Graduate Thesis Or Dissertation
 

Canyon Grasslands of the Hells Canyon National Recreation Area : How have they changed over time and what is their future trajectory?

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  • The canyon grasslands of the Hells Canyon National Recreation Area (HCNRA) are a unique ecosystem within the Pacific Northwest Bunchgrass Region (PNWBR) with a long history of natural and anthropogenic disturbances including fire, invasive species introduction, historical livestock grazing, and cultivation. Even with this history, these canyon grasslands contain some of the last remnants of the Pacific Northwest Bunchgrass Region. For thousands of years, these grasslands were occupied by the Nez Perce Tribe and have been grazed since the 1700s. In addition to grazing, settlers cultivated many parts of the HCNRA and some of these homesteads can still be seen today. Both historical and current land uses are strongly influenced by the natural topography of these canyon grasslands, with the highest concentration of land use centered on benchlands while steep canyon slopes avoided cultivation and were used less by livestock. The different plant associations of these grasslands are also influenced by the unique topography of the HCNRA due to the relationships between soil moisture and depth and abrupt alterations in aspect, slope, and elevation. Very few studies have examined the plant associations of the canyon grasslands of the HCNRA, even fewer have asked how they have changed over time, and there are no studies looking into their future trajectory by assessing the seed bank. The first study in my thesis (Chapter 2) focused on how canyon grasslands have changed over time using a repeated survey of vegetation from four different plant associations within the Lower Imnaha Subbasin. From the original study conducted in 1981, a total of 19 different plots in four plant associations were chosen to be resampled in 2014. Since the original study was used to classify seral stages within the plant associations, these successional stages were used to determine if the plant communities had transitioned between the seral classes over the 33 year-time-period. In addition, given the importance of topography to these grasslands, elevation, slope, and aspect were evaluated for their relationship to successional changes. Both Nonmetric Multidimensional Scaling (NMS) and Indicator Species Analysis were used to verify the seral stage classifications for each plot in 1981 and 2014. To evaluate how each association had changed between sampling years, Multi-response Permutation Procedures (MRPP) and NMS were used. Most of these plant associations were relatively stable and had, for the most part, remained at the same seral stage or transitioned to a later seral stage. Among the plant associations, slope was the topographical variable that appeared most related to the transitions in seral stages. Steeper slopes either remained at the same seral stage or transitioned to a later one, while gentler slopes (< 20%) tended to shift from later to earlier seral stages. A relatively new introduced annual grass to the region, Ventenata dubia (not present in the 1981 sampling), was found in three of the four plant associations and was most abundant on the benchlands. The results of this resurvey suggest that topography is related to both the distribution of plant associations and which sites will shift in seral stage over time across the canyon grasslands of the Lower Imnaha Subbasin. The second study of my thesis (Chapter 3) focused on using a seed bank study as one of the many ways to examine the future trajectory of the plant communities in the canyon grasslands of the Lower Imnaha Subbasin, with a particular focus on the benchlands. The seed bank contains the regenerative pool for plant communities and represents the potential for a community to respond to disturbances. The seed bank from benchland sites in one plant association was evaluated in relation to the standing vegetation, successional stage, and historical cultivation. To my knowledge, this was the first seed bank study for the grasslands of this region. Vegetation cover and soil samples were collected from 8 sites, including two previously cultivated and two reference noncultivated sites. NMS was used to extract the strongest community gradients, which naturally separated out the seral stage classifications of the vegetation. To evaluate differences between the vegetation and the seed bank, between successional stages, and between cultivation histories, MRPP was used. Results from the seed bank study are consistent with many other studies around the world in perennial grasslands showing that the vegetation and seed bank are often dissimilar. Similarities between the vegetation and seed bank were highest in the annual grass dominated stage compared to the early seral stage. The effects of cultivation appear to still be evident in the seed bank, where cultivated sites have significantly more introduced grasses compared to noncultivated sites (p < 0.05). There was an overall greater abundance of introduced annual forb and grass species in the seed bank on these benchland sites, suggesting that they may be native seed limited and could easily shift to invasive species dominance after further disturbance, especially on previously cultivated areas. The results of both of these studies suggest that benchlands and lower sloped sites surrounding them in the canyon grasslands of the Lower Imnaha Subbasin may less resistant and resilient to disturbance. Priorities for future research and management may also need to focus on the plant communities of benchlands in these unique grasslands.
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