Seedling emergence from seed banks of 15 New Zealand lakes with contrasting vegetation histories

Abstract

Submerged sediments were sampled from 15 New Zealand lakes of contrasting vegetation histories and seedling emergence was measured. Lakes either supported seed producing, indigenous plants (Native), vegetatively reproducing, adventive plants (Hydrocharitacean), or lake vegetation had declined (De-vegetated). Sediment was divided into three depth strata of 50-mm thickness and cultured in a shallow outdoor pond receiving 8% ambient light. Seedlings were recorded from 92% of samples, with emergence recorded throughout the trial (22–25 weeks). Initial emergence was earlier for Native lakes (mean 5.2 weeks), than Hydrocharitacean (8.7 weeks) or De-vegetated lakes (9.5 weeks). Overall numbers of emerging seedlings varied considerably (0–3 × 10 5 m −2 per sample), but could not be related to vegetation history ( p > 0.05, ANOVA). In contrast, seedling species richness was higher ( p < 0.05) for Native lakes than for either of the other two categories. Vegetation history was reflected by differing patterns of emergence across sediment depth strata ( p < 0.05), with reduced numbers of seedlings in deep sediments identified from Native lakes only. Seed banks (seedlings plus remaining viable seed) were recorded in all samples, representing up to 12 submerged species. Charophyte oospores dominated the seed banks, with higher recruitment of Chara species than Nitella species possibly related to a larger volume of reserves in the oospores of the former. The study suggests that submerged seed banks possess attributes of longevity and act to conserve seed density and species richness. These attributes, and the potential for a substantial and sustained seedling emergence response, demonstrate that lake seed banks offer a potential means to restore vegetation resources in degraded lakes.