Bottomland hardwood forest growth and stress response to hydroclimatic variation: Evidence from dendrochronology and tree ring Δ¹³C values

Wetland forests around the world have been reduced to a small proportion of their original expanse due to changing climatic conditions and intensification of human land use activities. As a case in point, the Columbia bottomland hardwood forests along the Brazos-Colorado coastal basin on the Gulf coast of Texas are currently threatened by an increasingly erratic hydroclimate in the form of both extreme floods and droughts and by urban expansion. In this study, we use dendrochronology and tree ring carbon isotopes to understand the effect of changing hydroclimatic conditions on the functional attributes of these forests. We examined the tree rings of Quercus nigra at four sites within the Columbia bottomlands, of which one site experiences frequent and prolonged flooding, while the other three are less flood prone. The objectives of this study were to (i) understand the impact of hydroclimatic variation on radial growth, using tree ring width analysis, (ii) assess the magnitude of physiological stress inflicted by extreme hydroclimatic conditions, using tree ring Δ13C measurements as a proxy, and (iii) evaluate the relationship between tree ring width and Δ13C values. Radial growth across the landscape was influenced most strongly by the midgrowing season climate, while the early growing season climate had the strongest effect on Δ13C. Growth inhibition was minimal, and tree ring Δ13C values were not affected in trees at the wetter site under extreme hydrological conditions such as droughts or floods. In addition, trees at the wet site were less sensitive to precipitation and showed no response to higher temperatures. In contrast, trees at the three drier sites experienced growth inhibition and had lower tree ring Δ13C values during dry periods. Our results indicate more favorable growing conditions and lower stress in trees growing under wetter hydrological conditions. Management and conservation strategies dependent on site-specific conditions are critical for the health of these wetland forests under a rapidly changing hydroclimate. This study provides the first dendrochronological baseline for this region and a better understanding of favorable conditions for the growth and health of these forests, which can assist in management decisions such as streamflow regulation and conservation plans.