Italy planted Norway spruce across the Alps in the 1930s, a deliberate-but-naive reforestation drive, but 90 years on, plant diversity is 50% lower than in native forests

During the 1930s, Italy’s government launched a sweeping reforestation effort in the Prealps region near Lake Como, at the foothills of the Alps, planting fast-growing Norway spruce on land that had been pasture and meadow for centuries. It was a conscious decision, made mainly to answer the demand for timber, but it did not involve much ecological thinking. Now, 90 years later, a new study has gone back to measure what that decision actually did to the landscape, and the results are not flattering.

According to the study, ‘Long-Term Ecological Impacts of Norway Spruce Plantations on Biodiversity and Microhabitat Conditions’ published in the journal Ecosystems by researchers at the University of Milan and the University of Lausanne, scientists compared century-old spruce plantations to nearby native deciduous forests and mountain grasslands at two sites in Italy’s Prealps.

The study found that plant diversity in spruce plantations was 50.3% lower than in nearby native forests and 74.5% lower than in the grasslands. The plantations have had close to a century to settle down and start acting like a proper forest, and they still haven't, according to the study.


Plant diversity took a huge hit and never recovered
In 2023, the researchers spent five months surveying plots of land across three habitat types: spruce plantations, native deciduous forests and grasslands. They counted all the plant species they could find, identified soil-dwelling insects and other arthropods caught in pitfall traps, and tested soil samples for acidity, carbon and nutrients.

The researchers' data indicated that a typical grassland plot contained a median of 37 plant species, a deciduous forest plot contained a median of about 18, and a spruce plantation plot contained a median of only 7.

Structurally, it appeared to be more than just a snapshot of that gap. The study found that when the researchers investigated which plant species were most representative of each habitat, the species in spruce plantations were not a separate community on their own. They were a smaller, nested subset of the species already present in native deciduous forests. Put another way, they were a thinned-out, impoverished version of the forest next door, not a truly different ecosystem. Researchers suggest one likely driver is the loss of early-blooming plants. The spruce is evergreen, and blocks sunlight from reaching the forest floor year-round, but native deciduous trees drop their leaves in winter, letting a burst of spring light reach the forest floor so wildflowers and other early bloomers can thrive before the canopy fills back in. Spruce never opens that window for the forest floor.

The bigger problem was the change in the soil itself
It’s not just what’s growing above the ground. The study found that the soil under spruce plantations had 25% more organic carbon than soil from deciduous forests and grasslands, mainly because spruce needles accumulate and decompose much more slowly than leaf litter from native trees. Soils under spruce were also consistently more acidic than soils under the other two habitats.

The more complex statistical modeling in the study showed that two specific soil elements, the carbon-to-nitrogen ratio and the amount of exchangeable aluminum, were directly associated with reduced plant diversity. Both tend to rise as acidic, slow-decomposing spruce litter builds up, giving researchers a plausible mechanism for how the plantation is actively altering the ground beneath it, rather than simply sitting on top of it.

The study also quantified functional evenness, a measure of how efficiently a plant community uses available ecological space (e.g., light, nutrients, soil conditions). Spruce plantations had 30% lower functional evenness than native forests, indicating a less stable and more disturbance-prone ecosystem (e.g., to drought or pests), according to the researchers.

Interestingly, soil-dwelling arthropods did not follow the same pattern. No statistically significant differences in arthropod diversity were found among the three habitat types in the study. That might be because these insects are mobile enough to move freely between nearby habitats, or because they’ve simply had an easier time recovering over the past century than slower-spreading plant species have, the researchers suggest.

Why this matters beyond one Italian mountainside
This isn’t just the story of one forest in Italy. The study, Restoring natural forests is the best way to remove atmospheric carbon, published in the journal Nature by ecologist Simon L. Lewis and co-authors at University College London and the University of Edinburgh found that almost half of the global land area committed to forest restoration under international climate pledges is being turned into commercial monoculture plantations rather than restored as natural, biodiverse forest, a distinction the authors say should disqualify those projects from counting as real “restoration” at all.

That difference matters for biodiversity, and more and more research is backing that up at scale. In the ‘Positive effects of species mixing on biodiversity of understory plant communities and soil health in forest plantations’ study published in the Proceedings of the National Academy of Sciences, researchers including Huili Feng, Jiahuan Guo, Daniel Kneeshaw, and Changhui Peng pooled data from more than 7,000 paired observations from 311 studies around the world, finding mixed-species plantations significantly outperformed monocultures on understory plant biomass, ground cover and species richness, alongside measurable improvements in soil health.

The authors of the Italian study note that Norway spruce isn’t even a non-native species worldwide; it’s native to parts of Europe, just not to the specific mountain range where it was planted. According to the researchers, that detail actually reinforces their concern. If a species native to the wider region can wreak this level of havoc on local biodiversity when planted outside its natural range, the risks posed by truly foreign fast-growing species such as eucalyptus and pine, widely used in global reforestation efforts, might be even greater.

The takeaway for climate-conscious planting
None of this means planting trees is pointless. It means the type of tree planting actually matters. According to the study, the researchers recommend that reforestation programs should aim to mix native species, rather than defaulting to fast-growing monocultures, even if monocultures are cheaper and easier to manage up front.

It’s a handy reality check for anyone who ever donated to a “plant a tree” campaign thinking it was a sure winner for the planet. It turns out that where and what you plant is just as important as how many trees you put in the ground.