Chemical Brush Control: Assessing the Hazard

Reprinted from the JOURNAL OF FORESTRY, Vol. 69, No. 10, October 1971 Reproduced by USDA Forest Service for official use.

PLEASE DO NOT REMOVE FROM FILES CHEMICAL BRUSH CONTROL: ASSESSING THE HAZARD

hazard from the use of any chemical requires consider- ABSTRACT—An adequate evaluation of the hazard asso- ation of both the likelihood of exposure and the toxicity ciated with the use of any chemical agent requires consid- of the chemical (15). eration of both the toxicity of the material and the potential for exposure of nontarget organisms. The hazard can be high only if both the toxicity of the chemical and the Likelihood of Exposure to Herbicides potential for exposure to a significant dose are high. The The likelihood that a nontarget organism will be relatively large doses of 2,4-D, amitrole, 2,4,5-T, and exposed to a significant dose is determined by the picloram required to produce acutely toxic responses in behavior of the chemical. Behavior is the initial dis- most nontarget organisms are not likely to occur from nor- tribution, subsequent movement, persistence, and fate mal chemical brush control operations on forest lands. The short persistence, lack of biomagnification in food chains, of chemicals in the environment. Chemical behavior and the rapid excretion of these herbicides by animals dictates the magnitude and duration of exposure and preclude chronic exposure and, therefore, chronic toxicity. thus the nature of a toxic response. A long history of field use and research shows our com- Herbicides applied aerially are distributed initially mon brush control chemicals can be used with minimum among four components of the forest environment—air, hazard to the quality of our environment. vegetation, forest floor, and surface waters. The amount of chemical entering each portion of the environment is determined by the chemical, equipment used, condi- P reoccupation with the projected needs of the nation tions of application and environmental factors. for wood fiber has obscured similarly increasing de- mands for forage, water, wildlife, and areas for purely Distribution in Air recreational purposes. If we accept the premise that all these needs must be satisfied, we must compensate for a Appreciable amounts of herbicide may be dispersed decreasing production base for wood products by in- I A toxic dose is one that causes an adverse effect; it is not creasing unit productivity of land devoted primarily to restricted to a lethal dose. timber production. Fertilizers, insecticides, and herbicides are important tools in increasing forest productivity, but the use of these chemicals has risks as well as benefits. Therefore, we must know in advance the consequence of each practice involving their use. The purpose of this paper is to assess the hazard to nontarget organisms from the routine use in forest brush control of four common herbicides: 2,4-dichlorophenoxyacetic acid (2,4D); 2,4,5-trichlorophenoxyacetic acid (2,4-5-T); 3-amino-1,2,4-triazole (amitrole); and 4-amino-3,5,6-trichloropicolinic acid (picloram). Logan A. Norris The hazard of using a herbicide is the risk of adverse effects on nontarget organisms. Two factors determine the degree of hazard: (1) the toxicity of the chemical and (2) the likelihood that nontarget organisms will be exposed to toxic doses. Toxicity alone does not make a chemical hazardous. The hazard comes from exposure THE AUTHOR is principal chemist, Forestry Sci. Lab., Pac. North- to toxic doses of that chemical. Even the most toxic west Forest and Range Exp. Sta., U.S. Forest Serv., Corvallis, chemicals pose no hazard if organisms are not exposed Ore. Adapted from a paper presented at the 1970 SAF National to them. Therefore, an adequate assessment of the Convention, October 13. OCTOBER 1971 715

Fig. 1. Recovery of 2,4-D, amitrole, 2,4,5-T, and picloram from red alder forest floor material. Amitrole, 2,4-D, and 2,4,5-T applied at 2 pounds per acre and picloram at 0.5 pounds per acre in water (19).

120

100

1c"-- 70 a) U cr) 60 LEGEND cc>- 50 w 0 2,4-D CD 40 U A Amitrole w CC El 2,4,5-T 30 Picloram 20

20 40 60 80 100 • 120 140 160 180 TIME (days) in the air in the vapor phase or as fine droplets called land. Air dispersion of herbicides can be minimized by "drift." Studies in western Oregon revealed that from taking full advantage of recent advances in spray 20 to 75 percent of some aerially applied herbicides did technology, equipment and chemicals. not reach the first intercepting surfaces.2 The hazard from this loss is probably minimized by atmospheric Distribution in Vegetation dilution and the fact that most chemical brush control The amount of spray material intercepted by vegeta- operations do not involve large, contiguous blocks of tion depends on the nature and density of the vegetation, the physical characteristics of the spray material, 2 Michael Newton, Logan A. Norris, and Jaroslav Zavitkov- and the rate of application. A single-level community, ski. 1966. Unpublished data. Sch. of Forestry, Oregon State like grass, may offer maximum concentrations of herbi- University, Corvallis, Ore. • cide to organisms which use this vegetation as a food source. Table 1. Residues of Herbicide 1 in Forage Gross. In tests with 2,4-D, picloram, and 2,4,5-T, highest Herbicide residue concentrations in forage grasses were found immediate- Weeks after treatment 2,4-D1 2,4,5-P Picloram3 ly after spraying (Table 1). The herbicide levels de- clined markedly with time due to growth dilution, parts per million 0 100 100 1 35 60 60 2 50 30 32 Selective control of competing species with herbicides can 4 30 15 increase the growth rate of suppressed conifers. 8 6 6 24 16 2 16 52 3

Rate of application-1 pound per acre. s Data from Fig. 4 of Morton et al. (16). _ _ ‘,, Data from Table 5 of Getzendaner (9). -- HERB IC ME \ APPLICATION Table 2. Disappearance of Herbicides from Mammals. \ ,. Herbicide Mammal Excretion Reference • — — — — percent ofdose in days 2,4-D Rat 95 1 (14) Sheep 96 3 (3) 2,4,5-T Mouse 76 (23) Cow 89 4 (21) Amitrole Rat 95 I ( 7) Picloram Cow 98 4 ( 8)

716 J OURNAL OF FORESTRY Table 3. Acute Toxicity of Herbicides.1 Organism 2,4-D 2,4,5-T Amitrole Picloram Birds: LD,,o, mg/kg 360-2000 300 2000+ 2000+ No effect, ppm2 7203 600 2000+3 1000 Rodents: LD,,o, mg/kg 375-800 400-950 5000+ 2000 No effect, ppm2 1500 8003 2000+3 3000 Ruminants: LD50, mg/kg 400-800 500-1000 2000 No effect, ppm2 24003 1200 20003 Other mammals: LD5o, mg/kg 100 100 1200+ No effect, ppm2 500 2003 2000+3 Fish: TL m, ppm, 1-60 1-30 325 13-90 No effect, pprn5 0.16 0.16 326 1.06 Other aquatics: TLm, ppm 1-5 0.5-50 20 1+ No effect, ppm, 0.1 6 0.056 26 0.16 A list of references for specific values in this table is available from the author. Concentration in diet for a limited exposure period which causes no acute effect. Assumes daily food consumption is 10 percent of body weight and that 20 percent of LD:,o in daily diet has no acute effect. 48-hour median tolerance limit, i.e., the concentration of herbicide in the water which will kill 50 percent of an exposed population of aquatic organisms in 48 hours. Concentration in water which has no acute effect following 48 hours exposure. 6 Assumes 10 percent of TL m has no effect. metabolism, excretion of herbicide from the plant roots, Distribution in Forest Floor (9, 16). The half- and weathering of surface residues The forest floor is a major receptor of aerially life of amitrole was about one day in sugar beet, corn, applied herbicides. The behavior of herbicides in the and bean leaves (16). Amitrole residues completely forest floor will determine the magnitude of the dose disappeared from sugar cane in eight weeks, even after and the duration of exposure to soil microorganisms, applications of up to 40 pounds per acre (10). These the amount of Chemical available for uptake by plants, reports and the resprouting of brush which commonly and the potential for movement of herbicide residues occurs a year following spraying on forest lands suggest into water. that high residues of 2,4-D, amitrole, 2,4,5-T, and picloram do not persist for long periods in vegetation. Chemicals in the forest floor may be either volatilized and reenter the atmosphere, adsorbed by soil colloids Intense competition for light, water and nutrients can and organic matter, leached through the soil profile, markedly reduce regeneration success. Herbicides can re- absorbed by plants, or degraded by chemical or biologi- duce vegetation density and permit conifer establishment. cal processes. The physical properties of amitrole and picloram prevent their evaporation from the forest floor. The phenoxy herbicide esters could volatilize, but the rapid hydrolysis of these esters to nonvolatile forms may restrict this process (2). Degradation is the only means by which the total load of an environmental pollutant can be reduced. Ami- trole, 2,4-D, picloram, and 2,4,5-T are degraded in forest floor material (19). In one study with red alder (Alma rubra Bong.) forest floor material, 80 percent of the amitrole, and 94 percent of the 2,4-D were degraded in 35 days (Fig. 1). One hundred and twenty days were required to degrade 87 percent of the 2,4,5-T. Picloram degradation was slow-33 percent in 180 days.

Distribution in Water Stream contamination is one of the most immediate, and perhaps the most important, expressions of contamination in the forest environment. Herbicides may be applied directly to surface waters either intentionally or due to spray materials drifting from adjacent areas. This type of contamination will occur for only a short time during application but may result in high concentrations of pollutants in streams. The movement of herbicides to streams due to leaching through the soil profile or in mass overland flow during periods of intense precipitation could introduce low levels of

OCTOBER 1971 717 Summary of Herbicide Residues in Forest What have these research efforts shown about the quantities of 2,4-D, amitrole, 2,4,5-T, and picloram which occur in the forest following chemical brush control operations? Significant quantities of aerially applied herbicides are dispersed in the air and do not reach the target area. About 100-300 ppm herbicide will be in vegetation shortly after application. These residues will decline to low levels in a few weeks. These herbicides are subject to degradation in the forest floor. Picloram may persist for more than one year while the rest are more rapidly degraded. Herbicide concentrations in excess of 0.1 ppm are seldom encountered in streams close to treatment areas even immediately after spraying. Res- idues higher than 1 ppm have never been observed in more than 7 years of monitoring efforts. Herbicides persist for only short periods in streams near treatment areas. The concentration is rapidly reduced with downstream movement of water. 5. Long-term, low-level pollution of forest streams with herbicide residues has not been observed except from treatment of marshy areas.

The analytical chemist can measure minute quantities of Toxicity of Herbicides herbicides in samples from the forest. Monitoring for her- • bicide residues is a necessary part of some spray projects. There are two kinds of toxicity. Acute toxicity is the rapid response of organisms to a few large doses of chemical received over a short period. Chronic toxicity is the accumulation of effects resulting from exposure to chemical for extended periods of time leading to injury many small doses over a long interval. Thus, the nature of aquatic organisms. of the response is determined by the magnitude and The levels of herbicides in the aquatic environment duration of the dose the organism receives. were determined in an extensive study of stream contam- It has been determined that aerial application of ination from spray projects on range and forest lands 2,4-D, amitrole, 2,4,5-T, and picloram for brush con- in Oregon (17). Some herbicide residues were found in trol resulted in certain levels of herbicide residues all streams near treated areas. Peak concentrations which persisted for various periods in different parts of occurred shortly after application but seldom exceeded the forest environment. We must now consider the 0.1 ppm. Peak concentrations in streams which ran toxicity of these herbicides to determine whether or not adjacent to but did not enter spray units seldom exceed- their residues pose a hazard to the inhabitants of the ed 0.01 ppm. Herbicide residues persisted for only a forest. few hours in nearly all streams. Application of herbi- I have estimated no-acute-effect3 doses for mammals cide to marshy areas may result in high-level, long and birds based on either doses in chronic feeding persistence of chemical residues in nearby streams. studies which have shown no effect on the organisms or Special care must be taken to avoid treatment of such a daily chemical dose equal to 20 percent of the oral areas. LD,„ 1 in the diet when daily food intake is equal to 10 The short-term, high-level contamination which re- percent body weight. The no-acute-effect dose for aquat- sults from the direct application of herbicide to the ic organisms is based on 10 percent of the TL„,.5 water surface can be markedly reduced by excluding These are conservative estimates, and the true threshold streams from treatment areas. in other words, if you level required for an acute response is probably greater. dont want herbicides in the water, then dont put them The dose required in a chronic exposure to cause a there. Long-term, low-level contamination of Oregon forest 3 No-acute-effect—no detectable effect during or for several days after the exposure period. streams by herbicides has not been observed even 4 LD,„—the size of a single oral dose necessary to kill 50 during periods of heavy precipitation in the first fall and percent of the test organisms. The LD is usually expressed winter after spraying (18). This supports our under- as milligrams (mg) of chemical per kilogram (kg) of body standing that surface flow or mass overland flow of weight of the organism. water (arid therefore, chemicals) seldom occurs on 5 TL,„—rnedian tolerance limit. The concentration of chemical in water necessary to kill 50 percent of the test aquatic forest lands and that herbicide leaching is a slow organisms during a specified exposure period. The is us- process which can move only small amounts of a ually expressed as parts of chemical per million parts of water chemical a relatively short distance. for 24, 48, 72, or 96 hours of exposure. 718 JOURNAL OF FORESTRY detectable effect depends on the distribution, persist- only for indicating that a particular compound has the ence, and fate of the chemical in the organism. Contin- potential for this type of biological activity. The forced uous exposure to doses of 2,4-D, amitrole, 2,4,5-T, and feeding, subcutaneous injection, and the use of exotic picloram which approach the threshold for an acutely solvents in dosing laboratory strains of animals bear no toxic response is necessary to cause a chronic response resemblance whatsoever to the type of exposure orga- in many animals. nisms may receive in actual practice. Amitrole, 2,4-D, picloram, and 2,4,5-T do not persist Based on LI350 values, dioxin is 10,000 to 600,000 or accumulate in mammals (Table 2). All four herbi- times more acutely toxic than 2,4,5-T. But in the cides are nearly completely eliminated in the feces and commercial formulations presently available, there is urine within a few days after feeding. Thus, the only over 2,000,000 times more 2,4,5-T than dioxin. Recent way a chronic response can develop is by continuous studies with production grade 2,4,5-T (less than 0.5 fresh exposure to large quantities of these chemicals. ppm dioxin) did not show statistically significant inci- Toxicity of 2,4-D dences of teratogenic effects in mice or rats at dosage The acute toxicity of 2,4-D is fairly uniform among levels up to 50 mg 2,4,5-T per kg body weight per day species (Table 3). Levels of 2,4-D from 500 to 1,500 on days 6 through 15 of the gestation period (12). ppm can be tolerated for short periods in the diet of the These animals should be able to tolerate levels of organisms listed, but these levels are not likely to occur 2,4,5-T near 500 ppm in their diet with no teratogenic in the forest environment from normal brush control effects if the daily food intake is 10 percent of body projects. The no-acute-effect level of 0.1 ppm for weight. Application rates of one or two pounds per acre aquatic organisms has seldom been approached in the of 2,4,5-T and the rapid decline of the herbicide field even for brief periods (1, 17). residues in vegetation preclude acute or chronic ex- Table 4 shows the dose of 2,4-D that animals can posure to 500 ppm 2,4,5-T in the diet of nontarget tolerate with no effect despite chronic exposure. The organisms in the forest environment. short persistence of 2,4-D prevents exposure to doses of these magnitudes. With cattle, for instance, intake of 50 Toxicity of Amitrole mg 2,4-D per kg body weight per day for 112 days The acute toxicity of amitrole is very low (LD 50 to produced no effect (20). On this basis, a 1,000-pound rats is 5,000 to 25,000 mg/kg). (Table 3). In 1959, cow could consume all the herbicide for more than amitrole was the subject of much controversy when low 1,120 square feet of freshly treated (2 pounds of 2,4-D levels of the herbicide were found on cranberries just per acre) land each day for more than 3 1 /2 months and before the Thanksgiving holidays. Some studies report- show no effect. Of course, in the field, all the herbicide ed that chronic exposure to high levels of amitrole which is applied does not reach the vegetation, and resulted in thyroid carcinoma. Total emphasis on toxici- 2,4-D residues will not persist for long periods following ty and failure to consider the likelihood of exposure to usual forest spraying practices (Table 1). a significant dose put the incident out of focus. Toxicity of 2,4,5-T Large doses of amitrole administered for prolonged The acute toxicity of 2,4,5-T is only slightly greater periods can seriously impair thyroid function which than the toxicity of 2,4-D (Table 3). may lead to malignancy (Table 4). Male rats fed 50 The no-effect or threshold level for an acutely toxic response is unlikely to occur in either the terrestrial or Table 4. Chronic Toxicity of Herbicides) aquatic portions of the forest en- Herbicide Equivalent vironment. Animals are able to tol- and concentration erate chronic exposure to fairly large organism Dose in diet Duration Effect doses of 2,4,5-T (Table 4). mg/kg ppm days The use of 2,4,5-T is under close 2,4-D: Mule deer 240 2400 30 Slight scrutiny because of reports that it is Cattle 50 500 112 None teratogenic (fetus-deforming) (4). Sheep 100 1000 481 None 2,4,5-T: These early studies were confound- Dog 10 100 90 None ed by high levels (27 ± 8 ppm) of Cattle 125 1250 15 None the highly biologically active con- Sheep 100 1000 481 None Amitrole: taminant 2,3,7,8-tetrachlorodibenzo- Rat 476 None p-dioxin (dioxin) in the samples of Rat 100, 730 Thyroid adenomas 2,4,5-T used in the experiments. adenocarcinoma Currently, available commercial for- Rat 5003 119 Normal followed thyroid mulations of 2,4,5-T contain less by 14 days than 0.5 ppm dioxin. Other tests are no amitrole interpreted by some toxicologists as Picloram: Sheep 110 1100 30 None indicating that both 2,4,5-T and di- Dog 150 1500 730 None oxin are teratogens, but others do Rat 1000 90 None Quail 500, three None not agree (5, 12). generations We must realize that the tech- A list of references for specific values in this table is available from the author. niques used to demonstrate terato- 2 Assumes food intake is 10 percent of body weight per day. genicity in the laboratory are useful Actual concentration tested.

OCTOBER 1971 719 - - residues expected to occur. The use of hazardous chemicals must be restricted. However, those chemicals which present minimum hazard to man and the inhabitants of the forest environment must remain available for proper use. I am satisfied after evaluating the toxicity and behavior of 2,4-D, amitrole, 2,4,5-T, and picloram that the proper use of these herbicides will not normally result in either an acute or a chronic hazard to nontarget organisms on forest lands. My conclusion has a broad base of research results and is also supported by a long history of safe use of these herbicides on forest lands.

Literature Cited

BOND, C. E., R. H. LEWIS, and J. L. FRYER. 1959. Toxicity of various herbicidal materials to fishes. In: Biological problems in water pollution. U.S. Dep. Health, Educ. Welfare, Pub. Health Serv. Tech. Rep. W60-3. p. 96-101. BURCAR, P. J., R. L. WERSHAW, M. C. GOLDBERG, and L. KAHN. 1966. Gas chromatographic study of the behavior of the isooctyl ester of 2,4-D under field conditions in North Park, Colorado. Anal. Instrum. 4: 215-244. LARK, UNT, I4 •16 . - C D. E., J. E. YOUNG, R. L. YOUNGER, L. M. H and J. K. MC LAREN. 1964, The fate of 2,4-dichlorophenoxyacetic Aerial application of herbicides for brush control is an acid in sheep. J. Agr. Food Chem. 12: 43-45. exacting task. Careful attention to selection of the herbi- COURTNEY, K. D., et al. 1970. Teratogenic evaluation of 2.4.5- T. Science 168: 864-866. cide, location of treatment unit boundaries, and conditions . 1970. Summary report from NIEHS. Nat. Ins.. during application are necessary to achieve both effective Environ. Health Sri., Research. Triangle Park. NY...C. control of target species and minimum impact on other FANG, S. C., E LIZABETH FAWN, and SUC II ITRA K HANNA. 1967. The metabolism of labeled amitrole in plants. Weeds 15: 343- parts of the forest environment. 346. , M. GEORGE, and T. C. YU. 1964. Metabolism of 3-amino-1,2,4-triazole-5-0 , by rats. J. Agr. Food Chem. 12: 219-223. ppm amitrole for 68 weeks had enlarged thyroids after FISHER, D. E., L. E. ST. JOHN, W. H. G UTTENM ANN, D. G. WAGNER, and D. J. LISK . 1965. Fate of Banvel T. ioxynil, 13 weeks (22), but these symptoms were reversible Tordon and trIfluorilin in the dairy cow. J. Dairy Sci. 48: since normal thyroids were found in rats fed 500 ppm 1711-1715. GETZENDANER, M. E., J. L. H ERMAN, and BART VAN GIESSEN. amitrole for 17 weeks, followed by two weeks of no 1969. Residues of 4-amino-3.5,6-trichloropicolinic acid in grass exposure to the herbicide before sacrifice (13). from applications of Tordon herbicides, J. Agr. Food Chem. 17: 1251-1256. The acute exposure levels of amitrole required to H ILTON, H. W., and G. K. H YEII ARA. 1966. Determination of 3-amino-1,2,4-triazole residues in sugarcane. J. Agr. Food inhibit thyroid function are probably not present in the Chem. 14: 90-94. forest environment even immediately after spraying. House. W. B., L. H. G OODSON, H. M. G ADBERRY, and K. W. DOCKTOR. 1967. Assessment of ecological effects of extensive The short persistence of amitrole precludes chronic or repeated use of herbicides. Midwest Res. Inst. Proj. 3103-B. exposure. Adv. Res. Prof. Agency Order 1086, Dep. Def. Contract No. DAHC15-68-C-0119. 369 p. JoitNsort, JULIUS E. 1970. Testimony before subcommittee on Toxicity of Picloram energy, natural resources and the environment of the com- mittee on commerce, Apr. 7 and 15, 1970. In: Effects of 2,4,5- Piclorarn is considerably less toxic than either 2,4-D T on man and the environment. U.S. Congr. Ser. 91-60, p. 360-404. or 2,4,5-T (Table 3). Picloram is used at Considerably JUKES. T. H., and C. B. S HAFFER. 1960. Antithyroid effects lower rates than the phenoxy herbicides, and the likeli- of aminotriazole. Science 132: 296-297. K HANNA, SUCH ITRA, and S. C. FANG. 1966. Metabolism of C"- hood that nontarget organisms will be exposed to labeled 2.4-dichlorophenuxyacetic acid in rats. J. Agr. Food acutely toxic doses is practically nonexistent (11). Chem. 14: 500-503. MONTGOMERY, MARVIN L., and LOGAN A. NORRIS. 1970. A pre- Studies of the chronic toxicity of picloram show orga- liminary evaluation of the hazards of 2,4,5-T in the forest environment. USDA Forest Serv. Res. Note PNW-116. Pac. nisms are able to tolerate high levels for extended Northwest Forest Range Exp. Sta. 11 p. periods without adverse effects (Table 4). MORTON, HOWARD L., E. D. ROBISON, and ROBERT E. MEYER. 1967. Persistence of 2,4-D, 2.4,5-T, and dicamba in range forage grasses. Weeds 15: 268-271. Toxicity to Soil Microorganisms NORRIS, LOGAN A. 1967. Chemical brush control and herbicide residues in the forest environment. In: Symp. proc. herbicides High concentrations of herbicides are required to and vegetation management in forests, ranges and noncrop lands. Oregon State Univ., Corvallis. p. 103-123. adversely affect soil microorganisms and their activities, 1968. Stream contamination by herbicides after which are important in maintenance of soil fertility fall rains on forest land. Res. Progr. Rep., West. Soc. Weed Sci.: 33-34. (11). A one pound-per-acre application will result in . 1970. Degradation of herbicides in the forest floor. about 3 ppm herbicide in the surface inch of soil. This In: Tree growth and forest soils. C. T. Youngberg and C. B. Davey, eds. Oregon State Univ. Press, Corvallis. p. 397-411. is well below the level harmful to microbial popula- PALMER, J. S.. and R. D. RADELEFF. 1964. The toxicologic ef- tions. fects of certain fungicides and herbicides on sheep and cattle. Ann. N. Y. Acad. Sci. 3, Article 2, p. 729-736. ST. JOHN, L. B., J R., D. G. WAGNER, and D. J. LISK . 1964. Conclusion Fate of atrazine, Kuron, Silvex, and 2,4,5-T in the dairy cow. J. Dairy Sci. 47: 1267-1270. An adequate assessment of the hazard associated WEIR, R. J., 0. E. PAYNTER, and J. A. ELSERS. 1958. Toxicology of 3-amino-1.2,4-triazole. The Hormolog 2: 13-14. with the use of any chemical tool must include consid- 23. ZIELINSKI,I, WALTER L., J R., and LAWRENCE EIS ILBEIN . 1967. Gas eration of the toxicity characteristics of the chemical chromatographic measurement of disappearance rates of 2.4-D and 2,4,5-T acids and 2,4-D esters in mice. J. Agr. Food Chem. and the magnitude and persistence of the chemical 15: 841-844. 720 J OURNAL OF FORESTRY