Some effects of aspect on grassland responses to elevated CO2

Grasslands are distributed globally across various topographies including hilly and mountainous areas where aspect (i.e. the direction that a slope faces) influences the amounts of radiation and consequent effects on temperature and soil moisture, all of which are important drivers of plant growth. In this experiment we tested the aboveground growth response to an important driver of climate change – elevated CO2 (eCO2)- of two temperate grass species grown under simulated north (unshaded) and south (shaded) aspects. We utilised the long-running New Zealand Free-Air CO2 Enrichment (FACE) experiment and established turves of Lolium perenne L. and Agrostis capillaris L. (two common temperate pasture species). The aboveground dry matter (DM) was regularly harvested over approximately 9 months. For the main effects, there was no overall response to CO2 but Agrostis produced about 50% more DM than Lolium while the north aspect produced about 15% more DM than the south. There was an interaction between CO2 level and aspect: for both species production was increased about 20% under eCO2 on the north aspect but not on the south aspect. Given that a large proportion of the world’s grasslands are on slopes and hence plant growth is likely to be influenced by aspect, this interaction causes us to reconsider the up-scaling of the CO2 responses from FACE experiments that have been universally carried out on flat terrain. In addition, this data will enable better modelling of the response of hill country pastures to climate change.