Divergent effects of defoliation intensity and frequency on tiller growth and production dynamics of Pascopyrum smithii and Hesperostipa comata

Abstract

Rotational stocking theoretically enables regrazing of regrowth, but regrowth may be limited in xerophytic vegetation. We tested the hypothesis that regrowth would be slow and fixed in Hesperostipa comata (a caespitose, drought‐tolerant grass), while growth would be flexible in Pascopyrum smithii (a rhizomatous, less drought‐tolerant grass) under increasing defoliation and moisture, by assessing tiller growth rates, population dynamics and plant yield on marked plants in a Dry Mixed Grass prairie. Plots were clipped in late summer to simulate a deferred control, or intermittently during the growing season (May–August) at high intensity–low frequency (HILF), low intensity–high frequency (LIHF) or high intensity–high frequency and crossed with two water treatments (ambient and addition) in both a xeric upland and a mesic lowland. Growing season defoliation increased tiller growth rates of P. smithii in the upland, whereas HILF and LIHF reduced growth in the lowland. All defoliation regimes increased tiller growth for H. comata. Tiller populations of H. comata increased with frequent defoliation, while tiller populations of P. smithii decreased regardless of defoliation regime. Frequent defoliation, regardless of intensity, reduced yield relative to the deferred control for both grasses. While water addition consistently increased growth rates and reduced differences in tiller number among defoliation treatments, the regrowth of both grasses remained similar under varied defoliation with ambient moisture. High moisture conditions also promoted regrowth more in P. smithii compared to H. comata. In summary, neither grass species displayed an inherently fixed or flexible tiller or plant yield response in response to defoliation or moisture.

Drought weakens the positive effects of defoliation on native rhizomatous grasses but enhances the drought‐tolerance traits of native caespitose grasses

“…Compared to other plant trait parameters measured, the number of rhizomatous tillers and shoot biomass was more closely linked with clipping treatment under relatively sufficient water conditions. On the one hand, the stable number of tillers is an important tolerance mechanism for maintaining the basic productivity under the drought stress (Busso & Richards, 1995;Zhang & James, 1995); on the other hand, the rhizomatous grasses may inhibit the increasing tillers to reallocate resources to contribute to compensatory growth of leaves (Broadbent et al, 2017;van Staalduinen & Anten, 2005;Zhao et al, 2008). In addition, we found that the number of leaves, root biomass, and rhizome biomass in two rhizomatous grasses correlated with the 100% and 85% water treatments under no clipping treatment, even though the result indicates that the rhizomatous grasses allocated greater biomass to the root system under wet conditions, for capturing more soil nutrient (Shipley & Meziane, 2002).…”

“…These different plant traits may result in various response mechanisms between the two grass types to defoliation and water stress. Present studies showed that the slower tiller regrowth of Hesperostipa species may be better adapted to drought, and the tiller growth rates responded positively to defoliation (Broadbent, Bork, & Willms, ). However, the effects of drought and defoliation on tiller growth rate of rhizomatous grasses are site specific and varied (Bryant, Matthew, & Hodgson, ; N'Guessan, ).…”

Different regrowth patterns after repeated clipping in two Mongolian pasture species

“…In our study, plants were watered every day, but the pot was relatively small (1.5 L vs. 5 L in van Staalduinen and Anten 2005); therefore, plants might have been susceptible to water de cit, and consequently the regrowth of rhizomatous A. cristatum was not enhanced. This speculation should be carefully considered, because very few studies have compared regrowth between rhizomatous and caespitose grasses, and one study showed conversely that the tiller growth rate after defoliation was higher in caespitose Hesperostipa comata than in rhizomatous Pascopyrum smithii (Broadbent et al 2017). Clearly, more studies are needed to understand the difference in regrowth ability between rhizomatous and caespitose species.…”

Belowground mechanism reveals climate change impacts on invasive clonal plant establishment

“…Repeated defoliation may negatively affect seedling establishment, as aboveground productivity and relative growth rates often decline with increasing defoliation frequency within the growing season and in consecutive years 50 . Repeated grazing initially increases P. smithii tiller recruitment 51 , but usually results in reduced tiller densities by the end of the season 52 . Although not seen in this study, clipping may shift biomass allocation belowground in B. inermis 47 , 53 , 54 .…”