A study of the seed ecology of two species of kowhai, Sophora microphylla and Sophora prostrata in Canterbury, New Zealand (1993)
AuthorsWebb, J. A.show all
The seed ecology of Sophora microphylla (common kowhai) and to a lesser extent Sophora prostrata (prostrate kowhai) from two Canterbury sites was studied. The aspects of the seed ecology examined were firstly, immature germination, embryo maturity and seed predation in S. microphylla followed by a study of the seed banks, seed fall, seed dispersal, the influences of temperature and light on germination and early growth, and natural mechanisms employed for the scarification of the seed coat in both S. microphylla and S. prostrata. Germination experiments revealed that S. microphylla seeds can germinate when they are immature and that embryo maturity is achieved in most seeds at around 16 weeks of age, although some mature in only 12 weeks. The only significant predator of S. microphylla seed was the larvae of the moth Stathmopoda aposema, which began to predate the seeds at about 8 weeks of age and continued to predate them from this time on. It was found that predation by this organism accounted for the death of about 28% of seeds in the population of trees studied in the 1991-92 year, although annual fluctuations probably occur. Wind was found to be the principle mechanism of seed removal from the trees of both S. microphylla and S. prostrata and the main, if not the only, dispersal mechanism in S. prostrata. Although wind does play a role in the short distance dispersal of S. microphylla seeds (particularly the whole fruit or pod) the chief mechanism of long distance dispersal in this species is water. S. microphylla possesses both buoyant (up to 27%) and non-buoyant seeds, attributable to the density of its embryo, which is thought to aid the long distance dispersal process. It was found that S. microphylla possessed two types of seed banks one on the tree (resulting from the retention of previous seasons seed) and the other in the soil. The soil seed bank was found to be extensive and deep although most seeds were contained in the top 10cm of the soil profiles dug. S. prostrata seemed to have no long-term seed banks as such although many seeds are assimilated into the humus layer beneath parent plants before being lost downslope or germinating, indicating a very temporary soil seed bank at most. The seeds of both S. microphylla and S. prostrata were sensitive to temperature when germinating and have an optimum germinating range of about 10-20°C. As expected there was no obvious sensitivity to light during the germination of either species and most seeds established more successfully after burial (lightless conditions). However, the light intensity appeared to affect young seedlings with both species showing a distinct etiolation pattern. The scarification of seeds in nature appeared to be different for each species. For S. microphylla, scarification of the seed coat did occur in the soil and this was supported by the discovery of germinated and imbibed seeds there. But how this happens is still not clear. It is thought that it may be a result of very gradual microbial breakdown of the micropyle, or influences from other soil organisms. It is also possible that many seeds are scarified in river systems by alluvium in river beds, however experiments showed that many seeds particularly those which are not buoyant may be destroyed in the riverbed environment. Mature S. prostrata seeds when relatively young appeared to have soft seed coats, and at this stage seeds would imbibe and germinate without scarification. As seeds get progressively older however, the seed coats harden and scarification was necessary before germination could take place. After hardening, breakage of the seed coat was thought to occur due to abrasion of soil particles as the seeds move downslope under the influence of gravity or by similar mechanisms to S. microphylla after assimilation into the soil.