Tree Density and Fire Scarring in Tree Density and Fire Scarring in Minnesota Oak SavannaMinnesota Oak Savanna

Implications for RestorationImplications for Restoration

James G. Mickley – Kalamazoo CollegeJames G. Mickley – Kalamazoo CollegeDr. Clarence Lehman – University of MinnesotaDr. Clarence Lehman – University of Minnesota

Cedar Creek Natural History AreaCedar Creek Natural History Area

Oak SavannaOak Savanna

Non-Contact ScarringNon-Contact Scarring

• Wind causes a temperature increase on the leeward side• Increases the fire’s residence time• Most commonly studied type of fire scarring

Contact ScarringContact Scarring

• Caused by high localized fuel supply (eg. fallen logs)• Logs burn long after fire has moved on

– Residence time increases drastically– Can scar even large trees with thick bark

• Not well documented in literature

Picture Credits: Clarence Lehman

Density Effect on ScarringDensity Effect on Scarring

• Originally savanna and scrub (Pierce 1954).

• Burn program started in 1964

• Compartments with varied fire intervals

Cedar CreekCedar Creek

• Intervals varied from annually to 1 in 8 years and unburned controls

• No treatment other than prescribed burns

• Originally farmland, grew back into savanna

• Restoration started in 1998

Olaf’s SavannaOlaf’s Savanna

• More actively managed– Bulldozed to remove aspen and sumac– Dead wood and dead trees removed

• Initial Density counts from old aerial photos• Survey 8 plots (3/8th ha each) at Cedar Creek

– Full Factorial Design:• Initial Density (High, Low)• Fire Interval (High, Low, None)

• Survey ~2 ha at Olaf’s Savanna• Census trees and measure scar variables such as height, width,

direction etc.

MethodsMethods

Plots

1 3 13 15 5 10 11 24

Den

sity

(ste

ms/

plot

)

0

25

50

75

100

125

150

175

200

High Fire IntervalLow Fire IntervalUnburned

Low Initial DensityHigh Initial Density

Current Plot DensitiesCurrent Plot Densities

Plots

1 3 13 15 5 10 11 24 Olaf's

Per

cent

age

of S

tem

s S

carr

ed

0

10

20

30

40

50

60

70

80

90

100

High Fire IntervalLow Fire IntervalUnburned

Low Initial DensityHigh Initial Density

Percentage of Scarred StemsPercentage of Scarred Stems

Distributions of Scar DirectionsDistributions of Scar Directions

• Outermost circle: 10 cm DBH• Innermost circle: 100 cm DBH

• Assuming that the largest size class is primarily contact scars, then contact scars…– Are farther from the ground– Cover a larger fraction of the tree’s

circumference– Result in less healthy canopies, and could

possibly have more severe effects

Predictors of Scar TypePredictors of Scar Type

Environmental Variables MatterEnvironmental Variables Matter

2xerf

TfToTfTc

Variables• w = flame front width• R = rate of spread• d = tree diameter• Tc = initial cambium temp• Tf = fire temperature• To = lethal cambium temp• x = bark thickness• α = thermal diffusivity const.• τ = residence time• erf() = Gaussian Error Function

Rd

Rw 2

Residence Time Predicting Scarring

Equations: Gutsell and Johnson, 1996

• Scarring is prevalent in savanna at Cedar Creek• Initial density still plays a role.• Scars on smaller trees are directional (non-

contact scars)• Scars on larger trees are not directional (contact

scars) and incidence is affected by initial density• Both types of scars could potentially be

minimized by different restoration methods.

ConclusionsConclusions

• Alternative management through structural manipulation

• Rate of spread measurements• Decrease in lifespan due to scarring• Closer look at contact scarring

– How do contact scars impact trees?– How drastic is the increase in residence time?– How close does a fuel source need to be?– Do higher fuel loads result in more scarring?

Future ResearchFuture Research

• Dr Clarence Lehman• Cedar Creek Staff• Dr. Binney Girdler• Interns:

– Allyn Dodd– Heidi Bulfer– Drew Ballantyne– Spencer Agnew

• Thesis review team at Kalamazoo College

AcknowledgementsAcknowledgements