The First 20 Years of The Hardwood Ecosystem Experiment
By Scott Haulton
This summer, the Hardwood Ecosystem Experiment – Indiana’s largest and most comprehensive study of forest management effects on flora, fauna, and ecological communities – celebrated the completion of its 20th field season. This remarkable achievement offers a great opportunity to look back on the project, the major lessons learned, and where we plan to take it over the next 20 years and beyond.
The Hardwood Ecosystem Experiment, or HEE, grew out of concern for the health and resiliency of Indiana’s forests and how future changes in forest development may impact the native species that call them home. In the decade preceding HEE, researchers throughout the Midwest had been documenting disturbing trends regarding the sustainability of oak and hickory dominated forests. Older oak-hickory stands were being slowly being replaced by maples and American beech, shade tolerant and fire intolerant species that were benefitting from human-caused changes in historic disturbance patterns that had previously maintained oaks and hickories in these stands. Forest ecologists throughout the Midwest, such as Purdue University’s Dr. George Parker, warned the widespread loss of oaks in particular - considered keystone species for their role in sustaining forest wildlife populations – would be devastating to Indiana’s forest and woodland communities.
Since the problem of oak-hickory sustainability was largely the result of changes in historic disturbance patterns, ecologists such as Dr. Parker and others argued reintroducing disturbance through forest management would be necessary to reverse the trends. Oaks and hickories are generally classified as moderately shade intolerant, meaning forest canopy disturbance is necessary to provide seedlings and saplings the necessary light levels to compete with more shade tolerant species in the understory. This could be accomplished by natural disturbance events such as windstorms, severe drought, or wildfire. Forest management activities such as timber harvest can also be used to mimic the effects of natural disturbances and create the appropriate light levels for development. The periodic use of prescribed fire is also recognized as an important management tool for oak-hickory stands as these species are uniquely adapted to survive and thrive following fire, while aggressive competitors such as American beech, are not.
Given the extensive scope of the problem, it was recognized early on that more information was needed to better understand and communicate the effects of this major ecological restoration effort. What would be effects on plants and wildlife from the various forest management techniques used? Which species would benefit from improved habitat conditions? What adjustments were needed to avoid negative impacts on species sensitive to disturbance? And, maybe most importantly, what would be the long-term effects of these actions? Conversely, what would be the long-term effect of not managing some stands?
To address these questions, the HEE project was born. The project is based at Morgan-Monroe and Yellowwood state forests, where 2,000 acres of forest are dedicated to HEE research activities. In addition to providing the study area, The Indiana Division of Forestry would provide the bulk of the financial support and on-site housing facilities for the researchers. The study areas consist of nine units divided among three treatments – uneven-age timber harvests, even-age harvests, and unharvested control units where no management occurs for the duration of the planned 100 years of study. Uneven-age treatments include single-tree selection and various sized group and patch cut openings. Even-aged treatments feature 10-acre clearcuts and shelterwoods, with additional 10-acre areas that would receive periodic prescribed burns starting in 2015.
Field studies commenced in 2006, allowing three years of pre-harvest data to be collected before conducting the timber harvests over the winter of 2008-2009. Post-harvest studies would continue on the research areas until two or three years prior to the next round of experimental harvests in 2028. Studies evaluating the effects of prescribed burning have occurred over the last several years, once each of the designated burn areas received multiple burns.
To study the ecological effects of forest management, researchers from across the Midwest have joined the project, mostly from Indiana (Purdue University, Indiana University, Indiana State University, Ball State University, University of Indianapolis), but also Illinois (University of Illinois, Southern Illinois University), Iowa (Drake University), Ohio (Miami University) and Pennsylvania (Indiana University of Pennsylvania). Additional conservation partners have included the Indiana Division of Fish and Wildlife, The Nature Conservancy in Indiana, and the USDA Forest Service.
The movements of Eastern box turtles and timber rattlesnakes were studied before and after timber harvesting at HEE. Photo credit John MaxwellOver the past 20 years HEE research partners have explored over two dozen major study areas, ranging from the inhabitants of forest soil (including invertebrates and salamanders) to those traversing terra firma (such as timber rattlesnakes and box turtles, Photo 1 and 2 ) and those flying in and around the forest canopy (birds, bats, and moths). The findings of these studies are reported in dozens of graduate theses and dissertations, scores of peer-reviewed scientific journal articles, and many informative extension publications and videos available on HEE’s webpage (https://www.heeforeststudy.org/). Detailing all these projects and their findings here would be impossible, so instead, we’ll focus on a few of the overarching themes from the first 20 years at HEE.
Forest Management Is Compatible with the Conservation of Endangered Species and Their Habitats
Given the importance of protecting endangered and threatened species on managed forests, this topic was a high priority throughout the first 20 years of the HEE project. One species in particular, the federally endangered Indiana bat, was an early focus of study since the Division of Forestry had been developing management guidance for its state forests and private forest owners concerned about the species’ conservation. Shortly after the project began, however, that focus widened to include other bat species susceptible to the emerging disease known as white-nose syndrome. First identified in New York in 2006, the disease quickly spread and was detected in Indiana in early 2011. Soon after, HEE bat researchers monitoring bat populations on the study areas began to observe declines in species that were once commonly detected on the two state forests, such as northern long-eared bat and tricolored bat. These declines were also occurring across the entire range of these species and eventually led to northern long-eared bat joining the Indiana bat as a federally endangered species, while tricolored bat has been recently proposed for federal endangered status. All other bats found at the HEE study area are state-listed with the exception of one, the big brown bat. Since all these bats use both live and dead trees for daytime roosting and forage throughout the forest at night, a thorough understanding of how forest management activities affect these imperiled species and their habitats is essential.
What researchers have found is that when simple precautions are taken to avoid impacts, such as retaining favorite roosting structure such as dead trees (especially for Indiana bats), managed forests can offer suitable – and in some ways improved – habitat conditions for bats. Over the 20 years of study at HEE, no bat species has been found to avoid any of the harvested treatments; in fact, most species exhibited higher activity levels or showed a preference for foraging in the harvest areas. Many bat species, including endangered northern-long eared bats and Indiana bats, readily foraged within patch cuts and along the edges of larger forest openings. Roosting preferences for Indiana bats and northern long-eared bats were extensively studied by temporarily attaching radio-transmitters (mostly to adult females) and tracking them to their daytime roosts (Photo 3). These “maternity roosts” are critical to bat populations since they shelter the newborn bat pups. Indiana bats, in particular, typically select maternity roosts at least partially exposed to direct sunlight, as they are often located in canopy gaps or along the edges of larger canopy openings, such as the HEE patch cuts. Maternity roosts of northern long-eared bats are less dependent on the presence of canopy gaps but are often located close to the forest roads and trails used by logging equipment during timber harvest and likely used by bats as convenient travel corridors while foraging.
Photo 3. Attaching radio-transmitter using surgical glue to back of female northern long-eared bat. Photo credit Scott Haulton.
The state endangered cerulean warbler has also been the focus of attention by researchers over the first 20 years of HEE (Photo 4). Range-wide, this species has experienced a greater decline in abundance than any other migratory warbler. At HEE, cerulean warbler studies were conducted by Dr. Kamal Islam and his many Ball State University graduate students. Dr. Islam and his students had been studying cerulean warbler ecology at Morgan-Monroe and Yellowwood state forests even before HEE started, so they were a natural fit for the project. Ceruleans were found to nest in all managed study units, as well as control areas. One major finding was the species’ preference for nesting in mature trees in the white oak group. Dr. Islam and his students found that a preferred food for nesting ceruleans and their young – lepidopteran larvae (i.e. caterpillars) – were more abundant on the oaks and hickories at these nest sites, compared to other tree species. They also found nest sites were often found near canopy gaps and openings. This, together with the fidelity of nesting pairs in the managed units over the course of the study, indicates this species is tolerant of managed forest habitat, as long as mature trees from the white oak group are retained following harvest. In fact, Dr. Islam believes the continued presence of breeding cerulean warblers in Indiana may be largely dependent on forest management planning that can both ensure the long-term maintenance of oaks and hickories in our forests, while also providing suitable mature forest as breeding habitat.
Photo 4. A cerulean warbler receives an identifying leg band. Photo credit Scott Haulton.
Variety is the Spice of Life
Across all the HEE studies, one of the most commonly reported refrains has been the benefit of habitat diversity provided by the HEE forest management activities. Probably no other group of studies demonstrates this better than those related to breeding birds. As other bird studies have shown, HEE researchers found species richness (i.e. the number of species) increased in managed treatments after timber harvesting (Figure 1). This was due to the increased variety of habitats available – regenerating young forest patches and open thinned forest in the uneven-age units and larger areas of regenerating forest and open forest found in clearcuts and shelterwoods within even-age units. Young forest specialists that had not been detected at HEE prior to harvesting were now finding suitable nesting habitat in the harvested areas. Importantly, species that nest in mature forests either continued to nest in the mature thinned forest in uneven units or nested in the mature forest surrounding the larger openings, such as clearcuts. Taken together, the combined number of these two groups of habitat specialists resulted in the higher species richness observed.
Figure 1. Bird species richness at HEE in the three pre-harvest years and siz years post harvest. Dotted vertical line identifies when harvest occurred. Keller, K.F., P.J. Ruhl, J.B. Dunning, J.K. Riegel and R.K. Swihart. 2016 Multiscale responses of breeding birds to experimental forest management in Indiana. Forest Ecology and Management, 382:64-75
Another important finding was that many bird species selected a variety of forest habitats at the HEE study areas. When mist-nets were erected in the HEE clearcuts during the summer, 6 to 8 years after harvest, the expectation was that young forest specialists would dominate the captures. Instead, the most frequently captured birds were those associated with mature, interior forest habitat. Researchers were surprised to find that a species associated with large areas of mature forest – the worm-eating warbler – was the most frequently captured species in clearcuts. Other mature forest nesting species frequenting clearcuts were scarlet tanagers, ovenbirds, and even cerulean warblers. Researchers concluded these birds were visiting the clearcuts to forage for insects and berries that they would provide to their nestlings in the surrounding mature forest habitat. These young forest patches were densely packed with regenerating trees, raspberry thickets, and other flowering plants hosting a wide variety of insects and fruit, necessary for rapidly developing nestlings and late summer migrants preparing for long journeys south.
Other study taxa found to increase in species richness post-harvest included invertebrates, such as long-horned beetles. Species specializing in the consumption of dead wood increased in harvested areas, contributing to the higher species richness noted in insect traps located closer to harvested areas compared to those further away. Researchers also noted the coarse woody debris created by the harvests provided important habitat structure for other taxa, including timber rattlesnakes, small mammals, and woodland salamanders. Retention of dead wood throughout the harvesting process by avoiding disturbance of the pre-harvest supply of logs and standing dead trees and leaving behind tops, large limbs, and unmerchantable portions of harvested trees, will help maintain and, in some cases, improve habitat conditions for a wide variety of forest wildlife.
The Benefits of HEE’s Long-term Approach
One of the greatest strengths of the HEE project is its long-term approach. Forests are complex and dynamic, and disturbance-related impacts can be subtle, sometimes taking years or even decades to become apparent. The long-term commitment of the HEE project is necessary to fully understand how our forests develop, with and without active forest management.
The forests of the Midwest have been continually shaped by a variety of natural and human-caused disturbance events. Fires, windstorms, insect pests, floods, and droughts have all played a significant role in how our forests have developed. Plant and animals that rely on forest habitats have also been affected by these events and have developed a resiliency to change and disturbance that has allowed their populations to endure over time. Only through long-term research can these resiliency strategies become apparent. Due to the natural processes of forest succession, habitat suitable for birds that nest in patches of young forest, eventually disappears as the stand ages over time. Conversely, habitat that may become temporarily unsuitable for a species due to canopy disturbance, will eventually regain its suitability as the forest canopy recovers. Documenting the availability of ephemeral habitats or the time required for habitat recovery are critical to our understanding of how disturbance affects our forest communities. Long-term projects like HEE greatly improve our ability to see the all-important “big picture” when it comes to forest development.
As HEE continues, new research topics continue to emerge. Back in 2006, few thought of using aerial drones to remotely measure carbon stocks sequestered in the various study areas; however, in recent years these studies have expanded greatly at HEE. Even the forest management techniques used at HEE continue to evolve as we build upon the lessons learned over the first 20 years. In the coming years, prescribed fire will likely play a bigger role in studies, as it will be used for the first time in the uneven-age units. In the even-age treatments, areas designated for the next shelterwood and clearcut harvests in 2028 have received repeated prescribed burns over the last decade as a preparatory effort to reduce competition with fire tolerant oak and hickory seedlings. Post-harvest studies will then reveal whether the repeated burns have succeeded in giving oak and hickory seedlings a competitive advantage in the developing stand.
HEE continues to address the original questions and concerns that gave birth to the project 20 years ago, by investigating the forest management activities best suited for maintaining oak and hickory in Indiana’s forests and how these activities affect the diverse flora and fauna that depend on them. So, stay tuned; there will be a lot more to report as the next 20 years of HEE unfolds.
Scott Haulton is a Forestry Wildlife Specialist with the Indian DNR Division of Forestry. Scott’s service with the Division of Forestry began in 2007 when he was hired to coordinate the Division’s state forest wildlife habitat management program and provide guidance on forest wildlife habitat to various sections of the Division, other resource management professionals, and the general public.