# 3-D Forest Structure

One of the things that most intrigues us about forest dynamics is how tree-to-tree interaction relates to stand-level forest structure.

## Background:  Spatially Explicit Tree-based Forest Structure

Processes such as regeneration and competition occur at the tree-level with interactions occurring within tree neighborhoods.  These neighborhoods can be mathematically delineated as polygons, encompassing all the surrounding trees with which a tree can be connected with a straight line without crossing any other lines.

Structure can be explored by looking at the size differential of trees in a neighborhood:  when the trees within a neighborhood are relatively similar in size, the mapped surface connecting them has a shallow slope (here in cool colors like blue), whereas the slope is steeper when there is a larger difference in size among the trees (here in warm colors like orange).  Calculated over the entire stem-mapped area (such as the 1 ha stand below), a measure of 3-D (x, y, and size) structural heterogeneity can be calculated, known as the Structural Complexity Index (or SCI).

The SCI can be applied to any stem-mapped stand and factors other than size can be used for the third dimension (e.g., z=monetary or habitat value).  The SCI is but one of many metrics and indices used to characterize forest structure in stands within the SCN, but is very promising because it can capture both horizontal (e.g., spatial pattern) and vertical (e.g., tree size) heterogeneity, is relatively scale-invariant, and can be flexibly tailored to desired ecosystem services.

Zenner, E.K. and D.E. Hibbs.  2000.  A new method for modeling the heterogeneity of forest structure.  Forest Ecology and Management 129:75-87.

## The Development of Forest Heterogeneity

How you characterize forest stand structure has a lot to do with where you are standing and what indices you use. Because many tools are dependent on spatial scale, using a diversity of metrics and reference points lets us more accurately characterize structure, which then provide clues to the development of forests subjected to various disturbances.

Publications:

• Zenner, E.K., J.E. Peck, M.L. Hobi, & B. Commarmot.  2016.  Validation of a classification protocol: meeting the prospect requirement and ensuring distinctiveness when assigning forest development phases.  Applied Vegetation Science 19:541-552.
• Peck, J.E., B. Commarmot, M.L. Hobi, & E.K. Zenner.  2015.  Should reference conditions be drawn from a single 10-ha plot? Assessing representativeness in a 10,000-ha old-growth European beech forest.  Restoration Ecology 23.6:927-935.
• Zenner, E.K., E. Lähde, and O. Laiho.  2011.  Contrasting the temporal dynamics of stand structure in even-sized and uneven-sized Picea abies dominated stands.  Canadian Journal of Forest Research 41:289-299.
• Zenner, E.K. 2005.   Investigating scale-dependent stand heterogeneity with structure area curves.  Forest Ecology and Management 209:87-100.
• Zenner, E.K. 2005.   Development of tree size distributions in Douglas-fir forests under differing disturbance regimes.  Ecological Applications 15:701-714.
• Zenner, E.K. 2000.  Do residual trees increase structural heterogeneity in Pacific Northwest coniferous forests?  Ecological Applications 10:800-810.

## Structural Complexity in Old-Growth

How do you characterize the structural complexity of a forested stand? One approach is to evaluate the relationship of a tree to its neighboring trees and determine the degree of difference (the gradient) in the magnitude of the diameter or height.

Publications:

• Zenner, E.K., J.E. Peck, M.L. Hobi, and B. Commarmot.  2015.  The dynamics of structure across a primeval European beech stand.  Forestry 88:180-189.
• Zenner, E.K., K. Sagheb-Talebi, R. Akhavan, and J.E. Peck.  2015.  Integration of small-scale canopy dynamics smoothes live-tree structural complexity across development stages in old-growth Oriental beech (Fagus orientalis Lipsky) forests at the multi-gap scale.  Forest Ecology and Management 335:26-36.
• Akhavan, R., K. Sagheb-Talebi, E.K. Zenner, F. Safavimanesh. 2012. Spatial patterns in different forest development stages of an intact old-growth Oriental beech forest in the Caspian region of Iran. European Journal of Forest Research 131:1355-1366.
• Zenner, E.K. 2004. Does old-growth condition imply high live-tree structural complexity? Forest Ecology and Management 195:243-258.
• Zenner, E.K., S.A. Acker, and W.H. Emmingham. 1998. Growth reduction in harvest-age coniferous forests with residual trees in the western central Cascade Range of Oregon. Forest Ecology and Management 102:75-88.
• Acker, S.A., E.K. Zenner, and W.H. Emmingham.  1998. Structure and yield of two-aged stands on the Willamette National Forest, Oregon: Implications for green-tree retention. Canadian Journal of Forest Research 28:749-758.

## Forest Change:  structure, growth, and risk

Whether we like it or not, our forests are changing.  Incorporating our understanding of the relationship between forest structure and composition can improve our ability to predict what change to expect, whether growth will rise or fall, and what regions will be most susceptible to disturbance.

• Mafi-Gholami, D., B. Mahmoudi, and E.K. Zenner. An analysis of the relationship between drought events and mangrove changes. Esturarine, Coastal, and Shelf Science. In press.
• Jaafari, A., D. Mafi-Gholami, and E.K. Zenner. 2017. A Bayesian modeling of wildfire probability in the Zagros Mountains, Iran. Ecological Informatics, 39: 32–44.

## Complexity & Cerulean Warbler Habitat

Jeanne Hickey, Scott Stoleson, Alissa Carlton

There's been a lot of talk about the importance of forest structure for wildlife, but do the birds really care? This project relates the use of forest habitat and nest density of the vulnerable Cerulean warbler to forest structural characteristics and evaluates the utility of using LIDAR to identify potential habitat.

## Neighborhood Complexity & Oak Success

Pennsylvania's forest are increasingly mixed in composition, but how do individual oaks respond to competition? This project examines height growth responses in established oak forests by sectioning entire trees and examining growth rings at different stages of stand development.

Publications:

• Zenner, E.K., D.J. Heggenstaller, P.H. Brose, J.E. Peck, & K.C. Steiner.  2012.  Reconstructing the competitive dynamics of mixed oak neighborhoods.  Canadian Journal of Forest Research 42(9):1714-1723.
• Heggenstaller, D.J., E.K. Zenner, P.H. Brose, & J.E. Peck.  2012.  Field Note:  How much older are Appalachian oaks below-ground than above-ground?  Northern J. of Applied Forestry 29(3):155-157.

## Overstory Structure of Mature Red Pine

To restore the landscape mosaic, we need structural targets for mature managed stands as well as old-growth.  This project provides data on the natural range of variation in mature red pine (MN) and eastern hardwood (PA) stands and evaluates our ability to characterize structural variation in plots up to 1 ha in area.

Publications:

• Zenner, E.K. and J.E. Peck. 2009. Characterizing structural conditions in mature managed red pine:  Spatial dependency of metrics and adequacy of plot size. Forest Ecology and Management 257:311-320.
• Zenner, E.K. and J.E. Peck. 2009. Maintaining the pine legacy in Itasca State Park. Natural Areas Journal 29:157-166.