COS 233-4 - Diversity alters allometry: measuring tree growth in a forest diversity experiment

Background/Question/Methods

Understanding how diversity affects tree growth is important for predicting successional dynamics and ecosystem services, like carbon sequestration. Here we examine how tree species and diversity are related to tree size in a young forest. We also assessed the relationship between tree height, area and volume and the relationship between size metrics over time. We expected that both tree species identity and diversity would be important in predicting tree size, and that, due to the transition from open to closed canopy conditions, tree volume metrics might lag behind tree height metrics.

Growth was monitored on a cohort of ~540 trees at the BiodiversiTREE forest diversity experiment at the Smithsonian Environmental Research Center in Edgewater, Maryland. The experiment was planted in 2013 and includes 16 species native to Maryland. Trees are planted in monoculture plots, 4 species polyculture plots and 12 species polyculture plots. We measured a cohort of focal trees five times between 2016 and 2021. Tree volume metrics included basal area at root collar (0.1 m) and breast height (1.3 m), trunk volume and canopy volume. With this dataset we were able to compare tree height and volume across species and diversity levels in this early successional forest.

Results/Conclusions

Tree species identity and plot diversity were both significant, interacting factors in predicting tree height, basal area and volume. Tree volume diverged by plot diversity in trees over 500 cm, with trees in higher diversity plots reaching greater volumes compared to trees of a similar height in monoculture plots. We see different patterns of growth across the diversity treatments, and size metrics that lead or lag over time. In the early years of the experiment, trees in monoculture were taller, on average, across species, with this gap closing over time. With volume metrics, we see the opposite pattern; trees in polyculture increased in volume at a greater rate than trees in monoculture. Trees in 4 species polyculture plots that have not reached 500 cm in height are smaller across volume metrics, possibly due to selection effects.

These results underscore the importance of considering diversity as well as species in allometric equations –and biomass and carbon estimates—and illustrate how the value and interpretation of different size metrics depends on the age of the forest.