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Herbivory and tree mortality across a pinyon pine hybrid zone
Kerry M Christensen1, Thomas G Whitham1, Paul Keim1
1Department of Biological Sciences, Northern Arizona University, 86011, Flagstaff, AZ, USA.
Oecologia
|March 18, 2017
Summary
Hybrid pinyon pines (Pinus californiarum x Pinus edulis) support more insect herbivores, leading to higher tree mortality. These hybrid zones are crucial for natural selection and understanding plant-insect interactions.
Area of Science:
- Ecology
- Evolutionary Biology
- Forest Science
Background:
- Pinyon pines (Pinus californiarum and Pinus edulis) hybridize in west-central Arizona.
- Insect herbivory plays a significant role in forest dynamics and tree health.
- Hybrid zones can experience unique ecological pressures and evolutionary outcomes.
Purpose of the Study:
- To investigate insect herbivore abundances on pure and hybrid pinyon pine stands.
- To test hypotheses regarding hybrid susceptibility and dominance in relation to herbivory.
- To assess the impact of herbivory in hybrid zones on tree mortality.
Main Methods:
- Established a 250-km transect across pinyon pine populations in Arizona.
- Classified trees into pure Pinus californiarum, hybrid, or pure Pinus edulis using a hybrid index based on six morphological traits.
- Quantified the abundance of three key insect herbivores: stem-boring moths (Dioryctria albovittella), scale insects (Matsucoccus acalyptus), and pitch moths.
Main Results:
- Stem-boring moths were more abundant on hybrid trees, supporting the hybrid susceptibility hypothesis.
- Pitch moth abundance varied, with higher levels on pure Pinus californiarum and hybrids compared to pure Pinus edulis, supporting the dominance hypothesis.
- Scale insects were found exclusively in the hybrid zone. Hybrid sites supported significantly more herbivores overall, correlating with a 35-fold increase in tree mortality.
Conclusions:
- Pinyon pine hybrid zones experience elevated herbivore pressure and increased tree mortality.
- Hybrid zones act as significant arenas for natural selection, influencing plant-insect interactions and tree survival.
- Understanding herbivory dynamics in hybrid zones is critical for forest conservation and predicting ecological responses to hybridization.
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