Regeneration and Recruitment for Resilience: Sustaining Aspen Ecosystems Threatened by Climate Change, Ungulate Browse, and Oystershell Scale

Quaking aspen (Populus tremuloides) ecosystems are highly valued in the southwestern United States because of the ecological, economic, and aesthetic benefits they provide. Concerningly, aspen has experienced extensive mortality in recent decades, and there is evidence that many areas in Arizona, USA lack adequate recruitment to replace dying overstory trees. Maintaining sustainable levels of regeneration and recruitment is necessary for facilitating resilience to biotic and abiotic disturbance agents and for maximizing aspen’s ability to adapt in an increasingly uncertain future. However, questions remain about which factors currently limit aspen regeneration and recruitment in Arizona and which strategies are appropriate for promoting aspen sustainability. Moreover, recent outbreaks of an invasive insect, oystershell scale (Lepidosaphes ulmi; OSS) pose a new threat to aspen forest health. Because these are the first documented outbreaks of OSS in aspen ecosystems, there is an urgent need to survey the extent and impacts of these invasions and to better understand the insect’s biology and ecology on aspen in Arizona. To fill these knowledge gaps, we conducted a systematic literature review of aspen in the Southwest to understand how biotic and abiotic factors, including management, influence aspen forest dynamics (Chapter 1). We also sampled aspen populations across Arizona to quantify the sustainability and drivers of aspen regeneration and recruitment (Chapter 2) and impacts and drivers of OSS invasions (Chapter 3). Finally, we used repeated measurements of OSS-infested aspen stands to quantify short-term rates of OSS intensification on trees, OSS spread among trees, and aspen mortality, and we collected OSS from these stands to document the insect’s phenology in northern Arizona (Chapter 4).

We found that many aspen populations in Arizona lack sustainable regeneration and recruitment. The status of recruitment was especially dire, with 40% of study plots lacking a single recruiting stem. Aspen regeneration was less abundant on warmer, drier sites, highlighting the threat that a warming climate poses to aspen sustainability. Aspen recruitment was significantly more abundant in areas with recent fire and more severe fire. The most important factors limiting recruitment were OSS and browsing by ungulates, especially Rocky Mountain elk (Cervus canadensis). OSS was widespread in Arizona and was associated with increased aspen crown damage and mortality. Climate was the most important driver of OSS abundance, with warmer, drier conditions resulting in significantly more OSS. OSS was also associated with less recent fire, presence of ungulate management strategies such as fenced exclosures, and stands with a greater density of aspen saplings. We also found that immature OSS life stages persist throughout the year and that there are two waves of first-instar crawlers in northern Arizona, one throughout the summer and the second in mid-winter. The first wave seemed to be driven by warming temperatures, but the cause of the second wave is unknown and might represent the initiation of a second generation. We also found that OSS causes high levels of mortality and spreads rapidly within aspen stands. We conclude by discussing how our findings can inform contemporary management of aspen and OSS (Chapter 5).