Ecophysiology of Two Native Invasive Woody Species and Two Dominant Warm‐Season Grasses in the Semiarid Grasslands of the Nebraska Sandhills

Abstract

Populations of Pinus ponderosa and Juniperus virginiana are expanding into semiarid Sandhills grasslands in Nebraska. To evaluate the physiological basis of their success, we measured the seasonal course of leaf gas exchange, plant water status, and carbon isotope discrimination in these two native trees and two native C 4 grasses (Schizachyrium scoparium and Panicum virgatum). Compared to the trees, grasses had higher net photosynthetic rates (A net ) and water use efficiency (WUE) and more negative predawn and midday water potentials (C) in June and July. While leaf C and rates of leaf gas exchange declined for all four species during August, the C mid of the grasses were significantly more negative than those of the two trees. The deeply rooted trees maintained water status during summer, in contrast to the grasses, which senesced. Juniperus virginiana in particular was well adapted to xeric conditions, with low stomatal conductance, high WUE, and positive A net at low C. The highest values of A net were observed in May for J. virginiana and in May and September for P. ponderosa. Both species maintained low but positive A net throughout the winter at temperatures above 0°C. Leaf carbon isotopic signature differed between tree and grass species but did not exhibit significant withinspecies seasonal variability. The semiarid grassland climate of Nebraska does not appear to limit P. ponderosa and J. virginiana, which use growth during the nongrowing season and access to deep soil moisture to compensate for growing-season drought.

Ecophysiological responses of native invasive woody Juniperus virginiana L. to resource availability and stand characteristics in the semiarid grasslands of the Nebraska Sandhills

“…Understanding the dynamics and peak of these parameters are critical for accurately modeling and forecasting energy budgets and NPP (Hamada et al 2016). J. virginiana is considered a drought tolerant species (Bihmidine et al 2010) and has the ability to maintain stomatal opening and photosynthetic activity at relatively low water potentials (Eggemeyer et al 2006, Willson et al 2008, reaching xylem cavitation and 50% loss of conductivity at Ψ w = -5.8 [MPa] in stems and Ψ w = -4.9 [MPa] in roots (Willson et al 2008), values that were not reached in this study. J. virginiana expands and survives in arid and semiarid environments because it has significant plasticity in water uptake (Eggemeyer et al 2009, Caterina et al 2014) and access to deeper soil layers (roots can reach 7 m deep) where water is available during the periods of low water availability (Walker and Noy-Meir 1982).…”

“…Conversion of C4-dominated grasslands with relatively high photosynthetic capacity, high nitrogen-and water-use efficiencies and short growing season to C 3 woodlands with lower photosynthetic rates, longer growing season, and lower nitrogen-and water-use efficiencies in low to mid-latitudinal regions of the Great Plains of the continental U.S. (Eggemeyer et al 2006, Eggemeyer et al 2009, Awada et al 2013) has implications on NPP, biogeochemical and ecohydrological cycles (McKinley andBlair 2008, Wilcox 2010), and fire regime (D'Antonio and Vitousek 1992, Brooks et al 2004). With respect to ecohydrology, woody species encroachment has been found to modify water distribution in soils through shifts in transpiration rates, more precipitation interception, and less water infiltration, potentially resulting in drier soils (Liao et al 2008, Rout and Callaway 2009, Boutton and Liao 2010, Awada et al 2013.…”

“…Limited studies have addressed the ecophysiology of J. virginiana in the Nebraska Sandhills (e.g., Eggemeyer et al 2006, Eggemeyer et al 2009, Awada et al 2013, and these studies did not investigate the impact of density or changes in plant-soil processes and resource availability associated with encroachment. The Sandhills offer a unique ecosystem due to their economic (i.e., livestock production) and ecological importance, and the potential implications of this encroachment on regional ecohydrology.…”

“…were determined with an infra-red gas analyzer, with a 6 cm 2 chamber mounted with a LED light source (LICOR 6400 -02B; LICOR Inc., Lincoln NE, US). Measurements were conducted at light saturation -PAR was set at 1,500 μmol(photon) m -2 s -1 (Eggemeyer et al 2006, Bihmidine et al 2010) -and were conducted between 10:00 and 14:00 h solar time in winter, and 09:00 and 14:00 h solar time during the growing season. After completion of each measurement, and due to the scale-like foliage of J. virginiana, foliage material in the chamber was collected and measured with a leaf area meter (LICOR 3000; LICOR Inc., Lincoln NE, US) to correct for gasexchange measurements.…”

Species-driven changes in nitrogen cycling can provide a mechanism for plant invasions

“…Specific invader success has been attributed to factors such as specific leaf area (5), water use efficiency (6), resistance to disturbance (7), herbivore resistance (8), escape from natural enemies (9), and plant-soil feedbacks (10,11). However, among invasive species, only a few general plant traits, such as vegetative reproduction, self pollination, phylogeny (i.e., other successful invaders in the family or genus) and native geographic range have been identified (12,13).…”