Plant community compositional stability over 40 years in a Fraser River Estuary tidal freshwater marsh

AbstractLong-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 α-diversity decreased while β-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.
MethodsSampling design & harmonization between observations Our main goal was to sample the vegetation in a representative way to allow comparison with the datasets collected in 1979 (Bradfield & Porter, 1982) and 1999 (Denoth & Myers, 2007). This publication will reference dates the data were collected, rather than publication dates of the corresponding studies. In the original 1979 study, eight transects (Q-X) were laid out in a subjective fashion to cross through the main features of vegetation diversity at Ladner Marsh (Fig. 1 in Bradfield & Porter, 1982). All transects spanned a similar elevation range across the marsh platform, with the three main plant assemblages (Sedge, Fescue, Bogbean) separated by apparent changes in hydrological conditions along transects. In the 1999 study, Bradfield & Porter’s (1982) Fig.1 was used to visually approximate the locations of transects to repeat the vegetation survey (Denoth & Myers, 2007). In this study (2019 survey), transect locations were determined by overlaying Bradfield & Porter’s (1982) Fig. 1 on a georeferenced basemap, aligning prominent features such as tidal channel tributary junctions, marking GPS locations in Avenza Maps (Avenza Systems Inc., Ontario, Canada, v. 3.2), and finding these points in the field (Fig.1C). Difficulties arising from the inexact relocations of transects in the 1999 and 2019 surveys, and aggressive shrub encroachment along transect Q, resulted in an incomplete resampling of all eight transects from the original 1979 survey (further explained below). To evaluate the potential for differences in transect relocation to affect trends observed in the data, or to evaluate marsh-wide spatial trends in plant composition, we calculated the percentage of plots clustered in each assemblage group for each transect. All three studies used a semi-systematic approach for determining locations of 1x1 m quadrats along transects. In the 1979 study, quadrats were mainly located at 10 m intervals along transects although this varied in places from 2–20 m depending on local changes in the vegetation (Bradfield & Porter, 1982). In the 1999 study, an attempt was made to follow the quadrat spacing shown in Bradfield & Porter’s (1982) Fig. 3 regardless of perceived vegetation changes along transects. For the 2019 study, quadrat placement was guided by visual assessment of vegetation patchiness along transects. If patches dominated (>50 % cover) by one or two species (not necessarily the three assemblage identifiers) continued more than 10 m of transect length, or if no dominant species was evident, we sampled every 10 m of transect length (Fig. 2D). No patches were so small that the 1 m2 plot was less than 1 m from the boundary of the next patch. Such fine-scale variations in decisions for quadrat placement among the three studies were considered inconsequential for the broader scale assessments of assemblage changes over time.  Plot-scale sampling Species were recorded if their most basal stem originated within the 1 m2 quadrat, and cover within the plot was considered for all above-ground vegetation that occurred within the quadrat boundary; vegetation overhanging the quadrat from individuals whose basal stems originated outside the quadrat boundary was not considered. In the instance where the basal stem was inside the plot, but aerial vegetation extended beyond the boundary of the quadrat, we only considered vegetation cover for portions of the plant within the boundary of the quadrat. We treated each ramet...