Effects of cutting sclerophyll brush on sprout development and Douglas-fir growth

Forest Ecology and Management, 13 (1985) 69--81 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

69

EFFECTS OF CUTTING SCLEROPHYLL BRUSH ON SPROUT DEVELOPMENT AND DOUGLAS-FIR GROWTH S T E P H E N D. H O B B S and K E N N E T H

A. W E A R S T L E R ,

Jr.I

Department of Forest Science, Oregon State University,Corvallis,O R 97331 (U.S.A.) iCurrent address: Boise Cascade Corporation, Medford, O R 97501 (U.S.A.) (Accepted 24 M a y 1985)

ABSTRACT Hobbs, S.D. and Wearstler, K.A., Jr., 1985. Effects of cutting sclerophyll brush on sprout development and Douglas-fir growth. For. Ecol. Manage., 13: 69--81. In southwest Oregon, varying amount of brush were removed from a sclerophyll brushfield dominated by canyon live oak (Quercus chrysolepis Liebm.), and greenleaf manzanita (Arctostaphylos patula Green) with scattered Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) saplings. Brush removal was accomplished by slashing (cut by chainsaw) near ground level at three intensities: (1) total removal, (2) partial removal, and (3) an untreated control. Sclerophyll brush species responded within three weeks of slashing by vigorous sprouting, which was greatest in total brush removal areas where 861 513 sprout stems ha -1 developed during the first year. Soil water potentials and predawn xylem pressure potentials of Douglas-fir were less negative in total removal areas than in partial removal and untreated control areas. Relative growth rates of Douglas-fir saplings temporarily increased in total and partial brush removal areas, but were n o t significantly different from the untreated control three years after treatment. Slashing of sclerophyU brush to release long-suppressed Douglas-fir as described in this study, is not recommended because of rapid brush recovery by sprouting.

INTRODUCTION

Brushfields dominated by evergreen sclerophyll hardwoods occur throughout southwest Oregon in areas once occupied by coniferous forests. Historically, fire has been a major cause of brushfield formation (Gratkowski, 1961; Franklin and Dyrness, 1973), but with the advent of extensive logging, the failure to quickly regenerate harvested areas has frequently led to site dominance by sclerophyll brush species. These plants possess morphological and physiological adaptations which make them effective competitors with young conifers for limited soil moisture and other site resources (McDonald, 1982). Many of these brush communities can be adequately controlled, however, with herbicides (Gratkowski, 1975, 1978) and suppressed conifers released from competition. This method of vegetation control is particularly useful in steep terrain where machine treatments are limited. Numerous reports,

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© 1985 Elsevier Science Publishers B.V.

70 summarized by Stewart et al. (1984), establish that conifers usually respond favourably to herbicide release treatments. Studies in the western United States have shown that soil moisture remains greatest during drought when sclerophyll brush species are controlled with herbicides (Tarrant, 1957; Petersen, 1980; Conard and Radosevich, 1982). In areas such as southwest Oregon where the climate is characterized by a prolonged dry season (May-September) with accompanying high air temperatures, conservation of available soil moisture is particularly important for conifer growth. Typically during the dry season, only about 12.8% of the average annual precipitation occurs (McNabb et al., 1982) with potential evapotranspiration exceeding precipitation in the interior valleys (Johnsgard, 1963). In recent years administrative and judicially imposed restrictions have severely limited the use of herbicides in Oregon, which has caused increased consideration of non-chemical methods of brush control. Unfortunately, little information is available regarding the release of suppressed conifers from evergreen sclerophyll brush competition in mountainous terrain without the use of herbicides. In the eastern Siskiyou Mountains of southwest Oregon, two evergreen sclerophyll brush species frequently found in association with Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) on xeric sites, are canyon live oak (Quercus chrysolepis Liebm.) and greenleaf manzanita (Arctostaphylos patula Greene). No information exists on the interaction and response of these sclerophylls to slashing (cut by chainsaw at ground level) and the effect of this treatment on suppressed Douglas-fir saplings. To examine these questions, this study compared two intensities of brush removal with an untreated control in a mature brushfield of canyon live oak and greenleaf manzanita with scattered, suppressed Douglas-fir saplings. STUDY AREA The study area was a brushfield dominated by evergreen sclerophylls in the Mixed-Evergreen Forest Zone (Whittaker, 1960; Franklin and Dyrness, 1973) of the eastern Siskiyou Mountains in southwest Oregon (lat. 42 ° 14' N., long. 123 ° 5' W). Principal species were canyon live oak and greenleaf manzanita, 3--4 and 0.5--1 m in height, respectively. These formed a mosaic of distinct single-species groupings across the area. Prior to the study, a brush canopy covered approximately 95% of the area. Canyon live oak and greenleaf manzanita ranged from 25 to 35 years old and had developed from much older, partially exposed root-crown burls (lignotubers). We were unable to determine the exact age of the burls of either species because of irregular growth patterns, but the burls were older than the stems they supported. Although canyon live oak frequently attains tree stature, its shrublike appearance on this study area probably reflected the combined influences of fire (Horton, 1960) and high environmental stress (Rundel, 1980). Douglas-fir saplings ranged in age from 25--35 years and were 3--4 m in

71 height. Other minor components were silk tassel (Garrya fremontii Torr.), Pacific madrone (Arbutus menziesii Pursh.), and ponderosa pine (Pinus ponderosa Dougl.), Herbaceous vegetation in the understory was almost nonexistent. Average elevation on the study site is 1097 m with a relatively uniform west.facing slope of 66%. There were, however, minor differences in areas occupied by greenleaf manzanita. In these areas, accumulations of ravel (loose rock fragments) and organic material were less than in those areas dominated by canyon live oak. The soil was classified as a typic Xerochrept averaging 49 cm in depth with a coarse fragment content of 50--60% of the total soil volume. Parent materials were metasediments. An unstable layer of ravel covered the soil surface irregularly with depths from 0 to 20 cm. Mean annual precipitation is approximately 1 0 2 0 m m (Froehlich et al., 1982) with only 130 mm occurring from May through September (McNabb et al., 1982). METHODS A systematic survey of slope, aspect, soil morphology, and ravel depth was c o n d u c t e d prior to treatment installation in order to find relatively uniform treatment areas. Consequently, this requirement limited the number of treatment areas to five, which necessitated unequal treatment replication. T w o areas were assigned total brush removal, two were assigned 50% removal, while the fifth area was used as an untreated control. Treatments were assigned at random. Within each 0.07-ha treatment area, an interior rectangular plot measuring 0.04 ha was established to minimize edge effect. To ensure uniform coverage of the plots, ten rectangular 0.0004-ha microplots were also systematically located within each 0.04-ha area along three evenly spaced transects to measure the number of stems and diameters of all species to the nearest 2.5 mm at 15 cm above ground level. These microplots were measured prior to brush removal, immediately after brush removal, and at the end of the first year. In early May, brush was cut near ground level with chainsaws and removed. In those areas designated for partial brush removal, an attempt was made to cut equally among all species and diameter classes. No Douglas-fir were removed. Post-treatment evaluation of partial removal areas showed, however, that only an average of 38% of the basal area had actually been cut from the two replications rather than the desired 50%. Predawn xylem pressure potential (xPx) and soil moisture data were collected simultaneously at three week intervals from May to October during the first year following brush removal. Xylem pressure potentials of five Douglas-fir saplings per treatment were measured with a pressure chamber based on the concept developed b y Scholander et al. (1965) and described by Waring and Cleary (1967). Three years later, these trees were used to measure basal area growth from cross-sectional areas taken when trees were

24 092 0 553 009

22 486 19 768 157 957

29 405 29 405 33 359

Partial removal Pretreatment Immediate posttreatment 1 year after treatment

Untreated control Pretreatment Immediate posttreatment 1 year after treatment

Stems ha -1

Canyon live oak

Total removal Pretreatment Immediate posttreatment 1 year after treatment

Treatment/measurement date

22.05 22.05 24.40

21.58 12.41 13.59

37.31 0 2.41

BA(m 2 ha -1)

55 597 55 597 59 057

73 389 56 092 208 098

40 895 0 308 504

Stems ha -1

4.69 4.69 4.73

7.54 4.65 4.72

4.01 0 1.40

BA(m 2 ha -1)

Greenleaf manzanita

Growth response of canyon live oak and greenleaf manzanita to brush removal treatments

TABLE 1

85 002 85002 92 416

95 875 75 861 366055

64 987 0 861 513

Stems ha -1

Total

26.74 26.74 29.13

29.12 17.06 18.31

41.32 0 3.81

BA(m 2 ha -1 )

t~

73 cut. These data were converted to relative growth rates ( R G R ) by a procedure described b y Hunt (1982) because of large size differences between individual trees. Gravimetric soil moisture from six samples taken from each treatment area at the 20-cm depth was later converted to soil water potentials (~s) from a moisture release curve (Black et al., 1965). Precipitation was measured weekly with a nonrecording rain gauge located in a total brush removal area. A soil trench 14 m long was also dug to fractured bedrock and the soil profile described at 2-m intervals. Brush rooting patterns were evaluated at these same points. Soil water potential (kos), Douglas-fir predawn ~x, and R G R were analyzed statistically for treatment effects b y one-way analysis of variance (Steel and Torrie, 1960). Treatment replication data were pooled b y treatment. RESULTS AND DISCUSSION

Brush response Emergence of suppressed sprout buds from root-crown burls of both canyon live oak and greenleaf manzanita were observed within three weeks of cutting. Sprout elongation for both species was slow initially b u t increased rapidly by 1 July, particularly in total removal areas (Table 1). In these areas sudden elimination of all vegetative cover (except scattered Douglas-fir saplings) stimulated prolific sprout development. Sprouts in full sunlight continued height growth 2--3 weeks after sprout growth ceased in partial removal areas. In partial removal areas, sprouting was 75 and 51% less for canyon live oak and greenleaf manzanita, respectively, when compared to sprouting in total removal areas. Conard and Radosevich (1982) speculated that shade m a y reduce sprouting in montane chaparral shrubs while Berg and Plumb (1972), citing several studies, have suggested that partial damage to aboveground portions of w o o d y plants m a y stimulate correlative inhibition of sprout bud activation by growth inhibitors. In our study, several stems of each multi-stemmed clump were usually left uncut in partial removal areas, hypothetically providing sites for growth inhibitor production. This, in addition to shading, may offer a partial explanation for reduced sclerophyll sprouting in partial brush removal areas. Sprout development in total removal areas was dramatic. In this treatment 861 513 stems ha- 1, with a basal area of 3.81 m 2 ha- 1, developed during the first year following cutting (Table 1). Canyon live oak had the greatest number of sprouts per root-crown burl with many clumps containing more than 100 sprout stems. This exceeds the number of sprouts reported for burls of tanoak (Lithocarpus densiflorus (Hook. and Am.) Rehd) and Pacific madrone (Tappeiner et al., 1984), t w o sclerophyll species sometimes found in association with canyon live oak. Greenleaf manzanita burls

74

developed fewer (5--20 per clump) and less vigorous sprouts than canyon live oak. Although brush growth was n o t remeasured after the first year, it was obvious from frequent visits to the study site, that growth differences between the sclerophylls were even more pronounced three years after cutting. In total removal areas, canyon live oak sprouts formed dense, overlapping clumps up to a meter in height while those of greenleaf manzanita were less dense and not more than a half a meter in height. Partial removal areas were visually indistinguishable from the untreated control after three years. Canyon live oak's more rapid response to mechanical damage m a y in part, be due to its deep, well~leveloped r o o t system (Fig. 1). R o o t s of this species were f o u n d in all parts of the soil profile and penetrated fractured bed rock which probably enabled sprout clumps to utilize deep sources of water during summer drought. Rendel (1980), citing numerous reports, states that deep rooting b y oak species in drought environments is n o t unusual and Hellmers et al. (1955) described canyon live oak as a species with deep, coarse roots. Our observations confirm these earlier reports. R o o t s of greenleaf manzanita were, however, restricted to those portions of the soil profile directly beneath sprout clumps and did not extend into areas occupied aboveground by c a n y o n live oak (Fig. 1). Greenleaf manzanita roots were also n o t f o u n d in fractured bed rock. The theory that canyon live oak was able to utilize deep sources of soil moisture unavailable to greenleaf manzanita during the summer drought was reinforced by predawn q~x

RAVEL

MINERAL SOIL

FRACTURED BEDROCK

Fig. 1. Rooting patterns of canyon live oak and greenleaf manzanita.

75 measured in both species. On 1.1 September, for example, mean ~I'x (+ s.e.) for five mature, undisturbed plants of each species were -- 1.19 + 0.08 and - - 1 . 7 5 + 0.21 MPa, respectively, for canyon live oak and greenleaf manzanita. These data are n o t conclusive, however, because leaf conductance was not measured and possible differences between the two species in stomatal control could have affected internal moisture status. Soil water potentials Soil water potential ( ~ s ) at the 2 0 c m depth remained relatively unchanged in all treatments from 30 May to 20 June (Fig. 2). During this period 26 mm of precipitation fell on the study site with no additional precipitation received until late September when only 6 mm fell in two separate events. By 10 July, ~s had become more negative in partial removal and untreated control areas. Partial brush removal, at the level used in this study (--~ 38% of the basal area removed), had little effect on xPs when compared to the untreated control. Increasingly negative ko~ values in total removal areas were not measured until 31 July. After 20 June, ~s were significantly different among treatments except on August 21 when a peak was measured in total removal areas. This pattern of sharply rising and falling ~s in the absence of precipitation during conditions of increasing drought is n o t typical of that measured on a similar site in southwest Oregon - 1.00

--'----0.50

--

UNTREATED CONTROL PARTIAL REMOVAL TOTAL REMOVAL

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DATE OF MEASUREMENT

Fig. 2. Mean (-+ s.e.) soil water potential (XIJs) at the 20-cm depth by treatment (*P =

0.05; NS = not significant).

76 (Hobbs, unpublished data). Normally, a more gradual change in ~ would be expected such as that reported by Conard and Radosevich (1982) at their Sattley site in 1977. The qs peak measured on 21 August most likely represented sampling error or a data anomaly associated with unknown microsite differences unique to two adjacent sampling points where ~s of -- 0.42 and -- 0.50 MPa were measured. Measurement of ~s three weeks later on 11 September at these same points showed potentials of only -- 0.08 and -- 0.12 MPa without additional precipitation during the intervening period. The general pattern of differential soil moisture depletion between various intensities of vegetation control were, however, consistent with those reported from other studies in California and Oregon (Tarrant, 1957; Petersen, 1980; Conard and Radosevich, 1982; Radosevich, 1984).

Douglas-fir response Mean predawn ~x of Douglas-fir saplings were not significantly different among treatments until 31 July when xPx in total removal areas was less negative than either of the other treatments (Fig. 3). This difference persisted and became more pronounced as the summer drought intensified. Xylem pressure potentials were almost identical in the other two treatments despite reduced transpirational leaf surface area of competing sclerophylls in partial removal areas. The peak ~ x in total removal areas on 10 July was the maximum measured for that treatment for all measurement dates. This occurred in early July under favorable ~s conditions (Fig. 2). In their monograph, Hinckley et al. (1978) compiled an impressive amount of information from other reports to show that in conifers conditions may periodically exist that result in a poor correlation between @s and predawn i i,a.i

ill:

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F i g . 3. M e a n (+ s . e . ) p r e d a w n x y l e m p r e s s u r e p o t e n t i a l ( ~ x ) in D o u g l a s - f i r s a p l i n g s d u r i n g t h e first f i v e m o n t h s f o l l o w i n g b r u s h r e m o v a l ( * P = 0 . 0 5 ; N S --- n o t s i g n i f i c a n t ) .

77 potentials within the tree. One such condition is continued transpirational water loss after sunset, the result of incomplete stomatal closure (Hinckley and Ritchie, 1973) which may be possible when warm, nighttime winds exist and ~s conditions are still relatively favorable (Hinckley et al., 1978); conditions that existed on the research site only on 9--10 July. Reinforcing this, Running (1976), and Blake and Ferrell (1977) have also shown that in conditions of favorable internal moisture, Douglas-fir stomata can remain open in darkness. Extended diurnal transpirational water loss and the relatively high frictional resistance to water movement in conifers (Heine, 1971; cited in Hinckley et al., 1978) may have delayed the temporary equilibrium of the water pathway usually associated with predawn measurements. This seems a reasonable explanation for the maximum ~x measured on 10 July, in total removal areas, although leaf conductance was not measured. Relative growth rates (RGR) of Douglas-fir saplings did not differ significantly between treatments for two years prior to brush removal (Fig. 4). However, during the first year after brush removal, saplings in partial removal areas showed a significant (P = 0.05) 48% increase in RGR over the previous year compared to other treatments. This increase in RGR was most likely a response to increased light with partial shading while ~s conditions were still favorable in June and sprout growth not yet complete. Failure of Douglas-fir saplings in total removal treatment areas to respond i

i

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0.40

? ~---

E 0.35 u

UNTREATED CONTROL PARTIAL REMOVAL TOTAL REMOVAL

u

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YEARS

Fig. 4. Mean (+ s.e.) basal area relative growth rates ( R G R ) o f Douglas-fir saplings before and after brush removal (*P - 0.05; NS = not significant).

78 similarly was not surprising. Sudden exposure of shade foliage to full sunlight produced a chlorotic appearance in many trees with a few needles actually turning red and eventually dropping. This response is consistent with those reported in other studies involving the sudden release of conifers from sclerophyll brush competition (Petersen, 1980; Conard and Radosevich, 1982; Newton et al., 1982). During the second year following brush removal, trees in total removal areas showed a large increase in RGR while a slight decline was measured from those in partial removal areas. These were not, however, significantly different from the untreated control because of substantial within-treatment variation. By the end of the third growing season mean RGR values converged and did not differ significantly between treatments. This may have been caused by rapid brush recovery and its probable effect on Vs. The general pattern of increasing RGR after temporary control of sclerophyll brush followed by a decrease in RGR in subsequent years because of brush recovery, has been measured on a similar site in southwest Oregon but with Douglas-fir seedlings (Hobbs, unpublished data). Tesch and Hobbs (1984), although measuring Douglas-fir seedlings, have also shown that increasing levels of sprout competition from canyon live oak and green leaf manzanita substantially reduce Douglas-fir growth. In addition, Petersen and Newton (1985) have reported that in Oregon, ten-year-old Douglas-fir released from snowbrush ceanothus (Ceanothus velutinus Dougl.) by slashing, showed no significant growth increase after five years compared to an untreated control. In their study, cut ceanothus plants nearly regained their original height after five years on some sites by sprouting (T. Petersen, pets. commun., 1985). CONCLUSIONS Manual release of sclerophyll brush by slashing under the conditions of this study produced, at best, only a short-term change in RGR of Douglasfir saplings. The lack of sustained increase in RGR was most likely the result of rapid brush recovery by vigorous sprouting; particularly by canyon live oak which may have resulted in progressively less favorable soil moisture conditions in the three years following manual release. The use of manual slashing to release long-suppressed Douglas-fir trees from mature sclerophyll brush that possesses the potential to produce numerous and vigorous sprouts, does not appear to be operationally real~3tic as currently practiced. We do not suggest, however, that this method of brush control be entirely eliminated as an alternative to chemical treatment. Studies of manual release have not been adequately tested in the Pacific Northwest (Newton, 1984). For example, the effects of early manual control of young sclerophyll sprouts (2--4 years old) shortly after plantation establishment, and before competition becomes severe, should be evaluated. There is also evidence to indicate that the timing of manual slashing treatments may influence sprout growth (Wenger, 1953; Longhurst, 1956;

79 J o n e s and L a u d e , 1 9 6 0 ; Fitzgerald and H o d d i n o t t , 1 9 8 3 ; H a r r i n g t o n , 1 9 8 4 ) . Our k n o w l e d g e o f sclerophyll brush biology, and particularly t h e p h y s i o l o g y o f s p r o u t e m e r g e n c e and g r o w t h , should be e x p a n d e d if m o r e e f f e c t i v e n o n chemical m e t h o d s o f c o n t r o l are t o b e d e v e l o p e d . ACKNOWLEDGEMENTS

This s t u d y was c o n d u c t e d as p a r t o f t h e S o u t h w e s t O r e g o n F o r e s t r y Intensified Research ( F I R ) P r o g r a m as a c o o p e r a t i v e research p r o j e c t b e t w e e n O r e g o n S t a t e University and t h e USDI B u r e a u o f L a n d M a n a g e m e n t , M e d f o r d District. We w o u l d p a r t i c u l a r l y like t o t h a n k Barbara B r y s o n , R o b i n Byars, V e r n o n C r a w f o r d , and E l i z a b e t h G r a n t h a m w h o h e l p e d us with d a t a c o l l e c t i o n u n d e r d i f f i c u l t field c o n d i t i o n s .

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