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B. Cohen, P. N. Oml, and M. R. Kaufmann 1
Abstract--Transpiring pine, spruce, and fir bt'l:mches were connected to potometers and senne were treated with transverse cuts to Induce a wide range of transport resistances. The branches then were heated in an oven at 225"<> C. During heating branches without transverse cuts transported water at significantly greater rates comparee! to their preheating transpiration rates, and branches which were treated with transverse cuts transported water at lesser rates than branches without transverse cuts.
A wildland fire in which the flames enter the vegetation canopy and consume its fue·ls is known as a crown fire. Crown fires account for only a small proportion of all forest fires, but because they are often so devastating, they are normally responsible for a disproportionately large share of the annual costs and damages associated with wildfires.
Conifer forests are a common setting for crown fire events.
The crown of a live conifer tre·e consists of an abundance of needles and twigs (fuels) which are continually undergoing change.s in their water relations (Chrosciewicz 1986, Fuglem and l\furphy 1980, Jameson 1966, Philpot and l\futch 1971,
Van Wagner 1967). In the annual water conte·nt profile of live conifer fuels there is commonly a period in spring when an apparent wate.r deficiency (stre.ss) is evident. Hough (1973),
Van \\Tagner (1977). and Springer and Van Wagner (1984) have speculated that historical occurrences of conifer crown fire outbreaks in spring is linked to this period of water stress.
The only published results from experiments relating the water status of live conifer fuels to ignitability are by Quintilio
(1977) and Van \\'agner (1963). In both experiments, severed saplings were subjected to flame·s in a laboratory and a high correlation was found between foliage water content (expressed as percent dry weight and often referred to as moisture content) and the amount of fuel consume.d. For dead fuels, water content may be a good me·asure of ignitabHity, but for live fuels both the mass of water and the mass of dry matter are variable (Gary 1971, Pharis 1967). Thus, a measure of live fuel water relations encompassing more than percent wate·r content is probably desirable for assessing ignitabi.lity of live fuels.
Water e.nters and leaves living foliage in response to environmental and plant conditions (Kramer 1983). Rothermel and Susott (pers. comm., 1986)2 hypothesized that live conifer fuels, when heated, may be capable of drawing upon water stored elsewhere in the tree and thereby forestall ignition. If live fuels have such a capability, a water relations index which is not only independent of dry matter variations, but which also incorporate.s a measure of water transport, may be a better indicator than water content of a live crown's susceptibility to ignition.
Using standard transport law, studies of water transport into tree· foliage and of evaporation of water from the foliage surface have indicated that flow in both cases is regulated by a gradient or driving force and by resistance to flow (Fiscus et aI1983). For wate.r supply to the foliage, the gradient typically is taken to be the difference in water potential between the point of supply and the foliage, and the resistances are those associated with liquid phase flow of water through xylem tissue. For eva.poration from the leaf surface, the gradient is a va.por pressure deficit or a. vapor concentration difference from leaf to air, and the resistances are those associated with vapor flow from the water surface in the tissue to bulk air outside the foliage.
In the field environment, the rate of water loss from foliage by transpiration is nearly equal to the rate. of water transport into the foliage by the xylem. Small differences in these rates during the course of a day contribute to changes in foliage wate·r content and to the development of midday water stress.
\\'e suspect that during a fire, foliage tissue rapidly becomes exposed to abnormal conditions i.n which the vapor gradient increases enormously because of leaf heating. In addition, hi.gh temperatures may sharply reduce the resi.sta.nce of the foliage
1Warren B. Cohen and Philip N. Omi, Graduate
ResE~arch
Assistant and Professor, respectively, Department of Forest and Wood SGiencE~s,
Colorado State University, Fort Collins, CC); M. R. Kaufmann, Principal
Plant Physiologist, Rocky Mountain Forest and Range Experiment Station, USFS, Fort Collins, CO.
:~R.
C. Rothermel and R. A. Susott, Project Leader and Research
Chemist, respectively, Fire Fundamentals Project, Intermountain Fire
Sciences Laboratory, Missoula, MT.
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surfa.ce to water vapor loss. Under these condi.tions, evapora·· tion from the foliage surface would increase dramatically. If the rate of water transport into the foliage remains the same as it was before heating, the tissue will dry rapidly, and i.f the temperature. be.comes high enough, the tissue will ignite ..
Based upon this interpretation, the capacity of tissue. to protect itself from ignition depends upon the ability of the transport system to supply wate.r to the foHage. In this study, the role of water transport into tissue exposed to elevated temperatures is related to the initial rate of transport into the tissue and the initial foliage water potential. In addition, visual characteristics of the foliage are. related to the transport rate during heating. The results from this study could provide guidelines for the development of an index relating foliage water relations within a conife.r canopy to its susceptibility to ignition. l\laterials and 1\lethods
During the period from February to June, 1987, freshly cut boughs from three conifer species, lodgepole. pine
var.
Engelm.), Engelmann spruce
Parry ex. Engelm.), and subalpine fir
(B ook.) Nutt.), were brought to a laboratory weekly and refrigerated in plastic bags. As needed, branches were cut from the boughs and placed in specially designed potometers
(figs. 1 and 2). The. potometers supplied water to the branches continually and permitted branch transpiration rate·s to be monitored.
Branches were chosen by spe.cies in groups of four. They were selected according to size and health crite.ria, with the intent of selecting branches which were similar in form and vigor; branch selection was somewhat limited, however, which
Figure 2.--An assembled potometer. Branch water uptake Is measured In the barrel of the uptake measurement syringe. As needed, water transported by the branch Is replenished with water from the recharge syringe. restricted application of the size crite.rion. The approximate ranges of length and diameter inside the bark of the branches were between 160 and 240 mm and between 2 and 4 mm, respectively. After attachment to the potometers, branches were allowed to stabilize for one day. To obtain a variety of water stress leve.1s in the branche.s, on the second day, some branches were given two opposing transverse cuts (after
Mackay and \Veatherley 1973) to a depth of approximately
70% of their diamete.rs. The separation of the cuts varied between 4 and 7 mm. The intent of the cuts was to inhibit flow and thus c.ause· a decrease in wate·r potential (an increase in stress). The other branches were left untreated. The laboratory environment included diffuse lighting, room temperature of approximately 23° C, and relative humidity which varied between 30% and 45%.
On the third day, branch transpiration rates were determined for a 6- to lO-minute interval. 'Vater potentials were measured using a pressure chamber technique; for spruce and fir, branch tips were used (removing less tha.n 10% of the leaf area) and for pine, needle fascicle.s were selected. Immediately after recording water potentials and transpiration rates, the branches were placed into the exhaust port of a convection oven which was set at a temperature of 225 0 C, with the potometers supported outside of the. oven. "Tate.r uptake. by the branches during heating was monitored every minute for twenty minutes. A total of 14 pine, 19 spruce, and 24 fir branches were heated. Visual observations during heating were made on 8 branches of each species. Foliage color and surface appearance were noted every minute.
Figure 1.··Potomete.· used te) monitor branch water uptake before and during heating. It Includes: (top, lefi to right) 'i ste)pwa,tch, a glass tee, (.clockwise) a sho.1plece of flexible tUbing, an a ute)·· matlc pipet tip, two rubber serum stoppers; (middle) an uptake measurement syringe with microliter resolution; and (bottom) a recharge syringe.
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Rf~SUItS
When branches were set up in potometers and given time to equilibrate, their transport ra.tes rarely varied more than
15(10 per hour. When branches without transverse cuts were

placed in the oven, theIr water uptake increased substantially.
Figure 3 shows the average rate of water uptake for the branches without transverse cuts, during 20 minutes of heatlng, normalized to the maximum rate during that time. Normalization minimized the effect of variability in leaf area and water potential from branch to branch. Initial transpiration rates prior to heating are sh(wln at minute O. Prior to heating, pine transported water at only 2% of the maximum uptake rate attalne.d, which occurred around minute 6. Spruce and fir had prior uptake rates of 8 and 19% of maximum, respectively.
While actual rates varied widely among species and with branch size, the pattern for normalized rates was similar for all three species when their capacity for flow was not initially inhibited by transverse cuts.
Foliage color during heating' changed consistently among species in relation to position on the normalized water uptake curves (figure 3). All branches were green when first placed in the oven and by minute 1 were glistening. Glistening was associated with a decrease in water uptake. After minute 1, the foliage began to turn yellow and rose its glistening appearance.
At this time, the rate of water uptake began to increase sharply.
As the foliage began turning brown, water uptake quickly levele.d off near the peak rate. After brown needle.s dried and became brittle, water uptake still occurred at rapid rates, de.clining only to 50% of the maxi.mum in 9 minutes for pine and 13 - 14 minutes for spruce and fir,
Branches which had been treated with transverse cuts increased theiI rates of uptake during heating compared to their preheating transport rates, but had much lower rates relative to branches without transverse cuts. Figure 4 shows the uptake rates for the cut versus uncut branches for each species. A t test on the. peak uptake rates for branches treated with transve·rse cuts against those. without cuts revealed that uptake differences were significant at the J)1 level for all species. The lower uptake rates by the cut branches are probably associated with increased resistances to flow from
Spruce
• uncut
+ cut
Pine
• uncut t cut
Fir
• uncut
+ cut
6 8 10 12
Time (minutes)
14 16 18 20
100
90
QI
'"
§"
80
70
~ 60
:a:
3
50
E
'E
40
%
':; 30
.... c
~
20 a.. 10
Figure IJ.--Average water uptake rates for cut and uncut branches. the cuts. The progression of color change during heating was generally more rapid for cut than for uncut branches.
In an attempt to determine if water loss from the branch's woody surface also influences water uptake during heating, a few branches of each species were heated just after removing all foliage. Impetus for this came from the fact that uptake continued even after all of the needles on the heated branches became brown and brittle. During heatjng, all defoliated branches transported wate.r at higher rate.s than prior to heati.ng, although rates were highly variable. Thus, it appears that the. branch surfac.e also may influence the amount of water transported during heating.
Discussion o
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 1R 19 20
Time (minutes)
Figure 3.--Rate.of water uptake during heating normalized over the maximum I'ate attained. AV(~l'age I'ates for uncut bmnches, lind foliage color chang(~s associated with regions on the uptake curves are shown.
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Pine, spruce, and fir branches inc.rease.d their water transport rates when heated. On a normalized basis~ all 3 species underwent similar rates of change during the full 20 minutes of heating. The capability to increase water transport rates during heati.ng supports the hypothesi.s that live. c.onifer fuels may be able to draw water from the tree bole during heating and thereby forestall ignition,

On a normaliz.ed basis, foliage color changes were associated with changes in rates of water uptake for all three species.
Because the progression of color change was more rapid on branches '~'i.th transverse cuts, it appears that restrictions of water supply during heating also may cause more rapid damage to canopy foliage. Additionally, since the transverse cuts caused reductions in both peak and total water flow, water uptake during heating may be closely related to resistances to flow within the xylem tissue.
Initial foliage water potential prior to heating was not correlated well with water uptake. during heating. Part of the reason for this is due to variations in branch size; ie, larger branches generally transport. more water than smaller branches, all else being equal. Laboratory conditions also may have contributed to the poor water potential effect. Lighting intensity in the laboratory was low relative to the natural foliage environment. Be.cause of this, foliage stomata probably remained partially closed, inhibiting transpiration, and thereby permitting the mainten~nce of a relatively high wate·r potential. Although the wate·r potential of branches treated with transverse cuts used in this study was not always lower, the cuts
~lpparent1y increased transport resistance enough to sharply reduce water uptake during heating. In observations on branches moved temporarily into full sunlight, water potential generally decreased and transport increased. In future experiments, branches wi.ll be kept outdoors in more natural lighting conditions for at least an hour prior to heati.ng, and water uptake will be expressed on a unit leaf area or xylem surface area basis. These improvements in technique should he.lp to improve the possibility of discovering if initial wate·r potential is a good indicator of water uptake during heating and/or ignition.
The study reported here will. be continued using a specially designed calorimeter which will ignite live branches while measuring the energy required for ignition. The calorimeter will be used both on detached branches connected to potometers and on in situ branches. Experiments on the branches in potometers should permit the development of a relationship between water transported during ignition and the energy required for ignition. Water transported by in situ branche.s can then be. inferred from the measured ignition energy.
A foliage water relations index which incorporates a measure of water transport may be useful in assessing a canopy's susceptibility to ignition. Field studies, using in situ branches, are needed to validate this assertion.
Fiscus, E. L., A. Klute, and 1\1. R. Kaufmann. 1983. An interpretation of some whole plant water transport phenomena. Plant Physiol. 71:819817.
Fuglam, P. L. and P. J. 1\1urphy. 1980. Foliar moisture and crown fires in Alberta. Alb. Energy and Nat. Res. Rep.
No. 158.
Gary, H. L. 1971. Seasonal and diurnal changes in moisture content and water deficits of Englemann spruce needles.
Bot. Gaz. 132(4):327332.
Hough, W. A. 1973. Fuel and weather influence wildfires in sand pine fore.sts. USDA For. Servo Res. Pap. SE 106.
Jameson, D. A. 1966. Diurnal and seasonal fluctuations in moisture content of pinyon and juniper. USDA For. Servo
Res. Note RM-67.
Kramer, P. J. 1983. Vt.' ate·r relations of Plants. Academic I'ress,
Orlando.
Mackay, J. F. and P. E. Vt.'eatherley. 1973. The effect of transverse cuts through the stems of. transpiring woody plants on water transport and stress in the leaves. J. Exp.
Bot. 24(78):15 28.
Pharis, R. P.1967. Seasonal fluctuations in the. foliage- moisture content of well-watered conifers. Bot. Gaz. 128 (3-
4):179-185.
Philpot, C. Vt.'. and R. W. Mutch. 1971. The seasonal tre.nds in moisture content, ether extractives, and ene.rgy of ponderosa pine and Douglas-fir needles. USDA For. Servo Res.
Pap. INT-102.
Quintilio, D. 1977. Lodgepole pine flammability. Can. Dept.
Env. For. Rep. 5(2):7.
Springer, E. A. and C. E. Van Wagner. 1984. The seasonal foliar moisture trend of black spruce at Kapuskasing,
Ontario. Can. For. Servo Res. Notes 4(3):39-42.
Van Wagner, C. E. 1963. Fla.mmability of christmas trees.
Can., Dept. For., Pub. No. 1034.
Va.n Wagner, C. E. 1967. Seasonal variation in moisture content of eastern Canadian tree foliage and the. possible effect on crown fires. Can. Dept. For. & Rur. Dev., For.
Br. Dept. Pub!. No. 1204.
Van Vt.'agner, C. E. 1977. Conditions for the start and spread of crown fire. Can. J. For. Res. 7:23-34.
Acknowledgement
Literature Cited
Chrosciewicz, Z. 1986. Foliar moisture content variations in four coniferous tree species of central Alberta. Can. J.
For. Res. 16:157 162.
The authors acknowledge the assistance of 1\1anuel H.
Martinez, of the Rocky Mountain Forest and Range Experiment Station, for his assistance in collecting branches and making them available for use in these studies. Debbie Vt.'right
Olson and Joseph Stevens also are thanked for their assistance throughout.
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