Summary
A bioRxiv preprint analysing more than two million tree measurements finds canopy height mostly increased across eastern US ecoregions, while changes varied in the West. Shifts in tree populations and in how tree dimensions relate to one another both contributed.
A bioRxiv preprint analysing more than two million tree measurements from about 60,000 remeasured forest-inventory plots reports that canopy height increased in nearly all eastern US ecoregions between about 2003 and 2025. In western ecoregions, the direction of change varied. The study attributes these patterns to two influences: changes in the mix and sizes of trees, and changes in the relationship between tree dimensions.
The researchers examined non-plantation forests across 19 ecoregions in the contiguous United States. They compared changes in canopy height with changes in aboveground live tree biomass, and assessed how demographic and allometric shifts contributed to canopy-height trends. Demographic shifts include changes in tree size distributions and in the abundance of short- versus tall-statured species. Allometry describes how traits such as height relate to other measures of tree size.
Tree populations and growth patterns had different effects
Demographic shifts tended to raise canopy height in most eastern ecoregions, but lower it in most western ones. Allometric shifts, averaged across species, had a different pattern: they increased mean canopy height or produced no significant change at the ecoregion level.
The distinction matters because a taller canopy can result from changes in which trees are present, changes in their sizes, or changes in the height-to-size relationship. The study found that aboveground live tree biomass was strongly correlated with canopy height across ecoregions, and that changes in biomass were strongly correlated with changes in canopy height over time. The authors say accounting for allometric shifts is important to avoid biased estimates of canopy-height and biomass change.
Findings on water stress were mixed
The researchers tested predictions from the hydraulic limitation hypothesis, which links water stress to limits on tree height. Several results aligned with it: across ecoregions, canopy height was positively correlated with precipitation and negatively correlated with vapour pressure deficit, a measure related to atmospheric dryness. Canopy-height change over time was also positively correlated with changes in precipitation. Within species, allometric shifts were more positive for understory trees than for canopy trees.
However, allometric shifts tended to be positive even in ecoregions where precipitation declined and vapour pressure deficit increased. The authors suggest that increased water-use efficiency associated with rising carbon dioxide could be one explanation, but the abstract presents this as a possible cause rather than a confirmed mechanism.
The results come from a preprint analysis of forest inventories in the contiguous United States. They show why canopy height and biomass trends can move together while still being shaped by changes in forest composition and tree growth patterns.