Plant Community Stability over 40 Years in a Fraser River Estuary Tidal Freshwater Marsh

Long-term data sets documenting temporal changes in vegetation communities are uncommon, yet imperative for understanding trends and triggering potential conservation management interventions. For example, decreasing species diversity and increasing non-native species abundance may be indicative of decreasing community stability. We explored long-term plant community change over a 40-year period through the contribution of data collected in 2019 to two historical datasets collected in 1979 and 1999 to evaluate decadal changes in plant community biodiversity in a tidal freshwater marsh in the Fraser River Estuary in British Columbia, Canada. We found that plant assemblages were characterized by similar indicator species, but most other indicator species changed, and that overall alpha-diversity decreased while beta-diversity increased. Further, we found evidence for plant assemblage homogenization through the increased abundance of invasive species such as yellow flag iris (Iris pseudacorus), and reed canary grass (Phalaris arundinacea). These observations may inform concepts of habitat stability in the absence of direct anthropogenic disturbance, and corroborate globally observed trends of native species loss and non-native species encroachment. Our results indicate that within the Fraser River Estuary, active threat management may be necessary in areas of conservation concern in order to prevent further native species biodiversity loss.