| This paper presents a
pre- and post-harvest comparison of stream temperatures collected in five
neighbouring streams (sub-catchments) over a period of
five years (1994-1998). The aim of the study was to determine whether land cover
changes from clear cutting in areas outside forest buffer
zones (applied to streams >0.5 m wide) might contribute to an increase in
summer mean stream temperatures in buffered streams downslope
by infusion of warmed surface and sub-surface water into the streams. Specific
relationships were observed in all five forest streams
investigated. To assist in the analysis, several spatially-relevant variables,
such as land cover change, mid-summer potential solar radiation,
flow accumulation, stream location and slope of the land were determined, in
part, from existing aerial photographs, GIS-archived forest
inventory data and a digital terrain model of the study area. Spatial
calculations of insolation levels for July 15th were used as an index of
mid-summer solar heating across sub-catchments. Analysis indicated that prior to
the 1995 harvest, differences in stream temperature could
be attributed to (i) topographic position and catchment-to-sun
orientation, (ii) the level of cutting that occurred in the upper
catchment prior to the start of the study, and (iii) the
average slope within harvested areas. Compared to the pre-harvest mean stream
temperatures in 1994, mean temperatures in the three
streams downslope from the 1995 harvest areas increased by 0.3 to 0.7°C
(representing a 4-8% increase; p-value of normalised
temperatures <<0.05). The greatest temperature change occurred in the
stream that had the greatest proportion of its upper
catchment harvested (16.8%), which also had the highest calculated potential
solar loading ( ~2749 MJ per stream cell). From the analysis
it was determined that the thinning applied to the forest buffer of that stream,
with a basal area removal of ~28%, was insufficient to cause
significant change in the observed stream temperature. Similar effects were
observed following a second harvest in 1997. In general, increases
in mean stream temperature coincided with forest harvesting activities outside
forest buffers, where conditions promoting stream warming
were greatest. In this study, no clear relationship existed between forest
buffer strip width (ranging from 30-60 m) and the level of stream
warming observed at the monitoring stations.
Keywords: terrain attributes, solar radiation, land cover, forest buffers,
New Brunswick regulations, spatial modelling, DEM, forest covertypes |