Coast Redwood Responses to Pruning

Coast Redwood Responses to Pruning Kevin L. O'Hara 1 Abstract A large-scale pruning study was established in the winter of 1999 to2000 at seven different sites on Green Diamond Resource Company forestlands in Humboldt County. The objective of this study was to determine the effects of pruning on increment, epicormic sprouting, stem taper, heartwood formation, and bear damage on these young trees. Pruning treatments varied pruning severity and were usually applied in conjunction with thinning treatments. Trees were assessed six years after pruning. Basal area increment decreased with increasing pruning severity but results were inconsistent from one study site to another. Height increment was unaffected by pruning. Six-year volume increment results resembled those for basal area increment: heavier pruning sometimes resulted in lower increment. Additionally, the negative effects of pruning on tree increment were probably short-lived in redwood because of the fast growth rates of this species. The number of epicormic sprouts was generally unaffected by pruning severity with the notable exception of the most severe pruning treatments. By year six, the number of sprouts was no different in the unpruned treatments than in most pruned treatments. The exception was the severe crown removal that left only approximately 15 percent residual live crown length. Epicormic sprouting does not appear to be a deterrent to pruning in redwood. Tree stem taper was also unaffected by pruning severity. Heartwood formation was expected to increase with pruning severity. However, no effects were evident in these data. Apparently, the greater heartwood expected in more severely pruned treatments was obscured by the same factors that minimized treatment effects on increment: in the six years following treatment, the pruned trees had rebuilt crown foliage and required similar sapwood for water transport as unpruned trees. Bear damage was observed at four of the seven study sites and was severe in several plots. However, no trends were evident relative to pruning treatment. In summary, pruning that leaves residual crown lengths of 40 to 60 percent should result in minimal levels of epicormic sprouting and no effects on tree increment. Key words: Sequoia sempervirens, forest pruning, timber stand improvement, silviculture, wood quality Introduction Forest pruning is a common means to enhance wood quality in young forest stands. By removing lower branches, wood is formed over the branch wound that is clear and usually straight-grained. This clearwood is generally expected to produce higher grade wood products and earn a premium. Pruning may also improve stem taper from pruned logs and enhance heartwood development. In some species, such as coast redwood (Sequoia sempervirens), heartwood has desirable appearance and decay resistance properties which make it more valuable than sapwood. 1 Professor of Silviculture, University of California Berkeley, 137 Mulford Hall #3114, Berkeley, CA 94720-3114. ( kohara@berkeley.edu). 529

GENERAL TECHNICAL REPORT PSW-GTR-238 Although the effects of pruning forest trees are generally positive, it can lead to excessive epicormic branch production in some species, including coast redwood. Epicormic sprouts respond to increased light or temperature on the pruned stem resulting in a profusion of branches. These can negate the value of the pruning treatment by producing wood quality defects in the clear wood produced in the outer tree boles. No previous study has provided an analysis of the effects of pruning on coast redwood. This paper describes a six-year study to look at the effect of pruning on tree increment, stem taper, epicormic sprouting, heartwood development, and bear damage. Project description Pruning plots were established during the winter of 1999 to 2000 in seven stands (study sites) that received pruning treatments in the fall of 1999. All seven study sites were located in Humboldt County, California on Green Diamond land. These plots were designed to measure the effects of pruning on individual trees. Hence plots were assemblages of approximately 30 trees per treatment rather than being of fixed area. All study sites included control plots and some included more than one pruning treatment. Of the seven study sites, all but two were designed to represent standlevel, operational pruning treatments. The other two study sites 299 Cutoff and M- line provided variable pruning treatments over small areas, but not on an operational scale. At time of initial measurement, trees were tagged, and marked at dbh. Measurements taken at time of plot establishment included dbh, height, height to live crown after pruning, height to base of live crown on unpruned trees, counts of epicormic sprouts, and some upper stem diameter measurements (either at 2.7 or 5.5 m height). Additionally, trees of common stump origin were noted. Tree characteristics at first measurement are presented in table 1. The plot remeasurements and data analysis in 2001/02 and 2006 included studies of epicormic sprouting, increment, heartwood formation and stem taper. Analysis of variance was used to compare treatment means within study sites. When ANOVA indicated significant differences between treatment means, Tukey s HSD multiple range tests were used to identify significantly different means. Growth response studies Trees were remeasured in 2001 and 2002 and spring 2006 for height and dbh. Nearly all trees were also measured for diameter at tree height of 2.7 m (9 ft). The original plot measurements in 1999 to 2000 included height and dbh measurements, but not all trees were measured for the 2.7 m upper stem diameter measurement. Cubic volumes were estimated assuming the tree base to bh was a cylinder, bh to 2.7 m a cone frustum, and above 2.7 m as a cone for smaller trees. Equations from Krumland et al. (1977) were used for larger trees. Relative growth was calculated as the 6-year volume increment divided by volume in 1999 to 2000. Results resemble those from pruning studies with other species except for an apparent lack of sensitivity to more severe pruning treatments in redwood than has been observed in other pruning studies (table 2). In other species, a strong decrease in 530

Coast Redwood Responses to Pruning increment with increasing pruning severity is usually found (O Hara et al. 1995). In terms of plot comparisons, there were abnormalities in both the 299 Cutoff and M- Line study sites. At 299 Cutoff, the control plot had much greater increment than expected. At M-Line, the 15 percent residual treatment had greater increment than expected. Table 1 Average pruning heights, initial tree heights and diameters by plot and study site (pruning heights for control treatments are live crown heights at beginning of study) Average residual crown lengths show actual average crown lengths following pruning. Basal area increment/tree was largely unaffected by pruning intensity. Many study sites experienced increased basal area increment following pruning as compared to controls (Maple Creek, Fortuna, and to a lesser extent M-150). However, at 299 Cutoff, the control basal area increment/tree was considerably greater than the pruned treatments. At M-line, no trend was evident and the 15 percent residual crown treatment had virtually the same basal area increment as the control. This suggests a combination of site irregularities across plots and the possible influence of more productive clones on some plots. For example, it is possible that one plot could be dominated by the sprouts of one stump that is very productive, whereas the next plot is dominated by a less productive sprout clump. An additional factor was pretreatment tree size variability and its affect on growth. Growth was therefore also expressed as relative growth by dividing 6-year basal 531

GENERAL TECHNICAL REPORT PSW-GTR-238 area increment per tree by pretreatment basal area. This analysis produced substantially different results than the absolute basal area increment/tree analysis. For the 299 Cutoff study site, the highest relative growth was in the 30 percent residual treatment whereas at M-line it was the 60 percent residual. There were few significant differences in height increment among study plots. This result was expected as only very severe pruning treatments generally affect height increment. Significant differences were found at other study sites between control and pruned height increment, but these may have been the result of bear damage preferentially affecting taller trees thereby reducing average increment. Cubic volume increment followed similar trends as basal area and height increment (fig. 1, table 2). Statistical significance was only found at Mitsui and these results were influenced by bear damage. Relative volume increment (volume increment per unit of pretreatment volume) was statistically significant for some treatments/study sites. For example, at 299 Cutoff, the 30 percent residual treatment was significantly greater than the 15 percent and 35 percent treatment. This suggests an affect related to clonal patterns within measurement plots rather than a positive effect of that particular pruning treatment. In summary, pruning apparently has relatively minor effects on increment in coast redwood as represented by basal area increment, height increment, and volume increment. There is some evidence of reduced increment with the most severe pruning treatments, but for the operational range in pruning severity (> 35 percent residual live crown) there is no effect 6 years after pruning. Figure 1 Cubic volume response after six years at 299 Cutoff. 532

Coast Redwood Responses to Pruning Table 2 Six-year cubic volume increment and relative growth rates. Relative growth rates were volume increment divided by volume at time of pruning. Means followed by common letters within a study site were not significantly different (α=.05). Standard errors (SE) are given in parentheses following means. Epicormic sprout studies Approximately 10 trees per plot were randomly selected in 2002 for epicormic sprout sampling. For pruned trees, the pruned section of the bole was divided into six sections and one section was randomly selected for sampling and marked with paint. For example, a tree pruned to 3 m would have six 0.5 m sections and one of these sections would be randomly selected for measurement. In these sections, all sprouts were counted and the highest and lowest sprouts in the section were measured for length and caliper (diameter at base). The estimated number of sprouts per tree was the section count multiplied by six. Unpruned trees were sampled in a similar way: the section of bole, equal to 5.5 m or the highest level pruned in adjacent pruned plots, was divided into six sections and one section was randomly selected for measurement. On control plot trees, only sprouts that originated at the time of the pruning or after the pruning were counted. This may be a potential source of error as sprout age was difficult to determine. Caliper measurements on small sprouts may also be biased because the presence of scale-like leaves at the base of these sprouts may have exaggerated their diameter. The presence of leaves on larger sprouts had little effect on caliper measurements. This sampling procedure was followed in both 2002 and 2006. The same stem section was therefore sampled at each measurement. Because of sprout mortality between 2002 and 2006, however, the same sprouts may not have been measured in both years. 533

GENERAL TECHNICAL REPORT PSW-GTR-238 The estimated number of sprouts per tree was unaffected by pruning severity except at most severe pruning (table 2). Numbers of sprouts declined from 2002 to 2006. Figure 2 shows the relationship between estimated number of sprouts and residual crown length for 2006 at 299 Cutoff. A flat trend in number of sprouts is apparent until a sharp increase with the 15 percent pruning. After 6 years, there were no significant differences in numbers of sprouts in any of the study sites except the most severe treatments at the 299 Cutoff and M-Line. However, variance was large in these samples because some trees appeared to be much more prone to sprouting than other trees in the same treatment. There may also have been a clonal component to these results where one clone dominated a plot and had a disparate influence on the plot average. In a separate analysis of these data, O Hara and Berrill (2009) showed a clonal effect to sprouting where some clones were more likely to produce epicormic sprouts than others. Figure 2 Estimated number of sprouts per tree in 2006 for treatments at the 299 Cutoff study site. The "5.5 m or 35 percent" residual crown length treatment was pruned to 5.5 m (18 ft) lift or no less than 35 percent residual live crown. Larger sprout length and diameter is indicative of the size of the resulting wood quality defect, and probably indicative of sprout persistence over time. Larger sprouts were found on the most severely pruned trees which suggested these sprouts are a more important component of photosynthesis needs and are therefore likely to persist. This result is consistent with other species including giant sequoia (O Hara et al. 2008). In this study, more severe pruning treatments resulted in a large increase in average sprout size in most study sites, and particularly in the 299 Cutoff and M-Line study sites that included severely pruned trees. For example, at 299 Cutoff, the average sprout caliper in the 15 percent residual treatment was five times the average in the 60 percent residual treatment. Because of the decline in number of sprouts from 2002 to 2006 (table 3), the sample sizes for sprout measurements also declined during this period. This contributed to the high variation in sprout sizes, the lack of consistent trends in sprout size with increasing pruning severity, and the lack of statistical significance when means were quite different. 534

Coast Redwood Responses to Pruning It should be noted that trees in unpruned plots also produced epicormic sprouts. Some of these plots received precommercial thinning treatments at the time of pruning while others were thinned up to three years before pruning. This indicates that some background level of sprouting is present on young redwood trees. In Table 3 Epicormic sprout data for pruned and unpruned plots in 2002 and 2006. The Carlotta plot D was not measured for epicormic sprouts because of extensive bear damage. Common lower case letters within study sites denote means that were not significantly different. Standard errors (SE) are given in parentheses following means. unpruned trees, this sprouting is not a major consideration since clearwood is not an expectation from these plantations. In pruned trees, development and persistence of sprouts, whether background level or not, is likely to cause wood quality defects and reduce potential returns from a pruning investment. However, the results in 2006 indicate no difference in level of sprouting or persistence of sprouts between the unpruned and all but the most severely pruned trees Stem taper A measure of stem taper was calculated by dividing tree diameter at 2.7 m (9 ft) by tree diameter at bh (1.37 m). Note that some plots were not measured at 2.7 m, or experienced so much bear damage by 2006 that samples were too small for analysis. Pruning generally reduces stem taper through reduced radial increment at the base of tree and only a minor effect further up the stem (O Hara 1991). Results from this study showed virtually no affect of pruning on stem taper. 299 Cutoff showed 535

GENERAL TECHNICAL REPORT PSW-GTR-238 increased taper in the control but M-Line had the greatest taper in the most severe pruning treatment. Only the results at 299 Cutoff were statistically significant. These results suggest that pruning has no effect on stem form as represented by the measure of stem taper used here. An upper stem diameter measurement higher on the stem may have yielded a stem form measure more sensitive to pruning. These results were also affected by the rapid growth rates and recovery of trees following pruning that may have obscured affects of pruning on stem taper. Additionally, all plots were thinned at about the same time as the pruning treatments. Since thinning generally increases stem taper, these two opposite effects were both affecting stem form of study trees. Heartwood formation Sample trees were cored at time of establishment of this study, and again in 2006. These trees were randomly selected. Cores were removed at bh and were immediately measured for sapwood thickness based on the visual translucence of the sapwood. Sampling was limited to the same 10 trees/plot sampled for epicormic sprouts. Reducing the size of the crown reduces the transpiration requirements of the tree and also the need for sapwood conducting tissues. However, contrary to expectations, pruning had no effect on heartwood formation in this study. Possible explanations for the lack of a positive heartwood response in these data include the small sample sizes. The only post-pruning significant response was at Maple Creek where sample sizes were also highest. However, when ignoring statistical significance, these results do not reveal the expected trends of increasing percent heartwood with increasing pruning severity. At both 299 Cutoff and M-Line the control treatments were among the highest in percent heartwood and heartwood expansion. A more likely explanation is the rapid growth of these trees over the 6 year period has allowed them to rebuild crowns (through both height growth and epicormic sprouting) therefore requiring comparable sapwood conducting tissues regardless of treatment. Affecting heartwood at final harvest would apparently require repeated pruning or more severe pruning to significantly reduce crown size at end-of-rotation. Bear damage Bear damage was noted in plots at both the 2002 and 2006 remeasurements. The circumference of the damage around the base the tree was estimated to the nearest 10 percent. Bear damage was found in four of the seven study sites: M-Line, Carlotta, M-150, and Mitsui. The worst damage was at Carlotta and Mitsui, but concentrated in several plots. In the case of the Mitsui study site, the plots were considerable distance from each other. There were also no patterns evident with regard to pruning treatment. At Carlotta, two of the pruning treatments experienced damage to greater than 50 percent of the trees whereas the control and another pruning treatment experienced relatively light damage. At M-150, the percent of damaged trees was constant across all treatments. The percent of bole circumference damaged is of limited value because of healing that might have taken place since the damage 536

Coast Redwood Responses to Pruning occurred that reduced the visible damage and the percent values were just visual estimates. Future studies to ascertain the effects of silvicultural treatments on bear damage should involve much larger treatment areas and larger measurement plots than those used in this study. Summary and recommendations The response of coast redwood to pruning varies from typical responses of conifers. The typical decreasing increment with increasing pruning severity pattern was not observed in these study plots despite the inclusion of very severe pruning treatments. Instead, redwood growth is less sensitive to pruning and reductions in crown size than other conifers. Epicormic sprouting occurred following pruning and resembled patterns for other species. These included an initial sprouting response after pruning but a decline in sprouting over time. Six years after pruning, the number of sprouts in the moderate pruning treatments had declined to levels that were comparable to the unpruned controls. Pruning at moderate levels (leaving 40 to 60 percent live crown) had no effect on epicormic sprouting. Pruning did not affect stem taper based on the ratio of diameter at 2.7 m (9 ft) and diameter at bh. A taper measure based on a higher stem diameter measure may have produced results that were consistent with other pruned conifers. Additionally, all plots were also thinned: pruned trees were therefore also reacting to the effects of thinning which generally increases taper. Similarly, there was no effect of pruning on heartwood development. The expectation was an expansion of heartwood because of reduced sapwood requirements for smaller crowns in pruned trees. The rapid recovery of foliage after pruning that contributed to the rapid growth recovery apparently required sufficient sapwood conducting tissues and negated the effect of crown reduction on heartwood development. Several pruning plots have experienced major damage due to bear girdling. There was no apparent pattern to this damage with regard to pruning treatment. However, these small plots were too small to assess bear preferences for pruning treatments. Without the threat of excessive bear damage to pruned trees, pruning would appear to be a viable treatment to enhance wood quality in coast redwood. The longterm effect of pruning on epicormic sprouting is minimal with moderate pruning regimes. Pruning has little effect on redwood volume or height increment making this species one of the least sensitive conifer species to pruning. References Krumland, B.E.; Wensel, L.C.; Dye, J.B. 1977. Young growth volume tables for California coastal conifers. Res. Note No. 3. Berkeley, CA: University of California Berkeley, Coop Redwood Yield Research Project. O'Hara, K.L. 1991. A biological justification for pruning in coastal Douglas-fir stands. Western Journal of Applied Forestry 6(3): 59-63. O'Hara, K.L.; Parent, D.R.; Hagle, S.K. 1995. Pruning eastern Cascade and northern Rocky Mountain species: biological opportunities. In: Hanley, D.P.; Oliver, C.D.; 537

GENERAL TECHNICAL REPORT PSW-GTR-238 Maguire, D.A.; Briggs, D.G.; Fight, R.D., editors. Forest pruning and wood quality. Contribution No. 77. Seattle, WA: Univ. of Washington Institute of Forest Resources: 216-237. O Hara, K.L.; York, R.A.; Heald, R.C. 2008. Effect of pruning severity and timing of treatment on epicormic sprout development in giant sequoia. Forestry 81(1): 103-110. O'Hara, K.L.; Berrill, J-.P. 2009. Epicormic sprout development in pruned coast redwood: pruning severity, genotype, and sprouting characteristics. Annals of Forest Science 66(409): 1-9. 538