Late winter light exposure increases summer growth in the grass Poa pratensis: Implications for snow removal experiments and winter melt events Mathew R. Vankoughnett, Danielle A. Way, Hugh A.L. Henry* Department of Biology, University of Western Ontario, London, ON, N6A 5B7, Canada A R T I C L E I N F O Article history: Received 5 May 2016 Received in revised form 17 June 2016 Accepted 20 June 2016 Available online 22 June 2016 Keywords: Carbon gain Cold Deacclimation Kentucky bluegrass Photosynthesis A B S T R A C T Reductions in snow cover over winter can increase frost exposure in herbaceous plants. Nevertheless, increased exposure to light can potentially increase plant carbon gain during periods of reduced snow cover. We used a combined field and growth chamber experiment to examine how variation in the timing and cumulative duration of light exposure over winter (from one to four 1-week incubation periods at 5  C) affected subsequent summer growth in the grass Poa pratensis. We also measured net photosynthetic rates, dark respiration and chlorophyll fluorescence both 48 h and 120 h after the start of each winter light exposure period. Summer biomass increased by up to 50% for tillers exposed to light during the final winter incubation period (mid-late February), and the timing of light exposure, not the cumulative duration, was the most influential factor in increasing biomass. In contrast, for tillers incubated in the dark, multiple weeks of incubation at 5  C resulted in the largest reductions in summer biomass. Leaf-level net photosynthetic rates were highest for the earliest and latest light exposure periods over winter, whereas dark respiration rates were highest in early winter and lowest in late winter. Thus, the gas exchange and biomass results were consistent in revealing that the last period of light exposure promoted the highest carbon gain. Overall, our results reveal that naturally occurring periods of snow melt over winter, or scenarios where snow is removed or melted as an experimental treatment, have the potential to benefit plant growth substantially, as opposed to simply rendering plants vulnerable to frost damage. ã 2016 Elsevier B.V. All rights reserved. 1. Introduction Snow cover plays a key role in the stress physiology of herbaceous plants in seasonally-frozen regions. Due to its insulative properties, snow buffers overwintering plants from cold air temperatures and freeze-thaw cycles (Henry 2008). The influence of snow cover on soil temperatures is sufficiently strong that reduced snow cover in warm years can paradoxically result in colder soils over winter (Groffman et al., 2001). Despite the potential for thick snow cover to benefit overwintering plants by protecting them from frost damage, a persistent snow pack in early spring can delay the onset of plant growth, thus reducing annual biomass production (Henry et al., 2015). Moreover, the effects of pathogens (e.g. snow molds), ice encasement and increased sub- nivian herbivore activity, which can be associated with snow cover, can reduce subsequent plant growth (Gaudet 1994; Rapacz et al., 2014), and the timing and extent of snow melt can influence subsequent plant water availability over summer. Therefore, the overall effects of snow cover on plant stress physiology and plant growth are multi-faceted (Fig. 1), and can vary substantially depending on the timing and depth of snow cover, and on plant community type (Kreyling et al., 2008, 2010, 2011). In addition to the important observations that have been obtained from studying naturally-occurring variation in snow cover, snow removal experiments have been employed to examine the effects of soil frost on plants in situ (Comerford et al., 2013; Vankoughnett and Henry, 2014). The rationale for such experi- ments has typically been that the effects of snow removal on subsequent plant growth over summer can be attributed to frost damage, provided that snow removal effects on spring melt water recharge are controlled for (e.g. by ceasing the snow removal before the end of winter, allowing snow to accumulate in the plots prior to spring melt). Likewise, warming experiments using overhead infrared heaters and heated soil cables have been used to simulate * Corresponding author. E-mail address: hhenry4@uwo.ca (H.A.L. Henry). http://dx.doi.org/10.1016/j.envexpbot.2016.06.014 0098-8472/ã 2016 Elsevier B.V. All rights reserved. Environmental and Experimental Botany 131 (2016) 32–38 Contents lists available at ScienceDirect Environmental and Experimental Botany journal home page: www.elsevier.com/locat e/envex pbot