A 3D perspective on the effects of topography and wind on forest height and dynamics

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Strong winds can unleash huge destructive power on forests. For example, Britain's Great Storm of 1987 blew over about 15 million trees. Most research into wind damage occurs in temperate plantations, because of their social and economic value, but much less is known about the impact of wind on tropical rainforest. We recently used survey forests in Sabah (a Malaysian state on the island of Borneo) using airborne laser scanning, as part of a NERC-funded programme into human-modified tropical forests. We spotted the tallest tree ever found in the tropics in the laser scans: an 89-m tall individual in the Dipterocarpaceae family of trees, which are renowned for producing giants. This discovery raises questions about the mechanical limits of height in tropical forests and the reason why Sabah's lowland forests grow so much taller than those of Amazonia and Africa. One idea is that trees are particularly tall in sheltered hillsides in Sabah because they seldom experience strong winds. Another idea is that Dipterocarps have evolved a unique set of mechanical properties that enable them to withstand strong wind better than other tropical trees. Both of these theories need critical evaluation.The project addresses these ideas, by studying wind flow and tree responses in 40 square kilometres of old-growth hill forest situated in Sabah, where NERC's airborne research facility has already mapped ground elevation, canopy height and foliar nutrient concentrations by laser scanning (ALS) and imaging spectroscopy. The tallest forests in the tropics juxtapose with stunted heath forests in these hills, making the landscape ideal for evaluating the role of wind in determining forest height and dynamics. We will compare wind flows in the immensely tall forests of Borneo with those found in much shorter forests of South America (French Guiana) and the hurricane-impacted Caribbean (Puerto Rico), including an analysis of how wind is funneled through hilly landscapes resulting in strong wind forces on forests in exposed sites. We will also quantify the susceptibility of individual trees to windthrow by a novel approach involving 3D reconstruction of hundreds of rainforest trees using terrestrial laser scanning and simulation modelling of stresses within their trunks when exposed to wind gusts. Combining the wind flow maps with the tree susceptibility will allow us to produce maps showing where we think trees are most at risk of windthrow in each tropical region. The key question is whether sites predicted to be at high risk and found to be wind damaged. To test this, we will use the latest advances in repeat-survey ALS to track growth and mortality of about 200,000 tall trees (>30 m). Measuring tree demography at this unprecedented scale will provide a unique and powerful data set with which to test whether wind is a major determinant of tree death and canopy height. Finally, we will put this work in a global context. Wood density, modulus of elasticity and tree architecture of Bornean trees will be compared with values from French Guianan and pan-tropical datasets, to evaluate whether the exceptional tall trees are mechanically distinct. Simulations will be run using wind regimes from other tropical regions to assess whether a lack of extreme wind events may have contributed to the development of such tall forests in Borneo. ALS-measured canopy height is closely related to the aboveground carbon density, so these analyses will provide a wall-to-wall perspective on carbon dynamics in the context of climate change.