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Increased aridity is associated with stronger tradeoffs in ponderosa pine vital functions




Gonzalez, Angela D., author
Redmond, Miranda, advisor
Pearse, Ian, advisor
Bauerle, William, committee member

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Trees must allocate resources to core functions, like growth, defense, and reproduction, for populations to persist in the future. A changing climate is leading to more extreme droughts and hotter temperatures, and it is uncertain how trees will allocate resources to these functions under more extreme climatic conditions. Resource allocation patterns vary among individuals and in response to drought stress, limiting our understanding of how forests will respond to climate warming. Yet the impacts of drought on long term trends of reproductive investment have been seldom compared with growth or defensive traits simultaneously. We conducted a 21-year survey of growth, defense (resin duct area), and reproduction (cone production) in 80 ponderosa pine individuals spanning 8 populations along the Front Range of Colorado, USA. We used linear mixed models to describe tradeoffs among these vital functions and to characterize variability among individuals and within individuals over time. Within individual trees, we found a tradeoff over time between growth and reproduction and defense and reproduction, where growth and total resin duct area were lower in years of high cone production. Trees located at sites with hotter and drier climates showed stronger annual tradeoffs between annual reproduction and growth. In contrast, when comparing among trees, we found no evidence that trees with high long-term reproduction invested less in growth or defense. In our study, we found local drought conditions change the year-to-year tradeoffs in allocation between reproduction and growth. Our results support the environmental stress hypothesis of masting, which predicts that greater interannual variation in tree functions (reproduction, growth, defense) is associated with more marginal environments, such as those that are prone to drought. With warming temperatures and increased exposure to drought stress, trees will be faced with stronger interannual tradeoffs, which could lead to further decreases in growth and defensive efforts, ultimately increasing mortality.


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resource allocation
drought resistance
carbon isotopes


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