Date of Award

Summer 8-18-2023

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)


Forest Resources


Jay Wason

Second Committee Member

Shawn Fraver

Third Committee Member

Yong-Jiang Zhang


Climate change is a threat to global forest ecosystems. In the northeastern United States, forest trees are facing rising temperatures and increasingly inconsistent moisture regimes. In addition to long-term changes in climate conditions, there is concern about the potential for more frequent and intense climate extremes, which can have severe and rapid negative effects on tree physiology and growth. Further, climate extremes may co-occur to produce a greater magnitude of effect than the sum of their parts, with a prominent example being hot droughts, which are increasing in occurrence and severity. The impact of these and other extreme climate interactions may be especially large at range margins, where trees may already experience climate conditions near their tolerance limits. A region of note is the temperate-boreal ecotone, where temperate trees near the northern limit of their range interact with cold-adapted species with a more northerly range. Because of the rapidly changing climate conditions in this region, it is critical to understand the mechanisms of how these trees respond to current and future climate conditions to better predict how they will respond to future climate extremes.

In this study, we used a greenhouse experiment to examine the effects of climate extremes on physiology and growth of seedlings, as well as a field study to assess how daily climate conditions drive the probability of daily growth (PD) for canopy trees. In the greenhouse experiment, we implemented a full factorial design of three levels of drought (short, long, and fully irrigated), two levels of temperature (2-week heat wave and ambient), and two levels of planting treatment (planted alone, and planted with an associate) on seedlings of red spruce (Picea rubens), northern red oak (Quercus rubra), and paper birch (Betula papyrifera). We measured growth and vigor in addition to metrics of physiological stress including midday leaf water potential and net photosynthesis on all seedlings. In the field study of forest trees, we installed point dendrometers on 10 mature trees of each species of red maple (Acer rubrum), red spruce, and eastern hemlock (Tsuga canadensis) at the Howland Research Forest in central Maine. We developed species level models using environmental conditions to predict PD.

In the greenhouse experiment, we found that the long drought significantly reduced growth of red spruce and paper birch, while red oak growth was relatively resistant to drought stress. The moderate heat wave treatment and interaction of heat wave with drought did not have a significant effect on growth or vigor of any species, although photosynthesis declined more rapidly with increasing temperature under conditions of low moisture for red spruce and red oak. We found that total diameter growth was significantly lower in red spruce and paper birch seedlings that experienced turgor loss during the drought. In the observational field study, each species showed responses to unique combinations of climate conditions, with red maple being the least sensitive to daily climate conditions, red spruce showing a lower PD during days with high vapor pressure deficit (VPD) and long days with low soil moisture, and hemlock showing a lower PD on days of high VPD only when soil moisture was low. Combined, these studies demonstrate the importance of climate interactions on physiological responses and growth, suggesting that longer term correlations between climate on growth do not capture the complexities of the physiological response to climate conditions. Predictions for tree responses to future climate change should continue to explore the important effects of interacting climate conditions that can drive physiology and growth in trees of this climate-sensitive region.