Effects of forest harvesting on summer stream temperatures in New Brunswick, Canada: an inter-catchment, multiple-year comparison

摘要

This paper presents apre- and post-harvest comparison of stream temperatures collected in fiveneighbouring streams (sub-catchments) over a period offive years (1994-1998). The aim of the study was to determine whether land coverchanges from clear cutting in areas outside forest bufferzones (applied to streams >0.5 m wide) might contribute to an increase insummer mean stream temperatures in buffered streams downslopeby infusion of warmed surface and sub-surface water into the streams. Specificrelationships were observed in all five forest streamsinvestigated. 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, inpart, from existing aerial photographs, GIS-archived forestinventory data and a digital terrain model of the study area. Spatialcalculations of insolation levels for July 15th were used as an index ofmid-summer solar heating across sub-catchments. Analysis indicated that prior tothe 1995 harvest, differences in stream temperature couldbe attributed to (i) topographic position and catchment-to-sunorientation, (ii) the level of cutting that occurred in the uppercatchment prior to the start of the study, and (iii) theaverage slope within harvested areas. Compared to the pre-harvest mean streamtemperatures in 1994, mean temperatures in the threestreams downslope from the 1995 harvest areas increased by 0.3 to 0.7°C(representing a 4-8% increase; p-value of normalisedtemperatures 0.05). The greatest temperature change occurred in thestream that had the greatest proportion of its uppercatchment harvested (16.8%), which also had the highest calculated potentialsolar loading ( ~2749 MJ per stream cell). From the analysisit was determined that the thinning applied to the forest buffer of that stream,with a basal area removal of ~28%, was insufficient to causesignificant change in the observed stream temperature. Similar effects wereobserved following a second harvest in 1997. In general, increasesin mean stream temperature coincided with forest harvesting activities outsideforest buffers, where conditions promoting stream warmingwere greatest. In this study, no clear relationship existed between forestbuffer strip width (ranging from 30-60 m) and the level of streamwarming observed at the monitoring stations. style="line-height: 20px;">Keywords: terrain attributes, solar radiation, land cover, forest buffers, New Brunswick regulations, spatial modelling, DEM, forest covertypes