Thickets are dense, tangled stands of woody plants, typically shrubs and small trees, whose canopies interlock to form a nearly impenetrable layer of vegetation. They occur on every inhabited continent, from the subtropical valleys of South Africa to the barrier islands of Virginia to the dry forests of Argentina. Ecologically, thickets punch well above their weight: they store enormous quantities of carbon in their soils, shelter species that cannot survive in open habitats, regulate water flow across landscapes, and act as critical buffers between other biome types. Their importance is often underestimated precisely because they look, to casual observers, like scrubby wastelands rather than functioning ecosystems.
What Counts as a Thicket
In vegetation science, a thicket is defined primarily by its structure rather than the specific species it contains. The key features are dense woody plants, usually multi-stemmed shrubs or short trees, with crown cover high enough that the canopy is closed or nearly so. A broad structural classification system used in southern African ecology distinguishes thicket from related vegetation types like bushland and shrubland based on growth form, projected crown cover, and height classes, with shrub density being the defining separator between thicket and more open woody formations.1Bothalia. A broad-scale structural classification of vegetation for practical purposes In practical terms, if you cannot walk through it without pushing branches aside, it is likely thicket. Forests are taller, with a clear trunk layer beneath the canopy. Shrublands are sparser. Thickets sit in the dense, low-growing space between the two.
This structural definition matters because thickets are not confined to a single climate or geography. You find thickets in semi-arid river valleys, on windswept coastlines, at forest edges, and in the understory gaps of savannas. What unites them is not rainfall or soil type but growth architecture: plants that branch near the ground, grow laterally into their neighbors, and form a collective canopy that shades out the ground beneath.
A Rich Flora Hidden in Plain Sight
One of the most striking things about thickets is how much biodiversity they pack into vegetation that many people dismiss as monotonous scrub. The subtropical thicket of South Africa’s Eastern Cape provides a well-studied example. A comprehensive mapping effort covering about 105,500 square kilometers identified 112 unique thicket vegetation types within the region, and a compiled species list yielded 1,558 plant species, roughly 20% of which are endemic to the thicket biome.2South African Journal of Botany. Acocks’ Valley Bushveld 50 years on: new perspectives on the delimitation, characterisation and origin of subtropical thicket vegetation That is a fifth of the flora found nowhere else on Earth outside these tangled stands.
The same mapping project revealed that 78 of those 112 vegetation types were actually mosaics: patches of thicket interspersed with grassland, karoo scrub, or other non-thicket vegetation. Recognizing these mosaics expanded the known extent of subtropical thicket by roughly two to three times previous estimates. The lesson is that thickets do not always announce themselves as continuous blocks on a map. They weave through landscapes in clumps and corridors, and their ecological footprint is larger than it looks from a satellite.
An Ancient Lineage
Southern African subtropical thicket is not a recent assemblage thrown together by modern conditions. Molecular phylogenetic work on the plant lineages that dominate these thickets has revealed three distinct evolutionary layers. The oldest elements, including cycads in the genus Encephalartos and members of the bird-of-paradise family, trace back to the Cretaceous period, over 65 million years ago. A second wave of lineages that now dominate contemporary thicket, including members of the staff-tree and soapberry families, evolved during the Eocene roughly 35 to 55 million years ago. A third group diversified more recently, branching out from adjacent biomes as climates dried during the Neogene.3South African Journal of Botany. On the origin of southern African subtropical thicket vegetation The upshot is that thicket is an ancient formation, derived originally from forest lineages that adapted as the continent became drier. It is not degraded forest or immature woodland. It is its own thing, with deep evolutionary roots.
Carbon Storage Below the Surface
Thickets store a surprising amount of carbon, and most of it is underground. In intact semi-arid subtropical thicket in South Africa, below-ground carbon stocks (counting soil and root carbon to a depth of about a meter) have been measured at roughly 93 tonnes per hectare for soil carbon alone, with an additional 11 tonnes per hectare in root biomass.4Journal of Arid Environments. Below-ground carbon stocks in intact and transformed subtropical thicket landscapes in semi-arid South Africa Those numbers rival what you find in some mesic (wetter) forests. The same research found that restoring degraded thicket stands could sequester around 70 tonnes of below-ground carbon per hectare, a figure that has attracted attention from carbon-offset and restoration programs.
The reason so much carbon accumulates underground is tied to how thicket plants grow. Succulent species like spekboom (Portulacaria afra) shed leaf litter that decomposes slowly under the dense canopy, building up organic matter in the soil over decades. Roots penetrate deeply. The closed canopy keeps the soil cool and moist enough to prevent the rapid decomposition that strips carbon from exposed soils in the same climate. When that canopy is removed, the carbon advantage evaporates.
Holding Water and Soil in Place
The dense canopy and litter layer of intact thicket play a direct role in regulating water movement across the landscape. In semi-arid succulent thicket, the proportion of the land surface that promotes water infiltration, thanks to a distinct litter layer, drops from about 60% in intact vegetation to just 0.6% in degraded areas.5Austral Ecology. Landscape dysfunction and reduced spatial heterogeneity in soil resources and fertility in semi‐arid succulent thicket, South Africa That is a hundredfold collapse in the landscape’s ability to absorb rainfall. Without the litter and root mat, rain hits bare soil, runs off carrying sediment, and the landscape dries out and erodes.
Field comparisons on the same hillslope, separated only by a fenceline between grazed and ungrazed land, have confirmed that losing thicket cover leads to dramatically lower soil infiltration rates, reduced soil moisture, increased runoff, and greater erosion.6Journal of Arid Environments. Hydrological implications of desertification: Degradation of South African semi-arid subtropical thicket The fenceline studies are especially convincing because they compare degraded and intact conditions on the same slope, same soil, same climate. The only variable is whether the thicket is still there. The hydrological consequences of losing it are stark and immediate.
Seed Dispersal and the Nurse-Plant Cycle
Thicket ecosystems often depend on a cyclic relationship between plants and the animals that move through them. In arid subtropical thicket, vertebrates eat the fleshy fruits of canopy trees and deposit the seeds beneath “nurse plants,” particularly spekboom clumps, where shade and accumulated litter create favorable conditions for germination. These animal-dispersed seeds would struggle to establish in the open, competitively harsh spaces between clumps.7Plant Ecology. Directed dispersal and decomposition drive cyclic succession in arid subtropical thicket The nurse plants eventually die and decompose, enriching the soil and creating gaps where the newly established trees take over. Then a new generation of spekboom colonizes the open ground nearby, and the cycle repeats.
This process means that thicket regeneration is not simply a matter of seeds landing in the right spot. It requires functioning animal communities (birds, small mammals, and sometimes larger herbivores) to physically move seeds to the safe sites where they can grow. Remove those dispersers, and the regeneration cycle stalls even if the adult plants are still standing.
How Herbivores Shape Thicket Structure
Large herbivores, particularly elephants, have a complex relationship with thicket vegetation. Across densely wooded African landscapes, elephants can reduce vegetation height by up to four times and alter the vertical profile of woody cover, while simultaneously increasing structural variability across the landscape.8PubMed. Megafaunal effects on vegetation structure throughout a densely wooded African landscape In other words, elephants do not uniformly destroy thicket; they create a mosaic of heights and densities that can benefit other species by opening up a more heterogeneous habitat.
Yet Albany thicket, a form of subtropical thicket in South Africa’s Eastern Cape, appears to be remarkably resilient to elephant browsing. A comparison of ten sites with and without elephants found that woody and succulent vegetation was structurally similar across all sites, though non-elephant sites were slightly taller, denser, and more complex. Elephant-induced damage to plants was generally low.9South African Journal of Botany. The composition and complexity of the woody and succulent components of Albany thicket with and without elephants The evidence here supports the idea that the woody component of thicket vegetation evolved alongside large indigenous browsers and can handle their pressure. The problems start when the browsing animals are not elephants but domestic goats and cattle, which exert a very different kind of pressure.
Overgrazing and Desertification
The replacement of indigenous herbivores with domestic livestock over the past two centuries has been devastating for many semi-arid thickets. In the Eastern Cape, intensive goat browsing has extensively degraded vegetation, leading to the loss of plant biomass and species, a shift from perennial to annual plants, and deterioration of soil structure and its capacity to support life.10PubMed. Desertification of subtropical thicket in the Eastern Cape, South Africa: Are there alternatives? Unlike elephants, which break branches and push over trees in a patchy, mobile pattern, domestic goats strip vegetation systematically within fenced areas. The plants cannot recover between bouts of feeding because the animals never leave.
The degradation cascades. Once thicket cover is gone, the soil loses its litter layer and organic carbon, infiltration plummets, and erosion increases. Recovery is painfully slow because the nurse-plant cycle described earlier cannot function without an initial canopy to shelter new seedlings. Some researchers have described this as a desertification trap: once the system tips past a certain threshold of degradation, it cannot return to thicket without active restoration. The fenceline comparisons mentioned earlier make the contrast vivid. On one side of the fence, intact thicket with rich soils. On the other, bare ground, sheet erosion, and almost no perennial plant cover.
Fire as a Double-Edged Sword
Thickets and fire have a complicated relationship that depends heavily on intensity. In African savanna-forest mosaics, thickets experience less tree mortality during fires than forests do, their dense structure and different bark composition providing some protection. But repeat fires can still push thickets toward a more open, savanna-like state over time.11Journal of Ecology. Pathways of savannization in a mesic African savanna–forest mosaic following an extreme fire
In Neotropical dry forests, the picture is even more dramatic. Research in the Caldén forests of Argentina found that the forest was resilient to fires of low to moderate severity, bouncing back without changing its fundamental character. But high-severity fires triggered an abrupt transition to a shrub thicket state, where top-killed trees resprouted as multi-stemmed shrubs and new shrubs established in the gaps. This shrub thicket was self-reinforcing: it enabled recurring high-severity fires that prevented the forest from recovering. Post-fire grazing and controlled burns compounded the problem by favoring shrubs over tree regeneration.12PubMed. Thresholds and alternative states in a Neotropical dry forest in response to fire severity This is a case where thicket formation is the problem rather than the solution: the shrub thicket is a degraded state that traps the ecosystem in a cycle of frequent, intense fires.
The takeaway is that “thicket” is not inherently good or bad. Whether a thicket is ecologically valuable depends on whether it is a natural, biodiverse formation or a degraded, species-poor state maintained by disturbance. Context matters enormously.
When Invasive Shrubs Build the Wrong Kind of Thicket
Not all dense shrub cover is equal. In the northeastern United States, invasive shrubs such as honeysuckle and buckthorn can form thickets that superficially resemble native ones but function very differently for the animals that depend on them. Research comparing insect communities across thicket habitats found that sites dominated by invasive shrubs supported an abundance of generalist and pest insects, while sites with native shrubs hosted more caterpillars, more herbivorous insects overall, and more rare species. When captive caterpillars were offered foliage from invasive shrubs, most of them refused to eat and died.13Forest Ecology and Management. Insect responses to invasive shrubs: Implications to managing thicket habitats in the northeastern United States Since caterpillars are the primary food source for nesting songbirds, invasive thickets can become ecological traps: birds nest in what looks like suitable habitat but cannot find enough food to raise their young.
A different dynamic plays out in South Africa’s fynbos region, where invasive Australian Acacia species form dense thickets in areas that were previously open shrubland. These alien thickets do support bird communities, averaging about 20 species per patch with densities comparable to some natural habitats. But they lack the nectarivore species typical of native fynbos, reflecting the absence of the nectar-rich indigenous plants those birds depend on.14Diversity and Distributions. Novel ecosystems support substantial avian assemblages: the case of invasive alien Acacia thickets These “novel ecosystems” are not wastelands, but they are not substitutes for native habitat either. Managing them requires deciding which ecological functions matter most in a given landscape, a question with no easy universal answer.
Thickets and Climate Vulnerability
The future of thicket ecosystems under climate change depends on water. Simulation work on shrub thickets growing on Virginia’s barrier islands found that thicket transpiration patterns were sensitive to climate change, landscape change, and sea-level rise. Soil water was the primary limit on transpiration, and capillary rise from groundwater influenced where shrubs could grow on the island.15Ecological Modelling. Simulation of transpiration sensitivity to environmental changes for shrub (Myrica cerifera) thickets on a Virginia barrier island As sea levels rise and freshwater lenses shrink on barrier islands, thickets in these coastal settings face an existential water squeeze.
Drought responses among thicket species are not uniform, either. A study of six common woody species from a dry evergreen forest in South Africa found mixed hydraulic responses to drought. Some species experienced high levels of embolism (blockage in their water-conducting vessels) during drought, and the severity of that embolism predicted how well they recovered afterward. Species with greater embolism showed less crown recovery, more dead leaves, and more discoloration months after the drought ended.16Tree Physiology. Mixed hydraulic responses to drought in six common woody species from a dry evergreen sclerophyll forest in South Africa The implication is that prolonged droughts could selectively remove the most vulnerable species from thicket communities, reshaping their composition even if the thicket as a whole persists.
People and Thickets
For rural communities living alongside thicket vegetation, these ecosystems are not abstract ecological assets. They are sources of food, fuel, medicine, and income. Ethnobotanical research in the Eastern Cape has documented at least 25 wild and tended plant species from thicket landscapes that contribute to cash income, food, and traditional medicines, including ethnoveterinary uses for livestock.17PubMed Central. Traditional uses of wild and tended plants in maintaining ecosystem services in agricultural landscapes of the Eastern Cape Province in South Africa The provisioning services, the tangible products people can harvest, were the most frequently cited ecosystem services by local communities. This creates a tension that runs through thicket conservation globally: the same communities that depend on thicket resources are often the ones whose livestock are degrading them.
Solutions that ignore this tension tend to fail. Fencing off thicket to exclude livestock may protect the vegetation but strips communities of grazing land they depend on. Carbon-offset programs that fund spekboom restoration can provide alternative income, but only if the economics work and the planting actually succeeds, which depends on rainfall that is becoming less reliable. The most promising approaches tend to integrate livestock management with thicket conservation, rotating grazing pressure and allowing recovery periods, rather than treating them as incompatible.
Riparian Thickets and Stream Health
Thickets that grow along rivers and streams perform a specific ecological function that disappears when they are removed. Research on a highly endemic river macroinvertebrate community in South Africa found that shade from riparian canopy, even when that canopy was composed of invasive alien trees, maintained the cool temperatures and high oxygen levels that the stream’s endemic species required. When the alien trees were cleared, water quality declined: temperatures rose, suspended solids increased, and oxygen dropped. The most sensitive endemic invertebrates only recovered after indigenous bushy vegetation re-established and began providing shade again.18Biological Invasions. Effect of alien riparian vegetation and its removal on a highly endemic river macroinvertebrate community
This finding complicates the straightforward narrative of “remove all alien vegetation.” In riparian zones, clearing invasive thickets without simultaneously establishing native canopy cover can cause more immediate damage to stream life than the invasive plants themselves were causing. The shade and bank stabilization provided by any dense riparian thicket, native or not, is a physical ecosystem service that cannot simply be turned off during the transition from alien to indigenous plant cover. Restoration in these settings requires careful staging: phased clearing, replanting, and monitoring of water conditions rather than wholesale removal.