The following is a text I wrote for tthe British Ecological Society Bulletin — that was published last month (June 2012). I wrote it to attract interest to a challenge that requires more attention from ecologists and others. I hope you find it of interest.
Science journals favour tidy theory and rigorous results, but ecological science can also be advanced through highlighting unfamiliar unknowns and quirky questions. For many of us such unknowns are what makes research fun. I want to share a problem (don’t worry, it is about ecology). Maybe you can help.
Our understanding of tropical forest dynamics has advanced considerably over recent decades. We now have vast data-sets tracking hundreds of thousands of tree stems over areas of 50 hectares or more. Numerous processes have been quantified in detail. Remaining unknowns may appear to be minor gaps that will soon be filled. But, for some locations, these impressions are misleading.
For the last few years I have been based in the Bwindi Impenetrable National Park – a World Heritage Site in Southwest Uganda. We can see Rwanda and the Democratic Republic of Congo from here. The site is known for supporting half the World’s remaining mountain gorillas; a major “see them before you die”-tourist-draw that brings in significant revenues to support conservation and the region’s economy. I help run a research station that focuses on the needs of local conservation practitioners (see ITFC.org). We have various day-to-day challenges ourselves – of which I may share more another time – but here I want to focus on our inadequate grasp of forest dynamics.
Living in a salad bowl: fewer than 800 mountain gorillas remain (Photograph Douglas Sheil)
What’s in a name
First, consider the name Bwindi Impenetrable National Park. That “Impenetrable” often leads to raised eyebrows, disbelieving laughter and accusations of ill-judged humour. But it is not a joke: “Impenetrable” is there in the official name. Ask why and you reveal you have not yet been here. Bwindi is rugged, steep and divided by cliffs but the key feature is the thick understorey. It is near impossible to walk through. The main element of this understorey challenge, superseding even the impressively rich flora of noxious nettles, barbed briars and spiny Acanthus, is its remarkable density. Understanding this forest by looking at the trees alone is probably harder than walking through it without tripping over the understorey.
Impenetrable forest – where are the seedlings? (Photograph Douglas Sheil)
Let me sketch out a few local features. The climate here is cool with altitudes up to 2,600m (we huddle at the fire at night). Being one degree south of the equator, seasonal variation is limited but we have two wetter and two drier seasons. Much of the forest canopy is open and there are extensive clearings – this likely reflects slow or stalled recovery from past disturbance (human activities, fires, landslides and elephants are all blamed). Small trees are scarce over large areas – suggesting limited regeneration. Many clearings are filled with persistent bracken fern Pteridium aquilinum (L.) Kuhn.reminiscent of the forests near where I grew up in Ireland. Other understorey species dominate other areas both with and without tree cover. Many of the common understorey species are synchronously monocarpic, living for a decade or more before flowering, seeding and dying en masse. . Examples include the African mountain bamboo (Yushania alpina (K. Schum.) Lin. Poaceae). Like many gregarious bamboo species elsewhere, it forms extensive stands and flowers only every few decades. Other monocarpic understorey plants include various Acanthaceae including several Mimulopsis spp.. One thicket forming species is Mimulopsis arborescens C.B. Clarke. It grows to 4 meters tall and bears multiple soft-woody stems with interlocking stilt-roots that are near impossible to walk through – this plant covers large areas of the forest. Along with another common monocarpic Mimulopsis species (M. solmsii Schweinf.) this is among the mountain gorillas’ most plentiful food plants. Both these Mimulopsis species flowered, seeded and died over the last two years.
ITFC researchers work among the woody remains and seedling carpets of the monocarpic liana Sericostachys scandens (Amaranthaceae): note the bamboo in the background (Photograph Douglas Sheil)
A liana that was abundant in Bwindi just three years ago is also monocarpic. Sericostachys scandens Gilg. & Lopr. (Amaranthaceae) used to cover almost every tree over large areas of forest. Then 3 years ago it flowered and for months its fluffy seeds were everywhere. It is now hard to spot a plant aside from seedlings. Large dead crumbling stems lie in heaps in parts of the forest.
So to recap: large areas of the forest are dominated by a dense understorey. Many of these plants are monocarpic and achieve high densities in cycles that must impact the recruitment opportunities of other plants. Trees are locally patchy and seedlings are often rare. The behaviour of the non-tree vegetation appears key in understanding the dynamics of these forests and, by implication, the animal species such as the mountain gorillas that they sustain.
These issues are not simply a matter of curiosity: major concerns have been raised about the long term management of the forest. These are practical questions. What, for example, should be done about the forest’s vulnerability to fire and to alien species (such as Lantana camara L. now spreading in the northern lower part of the forest)? Specific questions have been raised about maintaining conservation values. Recently the park authorities suggested it may be necessary to artificially maintain gorilla food species by cutting trees. We don’t have anything close to the understanding needed to address these concerns with confidence.
Fun fun fun
How can we get a handle on the key relationships within this complex patchy mountain forest vegetation? How can we, in only a few years of affordable research, grasp how outcomes are determined? If we can’t do that can we at least begin to clarify, gather and store the information that future researchers will need to better address these impenetrable problems? We have some plots with a few thousand tagged trees already. We also acknowledge valuable research elsewhere on seedling-understorey interactions, bracken control, and many other key issues. But we’ll need more, including the sustained funding to achieve it. So can we address this in easy bite-sized pieces? Ideally we would support local students and build capacity as we did it. I am hoping you may have suggestions. If you do please let me know, better still drop by and see just how impenetrable the forest here really is.
Left: Bwindi before, and right:, after last year’s understorey die-back (Photographs by Miriam van Heist)
Hope to hear your thoughts.