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Tree density and fire scarring in Minnesota oak savanna
- 1. Tree Density and Fire Scarring inTree Density and Fire Scarring in Minnesota Oak SavannaMinnesota Oak Savanna Implications for RestorationImplications for Restoration James G. Mickley – Kalamazoo CollegeJames G. Mickley – Kalamazoo College Dr. Clarence Lehman – University of MinnesotaDr. Clarence Lehman – University of Minnesota Cedar Creek Natural History AreaCedar Creek Natural History Area
- 3. 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
- 4. 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
- 5. Density Effect on ScarringDensity Effect on Scarring
- 6. • 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
- 7. • 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
- 8. • 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
- 9. Plots 1 3 13 15 5 10 11 24 Density(stems/plot) 0 25 50 75 100 125 150 175 200 High Fire Interval Low Fire Interval Unburned Low Initial DensityHigh Initial Density Current Plot DensitiesCurrent Plot Densities
- 10. Plots 1 3 13 15 5 10 11 24 Olaf's PercentageofStemsScarred 0 10 20 30 40 50 60 70 80 90 100 High Fire Interval Low Fire Interval Unburned Low Initial DensityHigh Initial Density Percentage of Scarred StemsPercentage of Scarred Stems
- 11. Distributions of Scar DirectionsDistributions of Scar Directions • Outermost circle: 10 cm DBH • Innermost circle: 100 cm DBH
- 12. • 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
- 13. Environmental Variables MatterEnvironmental Variables Matter = − − ατ2 x erf TfTo TfTc 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 R d R w 2 +=τ Residence Time Predicting Scarring Equations: Gutsell and Johnson, 1996
- 14. • 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
- 15. • 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
- 16. • 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
Editor's Notes
- Talk about what makes savanna: 2 layers (canopy and ground layer). Oak savanna used to cover a large swath of the midwest (including this area) and was a transition between eastern hardwood forest and prairie. Fire adapted ecosystem: fire maintains grass layer, of trees, only oaks can survive. At the time of settlement it covered about 11-13 million hectares, but this has dwindled below 3000 ha (0.02%) of quality savanna. This loss is due mainly to fire suppression or conversion to agricultural land.
- Two types of scarring. Non-contact is caused by higher temperatures on the leeward side of the tree. When there is wind or air flow, it pulls the fire around to the leeward side of the tree and creates vortices which hold the heat in. This also increases residence time. So one side of the tree gets a longer dose of hotter fire, which can often result in heat permeating through to the cambium and killing it.
- Happens when there is dead wood up against a live tree or closeby. Normal fires move through an area in minutes, but localized fuel can smolder and burn for half an hour or more, so there is a drastic increase in residence time, giving more time for heat to get to the cambium. With a big enough log burning for long enough, there is no limit to the size of tree scarred.
- Positive feedback of high density can lead to scarring in lots of trees, and “kill” the system before it becomes obvious because of the long lag time. What you end up with is a bunch of fallen logs in a prairie. Needs to be clicked through the animations to make sense. So what we wanted to do was look for density dependent levels of scarring. But you have to go back to initial density for that.
- This is an aerial photo from 2000 showing 4 of the 8 plots we surveyed. Note the drastic difference in density. In the 1930s, much of this area looked like the 3 plots on the right, which are currently burned frequently
- This area in the middle was open farmland in the 1930s, but by 1960 most of these trees were already showing up. By 1998 when restoration started, it was full of brush and aspens, which were removed. So we used this as a comparison that was highly structurally manipulated, and known to have very little scarring compared to Cedar Creek
- We wanted to see if high initial densities led to more scarring.
- Fire kills trees because higher fire intervals result in less trees. But there are density effects. Low initial density were significantly lower in both high and low fire interval.
- High initial densities did not have significantly higher scarring. But olaf’s is significantly lower than any burned plots at cedar creek.
- High initial density plots have uniformly distributed scar directions, while low initial density are more directional, especially in smaller size classes. Also, there are very few large trees that are scarred in low initial density plots. Talk about 3 size classes: 5-10cm, 10-30cm, 30-80cm.
- So using our scarring and tree health measurements, we explored whether or not there was a quantitative difference between contact and non-contact scars. This is done assuming that large trees are mostly contact and small trees are mostly non-contact.
- Without going into depth on these equations, I want to point out that here residence time is dependent on three things: flame front width, tree diameter, and rate of spread. You can see here that residence time increases as rate of spread decreases and as diameter increases. Now over here, we have an equation that links rate of spread to temperatures, thermal diffusivity of bark, and bark thickness. So this can be used to predict the residence time needed to reach Tc the temperature at which the cambium dies. And this leads us to the graph: Here we have rate of spread graphed vs the maximum diameter of tree scarred. And you can see how as rate of spread goes below 0.05 m/s, the size of trees scarred begins to increase drastically. There have been a few measures of rate of spread at cedar creek, and they show that rates do indeed go below 0.05 m/s. Since rate of spread is tied to wind velocity, this is telling us that we shouldn’t burn when the wind is too LOW or we’ll get large trees scarred via non-contact.