Aaron Snow and Dr. Matthew Bekker, Geography Department

Introduction

In July, 2016, I had the opportunity to attend the North American Dendroecological Fieldweek (NADEF) at Mount Rainier National Park, Washington. NADEF organizes the attendees into five different groups to undergo specific research projects all having to do with tree rings. My group, the intro group, hiked to the southern side of the park, assembled a chronology of mountain hemlock trees (Tsuga mertensiana), and observed notable attributes of the collected data. Because the fieldweek lasted only seven days, our research was minimal, which was why I applied for an ORCA grant in order to take the research to the next level. With the extra funds and time, I sought to do a climatic analysis using tree rings on mountain hemlock trees in the Pacific Northwest, with hopes to come to conclusions based on my findings.

Methodology

To begin, the NADEF group needed to core several mountain hemlock trees (Figure 1). After coring several dozen trees, we mounted, sanded, and cross-dated each core to determine which rings corresponded to which year. We measured and recorded each ring year’s width and used a program called COFECHA to see how well the tree rings on each core corresponded to each other.

After we gathered a vast collection of tree ring measurement data, a statistical computer program called TreeClim combined that data with temperature and precipitation data to see the relationship that mountain hemlock trees have with the environment. The results from analyzing this relationship would help identify climatic attributes with mountain hemlock trees in the Pacific Northwest.

Results

A statistically significant correlation index, which determines how well an individual tree core will correspond with the large collection of the rest of the tree cores, is 0.32. The collection of cores that we assembled at NADEF had a correlation of 0.61, which is very high. In other words, statistically, we can trust the given results of the analysis. These results allowed our group to extend an already existing tree ring chronology over 151 years. In other words, we found out that these trees were 151 years older than previously thought; our cores ranged from the year 2015 to 1551 AD.

My personal analysis with the TreeClim program produced even more effective results. Figure 2 displays a graph produced by TreeClim that reveals the strength of the response in tree growth for each of the months of the year compared with temperature. The left-most dark bar means that a high temperature in the previous year’s February results in a negative response in tree ring growth. The dark bars towards the right indicate that high temperatures in the late spring and early summer months result in a positive response in tree-ring growth. In other words, hot Februarys from a previous year will melt snow that is necessary for tree growth, and high temperatures in spring and summer mean there is a lot of energy and rainfall, also increasing the precipitation necessary for tree growth.

Figure 3 provides a graph that shows the strength of the response in tree growth for each of the months of the year compared with precipitation. As noted before, dark bars indicate a strong response. The first one toward the left indicates a strong positive relationship in tree growth in the previous growing season’s March months. In other words, if there was heavy snowfall in the previous growing season’s March, then enough precipitation was stored to produce a lot of growth for the current year. In contrast to that, the bar next to it and the very dark, low bars afterwards indicate that high precipitation in the previous and current year’s April months have a strong negative response to tree growth. This indicates that high precipitation in the previous and current growing season’s April months can actually inhibit tree growth. This is because in the April months’ precipitation falls in the form of rain rather than snow. If snow, which is the most significant component in tree growth, is washed away by rain, then it cannot be stored for the next growing season.

Discussion

The results indicate that mountain hemlock trees respond best to high snowpack conditions. Higher temperatures in the winter months will diminish potential snowpack, which is the most important limiting factor in tree growth. Increasing yearly temperatures brought out by climate change can ultimately be harmful for these trees, since they get the majority of their growth moisture from winter snow rather than rain. If higher temperature climatic trends continue as they have been in the past forty years then mountain hemlock trees will struggle to survive.

Conclusion

Mountain hemlock trees in Mount Rainier National Parks are clearly part of a snowpack-driven system. Based on the relationship with their tree-ring widths, temperature and precipitation data, we can tell just how well they respond to Pacific Northwest climate and understand how to best care for them.