Abstract: Large woody debris (LWD) provides structural complexity to small streams. Riparian buffers are intended to provide long-term vegetation cover and supplies of LWD, but post-harvest windthrow often occurs. To evaluate the impacts of windthrow in riparian buffers and identify the components for a small stream LWD recruitment model, we sampled 26 streams in immature and older stands in wind-exposed areas of southwestern and northern Vancouver Island. These treed buffer strips had been exposed following clearcut harvest of adjacent timber on both sides 1–20 years previously. For stream sections 100 m long in each buffer, all logs greater than 7.5 cm diameter that spanned at least part of stream channel were measured. A total of 658 logs were recorded. Windthrown trees were comparable in characteristics to the trees that made up the buffer. The majority of logs derived from windthrown trees were oriented perpendicular to the stream channel and were suspended above the stream channel. Even 20 years after harvesting, two-thirds of the logs were still suspended above the stream. Logs in older buffers were more decayed, and the decay rate depended on tree species and initial diameter. Log height above stream was negatively correlated with log decay class and time since logging. Log length declined with time since harvest exposure and decay class. Sediment was exposed on upturned roots and within mineral soil pits. The volume of soil retained on upturned rootwads declined over time, but some soil remained even after 20 years. Very little of this exposed sediment was close enough to the creek to result in sediment delivery.
Abstract: To document the impacts of windthrow in riparian leave strips and identify the components needed for small stream large woody debris (LWD) recruitment modeling, we monitored nine small streams at a temperate rainforest site in coastal British Columbia. This study was a component of a larger integrated study of forest management impacts on small streams. A series of small clearcuts were harvested in 1998 in a 70-year-old second growth stand that had regenerated naturally following logging and wildfire. Three cutblocks each were assigned to 10 m and 30 m buffer width treatments and three areas were assigned as unharvested controls. Seven years after the 1998 logging, all logs greater than 10 cm diameter that spanned at least part of stream channel width were measured. A total of 179 logs were recorded. Post-harvest windthrow was higher in the 10 m buffer treatment, while competition-related standing tree mortality was higher in the controls. The major windthrow events had occurred in the first and second years after logging of adjacent stands. There was no significant difference in the number of spanning and in-stream logs in the 10 m, 30 m buffer and control treatments. More than 90% of the LWD was in the 10–30 cm diameter classes. The majority of logs were oriented perpendicular to the stream channel. At the time of measurement, the majority of these trees were still suspended above the stream channel, indicating that the recruitment of logs into the stream channel is a long-term process. Time to recruitment into the channel is dependent on log and valley geometry, log size, species, and log condition prior to toppling. Log height above stream was negatively correlated with log decay class and valley width. Log length was negatively correlated with state of decay, and many windthrown logs were in an advanced state of decay before they entered the stream.
Abstract: The riparian buffers experiment at the University of British Columbia research forest was commenced
in 1998. In this experiment 0m buffer (fully harvested), 10 m, 30 m and unharvested
(control) treatments were replicated three times within 70 year old uniform structured stands
dominated by western hemlock (Tsuga heterophylla). Approximately 15 % of the retained trees
were windthrown in the 10 m buffer treatment, and 10 % in the 30 m wide treatment, primarily
in the first 2 years following harvest of adjacent timber. Windthrown trees were smaller than the
average tree. Stand self-thinning is an additional source of mortality, particularly in the control
and 30 m buffers. Eight years after harvest, 90% of the windthrown trees were still suspended
above the stream channel. The area of exposed soil is related to the diameter and number of
uprooted trees, but only 14 % of uprooted trees were in a position to deliver sediment to the
stream channels. Results of this study have been integrated into ForestGALES_BC to model
windthrow impacts on streams.
Abstract: A series of small clearcuts were harvested in 1998 in the
southeast corner of a 70 year old second growth stand in the
Coastal Western Hemlock dry maritime subzone at Malcolm
Knapp Research Forest. Each cutblock was bisected by a small
stream, and 0m, 10m and 30m reserve zones were established
adjacent to these streams. Each treatment was replicated 3
times and 3 unharvested streams were identified as controls.
In the summers of 2005 and 2006 all logs greater than 10cm
at mid-span diameter that were in decay classes 1 through 3
were measured and tagged.
Post-harvest windthrow in the 10 and 30m buffers in the
2 years following harvesting produced a number of large
spanning logs. The western hemlock logs have lost their
fine branches and some of their bark and have substantial
sapwood decay but are still fully suspended. Many of the
windthrown redcedar trees still have live branches and are undecayed.
Most of the LWD recruitment into the stream channel
to-date has been from small hemlock or deciduous trees
that had died standing and were in decay class 2 or 3 when
they fell. These trees break into short chunks when they fall.
We will continue to track the fate of LWD in this system and
will develop equations that relate years to in-stream recruitment
to the species, spanning length and mid-span diameter.
Abstract: Large woody debris (LWD) is an important component of forest ecosystems and provides structural complexity to small streams. Riparian buffers are intended to provide long term supplies of LWD, but post harvest windthrow often occurs. To document the impacts of windthrow in riparian buffers and identify the components needed for small stream LWD recruitment modeling, I sampled 39 small streams at the Malcolm Knapp Research Forest (MKRF) and on Vancouver Island. I took two basic approaches. In the small stream experiment at MKRF a series of small clearcuts were harvested in 1998 in a 70 year old second growth stand. I measured LWD in 10m and 30m buffer treatments, and in the unharvested control. I added samples in mature and old-growth stands for comparison. In the second approach, I retrospectively sampled buffers that were exposed by harvesting from 0-20yrs ago on southwestern and northeastern Vancouver Island. In both studies, all logs greater than 7.5 cm diameter at mid-creek, in decay class 1 to 4 that spanned at least part of stream channel width were measured. There was no significant difference in the number of spanning and in-creek logs in 10m and 30m buffer given the short term monitoring of woody debris in the buffers. The majority of windthrown trees were still suspended above the stream channel years after a windthrow event. The height above stream was negatively correlated with log decay class and the buffer age class. The number of logs was higher in immature stands than mature stands. As the stems per hectare in riparian stands increases, so does the frequency of spanning LWD. The frequency of logs in decay classes 3 and 4 was higher in older buffers, and deciduous LWD decayed more quickly than conifers. Interestingly, the log length was found to be shorter in advance stage of decay. Key elements in a conceptual model of LWD recruitment via windthrow are the geometry of initial log position, log size, species and decay rate.
Abstract: A series of small clearcuts were harvested in 1998 in the southeast corner of a 70 year old second growth stand in the Coastal Western Hemlock dry maritime subzone at Malcolm Knapp Research Forest. Each cutblock was bisected by a small stream, and 0m, 10m and 30m reserve zones were established adjacent to these streams. Each treatment was replicated 3 times and 3 unharvested streams were identified as controls. In the summers of 2005 and 2006 all logs greater than 7.5cm at mid-span diameter that were in decay classes 1 through 3 were measured and tagged. Post-harvest windthrow in the 10 and 30m buffers in the 2 years following harvesting produced a number of large spanning logs. Most conifer logs are still in decay class 1, while deciduous logs are mostly in decay classes 2 and 3. Most logs are still suspended above the stream. We will continue to track the fate of LWD in this system and will develop equations that relate years to in-stream recruitment to the species, spanning length and mid-span diameter.
