is fairly high (but less so than the tropics). The acid effect of conifer needles leads to a large O horizon and a deep E horizon as a result of podzolization.

Shrubland and grasslands form in temperate regions and this is again thought to be dictated by the timing of precipitation and weather extremes as well as disturbance regimes. In both shrubland and grassland, PET outweighs AET. Grasses are quite different in that they form the mollisols described earlier where shrub communities form aridisols with high carbonate and salt levels.

Boreal areas tend to have lower precipitation than tropical or temperate areas but also have much lower evaporation rates due to low temperatures. Production is low due to low temperatures and nitrogen cycling is low due to the coniferous nature of these areas. With low decomposition rates and few soil animals due to the poor quality of the O horizon, acidic podzolization takes control. Unlike other areas of dominant podzolization, the E horizons are often not well developed as it hasn't been that long since the last glaciation and then cold temperatures slow everything down. They are called Entisols or Inceptisols.

Histosols are found in lowland tundra where all processes occur extremely slowly. This is due to low temperatures and almost permanent frost. During the growing season the soils can be waterlogged which reduces any processes further. Rooting only occurs in the top O horizon, above the permafrost.

Deserts are the most dehydrated of all terrestrial ecosystems and their soils are typified by salinization and calcification.

Boundaries are never well defined though and ecosystems blend into eachother via ecotones. Environmental factors change and ecosystems change therefore over both space and time.

Disturbance can play a large role in the structure of an ecosystem. There are areas where forests would be expected but regular fires prevent this succession. Over exploitation of wood or over grazing can also hold back an ecosystem or alter it.

AET essentially measures a combination of moisture and temperature. High AET is always associated with high biomass production.

Remote sensing can be used to measure how much incoming solar radiation is absorbed by chlorophyll. This gives an approximate figure of biomass production.