New Zealand high-altitude tree limits are formed either abruptly by evergreen Nothofagus or by low forest of more frost-tolerant small trees reaching similar maximum altitudes. Whereas most tree limits are contiguous with low-growing alpine vegetation, in New Zealand a belt dominated by tussock grasses intervenes that is vulnerable to invasion by hardy introduced trees and seems ecologically equivalent to fire-maintained high-altitude tropical grasslands. New Zealand tree limits coincide with warmer growing-season temperatures than other tree limits, including deciduous Nothofagus in the southern Andes. They also correlate with coldest-month mean temperatures around 0°C, in accordance with the limits of broadleaved evergreen trees globally, unlike north temperate subalpine trees that withstand extreme winter cold. Adverse environments lead to krummholz that in temperate regions can form an attenuated belt above the forest limit, but in New Zealand Nothofagus krummholz develops only at or below the forest limit, in accordance with absence of Nothofagus seedlings beyond a few meters above the forest limit. The relatively low altitudes attained by New Zealand trees are related to isolation and the recent uplift of high mountains, and the differentiation between Nothofagus forest and low forest reflects historical and geological events.
Transitions from forest to alpine vegetation on tropical and southern hemisphere mountains are less extensive but floristically more diverse than on northern hemisphere mountains. Among these, New Zealand transitions are the most taxonomically and physiognomically complex and second only to Andean transitions in extent. Separated by 2000 km of ocean from those of Australia, they are among the most isolated, and situated on mountains that began to rise only in the late Pliocene, they are among the most recent in origin. They also have been little affected by human activities, except where destroyed by fire on rain-shadow mountains. In this review I compare the New Zealand situation with others, in respect of taxonomy, physiognomy, ecological dynamics, and limiting environmental factors.
Several components of the forest-alpine transition are considered. Tree limit can comprise trees isolated above the forest limit or coincide with the latter; cf. alpine treeline defined by Körner and Paulsen (2004) as “the connecting line between the uppermost forest patches in an area, with trees upright and at least 3 m in height and growing in groups.” The transition often also includes krummholz, i.e. trees stunted or deformed by environmental stress. Genetically dwarf forms, in which the most rapid growth occurs in peripheral stems instead of being concentrated in tall, vulnerable, leading shoots, are referred as to shrubs. Inverted forest limits are formed against treeless valley floors. Limiting altitudes cited refer to the highest elevations reached regionally rather than average positions that are likely to be lower because of local environmental irregularities or disturbances. I refer to the dormant season as winter, and to aspects as sunlit and shaded.Figure 1Figure 2
Description of New Zealand Forest-alpine Transitions
Subalpine Nothofagus Forest
Most New Zealand tree limits coincide with a forest limit of two Nothofagus species with evergreen, broad, coriaceous leaves scarcely 1 cm long. N. menziesii usually dominates where annual precipitation exceeds 2000 mm and N. solandri var. cliffortioides on drier mountains or shallower soils (Wardle, 1991). Altitudes attained on coastal mountains range from 1400 m at lat. 38°S to 900 m at lat. 46°S but are 200–300 m higher in summer-warm inland districts where, at lat. 42°S, they can exceed 1500 m. Locally, altitudes are highest on steep, convex slopes regardless of aspect, but average lower on shaded aspects. The forest is typically close-canopied and composed single-stemmed, erect trees, though the upper fringe can consist of multi-stemmed trees with procumbent lower trunks that can root adventitiously (Norton and Schoenenberger, 1984). At or below the forest limit, Nothofagus can be reduced to krummholz on wind-exposed sites and shallow soils. Where snow accumulates in the forest margin, krummholz develops through tipping of trunks and bending and breaking of branches; stems exposed above the snow pack are likely to die back during winter. Shrubs, especially Coprosma species, occupy gaps, and outliers of the grassland that predominates above the forest limit can occupy gentle slopes and benches.
Nothofagus limits dip sharply into gullies (Fig. 1), and Nothofagus does not enter U-shaped valley heads, except for short distances on mid-slopes. Nothofagus also forms inverted forest limits against flat valley floors, especially on recent soils. Finally, large areas completely lack Nothofagus, most notably west of the Southern Alps over a distance of >200 km centered on lat. 43°S and in contiguous headwaters of east-flowing rivers; other significant gaps are the dormant volcano Mount Taranaki (39°18′S), mountains on either side of the Manawatu Gorge (40°S), and Stewart Island (47°S).
Subalpine Low Forest
Where Nothofagus is absent, mid slopes support tall forest with Podocarpus, Libocedrus, Weinmannia, Metrosideros, etc. At its upper limits, which are 150–200 m below tree limit, this gives way to shorter, often very dense woody vegetation. Thirteen species become trees over 3 m tall on sheltered sites, but on wind-exposed sites and shallow, infertile soils, they become reduced to shrub stature, without