PRODUCERS

READINGS: FREEMAN, 2005

Chapter 54 Pages 1229-124

Producers are autotrophs.

• Autrotrophs are organisms that can make their own food - complex organic molecules - from CO2.

• Such organisms include green plants and cyanobacteria (blue-green algae).

• These organisms use the energy of the sun to produce their own food from CO2 and H2O.

• Ecologists call these organisms producers.

Producers and Photosynthesis

• The carbon (C) in organic molecules is found at very low concentrations in the atmosphere.

• The process by which producers use CO2 to make organic molecules is called photosynthesis.

An Overview of Photosynthesis

• Beginning in the 1770s experiments showed that the green parts of plants in the presence of sunlight, water, and carbon dioxide could release oxygen.

• By the 1840’s it was known that carbohydrates (sugars) were produced and a rough formula for photosynthesis could be written.

(CO2) 6(H20)12

C6H1206

(O2)6(H20)6

SUNLIGHT

Conversion of Light Energy into Chemical Bond Energy

• Photosynthesis concerts light energy into chemical bond energy by adding carbon, oxygen and hydrogen atoms to existing 5 carbon compounds

• This process increases plant mass, as measured by dry weight (biomass).

• Many other biomolecules and mineral elements are required for biomass production.

Photosynthesis Takes Place in Chloroplasts

• At the plant level, photosynthesis takes place primarily in the leaf.

• Each leaf contains millions of chloroplasts.

• The chloroplast is the site of photosynthesis.

Summary of Photosynthesis

• Green plants use light energy to convert carbon dioxide and water into sugar and oxygen.

• Sugars produced in photosynthesis are converted into biomolecules that make up the dry weight (biomass) of a plant.

• In short, plants are able to make themselves (organic molecules) from inorganic molecules (carbon dioxide and water).

Atoms, Biomass and Nutrients

• C,H,O,N are the major atomic building blocks of living things. Their rank order of mass is:

0 > C >> H > N >> 50 or so others

• A comparison of biomass with the make-up of the earth leads to the conclusion that life forms concentrate certain atoms.

ATOMIC COMPOSITION OF VASCULAR PLANTS

• Recalling that biomass refers to dry weight, around 90% of plant biomass comes from carbon dioxide (CO2) in the air.

• H from water is the most abundant atom, but it is only 6% of plant dry weight (biomass).

ATOM % of BIOMASS

O ~45%

C ~45%

H ~6%

N ~1.5%

50+/- ~2.5%

ESSENTIAL PLANT NUTRIENTS

• Macronutrients (those that make up 0.1% or more of biomass and, thus, required in relatively large quantities):

O, C, H, N, K, Ca, Mg, P, S, (SI)

• Micronutrients (those that make up 0.01% or less of biomass and, thus, required in small quantities):

CI, Fe, Mn, Zn, B, Cu, Mo, Ni, (Na), Co?, (Se)?

See Table 37.1 on page 854 In Freeman (2005) for a more complete description of essential nutrients

Phosphorus Deficiency in Corn

• Phosphorous in the form of phosphate is a major ingredient in plant fertilizers.

• P deficient plants may remain greener than normal and develop a purple discoloration on leaves.

• Phosphate is an important constituent of DNA, RNA, ATP, and NADP.

Nitrogen Deficiency in Corn

• Nitrogen deficiency results in young plants that are stunted in growth and pale green to yellow.

• N deficiency that occurs later results in a yellowing of the lower leaves.

• N is an important element in amino acids (proteins) and nucleic acids (DNA. RNA, ATP, NADP).

Nitrogen (N) is the 4th major contributor to biomass.

• The element nitrogen (N) makes up about 6% of plant dry weight.

• Nitrogen (N2) is approximately 80% by volume of the atmosphere. Yet, plants can not take in and utilize N2 by way of leaves.

• Only bacteria are able to fix and convert atmospheric nitrogen (N2) into forms that plants can use - ammonia (NH3) or nitrate (NO3) .

SOIL

• Soil is the environment that provides the mineral nutrients for plant growth and development.

• It is a complex of inorganic particles, organic materials and living and dead organisms.

• During the process of soil development, the residues of plants, microbes and animals return more than the green plants take away.

Soil Testing for Plant Nutrients

• Soil testing for macronutrients is a common practice among gardeners and agriculturists.

• Simple soil test kits give crude determination of N, P and K. The major ingredients of fertilizers.

SOIL NITROGEN

• Stores of soil nitrogen can be quite high. Total N can reach 760 gN/m 2 in a tallgrass prairie. Rich forest soils can be as high as 550 gN/m 2 .

• Two sources of nitrogen are lightning and nitrogen-fixing bacteria.

NITROGEN FIXATION AND LEGUMES

• Nitrogen fixation occurs when certain bacteria convert dinitrogen (N2) into ammonium (NH4

+) .

• Legumes are a large family of plants that form a mutualism with nitrogen fixing bacteria.

Plant biomass increases as N increases and then levels off.

• Plants of Old Field Goldenrod were grown in pots that contained total soil N that varied from150 to 1650 mg N / kg of dry soil.

• Higher soil nitrogen yielded greater plant biomass up to about 1000 mg N / kg of soil and then biomass remained more or less constant.

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Intraspecific Competition for Nitrogen

• Since nitrogen is an important nutrient resource, one might expect that individuals compete for N.

• These experimental results confirm this prediction.

• Note that high density plants remain small at all N levels. Why?

Interspecific Competition for Nitrogen

• The same experimenters examined two more species: stiff golden rod (top) and little bluestem.

• Of the three, stiff golden reached maximum size at lowest N concentration; little bluestem at highest.

• What would you predict concerning the outcome of interspecific competition between stiff golden rod and little bluestem?

MAJOR FACTORS THAT INFLUENCE PRODUCTION

• As we have seen, nutrient availability can influence production as measured by biomass or seed set.

• Given the importance of water in photosynthesis, it is also a major factor in influencing production.

• Lastly temperature, particularly associated with day length and seasonality, influences production.

PRECIPITATION AND PRODUCTION

• The effect of precipitation (water) is seen as one travels from east to west through the NA grassland biome.

• The tallgrass prairies of Illinois (top) receive about 36 inches per year.

• Those of western Kansas only about 15 inches.

BIOMASS AS A MEASURE OF PRODUCTION

• Biomass is a universal measure of production.

• The change in weight of a cactus plant over a year can be used as a measure of production.

• Also, the change in weight of vegetation in a square meter of a desert from one year to the next can be used as a measure of production.

NET PRODUCTION

• All plants not only increase in mass through photosynthesis, but like other living things they use some of that stored energy for respiration.

• That which goes unused is called net production.

NET PRODUCTION, GROSS PRODUCTION & RESPIRATION

• Plants accumulate matter (and energy) through photosynthesis (gross production).

• Plants use matter (and energy) during respiration (respiration).

• Net production = Gross production - Respiration• Production (Gross or Net) is either expressed

in units of mass (g /m2 /year) or energy (kcal /m2 /year).

MEASURING NET PRODUCTION

• Field measurement of net production entails random plot assignment, clipping and sorting vegetation, drying and weighing plant material.

• Data is often reported as grams of biomass per square meter per year.

Net production can be used to answer a variety of experimental questions.

• Question: Does fall burning decrease net production in a prairie community?

• Null Hypothesis: No difference between burned and unburned plots.

• Method: Burn a random sample of plots; do not burn a random sample.

• Conclusion: Reject null hypothesis. Results suggest that burning actually increases net production. Why?

NET PRODUCTION IN SOME MAJOR BIOMES

BIOME RANGE AVERAGE

Desert & Semi-desert 0-250 40

Artic & Alpine Tundra 10-400 140

Coniferous Forest 400-2,000 800

Deciduous Forest 600-2,500 1,250

Grassland 200-1,500 600

Tropical Forest 1,000-3,500 2,000

Net Production / Unit Area (grams/meter 2 /year)

PRODUCERS

READINGS: FREEMAN, 2005 Pages 1229-1242