The fluted western hemlock of Alaska. I. Morphological studies and experiments

Forest Ecology and Management, 60 ( 1993 ) 119-132 Elsevier Science Publishers B.V., Amsterdam

119

The fluted western hemlock of Alaska. I. Morphological studies and experiments Kent R. Julin a'*, Charles G. Shaw III b, Wilbur A. Farr c, Thomas M. Hinckleyd aHarding Lawson Associates, 7655 Redwood Boulevard, Novato, CA 94948, USA bRocky Mountain Forest and Range Experiment Station, 240 West Prospect, Fort Collins, CO 80526, USA CForestrySciences Laboratory, Pacific Northwest Forest and Range Experiment Station, 2770 Sherwood Lane, Suite 2A, Juneau, AK 99802-0909, USA dCollege of Forest Resources AR- I O, University of Washington, Seattle, WA 98195, USA (Accepted 4 February 1993 )

Abstract

Stem fluting on western hemlock (Tsuga heterophylla (Raf.) Sarg.) in southeast Alaska was examined. Morphological studies revealed that flutes occur between roots at the root collar, and are vertically aligned with non-functional branches in the lower crown. Trees with small angles of branch insertion have more pronounced fluting than trees with large angles of branch insertion. Such trees could be removed from stands during precommercial thinning. Manipulative treatments (branch girdling, stem drilling and plugging, and root cutting) suggested that fluted trees have a confined type of transport system that restricts transverse movement of growth substances (i.e. carbohydrates, water, nutrients and hormones) and results in differential stimulation of cambial tissues. Any factor or treatment that decreased the supply of growth substances to the stem reduced growth rates, thereby producing flutes.

Introduction Many western hemlock (Tsuga heterophylla (Raf.) Sarg.) trees in the coastal forests of southeast Alaska have unusually fluted stems. This characteristic, where vertically oriented fissures (flutes) and ridges (buttresses) spiral up the stem to well within the live crown, is atypical for most conifer species and for western hemlock in the southern portion of its range. Flutes severely reduce the product value of western hemlock logs (Harris and Farr, 1974). We surveyed numerous stands in southeast Alaska in 1984 to investigate the geographical distribution of fluted hemlock and to generate a series of hypotheses that could be tested. We found that fluted hemlock were widespread in the region, but most common near the coast. Moreover, most fluted *Corresponding author.

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Fig. 1. Fluted western hemlock on Prince of Wales Island, Alaska (A). Cross-section of a fluted western hemlock stem that had a breast height age of 88 years (B).

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VICINITY MAP

N

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SLAND

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0 I

1 SC,~

Fig. 2. Study locations on Prince of Wales Island, Alaska.

trees exhibited highly repeatable morphological patterns. Flutes ascended in relatively constant right-to-left spirals (Fig. 1 ), were deepest at the root collar, and became progressively shallower with increased stem height. Branches in the lower crown were vertically aligned with flutes. Flutes were not observed on branches. Major roots coincided with root collar buttresses. Literature on stem fluting provided clues on its causes. As fluted stems are initially circular (Francis, 1924 ) and cambium in flutes is alive (Day, 1964 ), fluting is theorized to result from a differential stimulation of the cambium (Day, 1964; Courts and Philipson, 1975; Fayle, 1981 ). This may be caused by an unequal distribution of substances translocated in xylem and/or phloem. This paper is one of three on the causes and occurrence of fluting in western hemlock. Here, we investigate, observationally and experimentally, branch-

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and tree-level factors responsible for flute formation. A second paper examines tree and stand factors related to the occurrence and development of fluting (Julin et al., 1993 ), and a third paper presents results of a guying study and a follow-up on the tree-level experiments (Segura et al., 1993 ). Fluted-tree morphology was studied on Prince of Wales Island (Fig. 2 ) to elucidate structural and growth factor(s) related to flute development. Two approaches were used: stem dissections and analyses of branch angles, and manipulative experiments to induce fluting. Stem-analysis protocols of Duff and Nolan ( 1953, 1957 ) were modified to examine specific aspects of fluted-tree morphology, rather than the growth rate parameters typically used in stem analysis (Farrar, 1961 ). We identified flute locations on stems in radial, longitudinal, and temporal contexts, and positional and temporal associations between stem flutes and branches, including relationships between branch size and function and flute initiation and development. Distribution of roots in relation to buttresses at the root collar also was investigated. On a much larger population of trees, the relationship of branch angle to fluting was assessed. Three treatments were conducted to relate our morphological observations more directly to causative factors. Specific treatments, hypothesized to induce flute formation, were carried out in a coastal and an inland stand. Methods

Observational studies As associations between branches and spiralling times were obscured at stem sampling intervals greater than 10 cm, we used this interval for dissection of a relatively small tree (Table 1, Tree 1 ). Once the technique was refined, three additional codominant trees (Table l, trees 2, 3 and 4 ) were cut into 10 cm sections from the root collar to the tip. Sections and branch stubs were sanded so that annual rings could be counted with a 10 × hand lens. Time of flute initiation was considered to be the year when an annual ring changed form from being convex to concave relative to the pith. Table 1 Summary data for four codominant western hemlock used for stem analysis of flute development, Prince of Wales Island, Alaska (Fig. 2 ) Tree no.

Age (years)

Height ( m )

Collection site

1 2 3 4

10 52 38 48

7.1 21.2 21.6 17.8

Naukati Winter Harbor Tuxekan Village Sarkar Point

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With stem pith as the axis, the azimuth of roots larger than 4 cm 2 and of root collar buttresses were measured in degrees. Similarly, azimuths of all branches and flutes contained within each section were measured. The two respective associations were examined by correlation analyses. The relationship between angles of branch insertion into the stem and the presence of flutes above that point was examined on 90 trees growing near Staney Creek (Fig. 2). Forty-five trees were sampled in a coastal stand and 45 in a stand 10 km inland. Both stands were naturally regenerated after clearcut logging some 12 years earlier. Older stands near the coastal site contained fluted western hemlock, but those near the interior stand did not. On two randomly selected branches within 0.5 m of breast height, a standard goniometer (Sammons, Inc., Brookfield, IL) was used to measure angles of insertion (the interior angle between branch and stem) to the nearest degree. The occurrence of flutes and time of their initiation were observed on stem crosssections taken 5 cm directly above the branches. Mean angles of insertion for trees with and without incipient flutes were compared by using Student's ttest. Stands were tested individually (i.e. coastal vs. inland) and together.

Experimental studies Experimental studies were conducted near Staney Creek (Fig. 2 ) in the two stands mentioned above. These sites were used to test whether treatment responses varied with distance from the coast, as we found in the earlier survey. A 60 m × 60 m portion of each stand was thinned from about 7400 trees h a (coastal) and 4700 trees ha -1 (inland) to 500 trees ha -1 in April 1985. We thinned the stand to prolong branch longevity for girdling studies; growth rates and exposure to wind were also expected to increase. Study sites were subdivided into nine 20 m × 20 m cells using a Latin Square configuration. This design allowed us to assess variation caused by microsite quality. There were 20 study trees in each cell which was assigned to one of the following treatments: ( 1 ) branch girdling; (2) stem drilling and plugging; (3) root pruning. All treatments were initiated before bud burst in April 1985. After two growing seasons (late August 1986), 15 randomly selected trees in each cell were felled for stem analysis (90 trees total for each treatment including both sites). Trees were aged at the stump and total height and diameter at breast height ( 1.37 m ) were measured to the nearest 0.1 m and 0.5 cm, respectively. The tree stem in each cell closest in height and diameter to that cell's mean height and diameter was used to assess treatment effects. The vertical extent of any incipient flutes were noted and ring widths were measured at 10 cm intervals to document growth patterns before and after treatment. Response differences between sites for a given treatment also were examined. Because ring widths were smaller at the inland site than at the coastal site, treatment responses were expressed as a percentage of control, where

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Response = 100 × ( C1986-- T1986 ) / C1986 Ci986 and TI986 were respectively, the average ring width by plot of control and treated trees in 1986. Student's t-test also was used to compare response differences between sites.

Branch girdling Branch girdling was used to define how loss of branch function might affect flute formation. One branch within 1 m of breast height on each sample tree (n = 20) was girdled by removing a 2 cm band of bark from the branch. To avoid desiccation, girdled areas were treated immediately with Tree Seal TM pruning compound. An un-girdled branch of similar size and within 0.5 m of the girdled branch served as a control. Stem sections containing girdled or control branches were cut from 15 trees. On these sections, ring widths were measured along radii aligned 5 cm above and below the branch. The paired sample t-test was used to compare ring widths on treated and control branches on all 180 trees for 1984, 1985, and 1986.

Stem drilling and plugging Resistance to xylem and phloem translocation that might occur at a branch collar was simulated by placing wooden plugs in drilled holes. A hole of 1.6 cm diameter was drilled to the stem pith near breast height in a section without branches or deformities. Maple doweling ( 1.6 cm in diameter) was driven into the hole and t r i m m e d to leave a 3 cm stub. A 10 cm section containing the plug was cut from each tree and ring widths were measured along radii located 5 cm above and below the plug. These values were compared with estimates of expected ring widths obtained by averaging 10 ring width measurements from 40 °, 70 °, 100 o, 130 °, 160 °, 190 °, 220 °, 250 °, 280 °, and 310 ° (the plug was at 0 ° and the pith was the rotation axis). Expected ring widths accounted for positional variation as a result of dimensional changes in the stem, and year-to-year changes caused by weather and thinning. Estimated and actual ring widths for 1984, 1985, and 1986 above and below plugs were compared using a paired sample t-test.

Root pruning Root pruning was used to define how water and mineral nutrient uptake might affect flute formation. One major root on each tree was severed at the root collar using a chainsaw. Exposed tissues were immediately treated with Tree Seal T M pruning c o m p o u n d to reduce desiccation. Stem disks were removed 5 cm above the soil surface and ring widths measured to the nearest

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0.5 mm. Ring widths at 0 ° (directly above the pruned root) were compared with estimated ring widths as described above. Estimated and actual ring widths for 1984, 1985 and 1986 above pruned roots were compared using a paired sample t-test. Results and discussion

Observational studies Even though we analyzed only four trees, the intensive dissections allowed us to evaluate morphological relationships between ( 1 ) basal buttresses and roots, (2) flutes and branch location, and (3) the timing of flute initiation and loss of branch function. The distribution of root collar buttresses was positively correlated ( y = - 1.01 + 1.00~ r2= 0.98) with positions of major roots. Thus roots are associated with buttresses around the tree base. The distribution of stress at the root collar may affect the arrangement of buttresses. Increased production of the growth regulator ethylene has been associated with mechanical stress (Telewski and Jaffe, 1981 ). If stress at the root collar is greatest where roots attach to the stem (Wilson and Archer, 1979), then ethylene production should be most pronounced in this region. As such, buttresses, indicative of more growth, would develop above roots. Mechanical implications of this structuring are discussed in a consecutive paper (Segura et al., 1993 ). The distribution of other plant-growth substances at the root collar also may increase growth directly above roots. Water, minerals, and cytokinins from roots enter the stem in discrete zones (Rudinsky and Vitr, 1959; Coutts and Philipson, 1975 ). A xylem structure that limits lateral movement of these growth substances would accentuate growth near the source of these substances and lead to asymmetric growth around the stem at root-stem junctions. Significant positive correlations were found between locations of branches and the nearest flutes above them (y = 5.04 + 0.97x; r 2= 0.94 ) or below them (y=6.55+0.96x; r2=0.94). This occurrence of flutes above and below branches has been described previously (Newman, 1955; Day, 1964; Fayle, 1981 ). Branches formed before flute initiation. As branches age and the yearly branch-to-stem junction is renewed (Shigo, 1985 ), a factor associated with either branch function or structure may somehow affect flute initiation and expression. For example, branches may affect the distribution of plant growth substances in the stem near the branch, and that in turn alters growth rates in the stem. Changes in branch productivity may affect the growth of associated stem tissues (Sprugel et al., 1991 ). Branch collar vasculature has been described in detail by Shigo ( 1985 ) for a variety of tree species. Shigo demonstrated that the primary translocation pathway from branches was downward into the

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stem. Thus, perhaps physiological decline of a branch creates localized carbohydrate and/or hormonal deficiencies in the stem beneath such branches. These deficiencies, in the context of a limited pattern of translocation, could result in differential rates of growth around the stem. Shigo (1985) also observed that branch and stem cambial activity is not synchronous. As a result, branch-xylem elements are not directly linked to stem-xylem elements, but are linked indirectly through a series of overlapping plates at the branch-stem junction. Growth changes in branches relative to stems may affect the nature of this indirect linkage. We hypothesized that flute initiation beneath a branch was caused by its physiological decline. The functional status of branches can be estimated by assuming that accretion of annual rings at the branch base indicates active carbohydrate transport from branch to stem (Underwood, 1967; Sprugel et al., 1991 ). Branch function is then determined by counting annual rings in the branch and stem directly above it. As branch and stem were formed in the same year, each could have the same number of annual rings. If they do, then the branch is considered functional. If there are fewer branch rings than stem rings, then the branch is considered non-functional. There was correspondence between the temporal loss of branch function and flute initiation (Fig. 3 ). On the three large western hemlock examined, 76% of the flutes initiated within 5 years after a loss of branch function. This is consistent with results on red pine (Pinus resinosa Ait.) (Fayle, 1981 ). Considering the subjectivity required to establish branch functionality and flute initiation, a 5 year difference is probably not meaningful. Even so, these 80 CO Z

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60

o

~ 4o rr U.I en

D Z

20

0

,

~

-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 NUMBER OF YEARS BETWEEN WHEN BRANCHES BECAME NON-FUNCTIONAL AND FLUTES INITIATED

Fig. 3. Correspondence between time of flute initiation and when branches became non-functional on sample Trees 2, 3, and 4 (n = 204). Positive values along the x-axis indicate that flutes formed after the branch became non-functional.

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results indicate that changes in branch productivity affect stem growth directly beneath these branches. Our branch girdling study allowed us to examine this supposition experimentally. Flutes initiate on western hemlock stems above points of branch insertion before their loss in function. Fayle (1981 ) suggested that histical pressures develop at the point of contact between branch and stem, resulting in flute formation. From this, we hypothesized that branches with small angles of insertion into the stem would be associated with flutes, whereas branches with large angles of insertion would not. It was found that trees with flutes above points of branch insertion had a significantly smaller mean angle of branch insertion than did trees without flutes (Fig. 4). This pattern was evident in each stand ( P < 0.001 ). For the entire sample, the mean angle of branch insertion was 52.8 _+2.5 ° for fluted trees and 64.5 +_2.3 ° for non-fluted ones. The coastal stand had a higher proportion of trees with small angles of insertion (e.g. less than 60 ° ) than did the inland stand (49% vs. 44%). These results support the hypothesis that branch angle affects stem growth directly above points of branch insertion. Perhaps growth is physically limited by pressures developing as the branch grows against the stem. Jankiewicz and Stecki ( 1976 ) reported that pressures of at least 10 N cm-2 can be exerted by developing tissues within such contact zones. Reduced growth above the point of contact is, however, less easily explained. In this area, growth may be limited by the availability of plant-growth substances. Perhaps branches interrupt acropetal translocation and tissues adjacent to them are incapable of supplying these substances to tissues directly above. Although plant growth substances flow primarily basipetally in tree phloem, acropetal carbohydrate transport can occur (Rangnekar et al., 40

/ ©

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NOT FLUTED

=>

20

0

10

20

30

40

50

60

70

80

90

100

BRANCH ANGLE OF DIVERGENCE(DEG)

Fig. 4. Relationship between angle of branch insertion (divergence from vertical) and occurrence of flutes ( n = 180).

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1969; Ross, 1972; Dickson and Isebrands, 1991 ). A physical interruption of acropetal transport in the phloem caused by a branch could reduce stem growth above the branch. An analogous situation may occur in xylem, where water, nutrients and perhaps hormones are physically diverted by branches. Fayle ( 1981 ) used this latter explanation to account for flutes forming in association with branches with small angles of insertion on red pine. This proposed relationship between angles of branch insertion and flute initiation may provide a means to identify young trees with the potential to develop severely fluted stems. A much larger sample of branch angles and stem growth responses will be needed before this method of early flute detection could be applied by field crews during precommercial thinning, but benefits could be substantial. Several generalizations on the gross morphology of fluted western hemlock arise from these investigations. Buttresses at the root collar occur above major structural roots. Flutes occur above and below main stem branches. Branch functional status affects flute development below points of branch insertion. Branch angle is associated with flute development above points of branch insertion. Further research on the nutritional dynamics at the branch collar of fluted western hemlock is needed. The following experimental studies provide some additional clues regarding changes in stem growth associated with the manipulation of branch and root function.

Experimental studies Branch girdling Branch girdling was designed to interrupt the supply of substances transported within the phloem from branch to stem. According to Shigo (1985), conduction to and from branches occurs along a vascular pathway called the branch collar, which encircles the branch and extends downward from the point of branch insertion. As such, we expected effects of branch girdling to be confined to stem portions beneath treated branches. In the year before girdling (1984), ring widths from stem sections associated with control branches did not differ significantly from those associated with branches that were subsequently girdled. Thus, any differences in ring width after girdling were considered to be a treatment effect. Ring widths directly above control and treated branches did not differ significantly in 1985 and 1986, the first and second growing seasons after girdling. In contrast, ring widths directly below treated branches decreased by 26% in 1985, and by 29% in 1986. Flute extent averaged 35.0 _+11.0 cm below girdled branches. Branches aligned with a girdled branch and lower on the stem bounded these flutes. These lower branches were probably functional. As such, they probably provided the stem

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with sufficient carbohydrates to overcome the deficit created by the girdled branch. These results indicate that loss of branch function leads to flute formation beneath affected branches. Girdling apparently creates a localized zone deficient in plant growth substances (hormones a n d / o r assimilates) in the stem directly beneath treated branches. Girdling did not prevent materials in stem xylem from moving into branches, as all girdled branches were living when the trees were cut in late summer 1986. Because of the nature of branch attachments (Shigo, 1985 ), substances normally translocated in branch phloem to a stem cannot be supplied easily by lateral translocation from another branch unless it is directly above or below that branch.

Stem drilling and plugging Stem drilling and plugging was used to investigate how barriers to vertical vascular pathways might affect flute formation. Fayle ( 1981 ) attributed flute development in red pine to changes in translocation links at branch-stem junctions. Using brass screws to simulate branch resistance to vascular flow, he was able to produce grooves both above and below points of screw insertion. Above screws, flute development was hypothesized to result from a localized water deficiency, whereas development below screws was hypothesized to result from localized carbohydrate and hormonal deficiencies. Ring width measurements did not differ significantly between controls and plugged stems in the year before treatment (1984). Average ring widths above and below points of plug insertion also did not differ significantly in 1984. Therefore, subsequent variations in the width of a ring around the stem and along the vertical plane coinciding with the plug were considered to be treatment effects. One year after treatment ( 1985 ) there was a significant reduction in average ring width directly above (51%) and below (62%) points of plug insertion. The average vertical extent of incipient flutes was 26.7 _+8.6 cm above and 20.0 + 6.6 cm below points of plugging. Branches aligned with the plug formed upper and lower limits of the flute. Reductions in ring widths below plugs were significantly greater than those above plugs in 1985. In 1986, ring widths were again reduced significantly, but less than in 1985, both above ( 14% ) and below ( 14% ) points of plug insertion. In contrast to the branch girdling and branch angle studies, responses to stem drilling and plugging did not differ significantly between coastal and inland sites. Stem drilling and plugging results support those from branch girdling. Restricted phloem translocation, whether associated with a branch's ability to translocate carbohydrates to the stem or the stem's ability to translocate, occurred in both experiments. In addition, stem drilling and plugging would have affected xylem transport. The plug, or the hole made to accommodate it, apparently created localized carbohydrate, water, a n d / o r nutrient deficiencies which could not be compensated for by lateral movement of these

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substances from the vertically oriented vascular system of the tree. Thus, the plug or hole diverted substance flow, which resulted in differential growth around the stem and subsequent flute initiation. Because ring widths in 1986 approached expected values, treatment effects may be ephemeral. Had the treatment period been longer, differences may have become non-significant. Perhaps young trees have a higher capability of re-establishing lateral translocation links after blockage of a section, particularly if the section is not associated with any branches. In addition, a larger-diameter natural plug (a branch) might have caused a greater or a more prolonged impact. These results also indicate that damage to cambial tissues may disrupt vertical translocation pathways and promote flute development. For example, insect damage caused by the hemlock bark maggot (Cheilosia alaskensis Hunter), or by the western hemlock beetle (Pseudohylesinus tsugae Swaine) could result in flute formation near egg galleries and feeding areas (Furniss and Carolin, 1977 ). Conceivably, thinning and yarding operations also could lead to fluting if there is mechanical damage to residual trees.

Root pruning Roots were pruned to study flute formation at the root collar. Day (1964) observed that buttresses are associated with actively functioning roots, and flutes occurred between roots and above dead roots. Our root pruning was designed to interrupt the local supply of water and mineral nutrients to the root collar. Elimination of growth regulators, such as cytokinins produced in roots, was also a likely treatment effect. Morphological observations identified a strong correlation between locations of roots and buttresses. Assuming that lateral translocation of plant growth substances in these trees is limited, we hypothesized that root pruning would result in incipient flute formation because of localized deficiencies in plant-growth substances. Root pruning did initiate stem fluting above these roots. Before treatment (1984), ring widths in controls and in stem portions above roots that were subsequently pruned did not differ significantly. Thus, future deviations from expected ring widths were considered to be a treatment effect. In 1985, ring widths for root-pruned trees decreased by 37%; this pattern continued in 1986 (48% decrease ). The average vertical extent of incipient flutes caused by root pruning was 18.3 _+ 13.9 cm. Branches that aligned with pruned roots bounded these flutes. These results imply that the distributional pattern of substances supplied by roots can cause differential growth rates, resulting in incipient fluting at the tree base. Perhaps flutes and buttresses develop as a result of the patterns of movement of water, nutrients and growth substances upward and downward. When stems are damaged growth patterns are altered.

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Conclusions Morphological studies revealed that flutes occur between roots at the root collar, and beneath and above dead and non-functional branches in the lower crown. Small angles of branch insertion also correlate with flute formation, as did loss o f branch function, Results from all three manipulative treatments indicate that fluting m a y result from tangential restrictions to m o v e m e n t of carbohydrates, hormones, nutrients a n d / o r water. Stem radial growth directly beneath branches was reduced by their girdling. Stem drilling and plugging reduced ring widths above and below plugs. Finally, stem ring widths directly above pruned roots were reduced. In each instance, interruptions of the vascular stream resulted in incipient flutes. These deficiencies apparently could not be adequately offset by tangential m o v e m e n t o f growth substances from other stem regions. Ring width responses between control and treated trees varied by site. Responses to branch girdling and root pruning were greater at the coastal site, but location was not significant for the plugging treatment. Western hemlock vasculature is known to be confined (Rudinsky and Vitr, 1959 ). Our studies support this observation in southeast Alaska. Although a confined type o f vasculature may be necessary for flutes to develop (as indicated by the manipulative t r e a t m e n t s ) , other factors also m a y contribute to the development o f asymmetric stems.

References Coutts, M.P. and Philipson, J.J., 1975. The influence of mineral nutrition on the root development of trees. I. The growthof Sitka spruce wi~ladivided root systems.J. Exp. Bot., 27:11021111. Day, W.R., 1964. The development of flutes o9 ho-~l~o~,son main stems of trees and its relation to bark splitting and strip necrosis. Forestry, 37:145-160. Dickson, R.E. and Isebrands, J.G., 1991. Leaves as regulators of stress response. In: H.A. Mooney, W.E. Winner and E.J. Pell (Editors), Response of Plants to Multiple Stresses. Academic Press, San Diego, pp. 3-34. Duff, G.H. and Nolan, N.J., 1953. Growth and morphogenesisin the Canadian forest species. 1. Controls of cambial and apical activity in Pinus resinosa Ait. Can. J. Bot., 31:471-513. Duff, G.H. and Nolan, N.J., 1957. Growth and morphogenesisin the Canadian forest species. II. Specific increments and their relation to the quantity and activity of growth in Pinus resinosa Ait. Can. J. Bot., 35: 527-572. Farrar, J.L, 1961. Longitudinal variation in the thickness of the annual ring. For. Chron., 37: 323-330. Fayle, D.C.F., 1981. Groove formation in the stem of red pine associated with branches. Can. J. For. Res., 11: 643-650. Francis, W.D., 1924. The developmentof buttresses in Queenslandtrees. Proc. R. Soc. Queensl., 36(3): 21-37. Furniss, R.L. and Carolin, V.M., 1977. Western forest insects. US Dep. Agric. For. Serv. Misc. Publ. 1339, 654 pp.

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Harris, A.S. and Farr, W.A., 1974. The forest ecosystem of southeast Alaska: 7. Forest ecology and timber management. Gen. Tech. Rep., PNW-25, US Dep. Agric. For. Serv., Portland, OR, 109 pp. Jankiewicz, L.S. and Stecki, Z.J., 1976. Some mechanisms responsible for differences in tree form. In: M.G.R. Cannell and F.T. Last (Editors), Tree Physiology and Yield Improvement. Academic Press, London, pp. 157-172. Julin, K.R., Shaw, III, C.G., Farr, W.A. and Hinckley, T.M., 1993. The fluted western hemlock of Alaska. II. Stand observations and synthesis. For. Ecol. Manage., 60:133-141. Newman, I.V., 1955. On fluting of the trunk in young trees o f P i n u s taeda L. (loblolly pine). Aust. J. Bot., 4: 1-16. Rangnekar, P.V., Forward, D.F. and Nolan, N.J., 1969. Foliar nutrition and wood growth in red pine: the distribution of radiocarbon photoassimilated by individual branches of young trees. Can. J. Bot., 47:1701-1711. Ross, S.D., 1972. The seasonal and diurnal source-sink relationships of photoassimilated laC in the Douglas-fir branch. PhD dissertation, University of Washington, Seattle, 99 pp. Rudinsky, J.A. and Vit6, J.P., 1959. Certain ecological and phyiogenetic aspects of the pattern of water conduction in conifers. For. Sci., 5: 259-266. Segura, G., Julin, K.R., Farr, W.A., Shaw III, C.G., Hinckley, T.M., Tierney, P., 1993. The fluted western hemlock of southeast Alaska. III. Results for six growing seasons following treatment, in preparation. Shigo, A.L., 1985. How tree branches are attached to trunks. Can. J. Bot., 63:1391-1401. Sprugel, D.G., Hinckley, T.M. and Schaap, W., 1991. The theory and practice of branch autonomy. Annu. Rev. Ecol. Syst., 22: 309-334. Telewski, F.W. and Jaffe, M.J., 1981. Thigomorphogenesis: changes in the morphology and chemical composition induced by mechanical perturbation in 6-month-old P i n u s taeda seedlings. Can. J. For. Res., 11: 380-387. Underwood, R.J., 1967. A study of the effect of pruning on the longitudinal distribution of radial growth in Douglas-fir. MS thesis, University of Washington, Seattle, 111 pp. Wilson, B.F. and Archer, R.R., 1979. Tree design: some biological solutions to mechanical problems. Bioscience, 29: 293-298.