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Harvard Forest Symposium Abstract 2015

• Title: Broad Tree-ring Networks Reveal Synchronous Dynamics Across a Largely Asynchronous Region
• Primary Author: Neil Pederson (Harvard Forest)
• Additional Authors: Daniel Druckenbrod (Rider University); James Dyer (Ohio University - Main Campus); Amy Hessl (West Virginia University); Dario Martin Benito (Eidgenössische Technische Hochschule Zürich (ETH-Zurich)); Ryan McEwan (University of Dayton); David Orwig (Harvard Forest); Ben Poulter (Montana State University)
• Abstract:

  Climate change is expected to have significant impacts on trees, disturbance, and, on a broader scale, forests and biomes. Much forest research has been conducted over short time scales or small spatial scales, which provides great information on near-term, local processes, but might lack in the ability to detect the influence of large-scale drivers and deep-time legacies. Broad-scale networks have identified previously unknown relations between climate and disturbance regimes that enrich our understanding of forest development. Early tree-ring networks have revealed hemispheric patterns of climate and tree growth as well as connections between ocean-atmospheric circulations and fire regimes. Many of these studies were conducted in regions where climate-forest interaction could be seen as more intuitive than in broadleaf-dominated forests, like those in the eastern US, where a common characterization of disturbance is asynchronous small-scale dynamics and where climate is less likely seen as a constraint on tree growth or a driver of disturbance. Here we present recent research indicating that even in the eastern US, climate appears to have a strong influence and legacy on extant old-growth forests (Pederson et al., 2014). A time-series of reconstructed canopy disturbance over the southern US region indicates the occurrence of asynchronous disturbance that was punctuated by broad-scale synchronous canopy disturbance. Periods of temporally and spatially synchronous disturbance are characterized by the formation of a greater ratio of large canopy gaps and preceded by dry conditions. Large gaps increase the potential for greater change in recruitment at broad scales versus asynchronous periods where gap formation is more commonly smaller in size. Supporting prior paleoecological research, our new research has also identified the possibility that forests previously seen as being quasi-stable at large scales have the potential to abruptly flip to new states with extreme climatic events. For example, the dominant Quercus trees in an 800 ha old-growth forest in Kentucky reached their current canopy status synchronously >200 years ago as a result of a climate-induced, regional-scale canopy disturbance event. Further, recruitment to coring heights of those same Quercus trees appears to have occurred synchronously in the late 1600s. Not only was the 1600s recruitment event observed in Kentucky, it was also detected over much of the eastern US including, provisionally, the great conifer-dominated Pisgah Tract in southern New Hampshire. This indicates that two regional to subcontinental-scale events, presumably driven partly by climate, imparted much legacy in a ‘classic’ old-growth oak forest in Kentucky (Fig 1). When viewing the time-series of reconstructed canopy disturbance from a theoretical perspective, it leads to the hypothesis that climate can synchronize disturbance at broad scales in forests where disturbance often occurs at small scales and is often asynchronous. These findings also provide some mechanism to the abrupt and synchronous forecasts of forest change in the eastern US over the coming century.
  
  
  
  
  
  
  
  Reference
  
  
  
  Pederson N, Dyer JM, McEwan RW et al. (2014) The legacy of episodic climatic events in shaping broadleaf-dominated forests. Ecological MonoGraphs, 84, 599–620.
  
  
  
  

• Research Category: Historical and Retrospective Studies
  Large Experiments and Permanent Plot Studies
  Regional Studies