Fire-Disturbed Soil and Intraspecific Competition Modulate Juvenile Performance of Invasive Hakea decurrens subsp. physocarpa

Fire has historically been an important component of Mediterranean ecosystems, where many invasive alien plant species (IAPS) spread by taking advantage of fire disturbance, out-competing native species and creating landscapes with novel fire regimes. In the serotinous woody IAPS Hakea decurrens subsp. physocarpa (hereafter H. decurrens for simplicity), fire promotes fruit opening and subsequent recruitment, but the contribution of fire-disturbed soil and conspecific density to its establishment remains poorly understood. Thus, the present study evaluated the responses of juvenile H. decurrens to local natural soil (NS) and fire-disturbed soil (DS), in the absence (NC) or presence (WC) of intraspecific competition, during a 12-month outdoor experiment. A preliminary assay also characterized seedling emergence under different light conditions (full, moderate and low) to mimic the gap effect caused by fires. Seedling emergence ranged from 78.9% to 97.4%, with the lowest values observed under low light conditions. Soil condition did not affect relative height growth rate, whereas intraspecific competition significantly reduced growth rate and most final growth traits. Compared with NS, DS selectively altered shoot biomass, root development, proteoid root formation, leaf pigments, carbon metabolism and mineral composition, and was associated with higher easily extractable glomalin-related soil protein values, but did not consistently enhance overall plant growth. Soil × competition interactions were particularly evident for biomass allocation and carbohydrate metabolism. Principal component analysis revealed distinct functional profiles among treatment combinations. Overall, DS did not consistently enhance juvenile growth of H. decurrens. On the contrary, plants maintained performance through coordinated belowground, physiological, biochemical and nutritional adjustments, whereas intraspecific competition emerged as the strongest constraint on individual performance.