Change in trade wind inversion frequency implicated in the decline of an alpine plant

Abstract

Background: Detailed assessments of species responses to climate change are uncommon, owing to the limited nature of most ecological and local climate data sets. Exceptions, such as the case of the Haleakalā silversword, can provide important insights into the complexity of biological responses to changing climate conditions. We present a time series of decadal population censuses, combined with a pair of early population projections, which together span the past 80 years of demographic history for this alpine plant. Results: The time series suggests a strong population recovery from the 1930s through the 1980s, likely owing at least in part to management actions taken on its behalf. In contrast, the population is estimated to have suffered a decline of approximately 60 % since the early 1990s. Fine-scale estimates of rainfall within silversword habitat are strongly correlated with these decadal-scale population changes over the past 50 years, with rainfall estimated to be substantially lower on average during the two most recent inter-census periods (after 1991). The reversal in the silversword population trajectory, and declines in rainfall in silversword habitat, coincide with an abrupt increase in the frequency of occurrence of the trade wind inversion (TWI) in Hawaiʻi around 1990. Conclusions: The shift in TWI incidence, which is linked to stronger subsidence in the Hadley circulation, has led to drier conditions in high elevation ecosystems in Hawaiʻi and appears to be eliciting ecological responses. Other regions influenced by the TWI could be similarly affected. The silversword case study reveals additional unexpected outcomes, such as the likely initial retraction from wetter, rather than drier, portions of the range in response to drying conditions. This pattern may stem in part from variation in drought tolerance across the range, highlighting the importance of detailed ecological and climatic information for making accurate predictions about climate change responses.

Clinal variation in drought resistance shapes past population declines and future management of a threatened plant

“…The most recent estimate of the total population size was roughly 40,000 plants in 2013 (Krushelnycky et al. ).…”

“…Owing to its ecological and cultural importance and protected status, an abundance of ecological information has been collected for this species, which has revealed that its population has undergone a substantial climate‐associated decline in recent decades, principally in the lower portion of its elevational range (Krushelnycky et al. , ). Although it has been hypothesized that the lower elevation limit of Haleakalā silverswords (hereafter, silverswords) could be controlled in part by damage to the apical meristem from high temperatures (Melcher et al.…”

“…), the available demographic and climate data suggest that recent mortality has been driven more by drier conditions than by increasing temperature (Krushelnycky et al. , ). The causes of the elevational pattern in mortality are therefore likely more complex than a simple upslope tracking of temperature and may involve interactions between changing climate gradients and local variation in drought resistance (Krushelnycky et al.…”

“…The causes of the elevational pattern in mortality are therefore likely more complex than a simple upslope tracking of temperature and may involve interactions between changing climate gradients and local variation in drought resistance (Krushelnycky et al. ). Clarifying the drivers of population decline is central to this species’ conservation, while also contributing to the broader goal of understanding the responses of rare plant populations to a changing climate.…”

“…Nevertheless, a ~60% decline in the silversword population size in recent decades has been strongly tied to reduced rainfall, which has coincided with other atmospheric changes such as higher solar radiation and vapor pressure deficit that appeared to result from an upward shift in the incidence of the trade wind inversion (TWI) around 1990 (Krushelnycky et al. , , Longman et al. ).…”