Plants are remarkably responsive in how they express leaf, wood, and ecophysiological traits in response to environmental gradients. Such plasticity is often functional in the stress responses of trees. Trees can alter their traits in response to the environment through phenotypic plasticity, and this rapid response may be beneficial for trees facing novel environmental stressors. However, plasticity is not necessarily adaptive. Further, the plastic variations in leaves within individual trees are not well understood. Plant traits are often studied on the scale of a species, but plant traits can vary dramatically within species and sometimes even within individual trees. This project will investigate plasticity in plant traits of mature trees along gradients of height, light, and microclimate. Much of the work will be in the Climate Interactions with Forest Fragmentation (CLIFF) experiment– a large-scale, novel forest fragmentation x precipitation manipulation experiment. Depending on the sub-project, students will have varying combinations of field and laboratory work; some projects involve using an aerial lift to measure leaf traits and physiology in the tree canopy. We seek four students interested in plant physiological ecology to work on three sub-projects; while each student will primarily work within a given subproject, there will be opportunities for students to collaborate across subprojects.
SUBPROJECT 1 - Environmental gradients in leaf traits within tree canopies
This project will focus on plasticity in leaf traits through the canopies of mature trees along gradients of height, light, and microclimate. Many leaf traits have been observed to shift throughout the canopy of a tree. Leaf mass per area, leaf area, stomatal density, and vein density have all been shown to vary with canopy height, but it is unclear which factors may drive these plastic shifts in leaf form. The student involved in this sub-project will measure leaf traits in response to height, light, and microclimate vertically through tree canopies. This student will use the aerial lift to place temperature and humidity sensors at intervals through tree canopies and will measure light quantity and quality at the same intervals. This sub-project will investigate the environmental gradients driving intracanopy leaf form and function. A typical week for the student on this project will involve about 50% field work using the aerial lift, 25% lab work and leaf measurements, and 25% data entry and analysis. Laura Ostrowsky will work with one student on this subproject.
SUBPROJECT 2 - Response of tree leaf ecophysiological process to forest edge effects and climate
Forest fragmentation creates abrupt transitions (i.e., forest edges) which induce large gradients in key environmental drivers of tree ecophysiology (e.g., light, temperature, and soil moisture) between the forest edge and interior. However, we are only beginning to learn how these changes in environmental conditions alter tree carbon and water exchange with the atmosphere. We know that trees along a forest edge can grow more than twice as fast as trees in the forest interior (Reinmann and Hutyra 2017) and there is evidence that trees near a forest edge also use more water (Herbst et al. 2007), which in turn accelerates soil drying during the growing season (Reinmann et al. 2020). Our previous work has also demonstrated that while conditions near a forest edge can greatly enhance rates of tree productivity, they can also make trees more negatively impacted by climate stressors such as excessive heat (Reinmann and Hutyra 2017). Despite these advances in understanding of how forest edge effects alter tree productivity and water use, we know very little about the underlying ecophysiological mechanisms. In this subproject we will conduct leaf-level measurements of gas exchange on canopy leaves from trees along forest edge-to-interior transects and across our precipitation treatments at CLIFF. In addition, we will conduct mini experiments on these leaves that will help us understand temperature sensitivities of gas exchange and to separate stomatal versus biochemical limitations to photosynthesis. This subproject will also incorporate measurements of tree sap flow (i.e., transpiration) using an automated sensor system. A typical week may entail some combination of field work to measure leaf-level ecophysiological processes, maintenance and data downloads from sap flow sensors, compile/review literature on forest edges and tree ecophysiology, curating and analyzing datasets, and preparation for a final presentation. Collectively, we expect these measurements will provide data that advances our understanding of how forest edge effects and water availability alter tree ecophysiology modulators of ecosystem carbon and water exchange. We also expect this work to provide new insight into how water availability mediates the response of tree ecophysiological processes to heat stress. Joy Winbourne and Evonne Aguirre will each be the primary mentor for one of the two students on this subproject.
SUBPROJECT 3 - Tree carbon storage dynamics responses to forest edge effects and climate
Trees rely on stored sugars and starches, collectively termed non-structural carbohydrates (NSCs), for metabolism, transport, defense, osmoregulation and as a buffer during stressful events when photosynthesis is impaired (Barker Plotkin et al. 2021, Richardson et al. 2013). Stress may mediate how plants allocate sugars and starches to growth versus storage; therefore, measuring NSCs is a key part of understanding how trees are responding to the interacting stressors of fragmentation and water availability in the CLIFF experiment. The student will process stem wood samples from the first three years of the CLIFF experiment to explore how tree carbon storage responds to forest fragmentation, drought, and their interaction. We expect that NSCs will be higher in trees closer to a forest edge because trees on edges have greater access to light and nutrients. Further, we expect NSCs will be lowest in trees in the water-limited treatment, and highest in the water-addition treatment, because water availability strongly controls photosynthesis. However, these relationships may be modified by growth-storage tradeoffs. This is a laboratory-based study; applicants to this sub-project should have some wet lab experience and the desire to spend most of the summer in the lab. However, there will be opportunities to collaborate on the larger project and field sampling if desired. In addition, the student selected will have the opportunity to hone a research question based on the larger study and incorporate other data streams from the CLIFF experiment in the study design. One student will work with Audrey Barker Plotkin on this subproject.
General requirements for all overall project:
1. Participate in field work off-trail in hot, humid, and buggy conditions
2. Stamina and humor to work collaboratively in sometimes challenging conditions (whether they be in the field, lab, or computer).
3. Conduct rigorous data collection and analysis
4. Learn the basics of R for statistics and data visualization
5. An inquisitive nature and comfortable asking questions
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