Grazing limits natural biological controls of woody encroachment in Inner Mongolia Steppe

Abstract

Woody 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.

Using an integrated social-ecological analysis to detect effects of household herding practices on indicators of rangeland resilience in Mongolia

“…Shrubs were associated with both F1ResRet and F2FunctDiv, and are important components of many Mongolian steppe ecosystems, their presence driven largely by climate and soil texture [61], although some species, such as Caragana microphylla, are associated with heavy grazing [62][63][64]. Many species of shrubs and sub-shrubs are important to pastoral production as protein-rich forage for browsing species (goats, camels) and as fuel for pastoral households.…”

Reproductive effort of Caragana microphylla under long‐term grazing exclusion in a semi‐arid grassland

“…The first was that the long‐term grazing exclusion increased plant diversity and plant cover (Good et al., 2013; Zhao et al., 2011) compared with the continuous grazing treatment, resulting in strong interspecific competition. Such strong competition could lead to reduced resource uptake for C. microphylla shrubs, thereby inhibiting their growth, resulting in decreased relative biomass (Guo et al., 2017). Thus, the decrease in sexual allocation, flower numbers, flower weight, fruit set and seed weight, and thereby low sexual reproduction might be due to insufficient nutrition uptake.…”

“…Therefore, grazing exclusion has been applied in many grasslands to increase herbaceous plant cover and prevent shrub expansion. Studies have shown that grazing exclusion could reduce abundance (Guo et al., 2017; Kirkpatrick & Bridle, 2016; Saatkamp et al., 2018; Xiong et al., 2011; Zhang et al., 2017) and dominance (Good et al., 2013) of shrub plants. However, the mechanism behind this remains unclear.…”

“…C. microphylla is the main shrub species in semi‐arid steppe in Inner Mongolia. Previous studies suggested that grazing exclusion could decrease the relative biomass of C. microphylla , and thereby preventing the expansion of this shrub species (Guo et al., 2017). But little is known about the underlying mechanisms.…”

Patterns and determinants of woody encroachment in the eastern Eurasian steppe

“…Fire and overgrazing significantly reduce the aboveground biomass, and livestock grazing preferences may also cause shrubs to become the dominant species of grassland communities. However, grazing also reduces the presence of rodents on grasslands, thus leading to more complex changes in the vegetation composition of grasslands (Calisher et al, 2005;Guo et al, 2017;Valone & Thornhill, 2001). The possible existence of different limiting factors for grasses and shrubs adds complexity to the uncertain conversion between two growth forms.…”