Grain size modulates volcanic ash retention on crop foliage and potential yield loss

Abstract

Abstract. Ash fall from volcanic eruptions endangers crop production and food security and jeopardises agricultural livelihoods. As population in the vicinity of volcanoes continues to grow, strategies to reduce volcanic risks to and impacts on crops are increasingly needed. This effort involves the use of quantitative relationships for anticipating crop damage from ash exposure. However, current limited models of crop vulnerability to ash rely solely on ash thickness (or loading) and fail to reproduce the complex interplay of other volcanic and non-volcanic factors that drive impact. Amongst these, ash retention on crop leaves affects photosynthesis and is ultimately responsible for widespread damage to crops. In this context, we carried out greenhouse experiments to assess how ash grain size, leaf pubescence and humidity conditions at leaf surfaces influence the retention of ash (defined as the percentage of foliar cover coated with ash) in tomato and chilli pepper plants, two crop types commonly grown in volcanic regions. For a fixed ash mass load (~570 g m-2), we found that ash retention decreases exponentially with increasing grain size and is enhanced when leaves are pubescent (such as in tomato) or their surfaces are wet. Assuming that leaf area index (LAI) diminishes with ash retention in tomato and chilli pepper, we derived a new expression for predicting potential crop yield loss after an ash fall event. A corollary result is that the measurement of crop LAI in ash-affected areas may serve as a useful impact metric. Our study demonstrates that quantitative insights into crop vulnerability can be gained rapidly from controlled experiments, thereby providing a mean to improve models that can predict ash risks to crops accurately. We advocate this approach to broaden our understanding of ash-plant interaction and to validate the use of remote sensing methods for assessing crop damage and recovery at various spatial and time scales after an eruption.