| This dissertation describes how climate variability, fire regimes, livestock grazing and logging have interacted and influenced structural and compositional changes in forests dominated by ponderosa pine (Pinus ponderosa) and Jeffrey pine (Pinus jeffreyi) and in upland mixed conifer forests in the southeastern Cascades, California. There are six main objectives: (1) to document and explain vegetation change at the meadow-forest ecotone, (2) to describe how forest stand structure and composition have changed since the late 1800s, (3) to reconstruct the disturbance histories of fire, grazing and logging, (4) to reconstruct interannual to multidecadal changes in climate since the late 1600s, (5) to relate global teleconnections (El Niño/Southern Oscillation, or ENSO, and the Pacific Decadal Oscillation, or PDO) to fire regimes, and (6) to theorize how vegetation dynamics that result from both anthropogenic and climate change can be integrated with vegetation management objectives. Tree invasion of meadows began during the late 1800s and peaked during the early 1900s following a decline in fire frequency. Establishment occurred during cool and/or normal to wet springs, but was delayed along stocktrails where grazing effects were most severe. Increased clumping in pine and mixed conifer forests resulted from a surge in establishment that followed the last fire. Subsequent logging did not trigger further establishment, but it accelerated earlier successional changes caused by fire suppression and grazing. Between 1675 and 1850, changes in fire frequency, extent and season were largely controlled by climate variability, but since then, fire and vegetation dynamics were increasingly the result of local anthropogenic factors. Before 1850, fires occurred during warm/dry years and were more extensive following cooler and wetter years. Fires were more extensive during El Niño events when the PDO was in a negative phase, but larger fires occurred during La Niña events. During the twentieth century, fires were more numerous during dry summers that followed cool autumns, and area burned increased following dry winters. Both the historical and contemporary forests developed from non-equilibrium dynamics imposed by continuous cultural and climate change. Realistic forest management goals should incorporate a temporal and spatial context that is provided by historical ecology. |
