transient maxima hypothesis


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Fire, N availability, and plant response in grasslands a test of the transient maxima hypothesis :

Fire, N availability, and plant response in grasslands a test of the transient maxima hypothesis AYOUB AL-LOUZI

Temperate Grasslands:

Temperate Grasslands A large area of grasses , forbs, herbs, and shrubs, with very few trees. Receive low to moderate precipitation, most as snow in the northern hemisphere. Temperatures vary according to the season. In winter, can decrease below freezing in some areas. In summer, can reach above 32 °C .

Temperate grassland vegetation :

Temperate grassland vegetation Grasses adapted to cold temperatures, drought, and fires. Temperate grassland vegetation either short or tall In areas that receive little precipitation, grasses remain low to the ground. Taller grasses can be found in warmer areas that receive more rainfall.

Location of Temperate Grasslands:

Location of Temperate Grasslands North America and Canada. South America Rangelands Australia Russia and Eurasia South Africa

Range of Prairies:

Range of Prairies Tall-grass Mid-grass Short-grass Three types of grasses which dominate Prairies: Short-grasses , mid-grasses , and tall-grasses .

Kinds of Prairies:

Kinds of Prairies Short-grasslands Long-grassland Dry areas of grass hotter summers colder winters Wet very humid areas This helps the grass grow faster and higher

Major factors that effect prairie formation & maintenance:

Major factors that effect prairie formation & maintenance Fire Grazing Variable climate

Fire in tallgrass prairies:

Fire in tallgrass prairies Fire naturally occurring, is very important for development and existence of this ecosystem. Effects plant species composition , productivity , and nutrient cycles.


Post-Fire Fires burn only the uppermost part of grass plants, roots are unharmed. After a short time, the plant will grow again .

N cycle & Fire frequency :

N cycle & Fire frequency Fire frequency affect soil availability of inorganic N. Volatilization of N during burning of above ground biomass (N loss). Plant productivity limited by N availability, which varies with fire frequency.

Fire frequency and productivity:

Fire frequency and productivity Factor limit productivity Annually burned Unburned N limitation Low tissue N concentration Higher (NUE) Greater Increase in productivity (N addition) - - Energy limitation Shading effect of accumulated detritus Low soil water content - Greater ANPP -

Fire frequency and ANPP:

Fire frequency and ANPP Infrequently burned prairie (after long time without fire ) having the greatest rate of ANPP. Two hypothesis explain greater ANPP Enhanced N mineralization hypothesis. Transient maxima hypothesis.

Enhanced N mineralization hypothesis:

Enhanced N mineralization hypothesis

Transient maxima hypothesis :

Transient maxima hypothesis Ecosystem switches from energy (light & temperature) limited to soil N availability limited. Transient release from both light and N limitation .

Transient maxima hypothesis :

Transient maxima hypothesis

Test of the two hypothesis :

Test of the two hypothesis Measure inorganic soil N. Net N mineralization rate Plant productivity and N content. In prairie site annually burned, infrequently burned, and unburned.

Objective of the study:

Objective of the study To determine if higher ANPP after infrequent fire due to: Greater net N mineralization after fire. Combination of availability of N and energy.

Result and discussion :

Result and discussion Soil temperature and moisture effect nitrogen mineralization.

Soil moisture:

Soil moisture Soil moisture Greater in unburned relative to annually burned or burned 1x. Annually burn drier than other.

Concentration of inorganic soil N:

Concentration of inorganic soil N 1994 (unburned>burned 1x>annually burned) Lowland>upland. 1995 Burn 1x=unburned>burn 2x= annually burned. Conc.

Daily Net N mineralization rate :

Daily Net N mineralization rate 1994 Unburned >burned 1x> annually burned. 1995 Unburned > annually burned Upland > lowland

Cumulative net N mineralization :

Cumulative net N mineralization Soil mineralization in unburned >soil in annually burned. 1x intermediate #

Plant response (ANPP):

Plant response (ANPP) Low land > upland. Annually burned >unburned in low land but lowest in upland (H 2 O limitation). ANPP greatest in the burned 1x. Lowland > upland due to increase water limitation during dry year.

Live root biomass:

Live root biomass Great live root biomass in annually and x2 and lower in burned 1x. Fire increase live root mass.

Grass shoot and root tissue N concentration:

Grass shoot and root tissue N concentration Fire lower root and shoot tissue N content.

Conclusion :

Conclusion Repeated frequent burning lower soil nitrogen availability, but increase the degree of plant productivity. Burned 1x and unburned have similar organic N pools initially. But decrease soil water availability after fire maybe reducing net N mineralization rate after infrequent fire.

Conclusion :

Conclusion Enhanced ANPP after infrequent fire

The end :

The end

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