PARENT SESSION
Poster Session 20: Invasive Species
Wednesday, August 10, 5:00 PM - 6:30 PM, Exhibit Hall 220 A-E, Level 2, Palais des congrès de Montréal

Physiological characteristics of two native invasive trees and two dominant C4 grasses in the Nebraska Sandhills.

Eggemeyer, Kathleen^*,1, Awada, Tala ¹, Wedin, David ¹, Harvey, F. Edwin¹, Zhou, Xinhua¹, ¹ School of Natural Resources, Lincoln, NE, USA

ABSTRACT- The expansion of woody species into the Nebraska Sandhills represents a change in resource acquisition and physiological functions. Changes were assessed by studying the ecophysiology of two native invasive woody species (Pinus ponderosa and Juniperus virginiana) and two dominant C₄ grasses (Panicum virgatum and Schizachyrium scoparium) over 1 year in the field. Winter photosynthesis (Anet) was positive at T>0^oC, averaging 2 and 3 molm^-2s^-1 for P. ponderosa and J. virginiana, respectively, and showing possible carbon gain. Anet peaked in May for P. ponderosa (10 molm^-2s^-1) and J. virginiana (8.8 molm^-2s^-1). A decrease in Anet was observed between May and Aug., before increasing again in Sept. with improved weather conditions. Grasses were measured in early June after emergence. Initial Anet values were 12 and 9 molm^-2s^-1 for P. virgatum and S. scoparium, respectively. Anet declined in Aug. by 75% in P. virgatum and 44% in S. scoparium due to water stress, high radiation and temperatures, and went dormant early in the season. Predawn water potential (pre) coincided with volumetric soil water content, indicating the onset of stress in May, the severe stress in Aug. and the recovery in Sept. The most negative midday water potential (mid) occurred in Aug., with -2.5 MPa reported for the C₄ grasses, followed by -2.0 for J. virginiana and -1.8 for P. ponderosa. ¹³C averaged 26‰ and 14‰ in trees and grasses, respectively. Trees displayed an 8% increase in ¹³C in response to stress, whereas ¹³C showed little directional change in grasses. The access to deep water by P. ponderosa and J. virginiana allowed them to avoid both winter and summer water stresses and to maintain physiological function to be successful invaders into the Nebraska Sandhills. We conclude that climate in the Nebraska Sandhills does not seem to restrict the establishment and expansion of these woody species as previously thought, and this invasion will likely impact ecosystem functions such as ground water recharge and carbon storage.

Key words: Nebraska Sandhills, tree invasion, ecophysiology, warm season grasses