Since then the grasslands have been the subject of an ecological murder mystery – why was such a dramatic invasion able to take hold, and what factors have sustained it? In our research, we ask how the invasion of non-native grasses has altered carbon cycling and storage. Broadly, we sought to understand how invasion biology and large-scale land-cover change has the potential to affect the greenhouse-gas content of the atmosphere, and thus global climate change.

To this end, we compared carbon-cycling processes between native and non-native grasses, using two study sites in coastal northern California where native perennial grasses still exist in large patches alongside non-natives. On average we found that grassland invasion has caused a drop in carbon storage of 40 Mg per hectare in the top half-metre of soil. The cause relates to differences in life-cycle strategy between the non-native grasses, which are annuals, and the perennial native grasses, plus the climatic constraints of an extended summer drought.

To survive the long dry-season, native perennial grasses possess long roots to tap into deep soil-moisture reserves. Many species also have a dense above-ground structure to prevent soil evaporation and conserve water inputs for use in summer. In contrast, non-native annual grasses grow only when soil moisture is available; they germinate when rains begin each autumn and die with the onset of summer drought. Their growth is primarily above ground, with little investment in deep roots and a relatively sparse above-ground architecture.

Our work revealed that the same traits that evolved in response to seasonal water scarcity have resulted in greater carbon storage in soils where perennial grasses are found. Because they are alive year-round, native grasses have higher annual growth rates, resulting in greater soil carbon storage when plant tissues are shed each year. They also have lower soil carbon loss rates relative to growth than non-native grasses, thanks to the influence of contrasting water conservation strategies on soil respiration. In summary, our study reveals that beyond impacts on native biodiversity, broad-scale land-cover change can have implications for climate as well.