Formidable Forest Shrubs Affect Tree Height and Biomass

Rhododendron blossoms are lovely, but the shrub can stunt trees and out-compete trilliums, ferns, and other understory species. CC0 photo.

Forests are made of three-dimensional life forms that constantly interact with each other and with the abiotic factors in their environment.

A recent study shows how complex those interactions can be.

“Trees growing within an evergreen shrub layer can be almost 20 feet shorter than trees without a shrub layer,” says USDA Forest Service scientist Katherine Elliott.

Evergreen shrubs such as rhododendron (Rhododendron maximum) and mountain laurel (Kalmia latifolia) are known suppress forest regeneration. The new study shows that these shrubs also suppress tree height and biomass – even in trees established before the evergreen shrub layer.

Paul Bolstad, a professor at the University of Minnesota, led the study, which was published in the journal Ecosphere.

The scientists combined field measurements and remote sensing data. “When we started this study in 2013, we wanted to show that LiDAR could be used to quantify forest structure in montane deciduous forests,” says Elliott. At the time, no one knew whether LiDAR could be used in complex, mountainous terrain. Since then, just a few other studies using LiDAR in deciduous forests have been published.

“Our study clearly shows that LiDAR can accurately measure tree canopy height in complex terrain,” says Elliott. “Coweeta is a great place to test this because we have numerous field-measured plots that can be directly compared to LiDAR measurements.”

The study site at Coweeta Hydrologic Laboratory has permanent long-term research plots: 282 of them were recently remeasured and used for the study. By combining LiDAR and other remote sensing tools, the scientists more fully understand the influence of the evergreen shrub layer and mountain terrain on forest structural components, including tree height, biomass, and leaf area.

The study shows that rhododendron and mountain laurel understories are associated with shorter trees and less tree growth, biomass, and leaf area. Trees that became established even before the understory subcanopy are shorter and have lower biomass than trees of the same age without an evergreen understory.

rhododendron thicket
Rhododendron forms dense thickets that are hard to walk through. Photo by James St. John, CC 2.0.

Terrain also affects tree species composition and growth and can mediate some of the shrub layer’s negative effects. For example, coves have concave terrain where soil moisture tends to accumulate. Trees growing in a cove, even with an evergreen shrub layer, are taller than trees growing on sideslopes and ridges.

Cove trees with a shrub layer are eight feet shorter, on average, than cove trees without a shrub layer; whereas, trees on ridges with convex terrain can be more than 20 feet shorter with a high density evergreen layer compared to trees on ridges without a shrub layer.

Previous research from Coweeta by Elliott and Vose has suggested that the loss of American chestnut in the 1930s led to rhododendron’s current distribution and abundance.

Dense evergreen shrub thickets can make it hard to enjoy a forest. They also create a bottleneck to successful establishment of a diverse understory. “Neither tree seedlings nor herbaceous plants regenerate under dense rhododendron thickets,” says Elliott. “If you walk through or under a dense evergreen subcanopy, somewhat of a difficult task, you will notice very few plants growing there compared to a forest without evergreen shrubs.”

Read the full text of the article.

For more information, email Katherine Elliott at

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