Drought, Ash and Soil Legacies Shape Seed Germination Responses in Pinus canariensis C. Sm. & DC. Forests

Fire plays a central role in shaping regeneration dynamics in Pinus canariensis C. Sm. & DC. forests, yet the relative importance of different fire-related cues and post-fire environmental filters remains poorly understood. We experimentally evaluated the effects of drought, heat, smoke, ash and soil microbial extracts on the germination of seven co-occurring species representing the main functional groups of these forests. Germination responses varied markedly among species, revealing that no single cue dominates early recruitment. Drought emerged as the strongest and most consistent filter, sharply reducing germination success and slowing emergence in P. canariensis and Fabaceae, while Poaceae remained largely insensitive under the imposed water potentials. Ash enhanced germination in P. canariensis and Cynosurus echinatus L., indicating that nutrient-rich post-fire substrates can facilitate emergence when moisture is available. Soil and microbial extracts accelerated germination in fast-emerging grasses, providing the first experimental evidence that post-fire soil legacies influence recruitment in Macaronesian pine forests. In contrast, heat acted as a species-specific cue, with only limited stimulation at moderate temperatures and widespread declines under extreme heat, while the smoke treatment used in this study produced negligible effects across all species. Across treatments, germination timing emerged as a key functional axis, structuring regeneration niches, with rapid-germinating Poaceae favoured under increasing aridity. Our findings highlight the dominant role of hydric stress, the selective relevance of ash and soil legacies, and the functional divergence among species, offering new mechanistic insights into post-fire regeneration under current and future climate scenarios.