Harvard Forest >

Summer Research Project 2023

• Title: Environmental Drivers of Tree Growth: Effects of Gravity, Light, and Climate
• Group Project Leader: Andrew Reinmann
• Mentors: Evonne Aguirre; Sophie Everbach; John Paul Hellenbrand; Andrew Reinmann
• Collaborators: Noel Michele Holbrook
• Project Description:

  Wood is a defining characteristic of trees that plays a critical role in light harvesting and is a key mechanism of carbon sequestration in forest ecosystems. Gravity can induce a form of stress on tree stems and branches that alters the wood anatomy of these organs while changes in environmental conditions such as light, temperature, and water availability can have important implications for the rate and timing of wood production in tree stems. Despite the importance of gravity and environmental conditions in wood anatomy and production, many knowledge gaps remain. Forest fragmentation creates abrupt transitions between forest and non-forest land covers that induce large gradients in growing conditions between the forest edge and interior. These gradients in growing conditions also create gradients in tree canopy architecture and wood production dynamics (Mourelle et al. 2001; Reinmann et al. 2020). This summer, we will use forest fragmentation experiments as a model system to study how wood production and anatomy respond to changes in important resources such as light and stress associated with gravity and excessive heat.
  
  
  Sub-project 1
  Living on the edge: Tree growth in fragmented forests
  
  Agriculture and development have made the temperate forests the most heavily fragmented forest biome in the world. Landscapes with fragmented forests are typically characterized by abrupt transitions between forest and non-forest land covers (e.g., meadows, agricultural land, development). These abrupt transitions (i.e., forest edges) induce large gradients in key environmental drivers of tree growth (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 architecture and wood production dynamics. The higher light conditions at the edge likely trigger a cascade of ecophysiological responses that, over time, result in a vegetative “walling off” of the forest edge. In particular, during this walling off period, tree canopy architecture likely becomes more complex from epicormic branching, lateral growth of tree limbs, and a proliferation of new leaves taking advantage of higher light conditions at the edge. As the walling off process progresses, penetration of light and heat into the forest interior likely subside. We know that trees along mature forest edges (i.e., after the walling off process is complete) can grow twice as fast or faster than trees in the interior, but these trees are also more adversely impacted by heat stress. This REU project will address two key remaining areas of uncertainty. First, how quickly can tree growth respond to the creation of a new edge? Second, how does the walling off process modulate the magnitude of heat sensitivity of trees near a forest edge? Field work will center around using manual and automated measurements of tree radial growth to quantify the magnitude and temporal patterns of tree stem wood production along transects between the forest edge and interior in young forest-edge plots (created 60 years ago). A typical week will entail: field work to manually measure tree stem diameters and and download data from automated tree growth sensors and microenvironment sensors, compile/review literature on forest edge effects, curating and analyzing datasets, and preparation for a final presentation. We expect this project will fill important knowledge gaps on environmental controls of wood production that support broader research goals of linking tree physiology to ecosystem carbon sequestration across human-altered landscapes.
  
  
  Sub-project 2
  Reach for the sky: The stress-modified woods of light-foraging branches
  
  Trees are always on the hunt for light. The variety of growth forms seen in the forest is a testament to this photic need; individuals differentially find, occupy, and maintain branches in available light gaps despite the mechanical stressors of gravity and wind. A tree’s structural ability to respond to mechanical stress, while also balancing hydraulic efficiency, resistance to drought damage, and sugar storage, is therefore key to its resource acquisition success and survival. Yet these mechanical-hydraulic-storage tradeoffs are not well understood, especially in the wood that forms in response to mechanical stress. This REU project, designed for one student, will investigate the mechanical side of these tradeoffs by determining the distribution of stress-modified woods in adult tree branches at Harvard Forest. A typical week will be composed of the following, with different priorities placed on each activity depending on weekly project progress: exploring the literature on stress modified woods and canopy structure, field work (branch identification, measurements, and harvesting), processing of field samples (wood anatomy staining and imaging), analysis of measurable features associated with stress modified woods, and final presentation of the findings. Ultimately, we hope to better understand the biological costs of maintaining productive foraging growth behaviors in forests battling drought, severe storm winds, and other disturbance conditions that have intensified under global climate change.
  
  
  General requirements for all overall project:
  
  -Students can expect to spend ~50% of their time in the field. The ideal candidate has a positive attitude in group environments and is comfortable working under a range of conditions (e.g., hot and humid, cool and rainy, buggy, etc.)
  -An inquisitive nature and comfortable asking questions
  -Experience using R or the motivation to learn. This will be the primary software used for data analysis and visualization
  -Capable of walking 2+ miles (in a day) on and off trails to visit field sites

• Readings:

  Badel et al. 2015. Acclimation of mechanical and hydraulic functions in trees: impact of the thigmomorphogenetic process. Frontiers in Plant Science 6: 266.
  
  Groover 2016. Gravitropisms and reaction woods of forest trees - evolution, functions and mechanisms. New Phytologist 211: 790-802.
  
  Reinmann AB and Hutyra LR. 2017. Edge effects enhance carbon uptake and its vulnerability to climate change in temperate broadleaf forests. Proceedings of the National Academy of Sciences 114(1): 107-112. DOI: 10.1073/pnas.1612369114
  
  Smith et al.. 2018. Piecing together the fragments: elucidating edge effects on forest carbon dynamics. Frontiers in Ecology and the Environment 16: 213-221.
  
  Reinmann, A. B., Smith, I. A., Thompson, J. R., & Hutyra, L. R. (2020). Urbanization and fragmentation mediate temperate forest carbon cycle response to climate. Environmental Research Letters, 15, 114036–114036.
  
  Mourelle, C., Kellman, M., & Kwon, L. (2001). Light occlusion at forest edges: An analysis of tree architectural characteristics. Forest Ecology and Management, 154(1–2), 179–192.

• Research Category: Physiological Ecology, Population Dynamics, and Species Interactions