Population dynamics of Hippophae rhamnoides shrub in response of sea-level rise and insect outbreaks

Abstract

The coastal vegetation of islands is expected to be affected by future sea-level rise and other anthropogenic impacts. The biodiverse coastal vegetation on the eastern part of the Dutch Wadden Island of Ameland has experienced land subsidence caused by gas extraction since 1986. This subsidence mimics future sea-level rising through increased flooding and raising groundwater levels. We studied the effects of this relative sea-level rise and other environmental factors (i.e. insect outbreaks, temperature and precipitation) on the population dynamics (i.e. cover and age structure and annual growth) of the shrub seabuckthorn (Hippophae rhamnoides L.) in young (formed after 1950) and old (formed before 1950) dune areas over a period of 56 years (1959-2015). We found an increase in seabuckthorn cover in the young dune areas since 1959, while over time the population in the old dunes decreased due to successional replacement by other species. With the increasing age of the young dunes, we found also a decrease in sea-buckthorn after 2009. However the sharp decrease indicated that other environmental factors were also involved. The most important determinant of annual shrub growth appeared to be five outbreaks of the browntail moth (Euproctis chrysorrhoea L.), in the last decade. Relative sea-level rise caused more frequent flooding and reduced growth at lower elevations due to inundation or soil water saturation. This study clearly indicates that sea-buckthorn is affected by relative sealevel rise, but that other ecological events better explain its variation in growth. Although shrub distribution and growth can be monitored with robust methods, future predictions of vegetation dynamics are complicated by unpredictable extreme events caused by (a)biotic stressors such as insect outbreaks.

The role of livestock grazing in long-term vegetation changes in coastal dunes: a case study from the Netherlands

“…After 1990, large parts of the Hippophae shrubland collapsed and returned to grasslands. This is due to the lifespan of Hippophae of around 40 years, which equals the timespan between the 1954 outbreak of myxomatosis and1990 (Van der Hagen et al 2020b) but note that the lifespan of Hippophae can vary between 18 and 56 years, incidentally up to 80 years (Zoon 1995;Provoost and Declerck 2020;Decuyper et al 2020). However, our permanent plot data showed that shrubland shifted mainly into tall shrubs (e.g., Crataegus monogyna; Van der Hagen 2020b) from 1972 to 2012 (Fig.…”