The remnant paper birch (Betula papyrifera) riparian forest of the Niobrara River Valley NE, represents one of the southernmost distributions of the species in North America, in a climate which is typically far warmer than its northern boreal counterpart. We hypothesized that while microclimate conditions allow currently established individuals to persist, climate change is causing a decline in population health and growth, and therefore regeneration of the forest. We used dendroecological (raw ring width, standardized ring width, and basal area increment increase), in combination with remote sensing techniques to investigate the responses of this species to key microclimate variables including current and previous year, monthly and seasonal temperatures, precipitation, and streamflow. Tree ring growth was also correlated with pixel-based LANDSAT5-derived peak-season normalized difference vegetation index (NDVI) satellite imagery to determine if this technique could be used as a proxy of tree health.
Statistical analyses using “R”, demonstrated a general positive relationship between tree ring growth and early to mid-growing season precipitation of current year, and a general negative relationship between tree ring growth and temperatures during both the early spring of current year and previous year’s fall season. Paper birch did not seem to be reliant on streamflow for growth. Phenologically this reflects the dependence of this species on temperature for bud formation and bursting and on water availability early in the growing season for growth. Tree ring growth was observed to show a high correlation (R2 0.96) with NDVI in the forest and adjacent grasslands, indicating the potential usefulness of this technique in studying the forest health. Our findings demonstrate that an increasing trend in fall minimum and maximum temperatures and a decreased spring precipitation contribute to the decline in paper birch health. Results from this study can be used in predicting dynamics of this species in its current dominant range of distribution.