Postglacial climate and fire-mediated forest diversity on the western Olympic Peninsula, Washington

Background/Question/Methods

The mode and tempo of forest compositional change during periods of rapid climate change is a long-standing theme of forest ecological research, especially with respect to the potential for fire to produce non-linear relationships between climate change and vegetation.  In the dense and old conifer forests of the coastal Pacific Northwest, fire disturbances are sufficiently rare that their ecological role is poorly understood.  We used a high-resolution sediment record of vegetation (from pollen and macrofossils) and fire (from macroscopic charcoal) from the Olympic Peninsula in conjunction with independent records of climate to examine the co-varying trends in climate, fire, and species turnover.   We hypothesized that neighborhood effects would result in inertia between late and early-successional local communities under a changing disturbance regime and that that community rate-of-change (species turnover) would track changing fire regimes more closely at regional than local scales.  

Results/Conclusions

We found strong evidence for inertia of forest communities at the local scale, with two distinct modes of forest dynamics during the past 14,700 years.  Before 7000 years ago, rapid species turnover at both local and regional scales was driven by climate changes and mediated by fire events while during the last 7000 years the local forest composition remained dominated by late-successional species despite periods of moderate fire activity.  In contrast, the species-level community rates-of-change tracked fire activity much more closely at the local scale than at the regional scale suggestive of spatially restricted fires and a shifting-mosaic steady state system.  In addition, the relatively minor climate fluctuations of the past 7000 years compared to the Early Holocene were insufficient to cause large-scale species turnover after fire.  Forecasted summer climate in this region is expected to be close to that of the Early Holocene.  A return to Early Holocene Pseudotsuga–Alnus forests would be a dramatic change for the western Olympic Peninsula.  The Yahoo Lake fire and vegetation record provides a model for such an anticipated ecosystem change.