Climate Variability Structures Plant Community Dynamics in Mediterranean Restored and Reference Tidal Wetlands

Dylan Chapple,Phyllis Faber,Adina Merenlender,Katharine Suding

doi:10.3390/w9030209

Abstract

In Mediterranean regions and other areas with variable climates, interannual weather variability may impact ecosystem dynamics, and by extension ecological restoration projects. Conditions at reference sites, which are often used to evaluate restoration projects, may also be influenced by weather variability, confounding interpretations of restoration outcomes. To better understand the influence of weather variability on plant community dynamics, we explore change in a vegetation dataset collected between 1990 and 2005 at a historic tidal wetland reference site and a nearby tidal wetland restoration project initiated in 1976 in California’s San Francisco (SF) Bay. To determine the factors influencing reference and restoration trajectories, we examine changes in plant community identity in relation to annual salinity levels in the SF Bay, annual rainfall, and tidal channel structure. Over the entire study period, both sites experienced significant directional change away from the 1990 community. Community change was accelerated following low salinity conditions that resulted from strong El Niño events in 1994–1995 and 1997–1998. Overall rates of change were greater at the restoration site and driven by a combination of dominant and sub-dominant species, whereas change at the reference site was driven by sub-dominant species. Sub-dominant species first appeared at the restoration site in 1996 and incrementally increased during each subsequent year, whereas sub-dominant species cover at the reference site peaked in 1999 and subsequently declined. Our results show that frequent, long-term monitoring is needed to adequately capture plant community dynamics in variable Mediterranean ecosystems and demonstrate the need for expanding restoration monitoring and timing restoration actions to match weather conditions.

Highlights

The practice of restoring native plant communities aims to facilitate transitions between degraded and ecologically functional landscapes by manipulating abiotic [1,2,3] and biotic conditions [4].transitions from a degraded state to a desirable state depend on actions taken at discrete sites, and on regional climate and weather [5,6,7]
We focus on California tidal wetlands, where salinity [31,40], channel structure [42,43], species dynamics [44], and elevation [34] interact to influence species composition, and shifts in salinity can alter productivity [28] and species composition [31]
Salicornia pacifica was dominant, comprising between 99 and 67 percent of total cover depending on the year

Summary

Introduction

The practice of restoring native plant communities aims to facilitate transitions between degraded and ecologically functional landscapes by manipulating abiotic [1,2,3] and biotic conditions [4]. Mediterranean climates and other arid and semi-arid systems around the world are characterized by high interannual weather variability (high coefficient of variation for precipitation) [5,15,16], which may impact plant species dynamics [17,18], and by extension, restoration outcomes [19] These changes in weather can lead systems down different trajectories depending on where and when projects are initiated [20]. To better understand the influence of interannual weather variability on restoration outcomes, we analyzed a long-term data set of plant community composition collected at one reference and one restoration site between 1990 and 2005 These studies were initially set up as part of an effort by the California Coastal Conservancy to better understand restoration trajectories over time in order to plan and manage future projects [47]. To explore how regional salinity, local rainfall, and site topography influence plant community trajectories, we look at SF Bay salinity adjacent to the sites, rainfall, and channel structure

Materials and Methods

Data Analysis

Results

Discussion