Lightning: An invisible driver of tree mortality in the tropics?

Tropical forests are one of the most important and diverse ecosystems on Earth; they act as a vast store for living carbon and, in doing so they help mitigate climate change by lowering atmospheric levels of the greenhouse gas carbon dioxide. However, in recent years, research has revealed an increase in the rate of tropic tree mortality, with the consequence that the strength of the carbon sink provided by tropical forests is reducing. It is therefore vital that we understand why tropical trees die and how this might change with climate change.

This project will provide the very first assessment of the number of trees that are killed by lightning in tropical forests. We know that lightning can, and does, kill large trees. We also know that lightning strikes are most powerful and frequent in the tropics. Our estimates indicate that lightning strikes could affect trees containing over 1 % of the tropical forest biomass every year. If all these trees died it would indicate that lightning was a major controlling factor of tropical tree mortality rates. Worryingly, research has predicted that the rates of lightning strikes will increase significantly with climate change. Based on the most recent climate model simulations, lightning could increase by as much as 22 % to 60 % by 2100; Such an increase in lightning could substantially increase tree mortality, altering forest dynamics, and reducing the efficacy of tropical forests as a carbon store.

Despite the potential significance of lightning induced tree mortality, very little is actually known about this process. This lack of knowledge arises from the simple fact that it is impossible to predict exactly when and where lightning will strike. This uncertainty makes the effects of lightning extremely hard to observe. An added complication is that trees damaged by lightning may not show any external signs of damage, making it impossible to attribute their death to lightning solely on the basis of visual observations.

We propose to address the knowledge gap about lightning induced tree mortality with a revolutionary approach to observing lightning strikes on trees. To study the impacts of lightning on trees we have selected two high biomass tropical forest sites located in regions of high lightning activity in Nigeria and Cameroon. Unlike past studies that relied on visual observations, we will, for the first time, deploy sensors on 20,000 trees to provide an unambiguous record of lightning strikes over a 4 year period. We have adapted a sensor commonly used by electrical engineers to monitor electrical current and lightning strikes (called the Rogowski Coil) to make it inexpensive and easy to deploy in the field in large numbers. We have successfully tested our new version of this sensor in Cardiff University's unique lightning laboratory.

By tracking a large cohort of trees we will be able to capture a large number of lightning strikes on trees and study these individuals to work out what happens following a lightning strike. We will use this information to determine which trees are struck by lightning, what happens to surrounding trees, how many trees are killed by lightning and how the carbon storage of the forest is affected. We will combine this information with environmental modelling to determine how lightning damages trees and induces mortality. Finally, we will estimate the tropical loss of biomass due to lightning strikes, and predict how biomass loss will be influenced by climate change.

This research will be the very first systematic study on the rates of lightning induced tree mortality in the tropics. This information is vital to our understanding of the terrestrial carbon cycle and its continuing efficacy as a carbon sink. Therefore, this research is a priority for making informed global policy decisions on climate change mitigation.

Lightning induced tree mortality will affect the forest structure and carbon storage potential of tropical forests. Given the role the ecosystem services provided by tropical forests play in moderating climate change through their globally significant stores of living carbon and the fact that they are biodiversity hotspots, it is highly likely that this research will be of great interest to the local governments, the Intergovernmental Panel on Climate Change (IPCC), the UK Met Office and Hadley Centre, governments internationally (particularly in tropical countries), NGOs and the general public. We have an established track record of creating impact with project partners and stakeholders through engaging with NGOs and governments to influence policy (see track record). Project investigators also have experience of delivering impact to other non-academic partners through media interviews, public lectures, and lectures to the UN's Environmental Programme.

The IPCC has a strong focus on improving simulations of the Earth system. In particular, there have been efforts to improve our understanding of climate-vegetation feedbacks within the carbon cycle. Our work addresses a novel aspect of the carbon cycle that is missing in Global Climate Models (GCMs). Therefore, the information we will gain about lightning induced tree mortality, and its response to climate change, is highly relevant to the work of the IPCC Working Group 1 in improving the scientific basis of climate change. Similarly, we will look to exploit the excellent links between the University of Exeter and the Met Office to ensure that our findings help them improve their vegetation simulation model, the Joint UK Land Environment Simulator (JULES).

We envisage close links to non-academic elements of the Smithsonian Institute, and will use their highly visited web site to convey our results to a wider non-academic audience. By extending the depth and breadth of the data available at their sites in Korup, Cameroon and Ngel Nyaki, Nigeria we will expand the impact research from these long term field stations has internationally. We also expect both the Nigerian and Cameroonian researchers, governments and NGOs to benefit from the understanding we will bring to the long term resilience of living carbon stored in tropical forests, by providing better foundations for sound forest management policy. For example, an increase in tree mortality due to lightning may increase the number of years required for sustainable harvest, and have negative financial impacts on the sector that could affect adoption of sustainable practices. Our results will therefore affect decisions by government and NGOs that feed in to the REDD+ programme, where it is vital to characterise the historical and future regimes of tropical forests in relation to 'natural' climate change (as opposed to direct land-use change). This work builds directly on previous impact engagement with stakeholders in tropical countries (see track record).

Finally we expect the project outcomes to be of interest to the general public and of particular interest to educators and pupils. The public awareness and interest in climate change-related issues is very high. This project will provide a powerful example of the influence of climate change on natural processes, and also illustrate the importance of stochastic processes in shaping the Earth system. We will seek to engage with schools through an existing programme of outreach by the research team. Within the University of Exeter, our team also has established links with The Eden project, the UK's number one eco-tourist attraction, and is directly involved in a project in which scientific findings about links the between weather and tropical forests will be displayed interactively to the public within Eden's Rainforest Biome. We will also make use of other media, such as project websites, video posts, blogs and tweeter feeds.