Kosola, K. and D. Dickmann
Presented at the All Scientist Meeting (1996-07-16 to 1996-07-17 )
This work is part of the grant “Ecosystem responses to catastrophic defoliation of Populus”, a collaborative effort among entomologists (Mark Scriber, Dan Herms, Dylan Parry, and Olin Silander), biogeochemists (Eldor Paul, Phil Robertson, Craig Russel, Dave Harris, and Tracey Hruska), and tree ecophysiologists (Don Dickmann, Kevin Kosola, and Marla Fisher).How do poplars respond to severe defoliation? Gypsy moth caterpillars feed during June and early July. During this time, tree carbon and nitrogen storage pools are at a yearly low, as most carbon and nitrogen has been placed in the leaves. If most of the leaves are eaten by caterpillars, the tree has lost a large portion of its nitrogen at the same time that it looses the potential carbon gain from those leaves.Severe defoliation should cause both short-term and long-term changes in patterns of carbon allocation within the tree and in leaf composition. These changes in tree physiology will likely have cascading effects on ecosystem processes, as they can substantially alter carbon, nitrogen, and water fluxes in the forest ecosystem.Bassman and Dickmann (1985) have shown that seedlings of Populus rapidly shut down transport of photosynthate to the roots following an artificial defoliation, as production of new leaves becomes the strongest carbon sink within the tree. What are the consequences of this on root composition, growth and mortality? We are monitoring root turnover with minirhizotron tubes. Regular sampling of roots from soil cores will allow us to track shifts in root composition, especially pools of non-structural carbohydrates. Will root mortality increase? Will mycorrhizal colonization decrease? Both are potential responses to lower carbon supply to roots that we will be able to detect with regular root sampling and minirhizotron observations.Long-term changes in tissue composition are also likely to be a consequence of defoliation. Increased levels of carbon-based defensive compounds are typically found in leaves produced after defoliation- a phenomenon commonly called “delayed induced resistance”. The symptoms are similar to nitrogen stress, and, in fact, fertilization of trees with nitrogen following defoliation can eliminate delayed induced resistance (Haukioja and Neuvonen, 1985). We are monitoring leaf and root phenolic contents in fertilized and unfertilized subplots to determine whether phenolic contents increase in both roots and leaves following defoliation in a manner consistent with delayed induced resistance. Studies of plant-insect interactions most often focus on leaves- I am not aware of previous studies of changes in root phenolic contents following defoliation.Defoliation may also alter ecosystem water fluxes. Loss of leaf area will reduce total tree transpiration, and can potentially increase water flow to groundwater. We are monitoring tree sap flow in one plot, and simultaneously measuring soil water content.Large-scale defoliation can have ecosystem-level consequences in a forest- the poplar stands on the LTER provide us with a simplified system in which we can examine defoliation-induced changes in fluxes of carbon, nitrogen, and water in trees, and follow the effects of these changes throughout the ecosystem.
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