Snow accumulation and ablation under fire-altered lodgepole pine forest canopies byPeter Brooks Skidmore A thesis submitted in partial fulfillment ofthe requirements for the degree of Master of Science in Earth Sciences Montana State University © Copyright byPeter Brooks Skidmore (1994) Abstract: Clearcut, burned, and undisturbed lodgepole pine forests in southwestern Montana were studied to determine if forest fire increased snow accumulation and ablation rates onthe forest floor. Snow depth, snow density, and snow water equivalence data were collected at each plot throughout the ablation period during the 1992snow season and the accumulation and ablation periods during the 1993snow season. Forest variables including percent canopy cover, basal area and tree heights were measured during the 1992summer season to assess the effects of forest cover onsnow variables. Results suggest that burned forest canopy reduction results in a 9percent increase in snow water equivalence accumulations as compared to mature forest stands; forest canopy reduction in burned forest stands produces ablation rates which are as much as 57percent greater than in mature forest stands; and forest fire produces a forest structure which approximates the effects ofclearcut openings onsnow accumulation and ablation. SNOW ACCUMULATION AND ABLATION UNDER, FIRE-ALTERED LODGEPOLE PINE FOREST CANOPIES by Peter Brooks Skidmore A thesis submitted in partial fulfillment of the requirements for.the degree of Master of Science in Earth Sciences MONTANA STATE UNIVERSITY Bozeman, Montana May 1994 SK32- ii APPROVAL of a thesis submitted by Peter Brooks Skidmore This thesis has been read by each member of the thesis committee and has been found to be satisfactory regarding content, English usage, format, citations, bibliographic style, and consistency, and is ready for submission to the College of Graduate Studies. Approved for the Major Department Date Approved for the College of Graduate Studies Date iii STATEMENT OF PERMISSION TO USE In presenting this thesis in partial fulfillment of the requirements for a master's degree at Montana State University, I agree that the Library shall make it available to borrowers under rules of the Library. If I have indicated my intention to copyright this thesis by including a copyright notice page, copying is allowable only for scholarly purposes, consistent with "fair use" as prescribed in the U.S. Copyright Law. Requests for permission for extended quotation from or reproduction of this thesis in whole or in parts may be granted only by the copyright holder. Signature — — -— • Date /vNcvc-U \fc\ , I iv ACKNOWLEDGEMENTS To my thesis advisor, Katherine Hansen, I extend my appreciation for her guidance and for her editing which greatly improved this text. William Quimby provided indispensable assistance in statistical design and analysis and both Andrew Marcus and Stephan Custer were an integral part of my research design and offered valuable insight into the intricacies of hydrology and snow science. I could not have completed this project without the friendly assistance of the Soil Conservation Service who provided much of the necessary data for my work. In the field, the assistance of numerous friends lent a measure of safety, enjoyment, and warmth to the many winter hours of sampling. To my faithful companion, Kodiak, I offer special thanks for pulling my equipment and supplies in and out of the field and for her quiet companionship during both frustrated and joyful moments. Field research and transportation costs were funded in part by grants from the Yellowstone Center for Mountain Environments and by Sigma Xi. Both the Department of Earth Sciences and the Yellowstone Center for Mountain Environments provided fellowships for my work at Montana State University. Finally, I wish to express my heartfelt appreciation to William and Patricia Skidmore for their financial assistance and continued moral support over the past two years. V TABLE OF CONTENTS Page LIST OF TABLES. vii LIST OF FIGURES viii ABSTRACT...... x INTRODUCTION............. Objective of Study.. Previous Studies.... Study Area and Plots METHODS...................... 13 Study Plot Design..... 13 Forest Studies:........ 15 Canopy Coverage... 15 Basal Area....... 16 Tree Heights..... 17 Ground Cover..... 17 Snow Studies........... 18 1992 Snow Season.. 18 1993 Snow Season.. 20 Additional Studies 21 Data Analysis......... 22 Forest Study Data. 22 Snow Study Data... 23 Weather Data..... 24 RESULTS AND DISCUSSION....................... ........ . . 25 Forest Comparisons. . ......... 25 Basal Area....................... 25 Tree Heights.............................. 26 Canopy Coverage............................... 28 Weather Conditions....................... 31 The 1992 Snow Season.......................... 3 3 The 1993 Snow Season...... 36 Spatial Autocorrelation of Points............. 38 Snow Accumulation................................... 39 The 1992 Snow Accumulation Period............. 39 Comparison of Accumulation Among Plots.. 40 The 1993 Snow Accumulation Period............. 41 Comparison of Accumulation Among Plots.. 42 Peak Accumulation Transects........ .............. 47 Snow Ablation............................. 50 The 1992 Ablation Period...... ........... . 50 Comparison of Ablation Rates...... 50 Snow Density Trends...................... 52 vi TABLE OF CONTENTS-Contlnued Pd.Cf€ The 1993 Ablation Period. ........ 53 Snow Samplers.............. ......... ;. . .* 53 Comparison of Ablation Rates.........” 54 Snow Density Trends................. 57 CONCLUSIONS.................... 59 Implications of Results............. 62 Further Studies............... .......... ....... * [ ] 63 REFERENCES CITED......................................... 65 APPENDIX...................... 72 vii LIST OF TABLES Table Page 1. Mean canopy coverage, basal area, and tree heights for plot A ..................... 25 2. 25 year average, 1992., and 1993 snow data for the Hebgen Dam weather station , and the study site............. ................. 33 3. 1993 accumulation period snow water equivalence and snow depths..................... 42 4. Statistical significance of the effect of canopy coverage on snow water equivalence .............................. 44 5. Statistical significance of the effect of canopy coverage on snow depth............... 46 6. 1993 accumulation period snow density............ 47 7. 1992 ablation period snow density.......... 52 8. 1993 ablation period snow density................ 58 9. Results of two-sample t-tests............ 73 10. Results of paired t-tests....................... . 73 11. Results of Tukey pairwise multiple comparisons.. 74 viii LIST OF FIGURES Figure ' Page 1. Study site location................................ 9 2. Location of plots within the study site......... n 3. Arrangement of sample points...................... 14 4. Photocanopyometer image and dot planimeter overlay.......................... 16 5. Location of snow measurements taken around sample points............................. 19 6. Histogram of basal areas................... 26 7. Histogram of tree heights........... 27 8. Scatter plot of densiometer and photocanopyometer measurements................. 29 9. Tukey sum-difference graph comparing photocanopyometer and densiometer measurements..................................... 30 10. 1992 and 1993 temperature deviations from the 25 year average.... ................... 34 11. 1992 and 1993 precipitation deviations from the 25 year average........................ 35 12. 1992 and 1993 snow depth deviations from the 25 year average........................ 36 13. Variograms of sample point independence for all plots.............................. 39 14. Snow water equivalence for 1992, 1993, and the 25 year average snow seasons........ 41 15. 1993 peak snow water equivalence and canopy coverage for all plots.............. 44 16. North-south snow depth transect............... .'. 48 17. East-west snow depth transect.................... 49 ix LIST OF FIGURES-Continued Page 18. Snow water equivalence during the ablation period, 199.2......................... 51 19. Snow water equivalence during the ablation period, 1993......................... 54 20. Snow depth during the ablation period, 1993....................... 57'