Stump Sprouting of Blue Oaks 19 Years After Harvest 1

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1 Stump ing of Blue Oaks 19 Years After Harvest 1 Doug McCreary, 2 Bill Tietje, 2 and Bill Frost 3 Abstract In 1987, a study was initiated to determine how the sprouting of harvested blue oak (Quercus douglasii) trees was affected by the season the trees were cut down, the height of the residual stumps, and whether the stumps were protected by fencing. Five sites throughout the range of this species were selected, and at each site 192 trees were harvested. After 19 years, 31 percent of the stumps had viable sprouts. The greatest differences among treatments were between stumps protected with fencing and those unprotected from browsing animals, with five times as many protected stumps surviving, compared to unprotected ones. Twice as many 90-cm-tall stumps than basal-cut stumps had surviving sprouts, and these sprouts were both taller and had larger diameters. Differences among harvest dates were relatively small, but there were significant differences among sites for most response variables. However, there were no clear site attributes to explain these differences. These results indicate that if the cutting of trees in densely stocked blue oak stands is necessary, it may be possible to alter stand age structure and promote the establishment of young sprout-origin trees. The replacement of even-aged stands with stands of varying ages may help mitigate the negative impacts of inadequate natural regeneration. Keywords: Browsing, Quercus douglasii, regeneration, stumps, thinning. Introduction Purpose of Original Study For nearly a century there has been concern that several native California oak species are not regenerating adequately to sustain populations (Jepson 1910). One of the species identified as having poor regeneration is blue oak (Quercus douglasii), a member of the white oak sub-genera endemic to the state (Bolsinger 1988, Muick and Bartolome 1987, Swiecki and others 1997). This species has a wide distribution, extending from the Siskiyous in the north to the Tehachapis in the south. It is commonly described as forming a bathtub ring around the Central Valley, with extensive stands along the lower elevation foothills of the east side of the coast range, the west side of the Sierra Nevada, and the foothills of the transverse ranges. Because natural regeneration of blue oak is not always adequate to replace mortality (Bolsinger 1988, Muick and Bartolome 1987, Swiecki and others 1997), we wanted to determine whether sprouting could successfully help reestablish a new cohort of trees in cases where trees were going to be harvested to meet other management objectives (firewood harvesting, thinning densely stocked stands, etc.). If it could, then tree harvesting might result in multi-aged stands, partially mitigating concerns about poor natural regeneration and helping to promote stand longevity. 1 An abbreviated version of the paper was presented at the Sixth California Oak Symposium: Today s Challenges, Tomorrow s Opportunities. October 9-12, 2006, Rohnert Park, California 2 Natural Resources Specialists, University of California Cooperative Extension (UCCE). 3 3 Natural Resources Advisor, University of California Cooperative Extension (UCCE.). 333

2 GENERAL TECHNICAL REPORT PSW-GTR-217 One of the principal objectives of this project was therefore to identify variables that influence sprouting. The variables examined in this study were stand location, season of harvest, stump height, and protection of the sprouts from browsing animals. Methods of the Original Study In 1987, we identified five densely stocked blue oak stands throughout its range in California to determine sprouting response in a wide range of environments. The stand characteristics at each of these sites are shown in table 1. The five harvest sites ranged from Glenn County in the north to San Luis Obispo County in the south. Three were located in the foothills of the coast range, and two in foothills of the Sierra Nevada. Four plots were established at each site. Each of these plots contained at least 96 trees with a diameter breast height (DBH) of 7 cm or larger. Plots ranged in size from 0.15 to 0.30 ha. Two plots per site were randomly chosen to be protected, and two were left unprotected. Protection consisted of erecting a 1.8- m-tall fence around the plot perimeter, effectively preventing access by deer, cattle, and sheep and eliminating browsing of any new sprouts that emerged following harvest. While the unfenced plots at all sites were grazed by domestic livestock, season of use and stocking rate were not recorded. Table 1 Site characteristics of the blue oak harvest sites 1 Basal area (m2/ha) Average DBH (cm) Annual precip. (cm) Soil depth (cm) Site Density (stems/ha) Soil texture Mendocino Silt loam Glenn Clay loam Butte Clay loam Amador Grav. loam San Luis Obispo Grav. loam 1 Precipitation data are from the nearest NOAA weather station. Soils information is from U.S. Soil Conservation Service reports for Glenn, Butte, Amador, San Luis Obispo, and Mendocino Counties. Within each plot, all trees were tagged with sequential numbers from 1 to 96. Half of the trees within each plot were randomly selected for harvest. Of these 48 harvest trees per plot, 12 were chosen randomly for harvest in each of the four seasons (summer, fall, winter and spring). Half of the 48 trees were selected to be cut 90 cm above the ground and half for cutting at the base of the tree. All plots were evaluated in 1988, 1989, 1997, and Each harvested stump was evaluated for presence or absence of living sprouts, number of sprouts, and length of the tallest sprout. During the last two evaluation dates, the diameter of the largest living sprout per stump was also recorded. Prior to the 2006 assessments, sprouts were also assessed for browsing damage. Results from the earlier evaluations for this study were presented in 1990 at Symposium on Oak Woodlands and Hardwood Rangeland Management at Davis in 1990 (McCreary and others 1991), and in 2001 at the Fifth Symposium on Oak Woodlands (McCreary and others 2002). The 1990 results indicated that harvest date had relatively little influence on sprouting, while stump height had a large effect, 334

3 Stump ing of Blue Oaks 19 Years After Harvest McCreary with a far greater percentage of taller stumps sprouting. Similarly, there were significant differences in percent sprouting and average sprout growth among sites, with the most favorable site having sprouts more than twice as long as sprouts from the least favorable site. Fencing had relatively little effect on the incidence of sprouting, but greatly influenced the lengths of longest sprouts, with fenced plots having longer lengths. Incidence of sprouting was negatively correlated with DBH. In the 2002 paper, we reported that site and stump height still had significant impacts on sprouting response 10 years after harvest. However, unlike the evaluations in 1988 and 1989, protection of the sprouts was the most influential variable with nearly four times as many protected stumps having living sprouts (54 percent) as unprotected stumps (14 percent). Methods of the 2006 Evaluation In summer 2006, all plots were revisited and all stumps that had living sprouts were measured. After 19 years, not all the original stumps that failed to sprout or whose sprouts had died could be located, due to decay and deterioration. We are confident we located all stumps with living sprouts because they were easy to see and find, especially with the assistance of plot maps that showed the location of all of the original stumps. Each stump with living sprouts was assessed for the number of sprouts emerging from the stump, the length of the longest living sprout, and the diameter of the largest living sprout at its base. The length of the longest (tallest) sprout was the distance from the base of the stump to the tallest point in the sprout clump. Except for sprout diameter, these same variables had also been recorded during each of the three previous assessments. As it was for the earlier assessments, data was analyzed using analysis of variance for a doubly nested randomized block design, with sites as the main plots, fences as the sub-plots, and combinations of harvest dates and stump heights as the sub-sub plots. However, due to missing data resulting from sprout mortality, by 2006 (as well as for the 1997 data) a simplified model was required to analyze all variables. Because few differences were found for harvest dates when this study was evaluated in earlier assessments, the simplified model we used combined responses among harvest dates. Results Survival (the percentage of stumps with living sprouts) Although sprout survival increased slightly during the first two years of the study (table 2), between 1989 and 1997 it dropped markedly. Between 1997 and 2006 there was a continued reduction in survival, but at a far diminished rate. Average survival for the entire study dropped from 34 percent at time of the last assessment in 1997, to 31 percent in 2006 (table 2). The greatest reduction in survival occurred at the San Luis Obispo site, dropping from 38 percent in 1997 to 32 percent in However, the reduction in survival for all sites combined was not equally apportioned among treatments, with the largest reduction in survival for stumps in unfenced plots. In 1989, average survival of unfenced stumps was 59 percent. By the 1997 assessment, this had dropped to 14 percent. By 2006 it had gone down further, to 11 percent, a reduction of more than one-fifth (table 4). In contrast, average 335

4 GENERAL TECHNICAL REPORT PSW-GTR-217 survival of fenced stumps was 67 percent in 1989, 54 percent eight years later, and 51 percent in There were also significant differences between the survival of basal stumps and those cut at 90-cm, with the taller stumps having more than twice the average survival of those cut at ground level (42 percent vs. 20 percent) (table 5). This was consistent with results from measurements taken nine years earlier. Table 2 Average survival (percent) for the five sites for each of the 4 measurement periods 1. Site Mendocino 62 a 72 a 29 a 26 a Glenn 34 b 37 b 28 a 28 a Butte 63 a 65 a 44 a 40 a Amador 59 a 65 a 32 a 29 a San Luis 73 a 76 a 38 a 32 a Obispo Total Values within a column are significantly different (P< 0.05) if they are followed by a different letter (Fisher s protected LSD test). Table 3 Average response of different sites 19 years after harvest 1. Site survival (%) Number of sprouts length (m) diameter (cm) Mendocino 26 a 15.0 a 1.9 a 2.8 a Glenn 28 a 7.8 b 2.3 ab 4.6 ab Butte 40 a 3.0 c 4.1 c 7.7 c Amador 29 a 3.9 c 3.4 bc 6.7 bc San Luis Obispo 32 a 8.2 b 2.7 ab 3.8 ab 1 Values within a column are significantly different (P< 0.05) if they are followed by a different letter (Fisher s protected LSD test). Table 4 Average response of different fencing treatments 19 years after harvest 1. Fencing Treatment survival (%) Number of sprouts length (m) diameter (cm) Unfenced 11 a 9.5 a 2.1 a 3.7 a Fenced 51 b 5.7 b 3.6 b 6.5 b 1 Values within a column are significantly different (P< 0.05) if they are followed by a different letter (Fisher s protected LSD test). 336

5 Stump ing of Blue Oaks 19 Years After Harvest McCreary Table 5 Average responses of different stump heights 19 years after harvest 1. Stump height survival (%) Number of sprouts length (m) diameter (cm) Basal 20 a 6.1 a 2.3 a 4.4 a 90 cm 42 b 9.1 b 3.5 b 5.8 b 1 Values within a column are significantly different (P< 0.05) if they are followed by a different letter (Fisher s protected LSD test). Number of s The average number of living sprouts per living stump declined steadily over time, a trend that continued into The average number of sprouts per living stump was 30.6 in 1989, 8.4 in 1997, and 7.6 in There were significantly more sprouts per stump in unfenced plots (table 4), and taller stumps had significantly more sprouts than basal stumps (table 5). There was also a significantly larger number of sprouts on stumps at the Mendocino site than at the other four sites (table 3). Length The lengths of the longest sprouts were significantly greater in fenced than in unfenced plots (table 4); lengths were also greater for taller stumps (table 5). There were significant differences in sprout length among harvest sites, with those in Mendocino being significantly shorter than those in Butte or Amador counties (table 3). Diameter diameter and sprout length followed a similar pattern. There were significantly larger diameter sprouts for taller stumps (table 5) and for stumps within fenced areas (table 4). As was the case for sprout length, sprouts in Mendocino were significantly thinner than those in Butte or Amador counties (table 3). Interactions There were several interactions among response variables that warrant comment. For survival, there was a significant interaction between fencing and stump height. Even though the taller stumps had greater survival in both fenced and unfenced plots, the magnitude of the difference was much greater in the fenced plots. There were also significant interactions for height and diameter. For height, there was a significant interaction between site and stump height and between site and fencing treatment. At all sites, taller stumps grew more than shorter stumps and fenced stumps grew more than unfenced stumps. However the magnitude of these differences varied considerably among sites, ranging from only being slightly greater, to being more than double (the heights were more than twice as tall). The interactions for diameter followed a similar pattern. At all sites except one (Glenn), shorter stumps had smaller diameters than taller stumps. At the Glenn site, however, the shorter stumps had slightly larger average diameters, though these differences were not significant. At all sites except one (Amador), fenced stumps had larger diameters than unfenced stumps. At this site, however, the diameters of stumps in fenced plots and unfenced plots were very similar, but were slightly larger though not significantly so in unfenced plots. 337

6 GENERAL TECHNICAL REPORT PSW-GTR-217 Finally, there was also a significant interaction between fencing and stump height for diameter. Taller stumps had greater diameters both within and outside fences, but these differences were greater in the unfenced plots. All in all, the significant interactions indicate that the responses to stump height and fencing at the different sites were quite similar with taller stumps and fenced stumps performing better than shorter or unfenced stumps but that the degree of difference between these treatments can and does vary somewhat by site. Discussion ing is common for many species of hardwoods and it is likely that many of the oak trees that are alive in California today originated from sprouts that grew from stumps after the top of the tree was killed by fire or felling. For example, studies reveal that harvested stands or stands destroyed by fire were replaced by multiplestem trees of similar age, presumably from sprouting (McClaran and Bartolome 1989, Mensing 1992). ing is apparently an adaptive strategy that allows trees to remain alive and reoccupy a site following disturbance. The ability of oaks to sprout from their base following death of the aboveground portion of the tree varies by species (Longhurst 1956), size of the individual tree (Johnson 1977), and environmental conditions at the site (Hannah 1987). Generally, sprouting is greater for evergreen or live oaks than for deciduous oaks, for smaller diameter stumps, and for trees growing in moister environments with more light. While blue oak is commonly thought of as a weak sprouter compared to tanoak and California black oak (McDonald 1990), the original assessment of the 960 trees harvested in this study found that a relatively large proportion (63 percent) had living sprouts two years after harvest (McCreary and others 1991). In general, the smaller stumps tended to sprout more, but we detected no differences in sprouting among the four seasons of harvest, in contrast to Longhurst (1956) who reported higher sprouting for blue oaks harvested in winter. Standiford and others (1996) also reported relatively high sprouting for blue oak in the northern Sacramento Valley, with 54 percent of the trees having sprouted, even though many stumps had originally been treated with herbicides to prevent re-growth. One of the initial goals of this study was to identify variables that influence sprouting so that when harvesting is done as part of other management objectives, it may be possible to promote sprouting in the resultant stumps. Encouraging sprouting could then be used to promote stand longevity and even if natural regeneration was insufficient to replace mortality under natural circumstances, lengthening the life of stands could allow more time for seedling recruitment to occur. However, because there would be more ladder fuels, such stands may be more susceptible to catastrophic fire and they would remain vulnerable to anthropogenic pressures including development, agricultural conversions, and firewood harvesting. It is also worth mentioning that many blue oak stands are regenerating adequately and sprout regeneration would not be necessary or encouraged. It was further speculated that opening the stands up through thinning, and fencing out grazing and browsing animals, might also promote natural regeneration, though these effects were not officially evaluated in the study. However, previous research has demonstrated that thinning of blue oaks can result in increased acorn production (Standiford and others 1999) that could lead to greater regeneration of 338

7 Stump ing of Blue Oaks 19 Years After Harvest McCreary seedlings. We also did observe many vigorous naturally regenerating oak seedlings in fenced plots, especially at the plot in Mendocino County. In contrast, few seedlings were observed in unfenced plots. The data from this study suggest that sprouting can be used to alter age structure in stands since, after 19 years, a sizeable number of the harvested trees have living, vigorous sprouts. However, it is also clear that sprouting is influenced by a number of variables, especially protection from browsing animals using fencing. In most locations, nearly two decades of exposure to browsing animals resulted in high sprout mortality and greatly reduced sprout growth. This effect was particularly evident at the plot in Mendocino County where high populations of deer from a resident herd, and additional pressures from a flock of sheep, resulted in the sprouts on all but one stump being killed in the unfenced plots. In addition to causing increased mortality at all sites, browsing in unfenced plots reduced sprout size, but tended to increase the number of sprouts per stump. Repeated browsing apparently caused the stumps to continue producing more young sprouts and interrupted the normal thinning response observed inside the fenced plots. The data also demonstrated that if the sprouts survive for a decade, there is a high likelihood they will remain alive for at least 19 years. While there were differences in sprout growth among sites, it is not clear what site variables were likely responsible. The site with the highest average annual precipitation (Mendocino) had the smallest average sprout length and diameter. Perhaps growth was influenced by environmental conditions associated with different elevations, since the sites with greatest height and diameter growth (Butte and Amador) were at the two lowest elevations. It is also possible that stump diameter adversely affected sprout response since the stumps at the Mendocino site had the largest average diameters. Another concern we had after the study was in place for only two years was that the tall stumps from which sprouts were growing might decay more rapidly, causing the sprouts to die. After 19 years there is no evidence that this is a serious problem, and fewer 90-cm stumps had sprout mortality between 1997 and 2006 than did basalcut stumps. By 2006, the average percent sprouting for 90-cm stumps was more than double that for basal-cut stumps, and the average height and diameter of sprouts from taller stumps were significantly greater than for sprouts growing from basal-cut stumps. It thus appears that retaining taller stumps during harvest promotes greater sprouting success. Finally, it should be mentioned that there are other potential drawbacks to relying exclusively on stump sprouting to regenerate harvested stands. While a substantial number of harvested trees in this study did sprout, we would expect the percentage to go down with each harvest cycle. That is, trees likely cannot be repeatedly cut down and continue to sprout. Also, sprout-origin trees are genetically identical to the harvested trees so there is no additional genetic diversity associated with sexual reproduction as there would be for acorn-produced trees. In forested stands where the environment is changing over time, as many predict from global climate change models, replacing trees from sprouting rather than from acorns may make it less likely that species will adapt to these changes. 339

8 GENERAL TECHNICAL REPORT PSW-GTR-217 Conclusions Results from this study indicate that even after 19 years, over 30 percent of harvested trees have viable sprouts. Whereas there was considerable attrition in sprout survival between assessments in year 2 and year 10, sprout survival of all originally harvested trees only went down by 3 percent in the subsequent 9 years. The majority of stump death during this interval appeared to be caused by repeated browsing damage since the increase in mortality outside fenced exclosures was approximately four times that inside. In areas where browsing pressures are intense, either from livestock or deer, it is critical to protect sprouts. Since the study was established in 1987, season of harvest has had relatively little impact on sprouting. However, there was considerable variation in survival of sprouts, as well as sprout growth, among harvest sites. s from stumps cut at 90 cm continued to perform better than those cut at ground level, and there was no indication that these stumps deteriorated rapidly or that their sprouts died. All in all, this study indicates that if blue oak stands are thinned for other management objectives, sprouting can help these stands replace themselves. ing can alter stand structure and promote the establishment of a new, younger, cohort of trees. In this manner, it may prolong the life of stands, even when there is poor natural regeneration. References Bolsinger, C.L The hardwoods of California's timberlands, woodlands and savannas. Resource. Bulletin PNW-148. Portland, OR: Pacific Northwest Research Station, Forest Service, U.S. Department of Agriculture; 148 p. Hannah, P.R Regeneration methods for oaks. Northern Journal of Applied Forestry 4: Jepson, W.L The Silva of California. University of California memoirs. Berkeley, Calif; 480 p. Johnson, P.S Predicting oak stump sprouting and sprout development in the Missouri Ozarks. Res. Pap. NC-149. St. Paul, Minnesota: North Central Forest Experiment Station, Forest Service, U.S. Department of Agriculture; 11p. Longhurst, W.M Stump sprouting of oaks in response to seasonal cutting. Journal of Range Management 9(4): McClaran, M.P.; Bartolome, J.W Fire-related recruitment in stagnant Quercus douglasii populations. Canadian Journal of Forest Research 19: McCreary, D.D.; Tietje, W.D.; Schmidt, R.H.; Gross, R.; Weitkamp, W.H.; Willoughby, B.L.; Bell, F.L Stump sprouting of blue oaks. In: Standiford, Richard B., technical coordinator. Proceedings, Symposium on Oak Woodlands and Hardwood Rangeland Management; 1990 October 31-November; Davis, CA. Gen. Tech. Rep. PSW-126. Albany, CA: Pacific Southwest Forest and Range Experiment Station. Forest Service, U.S. Department of Agriculture; McCreary, D.D.; Tietje. W.D.; Frost, B Stump ing of Blue Oaks 10 Years after Harvest. Proceedings of the Fifth Oak Symposium: Oaks in California s Changing Landscape. Richard B. Standiford, Douglas McCreary and Kathryn L. Purcell, Technical Coordinators. Held Oct , 2001, in San Diego, CA. Gen. Tech Rep. PSW-GTR Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture;

9 Stump ing of Blue Oaks 19 Years After Harvest McCreary McDonald, P.M Quercus douglasii Hook. & Arn. Blue Oak. In: Burns, Russell M.; Honkola, B.H. technical coordinators. Silvics of North America - Volume 2. Hardwoods. U.S. Department of Agriculture, Forest Service, Washington, D.C. Agriculture Handbook No. 450; Mensing, S The impact of European settlement on blue oak (Quercus douglasii) regeneration and recruitment in the Tehachapi Mountains, California. Madroňo 39(1): Muick, P.C.; Bartolome, J Factors associated with oak regeneration in California. In: Plumb, Timothy R.; Pillsbury, Norman H. technical coordinators. Proceedings of the Symposium on Multiple-Use Management of California's Hardwood Resources Nov ; San Luis Obispo, CA. Gen. Tech. Rep. PSW-100. Albany, CA: Pacific Southwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture; Standiford, R.B.; Phillips, R.; McDougald, N.K Fire history of blue oak rangelands in California's southern Sierra Nevada, and restoration of fire-related stand structure through thinning. In: Fire Management: Emerging Policies and New Paradigms, California Association for Fire Ecology (CAFÉ) 1999 Symposium, Nov , 1999, San Diego. Standiford, R.B.; McCreary, D.D.; Barry, S.; Forero, L.; Knight, R.L Impact of firewood harvesting on hardwood rangelands varies with region. California Agriculture 50(2): Swiecki, T.J.; Bernhardt, E.A.; Drake, C Factors affecting blue oak sapling recruitment and regeneration. In: Pillsbury, Norman H. ; Verner, Jerry; Tietje, William D. technical coordinators. Proceedings of a Symposium on Oak Woodlands: Ecology, Management and Urban Interface Issues Nov San Luis Obispo, CA. Gen. Tech. Rep. PSW Albany, CA: Pacific Southwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture; Continue