Seed supply, drought, and grazing determine spatio-temporal patterns of recruitment for native and introduced invasive pines in grasslands

Aim The rate of grassland invasion by trees depends on the ability of the species to invade, i.e. their invasiveness, and on the susceptibility of the environments to invasion, i.e. their invasibility. Knowledge of the invasiveness of native and introduced tree species and of the environmental factors that contribute to invasibility is necessary to understand landscape evolution and assess required management measures. Our main aim was to explore this by estimating the separate effects of propagule pressure and environmental factors on the spatio‐temporal patterns of sapling recruitment, a key stage in the tree life cycle. Location Causse Mejean calcareous plateau (southern France). Methods The effects of seed supply and environmental variables (grazing, geological substrate, and duration or intensity of drought) on the spatio‐temporal patterns of sapling recruitment were assessed for the native Scots pine (Pinus sylvestris L.) and the introduced black pine (Pinus nigra Arn. ssp. nigra). Estimates were derived by inverse modelling with data of locations and ages of 4‐ to 20‐year‐old saplings and seed‐bearing trees in 32 sites. Yearly indices of drought were derived from a soil–water content model. Results For both species, seed supply was as important as the whole set of environmental factors in explaining sapling recruitment rates. Grazing and the duration of drought from July to August decreased sapling recruitment rates, which were also lower on hard limestone than on dolomite. Dispersal distances and effective fecundities were higher for the introduced P. nigra, which was less susceptible to drought but more affected by grazing than the native P. sylvestris. In grazed grasslands, shrubs facilitated sapling establishment of both species. Main conclusions This study shows how seed supply and environmental factors shape spatio‐temporal patterns of sapling recruitment for trees invading grasslands. Implications for landscape evolution and management, of the difference in invasiveness of the two pine species and of the hierarchy of environmental factors in determining invasibility, are discussed.