Testing Hypotheses for Plant Species Distributions in the Mountains

By Annie Carew, Univ. Maryland Center for Environmental Science

This article, and others in the series "Parks in Science History", was written by a graduate student at the University of Maryland. The articles highlight the roles that national parks have played in the history of science and, therefore, the world's intellectual heritage. More articles and videos will be produced in the future.
An orange sky above hazy mountains with green trees in the foreground
Great Smoky Mountains National Park

NPS Photo

Astride the border of North Carolina and Tennessee is Great Smoky Mountains National Park. It is one of the most-visited national parks in the U.S., and protects a variety of plants, animals, hiking trails, and cultural landmarks.

The park supports hundreds of ongoing scientific research projects on a variety of topics. For example, the All Taxa Biodiversity Inventory (ATBI) has been underway since 1998. It seeks to identify and understand the distribution of all species living in the park; to date, the tally is 19,638 and includes -- at the time of writing -- 1,000 species not previously known to science. Other projects range from studying the genetic diversity of ash trees threatened by the emerald ash borer to the evolution of salamanders. Much of this research is coordinated by the Appalachian Highlands Science Learning Center, which also engages students of all ages in research projects.

The park has also been the site of science in the past. One such scientific landmark is the work of Robert Whittaker, whose PhD dissertation research in the 1940s illuminated the complexity of ecological succession. His work has had a lasting effect on the field of ecology ever since.

Robert Whittaker

In the late 1940s, Robert Whittaker was a PhD student at the University of Illinois. Whittaker’s dissertation and subsequent research established him as an influential community ecologist. Community ecology focuses on the interactions between species that share a habitat. Whittaker’s research examined the relationships between environmental variables and plant communities. His study laid the foundation for the field of plant ecology. It showed that plant communities can be incredibly varied, with nearly limitless possibilities. Because every species has different environmental requirements, no two plant communities are alike.

The Succession Debate

In biology, a “community” describes the group of species that lives in a particular place. Ecological succession describes the ways that biological communities change over time. Species come and go, populations grow and decline, and the character of the community shifts. The nature and result of ecological succession was the subject of much debate in the first half of the twentieth century.

Two schools of thought on ecological succession were prevalent . One camp, led by ecologist Frederic Clements, believed that communities changed over time as predictably as individual organisms develop from an egg to an adult. Eventually, the community would reach a stable mix of species regardless of which species were present in the beginning. The other side, headed by botanist Henry Gleason, had a more complicated explanation of succession. Gleason thought that each species preferred different environmental conditions. Therefore, each ecological community is a unique combination of species best suited to those conditions.

Robert Whittaker’s research on plant communities worked towards resolving this debate. Whittaker initially hypothesized that groups of species were “co-adapted” and would be distributed similarly along environmental gradients. That is, if you hiked up several different trails in the park, you would see the plant community change in the same way on all the trails as you gained elevation. This hypothesis was in line more with Clements’s ideas about how succession works. However, Whittaker’s findings showed that different species changed in abundance in different ways. This resolved the debate in favor of Gleason’s theory, and opened the door for data-driven research on species distribution.
Trees with green foliage with a golden meadow and elk in the foreground

NPS Photo

Plants in the Smoky Mountains

Great Smoky Mountains National Park was an ideal location for Whittaker’s research. The diversity of plant species in the Smokies and large changes in elevation provided Whittaker with a lot of sites that varied widely in elevation, temperature, and moisture. The variety of conditions allowed Whittaker to closely examine how the environment affects the distribution of many plant species. Whittaker expected to find that species had parallel distributions, meaning that species were grouped together in favor of certain conditions.

Whittaker’s study began with plots along an elevation gradient. In each plot he counted every tree, measured its diameter, and noted its species. He also randomly selected trees and measured their heights. In smaller plots, he measured the growth of all the mosses, herbs, and shrubs. From these data, Whittaker determined how plants change with environmental factors.

The results of Whittaker’s study indicate that plant species respond to the environment in different ways. Every species has particular moisture, light, and temperature requirements. Because these factors depend on elevation and compass direction (e.g. north-facing slope vs. south-facing slope), no two sites have exactly the same group of plants.


Whittaker began with a hypothesis that was consistent with Clements’s ideas about succession. However, the conclusions of his dissertation research rejected Clement’s ideas in favor of Gleason’s. Plants are distributed individually along environmental gradients, and every plant community is different. Whittaker’s assertion that plant species have continuous distributions unrelated to each other is widely accepted to this day.

It took a few years for Whittaker’s dissertation results to be published and accepted by the scientific community, but he remained busy. For several years, he returned to the Smokies every summer to conduct research. He was interested in the plant growth of forest communities, and he again relied on the park’s elevation gradients to test ideas about forests.

Beyond studying plant communities in the Smokies, Whittaker also investigated ecological succession and plant diversity in the Rogue-River Siskiyou National Forest and the Oregon Caves National Monument. He did more park-based research by studying the ecology of the saguaro cactus in Saguaro National Park. And he’s also famous for creating the five-kingdom classification system of all living organisms (Bacteria, Protists, Fungi, Plants, and Animals) that millions of people have studied in school. Scientists today no longer use his five-kingdom system, but it remains influential for understanding the diversity of life on Earth.

Robert H. Whittaker was an important and prolific figure in the field of ecology. His long and illustrious career, filled with important scientific advances and key ecological studies, began with his PhD research in Great Smoky Mountains National Park. Subsequent studies in the Smokies expanded on Whittaker’s work, ensuring that his influence on ecology persists to this day.
Red fruit and green leaves suspended from a branch

NPS Photo


Whittaker, R. H. 1956. Vegetation of the Great Smoky Mountains. Ecological Monographs, 26:1 – 80.

Westman, W. E., Peet, R. K., and Likens, G. E. “Robert H. Whittaker.” Biographical Memoirs: V. 59. Washington, D.C.: The NAtional Academies Press, 1990. 423 - 444. Print.

Last updated: November 7, 2018