New Charts for Spruce-Fir Stand Density Management
The Northern Research Station has been collaborating with university researchers and partners within the Pacific Southwest Research Station to develop new and improved versions of stand density management charts for spruce-fir forests. The new charts are broadly applicable to stands in the northeastern U.S. where red spruce and balsam fir predominate and represent a significant advance over previous versions published 40 to 50 years ago because they show that much higher stocking can be maintained than was previously recognized and use zones instead of threshold lines to show areas in which different stand responses to thinning can be expected.
Overview
Eastern spruce-fir forests, which are characterized by red spruce (Picea rubens) and balsam fir (Abies balsamea), cover millions of acres in the northeastern U.S. and adjacent Canada. Many spruce-fir forests are working forests that are managed for wood production and wildlife habitat, such as wintering areas for white-tailed deer (Odocoileus virginianus).
One of the most important tools forestry practitioners have when deciding whether or how best to manage a forest is a stand density management chart. These charts, called stocking guides or density management diagrams, are often developed for publications like the new Northern Conifer Silviculture Guide currently being written by scientists from the Northern Research Station, universities, and other state and federal agencies. Stand density management charts allow practitioners to determine the stocking (i.e., size and number of trees) that provide different levels of growth and canopy closure. They help managers determine when to thin a forest stand, how many trees to remove, and what the expected growth response will be. This is critical when aiming to avoid mortality from overcrowding or maintain high canopy closure for wildlife.
Key Findings
- The new stand density management charts show that much higher stocking can be maintained than was previously recognized.
- Instead of using thresholds (i.e., lines) to denote under- and over-stocked stands, the new charts recognize that there are zones in which these different conditions exist.
Research scientist Laura Kenefic and a cooperator examine regeneration in a spruce stand at Umbagog National Wildlife Refuge in New Hampshire.
Approach
USDA Forest Service researchers collaborated with university partners to produce updated stand density management charts for spruce-fir forests. They created both a density-management diagram, which originated in the western United States, and a stocking guide, which originated and is more common in the eastern United States. They relate the two stand density management charts using maximum stand density index (SDIMax), which relates the stem density to average stem diameter in a forest, to define the maximum size-density relationship for both charts. Published values were obtained from an earlier publication and harvest scenarios were simulated using the Forest Vegetation Simulator (FVS).
Compared to earlier versions of the stand density management charts for this forest type published between 1973 and 2007, the new charts have several advantages. First, the charts are based on more comprehensive datasets and more robust maximum size-density relationships than were previously available. The new charts are also less complex and can also be used in irregular stands. The creation of a corresponding density management diagrams and stocking guides can aid in translating across methods and ensure that more practitioners can use the relationships defined in the charts.
Outcomes
- Two corresponding stand density management charts – a density management diagram and a stocking guide – are now available for use in eastern spruce-fir forests.
- The new stand density management charts are based on more data and more robust analyses and can be used in a wider range of spruce-fir stand types, such as irregular stands.
Updated quantitative silviculture tools for the spruce – fir forest type in the Northeast include a stocking guide based on stand density index (SDI). This guide shows basal area (ft2/acre) on the y-axis and average stand diameter (QMD) isolines (3-17 inches). It uses relative density (RD), a function of maximum and observed SDI, to define upper and lower bounds of five management zones: zone of high density-dependent mortality (DDM) (RD 1.0 - 0.67); zone of imminent competition mortality (ZICM) (RD 0.67 - 0.55) corresponding to the average self-thinning trajectory; zone of optimal density management (ZODM) (RD 0.55 - 0.35) where net stand-level production is expected to be highest; zone of low density management (LDM) (RD 0.35 - 0.15) where growth rates of individual trees are optimized; and RD < 0.15, which is considered understocked relative to wood production objectives
Note that SDI.max=580 was chosen as a reasonable range-wide average value for calculating RD and establishing the density management zones. Different SDI.max values can be used.
Key Personnel
- Laura S. Kenefic, PhD
- Martin Ritchie, PhD
- David Ray
- Robert Seymour
- Shawn Fraver
- John-Pascal Berrill
- Nicole Rogers
- Aaron Weiskittel