The relationship between long term trends in climate and pinyon juniper forest ecotone change: Methods for understanding establishment and its potential drivers

Background/Question/Methods

Dynamics of long term vegetation change in mountainous regions are not well known due to lack of long term data. NASA's Landsat mission has been recording the Earth's surface for 40 years and this data can be leveraged to understand landscape change for this time period. The Walker Lake Basin in the western Great Basin provides an excellent location to test methods for understanding long term change in vegetation and the climatic drivers associated with the change. Yearly Landsat imagery covering a 30 year period was used to determine per pixel trends of coniferous land cover change. Imagery from late summer and early fall was first corrected and normalized, then used in the creation of several established vegetation indices. Leaf area index values (LAI) were collected in the field in the fall of 2013 and regressed against the indices to determine which was best representing real biophysical change in the watershed. Downscaled PRISM climate variables for a longer time period (1940 to 2011) were used to create seasonal indices of precipitation and temperature. Trends in climate were compared to vegetation trends using geographically weighted regression techniques.

Results/Conclusions

Pinyon-juniper forests throughout the Great Basin have seen extensive range expansion, but the drivers for that change are not well understood, as timber harvesting, grazing, and other land uses altered these forests historically. Neither does there exist viable methods for quantifying this change over large areas. This study not only provides a method for examining expansion of semi-arid forests, but also attempts to understand overarching climatic drivers of that change. The results show that an average increase in NDVI of 0.05 has occurred at lower tree line. At upper tree line, the increase is slightly lower, 0.04. This correlates to a 0.5 to 0.75 increase in LAI, a large change for these forests. This also indicates lower tree line is becoming denser more quickly than upper tree line. Geographical patterns of precipitation trends show that winter and spring precipitation have a strong relationship with this change, as well as spring temperatures. These methods could inform studies done at larger scales, and to develop better models for future vegetation change in arid ecosystems.