Plant Growth Forms

What Is a Tree, Exactly?

Most people know a tree when they see one until they encounter a palm, a bamboo, a tree fern, or a giant cactus.

The word tree describes a growth form rather than one evolutionary lineage. Tree-like architecture has evolved repeatedly, and scientific or practical definitions can emphasize different features such as height, persistent stems, secondary growth, forestry thresholds, or ecological function.

Tree TSAR therefore asks a broader question: what kind of plant is this structurally and biologically? Its answer is the Plant Growth Form (PGF) system.

PGFs provide a visual language for conventional woody plants, fibrous trunk-forming plants, stem succulents, herbaceous perennials, geophytes, epiphytes, parasites, aquatics, and other vascular-plant strategies.

Growth Form Is Different from Taxonomy

Taxonomy describes evolutionary relationships. Growth form describes something about how a plant is built and how it lives.

Close relatives can evolve different growth forms, while distantly related lineages can converge on similar architecture. Ecology has long used plant functional types for a related reason: large numbers of species can be grouped by shared structural or functional traits without implying close ancestry (Wullschleger et al. 2014).

Tree TSAR adapts that general idea for botanical communication. PGF is not another taxonomic rank. It is a biological descriptor attached to a taxon.

What Counts as Wood?

In the conventional anatomical sense used for most woody seed plants, wood consists principally of secondary xylem produced by a vascular cambium. This is the familiar process by which many trees and shrubs add secondary tissues and increase stem diameter.

That description works well for an oak, pine, maple, or ginkgo. It becomes less adequate for plants that attain tree stature through fundamentally different developmental systems.

Palms, for example, lack the conventional vascular-cambium-driven secondary growth of typical woody dicots and gymnosperms. Their stems can nevertheless become tall, persistent, and mechanically formidable through sustained primary growth and specialized stem construction (Tomlinson & Huggett 2012).

Calling both a palm and a ginkgo simply “trees” therefore hides an important biological distinction.

The Palm and Bamboo Problem

Palms and giant bamboos are readily called trees in ordinary language. They can form trunks or trunk-like culms, rise high into the canopy, and dominate landscapes.

Their construction differs substantially from conventional secondary wood. The Food and Agriculture Organization illustrates the difference between botanical anatomy and operational language: its forest definitions can count qualifying palms and bamboos as trees or forest components for practical inventory purposes even though their stem development differs from conventional woody seed plants (FAO 2018).

Tree TSAR preserves this distinction with a separate Fibrous Trunk-Forming Perennial PGF rather than forcing every tall persistent plant into the same woody category.

Cacti Blur the Woody-Succulent Boundary

Cacti demonstrate another complication. Many familiar species are visually dominated by persistent succulent stems that store water and carry much of the plant’s photosynthetic function, yet cactus stems can also produce substantial secondary tissues.

Mauseth (1993, 2006) documented the remarkable diversity of cactus wood and shoot anatomy, including strongly lignified tissues in some lineages and water-storage adaptations in others.

Tree TSAR therefore does not classify a plant as conventionally woody merely because secondary xylem is present. When persistent stem succulence dominates the organism’s architecture and function, Perennial Stem Succulent can be the more informative PGF.

The boundary between a tree and shrub is often less clean than field guides imply. Some species grow as a single-stemmed tree in one environment and a multistemmed shrub in another. Ecological treatments often rely on practical distinctions in stature and architecture rather than an absolute biological boundary (Götmark et al. 2016).

Tree TSAR therefore groups conventional trees, shrubs, and woody vines under Woody Tree, Shrub, or Vine. The ordinary words can still appear in descriptions and common names; the PGF identifies the deeper structural feature they share.

What Is a Subshrub?

Some perennials sit between woody and herbaceous architecture. Lavender, thyme, and various sages can retain a persistent woody base while producing shoots that are comparatively herbaceous or seasonally renewed.

Calling such plants ordinary shrubs can exaggerate their persistent woody framework, while calling them ordinary herbaceous perennials hides it. Tree TSAR therefore recognizes Perennial Subshrub From Woody Base as a distinct PGF.

The category makes visible the fact that woody and herbaceous growth form a biological continuum rather than a perfect binary.

What Happens Underground Matters Too

Many herbaceous perennials disappear above ground during an unfavorable season and return from persistent structures below the soil. Those structures differ biologically.

A perennial surviving from a crown or persistent root system is not constructed in the same way as a tulip surviving through a bulb or a fern spreading through a rhizome. Tree TSAR therefore separates ordinary herbaceous perennials from geophytes whose persistence depends on specialized underground stem organs such as rhizomes, corms, bulbs, or stem tubers.

These distinctions can communicate useful information about dormancy, regeneration, vegetative spread, and disturbance response.

Some Growth Forms Depend on Other Organisms

Not every vascular plant follows the familiar model of a rooted, fully photosynthetic terrestrial plant. Some grow as epiphytes, some parasitize stems or roots, some obtain carbon through fungal relationships, and others spend their lives floating or submerged in water.

These strategies can be more biologically informative than a simple woody-versus-herbaceous distinction. PGF therefore includes specialized categories for parasitic, mycoheterotrophic, epiphytic, and aquatic plants.

Can One Taxon Have More Than One PGF?

Yes. Most species have one dominant growth form, but some taxa legitimately express more than one important structural or ecological strategy. Tree TSAR can assign multiple PGFs when doing so communicates genuine biological information.

Multiple codes should not become a checklist of every trait a plant possesses. The purpose is to capture the growth forms that actually define how the taxon is built and lives.

How PGFs Appear on Tree TSAR Pages

Species pages display the appropriate PGF artwork and descriptive label. Numeric codes are primarily part of the full Plant Growth Form guide rather than the main public identity of a taxon.

At genus level, Tree TSAR can summarize the PGFs represented among the included species. This lets a morphologically diverse genus display its structural breadth without forcing one misleading growth habit onto every member.

The full PGF guide and taxon pages use the same authoritative PGF definitions, keeping the terminology consistent wherever a growth form appears.

The PGF System

Among the growth strategies represented are:

Woody Tree, Shrub, or Vine
Perennial Subshrub From Woody Base
Fibrous Trunk-Forming Perennial
Perennial Stem Succulent
Evergreen Perennial Herb
Herbaceous Obligate Epiphyte
Herbaceous Perennials and Annuals
Root-Hardy Geophytes using rhizomes, corms, bulbs, or tubers
Herbaceous Stem and Root Parasites
Herbaceous Mycoheterotrophs
Submersed, Emergent, and Floating Aquatic Plants

The full public guide presents the official icons, numeric identifiers, definitions, and representative examples used throughout Tree TSAR.

A Visual Language for Plant Diversity

Plant functional classifications are useful because they compress complicated architecture into categories that can be compared across unrelated taxa (Wullschleger et al. 2014). Tree TSAR’s PGF system applies a similar principle at a public-reference scale.

The symbols are intended to become recognizable. A reader should be able to glance at a page and understand whether the plant is conventionally woody, stem-succulent, geophytic, parasitic, aquatic, or structurally unusual before reading a full description.

The icons do not replace taxonomy. They add another dimension to it.

Why Tree TSAR Uses Plant Growth Forms

The seemingly simple question “Is this a tree?” quickly expands into better questions. Does the plant produce conventional secondary wood? Does it build a fibrous trunk? Is its architecture dominated by a succulent stem? What structure survives dormancy? Does it climb, float, parasitize another organism, persist underground, or live on another plant?

Those questions reveal far more about a plant’s biology than one familiar label.

Tree TSAR uses PGFs to make that diversity easier to recognize without making it artificially simple.

Explore Further

How to Read a Tree TSAR Page - Where PGF elements appear on species and genus pages.
Supertaxonomy and the Tree of Plant Life - Why growth form and evolutionary relationship are different dimensions.
Scientific Names and Common Names - Why everyday terms such as tree, palm, cactus, and bamboo do not map neatly onto taxonomic ranks.

References and Further Reading

Food and Agriculture Organization of the United Nations (2018) Global Forest Resources Assessment 2020: Terms and definitions. Forest Resources Assessment Working Paper 188. FAO, Rome. https://www.fao.org/forest-resources-assessment

Götmark F, Götmark E, Jensen AM (2016) Why be a shrub? A basic model and hypotheses for the adaptive values of a common growth form. Frontiers in Plant Science 7: 1095. https://doi.org/10.3389/fpls.2016.01095

Mauseth JD (1993) Water-storing and cavitation-preventing adaptations in wood of cacti. Annals of Botany 72(1): 81-89. https://doi.org/10.1006/anbo.1993.1083

Mauseth JD (2006) Structure-function relationships in highly modified shoots of Cactaceae. Annals of Botany 98(5): 901-926. https://doi.org/10.1093/aob/mcl133

Tomlinson PB, Huggett BA (2012) Cell longevity and sustained primary growth in palm stems. American Journal of Botany 99(12): 1891-1902. https://doi.org/10.3732/ajb.1200089

Wullschleger SD, Epstein HE, Box EO, Euskirchen ES, Goswami S, Iversen CM, Kattge J, Norby RJ, van Bodegom PM, Xu X (2014) Plant functional types in Earth system models: Past experiences and future directions for application of dynamic vegetation models in high-latitude ecosystems. Annals of Botany 114(1): 1-16. https://doi.org/10.1093/aob/mcu077