Tree growth thrives on diversity - especially in wet forests

Forest ecologists have known for decades that mixing many kinds of trees generally makes woodlands healthier and more resilient. Yet field studies have not always agreed on how strongly that diversity effect shows up in different climates.

A globe-spanning analysis led by the University of Michigan now clarifies the picture: biodiversity gives trees a noticeable growth boost almost everywhere, but the advantage is far larger in forests that receive plenty of rain.

“Diversity matters everywhere we look. But in our experiments, we see it matters more in the wetter climates,” said Peter Reich, director of the Institute for Global Change Biology at Michigan and senior author of the new study.

Forest trials across continents

The project tapped the Tree Diversity Network, a consortium of long-term “outdoor laboratories” where researchers plant seedlings in carefully designed mixtures and monitor them year after year.

For this paper, the team drew on 15 sites spread from temperate Minnesota to subtropical China and Mediterranean Spain. In all, they followed about 100,000 individual trees representing nearly 130 species.

At each site, the scientists recorded which species surrounded every single trunk rather than averaging diversity across a whole plot. That fine-scale approach revealed subtle neighborhood effects that previous broad-brush analyses missed.

“We find a tree growth boost in diverse tree neighborhoods with different species and functional traits – for example, specific leaf area and wood density,” explained postdoctoral fellow Liting Zheng, first author of the paper.

Wet forests gain the most

When the team compared growth rates across climates, a clear pattern emerged.

In forests where annual precipitation is high – places ranging from the Upper Midwest and New England to parts of northern Europe – individuals growing in species-rich neighborhoods gained substantially more wood each year.

Their counterparts surrounded by just one or two species saw much lower growth. The effect weakened, and in some cases disappeared, in drier sites where water limits productivity.

The implication for restoration projects is straightforward. “If you’re thinking of restoring or regrowing, it makes a lot of sense to plant diverse trees, especially in wetter areas,” Reich said.

That “wet” label is broader than many people assume; by the study’s criterion, Michigan’s hardwood forests qualify just as much as a rainforest in Panama.

Extreme weather dulls the effect

The researchers also compared wet and dry years within the same forests. Surprisingly, the diversity benefits all but vanished during unusually rainy years at already-wet forest sites. To Reich, that nuance matters for land-management planning.

“Diversity appears to be better suited for the average condition, which matters for management,” he said.

“There’s thinking that biodiversity is really going to help you when you have those extreme years … but in our data this was not the case.”

Mixing species boosts growth

Ecologists offer several explanations for the wetter-is-better pattern. In moist climates, light and nutrients – rather than water – limit growth.

Different species often excel at capturing those resources at different heights or seasons, so mixing them lets stands use sunlight and soil nutrients more completely. Where rain is scarce, however, all species are constrained by the same shortage, leaving less room for “complementarity” to operate.

Those mechanics have immediate relevance for climate-smart forestry. Fast-growing stands sequester more carbon; stable stands resist pests and windthrow.

Knowing where extra species will pay the biggest dividends helps agencies and private owners target limited replanting budgets. It also highlights the risk of losing diversity in humid regions, as invasive pests, shifting land use, or single-species plantations simplify forests.

Watching forests over time

Because the experimental plantings are young – between four and 14 years old – the authors caution against extrapolating to century-old forests without more data. Still, Reich sees no obvious reason why the basic pattern would reverse as canopies mature.

The team plans to keep monitoring the same plots for decades, turning today’s snapshot into a moving picture of forest dynamics.

Meanwhile the message for managers is already clear. Where rainfall is generous, every extra species in a planting plan represents not just a hedge against future uncertainty but an immediate gain in wood volume and ecological function.

In drier zones, diversity remains valuable for reasons such as disease resistance and wildlife habitat, but the growth payoff will likely be smaller.

“This provides a more nuanced insight into how biodiversity works in different environments,” Reich concluded.

The study is published in the journal Nature Ecology & Evolution.

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