Their team used drones to solve the problem. By repeatedly piloting drones over 1500 hectares of forest during a five-year period, STRI research technician, Milton Garcia, provided a series of very high-resolution images that could be used to visualize much smaller disturbances. Based on the photos, they came up with a model of canopy height.
They defined a disturbance as an area where canopy height decreased by more than 5 meters over a contiguous area of 25 square meters. And then they did an analysis of changes in canopy height for each time period, identifying 11,153 thousand canopy disturbances greater than 25 square meters in area. These disturbances included treefalls, branch falls and dead trees that were still standing.
Another recent study led by STRI post-doctoral fellow, Raquel Araujo, analyzed monthly data from the ForestGEO long-term forest dynamics study site on Barro Colorado and found that most trees fell rather than dying while still standing, and it was more common for trunks to break than for the trees to be completely uprooted. Treefalls were concentrated during periods of heavy rainfall.
Disturbance rates were three times greater in some areas than in others. Most of the difference in disturbance from place to place could be predicted by three factors: the age of the forest, the type of soil and whether the land was steeply sloping or flatter (topography). Disturbances were more common in older than younger forests, as would be expected because trees are older and because tree height varies more in older forests, leaving some tall trees more exposed to storm damage. Soil type may be important because of the ability of trees to form deep roots. And greater disturbance on steep slopes could be explained simply by the steepness or because steep slopes are often more exposed to weather.
The team was pleased with the outcome of drone image analysis to accurately predict forest disturbances across a large area.
"To follow up, we hope to find out why more trees growing in certain soils died, and to test whether some tree species are more common in areas with high, as opposed to low rates of disturbance," said Cushman. "Other collaborators with the Muller-Landau group want to place sensors on individual trees to find out how trees move during storms, and to create mechanistic models of mortality that capture the influences of windthrow, lightning, and more."
"If tree mortality rates increase, then trees will be on average smaller-;both shorter and smaller in diameter-;forest canopy height will on average be shorter and forest carbon stocks will be smaller," said Muller-Landau. Tree death is directly related to a decrease in the amount of carbon that a forest stores. A roughly 10% increase in mortality rate means a 9% decrease in forest carbon stocks, all else being equal."
"Drones have given us a whole new perspective on tropical tree mortality on Barro Colorado Island, an iconic, long-term study site, by enabling us to monitor large areas and precisely pinpoint when trees die," said Muller-Landau. Drone data complements traditional ground-based tree censuses here and remote sensing projects that have shown differences in forest structure across this landscape– but this is the first time that anyone has shown that these differences are driven by variation in tree mortality rates, which themselves are due to variation in soils, topography, and forest age."
The Smithsonian Tropical Research Institute, headquartered in Panama City, Panama, is a unit of the Smithsonian Institution. The institute furthers the understanding of tropical biodiversity and its importance to human welfare, trains students to conduct research in the tropics and promotes conservation by increasing public awareness of the beauty and importance of tropical ecosystems.