The research team, led by Jiao Jiejie and Yao Liangjin of the Zhejiang Academy of Forestry, together with colleagues at Ningxia University and the Chinese Academy of Forestry, established thirty experimental plots in 2020 across a secondary forest that had regrown after past disturbance. Ten plots were left untouched as controls, ten received light thinning that removed 10 to 15 percent of basal area, and ten underwent heavy thinning at 20 to 25 percent. Three years later, the team returned to measure everything from tree heights and stem diameters to the functional traits of every species present, building one of the most complete pictures yet of how management intensity reshapes the architecture of a recovering forest.

The headline finding concerns biomass productivity, the rate at which the forest converts sunlight, water, and carbon dioxide into wood. Heavily thinned stands increased their total biomass at a relative growth rate 66.7 percent higher than the untouched controls. That is a striking acceleration for a treatment that, on its face, removes material from the ecosystem. The explanation lies in what foresters call a release effect. By cutting away crowded, suppressed individuals, thinning reduces competition for light, soil moisture, and nutrients, allowing the remaining trees to expand their canopies and accelerate growth. In dense secondary forests that have regrown without management, this competitive pressure can be severe, and the study demonstrates just how much latent productivity is locked away inside an overcrowded stand.

But the growth surge was only part of the story. The researchers measured diversity across three distinct dimensions: structural, species, and functional. Structural diversity captures how uneven the forest is in size, quantified here through the Gini coefficient of basal area and the standard deviation of tree height. Both metrics rose in thinned stands, meaning the forests became more architecturally heterogeneous, with a richer mix of tall dominants and smaller understory trees. Species diversity also climbed under heavy thinning, as newly available light and space allowed additional species to establish and persist. These changes matter because structural complexity is closely tied to habitat quality, light capture efficiency, and the overall resilience of a forest ecosystem.

Functional diversity, the variety of traits that determine how species use resources, told a more complicated tale. The team assessed it using eight plant traits, calculating three complementary indices: functional richness, functional evenness, and functional divergence. Heavy thinning significantly increased evenness and divergence, indicating that the surviving species occupied resource-use strategies more uniformly and more distinctly from one another. Yet functional richness, the total volume of trait space filled by the community, collapsed by 61.9 percent relative to the control. This apparent paradox makes ecological sense: thinning preferentially removes certain individuals and species, pruning away extremes of the trait distribution even as the remaining species pack the reduced trait space more evenly. A forest can simultaneously become more functionally balanced and less functionally expansive.

The most consequential result, however, is the shift in the diversity-productivity correlation itself. In unthinned and lightly thinned stands, functional diversity was negatively correlated with productivity, echoing the long-running debate in ecology over whether diversity genuinely drives ecosystem function or merely correlates with other factors. In heavily thinned stands, that correlation turned significantly positive. In other words, the same forest type, on the same landscape, over the same three years, exhibited opposite diversity-productivity relationships depending solely on management intensity. The finding suggests that many published inconsistencies in the biodiversity-ecosystem functioning literature may reflect unmeasured differences in disturbance and management history rather than genuine ecological disagreement.

Why would heavy thinning unlock a positive diversity effect? The authors’ interpretation rests on resource utilization. In an overcrowded stand, competition is so intense that species with complementary traits cannot fully express their complementary advantages; the forest is saturated, and niche differentiation provides little benefit. Once thinning opens the canopy, species with contrasting light requirements, rooting depths, and leaf economics can partition resources effectively, and the classic complementarity mechanism that underpins positive diversity-productivity relationships finally has room to operate. Functional divergence, which measures how spread out species are in trait space, increased under heavy thinning and aligns with this mechanism, offering a trait-based window into why the correlation flipped.

The authors are careful to frame these results as transient dynamics rather than equilibrium states. Three years is a snapshot in the life of a forest, and early post-treatment responses are often dominated by the release effect, which may fade as canopies close again. Species that colonize newly opened gaps may later be shaded out, and the reduced functional richness could recover, persist, or deepen as succession proceeds. The team explicitly calls for long-term monitoring to determine whether the positive diversity-productivity relationship under heavy thinning persists, strengthens, or reverses over decades. This caution is scientifically important: it distinguishes a genuine management insight from an overgeneralized rule, and it flags the study as a baseline for future re-measurements rather than a final verdict.

The practical implications are nonetheless immediate. Secondary forests now dominate large swaths of subtropical China and much of the world’s forested land, and they are increasingly relied upon for carbon sequestration targets, timber supply, and biodiversity conservation. If heavy thinning can raise productivity by two-thirds while simultaneously boosting species diversity and structural complexity, it offers a rare win-win-win for managers, though the 61.9 percent loss of functional richness tempers any triumphalism. The study also provides a template for evidence-based silviculture: rather than applying uniform thinning prescriptions, managers could tune intensity to specific objectives, using heavier thinning where rapid carbon accumulation and structural diversification are priorities and lighter treatments where preserving the full breadth of functional traits matters more.

Scientifically, the work adds a crucial temporal and managerial dimension to biodiversity-ecosystem functioning research, a field built largely on experiments in planted grasslands and young tree monocultures. By manipulating a real, unmanaged secondary forest and measuring all three dimensions of diversity simultaneously, the Zhejiang team shows that the diversity-productivity relationship is not a fixed property of an ecosystem but a moving target shaped by how humans intervene. As forests worldwide face mounting pressure from climate change, restoration initiatives, and demand for wood, understanding that thinning intensity can flip the sign of a fundamental ecological relationship may prove one of the most actionable discoveries in modern forest ecology, and the hills of Zhejiang are now the place to watch as the next chapter of this experiment unfolds.

Subject of Research: Effects of thinning intensity on tree diversity and forest productivity in subtropical secondary evergreen broad-leaved forests

Article Title: Tree diversity and productivity in subtropical secondary evergreen broad-leaved forests under different thinning intensities in Zhejiang Province, China

Article References: Jiejie, J., Chuping, W., Guangyao, S., Bo, J., Chenggong, L., & Liangjin, Y. (2026). Tree diversity and productivity in subtropical secondary evergreen broad-leaved forests under different thinning intensities in Zhejiang Province, China. BMC Plant Biology. https://doi.org/10.1186/s12870-026-09825-8

Image Credits: AI Generated

DOI: 10.1186/s12870-026-09825-8

Keywords: forest ecology, thinning, biodiversity, functional diversity, productivity, subtropical forests, secondary forest, biomass, Zhejiang, forest management, species diversity, structural diversity

The stakes are considerable. Norway spruce is the second most abundant tree species in Germany after Scots pine and underpins the solid-timber, paper and pulp industries. In 2010 it accounted for roughly 60 percent of the land expectation value of European forestry, a share projected to fall to around 45 percent by 2040 as climate change erodes the species’ viability in much of its planted range. Disturbances are already taking a heavy toll: over the past twenty years, natural disturbances accounted for about 16 percent of the mean annual volume harvested in European forests, and wind alone was responsible for roughly 46 percent of that loss. Germany ranks among the European countries most affected economically by wind damage, particularly along the northern coast and in high-elevation regions.

What makes spruce forests especially vulnerable is the chain reaction that follows a storm. Wind-thrown trees provide fertile breeding ground for the European spruce bark beetle, Ips typographus, which can then erupt into standing timber. Pure spruce plantations, mature stands, exposed clearcut edges, warm temperatures and dry soils all amplify the risk. The study’s authors coupled three models to capture this cascade: the 3-PGmix process-based growth model to simulate how stands develop under changing temperature, precipitation and radiation; the ForestGALES hybrid mechanistic model to calculate the critical wind speeds at which trees break or overturn; and the IpsR bark beetle model to estimate how much wood beetles kill after a storm, using effective degree days and drought stress as key drivers.

Before running the national simulations, the team validated their approach in Baden-Württemberg by comparing ForestGALES against the statistical “Lothar” model, which was calibrated on damage from the 1999 storm of that name and rigorously tested in south-western Germany. The two models tracked each other closely over a 30-year simulation, both showing reduced damage probability under regular thinning compared with unthinned stands. ForestGALES proved consistently slightly more pessimistic, with differences of 14.7 to 15.8 percent between the models’ risk estimates — a smaller discrepancy than the 20 to 30 percent biases reported in earlier model comparisons. The authors note that the mechanistic model’s slight pessimism may reflect its sensitivity to stand density and soil type, factors the statistical model does not consider, and they caution that absolute damage figures should be interpreted with care.

For the national-scale experiment, the researchers drew stand data from the German National Forest Inventory and simulated three management regimes under both moderate (RCP 4.5) and high-emission (RCP 8.5) climate scenarios. The “Standard Thinning” regime removed 16 percent of stem density every ten years. The “No Thin” regime left stands to grow with only natural mortality. The “Adapted Thinning” regime, based on the risk-adapted spruce management system of Baden-Württemberg, applied thinning governed by height class: once stands reached 28 metres, a felling sequence began, with volume removed five times at two-year intervals before a final clearcut. Then, in each simulated year from 2020 to 2046, the team imposed the wind field of storm Kyrill — the January 2007 storm that swept almost the entire country, with a median maximum hourly wind speed of 29.5 metres per second across the simulated plots — and followed up with a bark beetle attack.

When disturbance risk was ignored, the results looked straightforward: “Standard Thinning” produced the highest total volume by 2046, reaching a median of 1,141 cubic metres per hectare under RCP 4.5 and 1,165 cubic metres per hectare under RCP 8.5, ahead of both “No Thin” and “Adapted Thinning.” The slightly higher yields under the warmer scenario suggest that, in the short term and on sites not limited by water, some German spruce stands may even grow faster. But the picture inverted once storms and beetles entered the equation. The “Adapted Thinning” strategy showed the lowest median wind damage probability, 0.46 under both climate scenarios, compared with 0.55 for “Standard Thinning” and 0.57 for “No Thin,” and it held the lowest damaged volume across the country in both climate pathways.

The bark beetle results reinforced the same message. Beetle damage was concentrated in central and southern Germany, where spruce is most abundant, and the “No Thin” regime suffered the highest absolute losses because more standing wood was available as host material. Notably, “Standard Thinning” and “No Thin” produced identical beetle damage rates as a percentage of volume — but because unthinned stands carried more timber, their absolute losses were significantly greater. Under “Adapted Thinning,” the damaged percentage declined with stand age and fell to zero by age 80 under both climate scenarios, simply because those stands had already been harvested. Under RCP 8.5, beetle damage increased over time in the conventional regimes, while it steadily declined under the adapted strategy. The conditional value at risk — the mean damage in the worst 5 percent of cases — was lowest for “Adapted Thinning” under both climate pathways, indicating that early, intensive timber extraction protects not just the average outcome but the tail risk.

The mechanism behind these results is physically intuitive. Wind damage scales with tree height, diameter and the sail area of the canopy, so taller, denser, older stands are more likely to break or overturn in a given gust. By removing volume before a storm arrives, adapted management reduces the amount of timber physically exposed to the wind and simultaneously eliminates the windthrown wood that beetles need to build up populations. The study also found that the differences between the two climate scenarios were minor over the 2020 to 2046 window, since the pathways diverge substantially only in the second half of the century — but the authors warn that rising heat sums and drought stress could still drive larger beetle outbreaks, and drought can decouple the wind-beetle link in complex ways.

The findings carry a pointed implication for German forestry policy. The latest national forest inventory shows that Norway spruce coverage has already declined, largely due to the droughts and beetle epidemics that began in 2018, and the authors suggest that earlier implementation of adapted management could have reduced some of that damage. They are careful to acknowledge limitations: the simulations used the third national inventory because the fourth was not yet available in detailed form, storm frequency changes were not modelled, and the probability of storm occurrence in any given year was not accounted for. They also stress that unmanaged forests can serve as effective carbon sinks over decades, so the optimal strategy depends on whether the goal is timber production, carbon storage or both.

Longer term, the outlook for spruce itself remains bleak, with studies projecting substantial range losses, and in some scenarios complete disappearance from lower elevations, by the end of the century. Species mixtures with beech, Scots pine, Douglas fir or birch offer another route to resilience, as mixed stands are generally more resistant to both wind and beetles. But for the maturing spruce stands that dominate German forests today, the message of this study is unambiguous: in a climate of intensifying storms and beetles, the safest harvest is the one taken before the storm arrives. A risk-averse strategy of early thinning and shortened rotations, the authors conclude, is one of the most effective tools available to keep Germany’s spruce forests standing — and their timber out of the windthrow — in the decades ahead.

Subject of Research: Modelling the effects of forest management strategies on the resilience of German Norway spruce forests to wind storm and bark beetle disturbances under climate change

Article Title: Impact of forest management on the future resilience of German Norway spruce (Picea abies (L.) H. Karst) forests to abiotic and biotic threats in the changing climate

Article References: Bourke, R., Gardiner, B., Yousefpour, R., & Hanewinkel, M. (2026). Impact of forest management on the future resilience of German Norway spruce (Picea abies (L.) H. Karst) forests to abiotic and biotic threats in the changing climate. Regional Environmental Change, 26(4), Article 197. https://doi.org/10.1007/s10113-026-02671-2

Image Credits: AI Generated

DOI: 10.1007/s10113-026-02671-2

Keywords: Norway spruce, bark beetle, wind storms, climate change, forest management, thinning, ForestGALES, 3-PGmix, Ips typographus, Germany, disturbance ecology, RCP scenarios