Germination Patterns of Soil Seed Banks in Relation to Fire in Portuguese Littoral Pine Forest Vegetation

Abstract

Germination behavior of maritime pine (Pinus pinaster Aiton) forests soil seed banks after fire treatments in controlled laboratory conditions was analyzed. Germination response of all tree and shrub seeds after wildfires in the Leiria National Forest, Portugal, was simulated by treating sample trays of soil seed banks with distinct combinations of: burning time (0 min, 5 min, and 15 min), seed depth (5 cm and 8.5 cm), and presence or absence of ash cover. The design included control samples for null hypothesis testing. During a time span of 20 months after treatments, the maximum number of seedlings observed every 30 day period, their taxonomical identity, the number of destroyed seeds, and the number of non-germinated seeds were analyzed. Six functional species groups, defined by germination response (i.e,. germinated, destroyed, non-germinated), were identified using minimum-variance hierarchical clustering and correspondence analysis. In addition, the influence of the three environmental design factors and three functional species groups based on their general germination response were then analyzed by means of general regression models. Clustering and ordination results suggest two obvious main groups in terms of post-fire germination response: 1) seeders—fast-growing pioneer shrubs that respond positively to post-fire germination, and 2) resprouters—slow-growing understory tall-shrubs in which germination is largely depressed. Each of the two main functional species groups were further subdivided into six sub-groups, which are distinct in mean germinated, destroyed, and non-germinated seed values. Burning time and seed depth were highly significant in explaining post-fire germination response, while the presence of ash cover was less significant. Results suggest that pioneer seeder behavior is largely promoted by wildfires that are, in turn, detrimental to slow-growing, late-sucessional tall shrubs. Forest fire hazard risk management can thus be put into perspective—seeder fuel beds are promoted by wildfire and are themselves highly fire prone, and resprouters are not (due to discontinuous horizontal and vertical structure and less flammable leaf composition).