Growth response of bigcone Douglas fir (Pseudotsuga macrocarpa) to long-term ozone exposure in southern California

Long-term radial growth of bigcone Douglas fir (Pseudotsuga macrocarpa) was studied throughout its range in the San Bernardino Mountains of southern California, where ambient ozone has been high for approximately the past 40 years. A gradient of both ozone concentration and precipitation exists from west (high) to east (low). Growth rates of bigcone Douglas fir are considerably lower since 1950 throughout the San Bernardino Mountains, with the largest growth reductions in the western part of the range where ozone exposure is highest. Needle retention is also somewhat lower at high ozone sites. Lower annual precipitation since 1950 may have some impact on long-term growth reductions, and short-term growth reductions induced by drought are an important component of long-term growth reductions at sites with high ozone exposure. An ozone-climate stress complex may be responsible for recent reductions in the growth of bigcone Douglas fir.     

Resource degradation: a subtle effect of bottom fishing

Populations of hermit crabs are critically limited by the availability of suitable gastropod shells that they utilise to reduce their risk of predation and environmental stress. Common whelks are the main source of shells for large hermit crabs in the northern Atlantic but are vulnerable to direct and indirect effects of fishing activity. This study examined the potential consequences of degrading shell resources for common hermit crabs. Laboratory trials demonstrated that hermit crabs avoid low-quality damaged shells throughout their life history. This laboratory preference was corroborated by direct field observations of shells preferentially occupied by hermit crabs, compared with shells available for occupation. In the field, 8 times as many empty shells had holes compared to shells occupied by hermit crabs. In the North Sea, the abundance and biomass of live whelks and hermit crabs collected at sites where they co-occurred were significantly related. However, whelks occurred at far fewer sites overall and were more patchily distributed at high abundance than hermit crabs, which were more widespread. At a subset of sites, whelks of the same body-mass range occurred in the Irish and North Sea. However, at these sites, hermit crabs sampled from the North Sea had a significantly lower biomass. This suggests that the shells available for occupation at the North Sea sites would not support crabs of a body mass comparable to that found in the Irish Sea. Using published data, we calculated that in some of the intensively fished areas of the North Sea, 24% of the available shell resource will be damaged each year. The reduction in shell quality in the North Sea may impose a physical constraint on the upper size limit currently attainable by hermit crabs and hence may have implications for population viability.Communicated by J. P. Thorpe, Port Erin