Responses of two understory herbs, Maianthemum canadense and Eurybia macrophylla, to experimental forest warming: Early emergence is the key to enhanced reproductive output

Abstract

Global warming might improve fitness of herbaceous species in deciduous forests, mainly by advancing their spring emergence. However, other impacts of global warming such as drier soils in the summer might partly reduce the carbon gain associated with early emergence.

Phenological mismatch with trees reduces wildflower carbon budgets

“…First, trees and herbaceous spring wildflowers respond to warming at strikingly different rates. The possibility of contrasting tree and wildflower phenological responses to temperature have been suggested in earlier studies (Lapointe 2001;Neufeld & Young 2014;Jacques et al 2015;Augspurger & Salk 2017), but has not been demonstrated. Second, as a result of these differing phenological responsiveness between forest layers, the window of high light availability in the spring is decreasing with warmer spring temperatures.…”

“…Our findings highlight unexpected consequences of phenological mismatch under climate change. Soil warming experiments have predicted fitness increases in forest herbs due to earlier spring emergence in warmed plots (Jacques et al 2015). Another study experimentally manipulated the emergence of spring ephemeral species, highlighting the physiological constraints of cold and shade on growth and reproduction for these species (Augspurger & Salk 2017).…”

“…) – a common shade avoidance strategy known as ‘phenological escape’ (Jacques et al . ). This phenomenon is most dramatic among spring ephemerals, which go dormant after overstorey leaves appear.…”

“…Plant growth in deciduous forest understories is controlled by this dramatic and predictable seasonal event, with full sunlight in the spring followed by low light levels soon after trees leaf out (Hutchison & Matt 1977). Many forest herbs (hereafter 'wildflowers') emerge and flower in the early spring to exploit this critical period of high light (Lapointe 2001;Kudo et al 2008;Augspurger & Salk 2017), reaching maximum photosynthetic rates that abruptly decline as the overstorey trees leaf out and the canopy closes (Rothstein & Zak 2001;Kudo et al 2008;Heberling et al 2019)a common shade avoidance strategy known as 'phenological escape' (Jacques et al 2015). This phenomenon is most dramatic among spring ephemerals, which go dormant after overstorey leaves appear.…”

Carbon gain phenologies of spring‐flowering perennials in a deciduous forest indicate a novel niche for a widespread invader

“…Increases in air temperature resulting from climate change has been found to advance flowering times in understory herbs (e.g. Miller‐Rushing & Primack, ) and result in a generally longer growing season for most species (Cleland et al ., ), including forest herbs (Jacques et al ., ). Budbreak in overstory trees is determined primarily by photoperiod and air temperatures (Polgar & Primack, ), while aboveground emergence in understory herbs is determined by the timing of snowmelt and soil temperatures (Routhier & Lapointe, ).…”