Correlations Between Elevation, Proportion of Coniferous Trees and Shannon’s Diversity Index

Author

Amira Steinberg

Introduction

Elevation is a critical abiotic factor influencing forest structure and species composition across diverse ecosystems. As elevation increases, environmental conditions such as temperature and soil fertility typically decline, creating gradients that shape plant communities. Much research has focused on high-elevation forests, such as those in the Himalayas where species richness and diversity vary between zones (Pandey et al., 2016), For example, the Lower Temperate zone in the Himalayas exhibits the highest Shannon’s Diversity Index and species richness, whereas the Upper Subalpine zone shows the lowest diversity (Pandey et al., 2016).

In temperate forests, the decrease in temperature and nutrient availability with elevation often favors tree species with adaptations to conserve resources. Coniferous trees, with their needle-like leaves that persist year-round, reduce the energetic cost of leaf regrowth compared to deciduous species, providing them an advantage in harsher, high-altitude environments (Bai, 2015). This physiological trait may lead to an increased proportion of conifers at higher elevations.

Furthermore, isolated mountain tops can act as “sky islands,” promoting unique species evolution and biodiversity hotspots (Lomolino, 2008). Understanding how forest composition changes along elevation gradients is therefore important not only for ecology but also for evolutionary biology, conservation and predicting responses to climate change.

This study aims to investigate how forest composition varies with elevation within Centennial Woods, a temperate forest in Burlington, Vermont, where elevation ranges from approximately 200 to 450 feet. Specifically, the study examines two variables: the proportion of coniferous trees and species diversity measured by Shannon’s Diversity Index, which accounts for both species richness and evenness. It is hypothesized that the proportion of coniferous trees will increase with elevation due to their resource-conserving traits, while overall tree species diversity will decline because of the more stressful conditions at higher elevations.

Methods

This study was conducted in Centennial Woods, a temperate forest located in Burlington, Vermont. The region experiences a Humid Continental Climate characterized by cold winters and warm summers. The study area spans an elevation gradient from approximately 200 to 450 feet above sea level.

Six circular plots, each with a radius of 15 meters, were established randomly within Centennial Woods to sample forest composition along the elevation gradient. Plot centers were selected by generating random coordinates on a 5.85-inch by 7.50-inch map of the area. These coordinates were then georeferenced and plotted using ArcGIS software. A digital elevation model of Vermoont available for public use on the Vermont Geodata Portal was added to determine the altitude of each plot’s center, estimated to the nearest 10 feet.

Within each plot, all trees with a diameter at breast height (DBH) greater than 1 inch (2.54 cm) were identified. The DBH of each tree was measured using a DBH tape. The basal area (BA) for each individual tree was calculated using the following equation:

\[ BA = DBH^2 * 0.005454\]

The Shannon’s Diversity Index was then calculated for each plot using the proportional basal area of each species using the following equation:

\[ H = -\sum_{x = 1}^{s} p_{i} lnp_i \]

The proportion of coniferous trees in each plot was calculated as the percent of trees in the plot that were a coniferous species. Linear regression analyses were performed to examine the relationships between elevation and the proportion of coniferous basal area and Shannon’s Diversity Index.

Results

Fig. 1: The percent trees in a 15 meter plot with a DBH greater than 1 inch that are coniferous compared to the estimated altitude of that plot.

The Pearson Correlation Coefficient of this data is 0.18, signifying a weak positive correlation between altitude and proportion of coniferous trees. However, the p-value of 0.74 indicates that this is not a significant relationship.

Fig. 2: The Shannon’s Diversity Index of trees in a 15 meter plot with a DBH greater than 1 inch compared to the estimated altitude of that plot.

The Pearson Correlation Coefficient of this data is 0.35, signifying a weak positive correlation between altitude and proportion of coniferous trees. The p-value of 0.50 signifies that there is no significant correlation.

Discussion

The absence of a significant relationship between elevation and both the proportion of coniferous trees and Shannon’s Diversity Index in Centennial Woods suggests that within this relatively narrow elevation gradient (approximately 200 to 450 feet), elevation alone may not be a strong driver of forest composition or diversity. This contrasts with studies conducted in mountainous regions with much larger elevation ranges, such as the Himalayas or the Rocky Mountains, where harsher abiotic gradients (temperature, moisture, soil nutrients) create more pronounced shifts in species composition and diversity (Pandey et al., 2016; Bai, 2015).

At smaller scales, habitat factors such as soil moisture, disturbance history, light availability and proximity to water sources may exert stronger influences on species composition than elevation. For example, the low-elevation plot had unusually high coniferous proportion and diversity in comparison to the other low-elevation plots. However, this plot was located by a stream, meaning these results likely reflects hydrological conditions rather than elevation effects. The presence of this potential outlier also underscores the need for a larger sample size that was difficult to obtain with the time constraints of this project. Future studies would benefit from increasing the number of plots that were sampled.

In terms of diversity, the relatively stable Shannon’s Diversity Index across elevations suggests that species richness and evenness are maintained throughout this gradient. This stability may reflect the presence of mainly generalist species capable of thriving across a range of conditions or a forest community.

Literature Cited

Bai, K., C. He, X. Wan, and D. Jiang. 2015. Leaf economics of evergreen and deciduous tree species along an elevational gradient in a subtropical mountain. AoB Plants 7.

Lomolino, MARK. V. 2001. Elevation gradients of species‐density: Historical and prospective views. Global Ecology and Biogeography 10:3–13.

PANDEY, K. P. 2016. Structure, composition and diversity of forest along the altitudinal gradient in the Himalayas, Nepal. Applied Ecology and Environmental Research 14:235–251.