QUANTIFYING THE BOTTLENECKS FOR WOODLAND RECOVERY

Abstract

Fire regimes across the western US are changing and climate is becoming hotter and drier, challenging the ability of seed-obligate trees to regenerate. The recruitment of plant species is reliant upon certain conditions, like moisture, temperature, sunlight, factors that can collectively be referred to as the recruitment niche. In many ways, biotic interactions, like plant-plant or plant-microbe facilitation and competition, can increase or decrease the amount of stress a young tree is exposed to, and these biotic interactions are vastly changed in burned environments. Piñon pine and juniper (PJ) woodlands are an ideal ecosystem to study these kinds of interactions. In Chapter 1, I investigated the natural recovery of 25–27-year-old burned woodlands, to explore the role of proximity to seed source and the developing vegetation on inhibiting or assisting piñon and juniper tree recruitment. In Chapter 2, I conducted a planting study, where piñon seedlings were inoculated with soil from an unburned woodland to introduce ectomycorrhizal fungi to seedlings to Inoculated seedlings, along with non-inoculated seedlings, were grown in a greenhouse and then outplanted in a recent and an old fire. Seedlings were grown in natural conditions for 2 years; seedling height growth, survival, and soil fungal communities were assessed in the fall and spring of each year. Lastly, in Chapter 3, I expanded my study region to the broader southwest to understand implications of fire-driven vegetation conversion from forest to grassland or shrubland. Here, I selected six large wildfires that burned in 2002 and compared spatially-derived pre-fire, 1-year post-fire, and 20-years post-fire monthly and annual evapotranspiration. In all, this dissertation investigates bottlenecks to woodland recovery, identifying factors that may cause prolonged piñon and juniper regeneration, and outlining how long-term changes to vegetation in the southwest influence ecosystem services like evapotranspiration. Broadly, the three chapters aimed to assess how woodlands are currently recovering, what we can do to assist recovery, and what happens if forest recovery does not occur or our interventions do not work. I found that PJ woodland recovery is happening, but minimally and restricted to burned edges. Piñon and juniper recruitment mostly occurred under Gambel oak and downed logs. When conducting planting, we can facilitate recovery by planting under Gambel oak and logs to increase piñon survival, but longer timescales should be assessed before this is incorporated into management actions. While microbial inoculation may be a potential action, our findings showed that the source soil seedlings are planted into is more important than inoculation, and that inoculation did not have lasting effects on seedling survival. Lastly, using remote sensing products, we saw widespread vegetation conversion from forest to grassland or shrubland, and this was related to reduced evapotranspiration 20-years post-fire. This work demonstrates that decisions about post-fire restoration will influence not only vegetation recovery, but also long-term ecosystem processes such as evapotranspiration. As fire regimes continue to shift, understanding the mechanisms that constrain or facilitate recovery are critical for maintaining the structure and function of piñon-juniper woodlands, as well as other forest communities. This dissertation provides evidence that while recovery is increasingly uncertain, it is not uniformly constrained and that strategic management that works with, rather than against, existing biotic relationships can meaningfully influence post-fire vegetation trajectories.