Foliar phenology of temperate tree species in spring (bud break and leaf expansion) is shifting earlier in response to global warming. Trends in fall phenology (nutrient resorption and leaf senescence), however, are not that clear. In these forests, tree seedlings begin their spring phenology several weeks before canopy closure. This is a time of high light and soil moisture availability when seedlings can assimilate a large portion of their annual carbon budget. Despite the importance of this high light-high moisture period for seedling growth, there is a lack of information concerning if seedlings are responding to climate change at the same rate as canopy trees. If this period of time decreases, seedling survival could be jeopardized; if it increases for some species but not for others competitive dynamics among co-occurring species might shift.
In this study, we aim to answer the following questions: 1) When do temperate tree seedlings leaf out with respect to the canopy? 2) How do respiration and photosynthetic activity vary over the season? 3) What is the combined effect of these two mechanisms on seedling recruitment performance, specifically growth and survival? We measured growth, survival, foliar phenology, and photosynthetic activity in seedlings of two dominant species in eastern North American forests, Acer saccharum and Quercus rubra. We analyzed four years of data using hierarchical Bayesian models to estimate the effects of shifting phenology on these species’ recruitment.
Results/Conclusions
A. saccharum and Q. rubra seedlings differed in terms of both their foliar phenology and their photosynthetic activity. Acer seedlings were typically much earlier than Quercus seedlings in their spring phenology, while Quercus seedlings’ leaf out period overlapped with canopy leaf out. Earlier spring phenology for Acer seedlings coincided with higher photosynthetic rates that eventually declined to low activity during the shaded parts of the growing season. Quercus seedlings, meanwhile, maintained similar photosynthetic and respiration rates during the entire growing season; however, this species had an extended activity in fall following canopy senescence. Overall, these results indicate that, under moderate climate changes forecasted for the region, A. saccharum might benefit from earlier phenology while Q. rubra’s performance during recruitment might decline.