New Zealand’s Strip Forest Tease: alpine edge tree line
- Ivy
- Apr 15
- 12 min read
New Zealand – Aotearoa – an idealized paradise where wildlife is among the most diverse and unique in the world, but without a bunch of venomous bitey things, with 80% of the plant species being endemic (found now only in New Zealand). Tragically, a handful of unique species have already become extinct since the arrival of the Māori people in the 1300s CE,
and more so since the arrival of Europeans in the 1840s CE.
Humans… we kill things. Prior to the immigration of the Māori, the several islands of New Zealand were 80% covered in native bush (dense forests of trees and shrubs) with stouter plant ecosystems in the harsher areas and highest elevations unfavorable to larger vegetation.
200 years later, most of the forests have been cleared on both the North and South islands and many species only persist just shy of extinction on smaller outlying islands where introduced predators like Australian brushtail possums, UK stoats, rats, and the domestic murder-fluffies (aka Cats) have not spread. As you traverse the country, which is still mostly non-urbanized and wild bush or grassland as far as you can see, or until seeing the sea, you may notice a sort of strip distribution of the forests across the highly varied topography, teasing of what it once was.
Steep but gentle hills remind you of the highly volcanic nature of the land formation with towering snow-capped mountains scattered here and there or in a nice spine down the South Island forming the aptly dubbed Southern Alps.
Forests are often seen as a starkly dense strip starting a bit up a hillside (where it’s awkward for livestock and clearing) and then end a bit up the slope, stopping sharply before the top of any moderately high peak. New Zealand forests exist today in a belt where it’s hard to clear and where its not too hard to grow. The bottom limit makes sense to me, but I’m used to seeing forests taper off with trees getting less dense and shorter until they trickle off into a few patches and some stubby shrub-like trees clinging against the cold winds and ice and snow and scant soil. New Zealand looks like a medieval monk’s ring of hair (called a tonsure, fyi, used then to signify religious devotion and humility).
So… New Zealand has a tease strip of forest tree line alpine edge WHY? How? (skip down to TLDR now if in a hurry)
Are there not the normal variety of cold hardy trees that can penetrate the cold extremes of
alpine zones; do the native alpine parrots eat all the tree seedlings up high; is it too cold and densely snow covered for any trees to extend beyond the protection of the thick forest cover; is a NZ version of Paul Bunyan mowing all the hill tops?
First, was I imagining this monk-do of the forests while driving all over New Zealand. In fact, no. It is true and has been researched (yay!) that tree lines on high-altitude (nearly-alpine) forests in New Zealand have a stark or “abbreviated” edge, as the Kiwi researchers put it. The top elevations of these tree lines range from 900 m to 1,400 m (3,000 ft to 4,600 ft), which is fairly familiar to me from the US PNW. The tree lines of North America are lower in the northeast, where forests are dominated by deciduous trees, while higher in the central Rocky Mountains and western mountain ranges that have more conifer forests, reaching from a short 370 m in the eastern United States up to 3,700 m in the western US (1,200 to 12,000 ft). The tree line of the European Swiss alps is about 2,000m (6,900ft), and a whopping 4,400m (14,400ft) in the Asian Himalayas.
This Himalayan tree line is made up of both a deciduous birch (Betula utilis) and the coniferous Abies spectabilis, east Himalayan fir) and the coniferous Himalayan pencil “cedar” (Juniperus polycarpos- not an actual cedar) in the highest area of the Western Himalayas.
So in theory, there could be trees that can reach higher in the New Zealand elevations if they had similar conifer assemblages or some super hardy hardwoods (aka the broadleafs / angiosperms, typically deciduous trees). We’ll see soon that there’s a lot of mixing of evergreen, conifer, deciduous, and broadleaf/angiosperm in this alpine zone analysis. Buckle your tree terminology belt!
Next to address- why do trees stop up a mountain anyway?
Alpine literally means where there aren’t trees, coined after the European Alps, which is also from the Latin word for “high mountains” (alpes). But sometimes ecologists and such still refer to high elevation trees as alpine trees- just one of those semi-accepted hypocritical sayings- but technically high up trees would be “subalpine” (or “transition zone”) and true alpine plants are non-trees.
There are limits to the conditions any plant or animal can survive and reproduce in, including temperature, moisture, nutrients, sunlight, and simply put- the alpine zone is the end of an area’s tree species’ limit and defined by the tree line. This is typically a feathered line, not a stark edge because there is enough diversity in a forest with several species just a little hardier reaching a little further up slope and in nooks, and often able to survive by staying a lot shorter thus less in the severe winds and cold, also needing less soil to stay put.
Above the tree line we find shorter plants with some shrubs and often just grasses and eventually only a few very small alpine plants that can cling to the mostly bare rock and will reproduce quickly only a few inches tall. In alpine biomes, the presence of snow pack can drastically limit the growing season because everything is literally covered up for a good half of the year. Trees require more resources to get all tall and sturdy and make whatever form of reproductive structure (fruit or cones or other), thus as the environmental stresses increase higher up in elevation, the ability to live this more demanding life model of trees just peters out. But in New Zealand it looks like less ‘petering’ and more of a ‘plunk’.
Getting Forest Technical: Tree line limitation is strongly related to both the warmest growing season temperature and the coldest winter/dormant season temperature for any tree species making up the forest, thus creating their temperature range of survival. The typical forest edge in New Zealand is created by the evergreen ‘false beech’ trees (broadleaf/angiosperms) of the Nothofagus genus, or by more frost-tolerant shorter species in lower warmer areas. And one 2014 study (#33) did find that hardier conifer species do form more gradual tree lines at lower elevations.
When compared to similar forests of Nothofagus species (but deciduous) in the Chilean Andes, the tree line limit is roughly 250 m higher in elevation that the evergreen Nothofagus tree lines in New Zealand, despite the Andes being generally cooler in seasonal temperatures. Researchers found that the NZ Nothofagus forest limit is related more to the warmth of the growing season than the cold of the dormant season like Chilean and other regions. Sounds like New Zealand forests are doing things their own way- found both unusual species composition (topped by broadleaf trees) and limitations for their forests and they are still evolving and going higher.
Often the cold and wind of alpine environments creates the krummholz effect on trees (German for “twisted wood”) so you’ll find subalpine trees molded by the winds and snow the further they feather out from the forest edge. New Zealand seems to lack this effect to the degree alpine transition zones of other regions, partly because the dominant subalpine trees- evergreen false beech trees (Nothofagus spp.) only seem to get krummholzed within the forest limit, thus not creating a tapered edge of the forest into the gnarled shorter upper trees fading to other shrubs above the tree line. There are great examples of NZ wind krummholzed trees and even whole island-like patches of tangled forest on a grassy hill. But that’s a bit special. We’re talking general alpine transition here.
Recap- NZ forest false beech trees are limited from climbing higher by the warmth of their growing season & they don’t tiptoe much out of the forest edge hunched in krummholz style.
But what’s holding back the New Zealand forests to be so stark a tree line no matter its elevation level? The Andes’ tree line also often has a sharp end but it’s more due to human activity and fire, as historical evidence shows a more subtle tree line edge long ago (5). Several studies have concluded the NZ Nothofagus tree line is due to low sapling survival beyond the protection of the dense forest and is also a limit of the biology of the broadleaf (angiosperm aka hardwoods) group of trees that New Zealand is unusual in having for its tree line species in a temperate climate. Tropic regions more often have broadleaf trees as their top elevation species, while conifers are more often the top for temperate forests.
Partly the limit in New Zealand is also due to the youth of its alpine environments, simply not having the long evolutionary time to adapt hardiness at climate extremes or for the more often hardier conifer species to establish in the high elevations. The false beeches are fairly new to the forests (yet a globally ancient genus) and still spreading in the forests, evolutionarily speaking. NZ has plenty of conifer species, but most are in a less common family (the neato Podocarps) and it lacks the largest conifer family there is.
Where are the real pines?
There are 20 native (and endemic) conifers of New Zealand, among four families:
Araucariaceae
Cupressaceae / cypress
Phyllocladaceae / celery “pine”
Podocarpaceae
These are across 10 genera with the vast majority being among the very unique Podocarp
family, which make fleshy cones resembling fruits more than the dry seed cones more people are familiar with of conifers like pines, firs, and spruce. New Zealand has no native pine (in the genus or family) despite multiple trees with “pine” in their common name and leaves looking nothing like pine needles (botanist’s sigh…). Pinaceae is the largest conifer family in number of species (~230) and mostly present in the northern hemisphere, while Podocarpaceae is the largest tropical or southern hemisphere conifer family (~170 species), so NZ is right in the norm there. New Zealand did once have a few other types of gymnosperms on top of the conifers, like gingko and cycads, but these died out long ago, only found in Cretaceous fossil deposits, possibly from before New Zealand’s land mass separated from the Australian-Antarctic landmass.
For reference- North American native conifer species total 63 (bigger land area), among 15 genera but only three families:
Cupressaceae / cypress
Pinaceae / pine
Taxaceae / yew
Of course the land mass of North America is larger, so to compare another large region’s conifer diversity in the northern hemisphere- India has about 24 species of conifers across 11 genera (could not find details for the number of families). Interestingly similar to New Zealand’s diversity.
For broadleaf aka angiosperm or flowering trees, New Zealand has between163 and 195 species categorized as “trees” (reports vary and definitions of ‘tree’ varies too), and these are among 42 or more families:
North America has 128 species among 58 genera of broadleaf trees in 29 families:
Making New Zealand extra special, there are also 8 “tree” species in the fern class (Polypodiosida) among the Cyatheaceae and Dicksoniaceae families. They don’t reach high in elevation though and some argue that ferns, lacking true wood, aren’t for-reals trees. I say, it serves the function of trees, it’s got Aristotle’s “tree-y-ness”, it’s a tree.
With this decent conifer diversity and normal for the global region, and a large diversity of broadleaf trees, it may be a bit odd to think of these forests as still evolutionarily young. But even the New Zealand mountains are still very geologically young and thus active and even growing. The country is positively riddled with volcanoes and crater lakes and hot springs, oh my!
The mountains have only been pushing up from the seabed starting in the late Pliocene, between 5 & 3 million years ago. North America’s central and tallest Rocky Mountains formed beginning in the late Cretaceous and ending about 35 million years ago. The shortest mountain range of North America is the very old Appalachian in the northeast corner and are some of the oldest mountains on earth, eroded greatly now from the ancient heights built over 480 million years ago in the Ordovician period. New Zealand mountains are steep beautiful young whippersnappers, still working stuff out.
Due to the fairly recent colonization by Europeans (compared globally) with their massive logging and mining and invasive species introduction, the native landscapes are still mostly healthy and very diverse, though timber harvest and animal grazing has removed a major portion of the lowland forest and introduced many invasives in the disturbed areas. In fact, nearly three quarters of land has been cleared in New Zealand, compared to very similar 70% forest loss in North America. So the Europeans (and to extent Maori before) have quickly made a huge dent in the forests, but the untouched sections are still looking good. The conversion to grassland/grazing land of lower areas in New Zealand and the stark start of alpine transition has created a sort of dense belt lumping along the mountains and steep hillsides where the forest is packed in- dense, diverse, complex, and so very beautiful.
Where the tree line stops, sometimes with some krummholzed trees, some shorter hardy shrubs like Coprosma (taupata) species will trail off to the alpine grass-dominated zone. Snowfall and ice, as well as sunlight and soil depth/fertility all combine to limit the extent of tall tree growth at alpine elevations, though it’s that warmth of the growing season that’s key. But recent research across global tree line formations has found that three main factors shape all forest edges: tree species growth limitations (what they can survive), tree dieback, and seedling survival, which are based on both the abilities of each tree, how many tree species make up the transition zone, and the environmental stresses of their specific mountain habitat.
While the transition to alpine environment is stark in many southern hemisphere forests, not just NZ, the area is also usually more botanically diverse than in northern hemisphere (similarly comparing tropical warmer forests to the less diverse temperate forests). New Zealand forest transition zones are the second-most taxonomically diverse in the world. Yet researchers found “Southern Hemisphere tree line taxa are less adapted to cold temperatures”. So less tree taxa will be in this zone, but lots more of other taxa persist. Is it possible the still rising nature (literally) of these mountains is sort of kidnapping more species and lifting them up to where they will just barely survive and the diversity may drop over more millennia? The steepness of the mountains creates more microhabitats for more species to inhabit? Or the magic of New Zealand being biologically special?
The current forest species of New Zealand are indeed a fairly recent evolution, with the Nothofagus beech trees beginning to increase only five to six thousand years ago and continues to increase in population today. Though most of New Zealand forests are still dominated by the tropical group of podocarp trees, covering the lower and mid-elevation zones, with the main competition being that evergreen angiosperm - Nothofagus menziesii (silver ‘false’ beech), which can survive in higher elevations and colder regions. In Chile, the related but deciduous Nothofagus antarctica is among only a few deciduous species that form the tree lines of mountain forests around the world, with most tree line trees being in the pine genus (Pinus species like Rocky Mountain and Great basin bristlecone pines, whitebark pine, and Swiss pine). Larches are also a common tree line species, which are uniquely a deciduous conifer (losing its leaves in winter but typical cone-bearing), giving it a helpful trick to survive extreme mountain climates by tossing its leaves for long winters.
TLDR
(too long didn’t read by what’s up with New Zealand's monk’s tonsure forest strip?)
New Zealand forests (and mountains) are young and still getting taller and made up of tree taxa common to tropic regions
The main tree line trees are broadleaf but evergreen- Nothofagus (false beech)- yet stays at lower elevations than its Chilean deciduous Nothofagus relatives that can take colder temps but also doesn’t get the summer warmth NZ Nothofagus are subject to.
The warmth of the growing season and exposure of alpine zone are the main limitations on the NZ tree line with Nothofagus unable to emerge too far from the dense forest’s protection due to seedlings low survival.
NZ lacks cold adapted conifers like Pinaceae (largest conifer family but limited to Northern Hemisphere)
The cool Podocarp family, the most common conifer and subalpine tree of the southern hemisphere and tropics are not as cold adapted among the many NZ species.
THUS- the newness (still adapting and spreading) and limited biological tolerances of the new-kids on the forest-block (Nothofagus) creates the stark tree line we see.
The belt-structure is due to a lower limit of forest being cleared grazing land.
We’ll still love all NZ forests because they are super diverse, comparatively healthy, and have giant ferns as trees too.
Alpine Fact to End On:
The Chilean monkey puzzle tree (Araucaria araucana), an iconic unusual conifer often grown as showpiece trees in my Pacific Northwest of US, can grow as full multi-meter trees towering above the krummholz line of the native evergreen angiosperm Nothofagus species forests. But the badge of very Highest Elevation Tree species in the world goes to the Polylepis tarapacana/tomentella (called “queñoa de altura” meaning the Polylepis/quenoa of high altitude in Spanish) growing in South American Andes mountains of Chile, Bolivia, and Peru, and surprisingly is in the Rosaceae plant family (related to roses, apple trees, blackberries, and hawthorns). The Polylepis trees can grow up to 5,200m (that’s 17,000 feet!). This tree is estimated to have evolved recently as the Andes mountain uplifted, around the Plio-Pleistocene eon, about one to two million years ago. They are small evergreen trees, sometimes shrubs, and can grow older than 700 years.
References:
https://royalsocietypublishing.org/doi/10.1098/rstb.2020.0497
https://www.tandfonline.com/doi/full/10.1657/1523-0430%2806-066%29%5BWARDLE%5D2.0.CO%3B2#d1e97
https://northernwoodlands.org/outside_story/article/krummholz-wood
https://www.americanforests.org/article/north-american-forests-in-the-age-of-man/
https://www.srs.fs.usda.gov/pubs/misc/ag_654/volume_2/silvics_v2.pdf
https://www.indianforester.co.in/index.php/indianforester/article/view/10398
**** Image map of NZ south
https://www.ultimatekilimanjaro.com/mountain-ranges-of-the-usa/
https://www.sciencedirect.com/science/article/abs/pii/S0367253022000731
https://www.guinnessworldrecords.com/world-records/86953-highest-tree
https://en.wikipedia.org/wiki/List_of_trees_native_to_New_Zealand
https://www.sciencedirect.com/science/article/pii/S2405844024168284
https://www.jstor.org/stable/27226320
***Himalayan tree line pic
https://www.treeguideuk.co.uk/conifer-families/
** Hawknsetl PDF Hawkinsetal14JBIcommunityphylogenetics
https://www.sciencedirect.com/science/article/abs/pii/S0168192323000692
*Nothofagus distribution figure
Comments