Why tall poppies don't grow on mountain tops

Forest and Bird, Issue 281, 1 August 1996, Page 46

Why tall poppies don't grow on mountain tops

Ann Graeme

AKE A TWIG of leaves from a tawa tree and look closely at one of the leaves. The leaf is an elegantly designed factory with an upper surface that transmits the sun’s energy to fuel the process of food production in the battery of cells within. The basic process of making food from the sun is known as photosynthesis — that wonder of evolutionary technology that sustains most plant and animal life. But take this tawa twig up into the mountains. The thin leaves would freeze in the cold; shrivel in the wind, dry air and intense sunlight; be torn by gales; and crushed by the weight of falling snow. Leaves that have evolved in lowland forests are unsuited to the alpine region, yet photosynthesis still has to take place if plants are to live and grow in these areas. Cold slows down the process of photosynthesis, and excessive water loss causes plants to wilt and even die. Cold and water loss go hand in hand in the

mountains where the chill wind whips away the warmth from the sunlit ground, and steals moisture from the pores on the lower surfaces of the leaves.

Alpine plants have developed ways of living that allow them to reduce the loss of warmth and water. Shrubs huddle together while cushion plants crouch close to the sun-warmed earth. The most extraordinary of the cushion plants are the vegetable sheep — species of Raoulia and Haastia. Every plant is a model of mutual cooperation. At the tip of each branch, tiny woolly leaves are tightly clustered. Then the branches are packed together so tightly that you could walk upon the surface, and the whole plant canopy is domed to shed the blast of the fiercest gale and the weight of the snow. Underneath this diminutive canopy is a different world, a miniature climate of

still, moist air, several degrees warmer than the winds raging overhead. The growing buds are nourished by a soggy mulch of the plant’s own dead leaves and branches. Nothing protrudes from the community of the vegetable sheep except after flowering when the thistledown fruits are raised for their seed to be carried away in the wind. But even if the form of many alpine plants provides a more favourable microclimate for photosynthesis, there is still no place for the broad, thin leaf of the lowlands. While a broad surface enhances photosynthesis, the price is high. Only the toughest of leaves, coated with waterproof wax or woolly with hairs can survive the brutal wind and snow. Shrub daisies, like Brachyglottis (formerly Senecio) bidwillii, have thick rounded leaves. Their glossy green surfaces endure hail and storm yet absorb the vital energy of the sunshine. The lower leaf surfaces wear a woolly carpet, not to warm the leaves, but to trap the water evaporating from the stomata — controlled portholes that also absorb carbon dioxide for photosynthesis. The stomata must stay open to let in the carbon dioxide, and in the process, water vapour escapes. If the stomata were

unprotected, the wind would constantly blow away the water vapour and the plant would wilt under the stress of dragging more water up from the frozen soil. The woolly layer over the stomata means that lost water molecules must first meander through the still air of a forest of hairs ensuring that evaporation is much reduced. Another daisy, Craspedia incana, also goes to great lengths to reduce water loss. Its leaves are like sheep’s ears, entirely covered with a white, woolly coat. So efficiently does this trap and still the air that the leaf surface may be as much as 10 to 15 degrees warmer than its surroundings. But the blanket comes at a cost: it also shields the leaves from the sun, and slows down photosynthesis and consequently the plant’s growth. The snow tussocks — various species of Chionochloa — with their tall, thin, relatively hairless leaves appear at first glance to be ill-equipped to cope with the drying winds. But their stomata are sunken in grooves and when water is scarce, each leaf will roll into a cylinder with the stomata inside, shielding the vulnerable pores from the outside weather. Their rapier-like leaves also draw water from the air in damp, foggy weather. So even when it isn’t raining, water droplets catch on the leaves and slide down to water the roots below. HESE ARE JUST SOME of the ways plant life has adapted to the challenges of living in the mountains. More than 600 species, most of them flowering plants, live above the treeline on New Zealand mountains. Of these, an amazing 93 percent are found only in the New Zealand region. Such a large suite of unique plants, with such elegant adaptations for conditions above the snow line, might suggest that our alpine plants have been evolving on the mountains for an immense period of time. But this is not the case. New Zealand’s mountains are young — less than two million years old — so our alpine plants are Johnny-come-latelys in evolutionary terms.

How has such a large and distinct alpine flora evolved here in such a relatively short time? Theories fall into two camps. Either we are seeing an explosion of evolutionary diversity, or somewhere, a nucleus of cold-tolerant plants persisted through the warm, mountain-less millennia, to form the basis of the present diversity of alpine plants. Possible refuges for the hypothetical ancestral plants include Antarctica, before it became ice covered; a southern extension of New Zealand; or infertile or rocky places in the lowlands that mimic some alpine conditions. Such refuges may account for species like the snowgrasses, hebes, spaniards and members of the genus Dracophyllum, which have few close relatives outside New Zealand, suggesting they have evolved here during a long period of isolation. Other species like the willow herbs, the sedges and the rushes have close relations overseas, and might possibly have mountain-hopped from Asia, Indonesia, New Guinea and eastern Australia — their seed blown by the wind or carried by birds. Other alpine species may be derived from warm-climate forest ancestors within New Zealand, such as the twiggy shrubs of the genera Coprosma and Myrsine. All these sources probably contributed to the alpine flora but the extra stimulus for its diversity may well have been the tumultuous birth of the mountains themselves, creating a host of new places for plants to grow. For mountain plants, New Zealand has truly been a land of opportunity.

ANN GRAEME 1s the national coordinator of Forest and Bird’s Kiwi Conservation Club.