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Summer Research Project 2017

• Title: Group Project: Faith in Oak: White Oak Trajectories in the Greater Walden Pond Woods
• Group Project Leader: Neil Pederson
• Mentors: David Orwig; Neil Pederson
• Collaborators:
• Project Description:

  How do white oak trees reach the canopy in New England forests? Do they directly and rapidly move from the seedling strata into the canopy or are there other pathways? Because New England forests have closed canopies, light is an important factor limiting growth, survival, and trajectory of trees from seedlings to canopy trees. Diversity of species and species traits, such as the ability to persist below the forest canopy, and centuries of ecological dynamics often result in multi-layered ecosystems. Thus, there are likely multiple pathways for a seedling to reach the canopy, some of which can be direct through the creation of large canopy gaps or arduous, tenuous, and episodic as a seedling competes against other trees for light, other resources, and space under the canopy.
  
  Across much of the eastern U.S. oaks are experiencing a seemingly tenuous situation in regards to their ability to endure the coming centuries. In many forests (but not all), there is little to no oak in the seedling to small tree stages. When centuries-old oaks are sampled, it is not uncommon for individual trees to have slow growth for decades and sometimes centuries. Growth of these individuals is so slow that it is often inferred that they were living under intense competition below the forest canopy. Much research has been focused on oak seedling recruitment and oaks currently in the canopy. It seems likely that less attention has been given to the growth of oak trees in between those positions. As many forests in New England are 100-140+ years post agriculture, there are opportunities to understand the dynamics of mid-story trees and the potential fate of oaks that are currently suppressed today. A natural experiment such as this is occurring within the Greater Walden Woods. Adding strength to this experiment, the ownership and detailed history of this landscape is often well-documented back to the early to mid-1600s. By examining the growth of suppressed white oaks across a range of local tree compositions and land-use histories, we can discern some of the pathways they travel to reach the canopy.
  
  Over the course of the summer, a student will examine disturbance dynamics in old, second-growth forests in central Massachusetts and, in particular, focus on the ability of white oak to persist in forest understories.
  
  1. White oak (Quercus alba), an important ecological and economic tree species native to North America, currently has a recruitment deficit over much of the eastern US (Abrams, 2003, McEwan et al., 2011). The reasons for the lack of recruitment are likely many (Lorimer 1993) and this phenomenon is an active research topic in forest ecology, e.g., (Chapman & McEwan, 2016, McEwan et al., 2014, Rentch, 2003). Research on the growth of white oak trees 300 years old or older indicates 1) slow initial growth that increases over time (Pederson, 2005) and
  
  2) that the initial growth of white oak is significantly lower than white oaks in today’s landscape of the same age (Black et al., 2008, Johnson & Abrams, 2009). These observations point to a pathway for young white oak that might be arduous and tenuous. In fact, one hypothesis for accelerating growth in old white oak is that it takes 100+ years for a white oak to reach the canopy and once it is in the canopy and expands its crown, growth accelerates (the late Robert Zahner, pers. com.).
  
  The primary hypothesis to be tested by the Walden White Oak student is to determine if suppressed white oak have been under the forest canopy for 70 years or more. A secondary hypothesis is that there is no difference in growth between today’s suppressed white oaks at the same ages as white oaks sampled in old-growth forests that were likely recruited in an understory position (see Lorimer (1985) and Lorimer & Frelich (1989) for inferred canopy position of trees centuries ago).
  
  Additional topics that can be pursued in parallel include:
  - Are there trees near Henry David Thoreau’s cabin site that overlapped in time with him at Walden Pond?
  - How does species competition and density around a white oak affect:
  o Growth rates?
  o Climate response?
  - How does land-use effect suppressed white oak growth?
  - Did white oak recruit synchronously across Walden Woods?
  
  Neil Pederson will have primary responsibility of supervising and co-advising the Walden White Oak student. Dave Orwig will be actively involved in the mentoring of this project. The student will collaborate with a quantitative ecology student investigating the impact of extreme events on the growth of trees in New England.
  
  To test the main hypotheses and answer questions above, the Walden White Oak student will collect field data (tree coring, vegetation sampling) to pair with vegetation plot measurements and land-use history within the Greater Walden Woods ecosystem as well as work with tree-ring data from 300-464 year old white oak growing across the eastern US collected by Pederson and other tree-ring scientists over the last 40 years. Data analysis will be conducted in the tree ring laboratory at Harvard forest and students will learn dendroecological techniques, crossdating, and data analysis and synthesis. The Walden White Oak REU student will become familiar with tree-ring software, the R programming language, and associated programs. The student should become familiar with Walden Pond, Henry David Thoreau, disturbance ecology, and have interest in forest ecology, working with tree rings, and dendroecology. The work will contribute to an understanding of the dynamics of old, second-growth forests in the northeastern US, of which there are many, and help understand life history traits for an important tree species.
  
  Student responsibilities will include:
  1) visit(s) to Walden Pond and, given time, a forest with mature white oak at Harvard Forest for a comparative analysis.
  2) assist in all aspects of dendroecological investigations from collection, sample preparation, analysis, and synthesis of results.
  3) results will be presented orally and in written form at the annual summer student symposium.
  4) must have a vaild driver's license to drive Harvard Forest vehicles

• Readings:

  structure and definitions: forest dynamics on Wachusett Mountain, Massachusetts. Ecological Applications, 11(2): 437-452.
  Pederson, N (2010) External Characteristics of Old Trees in the Eastern Deciduous Forest. Natural Areas Journal 30: 396-407.
  Rentch J (2003) Oak establishment and canopy accession strategies in five old-growth stands in the central hardwood forest region. Forest Ecology and Management, 184, 285-297.
  Sprugel, DG (1991) Disturbance, equilibrium, and environmental variability: what is ‘natural’ vegetation in a changing environment? Biological conservation, 58: 1-18.
  Thoreau, H.D. (1993, reissue). Faith in a Seed: The Dispersion Of Seeds And Other Late Natural History Writings. Island Press, 301 pp.
  

• Research Category: Physiological Ecology, Population Dynamics, and Species Interactions, Large Experiments and Permanent Plot Studies, Historical and Retrospective Studies, Conservation and Management