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Impact of ponderosa pine thinning on Abert’s squirrel (Sciurus aberti) populations in Boulder County, CO

Kelly J. Worden1 and Catherine Kleier2

(1) Red Rocks Community College, Lakewood, CO; (2) Regis University, Denver, CO

INTRODUCTION

DISCUSSION AND CONCLUSIONS

LITERATURE CITED

RESULTS

ACKNOWLEDGEMENTS

Abert’s squirrel (Sciurus aberti) is generally assumed to be ecologically dependent on southwestern ponderosa pine forests for food, cover, and nest sites (Nash and Seaman 1977; Keith 2003), but the structure of this habitat has changed dramatically since European settlement. Once open and park-like (Covington and Moore 1994; Moore et al. 1999), with logging, grazing, and wildfire suppression, ponderosa pine forests have typically become thick, uniform stands of even-aged young pines (Mast 2003). Efforts to restore ponderosa pine forests to pre-settlement conditions are widespread throughout the southwest; these efforts most often include aggressive thinning and prescribed burns (Covington and Moore 1994; Covington et al. 1997; Moore et al. 1999; Allen et al. 2002).

Intensive thinning treatments may be detrimental to Abert’s squirrel because they remove interlocking canopy trees, dramatically decrease tree density often resulting in densities of <60 trees/ha, decrease both the number of large trees and forest structure diversity, and alter the availability of fungi (Mast et al. 1999; Allen et al. 2002; Keith 2003; Dodd et al. 2006). On the other hand, restoration initiatives that involve only “thinning-from-below” (removal of saplings) may improve Abert’s squirrel habitat as characteristics of presettlement forests are achieved; recently, the recruitment and survival of Abert’s squirrel showed a negative correlation with the number of small saplings (Dodd et al. 2006).

METHODS

Figure 1. Study location.

This work was funded by Boulder County Parks and Open Space.Special thanks to Norris Dodd and Jennifer Frey for their expertise in

identifying feeding sign, to BCPOS personnel for their guidance, and to Rebecca Auger, Veronica Given, Geoff Greenfield, Nicole Hupp, and Patrick

Sawyer for their help in the field.

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Cell Bar ChartSplit By: C or TError Bars: ± 1 Standard Error(s)

Twenty seven 4-hectare monitoring plots were established in 9 different areas of Heil Valley Ranch (Figure 1). Each monitoring plot consists of 5 parallel transects, 50m apart, forming a 200x200m area. Within each plot, 1m2 quadrats were used to quantify the abundance of Abert’s squirrel over-wintering feeding sign. Quadrats were placed every 12.5m along each of the 5 transects, giving a total of 85 samples per monitoring plot.

General vegetation characteristics reported to influence Abert’s squirrel populations were measured using the point-centered-quarter method to collect data on 40 trees per plot. This allowed calculation of basal area and of density corresponding to size classes.

Feeding Sign Type Thinned UnthinnedPeeled Twigs 1.47 ± 0.53 1.30 ± 0.40

Terminal Branch Clippings 3.09 ± 1.11 3.96 ± 1.45Cone Cores 3.53 ± 1.79 5.64 ± 1.54

Twigs and Branches Only 3.09 ± 1.11 4.58 ± 1.51All Sign 5.29 ± 2.07 7.99 ± 2.12

Percent of Occurrence Per Monitoring Plot (mean ± SE)

Thinned UnthinnedCanopy Cover (%) 26.86 ± 5.25 43.17 ± 4.30*Ponderosa Pine:

Basal Area (m2/ha) 9.13 ± 2.00 14.55 ± 1.75Total Density (#/ha) 167.51 ± 48.18 320.51 ± 66.23Small Density 63.64 ± 50.54 168.49 ± 51.12Medium Density 101.58 ± 31.91 146.97 ± 20.95Large Density 2.30 ± 1.10 5.05 ± 1.39

Vegetation Characteristics (mean ± SE)

r p-valueCanopy Cover (%) 0.206 0.303Ponderosa Pine:

Basal Area (m2/ha) 0.202 0.312Total Density (#/ha) 0.020 0.919Small Density -0.049 0.807Medium Density 0.142 0.479Large Density 0.340 0.083

Only one plot in thinned forest (N=8) and one plot in unthinned forest (N=19) had no evidence of Abert’s squirrel feeding sign (see Table 1 for frequency of sign type). The highest squirrel density (0.315 squirrels/ha) was found in an unthinned plot. The second highest squirrel density (0.101 squirrels/ha) was found in a plot that had been thinned in 2006.

When all feeding sign is considered, there is a mean of 0.061 squirrels/ha. When cones are excluded, the density drops to 0.032 squirrels/ha. Feeding sign without cones was considered as other herbivores may also chew cones.

Plots located in unthinned stands have greater canopy cover than plots in thinned stands (Table 2). In areas that have been thinned, medium sized trees make up the majority of the stand while in unthinned areas, the proportion of medium and small sized trees is nearly equal (Figure 2).

The density of Abert’s squirrels tends to be higher in unthinned stands but the difference is not significant (Figure 3). When the sampling areas are ranked according to Abert’s squirrel density, Abert’s squirrel tends to prefer forest with relatively high density of large sized ponderosa pines (also see Table 3).

Table 1. Frequency of Abert’s squirrel feeding sign.

Table 2. Vegetation characteristics.Size classes are according to DBH measurements as follows: small = 0 to 20cm; medium = 20.5 to 50cm; large = >50.5cm. * indicates significant difference at p<0.05.

Figure 2. Relative density of ponderosa pine size classes.

Figure 3. Abert’s squirrel density.

Table 3. Relationship of squirrel density to vegetation chars.The direction and strength of the relationship is indicated by the correlation coefficient (r).

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We found no differences between Abert’s squirrel densities in thinned and unthinned ponderosa pine stands at Heil Valley Ranch in 2008. Many studies have indicated that Abert’s squirrels prefer forests that include trees over 40cm DBH (see Keith 2003). Importantly, Heil Valley Ranch showed no difference in the density of large trees between the thinned and unthinned stands. Therefore, although the overall tree density may differ, the major determining factor for the squirrels, large trees, was not different, and so a difference in squirrel density may not have been expected. The forest data also show that while there are more medium sized trees in the thinned plots, this difference did not affect squirrel densities.

So, while some authors have argued that thinning is good for squirrel densities (Keith 1965) and others have argued that thinning is bad for squirrel densities (Koprowski 2005), our study indicates that thinning may not have an effect if the density of trees with DBH over 40cm remains the same.

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Figure 3. Abert’s squirrel density as estimated by all over-winter feeding sign and by peeled twigs and terminal branch clippings only (mean±SE). Different letters indicate significant differences (p<0.05).

Allen, C.D., M. Savage, D.A. Falk, K.F. Suckling, TW Swetnam, T. Schulke, P.B. Stacey, P. Morgan, M. Hoffman, and J.T. Klingel. 2002. Ecological restoration of southwestern ponderosa pine ecosystems: a broad perspective. Ecological Applications 12: 1418-1433.

Covington, W.W., P.Z. Fule, M.M. Moore, S.C. Hart, T.E. Kolb, J.N. Mast, S.S. Sackett, and M.R. Wagner. 1997. Restoring ecosystem health in ponderosa pine forests of the southwest. Journal of Forestry 95: 23-29.

Covington, W.W. and M.M. Moore. 1994. Southwestern ponderosa forest structure: changes since Euro-American settlement. Journal of Forestry 92: 39-47.

Dodd, N.L., R.E. Schweinsburg, and S. Boe. 2006. Landscape scale forest habitat relationships to tassel-eared squirrel populations: implications for ponderosa pine forest restoration. Restoration Ecology 14: 537-547.

Keith, J.O. 2003. The Abert’s squirrel (Sciurus aberti): a technical conservation assessment. USDA Forest Service, Rocky Mountain Region. Available: http://www.fs.fed.us/r2/projects/scp/assessments/abertsquirrel.pdf.

Koprowski, J.L. 2005. Management and conservation of tree squirrels: the importance of endemism, species richness, and forest condition. Pages 245-250 in Gottfried, G.J., B.S. Gebow, L.G. Eskew,, C.B. Edminster, comps. Connecting mountain islands and desert seas: biodiversity and management of the Madrean Archipelago II. Proc. RMRS-P-36. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. Available: http://www.fs.fed.us/rm/pubs/rmrs_p036/rmrs_p036_245_250.pdf.

Mast, J.N. 2003. Tree health and forest structure. Pages 215-232 in P. Freiderici, ed. Ecological restoration of southwestern ponderosa pine forests. Island Press, Washington D.C.

Mast, J.N., P.Z. Fule, M.M. Moore, W.W. Covington, and A.E.M. Waltz. 1999. Restoration of presettlement age structure of an Arizona ponderosa pine forest. Ecological Applications 9: 228-239.

Moore, M.M., W.W. Covington, and P.Z. Fule. 1999. Reference conditions and ecological restoration: a southwestern ponderosa pine perspective. Ecological Applications 9: 1266-1277.

Nash, D.J. and R.N. Seaman. 1977. Sciurus aberti. Mammalian Species 80: 1-5.

Figure 2. Relative density of ponderosa pines by size class (see Table 2).