Agriculture is an economic and social pillar for the least developed countries. When these regions host volcanoes that exhibit explosive behaviour, a serious risk for agricultural production arises as crops endure various impacts from tephra fall. In order to gain new insights into the factors that govern tephra impacts on crops, we collected farmers’ perceptions of crop damage and production loss due to exposure to tephra fallout in 15 villages affected by the 1999–2014 eruptions of Tungurahua volcano, Ecuador. Crop type and developmental stage - both influenced by altitude - strongly modulate the level of tephra-induced impact. Using these observations, we illustrate how crop vulnerability fluctuates spatially and temporarily in the surveyed area. The study also highlights that fine tephra (<63μm) is more harmful to crops than coarser particles. Farmers have responded to the tephra hazard by favouring crops more resistant to tephra, a practice that has reduced crop diversity.

Abstract. Ashfall from volcanic eruptions endangers crop production and food security while jeopardising agricultural livelihoods. As populations in the vicinity of volcanoes continue to grow, strategies to reduce volcanic risks to and impacts on crops are increasingly needed. Current models of crop vulnerability to ash are limited. They also rely solely on ash thickness (or loading) as the hazard intensity metric 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 plants) or when their surfaces are wet. Assuming that leaf area index (LAI) diminishes with ash retention in tomato and chilli pepper plants, we derived a new expression for predicting potential crop yield loss after an ashfall event. We suggest that the measurement of crop LAI in ash-affected areas may serve as an impact metric. Our study demonstrates that quantitative insights into crop vulnerability can be gained rapidly from controlled experiments. We advocate this approach to broaden our understanding of ash–plant interactions and to validate the use of remote sensing methods for assessing crop damage and recovery at various spatial and time scales after an eruption.

Crops are regularly impacted by tephra from explosive volcanic eruptions, causing significant economic losses and jeopardizing farmers’ livelihood at the local to regional scales. Crop vulnerability to tephra remains poorly understood, impeding the construction of robust risk models for agriculture. Previous studies of crop vulnerability to tephra are semi-quantitative and consider tephra accumulation as the only hazard intensity metric. Here, we provide a robust evaluation of crop vulnerability based on the analysis of 700 sets of quantitative data, allowing for the assessment of the influence of various volcanic and non-volcanic factors. We collected farmers’ perceptions of damage to fodders, root and tuber crops, leafy crops, legumes, cereals, tree fruits, non-tree fruits, and estimations of their yield loss due to the August 16–17, 2006, October–November 2015 and February–March 2016 eruptions of Tungurahua volcano, Ecuador. Crop yield loss increased with tephra loads (48 ± 35, 69 ± 33 and 76 ± 34% for < 0.5, 0.5–5 and 5–50 kg m-2, respectively), and we found that exposure to tephra led to a greater decline in yield compared to existing predictions. The results further highlight the plant phenological stage as a key factor of vulnerability. Exposure to tephra during the flowering period of legumes, cereals and tree fruits caused a median yield loss ≥ 80%. Legumes, tree fruits and non-tree fruits are more vulnerable to tephra than onions. Quantitative knowledge on crop vulnerability to tephra can be obtained from post-eruption impact assessments provided that a large population sample is collected and careful uncertainty analysis is conducted.

&lt;p&gt;Volcanic ashfall negatively affects crops, causing major economic losses and jeopardising the livelihood of farmers in developing countries where agriculture is at volcanic risk. Ash on plant foliage reduces the amount of incident light, thereby limiting photosynthesis and plant yield. An excessive ash load may also result in mechanical plant damages, such as defoliation and breakage of the stem and twigs. Characterising crop vulnerability to ashfall is critical to conduct a comprehensive volcanic risk analysis. This is normally done by describing the relationship between the ash deposit thickness and the corresponding reduction in crop yield, i.e. a fragility function. However, ash depth measured on the ground surface is a crude proxy of ash retention on plant foliage as this metrics neglects other factors, such as ash particle size, leaf pubescence and condition of humidity at leaf surfaces, which are likely to influence the amount of ash that stays on leaves.&lt;/p&gt;&lt;p&gt;Here we report the results of greenhouse experiments in which we measured the percentage of leaf surface area covered by ash particles for one hairy leaf plant (tomato, Solanum lycopersicum L.) and one hairless leaf plant (chilli pepper, Capsicum annuum L.) exposed to simulated ashfalls. We tested six particle size ranges (&amp;#8804; 90, 90-125, 125-250, 250-500, 500-1000, 1000-2000 &amp;#181;m) and two conditions of humidity at leaf surfaces, i.e. dry and wet. Each treatment consisted of 15 replicates. The tomato and chilli pepper plants exposed to ash were at the seven- and eight-leaf stage, respectively. An ash load of ~570 g m&lt;sup&gt;-2 &lt;/sup&gt;was applied to each plant using a homemade ashfall simulator. We estimated the leaf surface area covered by ash from pictures taken before and immediately after the simulated ashfall. The ImageJ software was used for image processing and analysis.&lt;/p&gt;&lt;p&gt;Our results show that leaf coverage by ash increases with decreasing particle size. Exposure of tomato and chilli pepper to ash &amp;#8804; 90 &amp;#956;m always led to ~90% coverage of the leaf surface area. For coarser particles sizes (i.e. between 125 and 500 &amp;#181;m) and dry condition at leaf surfaces, a significantly higher percentage (on average 29 and 16%) of the leaf surface area was covered by ash in the case of tomato compared to chilli pepper, highlighting the influence of leaf pubescence on ash retention. In addition, for particle sizes between 90 and 500 &amp;#181;m, wetting of the leaf surfaces prior to ashfall enhanced the ash cover by 19 &amp;#177; 5% and 34 &amp;#177; 11% for tomato and chilli pepper, respectively.&lt;/p&gt;&lt;p&gt;These findings highlight that ash deposit thickness alone cannot describe the hazard intensity accurately. A thin deposit of fine ash (&amp;#8804; 90 &amp;#181;m) will likely cover the entire leaf surface area, thereby eliciting a disproportionate effect on plant foliage compared to a thicker but coarser deposit. Similarly, for a same ash depth, leaf pubescence and humid conditions at the leaf surfaces will enhance ash retention, thereby increasing the likelihood of damage. Our study will contribute to improve the reliability of crop fragility functions used in volcanic risk assessment.&lt;/p&gt;

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