Plant hydraulic traits regulating productivity and drought resistance in boreal crops

Plant hydraulics play a critical role in coordinating photosynthesis, water use, and hydraulic safety when facing water scarcity. The hydraulic traits of boreal crops and their relevance to productivity and drought resistance, however, remain poorly researched. In this study, we combined multi-year carbon and water flux measurements from two Finnish fields representing contrasting boreal cropping systems – a perennial forage grass mixture and an annual cereal crop (oats) – with a novel modelling framework coupling plant photosynthesis, hydraulics, and soil moisture dynamics to decipher the field-level effective hydraulic traits (i.e., apparent hydraulic traits) of the crops and their association with crop performance under varying drought conditions. The model reproduced the observed carbon and water fluxes well and revealed relatively low maximum whole-plant hydraulic conductance and conservative water-use strategies in both crops. Under drought, low hydraulic efficiency prevented soil water depletion and thereby supported high productivity through drought avoidance, whereas its advantage diminished under wetter conditions. Both crops were substantially more susceptible to atmospheric dryness than to low soil moisture, highlighting vapor pressure deficit as an important constraint on crop productivity under boreal conditions. The remaining discrepancy between the model-inferred apparent hydraulic traits and available empirical measurements for temperate crops indicates the need for more direct hydraulic measurements of boreal crops. Our findings further underscore the importance of incorporating crop hydraulic processes in process-based crop models to gain physiological insights into crop performance and to enhance their accuracy and predictive power under current and future climates.