Dendrochronological Methods to Examine Plant Competition with Dendrochronological Methods to Examine Plant Competition with Changing Fire Regimes in Desert and Forest Ecosystems Changing Fire Regimes in Desert and Forest Ecosystems

Human activities are changing wildfire regimes globally through ignition, spread of invasive species, fire suppression, and climate change. Because of this, ecosystems are experiencing novel fire regimes that may alter plant growth and patterns of succession. Annual growth rings are one metric that can track changes in tree and shrub growth patterns over time in response to changing fire frequency. In Chapter 1 we explored the effects of fire on resprouting native shrubs in the Mojave Desert. Fires are becoming increasingly frequent due to the spread of highly flammable invasive grasses in the region. We monitored growth and fruit production of Larrea tridentata D.C. (creosote bush) on burned and unburned transects from three independent 2005 wildfires. Even though creosote has a high fire mortality rate, we found that resprouting creosote produced 4.7 times the amount of fruit and had stems that grew nearly twice as fast compared to creosote in unburned areas. Our data suggest that creosote can resprout after fire and thrives in its growth rates and reproduction in post-fire environments. In Chapter 2 we used annual Basal Area Increment to investigate how fire suppression has altered facilitation and competition interactions through stages of succession in mixed aspen-conifer forests. We found that aspen had lower growth rates in mixed aspen-conifer stands compared to aspen dominant stands. We also found that aspen growing with an associated fir tree due to facilitation had increasingly lower growth rates over time than those growing independently. Fir trees in mixed stands were facilitated over time by associated aspen trees while fir trees growing in association and independently in aspen stands showed no statistical difference from each other but grew better than independent fir trees in mixed stands. Our data suggest that restoring a more frequent fire regime will balance competitive interactions between aspen and conifer in subalpine forests. ABSTRACT Human activities are changing patterns of ecological disturbance globally. In North American deserts, wildfire is increasing in size and frequency due to fuel characteristics of invasive annual grasses. Fire reduces the abundance and cover of native vegetation in desert ecosystems. In this study, we sought to characterize stem growth and reproductive output of a dominant native shrub in the Mojave Desert, creosote bush (Larrea tridentata (DC.) Coville) following wildfires that occurred in 2005. We sampled 55 shrubs along burned and unburned transects 12 years after the fires (2017) and quantified age, stem diameter, stem number, radial and vertical growth rates, and fruit production for each shrub. The shrubs on the burn transects were most likely postfire resprouts based on stem age while stems from unburn transects dated from before the fire. Stem and vertical growth rates for shrubs on burned transects were 2.6 and 1.7 times higher than that observed for shrubs on unburned transects. Fruit production of shrubs along burned transects was 4.7‐fold more than shrubs along paired unburned transects. Growth rates and fruit production of shrubs in burned areas did not differ with increasing distance from the burn perimeter. Positive growth and reproduction responses of creosote following wildfires could be critical for soil stabilization and re‐establishment of native plant communities in this ABSTRACT Facilitation and competition interact over time resulting in patterns of plant community succession. Human activities are changing historical fire regimes that alter these successional cycles. We investigated the long term facilitative and competitive relationships of quaking aspen and subalpine fir growing in association across two successional aspen stages. In aspen dominant and mixed aspen-conifer forests, we sampled trees growing independently and in association. We aged trees and compared annual Basal Area Increment from both species in both stand and proximity types across time. Aspen growth rates decreased by more than half for trees in mixed stands compared to aspen dominant stands. Aspen growth rates decreased over time for trees growing in association with fir trees. Growth rates for fir trees in aspen stands were similar for both associated and independent trees and increased over time in relation to those growing independently in mixed stands. Fir trees in mixed stands associated with aspen trees had higher growth rates over time than fir trees growing independently. Our data underscores the importance of restoring natural fire regimes in mixed aspen conifer forests in order to prevent competitive imbalance that favors fir dominance and reduced aspen vigor.