Effects of soil type, period of burial and moisture levels on the germination of Oncocyclus iris seeds

Abstract

Understanding germination of seeds constituting a persistent soil seed bank is important because the ability to predict longevity of a seed in soil and a particular combination of environmental conditions supporting dormancy break and germination can be critical for restoration of threatened species having poor regeneration. We studied the pattern of germination under experimentally manipulated environmental conditions in several threatened iris species of section Oncocyclus. In the first experiment, spanning 11 years, we manipulated soil type and precipitation regime, and in the second experiment, spanning 7 years, we tested an effect of cumulative amount of rainfall during a season on germination. In all studied species the germination process was intermittent, with distinct germination pulses separated by years with no or very low germination. High innate dormancy appears to be a general property of newly produced seeds of Oncocyclus irises, but there are exceptions. Although in three tested Oncocyclus species considerable (above 25%) germination fraction was observed only in the second and subsequent seasons regardless of the soil moisture experienced by a seed, in Iris atropurpurea it occurred in the first season under watering equivalent or exceeding cumulative seasonal precipitation of 100 mm. In the three other species, although high cumulative germination percentage was observed by year four under all experimental conditions a substantial fraction of seeds was still ungerminated and viable. Our results have important conservation implications. We recommend for mass seed germination creation of a soil seed bank under natural conditions with minimal modifications, monitored for more than two years. Supplementary watering (if feasible) will speed up the germination process and shading will have a similar effect but neither of these two is a requirement.