|December 2013. A photo was sent showing Merremia smothering all else in Pohnpei, Federated States of Micronesia.
An article in New Scientist, Planet of the Vines can be viewed here.
As a member said, the thought that vines will overrun tropical forests is scary. Introduced invasive vines will overrun forests. One can see Merremia peltata doing just that in eastern Melanesia and some Polynesian islands. Its changing the ecology of whole forests, particularly the survival of epiphytic flora and fauna, of which not too many people are aware.
According to the observations of the original sender of the message, Merremia peltata is spreading and taking over when areas are cleared, e.g. building roads or clearings for agriculture or constructions. Where is forest is intact we do not find it. Lately we have found African tulip in the forest which is another worry.
There are other causes, too, such as natural events: cyclones, lightning strikes and wildfires. Also, isn’t it true that native forest soils are declining in fertility because the clouds of natural fertilisers, seabirds, are missing from many or most islands? The reduced vitality makes it easier for invasive vines to dominate native trees. Does anyone know whether native vines are responding in a similar manner to invasive ones?
Any cyclone, lightening strike or fire that opens up the canopy allows Merremia, which is a pioneering species, to take hold and dominate, usually along boundaries and not in intact native forest, at least that is the case in Fiji and some of the older islands. It could be more invasive on some recent islands like those of Samoa, where after tropical Cyclones Vale and ‘Ofa in the early 1990s, Merremia took over with a vengeance.
In addition, a long post was received from a member in Wageningen, and this is what he wrote:
1. The natural development of vines
In a rainforest there usually is a stand of trees of different age. “Coming” or “future” trees struggle to reach the canopy and are in fierce competition with their neighbours. Dominating trees spread out and occupy as much as possible the top level, shading out the vegetation below. These two categories usually do not allow vines to take over their place or their space in the canopy. However with age, the vitality of dominant trees and of coming trees that lost the competition, deteriorates. Their foliage gets thin, opening up the canopy allowing more light to reach lower levels in the forest. This is what the struggling newcomers in the undergrowth are waiting for. It is in regressing trees that vines become more prominent and indeed after colonizing the canopy of a declining tree, vines will try to get more of the sunlight and the nutrients brought by rains by spreading into neighbouring healthy trees, blocking the access of the neighbours to these vital requirements and thus inducing their decline. This a natural ecological process.
2. Human induced development of viney forest
Vines are a problem in over-exploited forests. In the Guyanas a maximum of around 5 cubic meters of wood can be extracted per hectare without irreversibly damaging the forest ecosystem. If more is extracted, pioneers such as thorny Solanum spp., Cecropia and early secondary species will develop, but vines will be growing and climbing faster than these others (in Asia the rattans are examples of similar behaviour). In general it can be said that over-expoitation leads to a viney forest.
3. Remarks on dynamic drivers
We often think of rainforest as a kind of uniform ecosystem, which it is not. For instance, at Victoria falls (Zambia-Zimbabwe) a vegetation developed in the sprays of the falls and local people call that a rainforest, which is in no way comparable to equatorial rainforest. Between the forests of Asia, Africa and the Americas there are huge differences although there are also parallels: Cecropia in South America has a comparable ecological function as Musanga in equatorial Africa. However, there seems to be a little understood phenomena that in a rainforest the vegetation enters collectively into a generative phase for some decades or maybe even more than a century in one region, while in other regions, the forests remains mainly in a vegetative state. This was pointed out to me by the late John Procter, a forestry expert with extensive experience in many tropical regions, who was my colleague in Suriname in the late 1970’s. He told me that the South American forest is presently in such a type of regenerative state, a condition that seems to occur in every tropical rainforest every 300-400 years. As the rainforest of the Guyanas (and maybe also in the other rainforest regions of the Americas) is in this regenerative state now, natural rejuvenation can be used to prolong timber production, with a successive exploitation after some 25-40 years (the experience with this system is now entering the third exploitation phase). This so called CELOS Forest Management System (see Werger, M.A. (editor) 2011. Sustainable management of tropical rainforests. Tropenbos Series 25, Paramaribo, Suriname. ISBN 978-90-5113-101-7) makes use of natural regeneration, selective poisoning of unwanted trees, killing of vines in over-exploited spots and planned felling, reducing logging damage to the minimum. However, the system seems not be working as well in equatorial Africa or South-East Asia where rejuvenation of the vegetation is much less. This may explain the difference of viney forest development between tropical regions.
4. A view from Palau
Is it possible that Merremia and other vines are playing a beneficial ecological role following natural disturbances, such those that occur from cyclones? Perhaps their ecological role is to temporarily cover the land which has been violently cleared, moderating soil temperatures and preventing erosion, providing cover for small wildlife, adding organic matter to the soil, etc. This point was supported by the International Project Coordinator/CABI-SEA FORIS project, who said that perhaps Merremia has a role to play in Palau,. Sometimes we tend to be overzealous about “branding” a plant as an invasive without consdidering teh context. Merremia is a target for management in the Bukit Barisan National Park, Sumatra ().
Landcare, New Zealand are about to begin a molecular study in the Cook Islands to determine how Meremmia first came into the Pacific.