Intra-canopy leaf trait variation facilitates high leaf area index and compensatory growth in a grassland woody-encroaching shrub

Background/Question/Methods

Intra-canopy leaf trait variation enables plants to take advantage of large gradients of light availability that exist across canopies of high leaf area index (LAI), allowing for increased net carbon gain while reducing light availability for understory competitors. This is achieved by varying leaf morphology to increase photosynthetic capacity under high-light conditions and decrease light compensation point under low-light conditions. While these canopy dynamics are well understood in forest ecosystems, studies of canopy structure of woody shrubs in grasslands are lacking, despite many species developing dense canopies with LAI values that exceed most temperate deciduous forests. To evaluate the investment strategy used by these shrubs, we investigated the vertical distribution of leaf traits and physiology across clonal canopies of Cornus drummondii, the predominant woody encroaching shrub in the Kansas tallgrass prairie. We also examined the impact of browsing and grazing on these factors. We hypothesized that (1) the high LAI values of C. drummondii are facilitated by large plasticity in leaf morphology, (2) changes in leaf morphology will lead to large differences in leaf physiology that maximizes net carbon gain, (3) C. drummondii will modify leaf morphology and physiology in response to browsing and grazing, leading to greater photosynthetic rates.

Results/Conclusions

Our results revealed that C. drummondii canopies had a mean LAI of 8.0 and reduced light availability by 98% between the top and bottom of the canopy, despite having heights of only 2-3 meters. Leaf mass per area (LMA) and leaf nitrogen per area (Na) varied ~3-fold across canopies of C. drummondii, resulting in major differences in the physiological functioning of leaves. High LMA leaves had high photosynthetic capacity, while low LMA leaves used a novel strategy for maintaining light compensation point below ambient light levels. C. drummondii also modified its vertical allocation of leaf traits in response to browsing, which increased light availability at deeper canopy depths. As a result, LMA and Na increased at lower canopy depths, leading to a greater photosynthetic capacity deeper in browsed canopies compared to control canopies. This response, along with increased light availability, facilitated greater photosynthetic rates and resource-use efficiency deeper in browsed canopies compared to control canopies. Our results give a mechanistic understanding of how C. drummondii facilitates high LAI canopies and a compensatory growth response to browsing—both of which are key factors contributing to the success of C. drummondii and other species responsible for grassland woody encroachment.