Principal investigator: Douglass Jacobs, Fred M. van Eck Professor, Forestry and Natural Resources, Purdue University (djacobs@purdue.edu)
Co-authors: Don Carlson, forester, Forestry and Natural Resources, Purdue University; Ron Rathfon, Extension forester, Forestry and Natural Resources, Purdue University; Caleb Redick, research associate, Forestry and Natural Resources, Purdue University; Brianne Innusa, research associate, Forestry and Natural Resources, Purdue University; Michael Saunders, professor, Forestry and Natural Resources, Purdue University
Purdue technicians Scott Engwall and Chelsea Zaldivar walk between enrichment planting experimental plots at the Southeast Purdue Agricultural Center (SEPAC). Photo credit: Brianne Innusa.
Researchers have used an existing network of HTIRC/Purdue Department of Forestry and Natural Resources enrichment planting trials to establish a long-term study to examine the performance of enrichment oak plantings, including their responses to deer exclusion and a crown release from competing trees. These plantings also have been used in HTIRC/Purdue Extension field days to communicate research findings to forest managers.
Since 2021, researchers have visited eight sites across the Central Hardwood Forest Region that received enrichment plantings in the past nine to 21 years. They measured the growth, competition status, survival and vigor of these planted trees and analyzed the effects of various silvicultural treatments. A crown release also was performed at three sites.
In 2022, after eight years, planted northern red oak (Quercus rubra), white oak (Quercus alba), chinkapin oak (Quercus muehlenbergii) and American chestnut (Castanea dentata) all had taller heights, greater stem diameters and a higher percentage of trees in competitive positions in the canopy (codominant or dominant) when grown within fenced enclosures. Researchers also found that tree height decreased with increasing competition, but some competition may be needed to force the trees to allocate resources to height to maintain sun exposure.
Another site that underwent midstory and crown thinning in fenced and unfenced plots showed that 12 years after thinning, northern red oaks exhibited the most growth in unfenced crown thinned treatments. White oaks had the largest diameters in this treatment as well. The tallest white oaks were found in the fenced crown thinned treatments. On a site that underwent gap harvests of varying size, researchers found that larger gaps resulted in a higher probability of survival than small or medium gaps. Lastly, at another site, fencing increased the survival probability of northern red oak and white oak.
In 2024, researchers revisited a few of the sites to measure height, diameter, crown width, survival and crown status. Planted American chestnut and chinkapin oak had a higher percentage of mortality than white oak and northern red oak. American chestnut and northern red oak had more growth in terms of height and DBH compared to white oak and chinkapin oak across all levels of fencing and vegetation management. The planted trees receiving a crown release had greater groundline diameters and crown widths than those that received no crown release.
“This research provides a framework to supplement natural oak regeneration,” principal investigator Doug Jacobs said. “From our current results, we can recommend that forest managers crown thin stands that are 12-plus years of age to open more light in the canopy for oak growth — and to fence seedlings when possible. This research also shows that crown release can be successfully performed on both planted and naturally regenerating trees as young as 8 years.”
Researchers say next steps should include following up on the responses of released trees for two more years, as the effects of crown release often require a few years to manifest.
Figure 1: The average crown width of planted trees at the Southeast Purdue Agricultural Center (SEPAC) in 2024 from the control and release treatments.
Figure 2. Factors that help to explain relative success of underplanting and enrichment plantings.
Goals:
Oak is a keystone species that provides valuable habitat and food for wildlife and has a high economic value. This project examines eight enrichment plantings in the Central Hardwood Forest Region of Indiana. This research is important for maintaining oaks in the overstory and understory of our oak-hickory forests and it is one of the few existing experiments to document these changes for more than 10 years.
The primary goal is to increase the survival, growth and vigor of oak species while analyzing the effect of fencing and vegetation control. Analyzing these methods over a longer term allows landowners and foresters to make better informed land management decisions.
Researchers anticipated that the crown release would increase the growth of natural and artificial trees in the enrichment planting, allowing the enrichment planting to succeed, due to higher nutrients and sunlight available to the saplings.
Methods:
Some enrichment plantings were done in conjunction with a previous shelterwood harvest, a preparatory cut where the smaller trees in the understory and midstory are removed to increase the amount of sunlight reaching the forest floor.
Each site was revisited one time, 10-20 years after the original planting. The original experimental design and seedlings were identified and measured for DBH, ground diameter (where appropriate), competitive status, crown width, crown ratio and height. Using this data, researchers assessed the long-term performance of these sites and are currently making management recommendations.
At three of the eight enrichment plantings, researchers performed a crown release in 2022 on half of the planted trees with competitive crown positions to assess their ability to respond to the release and to maintain their status as part of the maturing stand.
Release treatments consisted of partial (two- or three-sided) crown release. Trees that were not competitive were excluded. In addition to the planted trees, natural regeneration (stump-sprouts and seeded trees) of a similar age and competitive canopy were selected for release, allowing researchers to compare the responses between planted trees and natural regeneration.
These experimental trees were all measured at the time of release and again after two growing seasons.
Key Findings in 2024:
Release of planted trees resulted in greater growth than the control (not released) treatments, which shows that crop tree release can be done successfully on enrichment planting as soon as eight years after planting.
At the Southeastern Indiana Purdue Agricultural Center (SEPAC), the release treatments showed an increase in the average crown width relative to the control treatments (Figure 1).
The Southern Indiana Purdue Agricultural Center (SIPAC) seedlings planted in 2002 showed that harvest gap size had a significant effect on planted tree survival. Probability of survival was higher in large gaps than in medium or small gaps. The effect of stock type was smaller but also significant. Probability of survival was higher for container northern red oaks seedlings than bareroot northern red oak seedlings.
At the Harrold property, deer exclusion fencing increased survival of red oaks and white oaks over 10 years. Without fencing, white oaks had lower survival than tulip tree and black walnut.
Unfenced seedlings had larger crown widths and larger crown ratios than unfenced seedlings on Nelson Stokes property seedlings planting in 2011. This is a surprise given that deer browse usually restricts initial crown growth.