George W. Koch, IOS- 1010769, A rapid assessment of post-fire changes in biophysical variables, carbon stocks, and soil microbial processes in the tallest angiosperm forest.
The project examines ecological impacts of the February 2009 fire in the tallest broad-leaf forest on Earth, the Wallaby Creek watershed in Victoria, Australia dominated by Eucalyptus regnans (?Mountain Ash?). Mountain Ash is among the fastest growing trees, and its forests are recognized for their high carbon sequestration potential, yet the prevalence of fires in eucalypt forests raises questions of the permanence of that carbon. The investigators have studied this forest since 2005 and have extensive information on pre-fire conditions, including highly accurate estimates of the mass of individual trees. This follow-up study assesses changes in carbon stocks of soils and trees killed in the fire and samples the crowns of dead trees to test fundamental ideas about height-related variation in wood anatomical and chemical properties. Samples of soils and trees collected from burned and unburned stands, and measurements of dimensions of trees studied prior to the fire, will be used to estimate changes in carbon stocks. Wood samples collected from different heights within the crowns of dead trees will be analyzed in the laboratory to test theories about the optimal design of the dimensions of the water-conducting cells in extremely tall trees. Analysis of the chemical composition (the relative abundance of two stable isotopes of carbon) of wood samples will be used to understand variation in long-term, integrated water stress. The study will produce new understanding of the constraints on tree height and size growth and of the role of fire in the forest carbon cycle, an issue of increasing importance as fire frequency and severity increases with climate change in some regions. The study will involve U.S. students (undergraduate and graduate), postdoctoral associates, and faculty who will collaborate with Australian technical assistants and university researchers.
The project’s goal was to take advantage of a short-term opportunity afforded by a catastrophic wildfire that occurred in February 2009 in a mountain ash (Eucalyptus regnans) forest in the state of Victoria, Australia. This forest type is known to have among the highest stocks of forest carbon in the world, mountain ash is the tallest broadleaf tree known, and eucalyptus forests have evolved in an environment characterized by fire. Previous work at this site by members of the project team involved climbing mountain ash trees over 300 feet tall to quantify growth rates and examine physiological features related to this species’ ability to grow to extreme height. This work documented that, contrary to previous understanding, the largest and oldest trees were producing the most new wood each year. The physiological studies from that earlier work indicated that leaves at the tops of the taller trees experience greater water stress and reduced rates of photosynthesis compared to shorter trees. Thus, it was the enormous quantity of leaves of the largest trees that enabled their very high growth rates. The fire of February 2009 killed nearly 100% of the mountain ash trees and much of the understory vegetation at our study site, the Wallaby Creek watershed adjacent to Kinglake National Park. Although an ecological catastrophe in the short-term, this event provided us the opportunity to climb the dead trees, which were still structurally sound, and make measurements that would have been overly destructive in the living trees. Specifically, we were able to cut out large wood samples from throughout the trunk and branch system that we needed to understand details of the design of the water conducting system of these giant trees as well as to examine in more detail the consequences of the increased water stress experienced by taller trees that we had observed previously. In addition, we took this opportunity to collect data on the amount of tree biomass that was killed by the fire and that was therefore destined to decompose and be released back to the atmosphere as carbon dioxide. The project activities were concentrated in a single 10 day visit to the burned forest site in March of 2010, followed by subsequent laboratory work and data analysis. To date, we have learned several things from the study: 1) analysis of chemical properties of wood from different heights in the tree indicate that wood growth throughout the tree is apparently supported primarily by photosynthesis of the leaves in the upper one-third of the tree crown; 2) although very little of the mountain ash trees burned (the fire that swept through the forest rapidly and burned the understory but killed the large overstory trees with its intense heat), there is an enormous quantity of dead tree biomass now poised to fall to the ground and decompose. There is nearly 1300 tons of dead tree biomass per hectare (1 hectare is about 2 ½ acres) in this forest; 3) Decomposition of leaves of mountain ash trees was very rapid, but it will require additional visits to the site to determine how rapidly the wood from the dead trees is decomposing. Although there has been vigorous germination and regrowth of new vegetation, including mountain ash trees, it will be several centuries before this forest re-accumulates the vast quantity of wood in living trees that it contained prior to the fire. Over the coming decades the rate of decomposition of wood from trees killed by the fire will likely exceed the rate of production of new wood by regenerating trees. Thus, this forest may act as a net source of carbon dioxide to the atmosphere for many years as a result of the wildfire of 2009. Wildfire in this system, as in other fire prone forest types worldwide, is affected by climatic conditions, increasing during hot, dry periods. Forest fires release heat-trapping gases (especially carbon dioxide) as a result of combustion during the fire. As this study has documented, they also leave large quantities of dead wood that is subject to subsequent decomposition, which further adds to carbon dioxide emissions to the atmosphere.
