LTREB: Rapid Ecotonal Shifts, Pest Outbreaks, and Mortality of a Dominant Tree Species in Response to Record Drought

Northern Arizona University, Flagstaff AZ

Investigators

Abstract

This LTREB research will examine four key predictions. First, plant resistance to herbivory affects community and ecosystem-level processes. Beginning in 1982 genetically resistant and susceptible trees were identified, and on 1/2 of the susceptible trees, 2 species of common insect have been removed each year. Studies involve tree fitness responses to chronic herbivory, mycorrhizal community responses, the community consequences of a cryptic keystone herbivore, and the use of resistant and susceptible trees in reconstructing past climates. Ongoing studies of soil bacteria, nutrient cycling, and costs of resistance make these long-term experiments very important to continue. Second, drought results in rapid ecotonal shifts, which alter community structure and ecosystem dynamics. In 1996 a record drought occurred that resulted in 60 sites that suffered 30-70% tree mortality and killed an estimated 75,000 mature trees. These sites underwent a rapid ecotonal shift from pinyon-juniper woodland to juniper woodland or grassland. Eleven of these sites, paired with adjacent low mortality sites, represent a valuable resource because they are the direct result of a climate change event. These sites are crucial for studying the causes and consequences of climate change on mycorrhizal mutualists, microbial decomposers, arthropod and vertebrate communities, and how environmental stress differentially affects biodiversity and ecosystem function. Third, repeated drought events have different effects than single drought events. This year, 2002, is already identified as the driest year in recorded history. This new drought following the 1996 drought allows us to determine if the same sites suffer repeated mortality and selection, which is likely to have very different long-term consequences than a single mortality event. Four, herbivore x environmental interactions affect insect outbreaks to alter community structure and ecosystem processes. Since 1982 we have been monitoring the distribution of moth and scale insects that were restricted to sites of chronic stress. However, in response to the record drought of 1996, scales have expanded from these epicenters to become the largest outbreak in the southwest. We have mapped their expansion and are studying how resistant and susceptible trees are affected by drought. Broader Impacts: The southwestern US is in the midst of a drought similar to those predicted to increase throughout much of the US by many climate change models. The principles learned by studying pinyon pine and its associated community can be used to make informed decisions regarding the consequences of these predicted climate changes. In addition, because NAU is located in an area with a diverse population including 42 Native American tribes, we propose to train and mentor students drawn from a diversity of under-represented groups. Our ability to do so will be enhanced by programs already in place, including an NSF-sponsored University Mentoring in Environmental Biology grant targeting under-represented groups and an NAU grants program that promotes undergraduate research.