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Abstract. Forest harvesting directly affects headwater systems, causing changes in catchment hydrology and riparian habitats. We investigated geomorphological impacts of clearcut harvesting on headwater systems and their recovery in the boreal mixedwood forests of northwestern Ontario, Canada. We studied 30 headwater streams (width ,3 m), 24 in previously clearcut sites and six in undisturbed mature forests as reference. Each sampled stream had two segments, i ) in clearcut 10 m away from the cut edge and ii ) in riparian buffer of the larger stream to which it flows. Using MANOVA and discriminant function analysis, we examined harvesting impacts on stream width, depth, number of stream channels, riparian width, ground exposure, canopy exposure and organic matter depths as disturbance index in clearcut chronosequence and reference forest. Among all these factors we found canopy exposure contributed most to the observed harvesting impacts, which remained significantly high up to 15 years after clearcutting compared to the reference forest. Stream width and number of stream channels were also significantly high in clearcut sites for up to three years after harvesting but the differences tapered within 10 years. Streams were shallower in recently harvested sites, a difference that remained detectable even 23 years after clearcutting. General impacts of harvesting adjacent to headwater streams and their riparian zones were detectable at least until 15 years after harvesting. Although it is likely that harvest related biophysical damage in the headwater systems have negative effects on the aquatic ecosystems of the larger streams no study in the boreal forests has yet evaluated this direct link. We argue that although protecting all small streams and their headwater system by buffers is not possible, biophysical damage during harvesting can be avoided by complying with guidelines that forbid harvesting equipments near waters edge and retaining residual vegetation along small streams.

Climate change will cause northward shifts of climate envelopes for boreal plants, however, the different photoperiod and soil moisture regimes at higher latitudes will likely influence the success of species migrations (natural and assisted). The objective of this study was to assess the effects of photoperiod regime and its interactions with soil moisture and carbon dioxide concentration ([CO 2 ]) on the morpho-physiological processes in jack pine Pinus banksiana. One-year old seedlings were exposed to two [CO 2 ] (400 and 950 mmol mol -1 ), two soil moistures (60-70% and 30-40% of field water capacity) and three photoperiod regimes (photoperiods at seed origin, 5° and 10° north of the seed origin) in environment controlled greenhouses. The impacts of photoperiod, soil moisture and elevated [CO 2 ] on growth and physiological processes in the seedlings were examined. The results suggest that the response of jack pine to climate change will be complex under the interactive effects of northward migration associated longer photoperiod, soil moisture stress and elevated [CO 2 ]. The longer photoperiod associated with higher latitudes under elevated [CO 2 ] significantly advanced the budburst at both high and low soil moisture regimes, which may likely increase the risk of late spring frosts damage prior to and during budburst. The interactive effects of longer photoperiod and low soil moisture significantly increased the water use efficiency under elevated [CO 2 ]. However, the significant 2-and 3-way interactions suggest that drought and longer photoperiods with northward migration will limit the positive effects of elevated [CO 2 ] on growth and physiological processes in the species. These results might have important implications in assisted migration/seed transfer of the species following climate change.

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