HYDROLOGIC DRIVERS OF RIPARIAN TREE GERMINATION AND GROWTH ALONG THREE TEXAS RIVERS

Abstract

With increased regulation of rivers for human use, an improved understanding of the needs of the riparian ecosystem is necessary to develop management practices that sustain these ecosystems. The primary objective of this study was to develop a low expenditure method for the Texas Water Development Board to assess the riparian condition by relating germination and growth to flow histories. My study sites were located along the lower reaches of the Brazos River, Colorado River and Guadalupe River in Texas, USA. Six target riparian species were chosen for this study representing fast maturing species (Acer negundo, Populus deltoides, and Salix nigra) and slow maturing species (Fraxinus pennsylvanica, Platanus occidentalis, and Taxodium distichum). Germination years and growth indices were derived from tree cores collected from across the three rivers. The germination response to seasonal average flows and flow pulses was analyzed using binary logistic regression. Relationships between growth and precipitation and flows across various periods were analyzed using simple linear regression. Results showed that probability of germination was positively correlated with spring and summer average flows, along with smaller flow pulses, for most species along the three rivers. For growth, precipitation had a stronger effect than flows on increasing growth as much as 0.04 per 10 mm of precipitation and 0.03 per 10 m3s-1. The period of late winter to early summer for both precipitation and flows was the most influential on growth, though in some instances these same conditions decreased germination and growth on different rivers. The results also suggest that there are detectable differences in species-specific responses across rivers with different flow regimes. The framework presented in in this study is the first of its kind to assess the effects of flows on germination and growth using tree rings and has promise for more widespread use on rivers. Further studies on these species covered here with more comprehensive sampling efforts would be beneficial in expanding our understanding of their growth and germination processes to ultimately improve management.