Alternative stable states of tidal marsh vegetation patterns and channel complexity

Intertidal marshes develop between uplands and mudflats, and develop vegetation zonation, via biogeomorphic feedbacks. Is the spatial configuration of vegetation and channels also biogeomorphically organized at the intermediate, marsh-scale? We used high-resolution aerial photographs and a decision-tree procedure to categorize marsh vegetation patterns and channel geometries for 113 tidal marshes in San Francisco Bay estuary and assessed these patterns' relations to site characteristics. Interpretation was further informed by generalized linear mixed models using pattern-quantifying metrics from object-based image analysis to predict vegetation and channel pattern complexity. Vegetation pattern complexity was significantly related to marsh salinity but independent of marsh age and elevation. Channel complexity was significantly related to marsh age but independent of salinity and elevation. Vegetation pattern complexity and channel complexity were significantly related, forming two prevalent biogeomorphic states: complex versus simple vegetation-and-channel configurations. That this correspondence held across marsh ages (decades to millennia) and at both high and low marsh elevations suggests the following: (1) marshes of shared physiography can exhibit highly variable ecosystem structures; (2) young marshes are not necessarily simple nor necessarily develop vegetation complexity with age and elevation; (3) Bay marshes should continue to exhibit both simple/complex configurations in the future despite a likely shift toward low marshes; (4) salt marshes may tend to occupy two alternative stable states characterized by linked complexity in vegetation and channel organization. This final point may help fill the gap at the marsh scale between biogeomorphic models explaining marsh occurrence at larger coastal and smaller vegetation patch scales. Copyright (C) 2016 John Wiley & Sons, Ltd.