Many features of the high country climate affect forest growth in alpine regions.

Oceanic influence
The surrounding ocean strongly influences our island climate. This oceanic effect strongly affects high country tree growth.

Rainfall
The high country possesses some of the steepest rainfall gradients in the world. In less than 40 km, rainfall can vary below 500mm in the east to more than 8000 mm/year in the west. This has a very significant effect on tree growth. In the west, species such as Douglas fir grow particularly well. At the dry eastern end growth rates are less than 20% as fast. This is slow indeed by New Zealand standards, but still better than in many major Northern Hemisphere forest growing regions. Despite the fact that the high country includes regions such as Central Otago and the Mackenzie Basin, well known for their dry summers, plant growth over much of the area generally profits from an adequate rainfall which is evenly distributed throughout the year.

Temperature
The oceanic influence moderates, but also increases the variability of , both summer and winter temperatures. This can lead to problems for trees in the form of ;

• milder winters with short periods of blanket snow cover. Snow does not lie long enough to protect plants from wind and the extremes of winter temperature. The greater exposure of bare soil surfaces leads to a higher incidence of frost heave which can lift young seedlings out of the ground.

• unseasonal temperature fluctuations. Warm spells over winter encourage premature growth which can lead to frost damage when cold temperatures return. In spring and summer, frosts can occur in any month and damage the new growth of less hardy plants. On December 18, 1991, parts of the Mackenzie Basin experienced a frost of -3°C followed later in the same day by a maximum temperature in the high 20's.

• more damage from extreme winter cold. Mature radiata pines and eucalypts were killed by frosts of -23°C and -17°C in winters of 1982 and 1991.

Snow
Snow in New Zealand often weighs well over 100kg/m³. Consequently snow storms causing tree damage are not uncommon, with worst damage occurring when wet snow freezes onto branches.

Wind
Being an island in the latitude of the Roaring Forties means that the high country frequently experiences strong, gusty winds which can be very dry as they descend from the western mountains. Strong winds are a major reason why many broadleaf species have difficulty growing on exposed high country sites. The Canterbury region is more windy than Central Otago (Omarama has almost twice the daily wind run of Alexandra).

Altitude and Latitude
Average temperatures decline quicker with increasing altitude in New Zealand than in alpine areas of a similar latitude overseas. As plants need a certain amount of warmth to complete growth and mature tissues before the onset of winter, this could be one reason for the high country's lower than expected timberline. However, contrary to popular belief, increasing altitude and latitude do not always affect tree performance adversely.

Species such as Douglas fir, Corsican pine, Ponderosa pine and European larch actually grow better in the high country than in similar rainfall areas in the North Island. The major reasons are thought to be;

• a lower incidence of pests and diseases, resulting in longer retention of foliage and higher stocking rates, and

• The warm day/cool night temperature pattern over the growing season. Plants gain carbon by photosynthesis during the day but levels are lowered by respiration during the night. Respiration rates are positively related to temperature and night temperatures are cool in the high country. Therefore night-time respiration losses of the carbon fixed by photosynthesis during the warm, sunny days are low. This means more wood volume can be grown per hectare.

Climate and choice of species
Woody species are the most efficient utilisers of the high country's soils and climate. Trees can produce over 30 tonnes of above ground dry matter per ha per year compared to up to 5 tonnes on improved pasture.

Certain trees perform better than others in the high country climate, so species choice is important.

The variability of the climate means that trees with short defined growth periods often do better than those with longer or multiple periods. For example, the shoot extension period of lodgepole, Corsican and Ponderosa pine only lasts a few weeks from late November to early January. These species then have a long 'resting' period when they concentrate on hardening tissues and setting aside reserves for the winter and the following season.

In the long term they are likely to survive better and suffer less damage than species such as the eucalypts and radiata pine which have poorly defined 'resting' periods and try to make use of all the warm summer months.

Few broadleaf tree species do well in the high country, buy many conifers survive and grow well. Conifers are hardier to frost and drought, and their form and foliage is better designed to cope with heavy snows and strong winds. Many people are surprised to learn that native species such as mountain beech, that have evolved locally, have less ability to withstand the extremes of the high country climate than many of the introduced conifer species and some introduced broadleaves (e.g. Alnus viridis, green alder).

Plant hardiness is commonly related directly to ability to withstand extremes of cold and drought. Therefore, species from the middle of continental areas such as N. America and Russia, should do well in our high country. The opposite is also true. Species of continental origin are used to distinct winter and summer seasons. Warm spells in winter and frosts in summer are foreign to them. For this reason Siberian larch has never survived more than a few seasons in the high country. Warm temperatures in winter and early spring encourage premature growth which is soon killed by a subsequent return to cold temperatures and frost.

The forest species that do best in the high country originate from areas of mid altitude and similar latitude where there is an oceanic influence. Good examples are Douglas fir and Ponderosa pine from the western coastal side of the Rocky Mountains, and Corsican pine from the coastal mountains to the north of the Mediterranean sea.