Abstract
Native sagebrush-steppe bunchgrass populations are threatened by the spread and dominance of exotic invasive annual grasses, in part due to low, episodic seed production. In contrast, the widespread exotic bunchgrass, crested wheatgrass, readily produces viable seed cohorts. The mechanisms underlying these differences are unclear. To address this, we measured seed head specific mass (gm-2) and net photosynthetic assimilation (A net) as a function of internal [CO2] (A/Ci curves) in pre- and post-anthesis seed heads and flag leaves of crested wheatgrass and four native bunchgrasses to determine if differences in allocation and photosynthetic characteristics of seed heads was consistent with differential reproductive success. Crested wheatgrass seed heads had 2-fold greater specific mass compared to the native grasses, concurrent with greater CO2-saturated photosynthesis (A max), mesophyll carboxylation efficiency (CE), and higher intrinsic water-use efficiency (WUE i ; A net/stomatal conductance (g s)), but with similar relative stomatal limitations to photosynthesis (RSL). Post-anthesis seed head A max, CE, RSL and g s decreased in native grasses, while crested wheatgrass RSL decreased and CE increased dramatically, likely due to tighter coordination between seed head structural changes with stomatal and biochemical dynamics. Our results suggest native sagebrush-steppe bunchgrasses have greater stomatal and structural constraints to reproductive photosynthesis, while the exotic grass has evolved seed heads functionally similar to leaves. This study shows elucidating reproduction-related ecophysiological mechanisms provide understanding of plant attributes that underlie restoration success and could help guide the development of native plant materials with functional attributes needed to overcome demographic bottlenecks that limit their restoration into degraded sagebrush-steppe.
Highlights
Conservation of ecosystem function is critical in the face of ongoing climate change, and a major challenge in ecosystem conservation is maintaining viable plant populations and diverse plant communities (Schwartz et al 2000; Luck et al 2003)
All plants in the study area were established from existing local seed sources; as such, we could not ascertain what cultivar of crested wheatgrass is present in the study area
Flag leaf an indicator of maximum photosynthetic capacity (Amax) was greatest in prairie junegrass (33.3 μmol m−2 s−1 ± 4.26 SE), significantly higher than in squirreltail (24.9 μmol m−2 s−1 ± 3.86 SE), crested wheatgrass (24.9 μmol m−2 s−1 ± 2.01 SE) and basin wildrye (22.9 μmol m−2 s−1 ± 2.42 SE)
Summary
Conservation of ecosystem function is critical in the face of ongoing climate change, and a major challenge in ecosystem conservation is maintaining viable plant populations and diverse plant communities (Schwartz et al 2000; Luck et al 2003). This is very much the case for North American sagebrush-steppe rangeland ecosystems, which face increasingly extensive degradation due to the combined effects of invasive annual grasses and ongoing climate change (Davies et al 2011; Svejcar et al 2017). In sagebrush-steppe, the rate invasive annual spread and accelerated fire frequency so outstrips the natural rate of recovery, and restoration is effectively the only means of conservation (Davies et al 2011). Elucidating the differences in the biophysical and physiological mechanisms between successful and less successful restoration species is critical to formulating effective ecosystem conservation strategies in these ecologically and economically important rangelands (Davies et al 2011; Madsen et al 2014; Svejcar et al 2017)
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