Danish Research Maps Carbon and Biodiversity Trade-Off

Planted forests deliver carbon returns far sooner than forests left to regrow on their own, according to a long-term study from the University of Copenhagen. Over roughly 21 years, plantations on former Danish cropland built woody biomass around 15 times faster than sites left to regenerate naturally. Those naturally colonised sites developed richer plant diversity and more complex structure.

A naturally regenerating Danish forest showcasing biodiversity, with people planting trees in a new forest area in the background. AI generated picture.

The study, published in Forest Ecology and Management, assessed four land-use types across 10 Danish landscapes: cropland as a baseline, naturally colonised and actively planted sites of about 21 years each, and mature managed forests of around 130 years. It measured tree species composition, biomass, soil organic carbon and total ecosystem carbon across 120 sample plots.

The carbon figures favour active planting in the near term. Plantations stored about 117 tonnes of ecosystem carbon per hectare. Naturally colonised land held roughly 56 tonnes and cropland 58 tonnes, leaving the naturally colonised sites close to farmland levels after two decades. Mature managed forests carried the largest wood biomass stocks, underlining how carbon accumulates over much longer horizons.

Fast biomass gains carry a caveat for project design. The researchers frame rapid growth as one measure among several, noting that mature ecological character takes far longer to develop. Naturally colonised sites built distinct, heterogeneous tree communities as they aged. Soil told a slower story again: topsoil organic carbon held steady across the two decades, a sign that soil carbon recovers gradually under both approaches.

The authors describe a trade-off between the near-term carbon efficiency of active planting and the longer-term biodiversity and carbon potential of natural colonisation. Passive natural colonisation offers a low-cost route that recovers diversity and carbon over time. Planting with native and mixed species can pair faster carbon gains with the structural complexity that supports resilient, higher-quality forests over the long term. Both findings give project designers clearer ground for matching restoration methods to their goals.