Effects of competition and habitat heterogeneity on native‐exotic plant richness relationships across spatial scales

Aim The biotic resistance hypothesis posits that greater native species richness limits invasions of exotic species. However, negative native-exotic relationships (NERRs) may reverse with increasing spatial scale, seemingly refuting the hypothesis. Here, we explore effects competitive interactions, environmental factors, habitat heterogeneity and vertical vegetation tiers on NERRs across scales in forests. Location New Zealand. Methods We combined vegetation, land cover climate data to predict from at different (ground canopy), cover, plant competition (tree basal area, ground cover), mean annual temperature total rainfall. considered four scales, single 20 × m plots an 8-km grid groups grids up 128 km. Habitat was measured using variance climatic conditions among within a group. Results A tree-exotic relationship (NTERR) observed plot level but reversed scale. Species interactions showed small/intermediate (≤32 km). Rainfall contributed positive NTERR largest Adjacent grassland had all did not prevent reversing. Main conclusions Our analysis shows importance considering structure adjacent covers, along when testing forest ecosystems. There is clear indication primarily driven by trees overstorey, probably limiting resources available tier plants. results support notion NERR interspecific small larger scales.