Reinvasion of restored California vernal pools reveals the importance of long‐term restoration planning

Ecological restoration often focuses on short‐term intervention efforts with the goal of creating restored ecosystems that do not require continuous human maintenance. Here, we ask: Do short‐term restoration efforts result in self‐sustaining native assemblages, or do these restored ecosystems require long‐term management to prevent reinvasion of exotic species? We address this question using restored vernal pool wetlands in coastal California. Restoration efforts in vernal pool ecosystems are often hindered because many restored vernal pools exist within a grassland matrix that is highly invaded by exotic annual grasses and forbs. To test whether restored pools experienced reinvasion, we assessed plant species abundance and diversity at varying times after intensive weeding had ceased. The central bottom of pools, where inundation duration is the longest, showed stable or even increasing native cover and no trends in exotic abundance over time. However, exotic cover and richness increased in the upland edges of the pools, where drier conditions allow exotic grasses from the surrounding unrestored grassland to grow. Our findings indicate that edges of restored ecosystems are susceptible to invasion over time, but that this depends on abiotic and biotic conditions within the ecosystem, such as pool shape and landscape matrix, that can potentially be manipulated through initial planning (e.g., constructing circular pools) and long‐term management (e.g., annual weeding). Our findings highlight the importance of ongoing monitoring and adaptive management and support a paradigm shift away from short‐term interventions and toward viewing restoration as a longstanding relationship with the land that may require continuous human management.

Methods
Approach 1: Multi-Year Monitoring Study ("np_percent_cover.csv")

From 2016 to 2019, we monitored seven restored vernal pools within UCSB’s North Parcel, which consists of vernal pools built amidst university faculty housing.  Within each restored vernal pool, we established a series of permanent monitoring quadrats.  We delineated each pool into central (experiencing longest inundation period), transition (inundated or hydric soil during longest inundation period), and upland (inundated during extreme storms but otherwise non-hydric soils) zones (Figure 2A).  Within each of these zones, we haphazardly placed three 1m2 quadrats, for a total of nine quadrats per pool.  We monitored the vernal pools monthly from November 2016 to December 2019.  Because the pools were different ages at the start of the experiment, sampling over three years allowed us to evaluate the vegetation community in pools ages two to nine years old.  Within each quadrat, we determined the identity and percent cover of all species present.  We also recorded the percent cover of bare ground, water, and thatch (dead plant matter).  Additionally, we estimated the number and percent cover of germinating seedlings for native species.  Because low-growing graminoids and forbs were often overlaid by taller species, total percent cover could exceed 100% in each quadrat.

Approach 2: Chronosequence Survey ("chronosequence.csv")

The 69 pools surveyed in this study were restored between 1986 and 2017.  In the spring of 2019, we conducted vegetation surveys in each pool when the majority of the native species were at peak biomass.  For each pool, we laid out two transects bisecting the pool along its elliptical major and minor axes (Figure 2B).  Every other meter along each transect, we laid down a 1m2 quadrat with 10% subdivisions.  We identified every plant species present and estimated its percent cover in each quadrat.  We also estimated percent cover of bare ground and thatch.  Because low-growing graminoids and forbs were overlaid with taller species, total percent cover could exceed 100% in each quadrat.  We also categorized each quadrat as being in the central, transition, or upland zone of the pool. 

To measure relative elevation, we used a laser level to calculate the elevation of each quadrat above the deepest point of the pool.  To determine pool hydroperiod, we installed 0.8m rulers in the deepest part of each pool in January 2019 and recorded the depth of the water in each pool every week beginning 11 January until all the pools dried up by 5 July.  To measure the site and pool area, we used a Trimble GPS to map out the perimeters of the sites and the pools.  We also used these data to calculate each pool’s perimeter-to-area ratio and the distance of each pool from the edge of the restoration site.