Oak Forests and Woodlands

This habitat is dominated by tree oaks, often accompanied by white pine, hickory and/or hemlock, above a ground cover of grasses, sedges, heaths, and shrubs. The habitat may be a closed-canopy forest or semi-open with dispersed trees. Most are fire influenced to some degree and occur across a diverse range of soil conditions.

This habitat is has been identified in the Massachusetts State Wildlife Action Plan because it supports a variety of plants and wildlife.

Habitat description

Oak forests and woodlands are broadly characterized by an overstory with a high proportion of tree oak, ranging from relatively open woodlands to closed canopy forests. They occur on a wide range of settings, but most often are found in areas of dry soils and/or exposed slopes. Oak forests and woodlands often occur in areas that have historically experienced disturbance, including fire. A variety of other tree species may be present depending on local conditions, such as soil moisture and richness, and may include hickory, sugar maple, hop hornbeam, and ironwood. Shade tolerant species such as white pine, eastern hemlock, and red maple often invade oak forest and woodlands lacking periodic disturbance. Understories typically consist of a mix of shrubs and herbs at varying densities, often including huckleberry and lowbush blueberries, and a rich herbaceous flora including many legumes.  Oak forests and woodlands are incredibly important for biodiversity conservation. Many hundreds of moths, butterflies and other insects depend upon oaks, and oak forests provide important habitat for numerous birds, reptiles, amphibians, mammals, and plants.

Associated natural communities

Oak forests and woodlands include a diversity of natural communities, and their distribution depends upon soil chemistry and moisture, slope, aspect, microclimate and other site conditions. For example, drier sites support black oak–scarlet oak woodlands and chestnut oak forest/woodland. Sites with richer, moister soils may support natural communities with more hickory, hop hornbeam, sugar maple, and red oak. Successional oak forest and white pine–oak forest are communities that are heavily influenced by a history of land clearing. The yellow oak dry calcareous forest is a rare natural community of more alkaline thin soils.

Learn more about the natural communities found in oak forests and woodlands:

• Oak–tulip tree forest (S1)
• Yellow oak dry calcareous forest (S1)
• Hickory–hop hornbeam forest/woodland (S2)
• Open oak forest/woodland (S3)
• Rich mesic forest (S3)
• Sugar maple–oak–hickory forest (S3)
• Black oak–scarlet oak woodland (S3S4)
• Chestnut oak forest/woodland (S4)
• Coastal forest/woodland (S4)
• Dry, rich oak forest/woodland (S4)
• Oak–hickory forest/woodland (S4)
• Red oak–sugar maple transition forest (S4)
• Oak–hemlock–white pine forest (S5)
• White pine–oak forest (S5)
   

Natural communities are given state rarity/imperilment ranks ranging from S1-S5 (S1: rarest/most imperiled).

Characteristic plants and animals

A wide range of animals use oak forests and woodlands, particularly when acorns are abundant. The leaves themselves support over 500 species of moths and butterflies and hundreds of species of oak gall wasps. As one of the most prevalent forest types, oak ecosystems support most common wildlife and many rare species found in the Commonwealth. Characteristic animals may include white-tailed deer, opossums, eastern box turtles, eastern towhees, vireos, eastern bluebirds, whip-poor-wills, wild turkey, mice, voles, red squirrel, gray squirrel, black bear, garter snakes, black racers, black rat snakes, frosted elfin butterflies, and oak hairstreaks. Oak and hickory woodlands support a rich diversity of plants on the forest floor including many rare and uncommon species.

Associated habitats

In areas of dry soils and/or frequent or severe disturbance, such as fire, oak forest and woodland, the black oak–scarlet oak woodland community in particular, can grade into dry woodlands and barrens and maritime forests and shrublands. With infrequent disturbance and moister conditions, oak forests and woodlands grade into northern hardwood and conifer forests as well as maritime forest and woodland. Because oak forest and woodland is a matrix habitat occupying a large proportion of the landscape, a variety of wetland and aquatic habitats (i.e., lakes, streams), including vernal pools, occur in association with this habitat.

Within oak forests and woodlands, there can be mature areas that have “maturing and old-growth characteristics.” These mature forests and woodlands are typified by older and larger trees of varying density, but where periodic fire reduces litter accumulation, maintains sandy mineral soils, and regularly resets the composition and structure of understory vegetation. 

Ecological processes

Oak forests and woodlands are disturbance dependent habitats that most often occur on dry, nutrient-poor soils, but can persist in a wide variety of conditions with periodic disturbance. A variety of natural disturbances, including ice damage, severe storms, tornados, insect and disease damage, fires, and the death of large trees, play an important role in creating canopy openings in which the next generation of oaks can grow into the overstory. Historically, tree cutting and harvest has also played a significant role in maintaining oak forests, but also in shifting species composition, for example by enhancing the growth and dispersal of white pine.

The highly flammable leaves of oaks and hickories, along with grasses and sedges in the understory, create conditions favorable to frequent, low-intensity fires. In the absence of fire, fire-intolerant shrubs and trees, such as red maple, beech, birches, and white pine, become established, shading out the grasses, sedges, and oak seedlings. Over time, a dense mat of moist flat leaves and needles replaces the more flammable understory and the other tree species overtake oaks for dominance in a process referred to as mesophication.

Threats

Although oak trees are very common in Massachusetts, the overall health of oak forests and woodlands is in decline. Numerous oak and hickory natural communities, such as yellow oak dry calcareous forest, hickory–hop hornbeam, and open oak forest/woodlands, are quite rare, and once common communities, such as oak-hickory forest/woodland and sugar maple–oak–hickory forest, are disappearing. Oak woodlands in general have become quite scarce. Across the commonwealth, there is a dearth of oak regeneration. Forestry practices that favor other tree species, combined with decades of fire exclusion and lack of other disturbance, have facilitated the expansion of red maple, white pine, birches, and other trees that are poised to take over canopy dominance when the oak overstory dies.

Development threatens oak forests and woodlands directly through destruction and fragmentation and indirectly by reducing the ability of managers to apply prescribed fire because of concerns about smoke impacts. 

Invasive species: Mature oaks are a favorite target of many non-native insects, including spongy moth and winter moth. Many oak forests once weakened by spongy moth, are vulnerable to a native insect pest the two-lined chestnut borer, which attacks and kills many oak species after they are weakened by spongy moth. Recent outbreaks by both insect species following droughts have caused massive oak die-offs. Because the moths prefer oaks, other tree species, such as white pine and red maple, are left with minimal damage, allowing them to replace the dead oaks in the canopy. Invasive, exotic plants such as barberry, shrub honeysuckle, and European and glossy buckthorn, often dominate the understory preventing canopy regeneration and pushing out native understory species. Exotic earthworms, including the more recently introduced wiggler worms, rapidly consume leaf litter and duff, limit storage of organic carbon and nitrogen, make areas prone to erosion and spread of invasive plants, and interfere with fire and forest ecosystem processes. Oak wilt is an exotic disease that severely impacts oaks and has been detected in neighboring New York and Connecticut. Once established, it could cause widespread mortality, especially in combination with other stressors.

Invasive species combined with a lack of disturbance and over-browsing by white-tailed deer result in natural system modifications that are hindering oak forest health and regeneration. Disturbances that create or maintain canopy and mid-story openings, such as wood-cutting, grazing, and most importantly fire, have declined in the region, and, as a result, fewer oak seedlings have become established. Deer prefer oak seedlings over most other tree species, so in areas where deer densities are high, oak regeneration is often absent. Species that do establish on the forest floor are those which deer do not readily prefer, such as white pine and ferns. 

Climate change: Under a climate change scenario of increased drought, oak forests and woodlands may not fare well. Despite being associated with dry sites, oaks may still die off during droughts. Furthermore, increased droughts may cause more frequent and severe outbreaks of spongy moth, a non-native insect that can decimate oaks. A wetter climate, however, would likely exacerbate the ongoing process of oak forests and woodlands becoming moister and conversion from oak overstory to other trees. A warmer climate is expected to make Massachusetts more suitable for the exotic disease oak wilt, which is a major threat to oaks. Additionally, because many oak forests and woodlands are dependent on management for their health and persistence, the impacts of climate change will in part be tied to the ability to effectively manage.

The establishment of other forest types on former and current oak forest and woodland is inevitable and can be viewed as a continuation of change driven by a combination of natural and anthropogenic processes taking place for hundreds of years. Nevertheless, the decline of oak forests and woodlands has profound impacts for biodiversity conservation as many native species are entirely dependent on these systems and have been for millennia. 

Conservation actions

• Proactive habitat protection. Protect oak forests and woodlands and other associated, interconnected habitats to maintain healthy and resilient landscapes for people and biodiversity conservation. Prioritize sites supporting state-listed animals and plants and other SGCN, as well as other protection priorities identified in BioMap and other conservation planning tools (e.g., municipal open space plans).
• Habitat restoration and management. See recommendations below
• Law and policy. Regulate and limit the impacts of development and consider innovative approaches to incentivizing compatible development and farm and forest preservation where applicable.
• Conservation planning. Include oak forest and woodlands and associated habitats in conservation planning efforts at multiple spatial scales. (See BioMap as an example.)
• Monitoring and research. Monitor the health and trends of SGCN populations, plant communities, and other wildlife. Monitor the effectiveness of habitat management efforts and conduct targeted research to improve habitat and population management.
• Public outreach: Include information about the role of oak forests and woodlands in biodiversity conservation, flood mitigation, water filtering, and climate resiliency as part of broader communication strategies. 

Restoration & management recommendations

Due to their importance to Species of Greatest Conservation Need and vulnerability to stressors, oak forests and woodlands  have been designated as very high priority (tier 1) for restoration and management.

Oak forests and woodlands occur on a continuum from relatively open to closed canopy in a mosaic across the landscape. Oak forests and woodlands generally need periodic management to promote regeneration of oaks and complimentary species, such as hickory, hop hornbeam and sugar maple, and to prevent conversion to other generalist, species such as white pine and red maple.

Planning phase

Before undertaking a project, it is important to complete a thorough planning process that establishes clear goals that are compatible with site conditions, as well as with other conservation goals that may be relevant to the site. For example, it is important to:

• identify unique and/ or otherwise sensitive species and natural communities, and evaluate how to benefit them;
• evaluate key threats, such as invasive species and mesophication;
• secure key resources required to implement the project, including subsequent monitoring and adaptive management; and
• secure community, stakeholder, and institutional support.

Timber income may be a priority for some landowners, in which case the approach to management may differ somewhat from a site managed solely for habitat values.

Several factors should be considered when setting goals for an oak forest and woodland restoration project. Oak forests and woodlands take many forms and vary greatly depending on site conditions and threats. As a consequence, the approach to short- and long-term management will also vary. If an oak forest is in good ecological condition and is being managed as a closed canopy oak forest, it will require little to no up front management and only infrequent maintenance management. A closed canopy site in poor ecological condition being managed to restore oaks and other complimentary species may initially require more intensive management to regenerate and restore the oak natural community, but then will likely require less frequent maintenance management in the long-term. On the other hand, sites being managed as oak woodland may require more intensive up front management and frequent long-term management thereafter. Simply having oak in the canopy does not mean that an oak forest is in a healthy ecological condition. Therefore, it is important to understand the species composition and structure of an oak natural community, what the key threats to that particular community are, and carefully determine how to restore and or maintain that natural community. Managing an oak forest for different age classes of oaks and complimentary species, like hickory, hop hornbeam, or sugar maple, should also ensure that the full suite of native shrubs and forbs are also maintained.

If possible, planning should begin years prior to initiating restoration and management activities. As oak forest and woodland natural communities vary based on underlying soils, disturbance history, and other factors, it is often important to evaluate the ecological condition and management history of surrounding lands. If prescribed fire is being considered, the site should be evaluated by an experienced fire practitioner to determine the feasibility. If needed, harvest areas, skid roads, firebreaks, and burn units should be established in coordination to limit conflicts and reduce overall cost. Prescribed fire is often not feasible, but can still be effectively managed for oak, hickory, and other complimentary species with other management techniques, such as mechanical treatments. Sometimes, threats at different scales must be addressed before management begins.

If selective tree removal is an appropriate restoration tool for a site, it should involve consultation with a forester who can help create plans tailored to the unique characteristics and goals for each site. In general, selective tree removal should focus on removal of generalist species, such as white pine, red maple, black birch, and similar species, while retaining healthy oaks and complimentary species, such as hickory, hop hornbeam and sugar maple. If any oak or hickory trees will be removed, healthy individuals that are highly productive for acorns and nuts should be retained. Oak needs open conditions with copious sunlight to regenerate. If you plan to use selective tree removal to regenerate oak, you should identify areas where regeneration is viable. Look for at least 10 mature oaks or hickories per acre and more than 150 seedlings and saplings per acre. If there are not adequate overstory trees, the amount of advanced regeneration should be at least double to ensure enough oaks will be present to take advantage of the conditions following harvest. Seedling establishment can be helped by thinning around mature oak and hickory trees, which will increase acorn and nut production. Soil scarification associated with tree cutting will facilitate seedling establishment by exposing bare mineral soil needed for acorn germination; if possible, time the harvest with mast years (years with copious acorn production) because the disturbance caused by logging equipment will greatly benefit seedling establishment.

Planning for selective tree removal should also account for additional overstory mortality following harvest caused by blowdown from reduced wind sheltering, disease, damage from logging, and prescribed fire. Use an estimate of 10-25% mortality based on site conditions. Areas with drier soils and/or more exposure to wind will likely experience higher rates of mortality, but it may be reduced by delaying harvest and prescribed fire during drought or insect/disease outbreaks. Keep up to date on the distribution of oak wilt and follow all best management practices, as appropriate.

If the forest overstory consists primarily of oak, hickory, and other complimentary species but there is little or no understory regeneration, consult with a deer biologist to see if deer are eating the seedlings. If deer predation is not the problem, deep leaf litter, light competition, or other factors that can be remedied through management may be preventing seedlings from becoming established. If deer populations are too dense, without population management, oak regeneration is likely to fail. Generally, larger management areas will distribute the impacts of browsers more widely and potentially allow more seedlings to mature. Population control is generally the best solution for deer over-browsing. Large fencing exclosures are effective but expensive and difficult to maintain, and slash walls are problematic in areas where prescribed fire will be part of restoration and management.

Initial restoration

The first step in restoration is often to control invasive plants that may be released by disturbance and light exposure. While mechanical removal of invasive plants can sometimes control small infestations, herbicide is generally most effective for managing widespread invasive plants. For sites without recent disturbance and where restoration includes selective tree removal, overstory thinning of generalist species is usually next. However, the management plan should be based on the existing species composition and forest structure, the target community type and structure, and soil characteristics. In oak forests and woodlands with a diversity of age classes and a healthy understory, selective tree removal may not be necessary and, instead, maintenance activities can be initiated.

The following recommendations for regeneration of oaks and other complimentary species through selective tree removal may be altered based on the target natural community, specific site conditions, and other factors. Work with a forester and a restoration ecologist to create a management plan for each forest stand. Drier sites generally regenerate oak more easily and with fewer trees cut, while moist oak sites may require a more rigorous approach. Sites on moist soils should average 60’-70’ spacing between trees of 20” diameter or larger. This will allow for enough light to reach the seedlings. Leave groups of trees and large open areas. Sugar maples occurring in drains or toe slopes should be retained. Rich mesic forests containing oak and sugar maple on calcium rich moist soils warrant special management consideration. While selective logging may be warranted to ensure tree regeneration, special consideration is needed to protect older growth trees, spring ephemerals and other herbaceous flora within these specialized forests and woodlands found on colluvial rich soils. Dry and nutrient-poor sites can have more trees retained, with ~15” diameter trees spaced every 30’-35’. Consider retaining any pitch pines. Intermediary sites should retain trees of ~17” diameter every 40’-50’. Non-native and generalist tree species, such as white pine and red maple, should be preferentially removed during overstory thinning, while healthy oaks and complimentary species should be retained. Whole tree removal is preferred to leaving treetops on the ground because the dead material can change soil characteristics and cause problems for prescribed fire implementation. Mowing or mulching should be minimized, when possible, for the same reasons. Where prescribed fire is planned, it should be applied as soon as vegetation is able to carry fire, usually 2-4 years after harvest. Oaks and hickories that establish shortly after a fire are more likely to survive subsequent fires because the rootstock is contained mostly in the soil rather than leaf litter or duff.

Following the initial mechanical treatment and prescribed fire, expect a flush of herbaceous vegetation, expansion of heath, and establishment of tree seedlings in the understory. Mowing and/or herbicide treatments may be required to control undesirable resprouting trees, shrubs, and invasive plants. In all sites, but especially wet and moderately dry sites, if fire is not an option or not able to control competition, mechanical or chemical control of generalists (i.e., beech, birch, red maple, white pine) may be needed to ensure oaks are dominant at canopy closure. Consider keeping or creating pockets of open, grassy areas nested within the oak forest or woodland matrix, as these areas provide important habitat for many plants and animals.

Maintenance

Prescribed fire is an important maintenance tool when feasible, but other maintenance approaches involving mechanical and/or chemical treatments to manage competing vegetation can also be effective. More open woodlands require more frequent fires to maintain their typical herbaceous plant diversity. More frequent fires promote grasses, heaths, sedges, and forbs; whereas, longer intervals allow trees and shrubs to proliferate. Fire return intervals of <5 years will reduce the ability of oak seedlings to mature, maintaining a more open canopy. In woodlands, burning during a variety of seasons will help promote open structure and plant diversity. Spring fires can be used to help establish native grasses and sedges; however, once they are established, growing season burns should be incorporated to help control woody stem proliferation and promote a diversity of herbaceous plants. Prescribed fires during periods of stress, such as drought or insect outbreaks, may cause high oak mortality, especially during late spring when temperatures are rising but vegetation has not yet greened-up. Burning in masticated or mechanically treated fuels during dry periods, particularly during summer or fall, may cause high intensity, extended smolder, and overstory mortality. Periodic mowing and/or herbicide treatments may be needed if fire alone does not achieve desired results. In a shifting mosaic of woodlands and forests, rotational harvest may be used to “reset” a forest to an open woodland once the overstory closes. Cordwood can be selectively harvested every 20-30 years, with low quality trees and undesirable species removed and the highest quality trees left behind. An open woodland is unlikely to produce quality timber because direct sunlight on tree boles promotes epicormic branching.

If managing for forest regeneration, after the initial one or two burns within the first 8-10 years, a fire-free period of roughly 8 years is often needed to allow some oak seedlings to grow large enough to be resistant to low-intensity fire (4”- 6" diameter). A fire-free period of 10-15 years may be required to maximize the number of oaks that will survive a fire, but the long interval may open opportunities for competitors to become established in the overstory. Just two or three burns may be enough to favor oaks and hickories to the point where they are the dominant overstory forest trees. When trying to establish oak forest, early spring is generally the preferred season to burn because it encourages vigorous resprouting and minimizes impacts to roots. Periodic fires in oak forests, including growing season fires, help maintain diversity, provide food for wildlife, and encourage the next generation of oaks.

In areas where fire is not feasible, selective thinning, mowing and herbicide treatments should be used to reduce competition from undesirable species. An initial foliar herbicide treatment targeting resprouting and generalist trees and shrubs within three years of canopy thinning can help reduce competition while the stems are still short enough to spray. Cutting or herbicide treatments of competing stems should be completed before they overtop the oaks, which usually occurs around 10-15 years after initial thinning. The trees that comprise the canopy at the time of crown closure will dominate the forest for decades.

Addressing invasive species when managing oak forests and woodlands is a priority, as invasive plant species are often able to colonize and spread in response to increased light availability and soil disturbance. Evaluating and controlling established invasive species prior to forest management activities - and monitoring and treating invasives after canopy tree removal has taken place - should aways be included in a management plan. Protocols to prevent the introduction or spread of invasive species during forest management, such as controlling potential vectors (contaminated soil, equipment, etc.) by powerwashing equipment prior to arriving on site, should be implemented to the greatest extent practicable. Finally, regular monitoring and rapid response to pioneering invasive species before they begin to impact habitat should be a priority. Fire can be helpful for discouraging some invasive plants, but herbicide is usually required for effective control, particularly in areas with widespread infestations prior to the initial treatment and canopy thinning.