| Improved projections of future climate trends, climate extremes and ecological responses are critical to inform policy and natural resource management aimed at sustainability and preparedness. Considerable attention has been focused on understanding drought, due to concerns over crop yield and forest mortality. However, some regions, such as the eastern US, have experienced increased summer precipitation over the past half century and the causes and ecological implications are not well understood. Greenhouse gases and regional effects of agricultural irrigation have been shown to be primary drivers of precipitation enhancement, although far-downstream effects of irrigation on moist, forested regions remain unclear. In addition, correctly anticipating forest responses to increasing precipitation, and other aspects of a changing climate, requires understanding the interaction of long-term successional processes across regional-to-continental spatial scales. Here I, 1) investigated how agricultural irrigation enhances regional and far-downstream precipitation; and, 2) developed a new data-driven model of forest dynamics that quantifies compositional and structural response to competition, disturbance, and climate anomalies. This work combined modeling experiments using the National Center for Atmospheric Research (NCAR) community earth system model (CESM) and analysis of large long-term datasets from the National Oceanic and Atmospheric Administration (NOAA) and US Forest Service (USFS) forest inventory and analysis (FIA). Results indicated that moisture advection from irrigation in the western U.S. enhances stratiform precipitation in the densely populated greater New York City region, where very-heavy precipitation events were 11% more intense and occurred 23% more frequently. Forest modeling provided validation and quantification of fundamental ideas underlying forest succession (e.g., self-thinning, resource limitation tolerance) and revealed increased summer precipitation may enhance forest net primary productivity up to +32% (+0.52+/-0.08 Mg C ha-1 yr-1). However, climate warming was associated with increased occurrence of weather disturbances and disease resulting in reduced productivity by as much as -29% (-0.55 +/-0.08 Mg C ha-1 yr-1). Well-known successional shifts in species-group dominance were attributed to disturbance susceptibility and a gradient of tolerance to resource limitation. Collectively this work provides new insight for climate mitigation efforts, robust comparisons for global vegetation models, and context for water resource and forest management. |
