Plant naturalizations are constrained by temperature but released by precipitation

Abstract

Aim: During naturalization, many species undergo climatic niche expansion, in which they spread into climates with which they have not been associated previously. This suggests that species are absent from some climatically suitable areas in their native range, but the reason for this is unknown. We aimed to evaluate whether the climatic conditions in which expansion occurs provide information about the causes of niche expansion. Location: Global. Time period: Contemporary. Major taxa studied: Terrestrial plants. Methods: We compiled native and naturalized occurrence data for 606 terrestrial plant species and compared their native and naturalized climatic niches to detect evidence of climatic niche expansion. Where species showed evidence of niche expansion, we used a variety of circular modelling techniques to investigate further whether species were more likely to expand, or expand further, along some climatic axes than others. We also asked, with or without expansion, whether species were more successful at colonizing the hottest, coldest, wettest or driest portions of their potential niche. Results: We found climatic niche expansion in 45% of naturalizations of 606 terrestrial plants. Species expanded predominantly into wetter climate than their native niche, somewhat less frequently into drier climate, and only in rare instances into hotter or colder climate. Species were least likely to naturalize in the hottest or coldest portions of their native climatic niche. Main conclusions: Our results could suggest that the wetter margins of native niches are limited by biotic interactions that are relaxed in the naturalized range. Our results could also suggest that evolutionary adaptation to novel precipitation regimes is occurring, and/or there are time lags caused by slow population growth rates in cold and hot conditions. Regardless of the explanation, range margins associated with precipitation might be the least predictable during naturalization or environmental change.