Spatial and temporal dynamics of old-growth forests on the British Columbia coast

Background/Question/Methods

Coastal temperate rainforests of North America span broad gradients in temperature, moisture and continentality.  These systems extend from southern Oregon (OR) and Washington (WA) to southeast Alaska (SE AK), including coastal British Columbia (BC).

Floodplain forests are particularly spatially and temporally dynamic and highly productive systems within these landscapes.  Studies of riparian forest structure and dynamics from the US highlight distinctive and complex disturbance regimes and successional pathways. However, few comparative studies have been published for coastal BC.

 We ask, How do old growth floodplain forests along BC’s coast compare to each other and to similar ecosystems described within the published literature? And, What processes drive any observed differences across latitudes and site types?

To address these questions, we compare the structure of four spruce floodplains from contrasting latitudes, site types and disturbance conditions on BC’s coast.  For each 0.25-1 ha plot, we measured and mapped all live trees, snags, and downed woody material (DWM). We compared old growth attributes reported elsewhere to those indices calculated for our plots.  The stands are part of a larger project, initiated in 1992-3, to describe coastal temperate old growth, evaluate temporal dynamics, and provide a baseline for measuring effects of climate change.

Results/Conclusions

Compared with riparian coastal old-growth forests in WA, OR, and SE AK, our riparian plots had similar or fewer total live stems (148 to 256 per hectare) , more large (>100 cm DBH) live stems (24-48 per hectare), a similar mean DBH for live stems (41.4 to 57.4 cm ), and slightly higher variability (SD) in live stem DBH (38.7-58.3 cm ). Our plots are less tree-species rich than WA plots (3-4 species). Western hemlock predominates in smaller diameter classes, Sitka spruce in larger diameter classes. Snags numbers are comparable to elsewhere (12-60 per ha), but large (>50 cm DBH, >5m tall) snags are absent from three of our four sites. Volume of DWM is skewed to higher decay classes, particularly for our northern site.

Our results support other work illustrating the unique nature of riparian versus upland old growth systems and the importance of riparian-specific old growth definitions.  Differences among our plots and to those in other studies suggest differing temporal dynamics and scales of spatial pattern.  Our ongoing research is examining stand dynamics and spatial structure from 1992-3 to 2007-9 to consider the population processes and disturbance dynamics influencing attributes commonly reported in old growth descriptions.