LB 9-95 - Consequences of climate change for the duration of the high light period for early spring flowering plants in northern forests.

Background/Question/Methods:

Early spring flowering plants contribute significantly to ecosystem stability as the only photosynthetically active plants at the start of the growing season in deciduous forests. The short period of high light in the spring, prior to canopy closure, is crucial for these plants to complete their yearly photosynthesis. However, if the vegetative phenology of deciduous trees and spring herbs respond differently to climate warming, a phenological mismatch between the two strata could change the duration of this high light period. In this study, we tested the impact of spatial and inter-annual climate variation on the duration of the high light period for a dominant early spring flowering plant, red trillium (Trillium erectum), growing in forests dominated by sugar maple (Acer saccharum) and yellow birch (Betula alleghaniensis). We used estimates of phenology derived from automated cameras from 2017 to 2020 across 10 sites along an elevational gradient in Mont Mégantic National Park, Québec, Canada. Across the 40 site-year combinations, we examined the effect of spring temperature on the timing of leaf expansion of understory species and of overstorey canopy closure.

Results/Conclusions:

We found that warming was associated with earlier leaf emergence and expansion for both strata, with both events occurring 5-6 days earlier per degree Celsius of warming. Contrary to a previous study, which suggested that the duration of high light would be reduced with warming, our results show no evidence that trees are more sensitive to warming than early spring herbaceous plants. Therefore, we do not expect the period of high light in the understory to be reduced with warming, with a weak signal that longer-term data might reveal in increased duration of high light. Overall, the indirect effects of climate warming on understory plants via phenological mismatch with the canopy are likely to be relatively small or even positive.