Seedling defoliation and drought stress: variation in intensity and frequency affect performance and survival

Authors: Denton, Elsie; SMITH, BRENDA - Oregon State University; Hamerlynck, Erik; Sheley, Roger

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/24/2017
Publication Date: 1/3/2018
Citation: Denton, E.M., Smith, B.S., Hamerlynck, E.P., Sheley, R.L. 2018. Seedling defoliation and drought stress: variation in intensity and frequency affect performance and survival. Rangeland Ecology and Management. 71(1):25-34. https://doi.org/10.1016/j.rama.2017.06.014.
DOI: https://doi.org/10.1016/j.rama.2017.06.014

Interpretive Summary: Our ability to restore rangelands is limited, and it is unknown if seedling herbivory on its own, or in interaction with other stressors, is a major contributor to restoration failure. To address this, we conducted two experiments: a No Defoliation experiment where seedlings from three perennial grasses (crested wheatgrass, bluebunch wheatgrass, Sandberg bluegrass) were subjected to wet and dry water regimes for four months, and a concurrent Defoliation experiment where seedlings additionally experienced a single defoliation at an intensity of 30% or 70% (low frequency); half of the seedlings were defoliated again at the same intensity (high frequency) four weeks later. Indicators of seedling performance were above- and belowground biomass, root:shoot ratio, tillering and mortality. Water stress reduced seedling performance across all measures, but less so for above ground biomass when also clipped at high intensity at the 70% level. Intensity and frequency of defoliation interacted so that high intensity and high frequency defoliation caused the largest reductions in performance measures. Crested Wheatgrass (an introduced species) and bluebunch wheatgrass (a native species) responded similarly to combinations of stressors, and were largely resistant to even intense grazing regimes. Sandberg bluegrass (a native) was more sensitive to defoliation and water stress and was the only species to experience extensive mortality. If species respond differently to herbivory at the seedling stage this may have implications for future plant population demography.

Technical Abstract: Our ability to restore rangelands is limited, and it is unknown if seedling herbivory on its own, or in interaction with other stressors, is a major contributor to restoration failure. To address this, we conducted two experiments: a No Defoliation experiment [ND] (n=48) where seedlings from three perennial grasses (crested wheatgrass [Agropyron cristatum (L.) Gaertn.], bluebunch wheatgrass [Psuedoroegnaria spicata (Pursh) Á. Love], Sandberg bluegrass [Poa secunda J Presl]) were subjected to wet and dry water regimes for four months, and a concurrent Defoliation experiment [D] (n=95) where seedlings additionally experienced a single defoliation at an intensity of 30% or 70% (low frequency); half of the seedlings were defoliated again at the same intensity (high frequency) four weeks later. Indicators of seedling performance were above- and belowground biomass (AGB & BGB), root:shoot ratio, tillering and mortality. The effect size statistic, Hedge's g, allowed for comparisons between performance measures. Water stress induced reductions in most performance measures: BGB (g= ND: '1.3; D: '1.6), root:shoot ratio (g= ND: n.s.; D: '0.2) and tillering (g= ND: '1.7; D: '1.2), though not significantly for all species. For ABG water stress interacted with defoliation, reducing performance less at an intensity of 70% (g = '2.0) as opposed to 30% (g = '3.0), but not always significantly in the former. Water stress also caused less reduction in AGB when no defoliation occurred (ND: '0.8; g = D: '2.5). Intensity and frequency of defoliation interacted; Seedlings were generally resistant to reductions in performance except at high frequency, high intensity defoliation. A. cristatum and P. spicata displayed similar sensitivity to treatments, mostly in terms of changes in AGB and BGB, while P. secunda also experienced increased mortality and reduced tillering. If these differences in sensitivity result in differential survival, herbivory could impact species population demographics post-restoration.