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Vegetation-environment relationships in a subalpine wet meadow and a brackish tidal marsh Drewa, Paul B.

Abstract

Community structure, community-environment relationships, the role of land surface elevation as a determinant of plant species composition, and aspects of scale were examined and compared between a brackish tidal marsh in the Squamish estuary and asubal pine wet meadow in Garibaldi Park in southwestern British Columbia. Vegetation abundance, soil variables, and ground elevation data were collected from 225, 0.5 X 0.5 m quadrats systematically located at five metre intervals in a 40 X 120 m sampling grid in both sites. The effect of changing the sampling scale was examined through simulation rather than by resampling in the field using different quadrat sizes. Larger quadrat sizes were simulated by aggregating adjacent quadrats in the grid and calculating average values for species and environmental variables. Five aggregation scales (referred to as 'agg' levels) were formed: aggl (0.5 X 0.5 m), agg4 (5 X 5 m), agg6a (5 X 10 m), agg6b (10 X5 m), and agg9 (10 X 10 m). At each scale, minimum variance cluster analysis and canonical correlation analysis were used to describe community structure by selecting a dendrogram level to segregate the vegetation data into subcommunities. Vegetation data were correlated with environmental data using canonical correspondence analysis. To evaluate which scale provided the clearest picture of community structure (yielded the largest between and smallest within-cluster variability estimate), an analysis of variance was performed on canonical correspondence analysis first and second axis scores using the selected dendrogram level for stratification at each scale. This helped to provide an overall between and within-cluster variability estimate for each scale. The role of elevation as a determinant of vegetation pattern was investigated by regressing a canonical axis representing species variables against a canonical axis representing elevation. Residuals, representing that proportion of the variation in vegetation unexplained by elevation, were saved and correlated with the environmental variables to examine if other variables unrelated to elevation shared strong relationships. At most scales the marsh study site is composed of two subcommunities: upper and lower. The upper zone, characterized by soils of greater sand and organic content but less clay than the lower, is resident to many species common to Pacific coastal marshes. The lower zone is a mono specific stand of Carex lvnqbvei that is exposed to high-low tide alternation which may remove organic content, sand, and deposit clay. Salinity did not share strong correlative relationships with the vertical distribution of plant species. However, soils were most saline and acidic in a low area near the upper marsh that was apparently not exposed to tidal flushing. Strong correlations’ between residuals and carbon, sand, and clay content suggest sources other than elevation such as tides and the species themselves may influence edaphic factors which in turn share relationships with vegetation pattern. Generally, the subal pine meadow is composed of three subcommunities: forb meadow (upper), heath (middle), and sedge meadow (lower). Greater sand and electrical conductivity in upper meadow soils suggest a well-exposed and well-drained area. The lower subcommunity characterized by mostly Carex niqricans, possess soils of greater clay and organic content. Soils generally tend to be less acidic than the upper zone suggesting that leaching may be occurring as water drains from the upper meadow into the lower. Simulation sampling with a rectangular quadrat positioned perpendicular to vegetational banding (agg6b), defined eight subcommunities in both sites. In addition, overall within-assemblage variability was least and between-assemblage variability was greatest suggesting that observation clarity is maximized atagg6b. Correlations among environmental variables and species axes generally become stronger at progressively coarser scales. In particular, subcommunity-pH relations were unnoticeable at aggl but strengthened at agg4 in both sites. However, a strong agg4correlation weakened at agg9 in the tidal marsh, recognizing exceptions. An hierarchical approach reminds one to be cautious when assessing the 'importance' of environmental variables. Results of this study suggest the importance of environmental factors, estimated by their correlations with vegetation pattern, may depend on the scale at which the data are analyzed.

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