Distance-dependent shifts in net effects by an unpalatable nettle on a palatable plant species

We examined whether the relative importance of facilitation and competition effects by an unpalatable perennial (Urtica thunbergiana) on a palatable annual (Persicaria longiseta) change with the spatial distance between them in a long-term deer-grazing habitat. Survivorship, growth, size, and fecundity of Persicaria were recorded at 1–2-month intervals during growing seasons in 2 successive years and were compared among individuals located in different positions relative to the canopy of Urtica: at the centre, internal edge, external edge, and far from the canopy. Survivorship of Persicaria was significantly higher at the centre of the Urtica canopy than outside the canopy early in the growing season in both years. No significant differences in Persicaria growth were observed among the four positions in most periods, except in one when growth was significantly higher at the centre, internal, and external edges of the canopy compared to outside the canopy. We found spatial shifts in the net effects of Urtica on Persicaria fecundity, from positive effects under the canopy centre to negative effects under the external edge of the canopy in the first year, and from negative effects under the centre to positive effects under the external edge in the second year. These results demonstrate that the relative importance of positive and negative effects of Urtica on Persicaria vary temporally within and among years and spatially around a single Urtica plant. Spatiotemporal variation in plant interactions may be attributable to annual and seasonal variation in vegetation productivity and grazing pressure.

► We examined the net effects by an unpalatable plant [UP] on a palatable plant.
► Spatial pattern of the net effects under UP canopy was analysed.
► The net effects shifted from positive to negative even under UP canopy.
► This means relative importance of positive and negative effects changed spatially.