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The Heat is On: Texas Tree Response to Fires and Drought
Joseph D. WhiteProfessor of Biology
Department of BiologyBaylor University
Drying from West to East (approximately less 5″)
Average Precipitation 1951‐2006 Average Precipitation 2050
Heating from Northwest to Southeast (approximately 10°F)
Average Temperature 1951‐2006 Average Temperature 2050
Low precipitation + Higher temperature = Low humidity
WaterForestWoodlandSavannaShrub/grasslandBarrenCloud
Unsupervised Classification of MODIS data
Area of conversion from woodland to savanna
Tree Mortality• Drought induced die back• Disease susceptibility• Sapling failure
And then there is fire…
Waco
0
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1500
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Acres (10
0x)
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Acres (10
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2% of Travis Co. woodlands2011 Drought Impacts
2011: Satellite Detection of Global Fires
Fires follow drought. Woodlands can go extinct with long term drought.
Fire Effects on Urban Trees
• Part I– Fire in the Texas landscape– Direct Effects– Indirect Effects
• Part II– Fire adaptation in the wildland urban interface– Fire‐sensitive arboriculture
The Fire Triangle and Trees
• Fuels– Dead branches– Downed branches and
dead vegetation litter near the bole base
– Vines • Insolation or exposure
increases temperature of the tree environment
• Wind and loosely packed fuels and/or litter increase aeration
Fuel Oxygen
Temperature
• Surface fires generally burn hottest on the opposite of the tree from the direction of the fire– Chimney effect
• This super‐heating is lethal to cambial tissue at specific locations
• Living cells surrounding the wound grow around dead tissue resulting in “catface” scars
• Bole wounding due to fire may result in long‐term heart rotTexas red oak collected at the
Balcones Canyonlands National Wildlife Refuge near Austin, Texas
Fire scarsInterior rot
Bole Damage due to Fire
Bark Thickness, Bole Diameter, and Fire Survivorship
Data redrawn from Harmon, 1984. Ecology, 65:796‐802.
Canopy Scorch
• Surface fires will also consume lower foliage either due to direct fire contact or from the thermal radiation from the fire
• Scorch height is predicted from the intensity of the fire (as an indirect measure of the fuel), temperature, and wind speed
Scorch model from Van Wagner, 1973. Canadian Journal of Forest Research, 3:373‐378.
Indirect, Post‐Fire Effects
• Low intensity fires– Reduce understory vegetation and increase mature tree vigor by reducing competition
– Release nutrients from litter and downed woody materials
– Induce resprouting from root stock of basal sprouting species (fire as a signal?)
– Produce charcoal, 10% of which will be incorporated into the soil with improvements in soil water holding capacity and nutrient recycling
Woodlands, Fire, and People
• Fire return intervals range from 5‐20 years across Texas landscapes
• The wildland urban interface (WUI) is an expanding front of housing development from cities and towns which are facing increasing influence from wildland fires
• Increased personal wealth and the desire to live near or in a woodland setting are positively correlated
Mandate for Fire Adapted Communities
• Community wildfire protection plans (CWPP) may become mandatory for the near future as insurance losses associated as houses destroyed by fires in the WUI increase with time
• CWPP may use Firewise protocols that mandate clearing trees from within 30’ from homes– http://firewise.org/
• In addition, tree species (and other landscape plants) may need to be more fire resistant both as part of a Firewise plan with houses fire adapted
Defensible Space
• Within the 30’ space– Trees >10’ (no overhang)– Tree and woody vegetation spacing 15’
– Prune trees minimum 6’ and remove understory vegetation
• Keep landscape well‐watered
10’
30’
15’
6’
Firewise: An Arborists To‐Do List• Removing trees close to
housing structures• Safe pruning of lower
limbs that act as ladder fuel for potential wildfires– Prudent disease
management• Selection of low fire hazard
tree species• Consideration of trees that
survive various types of fire events
What is a Low Hazard Tree?
• Low volatile oil composition in foliage• High water content of leaves during drought• Low energy content of wood
Definition: hazard – condition that increases the chance of loss
Volatile Oils
• Terpenoid compounds produced by tree to protect the leaves from high temperature stress in open sunlight
• In a distilled, pure form, the average flash temperature of plant‐secreted oils is 95°F!– Fortunately most oils are in low concentration and diffuse in the canopy
HIGH
Bald cypressJuniperAcaciaPersimmonMagnoliaLoblolly Pine
MEDIUM
HornbeamMaple/BoxelderSweetgumElmWalnutDogwoodPecanChinaberryCottonwoodBeechOakAshCherry
LOW
LocustWillowHackberryTulip treeSassafras
Tree Leaf Oil Content and Ignitability
Derived from:Geron, et al. 2000. Atmospheric Environment, 34:1761‐1781
HIGH
HackberryDogwoodMapleBald cypressWillow
MEDIUM
OakElmSassafrasAshPecanLoblolly pineSweetgum
LOW
JuniperCottonwoodCherryMagnoliaAcacia
Tree Leaf Water Sensitivity to Drought
Derived from:Abrams et al., 1994. Ecology,75:123‐133Bazzaz, 1979. Annual Review of Ecology and Systematics, 10:351‐371.Petaki et al., 1998. Tree Physiology, 18:307‐315Yordanov, et al., 2000. Photosynthetica, 38:171‐186
HIGH
PineJuniperDogwoodOakOsage orange
MEDIUM
Maple/BoxelderCottonwoodHornbeamLocustBeechCherryElmAsh
LOW
WalnutCatalpaHackberryBasswoodHickorySycamoreTulip treeSassafras
Tree Wood Energy Content
Derived from:California Energy Commission, www.consumerenergycenter.org
Wood EnergyWood Energy
Dogwood
OilOil
Juniper
Low Leaf Water Low Leaf Water
Bald cypress
Potential High Fire Hazard Fire Trees?
Some Candidates for Low Fire Hazard Species?
• Tulip tree (Liriodendron sp.)– Oil (L), Wood Energy (L)
• Sassafras (Sassafras sp.)– Oil (L), Wood Energy (L)
• Locust (Robinia sp.)– Oil (L), Wood Energy (M)
• Cherry (Prunus sp.)– Drought (L), Wood Energy (M)
• Cottonwood (Populus sp.)– Oil (M), Drought (L), Wood Energy (M)
• Walnut (Juglans sp.)– Oil (M), Wood Energy (M)
Crown Bulk Density: Another Element of Fire Hazard of Trees
• Mass of leaves per unit volume of canopy
• Measure of packing density of live fuels
• Species/canopies with higher density will have lower rates of fire spread in canopies
Influence of Species on Crown Bulk Density
Ashe’s juniper
Plateaulive oak
Shin oak
Texas red oak
CBD (g/m3) 18.15 21.16 24.29 26.04MaximumCBD (g/m3) 64.88 66.81 71.60 86.41
worse betterFire Hazard
However! Pruning within a species reduces hazard due to total fuel load reduction (e.g. dead branches)
Summary• Long term future drought will continue woodland die back
in Central Texas• Fire follows drought• Damage from fire:
– Dependent on bark thickness– Windspeed and temperature during fire– May induce heartwood disease
• Arboriculture in the WUI is an ongoing and expanding future concern– Will include selective tree harvesting, pruning, and canopy
modification– Some species less hazardous than others based on leaf oil,
response to drought, and wood energy (need research!)