Lauren Jenkins, Duke University, discusses her article: Fecundity and terrain-climate interactions that shape tree niche differences across North America and Europe

Terrain shapes tree reproduction

When you think about climate change threatening forests, you may picture species migrating miles northward or upslope. But what if refuge is just around the hillside? In forests of the northern hemisphere, slopes facing northeast are often cooler and moister than their south-facing counterparts. These small-scale differences in sunlight, water drainage, and terrain create a patchwork of microclimates, and this affects tree reproduction.

In our study of nearly 3 million tree-years across 292 species in North America and Europe, we show how subtle topographic features can shape where and how successfully trees reproduce under a changing climate. Our data come from the MASTIF network, a long-term monitoring effort that tracks seed production using two methods: seed traps that physically catch seeds, and crop counts, where observers directly count seeds on individual trees. Together, these give us individual tree estimates of seed production across hundreds of species, at the scale needed to detect terrain-driven patterns.

Terrain effects vary by region

Reproduction is vital for population persistence. Trees may survive drought, but without viable seed production, populations won’t persist. Terrain shapes this critical demographic process in regionally distinct ways. In dry parts of the southeastern US and western Europe, trees produce more seeds on cool, moist northeast-facing slopes. But in the arid southwestern US, high solar radiation on south-facing slopes stimulates seed production.

Species partition the terrain mosaic

If all species responded identically to terrain, they would compete intensely for the same microsites. Instead, we find that species partition the landscape through their reproductive responses. Some species maximize seed production on ridgetops, others in bottomlands, some on steep north-facing slopes, others on gentle southern exposures.

Surprisingly, the southeastern US, with subdued terrain relative to western North America and southern Europe, shows the greatest diversity in these responses. We quantify this partitioning using “community hypervolume”, a measure of how much niche space species collectively occupy. The Southeast’s exceptionally high hypervolume means its tree community spans a wider range of terrain responses than any other region. This diversity may be why it supports such remarkable tree biodiversity.

Not all mountains offer refuge

Not all regions have equal refuge potential. The rugged southwestern US supports terrain-sensitive species but lacks a community-wide diversity of responses. This tells us that simply having mountains isn’t enough; it’s about the match between landscape variation and species’ sensitivities.

As climate warms, topographically diverse regions may offer built-in climate refuges where populations can shift locally, an alternative to long-distance migration.

Understanding these reproductive differences helps us identify which landscape features matter most for conservation. In regions where terrain buffers reproduction from climate stress, preserving topographic diversity (e.g., ridges and valleys) becomes as important as preserving area.