The Relative Roles of Precipitation and Vapor-Pressure Deficit in Explaining Interannual Variability of Tree Growth in Eastern North America

Atmospheric water supply and demand are key drivers of drought events that often lead to tree growth decline. Here, to assess how these two factors affect tree growth within the deciduous forests of eastern North America, we use the Standardized Precipitation Index (SPI), which focuses solely on precipitation (P), and the Standardized Vapor Pressure Deficit Index (SVPDI), based on vapor-pressure deficit (VPD), to evaluate the impacts of atmospheric water supply and demand on interannual variations in the growth of common eastern North American tree species: Liriodendron tulipifera , Quercus alba , Quercus rubra , and Acer saccharum . We first examine the functional form of the relationship with standardized P and VPD to determine the linearity of the growth response. Then, using a variety of drought thresholds, we evaluate the difference in the magnitude of growth reduction during SPI and SVPDI drought events. While SPI generally has a stronger linear relationship with growth compared to SVPDI when assessed across the full range of moisture conditions, further analysis reveals that, when the indices indicate drought conditions, SVPDI better captures growth declines compared to SPI. As an additional comparison, we show that moderate droughts as determined by SVPDI resulted in larger growth decreases than droughts indicated Standardized Precipitation Evapotranspiration Index (SPEI), which uses the difference between P and potential evapotranspiration. Further, simple linear regression analyses indicate that, when isolating the effect to a single drought index, explained variance in tree growth is larger for SVPDI than for SPI or SPEI. In line with these results, a multiple linear regression confirms that SVPDI captures more growth reduction than SPI while quantifying the compound effect of atmospheric dryness and meteorological drought. These results suggest that VPD has a greater limiting effect than precipitation on tree growth, which is important for overall forest ecosystem function in this region. Given that further temperature-driven increases in VPD are likely, they may lead to declines in tree growth even when precipitation is not limiting, which may impact carbon storage in eastern North America.