The Fading of Fucnsia

Forest and Bird, Issue 299, 1 February 2001, Page 28

The Fading of Fucnsia

is a PhD student.

— LAURA A. SESSIONS

LAURA A. SESSIONS

wonders if the fate of tree fuchsia is an early-warning signal.

ative tree fuchsia, once common throughout New Zealand, is becoming increasingly rare in many areas. Scientists trying to figure out why it is disappearing have recently been led to the startling realisation that its decline may signal a similar decrease in numbers of other native plants. Like a canary in a coal mine, tree fuchsia may warn us of where the dangers are the greatest for these other species. New Zealand tree fuchsia, or kotukutuku Fuchsia excorticata, is the most common of New Zealand’s three fuchsia species and one of our few deciduous native trees. Of the 100 fuchsia species worldwide (all are confined to the

Americas except the New Zealand species and one in Tahiti), kotukutuku is the largest, occasionally growing up to 12 metres high. In 1910, Cockayne noted that "There is hardly a forest in New Zealand where the native fuchsia, the kotukutuku of the Maoris, may not be seen. Now, almost a century later, tree fuchsia is becoming increasingly rare in many areas, though it still has a wide geographical distribution from North Cape in the subtropics to Auckland Island in the subantarctic. The most obvious threat to tree fuchsia comes from introduced possums, which devour its tender leaves. A study in South Westland in the 1990s found that the amount of damage to fuchsia depends directly on the number of possums at a site, so fuchsia suffers the most "die back’ in areas with high possum densities. Similarly, the longer possums occupy an area, the less likely large tree fuchsia and young seedlings will survive. On the other hand, once possums are controlled, fuchsia can recover, suggesting that pest control is an effective — if temporary — solution to its decline. Possums threaten many of our native plant species but, as a highly preferred possum food, fuchsia can be especially sensitive to possum damage in some areas. At these sites, fuchsia can serve as an early indicator of how severe overall

How good is pollination in your area? ith a little knowledge of how fuchsia pollination works, you can find out well pollination is working in your area. Fuchsia pollination is easy to study because flowers have bright yellow stigmas (for receiving the pollen) and bright blue pollen (you may have noticed it before on the head of a tui or bellbird). You can examine the stigma to see how many blue pollen grains are glued to its sticky top. The more pollen grains on the stigma, the more likely the flower is to produce a fruit, and the more seeds it will produce within that fruit — up to 500 of them. After looking at a number of flowers from different plants, you can get a feel for the success of pollination by counting the percentage of stigmas with lots of pollen. If you rarely see pollen on the stigmas, the plants may be in trouble.

ermaphrodite flowers (right) are 4 generally larger than females, and the anthers in female flowers are shrivelled into short stumps (left). Hermaphrodite flowers with close anthers and stigmas usually have more pollen on their stigmas, because they can receive pollen from their own anthers as well as from pollinators. This makes it easier for them to produce more seeds. Hermaphrodite flowers with widely separated anthers and stigma rely on a pollinator to bring pollen from another flower, just like female flowers do.

possum damage is in that habitat. However, this alarm system does not always work, because healthy populations of fuchsia survive in some areas that have been occupied by possums for many years. Other plants at these sites are often damaged by possums more than fuchsia, and why these fuchsia populations have mostly escaped the possum threat is still

largely a mystery. Two Landcare Research scientists, Peter Sweetapple and Graham Nugent, have found that the browse-tolerant populations are just as palatable to captive possums as the browsed fuchsia populations, and it is unlikely that the unbrowsed plants are genetically different from the browsed ones. Instead, the difference is more likely to be related to variations in the relative abundance of different food sources between sites.

t now seems that, in addition to possums, fuchsia may also be at risk from a less-obvious but potentially more-dan-gerous threat that scientists around the world are calling ‘the pollination crisis. This crisis, which has been found on every continent, involves the dismantling of complex ecological relationships between plants and their animal pollinators. In most cases, the animal pollinators have declined due to diverse ecological pressures (such as habitat destruction and the introduction of exotic predators). Many plant species rely on these pollinators to move pollen between flowers, and without them, plants cannot produce new seeds or plants to replace old and dying ones. Scientists have begun documenting this crisis in New Zealand, and fuchsia is one of the first potential victims that they have found. It is likely that other native plants are suffering similarly, but fuchsia may provide clues to help identify where pollination is most limited. Tree fuchsia has a complex reproductive system with two different genders of plant. Female flowers have no functioning male anthers to produce pollen, and they must receive pollen from another plant to produce seeds. In contrast, hermaphrodite plants have flowers with both functioning male anthers, that produce pollen, and a female stigma that receives the pollen. Many of these hermaphrodite flowers can pollinate themselves just by transferring the pollen from their anthers to their own stigma. Some hermaphrodite flowers are unable to self-pollinate, however, because of the distance between their anthers and stigma. Additionally, gene exchange through pollination also helps populations maintain their genetic diversity, which would be limited if plants only reproduced through self-pollination.

ui and bellbird are the primary animal pollinators of fuchsia and they also help disperse its seeds to sites that are suitable for germination. Silvereyes and bumblebees may rob nectar from fuchsia flowers, but they do not help pollination by spreading pollen. Both are too small to reach the nectar from the mouth of the flower, so they usually steal the nectar by cutting holes near the flower’s base. Limited dispersal is probably less of a problem for fuchsia than limited pollination, because the fruits (known as konini) have more time to be dispersed than flowers have to be pollinated. Also, the fruits are dispersed by a wider range of birds, including kereru and silvereyes. A team of ecologists led by Alastair Robertson (Massey University), Dave Kelly and Jenny Ladley (both of Canterbury University) have found that the more pollen a fuchsia flower receives the greater its chances are of making a fruit, and producing more seeds within that fruit (fruits can contain up to 500 tiny seeds). Knowing this, they have compared the amount of pollen that flowers are receiving through pollinators to the amount received when no pollinators can reach the flowers (by placing flowers in bags). The results show that in many areas there were so few natural pollinators still living in the forest that they did little to help fuchsia reproduce. This low level of cross-pollination by birds at many sites may be a result of the recent decline in the number of hon-

eyeaters due to an increase in introduced predators such as stoats, cats and rats. The densities of tui and bellbird on the mainland are now much lower than those seen on island sanctuaries such as Tiritiri Matangi, Little Barrier and Kapiti. It is not known how many other plant species might be affected by the loss of these bird pollinators, but at least one other species, red mistletoe Peraxilla tetrapetala, is also suffering from lack of natural pollinators which suggests that the crisis extends beyond fuchsia alone. Ecologists are currently studying other plants that rely on similar bird pollinators to find out whether these species also have low pollination rates. Perhaps because fuchsia is still relatively common, especially along roadsides and in urban areas where they are easy to see, we have not fully appreciated how significant its decline in many areas could be. Its flowers and fruits are an important food for many native birds, it is an important coloniser of recently disturbed areas, and it may help prevent erosion near rivers and streams. Most importantly, as a species sensitive to threats such as possum damage and loss of bird pollinators, tree fuchsia may provide an alarm system to warn of the potential dangers many other plant species may face.

Why fuchsia flowers turn red he flowers of many plants T colour to signal to their pollinators that they are mature and ready for visitors to take nectar and pollen. Scientists have long thought that red flowers evolved to attract birds to a plant, because most birds worldwide have very poor senses of smell but good colour vision. Fuchsia illustrates an opposite phenomenon, however: the red flowers dissuade rather than attract birds. Fuchsia flowers change from green to red like other flowers, but turn red only after they stop producing nectar and can no longer be fertilised. Thus, the green flowers are attractive and are visited, while red flowers have virtually no nectar and are mostly ignored by birds and bees. The strange colour change probably occurs because it allows birds to spend the maximum time possible on receptive (green) flowers that can still be fertilised, rather than wasting time on (red) flowers that have already been pollinated. The red colour might not be needed as an attractant to birds in New Zealand, because our honeyeaters spend enough time in one area to learn the location of trees without flashy floral displays. You might wonder why the plant doesn’t just drop the flower petals once the flowers are pollinated rather than wasting the energy needed to change colour. The further colour change makes sense because once a flower receives pollen, it can take four or five days for the pollen to grow down through a pollen tube in the female style to reach and fertilise the female ovary. Because the style is attached to the flower petals, the pollen would be lost if the petals were to fall off during this period.