Phenological responses of herbaceous plants, shrubs, and tree seedlings to experimental climate change conditions in northern Minnesota

Changing climate, specifically warmer temperatures, has been linked to changes in phenology, the timing of biological events such as leaf out and flowering. Despite the growing use of phenology as a tool to assess the degree of climate change or understand potential changes to species distributions, among others, phenological changes of herbaceous plants and shrubs remain less studied and thus less understood. In northern temperate and boreal forests, the understory and groundlayer composes the majority of plant biodiversity, is a major component of nutrient cycling, and is a filter for tree regeneration, among other important ecosystem contributions. Understanding the potential phenological changes of the forest groundlayer to predicted changes in climate, such as warming and altered precipitation, can inform how these processes and contributions may change. In this thesis I examine how experimental warming and warming coupled with decreased rainfall impact several phenological stages of herbaceous plants, shrubs, and tree seedlings over multiple years. This study takes place within the Boreal Forest Warming at an Ecotone in Danger (B4WarmED) project in northern Minnesota. First, to assess the phenological response of 13 herbaceous plants and 3 shrubs to warming I analyzed data collected over five years (2009-2013) at two sites, two canopy covers, and in three warming treatments: ambient temperature, +1.7°C and +3.4°C. Overall, warming extended the growing season, primarily through earlier leaf unfolding in the spring. Notably warmer, earlier springs advanced leaf unfolding more than cooler springs. Compared to advance of budburst of tree seedlings, the groundlayer advances more in warming, possibly allowing for more carbon gain in the spring. Flowering iv advanced under warming, though more so for fall blooming species than for spring blooming species. This advance may be attributed to aforementioned carbon gain from earlier leaf unfolding, as the two events appear correlated. Warming did not alter senescence for most species, though several species did delay senescence with warming. Finally, the length of the growing season differed between canopies for several species, primarily through delayed senescence in the closed canopy habitat compared to the clearcut. Second, I compared the phenological responses of the understory community and tree seedling community under manipulated rainfall and temperature treatments. For both communities I analyzed data collected in 2012-2014 at two sites and one habitat, open canopy. Experimental treatments included the three warming treatments crossed with two rainfall treatments (ambient and 40% reduction). Combining the two sites, there are six replicate plots of each warming treatment (ambient temperature, +1.7°C and +3.4°C) crossed with each rainfall treatment, for a total of 36 plots. Overall, community level phenological responses of the groundlayer and tree seedlings were not altered by rainfall treatments. Thus the larger springtime advance of the groundlayer relative to the tree seedling community in warming (reviewed in Chapter 1) is maintained even when soil moisture is lower. Finally, I examined the difference in flowering responses of spring and fall blooming herbaceous species to warming and rainfall treatments. Only fall blooming species altered flowering phenology to rainfall manipulation. Two species (Eurybia macrophylla and Solidago spp.) flowered at the same time in ambient rainfall and v ambient warming conditions as well as ambient rainfall and warmed conditions, but diverged in flowering time in the warmest, driest treatment. This divergence led to a month long gap between when each flowered These results highlight potentially sensitive species or altered competitive interactions under future climate change conditions. Such changes may have consequences for increased fitness, changes in species persistence and community composition.