Harvard Forest >

Summer Research Project 2024

• Title: Impacts of Forest Fragmentation on Ecosystem Structure, Function, and Diversity
• Group Project Leader: Andrew Reinmann
• Mentors: Evonne Aguirre; Laura Figueroa; John Paul Hellenbrand; Andrew Reinmann; Shersingh Joseph Tumber-Dávila
• Collaborators:
• Project Description:

  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 between forest and non-forest land covers induce large gradients in environmental conditions (e.g., light, temperature, humidity, and soil moisture) between the forest edge and interior. For example, forest edges are typically hotter, drier, and have greater exposure to wind and light than the forest interior (Smith et al. 2019. Gradients in environmental conditions across the non-forest-to-forest interior ecotone can be large and create gradients in carbon and water cycling, forest structure (e.g., above and belowground biomass, rooting behavior, and canopy architecture), and diversity of soil invertebrates (Reinmann and Hutyra 2017; Harper et al. 2005; Herbst et al. 2007). This summer, we will use a new forest fragmentation experiment as a model system to study how carbon and water cycling, tree growth, rooting behavior, and soil invertebrate communities respond to changes in important resources and stressors related to temperature and availability of light and water.
  
  Sub-project 1
  Living on the edge: Tree growth in fragmented forests
  
  Forest fragmentation creates 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 three 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? Third, does the response of tree growth respond before or after changes in leaf area (e.g., walling off)? Field work will center around using dendrometer bands and automated point dendrometers to quantify the magnitude and temporal patterns of tree stem wood production along transects between the forest edge and interior in new forest-edge plots (created in 2023) and older forest-edge plots (created >60 years ago). Additional measurements will include quantify leaf area index and possibly measurements of soil respiration and tree sap flow. A typical week will entail: field work to measure dendrometer bands and download data from point dendrometers 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 tree growth that support broader research goals of linking tree ecophysiology to ecosystem carbon sequestration across human-altered landscapes.
  
  Sub-project 2
  Thirsty roots in wide-open spaces: investigating the impact of forest fragmentation on root system dynamics
  
  The large environmental gradients present between forest edges and interior forests allows us opportunities to investigate the impacts that forest fragmentation has on the belowground carbon allocation of forest trees. Forest trees at the edge of new forest fragments are introduced to a suite of new environmental conditions that could alter their overall growth and resource acquisition strategies. More is known about the aboveground response of trees to new edge conditions, such as increased heat stress, along with an increased abundance of lateral branching and leaf biomass to take advantage of the newly abundant light available at the edge, creating the “walling off effect”. However, we know little about the changes that occur belowground at forest edges. Plant root systems are the primary organ responsible for nutrient and water acquisition, and forest edges greatly impact belowground dynamics. For example, new edges lead to reduced competition as there is open space available for trees at the edge to now occupy. Additionally, the decomposition of necromass from plants previously occupying the new open areas at the edge along with the additional thermal energy in open areas that could speed-up microbial activity may lead to an increase in nutrient availability. Therefore, we seek to understand how belowground resources, water and nutrients, are altered along the transition from open-edge-interior forests, and how these dynamics alter the plant root systems of dominant trees in this altered environment. We will collect a transect of soil cores from the clearing adjacent to the forest through to the interior forest to measure the vertical distribution of root biomass for dominant tree species. Additionally, we will measure the fine root traits, such as specific root length, root tissue density, root diameter, root nitrogen, and the ratio of absorptive to transport fine roots to see if fine root strategies differ across the environmental gradients. We will also measure soil characteristics such as soil nitrogen, soil organic matter content, soil texture, and soil water content to evaluate the connections between belowground resource availability and rooting dynamics. The project will begin by taking a transect of soil cores and installing root ingrowth cores in the same locations during the first few weeks. Following this, the majority of the laboratory analyses will be conducted to measure the root and soil traits during the summer. We expect this project will fill important knowledge gaps on environmental controls belowground tree growth that support broader research goals of linking tree ecophysiology to ecosystem carbon sequestration across human-altered landscapes.
  
  Sub-project 3
  Brown food web responses to forest edges and water availability
  
  Soil invertebrates are essential for ecosystem functioning, contributing to decomposition and nutrient cycling, as well as numerous trophic interactions below and above ground. Invertebrates are impacted by numerous interacting stressors, including habitat loss and climate change, yet disentangling factors at a large spatial scale has been logistically challenging and limits our ability to forecast future impacts of climate change. Previous work in this system has shown that warming can affect both abundance, diversity, and composition of soil invertebrates, as well as decomposition rates (Figueroa et al. 2021), yet how edge effects and water availability influence patterns is much less known. In this project we will evaluate how edge effects and water availability influence invertebrate abundance, diversity, functional role, and ecosystem service provisioning. To evaluate this the students will collect red maple leaves due to their widespread representation in temperate forests and relative quick rate of decomposition and measure dry weight. They will then deploy the leaves in metallic chambers that limit vertebrate access while enabling invertebrate access. A subset of the leaves will be collected at the end of the summer to measure leaf litter loss, while another subset will be left for longer term assessment (i.e., over a year for future REU students). Simultaneously, the students will collect weekly leaf litter samples, and using Berlese funnels, will evaluate the soil invertebrates present in the six different treatments: forest interior low water, forest interior high water, forest interior water control, forest edge low water, forest edge high water, and forest edge water control. The students will learn to analyze the patterns in R, evaluating the effects of forest edge and water availability on invertebrate abundance, diversity, and functional role (namely predators vs not), as well as evaluate knockdown consequences for leaf litter loss at the end of the field season. This project will expand our understanding of how interacting anthropogenic stressors, namely climate change and habitat alteration, influence invertebrate communities and ecosystem service provisioning.
  
  General requirements for all overall project, regardless of sub-project:
  1. 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.)
  2. An inquisitive nature and comfortable asking questions
  3. Experience using R or the motivation to learn. This will be the primary software used for data analysis and visualization
  4. Capable of walking 2+ miles (in a day) on and off trails to visit field sites

• Readings:

  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.
  
  Figueroa, Laura L., Audrey Maran, and Shannon L. Pelini. Increasing temperatures reduce invertebrate abundance and slow decomposition. Plos one 16.11 (2021): e0259045.

• Research Category: Soil Carbon and Nitrogen Dynamics, Physiological Ecology, Population Dynamics, and Species Interactions, Large Experiments and Permanent Plot Studies, Biodiversity Studies