Glossary of Technical Terms for Forestry Operations

This page contains a glossary of terms for forestry operations of a technical nature.

This page contains technical forestry terms that can typically be found in forest resource management plans, silviculture prescriptions, forest research and monitoring reports.

Glossary of terms

Acceptable growing stock (AGS) - Trees that now, or could have in the future, one or more desirable characteristics making them suitable for allocation of growing space in a stand.  Historically the desirable characteristics have included crown class, form (e.g., stem sweep or crook; large branches), species, and vigor; and was oriented toward timber characteristics.  The scope is expanding to include other societal values and objectives deemed acceptable such as mast production, cavities, etc.  See also Unacceptable Growing Stock (UGS).

Adaptive management - A systematic and iterative approach for improving resource management by emphasizing learning from management outcomes.  DSPR management activity is not explicitly enacted under an experimental design, as in a pure application of adaptive management.  However, careful observations, thoughtful judgement, and learning from prior experiences, are critical to the applied science of silviculture.

 Age class - An interval of ages into which the age range of trees is divided for classification or use.  An age class may be a single cohort, but a cohort is not always an age class.  See also cohort.

Angle gage - A device that projects a known horizontal angle, originating from the eye, that can be used to carry out horizontal point sampling.  See also basal area, horizontal point sampling, prism, Relaskop.

Aspect - The direction land faces.  South and west facing slopes typically receive more solar radiation than north and east facing slopes; aspect and other geographic and meteorologic factors can influence growing conditions and suitability of a site for certain tree species.

Basal area (BA) - This can refer to (1) the cross-sectional area of a tree at breast-height; (2) the total sum of the cross-sectional areas of trees, within an area of interest (e.g., a stand); and (3) the mean of the sum of those cross-sectional areas within an area of interest per unit area (e.g., ft² per acre).  As a mean value per acre, it is a very useful measure in understanding forest growth and development.  It is related to stand volume, biomass, density, and competition.  (2) and (3) can be calculated over specific groups of trees (e.g., species, size classes, etc.).  See also diameter at breast height.

Basal area factor (BAF) - Used in horizontal point sampling, BAF is a combination of a conversion factor of areal units, and a trigonometric function of the angle selected for sampling; it sets the ratio between the square of the radius of a tree and its probability of inclusion in the sample.  It is usually selected so that the basal area represented by each tree selected into the sample is a round number – like 10 ft²/acre, 20 ft²/acre, 40 ft²/acre, etc., and eliminates the need for measuring specific angles in the field.  See angle gage, Relaskop, horizontal point sampling.

Best Management Practice (BMP) - A method that has been determined to be the most effective and practical means of achieving an objective.  In the context of forestry and water quality, these are practices like installing waterbars to divert water off skid trails, or seeding landings to help stabilize bare soil; and may have the force of law or regulation in meeting clean water and wetlands protection goals.
Bucking: The act of cutting the stem of a tree into separate logs.  This involves very critical decision-making and sound judgement on the part of professional loggers to avoid having grade and form defects, both seen and unseen, like knots, seams, sweep, and crook, negatively affect log value and usability.

Coarse woody debris (CWD) - As defined in this context, CWD are pieces or portions of pieces of down dead wood with a minimum small-end diameter of at least 3 inches and a length of at least 3 feet.  CWD pieces must be completely detached from a bole and/or not be self-supported by a root system with a lean angle more than 45 degrees from vertical.  CWD also corresponds with the 1,000 hour lag-time fuel class.  Both individual pieces, and piles comprising predominantly CWD pieces (as opposed to FWD), are sampled in the post-management monitoring protocol.  Apart from its importance in fire and fuel management, it plays an important role as habitat for many types of organisms, biogeochemical cycling, tree regeneration, and biodiversity.  Volume of CWD is most commonly reported in cubic feet.  See also down woody material, fine woody debris, line intersect sampling, transect.

Cohort - An aggregation of trees in a stand that starts as a result of a single disturbance.  If the range of ages of trees in a cohort is narrow, the aggregation may be regarded as a single age class which is also even-aged.  See also age class.

Continuous Forest Inventory (CFI) - A forest monitoring system based on methods where permanent sample plots are measured periodically.  Changes in the dimensions (growth or other changes in diameter and height) and fate (death or new trees growing in to plots) are tracked over time; other variables (e.g., forest condition; CWD are assessed as well).  CFI systems allow for an assessment at the strategic level (e.g., ownership and subregions, 10,000s to 100,000s of acres) of the current character and condition of forest land, and trends in growth, mortality, and harvesting.  See, in contrast, stand exam.

Crown class - A classification of the position of the crown of a tree relative to the crowns of adjacent trees.  Strictly speaking it is only applicable in even-aged, single-species stands, or stands or canopy strata comprising species of the same height growth regime.  Colloquially, though, it is extended to mixed-species and uneven-aged stands through comparing trees with their immediate neighbors.  The classes include:

• Open grown: Trees with crowns that have received full light from above and all sides throughout their lifespan, and especially during their early developmental period.
• Emergent: Trees with crowns completely above the general level of the main canopy receiving full light from above and from all sides.
• Dominant: Trees with crowns extending above the general level of the main canopy of even-aged stands or, in uneven-aged stands, above the crowns of the tree’s immediate neighbors, and receiving full light from above and partly from the sides.
• Codominant: Trees with crowns forming the general level of the main canopy in even-aged stands or, in uneven-aged stands, the main canopy of the tree’s immediate neighbors, receiving full light from above and comparatively little from the sides.
• Intermediate: Trees with crowns extending into the lower portion of the main canopy of even-aged stands or, in uneven-aged stands, into the lower portion of the canopy formed by the tree’s immediate neighbors, but shorter in height than the codominants.  They receive little direct light from above and none from the sides.
• Overtopped/suppressed: Trees of varying levels of vigor that have their crowns completely covered from the top and sides by the crowns of one or more neighboring trees.

Cutting cycle - The length of time between harvests in, typically balanced, uneven-age silvicultural systems.  See also stand structure (uneven-aged).

DCR - Massachusetts Department of Conservation and Recreation.

Decay class - An ordinal classification of the state of decay of standing and down dead trees; with 1 representing material that is relatively sound and recently deceased, and 5 representing material that is in advanced stages of decay.

Diameter at breast height (DBH) - A measurement of trees defined (in English units and normal situations) as the average stem diameter in inches, outside bark, at a point 4.5 feet above the ground; from the uphill side along with many other rules.  This measurement is a compromise between accessibility (i.e., where people can reach) and representativeness of the size of the stem (i.e., avoiding the effects of swelling and taper).  A wide variety of tree parameters can be related to tree diameter.  See also basal area.

Directional felling - A suite of specialized techniques, employed by professional timber harvesters. that allow the logger to exert more control over where the tree will fall.  These include using proper notching, boring, strapping, and boxing techniques for hand-felling; and use of specialized equipment and its operation in mechanized harvesting.  Use of directional felling can help minimize damage to regeneration and large trees to be left behind by a harvest, and erosion. See also logging system.

Down woody material (DWM) - Down and dead forest materials on the forest floor and in the understory; it includes dead material completely detached from living stems, not self-supported by a root system, and leaning >45° from vertical; or live herbs and shrubs.  DWM encompasses logging slash, pieces and piles of coarse woody debris (CWD), fine woody debris (FWD), duff, litter, herbs, shrubs, and fuelbed depth.  See also coarse woody debris, duff, fine woody debris, line intersect sampling, litter, transect.

DSPR - Division of State Parks and Recreation, within DCR.

Duff - Organic forest-floor layer consisting of decomposing leaves and other organic material in which individual plant parts are not recognizable.  See also down woody material, litter.

Even-age(d) stand structure -  See stand structure.

Fine woody debris (FWD) - Pieces or portions of pieces of down woody debris with a diameter less than 3 inches at the point of transect intersection; excludes dead branches attached to standing trees, dead foliage, bark fragments, and cubical rot.  See also coarse woody debris, down woody material, line intersect sampling, transect.

Fixed-radius plot sampling - In the context of sampling trees, all trees defined as within a circular plot of fixed radius are measured for various attributes that can then be used to estimate population parameters.  It is a form of probability proportional to frequency sampling – trees of a given set of characteristics that are more common are selected with greater probability.  See, in contrast, horizontal point sampling.

Horizontal point sampling (HPS) - Also called angle-count sampling, Bitterlich sampling, plotless sampling, point sampling, prism sampling, variable plot sampling, and variable-radius plot sampling.  In the context of sampling stands and trees and field application of the method, a fixed horizontal angle, which is often selected for its numerical relationship to convenient multiples of basal area per acre (basal area factor), is rotated around a sample point.  Trees appearing to be larger in width than that the projection of that angle are selected in to the sample, and measured for various attributes that can be used to estimate population parameters.  It is a form of probability proportional to size sampling (in this case, the square of tree radius, DBH, or basal area) – larger diameter trees have a greater probability of being selected than smaller diameter trees.  Devices used to implement this method of sampling include a wedge prism (also called simply, prism), an angle gage, and a Relaskop.  See also angle gage, basal area, basal area factor; and in contrast, fixed-radius plot sampling.

Interfering vegetation - A wide variety of plants that interfere with desired patterns of stand development.   It can include native plants, like American beech, grape, mountain laurel, striped maple, and witch hazel, all of which can exhibit invasive tendencies.  It can also include non-native invasive plants, like glossy buckthorn and Japanese barberry.  See also invasive species.

Invasive species - Invasive species are that may spread to a rate and extent that is deemed to cause harm to the desirable functioning of natural communities and ecosystems.  See also interfering vegetation

Landing (also called header) - The name for the location where, during a timber harvest, logs are brought by off-road equipment from the forest to a point where they can be loaded on to on-road log trucks and trailers.

Landscape designation - A system implemented by the Bureau of Forestry in 2012 to determine land use and management based on the characteristics of the land, suitability for certain uses, and a well-defined public process.  The three designations are: 

• Reserves (landscape designation; see also reserves in silviculture/harvesting): Large contiguous blocks of high‐value ecosystems.  These are areas where the dominant ecosystem service objectives will be biodiversity maintenance, nutrient cycling and soil formation, and long‐term carbon sequestration.  Forest management will generally consist of letting natural processes take their course, although under specific circumstances, more active management might be permitted.
• Parklands, landscape designation: Areas that conserve unique natural and cultural resources while focusing on the provision of recreation.  Parkland management approaches are expected to range from areas where natural processes dominate to highly modified environments where use is intensively managed.  While wood production will not be a management objective in Parklands, some vegetation management to support recreational use, or to ensure public safety or ecological integrity may take place.
• Woodlands: These areas will provide a range of ecosystem services, including:  production of high‐quality, local, renewable wood products, protection of water quality, carbon sequestration, and both late forest successional structures, and in focused areas, early forest successional stages to promote habitat diversity.  Commercial timber harvesting, primarily uneven-age management, that demonstrates a range of excellent forestry through the application of best management practices will be the applied management standard, and there will be demonstration forest opportunities to educate landowners and the general public.

Line intersect sampling (LIS) - Also called line intercept sampling and transect sampling.  In the context of its use for estimates related to coarse and fine woody debris, a sample line, called a transect, of fixed length is observed for pieces of DWM that cross it; other pieces are ignored.  Pieces that cross the transect are measured for various attributes that can be used to estimate population parameters.  It is a form of probability proportional to size sampling (in this case, length) – longer pieces of DWM are selected with greater probability than shorter pieces.  See also down woody material, coarse woody debris, fine woody debris, transect, transect depth sampling.

Litter - Forest-floor layer of freshly fallen leaves, needles, twigs, cones, bark chunks, dead moss, dead lichens, dead herbaceous stems, and flower parts.  See also down woody material, duff.

Logging system (equipment) - The method by which trees are cut and harvested.  Methods of classification include the level of mechanization/automation, and the length of stems which are hauled through the woods.  Many of the machines described can use either wheels or tracks, which present tradeoffs in terms of mobility and ground pressure.  Chains can be installed on wheeled equipment to help increase traction.  Note that rigged cable logging systems (e.g., highlead, skyline) and tethered systems are not commonly used in Massachusetts.

• Level of mechanization in felling and equipment types
  • Hand-felling: A logging system where a logger walks to each tree to be harvested and, using a chainsaw, notches and back-cuts the stem to lay the tree on the ground.  A piece of equipment is then required to move the trees or logs from where they fall to the landing.  At the lowest levels of mechanization, this would be a cable skidder, where the logger is required to set chokers around each tree that the skidder drags.
  • Mechanized harvesting/felling : A logging system using equipment that generally does not require the logger to leave the machine cabin.  Types of systems here include:
    • Equipment to cut the tree:
      • Feller bunchers & delimber
        • Feller bunchers: These are machines that perform the tasks of cutting the trees and gathering them.  Swing-boom units resemble an excavator with a cutting head on a boom; drive-to-tree units resemble dozers, skidders, or three-wheeled units like Bells, with a cutting saw mounted on them.  Cutting heads may be discs and hot- or intermittent saws, bars, or shears.  Each style of machine and head typically has arms that can grip the tree while the saw cuts it, and it has the ability to move the tree around and group, or bunch, them together.
        • Slasher/delimber: A piece of equipment designed to remove limbs from trees.  Sometimes there are dedicated in-woods machines like stroke boom delimbers; but and in other cases there are fixed machines at the landing like pull-through delimbers.
      • Cut-to-length (CTL) harvester: This may also resemble an excavator with a cutting head on a boom.  The head may be fixed (controllable and lockable in many dimensions) or dangle with limited control by comparison.  The head is locked on to a tree, cuts it, delimbs it, and bucks the tree into logs.  It uses rotating drums that pull the tree through the head while scanners measure its diameter and length to allow the operator to cut the stem at precise lengths into logs; a saw; and delimbing knives.
  • Other equipment to move the tree or cut logs:
    • Skidder: A machine with a low center of gravity that operates in the woods and drags logs between locations.  It may be either a cable skidder, where the logger must physically set chokers (wire ropes or chain) around each log and connect them to a cable and winch on the skidder; or a grapple skidder, that has what resembles large ice tongs that can pick up several logs.
    • Forwarder: A machine resembling a very large pickup truck with a boom, that can carry logs clear from the ground in its bed, and uses a grapple mounted on a boom to pick logs up and place them in its bed/bunks and carry them.
• Length of stems/location of bucking
  • Stem-only: The entire merchantable stem, from the top of the stump to its upper-most merchantable portion, is hauled, intact, from the stump to the landing.  Excessively tall trees may be cut in half to increase maneuverability and decrease residual stand damage, while still being considered stem-only logging.  Branches are cut off and are left where the tree lands when it is felled.
  • Whole-tree: Trees are transported with their branches intact from where they fall to the landing.  The tree is delimbed at the landing (sometimes with a machine called a slasher) and the resulting limbs, or slash, may then be chipped and utilized, or brought back into the woods and distributed.
  • Cut-to-length (CTL): Trees are felled, delimbed, and then cut into logs of the desired length by their stump in the woods.  This is done by a specialized processing head on a piece of logging equipment called a harvester.

Mast - Fruits and seeds of trees and shrubs.  It can be further categorized into hard mast, including nuts and seeds; and soft mast, including berries and other fleshy fruits.

Mulch (silvicultural) - A silvicultural mulching operation uses heavy equipment such as a skid-steer or excavator, mounted with a head with teeth or chains rotating at a high speed that is designed to mow woody vegetation in place.

Prism (wedge prism) - A piece of glass shaped like a wedge, which can be made to varying angles (or diopters), that causes the deviation of light rays a known amount and can be used to carry out horizontal point sampling.

Regeneration - Seedlings and saplings, or the act of establishing them.  This can be through germination of seeds that fall from other trees; or, less commonly in Massachusetts, by planting seeds or saplings.  When seedlings or saplings develop in the understory during a rotation or between cutting cycles, they may be called advance regeneration.

Regeneration method - The cutting method that creates a new age class.  Primary classifications are even-age, irregular, and uneven-age methods.  Coppice methods, where regeneration is intended to be primarily from root or stump sprouts, are not commonly practiced in Massachusetts.  Any of these methods may also be implemented with trees reserved individually or in groups (see reserve (silvicultural/harvesting)).  See silvicultural system.

• Even-age methods
  • Clearcutting: A method of regenerating an even-aged stand in which a new age class develops after removal of all trees in the previous stand in a single operation.  Regeneration is from natural seeding, direct seeding, planted seedlings, and/or advance reproduction.  Cutting may be done in groups or patches (group or patch clearcutting), or in strips (strip clearcutting).
  • Seed tree: An even-age regeneration method in which a new age class develops from seeds that germinate after removal of nearly all of the previous stand, except a very small number of trees left to provide seed, called seed trees.  The seed trees are removed after the regeneration is established.
  • Shelterwood: A method of regenerating an even-aged stand in which a new age class develops beneath residual trees from the prior stand. The sequence of treatments can include three distinct types of cuttings: (1) an optional preparatory (also called preparation or prep) cut to enhance conditions for seed production and germination; (2) an establishment cut to prepare the seed bed and to create a new age class; and (3) a removal cut to release established regeneration from large, residual overstory trees.  Cutting may be done uniformly throughout the stand (uniform shelterwood), in groups or patches (group shelterwood), or in strips (strip shelterwood).
• Uneven-age methods
  • Group selection: A method of regenerating uneven-aged stands in which trees are removed, and new age classes are established, in small groups (often called gaps, patches, or openings) or less frequently regular geometric strips.  Openings that are more shaded favor shade-tolerant species, and openings that are larger or oriented to receive more full sun favor shade-intolerant species.  Thinning is sometimes conducted between groups to regulate growing stock.
  • Single tree selection: A method of creating new age classes in uneven-aged stands in which individual trees of all size classes are removed more-or-less uniformly throughout the stand to achieve desired stand structural characteristics.
• Irregular shelterwood methods:
  • Extended irregular shelterwood: A method of creating a new age class similar to the shelterwood system but where a portion of the previous stand is left in the overstory over an extended period of time, at least 20% of the rotation length, after seeds have germinated.
  • Expanding-gap shelterwood: A method of regenerating uneven-aged stands where gaps created to establish regeneration are expanded upon periodically as in a series of concentric rings.
  • Continuous cover irregular shelterwood: A method of regenerating an uneven-aged stand where the types of cuttings in a traditional shelterwood sequence are applied irregularly in space and time, resulting trees of many different ages in place simultaneously. 

Regulated forest - A forest which is managed to generate something, for society, the same amount it produces.  Traditionally the regulation has been of wood products; but the concept is being expanded to include non-traditional forest products as well; for example, to ensure not only a steady flow of wood products, but recreational opportunities, habitat, clean water, carbon sequestration and storage, etc.

Relaskop - The Spiegel Relaskop is an instrument that can be used to measure stand basal area, tree height, stem diameter at any tree height, height, distance, and slope, while compensating for ground slope. It projects multiple horizontal angles thus offering flexibility as an angle gage with multiple basal area factors in horizontal point sampling.  See also angle gage, basal area, horizontal point sampling, prism.

Reserve, silvicultural/harvesting - Trees, selected during the regeneration cutting, that are retained indefinitely after the establishment of regeneration, for purposes other than as a seed source (e.g., carbon storage, genetic diversity, wildlife habitat, etc.).  Sometimes referred to as legacy tree(s).

Rotation - The length of time, in years, between regeneration establishment and final cutting in an even-age silvicultural system.  See also stand structure (even-aged).

Scarification - Mechanical disruption of the forest floor, including small vegetation, and the duff and litter layers, to expose bare mineral soil, to facilitate the germination and survival of seeds of certain tree species.

Silviculture- The theory and practice of controlling forest establishment, composition, structure, and growth; applied forest ecology; the art and science of producing and tending a forest.

Silvicultural system - The program for the treatment of a stand during the whole rotation.  Generally, it assumes the same name as the method of regeneration and can be broadly divided into even- and uneven-aged systems.  Below is a brief description of types of cuts in each system; see regeneration method for additional information on each method:

• Even-age systems: 
  • Reproduction or regeneration cuttings both remove old trees and create conditions that help to establish a new generation of trees, and may occur in a single (e.g., as the clearcut and seed tree methods), or multiple (as in the shelterwood method) operations.
  • Intermediate cuttings (tending, thinning) are cuts made partway during the intended lifespan of a cohort, or rotation in an even-age system.
• Irregular shelterwood systems:
  • These systems occupy a middle ground between traditional even- and uneven-age systems where multiple age classes may occupy a site over a variable area and time frame.  Trees are removed using various methods in each entry.
• Uneven-age systems:
  • Methods in this system include single tree selection and group selection.  Thinnings may also occur within groups.  A balanced uneven-age stand is one where every age class up to rotation age is represented by equal area such that the allowable cut is the volume which is produced each cutting cycle.  This concept is similar to but distinct from a regulated forest.

Slash - Residue and unutilized portions of trees cut during a forest management or other human activity.

Snag - A standing dead tree; sometimes to which a minimum diameter and height threshold is applied.  Standing snags can be categorized as hard snags, which are less decayed; and soft snags, which are in advanced stages of decay.  This term can also refer to a dead branch in the crown of a tree.

Stocking - The amount of anything in a given area relative to some predefined standard; often expressed as a percent and using trees per acre, basal area, or volume as units; and sometimes as a unitless index based on long-term research of stands across a range of ages and histories.

Stand - A spatially continuous group of trees and associated vegetation having similar structures and growing under similar soil and climatic conditions. 

Stand development stages - A given group of trees – whether an even-aged stand, or a small patch in an uneven-aged stand – proceeds through a series of developmental steps as follows:

• Stand initiation: occurs when occupied growing space (both of sunlight and available soil water) is vacated, regeneration is established, and completely fills the available growing space.
• Stem exclusion: occurs when the crowns of regenerating trees completely close, or touch; their lowermost branches and foliage begin to become shaded out and die; and the trees compete with each other.
• Understory re-initiation: occurs as trees that successfully competed through the earlier stages begin to die from external agents (e.g., not old age), their crowns do not fully close, and the light and soil water thus made available allow for establishment of new plants on the forest floor.
• Old-growth: occurs in the absence of stand-replacing disturbances as a majority of the original trees are gone, and trees in new age classes or cohorts occupy the main canopy.

Stand exam - An inventory, or sample, that is designed to answer questions about the composition and structure of a forest stand at the point in time the examination is conducted.  This type of sampling typically involves measuring temporary plots and is done at a tactical level, usually for a single or small group of stands in a small area (10s to 100s of acres in Massachusetts).  See, in contrast, continuous forest inventory.

Stand structure - The physical and temporal distribution of trees and other plants in a stand.  Stand structure is commonly classified by species composition and age classes.

• Species composition
  • Pure stands: Stands where there is a single, dominant species.
  • Mixed stands: Stands where there are multiple species playing important roles in the  development of the stand
• Age structure
  • Even-aged stands: Stands where there is a single cohort, or age group, of trees; sometimes defined as stands where the range of tree ages is less than 20% of the rotation length.  
  • Two-aged stands: Stands where management occurs explicitly for the presence of two cohorts simultaneously for at least some period of time.
  • Uneven-aged stands: Stands where there are at least three distinct age classes closely intermingled, either as individual trees or in small groups.

Succession, seral community - Ecological succession is the theory of the process of change in species structure and community composition over time toward a steady-state or equilibrium climax community; seral communities are those intermediate stages and groups and compositions of species in that process.  It is important to note there is not always a straightforward relationship between seral stages, successional stages, and stand structural or developmental stages.

• Early seral/successional: This stage occurs after disturbance and includes pioneer species.  It is commonly thought that this stage is dominated by tree species that are shade-intolerant, fast-growing, and short-lived; though this is not always the case.
• Mid-seral/successional: Occurring as a transition between pioneer and late-successional stages, a mid-seral stage is often associated with dominance by longer-lived, mid-tolerant tree species that do well in a wide range of conditions.
• Late seral/successional: This stage occurs after a long absence of severe disturbances from a site; and is associated with long-lived, more shade tolerant species that require complex conditions to survive.

Systematic sample - A system of sampling where sampling units are spaced at fixed intervals throughout the population.  In the context of forest inventory and stand exams in particular, a systematic sample constitutes a selection of points at regular intervals (typically on a square grid) throughout the area to be sampled at which measurements of stand characteristics are made.  The starting location of the grid is selected randomly with uniform probability from within the area of interest.  A properly designed and vetted systematic sample has many advantages over a random sample, including efficiency of navigation, better coverage of the area; and virtually always a more stable estimate of population parameters and conservative estimates of precision.  Constrained random sampling is a technique used by some staff where sample locations are selected randomly with uniform probability within the area of interest, but constrained to be no closer than a certain distance so as to afford more uniform coverage of the area.

Thinning (also called tending, intermediate treatment) - A treatment made partway during the expected lifespan of a cohort or rotation in even-aged systems, or in groups in uneven-aged systems, intended to control growth of residual trees by adjusting stand density or species composition; and for which establishment of regeneration is not a primary goal.  Thinnings are often classified as to whether or not the cut trees have financial value, and how the trees are selected for removal and retention.  Note that the word thinning is sometimes interchanged with cutting in the names below.

• Financial aspect:
  • Pre-commercial thinnings (PCT):  These are thinnings made purely as investments in the future growth of trees so young that none of the cut trees have any financial value.  These types of operations may also be called timber stand improvement (TSI).
  • Commercial thinnings: These are thinnings where at least part of the felled trees are able to be utilized for some financial return.
• How trees are selected for removal or retention:
  • Crown thinning (also called thinning from above, high thinning): The removal of trees from the dominant and codominant crown classes in order to favor the best trees of those same crown classes.
  • Free thinning: The removal of trees to control stand spacing and favor desired trees using a combination of thinning criteria without regard to crown position.
  • Low thinning (also called thinning from below): The removal of trees from the lower crown classes to favor those in the upper crown classes.
  • Mechanical thinning (also called geometric thinning, row thinning, strip thinning): The thinning of trees in either even- or uneven-aged stands involving removal of trees in rows, strips, or by using fixed spacing intervals.
  • Selection thinning (also called dominant thinning): The removal of trees in the dominant crown class in order to favor the lower crown classes.

Timber stand improvement (TSI) - See pre-commercial thinning.

Tolerance, shade - A characteristic of how well-adapted a plant species is to surviving under shade, without direct sun.  A species can have a different tolerance for shade at different ages.  The height at which the shade is cast (e.g., from high above in the overstory; or from lower canopy levels) can affect the characteristics of the shade.  Broad classifications include tolerant, mid-tolerant or intermediate, and intolerant. Examples include:

• Tolerant: sugar maple, American beech, striped maple
• Mid-tolerant: red maple
• Intolerant: quaking aspen

TPA: Trees per acre - This is nearly always expressed as a mean value – each measurement in a sample or inventory estimates a total number of trees; these are averaged together to create the estimate of the total; and this total is divided by the area sampled to estimate the mean (average) trees per acre in the area.

Transect - A line that is used in a sampling method.  See also down woody material, coarse woody debris, fine woody debris, line intersect sampling and transect depth sampling.

Transect depth sampling - In the context of its use for estimates related to down woody material, a sample line, called a transect, of fixed length is observed at regular intervals for a depth of something (e.g., duff, litter, or piles of CWD for which it is impractical or unsafe to measure individual pieces).  The average depth of the item of concern along the line may be converted to a volume by multiplying the average depth (e.g., feet), by the unit area base (e.g., feet²).  See also transect, line intersect sampling.

Tree/log product grades - 

• Cull: Trees and portions (logs) within trees that have no financial value.  These are typically left in the woods as whole trees or chunks during timber harvests.  Cull can typically be divided into rotten cull, where the wood in the tree has some kind of decay; and form cull, where the tree is too poorly formed (e.g., broad sweeps or bends, or sharp crooks) or of excessive knottiness.
• Firewood (fuelwood): Trees and portions within trees that are so small or poor quality as to only be useable for firewood.  Firewood in Massachusetts is typically hardwood.  Often there will be firewood logs in the upper portions of trees that have higher-quality products.
• Pulpwood: Trees and portions within trees that are so small or poor quality as to only be mechanically or chemically separated and used in products like paper, mulch, chips, or as feedstock for energy generation.  Often there will be pulpwood logs in the upper portions of trees that have higher-quality products.
• Sawlog: A log or tree of large enough size and high enough quality (in terms of branches, knots, other defect, sweep, crook, heartwood/sapwood ratio, etc., to be sawn in to boards.  Typically connotes a higher-value product than firewood or pulpwood; but a lower-value product than veneer.  Sawlog-quality trees may contain other logs that could be used for veneer; and logs that are so poor as to only be usable for firewood or pulp.  Pallet and tie logs are a sub-category of sawlogs and are the lowest quality sawlogs.
• Veneer: A log of large enough size and quality that it could be used as veneer.  Veneer logs can be further separated into two categories, of which is slicer is typically a higher, better grade than peeler.

Unacceptable Growing Stock (UGS) - A classification of trees that do not now, and cannot or are highly unlikely to in the future, possess characteristics that would cause them to be classified as AGS.  These may be also very sick trees or trees of low vigor.  Typically these trees are deemed unworthy of continued allocation of growing space in a stand unless needed for control of overall density.  See also acceptable growing stock (AGS).

Uneven age(d) stand structure - See stand structure.