Dioecious plants

Coprosma lucida female

Coprosma lucida male

Dioecious plants.
One of the intriguing features of the New Zealand flora is the high percentage of plants that have separate male and female individuals. While this is the norm in animals, plants that have male and female sex organs on different individuals is unusual.
Male flowers are those whose gametes (pollen) leave the flower, while female flowers are those whose gametes (ova) remain in the flower and are receptive to the male gamete.When the flowers are on separate trees it is termed dioecious

Exotic plants that this occurs in include Kiwi fruit and Cannabis sativa which have male and female plants. A characteristic of the The New Zealand flora is the high % of plants that have the sexes on separate plants (dioecious). Approx 12-13% of our flora are dioecious. (Britain 2-5%, South Australia, 3.9%., Hawaii 5%). However in many plant association's in the forest  there may be up to 50% of the individuals who exhibit dioeciousness
Dioeciousness of plants account for the variability in seeding from year to year that some species exhibit because seeding will be determined by synchronising the time at which the male and female parts are functional. Rimu may go 7-12 years before seed on females is produced in abundance. These years are termed mast years.
As a male tree may be some distance from a female tree, the environmental conditions that determine flowering would be more variable in separate trees than the same tree. There is more likelihood of reproductive failure if the nearest tree of the opposite sex is some distance away. The fact that pollen from a distant tree must fertilise a female tree may put the successful reproduction and production of viable seed under stress. Logging of a population, and a reduction in total density of the adult trees and total gamete production will reduce the likelihood of fertilisation . This seems to be the case in Monoao( Dacrydium kirkii), a dioecious species of conifer that grows in Northlands kauiri forests.Monoao has become relatively rare in the wild with its natural range vastly reduced to remnant populations in Coromandel, Northland and Little Barrier Island.
A feature related to separate male and female trees is Sexual dimorphism. This is that male plants have different form or shape than females. Of course in most animals this is obvious with the male having a different body shape to females. In plants however it is less obvious but does exist. Male Coprosma show broader leaves to female Coprosma.

Divarication 

A curious feature is the large number of shrubs with small leaves and flexible interlacing branches. This form is termed divarication or filliforme.
Divarication occurs across many plant families and is found in some genera or species, but may not be prevalent in all the members of the genera.  
One idea put forward to explain the prevalence of this form is that it is a defensive mechanism against browsing Moa. Moa did not browse in the same way as other mammal browsers (of whichNew Zealand had none). Small leaved woody tangled shrubs were unpalatable to Moa, so plants with this divaricating form were successful and reproduced.
Another theory is that this shape is advantageous in a dry, cold, windy, environment as may have been present in New Zealand in a previous ice age or, this twisted small leaved form creates an ideal habitat for lizards which are the primary agent of fruit and seed dispersal. The lizards are readily able to climb throughout the plant to digest the fruit and are hidden from predators or the interwoven branches.

Some species are divaricating as juveniles but adopt a more  regular leaf and branching habit as adults e.g. totara and matai

Divaricating plants

Dacrydium kirkkii monoao showing both juvenile and adult foliage on the same branch.

Juvenile & Adult forms

Juvenile and adult forms

are one of the most intriguing features of New Zealand plants. The juvenile has a leaf quite different in shape from the adult.

The advantage of this feature is the juvenile leaves are best adapted to an environment close to the ground where the leaf may be in shade or be food for browsing herbivores (suggested to be the Moa and othere flightless extinct birds), while the adult leaf on a tall trunk may now be in full sunlight where its shape maximises photosynthesis opportunities for the energy rich processes necessary for successful reproduction.

Kanuka Kunzea ericoides flowers in early summer.

Small white inconspicuous flowers on many species are attractive to pollination by small insects such as bees and ants. The white petals act as solar collectors creating a warm microclimate that encourages the small insect to visit on a chilly morning.

Hoheria populnea loses its leaves over winter.

Evergreen and lack of deciduous plants

Only 11 new Zealand species completely lose their leaves over winter including Sophora kowhai, hoheria lacebark and kotukutuku tree Fuchsia. In Northland these species are only deciduous for a week or two and in Southland they may be deciduous for the 4 months of winter. The other trees and shrubs hold leaves for all the year.                                                                        

Colensoa physaloides is an attractive shrub limited in its natural distribution to Whangarei  north

Pittosporum cornifolium an epiphyte of Northern forests. The capsule.presents  the sticky seeds to birds for dispersal. Pittosporum literally translates to sticky seeds.

Regional biological hotspots that lead to plant diversity.

The present day distribution of native plants is due to past climatic and geological processes.

The New Zealand landmass has existed in isolation from other landmasses for 80 million years which has led to forms and species uniquely adapted to the environment of these islands.

Over time New Zealand has  been a cluster of islands joining and dispersing , as sea level rise and lower creating islands of isolation or  broader combined landmasses..

New Zealand has the wet west side and the drier eastside while ranging into the subtropics in the north to the sub Antarctic in the south and the geological uplift created by tectonic plates colliding leads to rock and soil types changing over short distances. From the mountains to the coast, the New Zealand landscape is broken, variable and diverse.

These factors create a range of microclimates to which a range of plants are best adapted.

Now we find hotspots of plant biodiversity such as North West Nelson, The Three Kings and the offshore Northland Islands, along with Hokianga and Whangaroa. Each of these areas posses plants endemic to their area. In fact every province in New Zealand will have plants limited to their local distribution.

Pittosporum cornifolium

Special features

New Zealand’s unique plant forms or plants with their headquarters in New Zealand.

Cordyline Cabbage trees

Phormuim Flax,

Tree ferns Ponga

Nothofagus beech

Kauri the northern aristocrat

Nikau the southern most palm

Pittosporum,

Hebe,

Carmichaelia  leafless broom

Pseudopanax. Five fingers

Coprosma

The attractive foliage and wide range of form have made these genera the plants of choice to instil a native element within ornamental gardens.

Carmichaelia leafless broom.  Seed and pod