theories of vegetation change

Theories of Vegetation Theories of Vegetation ChangeChange

Mort KothmannMort Kothmann

Texas A&M UniversityTexas A&M University

Managing Plant SuccessionManaging Plant Succession

August 2003

October 2007

Fire + Herbivory

Vegetation change

Plant SuccessionPlant Succession

Plant succession is a result of the competitive Plant succession is a result of the competitive interactions of plant species and interactions of plant species and populations. Its effects are predictable. It populations. Its effects are predictable. It happens everywhere, not just on happens everywhere, not just on rangelands. Understanding plant succession rangelands. Understanding plant succession is a key concept to designing effective is a key concept to designing effective grazing management.grazing management.

Rangeland ConditionRangeland Condition

Composition of plant community at a specified Composition of plant community at a specified site site relative to potential benchmark relative to potential benchmark community community (climax or a desired plant (climax or a desired plant community) that could occur on that sitecommunity) that could occur on that site

Ecological SiteEcological Site

Distinctive kind of land with physical Distinctive kind of land with physical characteristics (climate, soils and topography) characteristics (climate, soils and topography) which differ from other sites giving rise to which differ from other sites giving rise to significantly different potential for the kinds and significantly different potential for the kinds and amount of vegetation it can produce. amount of vegetation it can produce.

(Note: this is essentially the same definition as (Note: this is essentially the same definition as Range Site except that it uses ‘land’ not Range Site except that it uses ‘land’ not ‘rangeland’)‘rangeland’)

Ecological Site DescriptionsEcological Site Descriptions

Adaptation of “poly” climax theoryAdaptation of “poly” climax theory Description of single plant community (Description of single plant community (edaphic edaphic

climax)climax) Geared towards grazing interpretationsGeared towards grazing interpretations Described at plant community scale and Described at plant community scale and

correlated to soil map unitscorrelated to soil map units

Names of Ecological (Range) Sites Names of Ecological (Range) Sites

Based on permanent Based on permanent physical features: physical features: Soil type Soil type TopographyTopography

Help users remember Help users remember significant rangeland in significant rangeland in their localitytheir locality Clay Upland, Saline, Clay Upland, Saline,

Limestone Breaks, Limestone Breaks, Claypan Savanna, Loamy Claypan Savanna, Loamy BottomlandBottomland

Time

Fire FrequencyGrazing Pressure

Probability & rate of woody plant establishment

A BC

Herbaceousretrogression

Perennialgrasses

Woodyplants

Com

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ity C

ompo

sitio

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Tim

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erg

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crem

ents

R

eq

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dri

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Shrubdriven succession

Graminoid-drivensuccession

A = Tall / mid-grasses

B = Mid / short grasses

C = Short grass / annuals

= Transition threshold

high lowhighhighlow

lowhigh

Archer

Juniper Replacing GrasslandJuniper Replacing Grassland

Note multiple age classes of juniper and weak stand of grasses

Prescribed BurningPrescribed Burning

FireFire

Drought – Sept 1999Drought – Sept 1999

Shrub dominated rangeland in transition towards Shrub dominated rangeland in transition towards grassland after a reclamation prescribed burngrassland after a reclamation prescribed burn

Grasses Replacing Juniper following FireGrasses Replacing Juniper following Fire

Sideoats gramma

Seasonal Vegetation DynamicsSeasonal Vegetation Dynamics(Aspect)(Aspect)

The following series of slides are photographs of The following series of slides are photographs of the same site made at 4-week intervals from the same site made at 4-week intervals from early March until mid December. Note that the early March until mid December. Note that the dominant species change as the seasons dominant species change as the seasons progress. In years with different weather progress. In years with different weather patterns, vegetation dynamics would differ.patterns, vegetation dynamics would differ.

Rolling Plains – Early MarchRolling Plains – Early March

Rolling Plains – Late MarchRolling Plains – Late March

Rolling Plains – Late AprilRolling Plains – Late April

Rolling Plains – Late MayRolling Plains – Late May

Rolling Plains – Late JuneRolling Plains – Late June

Rolling Plains – Late JulyRolling Plains – Late July

Rolling Plains – Late AugustRolling Plains – Late August

Rolling Plains – Late SeptemberRolling Plains – Late September

Rolling Plains – Mid OctoberRolling Plains – Mid October

Rolling Plains – Mid DecemberRolling Plains – Mid December