In dune fescue, mortality was mostly caused by:
In the cited dune-fescue example, drought stress is identified as the main source of mortality. Limited soil moisture can prevent seedlings from establishing and can kill established plants through loss of turgor, reduced photosynthesis, and hydraulic failure. Because weather-driven moisture deficits may strike across densities, this case illustrates how an abiotic event can dominate mortality without providing reliable population-regulating feedback. Demographic mechanisms are linked by the balance dN/dt = births + immigration - deaths - emigration. Density-dependent changes in any of these terms can alter net growth. To infer regulation, ecologists compare per-capita rates across densities and distinguish causal feedback from coincidental correlations produced by weather, age structure, or habitat quality. This reasoning also explains why field observations may be approximate even when the underlying textbook classification is useful. This interpretation connects individual-level processes with measurable changes in survival, reproduction, recruitment, or abundance across the population. Field evidence should therefore be compared with the model assumptions before extending the conclusion to every species, habitat, or time period.
Ref: Ecology: From Individuals to Ecosystems, Begon et al., 5th Ed., Ch. 5