3.2 Effects on co-occurring plants
It is commonly believed that toxic weeds have negative effects on the
quantity of forage via allelopathy, thereby decreasing grassland
productivity (Pan et al. 2015). However, toxic weeds actually
provide biotic refuges and keep surrounding herbaceous species away from
livestock in overgrazed grasslands (Fig. 4). The number of species and
the coverage of neighbouring plants are noticeably higher in plots with
toxic weeds than in those in open grasslands (Cheng et al.2014b). There are two principal means by which toxic weeds can
facilitate the proliferation of neighbouring plants in overgrazed
pastures. First, the toxic smell could repel livestock and thus reduce
the ingestion and trampling of edible forage surrounded by toxic weeds
(Oesterheld & Oyarzabal 2004). Second, toxic weeds alter the
surrounding micro-environmental conditions. For example, toxic weeds can
redistribute soil nutrients, form fertility islands (Sun et al.2009) and create a cool environment that promotes soil moisture
retention via the height of the plant canopy (Rebollo, Milchunas &
Chapman 2002). All of these micro-environmental changes provide better
soil conditions and microclimates for plant growth. Additionally, the
niche overlap between toxic weeds and fine herbage is smaller than that
between toxic weeds and unpalatable weeds, reflecting the lower degree
of competition between toxic weeds and edible forage (Ren, Zhao & An
2013).