Performing Department
School of Forest Resources
Non Technical Summary
Climate change is expected to increase the likelihood of drought and extreme heat events that can damage the water transport system in trees leading to tree mortality and subsequent shifts in forest community composition. Much of our understanding of drought impacts in forests is from arid systems such as the western United States (US) where drought has been implicated in large scale forest community change and tree mortality. However, global warming is expected to lead to increased frequency of drought and other negative climate effects in Maine. This is particularly important because many northeastern forest trees are not adapted to tolerate drought and are therefore vulnerable to minor reductions in moisture availability. Indeed, there are consistent negative responses of tree growth to moisture limitation in Maine and the broader region.The goal of this research is to improve our understanding of how plant physiology and ecology drive forest tree responses to changing climate.Using a combination of observational and experiemental approaches, I will examine relationships between plant structure and function to improve our understanding of how trees respond to extreme climate conditions. This research will improve our knowledge of how forests of the northeastern US will respond to future climate conditions benefiting forest managers and resulting in more resilient forest ecosystems.
Animal Health Component
0%
Research Effort Categories
Basic
75%
Applied
25%
Developmental
(N/A)
Goals / Objectives
The goal of this research is to improve our understanding of how plant physiology and ecology drive forest tree responses to changing climate. This goal will be accomplished via the following objectives:1. Relate plant structures with their function for water transport and storage during drought: Link leaf and xylem structural traits with physiological responses to water deficits. This research is important for identifying the mechanisms that make species more or less resistant and resilient to climate change and will improve our basic understanding of how plants function during drought.2. Tree persistence and tree colonization success in a changing climate: Determine the climate change thresholds for trees to better understand their ability to survive in novel future climates and determine the establishment and survival ability of new trees into forests experiencing a shift in composition. This research will improve our ability to predict future forest change in the region.3. Forest ecology and physiology to inform natural resource management: Describe natural forest ecological processes and their physiological underpinnings to better inform ecological and sustainable forest management practices. This research will allow for better management of ecosystems and provide management guidelines that can reduce the negative impacts of climate change on forests.
Project Methods
Relate plant structures with their function for water transport and storage during droughtThis objective will examine how the xylem of woody plants in Maine relates to their ability to transport and store water. For this project, I will expand on my previous research to quantify water storage and release in species and wood types such as Picea rubens, Abies balsamea, Thuja occidentalis, Betula papyrifera, Populus tremuloides, and Acer saccharum. I will determine how wood anatomy and water storage relates to the ability to withstand droughts. Although stored water typically makes up less than 10% of the daily water use of trees, this water becomes critical during extreme drought when transpiration stops.Water release curves relate a decline in water potential of a segment of wood that is drying to the amount of water it has lost. To generate a water release curve, we saturate a xylem sample with de-ionized water then progressively dry the sample and measure the samples water potential. From these data we can determine (i) the maximum amount of water available for storage in wood and (ii) the ability of that water to be released during drying (capacitance). Samples for these experiments will be collected locally in the University Forest. The physiology of the xylem will be linked back to the 2D (via light microscopy)and 3D (via collaborators using x-ray microtomography) anatomyto improve our understanding of xylem structure and function. For example, we will test how the ability of trees to access water stored in previous year growth ringsimpacts the ability to use stored water during drought. Furthermore, we will examine which species are able to store and access water in other cells such as fibers (Knipfer et al., 2019).Tree persistence and tree colonization success in a changing climateAs climate changes, we need to better understand how existing trees will survive new conditions and how new tree species will colonize these areas. This research objective will be addressed through experimental manipulations of environmental conditions for seedlings and saplings of tree species native to the Eastern US. We will continue our DroughtTIME study (Drought Timing Impacts in ME) that was established in 2019 on the University of Maine campus. In this study, we have experimentally manipulated soil moisture for 288 sapling trees of 6 different tree species that are native to Maine or expected to become more common in Maine with climate change. We will determine the impact of drought timingon tree growth and survival. On each tree we will measure growth (height, diameter, above and below ground biomass), survival, and water status (water potential and relative water content of leaves) throughout the drought. We will monitor soil moisture weekly on a subset of trees during each drought and continuously monitor air temperature and humidity with HOBO dataloggers.A major knowledge gap for managers is to better understand how large canopy trees will respond to climate change. Although it is logistically prohibitive to experimentally modify the environment around large trees, we will use measurements from large trees both in lab-based experiments and in the field to quantify how they respond to extreme drought and dry air. For example, whole-branch dry-downs have emerged as an effective method for rapidly measuring an integrated response of a plant to drought (Rosner et al., 2019). We will collect and rehydrate large branches of native tree species and experimentally dry them while tracking stored water use and leaf physiology (e.g., stomatal conductance, photosynthesis, embolism). We will also periodically monitor a subset of large canopy trees throughout the growing season to quantify seasonal changes in water potential as well as branch and leaf relative water content. These measurements on canopy trees will be linked to the results from the sapling scale experiments on the same species to better inform management.As the climate warms, there are potential benefits from longer growing seasons for cold-limited plants however this could tradeoff with the negative consequences of extreme heat and soil moisture deficits. For example, during hot days, many plants close their stomata and stop photosynthesis even with adequate soil moisture available. Therefore, it is currently unknown the extent to which warming may be a benefit for some tree species in Maine and detriment to others. We will conduct leaf-level measurements to quantify the species-specific vulnerabilities to climate change for trees native to the northeastern US. This research will take advantage of periods of hot and dry weather as well as natural soil moisture deficits. Our preliminary data on leaf physiology in Maine suggests that hot and dry midday conditions already occur consistently and reduce photosynthesis. We will take advantage of these conditions to measure photosynthesis and stomatal conductance at midday in trees native to Maine. These will include some trees from our other experimental work described above.However, as the climate continues to warm, we expect these midday declines in photosynthesis and stomatal conductance to intensify. If we are able to acquire the external funding to do so, we will establish a common garden tree nursery experiment on campus that will be able to isolate the benefits of longer growing seasons from the potential negative effects of high heat and soil moisture deficits. This experiment would include temporary greenhouse structures and use passive heating and controlled ventilation to regulate temperatures and simulate warmer climates. We currently have the capacity for the field-based measurements, however to fully address this objective with manipulative common garden experiments, external funds will be required and are currently being sought.Forest ecology and physiology to inform natural resource managementThe final objective of this proposed project aims to better understand the ecology and physiology of forests in Maine to inform natural resource management. To achieve this objective, I will focus on sensitive forest ecosystems. One aspect of this research will focus on identifying the biotic and abiotic factors that impact successful regeneration of northern white cedar and its ability to recruit to the canopy. Research sites include the Penobscot Experimental Forest. We are using xylem anatomy to determine the regeneration modes of northern white cedar. To successfully manage this species, managers need to know how this species naturally regenerates and the impact that different modes of regeneration have on the ecology and response to management. We will expand on this research to determine the effect that this has on the ecology of the species and management of this valuable forest type.We will also expand our research in spruce-fir forests and tree growth (Wason et al., 2017b, 2019a). We will leverage an existing network of research sites (Commercial Thinning Research Network) to determine how thinning impacted red spruce and balsam fir responsesto climate. We will measure ring widths and correlate them to past climate to determine if reducing forest density lowered the climate sensitivity of trees. We will also conduct preliminary studies to better understand potential climate change refugia for spruce-fir forests. We plan to perform forest inventories and collect tree rings for climate-growth analysis and stand reconstructions at 6 sites in Acadia National Park. At these sites research will include dendrochronology (colleting 30 tree cores for tree ring analysis), microclimate monitoring using iButton temperature sensors, and characterizing forest structure and dynamics by resurveying historic forest plots. Full realization of this objective will also require additional external funds that will be sought through the National Park Service and National Science Foundation.