Anthropogenic climate change risks significant changes in the global distribution of precipitation. Across the western United States, modelling studies show significant reductions in wetness that imply weighty societal and ecological impacts. But the validity of the model projections need to be ground-truthed. Paleo-hydroclimate data are useful reference points to assess a model’s ability to hindcast past hydroclimate. If the hindcast matches the paleodata, it brings confidence to a model’s ability to predict future hydroclimatic change. The foremost metric of hydroclimate in the geologic record is the surface area of lakes in hydrologically closed basins. In such basins, a lake’s surface area is determined by the balance between precipitation and evaporation. The lake will expand when the balance is positive, and it will contract when the balance is negative. In this dissertation, I develop a 25-9 ka record of lake fluctuation from the Mono Basin, a hydrologically closed basin in east-central California. I deduced the fluctuations using three pieces of evidence: stratigraphy; geomorphology; and geochronology. These pieces of evidence were determined from a study of the Mono Basin’s Late Pleistocene lithostratigraphic unit: the Wilson Creek Formation. There are 19 tephra intercalated in the Wilson Creek tephra. They are named by their reverse depositional order (Ash 19 is the oldest and Ash 1 is the youngest). Uncertainty on their ages cause confusion as to the paleo-hydroclimate record of the Mono Basin. The age of Ash 19, for example, is important because its deposition marks the onset of relatively high lake levels that occurred during the last glaciation. There are two principal interpretations of Ash 19’s age: 40 ka, which is based on lacustrine macrofossil 14C data; and 66 ka, which is underpinned by paleomagnetic intensity data. In chapter 2, I tested these end-member interpretations. I used the U/Th method to date carbonate deposits that underlie and cut across Ash 19. The U/Th data show that Ash 19 must have been deposited between these two dates: 66.8 ± 2.8 ka; and 65.4 ± 0.3 ka. These dates are, therefore, more consistent with the 66 ka interpretation of Ash 19’s age. Thus the onset of relatively high lake levels in the Mono Basin corresponds with the rapid drawdown of atmospheric CO2 during Marine Isotope Stage 4. The coincidence between the drop in atmospheric CO2 and lake level rise is suggestive of a causal link. In chapter 3, I determined Mono Lake's fluctuations 25-9 ka. This time encompasses three climatic intervals: the coolest time of the last glaciation, termed the Last Glacial Maximum (LGM); the period corresponding to the rapid termination of the last glaciation, termed the deglaciation; and the early Holocene, a period of inordinate warmth that immediately followed the last glaciation’s termination. In this study, I used stratigraphic and geomorphic evidence in conjunction with 14C and U/Th dates. I measured the 14C dates on bird bones and charcoal. And I measured the U/Th dates on carbonates. Together the data showed that the lake's rises and falls concurred with North Atlantic climate. Periods of aberrant warmth in the North Atlantic concurred with low stands of Mono Lake. On the other hand, extreme cooling in the North Atlantic correlated with Mono Lake high stands. The timing of these lake fluctuations also corresponds with variations in other tropical and mid-latitude hydroclimatic records. The global harmony in the hydroclimatic records suggests a unifying conductor. I hypothesize that the conductor is tropical atmospheric circulation. In chapter 4, I present evidence on the peculiar case of an extreme low stand of Mono Lake. The low stand is dubbed the “Big Low”. The principal evidence underpinning the Big Low derives from a sedimentary sequence exposed along the canyon walls of Mill Creek. The strata show that the lake fell below 1,982 m between the deposition of Ashes 5 and 4—making this low stand the lowest recognized level of Mono Lake during the Wilson Creek Formation. Observations from dispersed sequences corroborate this interpretation. And three data constrain the age of the Big Low to be between ~24.4-20.5 ka: a carbonate U/Th date on a littoral conglomerate associated with the Big Low; a carbonate U/Th date that underlies Ash 4; and a carbonate U/Th date that cuts across Ash 5. Thus the interval that the Big Low must occur within encompasses the LGM. The timing of this low stand, therefore, corresponds with summer temperature minima, suggesting that the fall was due not to an increase in evaporation but due to a decrease in precipitation. This finding is counter to conventional wisdom: that the LGM was a relatively wet interval. In addition, both the documentation of a low stand during glacial maximum conditions and the inference that precipitation must have been reduced are contrary to previous published interpretations from model and paleoclimatic data. These discrepancies raise significant questions about our understanding of the regional expression and forcing of hydroclimate across the western United States during the LGM. Because of this period’s importance to ground-truthing climatic models, additional evidence on the geographic extent of this unexpected result is essential.

Climate models predict changes to the frequency and intensity of extreme events, with large effects on tropical forests likely. Predicting these impacts requires understanding how landscape configuration and land-use change influence the susceptibility of forests to disturbances such as wind, drought, and fire. This is important because most tropical forests are regenerating from anthropogenic disturbance, and are located in landscape mosaics of forest, agriculture, and other land use. This dissertation consists of four chapters that combine remote sensing and field data to examine causes and consequences of disturbance and land-use change in tropical second-growth forests. In Chapter 1, I use satellite data to identify factors associated with permanence of second-growth forest, and assess how estimates of carbon sequestration vary under different assumptions about second-growth forest permanence. I show that most second-growth forest is cleared when young, limiting carbon sequestration. In Chapter 2, I combine data from weather stations, remote sensing, and landowner surveys to model fire activity on 732 farms in the study area over ten years. The relative importance of these factors differs across scales and depending on the metric of fire activity being considered, illustrating how implications for fire prevention and mitigation can be different depending on the metric considered. Chapter 3 combines Landsat imagery and field data to map wind damage from a severe convective storm, providing strong empirical evidence that vulnerability to wind disturbance is elevated in tropical forest fragments. Finally, in Chapter 4 I integrate annual forest census data with LiDAR-derived topography metrics and tree functional traits in a hierarchical Bayesian modeling framework to explore how drought, topography, and neighborhood crowding affect tree growth, and how functional traits modulate those effects. The results from these studies demonstrate innovative approaches to understanding spatial variation in forest vulnerability to disturbance at multiple scales, and the results have implications for managing forests in a changing climate.