Riparian Forest Succession and Changes in Stand Diversity and Above Ground Biomass on the Nyack Floodplain in Northwestern Montana

Walton, Kelly

Floodplains are biologically diverse areas shaped by environmental gradients and variation in topography, hydrological regimes, and substrates. Human regulation of river systems has changed this variation in natural floodplains, resulting in changes in habitats and riparian forest species compositions. The Nyack floodplain, on the Middle Fork of the Flathead River is an unregulated floodplain in northwestern Montana. Data examining the relationships between riparian tree diversity, above ground biomass, stand age, and elevation were collected in 2005 and 2006 from fifty-one 20 X 20 meter plots ranging from 2 to 130 years old. The species composition shifted from dense stands of black cottonwood (Populus trichocarpa) that colonized recently scoured areas to Engelmann spruce (Picea engelmannii) dominated forests. Riparian tree diversity was highest in intermediate age stands (40 years), which is supported by Connell’s intermediate disturbance hypothesis. Above ground biomass increases with stand age initially following a logistic curve (R2 =0.771, P < 0.001) due to cottonwood resource allocation, but the older stands fit a polynomial curve (R2 = 0.776, P <0.001). Cottonwoods make up more than 50% of the biomass in stands under approximately 110 years old, then conifers, primarily Engelmann spruce, dominate in biomass; however, conifers establish after 40 years and dominate in number of individuals after approximately 55 years. Scouring frequency is strongly correlated to elevation. Both biomass and stand age increase linearly with elevation (R2 =0.686, P <0.001; R2 =0.7959, P <0.001) due to less frequent scouring events, causing older, larger trees to grow at higher elevations

Riparian Forest Succession and Changes in Stand Diversity and Above Ground Biomass on the Nyack Floodplain in Northwestern Montana

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