Lecture 5. Environments of the Earth

-why does all this discussion of climate matter?

-recall that weather, and climate more generally, is a state factor

- which controls the kinds of ecosystem that can occur at different places.

-at the largest scale, uneven heating of the earth means that the tropics are always warm

-poles are always cold, with other latitudes in between

-ocean currents and arrangement of continents create regional differences in climate

-leading to large areas of the Earth’s surface that support ecosystems of the same type

-we call these regions biomes:

-large areas of the earth’s surface supporting a single type of vegetation

-can be viewed as a collection of similar ecosystems

-biomes are defined primarily by the vegetation:

-on land, vegetation determines both the physical structure of the ecosystem

and the primary energy source

-therefore determines the other species that live there too.

-concept of biomes was originally intended for terrestrial vegetation,

-has been extended to marine and freshwater habitats, not very successfully

-look at the Table (1.1, p. 9 in Morin) which shows the distribution of world biomes

-according to Whittaker (1975), a prominent ecologist

-terrestrial biomes are defined more precisely than aquatic ones

-if Whittaker had been an aquatic ecologist, the situation might be reversed

** I disagree with Whittaker’s assignment of freshwater lakes and ponds as one biome

-and the same for freshwater rivers and streams

-original idea of a biome is area of the earth’s surface that supports same kind of ecosystem

-such as boreal forest or hot desert

-simply not true for lakes and streams

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-as much difference between a tropical lake and a freshwater lake

as between a tropical and a temperate forest

-lakes and ponds are similar habitats but not similar biomes

-World Wildlife Fund defines 13 kinds of freshwater biomes

-so definition of what constitutes a biome is fuzzy

-Textbook, after p. 52-56, has some fancy colour pictures of the earth’s biomes

-Text figure 2.24, p. 52, shows world distribution of biomes matches temperature, rainfall

-influence of temperature and especially precipitation is apparent

-classic classification scheme for terrestrial vegetation shown in this figure

-if we know the annual temperature and precipitation range for any region

we can predict the vegetation type without fail

-shown in even simpler form in Text Figure 2.22, p. 50 (this from Whittaker again)

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-Look at Text figure 2.24, p. 52 (world map of biomes)

-within 12 of the equator, we have tropical wet forest, o

-in the zone of convergence,

-where temperatures are constantly high

-and convective uplift brings heavy rainfall

-farther north wet forest grades into tropical dry forest

-and then savanna, along a gradient of declining rainfall

-notice that hot deserts occur, not at the equator, where solar radiation is strongest,

-but at about 30 N or S, where the Hadley cell descends, o

-bringing warm, dry air to the surface

-thereby creating a semi-permanent high pressure cell

-otherwise, deserts and steppes tend to occur in the lee of major mountain ranges

-in the mid-latitudes, the climate is dominated by the Ferrell cell

-which is less well-defined and more complicated than the tropical Hadley cell

-consequently biomes are more complicated as well

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-here we have deserts, grasslands, shrublands and forests, depending mostly on rainfall

-finally Arctic tundra in the far north where weather is too cold to support trees

-absence of trees in high Arctic also due to insufficient available water because of freezing

-remember, biomes are regions that support similar kinds of vegetation

-boundaries between them are not sharply defined

-rather, a gradual transition from one kind of ecosystem to another

-especially along precipitation gradients within a temperature zone

-Text Figure 2.35, p. 58, gives one example

Aquatic Versus Terrestrial

-concept of biomes was invented for terrestrial vegetation

-doesn’t apply as well to marine environments,

-though Whittaker and others have tried

**Perhaps extending concept of biomes to the whole planet is the wrong approach

-goal of ecology is to find unifying principles that apply to all ecosystems

-or, if that is not possible, to discover how many different environments the planet supports

-so that we may erect a set of generalizations appropriate for each

-therefore the important question becomes:

**how many fundamentally different kinds of environments are there on the planet?

-below I suggest one simple answer: not necessarily the right one

-certainly there are two kinds of environment: aquatic and terrestrial

-these environments differ because water, a liquid, replaces air, a gas

-therefore, physical properties of these environments are fundamentally different

-the rest of this classification is my own surmise with which you may freely disagree

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ENVIRONMENTS OF THE EARTH

(as proposed by Barry R. Taylor)

1. Terrestrial

Warm (Tropical) >>> Cold (Polar)

Wet (Rainforest) >>> Dry (Desert)

2. Aquatic

2.1 Marine

2.1.1 Continental Shelf

2.1.2 Surface Pelagic

2.1.3 Deepwater Pelagic

2.2 Freshwater

2.2.1 Lotic (Flowing Water)

2.2.2 Lentic (Standing Water)

3. Transitional?

3.1 Wetlands

3.2 Estuaries

3.3 Coral reefs

3.4 Intertidal Zones

4. Subterranean

1. Terrestrial Environments

-all terrestrial environments are fundamentally similar

-they have air on top and soil below,

-plants root in soil and grow upward toward the light

-difference between two biomes, e.g., desert and rainforest, is a difference of degree, not kind

-that is, they are both extremes on a continuum of the same basic kind of ecosystem

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I have indicated two kinds of aquatic ecosystems: freshwater and marine

-similar in that they are ecosystems based on water, instead of soil and air

-so we may expect broad similarities in ecology

-different groups of scientists study fresh waters (limnologists) and oceans (marine biologists)

-so there has often been little sharing of ideas and perspective

-are fresh waters and marine environments truly different kinds of ecosystems?

-yes and no

-fresh water and oceans differ according to the three S’s: size, salinity and stability

Size

-first and most important feature

-oceans are immense: they cover millions of square kilometres of the earth’s surface

-and since they are all connected, they are really one giant body of water

** most important: oceans are immensely deep

-average depth of the world’s oceans is about 3.5 kilometres

-deepest points on the earth surface are >6 kilometres deep

-one deep canyon in the Marianis Trench that goes down 11 km

-hence, the volume of the oceans is also great

-so whenever we think about the oceans, we have to think on a very large scale

-by comparison, freshwater lakes are tiny

-only about 20 lakes worldwide are extremely deep, with mean depth exceeding 400 m

-biggest lake of all is Lake Baikal in Siberian Russia,

(mean depth 740 m, max. depth 1600 m)

-which contains about 20% of all the fresh water in the world

-Lake Tanganyika in Africa is also very deep (max depth >1000 m)

-one basins of Lake Superior is about 300 m deep

-great majority of lakes cover perhaps 10-1000 ha and are less than 10 m deep

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Salinity

Variable Seawater Fresh Water

Total Salinity (g/L) 35 <1

3Main Ions Na, Cl Ca, Mg, HCO

-fresh water is called that because it is very low in dissolved salts

-by definition, fresh water contains less than about 1-3 g/L of salt

-most fresh water is very dilute, anywhere from zero to 0.5 g/L

-seawater has been accumulating salt for eons

-away from the continents, ocean water always has a salinity near 35 g/L

** ocean water is 10 x to 100x more salty than fresh water

** ocean water salinity is much less variable through space and time than fresh water

-every lake and river differs in salinity, which makes a big difference to biology

-but almost all the world’s oceans fall within a very narrow range of salinities

-ions composing the salts also differ, but composition is less important

Stability

-stratification of the oceans into three thermal layers has already been described

-stratification is driven by differential heating of surface waters and differences in salinity

-fresh water lakes also stratify,

-but this stratification is usually driven only by temperature

** most of the ocean rarely, if ever, mixes surface and deep waters

** most mixing occurs at downwellings in the polar regions in winter,

-and at coastal and equatorial upwellings

-most freshwater lakes mix completely twice to many times every year

-again, the scale of stratification is different:

-surface layers in oceans is ~100-200 m thick

-intermediate layer is about a kilometre thick

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-Lochaber Lake, Antigonish County, has a surface layer 10 m thick, typical for a deep lake

-many freshwater lakes are too shallow to stratify at all (average lake is <10 m deep)

-so vertical separation of communities in the ocean is much more fixed than in fresh water

Other Differences

-the three S’s are the big ones, but there are other differences:

Tides: significant tides are unique to marine ecosystems

-important because they cause a regular pulse of stress (desiccation) once or twice per day

-no freshwater habitat equivalent to the intertidal zone

-where immense amounts of ecological research has been carried out

-though as proportion of volume or area of the oceans, intertidal zone is very small

Running Water

-there is no real marine equivalent of streams and rivers

-freshwater environments with constantly moving water in a confined channel

-marine water flows sometimes, at upwellings, but the system is not really equivalent

2.1 Marine Environments

-I have subdivided the marine environment into three broad classes:

-continental shelf is the area of relatively shallow water off the coasts of the continents

-up to about 150 m deep

-may also be described as the subtidal zone (for benthos)

-or the neritic zone (for fish)

-continental shelves differ from the open ocean because:

(1) relatively shallow water is well mixed, and often has strong tidal currents

(2) nearness of the bottom allows seaweeds and other benthic organisms to create habitat, and

(3) river runoff from the continents lowers salinity and brings nutrients,

supporting high productivity

-these areas also have warmer and more variable temperatures than the open ocean

-all the world’s great fisheries are located on the continental shelf

** continental shelf is probably the nearest marine equivalent to a freshwater lake

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-open water, away from the continents is referred to as the pelagic zone

-surface waters here (epipelagic) supported entirely by phytoplankton, many extremely small

-these waters tend to be uniformly cold and unproductive

-because of the lack of nutrients from continents,

-and poor nutrient recycling in the unmixing ocean

-once nutrients are lost from the pelagic zone, they can sink four kilometres into the depths

-especially true in the mid-ocean gyres, far from land or upwelling zones

-This spooled image from NASA satellites shows the distribution of oceanic production

-mid-ocean epipelagic zones (deep violet) are great marine deserts,

-limited not by water but by nutrients

-notice the high productivity areas on the continental slopes around the world

-everything below the epipelagic zone, is the deepwater pelagic zone, or the deep sea

-from about 200 m depth all the way to the bottom

-marine biologists subdivide this zone into smaller sub-categories that need not concern us

-deep see comprises an enormous volume of water

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-this figure gives some perspective on size

-whether the twilight zone below the epipelagic is truly deep-sea or not is an open question

-deep sea is perpetually cold, dark, and under immense hydrostatic pressure

-there is no photosynthesis,

-life depends entirely on detritus from the epipelagic zone

-one of the least productive places on the planet

-also one of the least well known,

-and surprisingly, one of the most diverse

-although most of that diversity lies in the bottom sediments, the benthos

2.2 Freshwater Environments

-I have subdivided freshwater habitats into just running water and standing water

-why not subdivide surface and deep water as in the oceans?

-a matter of scale again

-deepest water of virtually all lakes is still shallow by marine standards

-and most lakes mix their entire volume, at least briefly for a few weeks every year

-so the whole lake is more or less like a continental shelf ecosystem, only shallower

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3. Transitional Environments

-finally, we have what I call the transitional habitats

-systems that have characteristics of both terrestrial and aquatic ecosystems

-inclusion of coral reefs here may be questionable,

-but complicated 3-D structure created by coral may make these reefs comparable to forests

-transitional environments have a number of unique characteristics that we will examine later

4. Subterranean Environments

-if we are being thorough, we should include life underground, down in the rocks

-controversy about how much subterranean life there really is

-some researchers claim to have found active bacteria living within inclusions in deep rock

-from the bottom of South African gold mines, never exposed to the surface

** recent work suggests even multicellular organisms may be found in water-filled inclusions

-if true, simple extrapolation suggests rock is by far the largest habitat on Earth

-it is certainly a very different habitat

** we might include deep-sea hydrothermal vents in this category

Question: I have lumped all terrestrial habitats together, and subdivided aquatic habitats

-I argue that differences in terrestrial habitats are a continuum,

-whereas aquatic habitats are qualitatively different

-is this decision valid? Are there more distinct kinds of aquatic habitats?

-or is this a result of my bias, or our lack of understanding of marine environments?

-certainly there are gradients between all the habitat types I defined for water

-is the difference between continental shelf and open ocean any greater

than between tropical forest and desert?

-I think so, but I am not sure.

-feel free to form your own opinion

-we know so little about the ecology of the open ocean that our ideas are likely to change

profoundly in the years ahead

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Lecture 6. The Water Cycle

-recall the four things that sunlight energy accomplishes on Earth:

(climate, oceanic circulation, water cycling, photosynthesis)

-we have dealt with the first two, climate and ocean circulation

-now let us look at the water cycle

Why does it matter?

-First, life on Earth is water-based

-therefore, distribution of water and its abundance are critical for terrestrial life

** terrestrial ecosystems are water-limited

-productivity of essentially all terrestrial ecosystems increases if one adds more water

-degree of water limitation varies, from minor (tropical rainforest) to extreme (desert),

-but it is a feature of all dry-land ecosystems

(some tropical rainforests are limited by rainfall but the cause is leaching of nutrients,

not too much water)

-by definition, water distribution is not an issue for aquatic life

** in all ecosystems, water dissolves essential nutrients

-and makes them available in dissolved forms to organisms

-water also transports nutrients in solution

-within ecosystems and from one ecosystem to another

-water also transports particulate matter, including organisms

Second, evaporation of water is a powerful processes transferring sunlight energy

to the atmospheric climate engine

-therefore evaporation is a key part of the Earth’s energy budget

Third, the hydrologic cycle has been severely altered by humans

-especially the amount and distribution of fresh water

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-both directly and indirectly,

-humans have changed water allocation across the continents

-sufficiently that nutrient cycles have been altered, vegetation has changed,

-and global climate has been permanently altered

Look at Text Figure 4.4, p. 101, for a summary of biologically pertinent features of

the hydrologic cycle

-cycle consists essentially in the transfer of water from the atmosphere to the soil

-and then back to the atmosphere

-water is transferred from the atmosphere to the ground by precipitation (rain or snow)

-for most terrestrial ecosystems, precipitation is the principal form of water input

-fog and condensation (dew) can be important in some special instances

-rain is liquid water that is immediately available

-snow is crystallized water that is not immediately available

-snow represents a means of water storage that becomes available in the spring

(or in Nova Scotia, in the middle of January)

-rainfall or snowfall has a number of possible fates

(1) it can immediately evaporate back into the atmosphere

-water has a high heat of vaporization: the energy needed to change from liquid to vapour

-therefore evaporation transfers both water and energy to the atmosphere

(this energy is released when the water condenses into clouds in the upper atmosphere)

-evaporation of water is an important mechanism of atmospheric warming

(2) water can infiltrate the soil, moving downward to join the soil water

-more often referred to as subsurface flow

-this is water in the soil near the surface, that is not permanently saturated

-some water may percolate downward to join the ground water

-this water is in deep soil and bedrock that is permanently saturated with water

-top of the ground water is the water table

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-amount of a particular rainfall that enters the groundwater depends on the infiltration rate

-how fast the soil can accept water

(3) when soil is saturated, or rainfall exceeds the infiltration rate, rainfall can pool

on the surface of the soil

-it then moves down hill as overland flow, which is here called surface runoff

-perhaps reaching a river or stream, which continues the downstream movement

-overland flow occurs only during and immediately after rainfall and snowmelt

-but ground water moves downhill as well, although much more slowly

-typical ground water velocities are measured in centimetres to metres per day

-if it reaches a stream or river, it contributes to the base flow

-base flow from ground water is the reason that streams keep flowing even when it isn’t raining

-base flow carries elements, nutrients and carbon dissolved from the soil

-therefore strongly effects the nature of the aquatic ecosystem

** overland flow and base flow to streams constitutes a one-way transfer to a different ecosystem

-though some ecologists would argue that small streams are not a different ecosystem,

-but rather a different environment within the larger ecosystem of the drainage basin

(4) water in the soil may be absorbed by plant roots

-it then moves upward in the xylem,

eventually being lost as transpiration from the leaves

-completing the hydrologic cycle

-transpiration can be a major mechanism of water transfer to the atmosphere

-flux from soil to air is usually combined as evapotranspiration

-evapotranspiration occurs before water reaches streams

-consequently, streamflow is more sensitive to annual differences in precip than is evapotrans

-see Text Figure 4.19, p. 118, for example from Hubbard Brook, New Hampshire

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Rainfall on Vegetation

-where the land is covered with forest, most rainfall hits trees before it hits the ground

-referred to as interception

-in closed-canopy forests, interception is significant,

-10-50% of rainfall, says the text

-substantial part of intercepted moisture may evaporate from the surfaces of leaves,

-never even reaching the ground

-so interception reduces the effective rainfall to a mature forest

-interception varies with the kind of forest, density, canopy complexity and season

“Interception” is also the title of a new spy thriller, probably staring Matt Damon

-intercepted rainfall that doesn’t evaporate continues to ground as throughfall and stemflow

-these sources are chemically different from rainfall

-they have acquired nutrients, salts and soluble organics from the leaves and stem

-amounts are typically small, but may be significant nevertheless

** epiphytes, such as lichens on tree trunks, depend entirely on stem flow for their nutrition

-so significance of stemflow depends in large part on the scale being considered

-what about non-forest ecosystems?

-we have studied water cycling in forests intensely

-much less known about grasslands, meadows, which are more difficult to study

-canopy interception and evaporation, stemflow etc. probably work same way in meadows

-in grasslands, who knows?

-in scrub and deserts, canopies are always too open to intercept significantly

Water in Soil

-will deal only briefly with behaviour of water in soil

-because this is covered in detail in Environmental Biology of Soils (Biology 474)

-water is stored in soil in the spaces (pores) between soil particles

-in larger pores, macropores, most water is held by gravity, like water in a cup

-hence called gravitational water or free water

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-in smaller, micropores, water is held by matrix forces

-recall water is a polar molecule,

-slightly positive at the H end, slightly negative at the O end

-soil particles tend to have net negative charges at surfaces

-this is particularly true of clays and soil organic matter

-these charges bind water molecules in layers around the soil particle

-this capillary water fills the micropores between particles

-when every pore in the soil is full, it is saturated

-happens in spring or after a heavy rain

-free water in macropores gradually drains down to the water table

-when all gravitational water has drained out, the soil is at field capacity

(an agricultural term reflecting the capacity of a field to hold water)

-at field capacity, only capillary water remains, though some is not strongly bound

-as soil dries, remaining water is held in progressively thinner films about soil particles

-water closer to the particle more tightly bound by matrix forces

-at permanent wilting point, remaining water cannot be extracted by plant roots

-water between field capacity and permanent wilting point defines the available water

-Text Figure 4.8, p. 105, shows relationship between total water and available water

for soils of different textures

-clay-rich soils hold more total water, but much of it is tightly bound, unavailable

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Water Movement Through Plants

-Textbook (p. 106-114) talks in detail about water movement through plants

-we will skip most of this because it is the province of physiologists

-instead, look at this from an ecosystem perspective

** plants are efficient conduits transporting water from soil to air

textbook says (p. 106):

“Water transport from the soil through the plant to the atmosphere takes place in a soil-plant-

atmosphere continuum that is interconnected by a continuous film of liquid water.”

(emphasis mine)

-even dry-land, terrestrial plants grow in a mileau of continously flowing water

-in a forest or a meadow, a constant flux of water from soil, through plants, to the atmosphere

-intermittently replenished by precipitation

-hence soil water is constantly being moved, exchanged, renewed

-plants are an intimate part of the water cycle

-plants actively extract water from soil,

-and transport water from root depth (to ~2 m) rather than the surface

-where there are no plants, relatively little loss of deep water to the atmosphere

** more importantly, all interactions between plants and environment are based on water

Roots and Water

-plant roots, besides providing anchorage, provide water and nutrients to the plant

-growing roots explore the soil, with the help of root hairs and mycorrhizae,

-and draw water up a water potential gradient

implications:

(1) only nutrients plants can acquire are in dissolved form

-solid nutrients are useless, regardless of concentration

-plant roots can bring nutrients into the root only dissolved in water

** above is not quite true: roots with mycorrhizae may have access to nutrients in organic matter

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(2) maximum rooting depth of plants in an ecosystem determines the volume of soil

supporting plant growth

-this limit has implications for both water supply and nutrition of the ecosystem

(3) water supply provides powerful medium of interaction among plants in a community

-competition for water among roots is nearly universal and easily demonstrated

-interactions over water may be far more complex, however

hydraulic lift is a good example

-see Text Figure 4.11, p. 108

-large trees like maples typically have deep roots for acquiring water

-and shallow, surface roots for acquiring nutrients

-during the day, water flows from deep roots, up a water potential gradient, out the leaves

-at night, when the stomata close, water flows up a truncated gradient:

-from the deep roots and out through the shallow roots into the surrounding soil

-this hydraulic lift is an important source of summertime water for understorey plants

-especially important in arid ecosystems,

-where grasses may depend on deep water lifted by creosote bushes (Larrea tridentata)

- also important for decomposition of organic matter, which is frequently water-limited

Large-Scale Water Cycling

-better perspective on the hydraulic cycle would look at movement of water on a larger scale

than shown in the earlier figure

-water enters a drainage basin (watershed) as precipitation,

-some is evapotranspired, some is stored in soil

-remainder moves overland or in ground water, to a stream

-stream water flows downstream to a lake or the ocean

-transporting energy, nutrients, organic matter and organisms along with it

**aquatic ecosystems still depend on the water cycle

-even though they are water limited

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-water cycle is essential to provide an inflow of fresh water

-rainfall is critically important to streams and small rivers,

-in which flow may be very seasonal

-most lakes depend on runoff from the basin, delivered by streams, for their water renewal

-direct precipitation on the lake surface is rarely important (exception: Lake Superior)

** just as important as water supply are nutrients and organic matter carried in it

-hydrologic cycle is responsible for the nutrition of virtually all aquatic ecosystems,

-from the smallest streams to the ocean

-even in oceans, production is much greater along the continental shelves,

-where runoff from rivers enters the oceans, bringing limiting nutrients with them

in the other direction:

-large surface area of lakes, and especially oceans, produces rapid rates of evaporation

-for most water bodies evaporation does not change volume significantly

-may be important in shallow, prairie lakes

-it certainly makes no difference to marine ecosystems

-but evaporation important for recharging atmospheric supply of water vapour to rain on land

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Lecture 7. Primary Production on Land

Background and Terminology:

-with few exceptions, all ecosystems depend on photosynthesis by green plants

-grass and trees and herbs on land, algae (phytoplankton) in water

-accumulation of organic material by photosynthesis (or similar) is primary production

-rate of accumulation of organic material is primary productivity

-organic material produced by photosynthesis is gross primary production or GPP

-material remaining after respiration is net primary production, or NPP

-material that is produced by primary production is biomass

-total amount of living plant material at a location is standing crop or standing stock

or standing crop biomass

** because most ecosystems rely on primary production as the ultimate energy source,

-rate of NPP matters a great deal,

-because that determines energy base of the ecosystem

** flow of carbon and energy through ecosystems are inextricably linked:

-carbon is vehicle through which energy flows through ecosystems

-so when we speak of NPP or GPP in units of carbon, we are also speaking of energy

-organic matter surprisingly constant in its carbon content:

-virtually all OM is 48% C, once the mineral component is removed

-hence units of carbon, energy or organic matter (biomass in text) used interchangeably

to describe primary production and energy flow in ecosystems

Net Primary Production

-Textbook devotes much space to GPP, which we will skip entirely

-GPP is almost impossible to measure directly in terrestrial ecosystems

-and all generalizations that apply to GPP apply equally to NPP

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-NPP is net carbon accrual by an ecosystem:

NPP = GPP minus plant respiration

-all plants respire to maintain tissues and carry out other functions associated with growth

**respiration a more or less constant fraction of GPP when measured at the ecosystem level

-despite the wide range of temperatures across the globe

-Respiration accounts for roughly half (48-60%) of GPP

Text Figure 6.4, (p. 161) illustrates this constancy

-constancy arises because plants adapt to temperature environment where they live

-Arctic plant at room temperature respires much more than a tropical plant at same temp.

-as a result, over the whole vegetation, respiration rates tend to be similar in all terrestrial

ecosystems at the mean temperature of the site

-NPP includes all the C produced by the plant, whether or not it gets to keep it

-plants are always losing tissue through herbivory, dropping leaves, flowers, fruits,

-roots, especially fine roots, die and are replaced very rapidly

-perhaps half the fine root volume of a tree is replaced each year

-roots release soluble organic carbon into the surrounding soil

-most root exudation is rapidly respired by soil microbes

-some goes to supporting symbiotic organisms, especially mycorrhizae

-finally, many plants release volatiles to the atmosphere

-small quantities, relative to other losses, perhaps 1-5% of NPP

-volatiles are responsible for the characteristic odours of the woods

-recall blue haze over pine forests: from volatile terpenes released from the needles

-most measurements of NPP measure only standing crop biomass

-that is, the amount of plant material harvested at a particular time

-these measures underestimate true NPP by at least 30%

-loss of tissue to herbivores, root exudates, fine root turnover are biggest sources or error

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-Text Table 6.2, (p. 162) shows some typical proportions of NPP losses

-each of these components varies widely

** losses to root exudation and supporting mycorrhizae can be substantial, but invisible

-consequently, measuring NPP of a terrestrial ecosystem is an enormously difficult task

-usually, small sources or error are ignored, and simplifying assumptions are used

e.g., total root production is assumed to equal aboveground production

Review:

-some important global patterns in NPP:

1. NPP is greatest in the tropics and decreases with increasing latitude

2. At all latitudes, production in forests > grasslands > deserts

-difference among ecosystem types approximately as great as difference among latitudes

3. Wetlands (swamps and marshes) are among most productive terrestrial ecosystems

-NPP comparable with tropical rainforest

4. Fresh waters are only moderately productive compared with dry land ecosystems

Controls on Production

-rate of NPP determined by balance between GPP and Respiration

-but because respiration appears to be a constant fraction of GPP,

then NPP and GPP are controlled by the same things

In terrestrial ecosystems, primary production is controlled by these factors:

Light

Water

Temperature

Nutrients

-which of these is most important depends on the place and the scale being considered

-all four factors interact in complicated ways that makes it difficult to sort them out

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Light

-gradient of light intensity from tropics to Arctic

-caused by shape of the earth and tilt on its axis

-sunlight at high latitudes in spread over a larger area, passes through more atmosphere

** but longer days at high latitudes compensate for lower light intensity

-therefore light intensity a relatively minor factor controlling NPP

-more important factor is season length

-in tropics, carbon acquisition occurs all year round

-at higher latitudes winter shuts everything down

-farther north or south we go, shorter the growing season

-season length is major environmental factor determining C acquisition among systems

-however, winter is a function of temperature

-and temperature itself is not primary factor: it is loss of free water to ice

-above freezing, effect of temperature less important because of compensation

-physiological (adaptation) or ecological (species replacements between ecosystems)

-coniferous trees, for example, are adapted to low temperatures,

-may photosynthesize on a mild day in winter

-Text Figure 6.8, p. 169 (right), shows correlation of NPP with temperature across

many mesic or hydric biomes

(hydric = wet; xeric = dry; mesic = moist)

-that is, dry ecosystems have been left out

-relationship is strong, but the slope is not steep

-because of compensation by species replacement and physiological changes

-secondarily, moisture availability creates the gradient within latitudes

Text Figure 6.8 (left) shows relationship between precipitation and NPP

-at any given latitude, leaf surface area (canopy closure) may be limited by water

-correlation with moisture is stronger than with temperature

-note relatively steep increase in NPP with increasing rainfall

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-primary production in most ecosystems is water-limited

-at very high rainfall, (>3 m/yr) NPP declines due to soil saturation, leaching of nutrients

-applies only to a small number of sites in wet tropics

-global distribution of water explains great range of production

-at same latitude as extremely productive sub-tropical forests, also find barren deserts

-hot deserts have lowest productivity on the planet, except for ice caps

2-water limits because plants must open stomata to take in CO and thereby lose water

-even with adaptations (e.g., CAM metabolism), water severely limits production in deserts

** dry periods shut down C acquisition as effectively as freezing temperatures

-effect is the same: a reduction in season length for photosynthesis

-but remember high temperatures at least partly cause lack of water in hot deserts

-deserts have highest temperatures on earth, but without water, NPP is almost nil

-illustrates again difficulty in separating these interconnected factors

-taking temperature and water together, climate is the dominant force controlling NPP

-NPP highest in warm, wet environments (tropical rainforest), least in cold or dry environments

-Nutrients perhaps least important factor, but can still be significant

- nitrogen, and less commonly phosphorus are the most commonly limiting nutrients

-but trace elements such as Zn, Fe, or K may also be limiting

** nutrients limit production on a local scale, less on a global scale

-they make difference between a bald hill and a fertile valley

-and difference between one ecosystem and the next

** much more influence from nutrients in aquatic ecosystems

Interactions among Controlling Factors

**environmental factors are always interconnected

-exact mechanisms of influence may be difficult to sort out

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Textbook gives excellent example of work in temperate grasslands:

1. Compared across regions, mean NPP increases with increasing precipitation;

-adding water (experimentally or in a wet year) to any grassland site augments NPP

-therefore, grasslands are strongly water-limited, which of course we knew

2. At least part of effect of moisture on NPP is indirect through decomposition

-fungi and bacteria in dry soil limited by moisture, which slows organic matter decay

-slows rate of nutrient release, which slows plant growth

-adding water speeds up nutrient recycling and augments production

3. Normally arid grasslands do not respond to additional water as much as mesic grasslands

-probably because they lack capacity to take advantage of high water supply

-insufficient photosynthetic biomass, or

-nutrient supply too low, or

-plant species adapted to dry conditions cannot grow faster when it’s wet

-therefore, long-term climate limits the ability of grasslands to respond in short term

** effect of moisture on NPP, in grasslands at least, is partly direct,

-and partly mediated through effects on soil moisture, nutrient supply, species composition

** work in other biomes suggest similar patterns:

-moisture, nutrients, temperature combine in complicated ways to determine NPP

-in northern ecosystems, boreal forest and tundra, soil N supply key:

-warming these cold ecosystems stimulates NPP,

-mostly through enhanced decomposition, which releases more N

-extension of the growing season also

Global Distribution of Biomass and NPP

-just as difference between GPP and Respiration determines NPP

-so the difference between NPP and tissue turnover determines ecosystem standing crop

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-among major terrestrial biomes of Earth, standing crop varies by 50 times

-Look at Text Table 6.5, p. 178

-note differences in root/shoot ratios, and low standing crop of farmland

-fields are harvested regularly before any biomass accumulates

-relatively high proportion of shoots in forests because forests grow where moisture is available

-drier sites support grassland and desert that have characteristically large root biomass

** enormous differences in standing crop largely vanish when expressed as NPP per day

-that is, correcting for differences in the length of growing season

-Text Table 6.7, p. 179 illustrates this relationship

-range of NPP/day (productivity) is only 3-fold, compared with 50-fold for biomass

** Therefore, length of growing season is biggest control on NPP

-if we adjust for leaf area, difference is even smaller

-differences in leaf area are consequence of climate and soil factors

-in the tropics, abundant nutrients, water, warm weather that support high leaf area

-limitations in deserts, grasslands or tundra lead to less leaf area and less NPP

** largest standing crops produce the largest NPP

Net Ecosystem Production

-from whole-ecosystem point of view, important question concerns ecosystem accrual of C

2-and its loss to the atmosphere as CO

-accumulation of organic carbon is referred to as Net Ecosystem Production

-Text Figure 6.1, p. 158, shows fluxes and pools of C in a terrestrial ecosystem

2-at the centre of it all is GPP, producing organic C from CO

-that fixed carbon has a number of fates:

(1) almost half is respired to maintain the plant tissues (including root respiration)

(2) rest constitutes net primary production,

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(3) which is ultimately shed or dies to enter the soil organic matter pool

2(4) SOM either enters long-term storage or is respired back to CO by microbes

(5) some may also be consumed by heterotrophs (grazers) which is ultimately also respired, and

(6) some lost from the ecosystem through leaching or disturbances (fire, flood, weather)

NEP equals GPP minus (respiration + leaching + disturbance)

note: (1) all animal consumption is lumped together here as heterotrophic respiration

(2) leaching and disturbance are smaller losses, most of the time, than respiration

-GPP that is not lost accumulates, as standing biomass or soil organic matter, as NEP

-at steady state, C inputs should equal C losses, and the NEP should be zero

(~50% plant respiration, 40-50% heterotrophic respiration and 0-10% leaching and disturbance)

-we expect NEP >0 in early successional ecosystems

-in practice, almost all ecosystems show either NEP > 0 or NEP < 0 at any given moment

-true across all time scales, from seasonal to many centuries

-seasonally, NEP > 0 during the growing season, NEP < 0 during the winter or dry season

-many (most?) ecosystems not fully recovered from last disturbance, still accumulating biomass

-some ecosystems undergoing continual disturbance show long-term NEP < 0

-Examples: farmland and recently drained peatlands

-bogs and other wetlands in which decomposition is inhibited, may have NEP>0 for millennia

** textbook cites example of a network of stations around the globe designed to measure NEP

2(actually, they measure net carbon flux as CO , so leaching is ignored)

-virtually all temperate ecosystems are showing NEP > 0, i.e., ecosystems accumulating C

-Why?

2- could be because of increased CO in the atmosphere

-or most sites are vigorous young forests that have not reached steady state

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