Differential Impacts of Climate on Tree Rings across a Topographic Gradient

In 2012 and 2013 annual tree rings were measured in two co-occurring, yet contrasting tree species, the mesic red oak (Quercus rubra) and the more xeric chestnut oak (Quercus montana) across a topographic gradient to access differential impacts of climate and disturbance events. The study sites were located on the lower slope, southeast aspect, northwest aspect, and ridge top in the Ridge and Valley province of central Pennsylvania, USA. During the 20th century, but prior to 1980, growth rates on most sites were statistically similar. After 1980, both species experienced significantly higher basal area increment (BAI) on southeast aspects than northwest sites. Increased growth rate was significantly correlated with increasing tree size. However, this relationship varied across site types and was impacted by disturbances from severe storms in the mid-1950s, 1966, and mid-1990s and gypsy moth outbreaks in the 1980s. Both species exhibited the highest frequency of tree ring releases on southeast aspects. Tree growth was also affected by temperature, precipitation and topographic position. Red oak growth was negatively correlated with current year temperature, particularly on northwest and ridge top sites. Chestnut oak growth responded positively to increasing temperature on southeast and lower slope sites. Moreover, red oak growth on northwest and ridge top locations were negatively correlated with winter temperatures while chestnut oak growth was positively correlated with winter temperatures on lower slope and southeast sites. We conclude that changing climate patterns, including increased temperature, precipitation, and extreme climatic events and site variability affected the growth rates of red oak and chestnut oak.