Predicted Range Shifts of Non‐Native Grasses in Response to Climate Change Are Influenced by Photosynthetic Pathway: A Case Study in the Hawaiian Islands

ABSTRACT Aim Grasses comprise three main photosynthetic pathway variants (C 3 ‐BOP, C 3 ‐PACMAD and C 4 ‐PACMAD hereafter referred to as C 4 ). We sought to confirm climate niche differences among these photosynthetic pathway variants and assessed whether predicted non‐native grass range shift patterns with climate change differ among photosynthetic pathway variants. Location Hawaiian Islands. Methods We used a species distribution modelling (SDM) approach that uses global occurrence records to inform local SDM based on local (Hawaiian Islands) occurrences. We compared climate niches and projected climate‐driven range shifts, assuming moderate climate change (RCP 4.5, end of century), among 22 non‐native grasses representing C 3 ‐BOP, C 3 ‐PACMAD and C 4 photosynthetic pathway variants. Results C 4 grasses exhibited the warmest temperature niches on average, but did not differ substantially in rainfall niche versus C 3 ‐BOP grasses. C 3 ‐PACMAD species averaged high suitability across a broad range of temperatures and rainfall conditions, except extreme aridity. In response to projected climate change, C 4 grasses had projected range increases. C 3 ‐BOP grasses typically responded with net range decreases, while C 3 ‐PACMAD grasses had variable range responses. However, patterns were contingent on elevation: for instance, the projected expansion of C 4 grasses was generally limited to elevations below 2000 m, with the largest increases in areas up to ~750 m. Areas of greatest reduction for C 3 ‐BOP and C 3 ‐PACMAD were projected at 750–1900 m and 100–1100 m elevation, respectively. Above 2000 m, range increases were projected for both C 3 grass variants. Main Conclusions Our projections suggest that non‐native C 4 grasses pose the greatest risk for increasing spread and impacts under RCP 4.5, while certain C 3 ‐PACMAD grasses may endanger valuable high‐elevation habitats. Photosynthetic pathway may be a useful component of weed risk assessment to evaluate how species may respond to climate change as similar range response patterns may be expected for other non‐native grasses in other tropical and subtropical regions.