Origins of New Zealand
New Zealand, Australia, India, South Africa and Antarctica were originally joined together in a super-continent called Gondwanaland which broke up into separate territories about 60 to 80 million years ago.
The forests of New Zealand at this time included coniferous trees like Kauri (Agathis) and Podocarps (Podocarpus) which are still important there. About this time the first flowering plants such as the Southern Beeches (Nothofagus) appeared in the flora.
The Ice Age
After a warm period in which tropical plants grew both in New Zealand and the British Isles, the climate of both hemispheres cooled down and the Ice Age began about one million years ago. The British Isles were glaciated much more severely, with most of the land covered by ice and no forests left at all. This meant that after the Ice Age the British Isles were left with a much more reduced flora compared to New Zealand.
The Alpine Flora
The mountains of the British Isles are lower and less extensive than those of New Zealand; they were so heavily glaciated that they lost their entire flora during the Ice Age. The result is that the British Isles have a much poorer alpine flora: 130 compared to 500 species.
Among the New Zealand alpines, members of the Daisy Family (Compositae) are particularly important. The largest genus is Celmisia with 56 species. Leucogenes (New Zealand edelweiss), Helichrysum, Haastia, and Raoulia are also important.
Before the arrival of the Maoris, forest cover was 75% of the total land area. Between them, the Maoris and the later European settlers have reduced the cover to about 23%. Other vegetation was also affected but even so, 60% of New Zealand is still covered by native or modified native vegetation.
The Endemic Flora
Because of its remoteness many of the plants in New Zealand have evolved in isolation and occur nowhere else on earth. Such plants are called endemics. 85% of the flora of New Zealand is endemic in contrast to the British Isles where there are virtually no endemics at all.
One of the most characteristic New Zealand plant groups is the genus Hebe or "shrubby veronicas". All except 2 of the 70 or so species of Hebe in New Zealand are endemic and the genus seems to have originated there.
Of the endemics, perhaps the most spectacular are those whose northern hemisphere relatives are well known as insignificant plants, which in New Zealand have evolved to strange proportions. The Chatham Islands lie 500 miles to the east of New Zealand proper. The Chatham Island forget-me-not, Myosotidium hortensia, is related to the British forget-me-nots, but has leaves up to 3 ft long!
Some other comparisons
New Zealand and the British Isles are of similar area but New Zealand is closer to the Equator. There is a sub-tropical element in the New Zealand flora which is absent from the British flora.