Climate change alters temperate tree and shrub spring phenology and false spring risk

Abstract

Temperate tree and shrub species are at risk of damage from late spring freezing events. Individuals that initiate budburst before the last spring freeze risk leaf tissue loss, damage to the xylem, and slowed, or even stalled, canopy development. These damaging events are often called false springs and have the potential to detrimentally affect forest growth, which can result in highly adverse ecological and economic consequences. In the Introduction I combine theory from ecology, climatology, physiology, biogeography and crop science to examine the effects of false springs, and the complexity of factors that determine plants’ risk to frost damage. I then aim to explore how false spring risk and phenology are changing across species, local and regional climates, and with climate change. In Chapter 1, I ask how these dominant factors laid out in the Introduction operate across Europe using long-term phenology and climate data (from 1950 to 2016). Specifically, I examine which climatic and geographic factors are the strongest predictors of false springs across six tree species, and how these predictors have shifted with recent climate change. By investigating leafout observations, I unravel the effects of species, spring temperature, elevation, distance from the coast and NAO index on false spring risk with climate change. I found that recent warming has reshaped the influence of these factors and magnified species-level variation in false spring risk, with early-budbursting species generally experiencing more risk with climate change and late-budbursting species experiencing less. In Chapter 2, using a controlled lab experiment, I explore the species-level consequences of false springs coupled with warmer winters (generally expected to reduce chilling) across eight temperate deciduous tree species on a suite of phenological, growth and leaf tissue traits. I found that false springs increased shoot apical meristem damage and slowed budburst to leafout timing—extending the period of maximum freezing risk. Chilling, however, shortened this period of maximum risk, even under false spring conditions, thus compensating for some of the more adverse phenological effects of false springs. The results suggest climate change could reshape forest communities through impacts on growth and phenology from the coupled effects of false springs and warmer winters under future climate change. Finally, I use observations from one urban arboretum and one rural forested site to assess growing degree days (GDDs) until budburst using two methods for measuring climate data (i.e., weather station data and hobo logger data) to better predict spring phenology, which is essential for accurate false spring forecasting. I additionally incorporate simulations to better interpret the results and conclude with a series of simulated forecasts to estimate changes in these GDD model estimates. The results suggest the urban arboretum site required fewer GDDs until budburst and may have stronger microclimate effects than the rural forested site but, overall, I found that GDD models may become less accurate with warming. Understanding and predicting spring plant phenology is essential for determining growing season length and predicting an individual’s risk of false spring under climate change.