Diverging Sex Ratios in Dioecious Proteaceae are Exacerbated by Anthropogenic Disruptions to the Fire Cycle

Background and Aims Diverging secondary sex ratios in dioecious plant species often deviate from the expected 1:1 primary male-to-female ratio due to differential survival rates. Such deviations in life-history strategies, along with diverging reproductive trade-offs, have been used for assessing reproductive costs in plants. In the fire-prone Fynbos biome, previous studies on sex ratios and reproductive costs in dioecious Proteaceae have produced conflicting results, warranting further investigation. We examined whether obligate reseeding serotinous Leucadendron and Aulax species (Proteaceae) experience higher reproductive cost in males, females, or both equally. Methods We analysed sex ratios across populations of varying ages and assessed individual health through canopy cover scores. In addition, we conducted nutrient analysis to quantify allocation to vegetative versus reproductive structures. Key Results We found no evidence that primary sex ratios differ from 1:1, but clear evidence of secondary sex ratios becoming increasingly male biased with age. Predictions indicated that a typical 30-year-old population would have a sex ratio of 0.67 (95% CI, [0.52, 0.81]), corresponding to a twice as many males than females. In older, more male-biased populations, females exhibited lower health scores. While total nutrient content did not differ between the sexes, females allocated a greater proportion of total nutrients to their reproductive cone structures. Conclusions Our results suggest that females experience higher reproductive costs which contribute to increased female mortality over time, resulting in male-biased sex ratios in older populations. Anthropogenic fire suppression likely contributes to this trend by allowing populations to survive beyond the natural fire-return interval for fynbos vegetation. These altered population dynamics could undermine long-term population viability and ecosystem stability in fire-adapted dioecious Proteaceae of the fynbos.