Channel Islands National Park: Terrestrial vegetation monitoring annual report - 2020

Williams, Cameron

doi:10.36967/2299696

This report presents the data collected in 2020 as part of the long-term terrestrial vegetation monitoring program at Channel Islands National Park. The purpose of this monitoring program is to document long-term trends in the park’s vegetation communities. Data are collected from 30-m-long transects using a point-line intercept method. In the past, each transect was sampled annually. However, beginning in 2012 the program began adding randomly located transects to improve the representativeness of the sampling, and transitioned to a rotating panel design. Now only a core subset of the transects are surveyed annually. Non-core transects are assigned to one of four panels, each of which is surveyed once every four years. Coronavirus disease (COVID-19) caused a global pandemic that prevented field data collection from most of the transects scheduled for 2020. As a result, just 29 of the 127 transects scheduled were visited, distributed as follows: Santa Barbara Island (n = 18), Santa Cruz Island (n = 11). The small size of this 2020 dataset offers limited comparisons to previous years. The program also planned to install 19 new transects on Santa Rosa and San Miguel islands to complete the probabilistic component of the study design (Tiszler et al. 2016a), but the safety response to COVID-19 did not permit these installations. Key 2020 results and interpretation: For the transects visited at both islands in 2020, a long-term decline in mean absolute cover, measured as the mean number of taxa intersecting each point along a transect, was apparent. Annual rainfall correlated positively with mean absolute cover of vegetation. Six vegetation communities were visited at Santa Barbara Island compared to nine at Santa Cruz Island, despite the smaller number of transects surveyed at the latter. Mean absolute cover at both islands together averaged 87%, with native plants covering 44% and non-natives covering 39%. Among vegetation communities, mean absolute cover ranged from a high of 137% in a Lyonothamnus grove to a low of 32% in a riparian community where few plants were encountered along one transect. Mean absolute cover was higher at Santa Cruz Island (105%) than Santa Barbara Island (77%), and mean absolute native cover followed the same pattern (57% vs 36%). For both islands combined, mean richness per transect was 11 taxa. Compared to Santa Barbara Island, Santa Cruz Island supported higher overall mean species richness (14 vs 9 taxa) and higher species evenness (0.70 vs 0.55). Santa Cruz Island’s higher absolute cover, higher native species cover, higher species richness, and higher species evenness indicate that its vegetation is in better overall condition compared to Santa Barbara Island, and that the rate of recovery after overgrazing is faster at Santa Cruz Island than Santa Barbara Island.

Channel Islands National Park: Terrestrial vegetation monitoring annual report - 2020

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