Ivy Chatwin and Dr. Sam St. Clair, Plant and Wildlife Sciences

Introduction

Quaking aspen, Populus tremuloides, is a keystone species in forest ecosystems and is the most widely distributed tree in North America (St. Clair, Guyon, and Donaldson, 2010). Although it covers less than 10% of the forested landscape, aspen contributes disproportionately to water yield and plant and animal diversity (Kaye, Binkley, and Stohlgren, 2005). Severe drought and increased temperature in recent decades have contributed to largescale aspen die‐offs, (Worrall et al. 2007, Anderegg et al. 2012) likely through a process called hydraulic failure. Hydraulic failure occurs under drought conditions when the pressure required to draw water up a tree’s vascular tissue produces air bubbles (cavitation) that block water and nutrient flow and slowly kill the tree from the top down (Sperry et al. 1994). Field observations (Worral et al 2007) suggest that hydraulic failure primarily affects older aspen trees, implying that younger trees might better survive drought and thus perpetuate the clonal stand. Increasing evidence shows that young trees in many aspen stands have been reduced or eliminated through excessive browsing by livestock and wildlife (Kaye, Binkley, and Stohlgren, 2005). If data show that young trees enhance an aspen clone’s ability to cope with drought, then management measures should be taken to protect young trees from heavy browsing pressure. Previous studies have examined age influence on aspen leaf traits (Smith et al. 2011), compared cavitation among different species of trees (Sperry et al. 1994), and assessed the role of hydraulic failure in drought-stressed mortality in aspen (Anderegg et al. 2012). Unpublished data from the St. Clair lab show that cavitation is more prevalent among aspens at lower elevations than at higher elevations. However, none of these studies compared cavitation among different age classes in an aspen clone, which will help us better understand the clone’s ability to cope with hydraulic stress and the consequences of losing younger aspen stems to heavy browsing pressure.

Methodology

Under the direction of Dr. Sam St. Clair, I visited four aspen stands on the Alpine Loop area near Provo to collect branches from three trees from each of five age classes in each stand. Tree locations were marked using a GPS unit and flagging tape. Sample branches were selected from mid-canopy using a pole pruner, shotgun, or pruning shears, and a 10-20 cm segment was immediately cut from each and submerged in a water-filled cooler. These segments were transported to the Sperry Lab at the University of Utah for hydraulic measurement following the protocol described by Sperry et al. (1994). Stems were then flushed under vacuum pressure for one hour, and measurements were taken again to determine maximum conductivity. Native hydraulic flow divided by maximum hydraulic flow yielded the percent cavitation of each segment.

Results and Discussion

Unfortunately, the data I collected over the summer were inconsistent. At the time, I was struggling with health issues that made lab work difficult; I was unable to concentrate enough to properly use the equipment. Computer issues and equipment malfunctions plagued the project from the beginning. When I finally had everything figured out, an early frost cut the sampling season short.

Conclusion

Even though the results were inconclusive, I feel like I learned invaluable things about myself throughout this project and was able to further develop skills I first learned in the classroom. I enjoyed the beauty and challenges of field work in the mountains during the summer and cherished the peace I felt in the forest. I became a better driver through the many hours of commuting on mountain passes and city streets. As I struggled with mental health issues exacerbated by the stress and monotony of indoor lab work, I realized that a career in research may not be the best fit for me. I plan to develop my writing skills so I can help researchers share their findings with the public. I also hope to spend more time outdoors sharing my knowledge and adventures with the rising generation. I have a greater appreciation for the amazing natural processes around us and for the dedicated individuals who work to understand them. I will forever be grateful for this opportunity to experience the research process and find out more about my strengths and interests.

References

• Anderegg, W.R.L., Berry, J.A., Smith, D.D., Sperry, J.S., Anderegg, L.D.L., Field, C.B., 2012. The roles of hydraulic and carbon stress in a widespread climate‐induced forest die‐off. Proc. Natl. Acad. Sci. 109, 233‐237.
• Kaye, M.W., Binkley, D., Stohlgren, T., 2005. Effects of conifers and elk browsing on quaking aspen forests in the central Rocky Mountains, USA. Ecol. Appl. 15, 1284–1295.
• Smith, E.A., Collette, S.B., Boynton, T.A., Lillrose, T., Stevens, M.R., Bekker, M.F., Eggett, D., St
• Clair, S.B., 2011. Developmental contributions to phenotypic variation in functional leaf traits within quaking aspen clones. Tree Physiol. 31, 68–77.
• Sperry, J.S., Nichols, K.L., Sullivan, J.E.M., Eastlack. S.E., 1994. Xylem Embolism in Ring‐Porous, Diffuse‐Porous, and Coniferous Trees of Northern Utah and Interior Alaska. Ecol. 75, 1736‐1752.
• St. Clair, S.B., Guyon, J., Donaldson, J., 2010. Quaking Aspen’s Current and Future Status in Western North America: The Role of Succession, Climate, Biotic Agents and Its Clonal Nature. Prog. in Bot. 71, 371‐400.
• Worrall, J., Egeland, L., Eager, T., Mask, R., Johnson, E., Kemp, P., Shepperd, W., 2007. Sudden aspen decline in southwest Colorado: site and stand factors and a hypothesis on etiology. Pro. West. Int. For. Dis. Work Conf. (WIFDWC) 55, 1‐4.