Aim

Grasses comprise three main photosynthetic pathway variants (C₃-BOP, C₃-PACMAD and C₄-PACMAD hereafter referred to as C₄). 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 C₃-BOP, C₃-PACMAD and C₄ photosynthetic pathway variants.

Results

C₄ grasses exhibited the warmest temperature niches on average, but did not differ substantially in rainfall niche versus C₃-BOP grasses. C₃-PACMAD species averaged high suitability across a broad range of temperatures and rainfall conditions, except extreme aridity. In response to projected climate change, C₄ grasses had projected range increases. C₃-BOP grasses typically responded with net range decreases, while C₃-PACMAD grasses had variable range responses. However, patterns were contingent on elevation: for instance, the projected expansion of C₄ grasses was generally limited to elevations below 2000 m, with the largest increases in areas up to ~750 m. Areas of greatest reduction for C₃-BOP and C₃-PACMAD were projected at 750–1900 m and 100–1100 m elevation, respectively. Above 2000 m, range increases were projected for both C₃ grass variants.

Main Conclusions

Our projections suggest that non-native C₄ grasses pose the greatest risk for increasing spread and impacts under RCP 4.5, while certain C₃-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.