The first living things to colonize bare ground aren’t plants at all. Bacteria and cyanobacteria arrive first, sometimes within just a few years of new ground being exposed. Lichens and mosses follow, and only after these organisms have started breaking down rock and building up nutrients do the first true plants take root. This process, called primary succession, can take decades or centuries depending on the environment.
Microbes Arrive Before Any Plant
When a glacier retreats or a volcano buries a landscape in ash, the exposed surface is essentially lifeless rock or mineral debris. The very first colonizers are bacteria and cyanobacteria, microscopic organisms that can survive with almost nothing. Research on recently deglaciated soils at high elevations found that cyanobacteria established diverse communities within the first four to five years after ice melted away. These organisms don’t need soil to exist. They live on bare rock surfaces and glacier edges, pulling nitrogen from the atmosphere and carbon from sunlight.
This microbial activity matters enormously. Nitrogen-fixation rates in those newly exposed glacial soils increased almost a hundredfold during the first four to five years, all driven by microbes, well before any mosses, lichens, or vascular plants appeared. Without this invisible groundwork, nothing else can grow.
Lichens and Mosses Build the First Soil
Lichens are typically the next to arrive. They’re not technically plants but partnerships between fungi and algae that form crusty, colorful patches on rock. Lichens produce weak organic acids, including oxalic acid and a group of compounds unique to lichens called “lichen acids,” that slowly dissolve rock minerals. This chemical weathering, combined with physical pressure from lichen growth working into tiny cracks, gradually breaks solid rock into fine particles.
Mosses follow a similar strategy. They produce acids that continue breaking down rock, and their small, dense structures trap dust, moisture, and organic debris. When lichens and mosses die, their decomposing material mixes with the mineral particles they helped create, forming the earliest layers of true soil. On the volcanic island of Surtsey, which emerged from the ocean off Iceland in 1963, much of the vegetation even decades later still consists mainly of lichens and mosses.
Together, these organisms form what scientists call biological soil crusts: thin, living layers on the soil surface made up of cyanobacteria, algae, fungi, lichens, and mosses in varying proportions. These crusts act as ecosystem engineers in barren landscapes, stabilizing loose sediment against wind and water erosion, retaining moisture, and cycling carbon and nitrogen to prepare the ground for larger plants.
The First True Plants to Take Root
Once a thin layer of soil exists, the earliest vascular plants move in. These pioneers share a few key traits. They tend to produce huge numbers of small, lightweight seeds that travel easily by wind or water. Small seeds sacrifice individual survival chances for sheer numbers, maximizing the odds that at least some land in the rare favorable microsites on an otherwise hostile landscape.
Many of the first vascular plants also have a special advantage: they partner with soil bacteria that convert atmospheric nitrogen into a form plants can use. This matters because brand-new soil is extremely low in nitrogen, a nutrient essential for growth. Alder trees are a well-known example, forming root nodules with bacteria of the genus Frankia. Over 200 species of flowering plants across 25 genera share this ability, and they play an outsized role in enriching early soils so that other, more demanding species can eventually establish.
The specific pioneers depend on the environment. After Mount St. Helens erupted in 1980, fireweed appeared as early as the summer of that same year in areas where some biological material survived. On the Pumice Plain, one of the most devastated zones, prairie lupine was the first plant to gain a foothold. Lupines are nitrogen-fixers, and their presence gradually made the barren pumice hospitable to grasses, shrubs, and eventually trees.
How Long Soil Formation Takes
The conventional estimate is that it takes centuries to produce a single inch of topsoil. In many stable landscapes, that’s accurate. But the timeline varies wildly depending on conditions. Researchers at the University of Washington measured soil production rates on mountainous ridgetops covered with low, dense vegetation and found rates as high as 2.5 millimeters per year. At that pace, an inch of soil forms in roughly 12 years. Plant roots were likely responsible: they grow into rock fractures, physically breaking stone apart and exposing it to rainwater and chemical weathering.
In harsher environments, the process is far slower. High-altitude glacial terrain or lava fields with no existing biological material can take decades just to accumulate enough microbial life to support mosses, and decades more before the first grasses or shrubs appear.
Primary vs. Secondary Succession
Everything described above is primary succession, where life colonizes a surface with no existing soil or biological legacy. But most bare patches of ground people encounter, such as an abandoned field, a burned forest, or a cleared lot, still have soil intact. That’s secondary succession, and it follows a very different script.
When soil already exists, it contains seeds, root fragments, fungal networks, and microbial communities ready to reactivate. The first plants in secondary succession are typically fast-growing grasses, weeds, and wildflowers rather than lichens and mosses. Factors like which seeds are already in the soil, which arrive first from nearby, and interactions with soil microbes drive the process. Secondary succession moves much faster because the slow, centuries-long work of soil creation has already been done.
So the answer depends on the starting point. On truly bare rock or volcanic debris, bacteria, cyanobacteria, lichens, and mosses come first, sometimes working for decades before the first rooted plant can survive. On ground where soil already exists, grasses and herbaceous plants can establish within a single growing season.