Germination niche of a pioneer woody species (Manihot grahamii hook.): a strategy of seed heat stimulation to cope with disturbance in dry subtropical forests

Abstract

Light and temperature conditions trigger germination in specific temporal windows and microhabitats, thus determining the germination niche of plant species. Manihot species grow in fire-prone ecosystems and their seeds show heat tolerance. Successful establishment in disturbed environments might be related to seed attributes that allow seeds to cope with or avoid stressful environments. We studied some characteristics of the germination niche of Manihot grahamii, a pioneer woody species present in dry subtropical forests of central Argentina, to understand its successful establishment in disturbed environments. We evaluated the germination ecology of the seeds of M. grahamii with the aims to (1) characterize seed traits (viability, mass and moisture content); (2) determine whether it has dormancy and if it is physical or physiological; (3) evaluate the effect of several pre-treatments (gibberellic acid, after ripening, dry prechilling and dry prechilling + warm) on seed dormancy; and (4) assess the effect of different environmental events of high temperatures on the germination process simulating two treatments: fire intensities (with three levels of heat shock) and a gap temperature. M. grahamii seeds have large mass (0.24 g), low moisture content (8%), physiological dormancy, negative photoblastic behaviour and high heat tolerance. Dormancy was alleviated and seeds became insensitive to light when they were exposed to pre-treatments of dry prechilling + warm and high-temperature treatments. This germination strategy promotes secure germination timing into the rainy season on undisturbed habitats as well as a cue for competition-released gaps which in turn favour recruitment in open, disturbed and dry habitats, respectively. In the context of global change, with an increasing habitat fragmentation and fire frequency, M. grahamii could become more abundant and extend its geographic distributional range in central Argentina.