Channel Islands National Park: Terrestrial Vegetation Monitoring Annual Report, 2023

Williams, Cameron

doi:10.36967/2315223

Long-term ecological research is critical for evaluating past resource management actions and guiding future decisions. This report presents the data collected at Channel Islands National Park (CHIS) in 2023 as part of its terrestrial vegetation monitoring program. The program’s objective is to document long-term trends in the park’s vegetation communities as part of the Mediterranean Coast Inventory and Monitoring Network, which also includes Santa Monica Mountains National Recreation Area and Cabrillo National Monument. Monitoring at CHIS began in 1984 to observe vegetation recovery after the removal of nonnative ranched animals. In 2023, field data were collected from 143 transects. Analyzing all years of data together showed that annual rainfall correlated positively with percent cover of vegetation, and an apparent long-term decline in average percent cover of vegetation that started in the mid-1990s continued in the 2000s, with a slight increase in 2023. Analyzing the 2023 dataset by itself provided more nuanced results. Valley/foothill grassland had consistently low proportions of native species richness and relatively low overall richness. This community may have been more susceptible to grazing in the past and may currently be resistant to passive recovery. Vegetation was, on average, tallest on Santa Cruz and Santa Rosa islands due to the presence of tree-dominated communities. Perennial plant species and native plant species achieved significantly higher percent cover on Santa Cruz, Santa Rosa, and San Miguel islands compared to Santa Barbara or Anacapa islands. Shrubs at the seedling and juvenile developmental stages achieved more plants per square meter (i.e., density) on Santa Barbara, Santa Cruz, and San Miguel islands, while the highest adult shrub density occurred on San Miguel Island. Islands at more northwesterly positions generally supported higher biodiversity. The tendency for vegetation height, cover of perennials and natives, shrub density, and biodiversity to each decline along a gradient from northwest to southeast indicates that moisture limitation is a strong driver of vegetation recovery after the removal of ranched animals. Moreover, moisture limitation at the park’s most southeasterly islands may interact with poor soils and seabird abundance on Santa Barbara Island and seabird abundance on Anacapa Island to further inhibit recovery. Therefore, Santa Barbara and Anacapa islands may benefit greatly from targeted native plant restoration, especially given their low richness and Shannon diversity combined with a relatively large proportional contribution from nonnative species.

Channel Islands National Park: Terrestrial Vegetation Monitoring Annual Report, 2023

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