Forest Structure Trumps Topography in Shaping Plant Diversity in Italian Old-Growth Forests

In the remote forests of Pollino National Park, tucked into the Southern Apennines of southern Italy, a team of researchers spent months moving through difficult terrain to answer a deceptively simple question: what makes an old-growth forest so biologically rich? The answer, it turns out, has less to do with the slope of the land than with what grows on it—the massive trees, the layered canopies, the rotting logs in various stages of decay. These structural features, the researchers found, shape plant communities far more powerfully than topography alone.

Old-growth forests are vanishingly rare across Europe. They store carbon, buffer ecological systems against climate stress, and harbor species found nowhere else. Yet scientists still struggle to understand precisely which features drive their extraordinary diversity, especially in the Mediterranean mountains where conditions are harsh and variable. A study published in Ecology and Evolution, led by Danilo Travascia of the University of Basilicata and involving researchers from multiple Italian institutions and the park itself, set out to untangle these relationships by examining eight old-growth forest sites across different elevations and forest types—from Mediterranean oak stands to high-altitude beech forests.

The fieldwork was painstaking. Across 142 subplots, the team recorded everything: the diameter and species of living trees, the structure of the canopy, the amount and decay stage of deadwood on the ground and standing dead trees, and the complete inventory of vascular plants. They derived 24 structural variables from these measurements and combined them with topographic data—elevation, slope, aspect—to see which factors best explained the composition of plant communities. The robustness of their findings was tested through multiple statistical approaches, including canonical correspondence analysis and variation partitioning, ensuring that the patterns they identified were real and not artifacts of their methods.

The results reframed the question. Topography does influence forest structure—it accounts for about 18.8% of the variation in structural attributes. But when the researchers looked at what actually determines which plants grow where, the picture shifted dramatically. Forest structure alone explained 23.5% of the variation in plant composition, compared with just 12.8% for topography by itself. Another 31.3% of variation was jointly associated with both sets of variables. In other words, the physical complexity of the forest—the abundance of large trees, the diversity and coverage of the canopy, the density and heterogeneity of trees, and crucially, the variety of deadwood in different stages of decomposition—created a mosaic of light, temperature, moisture, and substrate conditions that determined which plants could thrive.

Across all 142 subplots, the researchers documented 153 vascular plant taxa, with sampling coverage so complete that they estimated the true richness at around 170 species. But here the study revealed something conservation biology often overlooks: which plants matter as much as how many. High-elevation beech forests supported cold-adapted and biogeographically distinctive species, including endemic plants found only in southern Europe. Lower mixed forests hosted different assemblages. Critically, sites that had experienced disturbance often contained more total species but included a higher proportion of common, weedy plants that thrive in disturbed conditions. The undisturbed old-growth stands, by contrast, preserved distinctive communities—assemblages that reflected centuries or millennia of ecological continuity and could not be quickly reassembled if lost.

The implications for conservation are substantial and specific. Protecting old-growth forests cannot mean simply maintaining forest cover. It requires preserving the large and ancient trees themselves, the multilayered canopy structure, the standing and fallen deadwood in various states of decay, and the long-term ecological stability that allows distinctive plant communities to persist. The researchers advocate for long-term monitoring that integrates structural surveys with dendrochronology—the study of tree rings to understand growth history—microclimate measurements, soil analysis, and remote sensing. Pollino National Park, with its old-growth forests spanning radically different elevations and bioclimatic zones, offers an exceptional opportunity to understand how Mediterranean mountain forests will respond as climate change accelerates. The structure of the forest, it turns out, is not merely a backdrop to biodiversity. It is the engine that drives it.