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Crown (botany)
Crown (botany)
Crown (botany)
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Tree crown

The crown of a plant is the total of an individual plant's aboveground parts, including stems, leaves, and reproductive structures. A plant community canopy consists of one or more plant crowns growing in a given area.

The crown of a woody plant (tree, shrub, liana) is the branches, leaves, and reproductive structures extending from the trunk or main stems.

Shapes of crowns are highly variable. The major types for trees are the excurrent branching habit resulting in conoid shapes and decurrent (deliquescent) branching habit, resulting in round shapes. Crowns are also characterized by their width, depth, surface area, volume, and density. Measurements of crowns are important in quantifying and qualifying plant health, growth stage, and efficiency.

Major functions of the crown include light energy assimilation, carbon dioxide absorption and release of oxygen via photosynthesis, energy release by respiration, and movement of water to the atmosphere by transpiration. These functions are performed by the leaves.

Crown classes

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Trees can be described as fitting different crown classes. Commonly used are Kraft's classes.[1] Kraft designated these "social classes" based on temperate and boreal forests in Central Europe, so they do not necessarily work with every forest type in the world.

Kraft wrote in German so here are his classes with translations:

  • 1 v vorherrschend (predominant)
  • 2 h herrschend (dominant)
  • 3 m mitherrschend (co-dominant)
  • 4 b beherrscht (dominated / suppressed)
  • 5 u unterständig (inferior) this is then split into 2 subclasses 5a (shade tolerant trees) and 5b (dying crowns / dying trees)

Often it has been simplified to Dominant, Co-dominant and Suppressed.[2]

Also IUFRO developed a tree classification it is based on three components with numbers that then aggregate to give a coded classification thus:[3]

Ecological criteria

Height component (Stand layer / Height class):

  • 100 Overstorey / Overlayer
  • 200 Middlestorey / Middlelayer
  • 300 Understorey / Underlayer

Vitality component (Tree vigor / vitality):

  • 10 Lush
  • 20 Normal
  • 30 Retarded

Future growth potential component (Developmental tendency / conversion tendency):

  • 1 High
  • 2 Average
  • 3 Lagging

and then additionally

Silvicultural criteria

Commercial worth

  • 400 Valuable, outstanding tree
  • 500 Usable, wood
  • 600 Poor to Unusable Quality

Trunk class

  • 40 Valuable wood (≥50% of the trunk is high-quality timber)
  • 50 Normal wood (≥50% of the trunk is normal-quality timber)
  • 60 Substandard wood (<50% of the trunk is normal-quality timber)

Crown class

  • 4 Deep crown (>½ the tree length)
  • 5 Medium crown
  • 6 Shallow crown (<¼ the tree length)

While both Kraft and IUFRO classifications are aimed at describing individual tree crowns both can and are applied to describe whole layers or storeys.[4]

See also

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References

[edit]

Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Crown (botany)

View on Grokipedia
from Grokipedia
In , the crown of a plant primarily refers to the aboveground portion consisting of stems, branches, leaves, and reproductive structures, particularly in trees and shrubs where it forms the canopy. This structure is crucial for , converting solar radiation into energy for growth, repair, and maintenance. In herbaceous perennials and other non-woody plants, the crown alternatively describes the compressed basal region at the soil surface where roots and shoots connect, serving as the origin for new growth and overwintering buds. The architecture and shape of a plant's crown vary widely across species and are influenced by genetic, environmental, and ecological factors, ranging from rounded and spreading forms in open habitats to narrow, columnar shapes in dense forests. morphology affects interception and competitive interactions. For instance, broader crowns are more common in tropical regions to maximize capture, while narrower crowns predominate at higher latitudes to reduce damage and load. morphology also influences water use efficiency. Beyond individual , crowns contribute to dynamics in forests and grasslands, influencing by providing and modulating microclimates. In and , understanding crown structure aids in , , and management, such as preventing crown rot in perennials. on crown plasticity highlights its importance in responding to stressors like and herbivory, underscoring evolutionary pressures on form.

Definition and Overview

Definition

In , the crown of a is defined as the uppermost branching portion, typically encompassing the branches, leaves, and reproductive structures located above the trunk or . This structure represents the primary aboveground growth zone responsible for capturing , with the crown in trees forming a distinct top section of branches and foliage. The crown is differentiated from the bole, which is the main trunk below the branching point, and from root systems, which the underground and absorb nutrients. The term "crown" in botanical contexts derives from the Latin corona, meaning "garland" or "wreath," reflecting the encircling form of the upper plant parts, and entered systematic botanical usage during the 18th century amid the rise of modern plant classification. It is also distinct from the canopy, which refers to the collective layer formed by the crowns of multiple trees in a forest stand, creating an overarching vegetation stratum. Examples illustrate the crown's variability across plant types: in trees such as oaks (Quercus spp.), the crown expands outward as a broad array of branches and leaves above the trunk, while in herbaceous plants like dandelions (), it manifests as a basal rosette of leaves emerging from a compressed stem at or near soil level. The crown briefly supports essential processes like by positioning leaves for optimal exposure.

Ecological and Physiological Role

The crown of a plays a crucial physiological role in capturing light for and facilitating , which are essential for plant survival and growth. In shaded understories, crown architecture converges to optimize light absorption efficiency, typically achieving 43–64% light capture despite varying structures, as seen in species like , where longer internodes enhance carbon gain but are balanced against biomechanical costs. In high-light environments, steep angles and self-shading in crowns, such as in Heteromeles arbutifolia, minimize excessive radiation exposure, thereby maintaining efficient and reducing that could limit daily carbon assimilation by up to 9% in species like . These adaptations ensure that the crown supports vital processes by maximizing resource acquisition while mitigating environmental stresses. Ecologically, plant crowns are vital for habitat provision and biodiversity support, creating complex three-dimensional spaces that host diverse organisms. Forest canopies, formed by tree crowns, support over 27,000 epiphyte species—representing 9% of global diversity—and an estimated 5.9–7.8 million species, with examples like New Guinea forests where ~200 species sustain ~9,600 herbivorous . They also promote through vertical stratification, as observed in communities where peaks in canopies, with 113,952 individuals across 5,858 species in stratified samples. Furthermore, crowns regulate microclimates by buffering climatic extremes, generating vertical gradients in and moisture—such as greater changes over 20 m in height than 200 m in elevation in Philippine dipterocarp forests—and contributing 34% to global terrestrial gross through rainfall interception and . For instance, broader crowns with lower branching heights more effectively reduce mean radiant and enhance in urban settings. Crown architecture mediates key interactions with pollinators, herbivores, competitors, and influences , shaping dynamics. In like , crown height affects pollen dispersal, with lower crown levels showing higher self-fertilization rates (~60%) and shorter dispersal distances (113–227 m on average), ensuring reproduction in low-density populations despite reduced . For herbivores, crown structure influences exposure and ; genetic susceptibility to herbivory alters architecture, impacting and herbivore access, while lifetime herbivory shapes allometries that affect growth and defense. Crowns also facilitate competition by optimizing light interception against neighbors, with taller, umbrella-shaped crowns enhancing space-filling and resource access in mixed stands. Regarding , architecture correlates with strategies: sparse, elevated crowns in wind-dispersed species like dipterocarps (e.g., 60 m height, 30 m crown diameter) promote airborne spread, whereas broad crowns in animal-dispersed trees like moabi (43 m height, 53 m diameter) position fruits for visibility to dispersers. Loss of crown integrity, such as through defoliation, significantly reduces plant vigor by depleting carbohydrate reserves and impairing recovery. In trees, up to 50% foliage loss in a single year typically causes no lasting harm, but 75–100% defoliation triggers refoliation at the expense of reserves, leading to smaller leaves, twig dieback, root impairment, and increased pathogen susceptibility, with repeated events over 2–3 years potentially killing even healthy individuals. In desert species like big sagebrush and squirreltail, defoliation depletes total available carbohydrates (TAC) by 0–28 mg/g dry weight for regrowth, with early spring clipping causing the greatest reserve loss and autumn TAC levels serving as indicators of subsequent vigor decline, often more pronounced in roots than crowns. Such impacts underscore the crown's centrality to overall plant resilience.

Anatomy and Structure

Components of the Crown

The crown of a or consists primarily of branches, twigs, leaves, , and reproductive structures such as flowers and fruits, which collectively form the above-ground framework supporting and reproduction. Branches are categorized into primary ones, which emerge directly from the trunk and provide main , and secondary branches, which extend from primary branches to further distribute the crown's mass. Twigs represent the youngest, most distal portions of branches, typically comprising the previous season's growth and serving as sites for and leaf attachment. Leaves arise from twigs and branches, arranged in patterns such as alternate, opposite, or whorled to optimize light capture, while —containing undeveloped shoots, leaves, or flowers—enable seasonal expansion. Flowers and fruits develop from specialized buds, varying by species to facilitate and within the crown. Structurally, the crown features branch forks where lateral branches diverge from main axes, often at angles that influence mechanical stability, with wider forks (around 45 degrees) providing stronger attachments by allowing bark inclusion avoidance. Apical meristems, located at the tips of branches and twigs, drive longitudinal growth through , exerting dominance over lateral development. Vascular connections link the crown to the trunk via tissues, which transport water and minerals upward, and , which distributes sugars produced in leaves downward, forming a continuous network of vascular bundles. Within the crown, a distinct layering exists between the outer foliage layer—dominated by leaves and fine twigs that intercept sunlight—and the inner woody skeleton of thicker branches that offer support but contribute less to light absorption. Certain species exhibit adaptations in these components for defense or climbing, such as thorns on branches and twigs in acacias to deter herbivores, or tendrils in vining plants integrated into crown-like structures for anchorage.

Variations Across Plant Types

In trees, crowns are typically broad and layered, facilitating extensive capture and structural support for foliage. Coniferous often develop conical crowns, which taper upward to optimize vertical growth and snow shedding in temperate and boreal environments. In contrast, trees like oaks exhibit rounded crowns, with wide-spreading branches forming a more spherical or dome-like structure that enhances lateral expansion in open woodlands. These layered architectures in trees allow for stratified foliage distribution, with denser layering in early successional stages transitioning to sparser arrangements in mature individuals. Shrub crowns differ markedly, being compact and multi-stemmed with frequent basal branching that arises directly from the root crown or lower stems. This configuration promotes resilience to disturbance, as multiple stems enable rapid regeneration after browsing or fire, and the dense, rounded form minimizes exposure in understory habitats. Unlike the singular trunk-dominated crowns of trees, shrubs often display basitonic growth patterns, where branching is more vigorous at the base, contributing to their low stature and clumping habit. Herbaceous plants feature crowns that are generally less woody and more ephemeral, often manifesting as rosettes or basal clusters close to the ground. In herbaceous perennials, the crown is a compressed basal region at the surface where and shoots connect, serving as the origin for new growth and containing overwintering buds. For instance, dandelions form a tight rosette of leaves at the crown, which serves as a compact hub for radial arrangement and efficient in short-lived perennials. In contrast, some herbaceous species develop upright inflorescences rising from this basal crown, as seen in many composites where flowering stems elongate vertically while maintaining a clustered base for photosynthetic support. Differences between monocots and dicots extend to morphology, with monocots like palms displaying fan-shaped or pinnate crowns adapted to tropical conditions through a single apical producing radiating fronds. Dicot crowns, by comparison, typically involve more complex branching from multiple lateral buds, leading to irregular or rounded forms that accommodate diverse phyllotaxes for varied light environments. Environmental factors further shape crown forms across plant types, such as wind exposure leading to pruned, flag-like or dwarfed s in exposed sites, known as in alpine trees and shrubs. Similarly, high winds can compact s in coastal herbs and shrubs, reducing height and promoting denser basal growth for stability.

Functions

Primary Functions

The crown of a , particularly in trees and shrubs, serves as the primary site for , where within leaves captures to convert and water into sugars, releasing oxygen as a . This process is optimized by the crown's architectural features, such as orientation and , which adjust to maximize exposure to and minimize self-shading, thereby enhancing overall interception efficiency. In temperate , for instance, leaves often exhibit angles that balance capture across varying canopy depths, supporting sustained production throughout the . Transpiration, another core function, occurs predominantly through stomata in the crown's leaves, regulating water loss to the atmosphere while facilitating cooling and . Stomatal opening allows to escape, creating a cooling effect that prevents overheating and maintains optimal temperatures for enzymatic reactions in . Simultaneously, this evaporative pull drives the ascent of water and dissolved minerals from via vessels, ensuring nutrient delivery to metabolically active crown tissues. In water-stressed conditions, stomatal closure in the crown reduces transpiration rates, balancing hydration needs with carbon gain. The crown also regulates plant growth through mechanisms like , where hormones produced at the shoot tips inhibit lateral branching to promote vertical elongation. , synthesized in the apical meristems of the crown, diffuses basipetally to suppress outgrowth, directing resources toward height gain and competitive light access. This hormonal control shapes crown form, influencing overall architecture and resource allocation for sustained development. Through these functions, crowns contribute significantly to , with temperate tree crowns fixing an average of approximately 0.95 kg C m⁻² annually via net primary productivity, varying from 0.7 to 1.3 kg C m⁻² year⁻¹ depending on site conditions and . This fixation rate underscores the crown's role in capturing atmospheric carbon, supporting both plant biomass accumulation and broader carbon balances.

Secondary Roles

In addition to its primary physiological roles, the plant crown facilitates by elevating flowers and s to optimal positions for and . In many species, the architectural complexity of the crown, including layering, influences flower production and the subsequent development of s and , enhancing by exposing reproductive structures to pollinators or dispersal agents. For instance, in wind-pollinated species like certain , the crown's height and openness allow to be carried efficiently over distances, while in animal-mediated systems, such as those in flowering s like oaks, the crown positions blossoms at heights that attract birds and , thereby increasing cross- rates. Similarly, fruit placement in the outer crown layers promotes dispersal by vertebrates, which consume and transport away from the parent plant, reducing competition and predation risks for seedlings. The also serves defensive functions against through both chemical and physical mechanisms integrated into its foliage and structure. Leaves within the often produce secondary metabolites like , which deter feeding by binding to proteins in the 's digestive tract, reducing nutrient absorption and causing toxicity or aversion; this is particularly evident in species such as oaks, where concentrations in leaves correlate with lower insect herbivory rates. Physically, the dense foliage and branching of the create barriers that hinder access for larger , such as by obscuring vulnerable buds or making navigation difficult for folivores, thereby minimizing damage to reproductive and photosynthetic tissues. These defenses are dynamically allocated, with higher investments in outer layers exposed to greater pressure. Furthermore, the crown provides structural support for and , bolstering local . In tropical and temperate , tree crowns host diverse epiphytic , such as orchids and bromeliads, which anchor to branches and derive moisture and nutrients from the air and canopy debris, with epiphyte abundance increasing in the lower to middle crown zones where and are balanced. This complexity extends to animals, including birds that nest in crown foliage for protection from predators and access to food resources, contributing to canopy diversity in some ecosystems through the layered microhabitats formed by branches and leaves. Such interactions enhance overall forest by creating refugia and food webs independent of the ground layer. Seasonal adaptations in the crown, particularly in deciduous species, enable during unfavorable periods. In temperate regions, deciduous trees undergo leaf abscission in autumn, shedding crown foliage to minimize water loss through when soil moisture is limited by frozen ground and to reallocate resources like from leaves to woody tissues for storage over winter. This , triggered by hormonal changes and shortening photoperiods, reduces metabolic demands, allowing the plant to survive stress with lower expenditure; for example, maples and birches exhibit this , resuming growth in spring with nutrient-rich buds. Evergreen species, by contrast, retain crown leaves but adjust to withstand winter, though deciduous shedding provides a net energy advantage in highly seasonal climates.

Classification and Crown Classes

Crown Classes in Forestry

In forestry, crown classes provide a standardized system for categorizing individual trees within a forest stand based on their relative position in the canopy and access to , which influences growth, vigor, and competition dynamics. This classification is essential for assessing stand structure, silvicultural planning, and inventory purposes. The system originates from the work of German Gustav Kraft, who introduced it in 1884 to evaluate tree quality and competitive status in even-aged stands, though it has since been adapted for broader applications including uneven-aged and mixed-species forests. Kraft originally defined five classes, including a predominant class for exceptionally tall trees with well-developed crowns; modern systems, such as those used by the USDA Forest Service, often employ four primary classes by merging or omitting the predominant category. The four primary crown classes—dominant, codominant, intermediate, and suppressed (also termed overtopped)—are defined by tree height relative to the surrounding canopy and the amount of direct light received by the crown. These classes reflect the vertical stratification of the forest and help predict individual tree responses to management interventions.
Crown ClassDescription
DominantTrees whose crowns extend well above the general level of the surrounding canopy, receiving full direct from above and considerable from the sides; these are typically the tallest individuals with well-developed, expansive crowns that dominate the stand.
CodominantTrees with crowns forming the main canopy layer at a height similar to adjacent trees, exposed to full overhead but limited lateral due to crowding; crowns are often medium-sized and intermingled, contributing to the bulk of the stand's overstory.
IntermediateTrees shorter than dominants and codominants, with crowns extending into the upper canopy but receiving only partial direct through gaps, and minimal side ; these trees experience moderate and have smaller, more confined crowns.
Suppressed (Overtopped)Trees whose crowns are largely below the main canopy, receiving little to no direct and relying on diffuse ; these are often stunted, with slow growth and high mortality risk due to intense and from taller trees.
Originally designed for pure, even-aged stands in European contexts, the Kraft classes have been refined by organizations like the USDA Forest Service to incorporate species-specific tolerances and stand variability, ensuring practical utility in modern forest assessments.

Factors Influencing Classification

The classification of a tree's crown class is significantly influenced by for , which is modulated by the of surrounding trees and of the stand. In denser stands, increased leads to stratification into distinct crown classes, where dominant and codominant trees capture most while intermediate and suppressed trees receive limited exposure, resulting in reduced growth and vigor for the latter. As stands age, self-pruning and crown recession intensify due to prolonged shading, further differentiating classes by reducing live crown ratios in lower strata. Stand indices, such as above 45, accelerate this by heightening inter-tree , particularly in even-aged forests. Site conditions, including , moisture availability, and , play a critical role in determining crown vigor and thus class assignment. Fertile soils with high nutrient content support larger, more vigorous crowns, enabling trees to maintain higher classes even under moderate competition, whereas nutrient-poor sites limit foliage density and promote downward class transitions. Adequate enhances crown development by sustaining and branch retention, but drought-prone or excessively wet conditions degrade crown health, increasing transparency and dieback that affect classification. influences these dynamics through variations in and ; higher elevations often result in smaller crowns due to shorter growing seasons and wind exposure, shifting trees toward intermediate or suppressed classes. Species-specific traits, particularly , determine how long a can sustain a given crown class amid competition. Shade-tolerant species, such as American beech (), can persist in intermediate crown positions longer than intolerant species by efficiently utilizing diffuse light and maintaining slow but steady growth in conditions. This tolerance allows beech to gradually ascend classes in mixed stands, whereas shade-intolerant species rapidly decline to suppressed status without canopy openings. Disturbances like , , and can abruptly alter crown class transitions by releasing suppressed trees or damaging upper canopy layers. Thinning operations, by removing overtopping trees, promote the upward mobility of suppressed individuals into codominant or intermediate classes through increased light access and reduced competition. and disturbances often top-kill dominant trees, enabling formerly intermediate ones to dominate post-event, while severe events can homogenize classes by affecting all strata uniformly.

Measurement and Applications

Measurement Techniques

Field methods for assessing tree crown size, shape, and health primarily involve direct observations and basic instrumentation conducted on-site. Visual estimation is a common technique where trained observers assess attributes such as crown density, foliage transparency, and live crown ratio by mentally outlining the crown and comparing it to reference cards or scales, typically recorded in 5% increments from 0 to 100%. This approach is efficient for large-scale inventories but relies on observer experience to minimize variability. For more precise measurements, crown diameter is determined using a logger's tape to measure the widest horizontal spread along the drip line and at a perpendicular angle, averaging the two distances after correcting for slope if necessary. Crown height and depth are gauged with a clinometer, which measures angles to the tree top, base, and bottom of the live crown from a known horizontal distance, applying trigonometric calculations to derive vertical dimensions. Remote sensing techniques provide non-invasive, scalable alternatives for mapping crown attributes over extensive areas. (Light Detection and Ranging) generates dense 3D point clouds from airborne or terrestrial platforms, enabling detailed reconstruction of crown through algorithms like or voxel-based methods that segment and quantify canopy structure. This is particularly useful for estimating crown and in complex forests, with reported accuracies achieving R² values up to 0.996 for coniferous . , often at very high resolutions (e.g., 0.59 m ), assesses canopy cover by analyzing spectral data to delineate tree crowns and compute coverage proportions, trained against references for and validation. Such methods yield mean absolute errors as low as 2.8 m for canopy mapping across large jurisdictions. Destructive sampling, though labor-intensive, offers direct insights into internal crown dynamics. Branch coring involves extracting increment cores from selected branches using a small borer to analyze tree ring patterns, revealing growth rates, age, and responses to environmental stressors like drought. This technique complements non-destructive methods by providing histological data on branch development and vigor. Digital tools enhance analysis by processing data from field or remote sources into quantitative models. Software like TreeQSM reconstructs crown architecture from LiDAR point clouds via cylinder-fitting algorithms, generating 3D quantitative structure models (QSMs) that estimate volume, diameter at breast height, and height with high precision, such as R² = 0.96 for volume in broadleaf species. Accuracy varies by method, with visual estimations prone to higher errors due to subjectivity—quality objectives target 90% agreement within ±10% or two classes for density and transparency—while instrumental techniques like tape measures for diameter achieve ±10% of the mean value through direct quantification. Experienced observers reduce discrepancies, as seen in diameter estimates differing by 5.4 cm on average compared to direct measurements.

Practical Uses in Ecology and Management

In forestry management, crown class analysis guides thinning operations to selectively remove suppressed or intermediate trees, thereby promoting dominant and co-dominant individuals that contribute to higher timber yields and stand stability. For instance, low thinning targets smaller crown classes to reduce competition and enhance growth of upper-story trees, as demonstrated in Douglas-fir stands where such practices increased volume growth over control plots. This approach not only optimizes wood production but also mitigates risks like windthrow by fostering structurally resilient crowns. Crown vigor indices, derived from visual assessments of foliage density and dieback, serve as key tools for ecological monitoring of , particularly in evaluating climate change impacts such as stress and temperature extremes. Studies in boreal and temperate forests have shown that declining crown conditions correlate with reduced radial growth. These indices enable early detection of widespread stressors, informing adaptive strategies like assisted migration or in vulnerable ecosystems. In , crown metrics such as projected area and are integrated into designs to maximize shading and air quality benefits, reducing urban heat islands by up to 3.7°C under optimal canopies. Research highlights that with broad, dense crowns, like oaks, intercept particulate matter and more effectively than narrow-crowned alternatives, supporting guidelines for equitable green space allocation in cities. This data-driven approach enhances management and by prioritizing plantings that balance aesthetic and functional outcomes. Crown data from underpins research applications in modeling carbon dynamics and , where multi-temporal lidar-derived metrics predict sequestration rates and structural diversity. For example, crown structural complexity indices explain over 60% of variation in aboveground carbon storage across diverse stands, outperforming alone in projections. Similarly, these models integrate crown cover to forecast responses to disturbances, aiding in the design of conservation corridors that sustain connectivity. Case studies in tropical rainforests utilize crown-based assessments via airborne to track , delineating individual crowns to quantify canopy gaps from and . Such applications support real-time monitoring programs, enabling targeted interventions to curb habitat loss in biodiversity hotspots.

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