Regrowing tropical forests push invasive trees back

Regrowing tropical forests are doing quiet work against invaders. A large study of secondary forests across the Americas found that regrowing tropical forests first fill with fast growing non-native trees, then lose them as native trees take over the shade. The finding matters for restoration projects that worry an early flush of foreign trees means a forest is lost. According to the researchers, that early flush is a phase, and the canopy closing above it is what ends the phase. The work was published in Nature Plants and draws on plots in forests that grew back after farming or pasture use ended.

The team behind the study examined 1,561 plots of secondary forest in ten Neotropical countries, with researchers from around 60 institutions contributing data. Instead of watching one site for decades, they compared plots of different ages in the same region, from land recently abandoned to older forest. In each plot, they identified and measured trees above a set diameter, which let them reconstruct how the mix of species changes as a forest ages. The approach covered humid and dry forests, and the researchers classified 3735 woody species by origin and invasiveness. That scale is what lets the pattern in regrowing tropical forests stand out from local quirks in any single site.

Early dominance in open ground

In the first years after abandonment, sunlight reaches the ground and fast growing trees rush in. Non-native trees behaved much like native pioneer species in that stage, growing quickly and demanding light. In moist forests, they accounted for 28 percent of stems and 22 percent of species during the early stage. In dry forests, the share was lower, at 9 percent for both measures. The researchers say the concern in that stage is real, because a foreign tree can take the place of a native one and change food webs and forest structure. First author Angelica Faria de Resende, who carried out the work during postdoctoral research at the University of Sao Paulo, said the interference fades as the forest matures. Regrowing tropical forests show the same early rush in both forest types, at different levels.

The study separated foreign trees that merely persist from those that spread on their own. Pine grown for wood and resin in Brazil spreads by wind borne seed and takes over ground without planting. The researchers report pine is invasive in that setting. Eucalyptus, also foreign to Brazil, stays largely where it is planted for timber, paper and pulp. The study reports eucalyptus is non-native but not invasive in that setting. Resende said classification can turn political in some countries, because agencies issue official lists and a widely used farm or forestry tree may escape the invasive label even when it spreads. That distinction matters for anyone reading a species list and assuming every foreign name on it behaves the same way.

Shade changes the contest

As native trees grow taller, the forest floor darkens and light hungry trees lose their edge. The study found non-native trees were present in 41 percent of humid forest plots and 18 percent of dry forest plots, yet their density and richness fell sharply after the early peak. Slow growing native trees that germinate in shade gradually dominate, and foreign trees persist only at low densities in mature forest. The researchers link that shift to canopy closure, which makes the ground less suitable for species that need full sun. Regrowing tropical forests, in this reading, do not need to be cleared of every foreign tree at the start to end up native dominated.

In regrowing tropical forests, the pattern was not uniform everywhere. Non-native species richness rose where the Human Development Index was higher, which the researchers read as a sign of greater movement of people and planting material near settled areas. Density and abundance fell as surrounding forest cover, rainfall, and proximity to farmland rose, a combination that points to intact landscapes holding foreign trees in check. Soil with more organic carbon tended to hold non-native trees longer, which suggests ground conditions can slow the fade even as shade increases. Only 11 species in the full sample were counted as non-native and invasive, most of them multifunctional trees tied to human use.

For restoration work, the message is practical. Letting regrowing tropical forests develop, and keeping the landscape around them intact, can curb foreign trees without constant clearing. That approach protects native biodiversity, forest structure, and local livelihoods, according to the study authors. It does not mean foreign trees vanish entirely, and mature forests still hold some at low density. It means time and shade do much of the work that managers once assumed only removal could do. A forest left to close its canopy, the study suggests, can correct its own early imbalance. The findings were reported by Phys.org and the study paper, with source material at https://phys.org/news/2026-10-natural-forest-regeneration-curb-invasive.html.