Long-Term Monitoring and Management of American Chaffseed (Schwalbea americana) in the New Jersey Pine Barrens: Population Ecology of an Endangered Hemiparasite

Abstract

We studied the long-term demographic structure of the endangered hemiparasitic plant, Schwalbea americana (American chaffseed), in the New Jersey Pine Barrens from 1991 to 2020 in order to address important questions about its population biology, life history, and responses to management. These included the relative contributions of dormancy, recruitment, and other life stages to annual population changes, as well as its ecological responses to fire, mowing, and other factors over time. Following baseline surveys in 1991, we conducted annual censuses from 1993 to 2020 and individualistic plant monitoring from 1999 to 2020. The Schwalbea population initially exhibited positive growth, with major increases observed in the early 2000s, followed by steady, long-term declines. We used generalized linear models and mixed effects models to analyze the relationships between demographic and environmental variables at the population and individual levels, respectively. Dormant season prescribed burning and mowing yielded significant increases in flowering, stems, and survival, with apparent residual benefits in subsequent years following repeated treatments. Germination declined to 0% after 4 y in seed viability experiments, and recruitment was more strongly correlated with estimated seed bank sizes than with flowering alone. Dormant plants represented an average of 21% of the total population, with 20% of above-ground plants entering dormancy annually. Most dormancy events (92%) were 1–3 y in duration, indicating that mortality cannot be distinguished from dormancy for a minimum of 3 y after disappearance. Annual survivorship was high (mean = 88%), with 9% of individuals surviving 18 y on average, but differed significantly by cohort and increased with management. Significant differences were observed in size and flowering by age/stage class. However, size alone was insufficient to identify Schwalbea recruits from older plants, due to overlap of their lower size ranges. Long-term declines in survival and recruitment persisted despite suppression of woody succession by hand clipping and intermittent mowing and burning. Increased frequency of mowing or burning may be needed to sustain this population in the future. The relationships of growth and vital rates to other factors are provided, and the benefits of individualistic data for informing Schwalbea monitoring and management are discussed.