The Science

Fire and herbivory restrict survival of most woody plants in grasslands. However, some woody species have strategies to overcome these disturbances. Many shrubs form dense canopies which displace grassland species, resulting in reduced fire intensity. While dense canopies play a key role in the survival of many woody species in grasslands, the mechanisms enabling them to maintain dense canopies are not well understood. In this study, scientists evaluated the vertical canopy structure of rough-leaf dogwood, the predominant encroaching woody shrub in the Kansas tallgrass prairie. The results show that these canopies contain: (1) large vertical variation in leaf morphology and physiology, enabling rough-leaf dogwood to deal with limitations of self-shading to form dense canopies, and (2) temporal variation in leaf traits, allowing rough-leaf dogwood to respond positively to periodic grassland disturbance.

The Impact

In grasslands worldwide, trees and shrubs are increasing at unprecedented rates, causing a loss of grassland ecosystems. In any given grassland, the increase of woody plant abundance is typically the result of a few woody species. Understanding the mechanisms that enable these species to survive in the open grassland is critical to understanding the complex phenomenon of woody plant encroachment. This study reveals the growth investment strategy of rough-leaf dogwood to achieve dense canopies, respond positively to periodic grassland disturbance, and ultimately facilitate successful encroachment in grassland ecosystems.

Summary

Leaf trait variation enables plants to utilize large gradients of light availability that exist across canopies of high leaf area index (LAI), allowing for greater net carbon gain while reducing light availability for understory competitors. To better understand how mesic woody encroaching shrubs achieve high LAI canopies, researchers investigated vertical distribution of leaf traits and physiology across canopies of Cornus drummondii, or rough-leaf dogwood, the predominant woody encroaching shrub in the Kansas tallgrass prairie. This study revealed that leaf mass per area (LMA) and leaf nitrogen per area (N_a) varied approximately threefold across canopies, exceeding that of most deciduous tree species, and leading to large differences in the physiological functioning of leaves in different light environments.

The vertical allocation of leaf traits in C. drummondii canopies was also modified in response to browsing. This response, along with increased light availability, facilitated greater photosynthesis and resource-use efficiency deeper in browsed canopies compared to control canopies. These results illustrate how C. drummondii facilitates high LAI canopies and a compensatory growth response to browsing—two key factors contributing to the success of C. drummondii and other species responsible for grassland woody encroachment.

Principal Investigator

Jesse Nippert
Kansas State University
nippert@ksu.edu

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

This research was funded by the Biological and Environmental Research (BER) program within the U.S. Department of Energy’s (DOE) Office of Science under Award DE-SC0019037, the Konza Prairie LTER program NSF DEB: 2025849, and the Division of Biology at Kansas State University.

References