Abstract
ABSTRACTWoody encroachment in grasslands has become increasingly problematic globally. Grazing by domestic animals can facilitate woody encroachment by reducing competition from herbaceous plants and fire frequency. Herbivorous insects and parasitic plants can each exert forces that result in the natural biological control of encroaching woody plants through reducing seeding of their host woody plants. However, the interplay of grazing and dynamics of herbivorous insects or parasitic plants, and its effects on the potential biological control of woody encroachment in grasslands remains unclear. We investigated the flower and pod damage by herbivorous insects, and the infection rates of a parasitic plant on the shrub Caragana microphylla, which is currently encroaching in Inner Mongolia Steppe, under different grazing management treatments (33-year non-grazed, 7-year non-grazed, currently grazed). Our results showed that Caragana biomass was highest at the currently grazed site, and lowest at the 33-year non-grazed site. Herbaceous plant biomass followed the opposite pattern, suggesting that grazing is indeed facilitating the encroachment of Caragana plants in Inner Mongolia Steppe. Grazing also reduced the abundance of herbivorous insects per Caragana flower, numbers of flowers and pods damaged by insect herbivores, and the infection rates of the parasitic plant on Caragana plants. Our results suggest that grazing may facilitate woody encroachment in grasslands not only through canonical mechanisms (e.g. competitive release via feeding on grasses, reductions in fires, etc.), but also by limiting natural biological controls of woody plants (herbivorous insects and parasitic plants). Thus, management efforts must focus on preventing overgrazing to better protect grassland ecosystems from woody encroachment.
Highlights
The encroachment of woody plants into grasslands has become one of the most significant biological phenomena globally over the last century (Eldridge et al, 2011; Naito and Cairns, 2011)
We investigated whether natural biological controls of woody encroachment in grasslands, herbivorous insects and parasitic plants on woody plants would be altered by different grazing management approaches
Our results showed that the herbivorous insect abundance per flower, and flower and pod damage by herbivorous insects, in Caragana plants decreased from the 33-year non-grazed site to the 7-year non-grazed site, and to the currently grazed site, suggesting that grazing had a negative impact on the herbivorous insects on Caragana plants, limiting natural biological controls of herbivorous insects on the establishment and expansion of Caragana plants in Inner Mongolia Steppe
Summary
The encroachment of woody plants into grasslands has become one of the most significant biological phenomena globally over the last century (Eldridge et al, 2011; Naito and Cairns, 2011). The effects of grazing by mammalian animals on woody encroachment across different rangeland ecosystems are complex (Van Auken, 2009; Sankaran et al, 2013). Grazing by domestic animals can reduce biomass of herbaceous plants, resulting in increased resource availability for the establishment of woody plants in grasslands (Coetzee et al, 2008). Grazing by domestic animals leads to reduction in fire frequency and intensity by reducing the biomass of herbaceous plants (Oba et al, 2000; Roques et al, 2001), favoring woody encroachment in grasslands
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