Soil moisture thresholds and seedling survival in Mediterranean environments

Seedling survival is one of the most critical stages in a plant’s life history and is often threatened by drought and soil desiccation. It is assumed that root systems accessing moist soil layers are critical for plant establishment, but actually very little is known about seedling root growth in the field. We related seedling mortality to the presence of deep roots in a field experiment in which we monitored soil moisture, root growth, and seedling survival in five Mediterranean woody species from the beginning of the growing season until the end of the drought season. We found strong positive relationships between survival and maximum rooting depth, as well as between survival and soil moisture. Species with roots reaching moist soil layers withstood prolonged drought better, whereas species with shallow roots died more frequently. In contrast, biomass allocation to roots was not related to establishment success. Access to moist soil horizons accounted for species-specific survival rates, whereas large R:S ratio did not. The existence of soil moisture thresholds that control establishment provides insights into plant population dynamics in dry environments.