United States Department of Agriculture
Pacific Southwest Research Station
General Technical Report
|Cut age||Pct. vol.|
|11: Original survey; 2: stand map|
|2 Distance between edge of clearcut and center of plot|
|3 Only the portion of the buffer within 15 m of the channel was mapped|
|4 This portion of the buffer strip includes only the area of the original survey that was not mapped later|
|5 Located on an old landslide|
|6 Located on a floodplain created by a splash-dam|
For each downed tree, a live control tree was selected at approximately the same distance from the bank by pacing a random number of paces upstream or downstream along the contour and choosing the nearest live tree. Data collected for each tree include species, diameter at breast height, damage class (snapped, thrown, or control), fall direction, whether the tree was dead when thrown or broken, estimated time since damage, distance to stream channel, distance to buffer-strip margin (if present), amount and type of wood input into the stream channel, amount of soil input to the stream channel, and the presence of factors that might be expected to increase the probability of a tree being thrown or broken. Possible contributing factors included seasonally saturated soils, steep slopes, root rot, whether a redwood was part of a sprout clump, bank and slope failures, logging damage, damage from another falling tree, and nearby road or trail construction.
Time since damage was known for most trees that fell in 1995 and for some trees that had fallen close to the channel in 1992 and 1993. For other trees, age was estimated from root sprouts or other vegetative evidence, or the tree was classified into one of five decay classes according to the condition of its foliage and branches: (1) green leaves or needles present, (2) dead leaves or needles present, (3) leaves gone but small twigs present, (4) small twigs gone but branches present, and (5) branches gone or bark sloughing off. Additional evidence, such as the age of vegetation on or around the root-wad, was used to eliminate trees that had fallen more than 5 years before the surveys. The decay classes were converted to approximate years since damage on the basis of comparison to fallen trees for which dates of fall were known by either direct observation or vegetative evidence.
The second study phase was carried out between May 1994 and December 1995. All standing and recently downed trees were mapped using a survey laser in 12 of the original survey plots (nine buffer zones and three controls) and in three additional plots, two in un-reentered stands and one in a small section of buffer (fig. 3, table 1). Species, diameter, condition (live, snag, snapped when living, snapped snag, thrown), distance from channel, and distance from clearcut edge were recorded for each tree. The channel edge was also mapped, and the outer edge of the selectively logged buffer zone was defined as a line connecting the outermost trees at approximately 20-m intervals. The first two phases of the study together characterized tree falls representing a 6-year period.
A storm in December 1995 resulted in abnormally severe blowdown in the study area and provided an opportunity to collect an additional set of data for the third phase of the study. All trees were counted that were felled by the storm and contributed wood or sediment to the channel along 3 km of the mainstem of North Fork Caspar Creek. Each tree was characterized by species, diameter, sediment input, wood input, distance along the channel, distance from the channel, whether it was alive or dead when it fell, and whether it was hit by another tree. Trees felled by the December storm were not included in data sets from the other phases of the study.
Comparison of the populations of standing and fallen trees in the uncut study plots suggests that tanoak is the most susceptible species to tree fall, while redwood falls at a lower than average rate. Fall rates are distributed approximately according to the proportional representation of size classes. Data from the post-storm channel survey show no statistically significant differences in the species or size distribution between trees that entered the channel from buffer strips and those from un-reentered forest.
Two styles of tree fall were evident. Most common is failure of individual trees, which then may topple others as they fall. However, the sequence of storms since logging progressively contributed to extreme rates of tree fall in at least four locations in the watershed, suggesting the possibility that high-intensity wind eddies may have locally severe effects. In each of these cases, the area of concentrated blowdown was located within about 150 m of a clearcut margin. None of these sites were included in the sampled plots.
Most tree fall occurred by uprooting, with only 13 percent of failures caused by snapping of boles. Failure by snapping was particularly common among grand fir, but both Douglas-fir and redwood were also susceptible. The majority of trees fell downslope even when the downslope direction was opposite the direction of prevailing storm winds.
Measurements from the long-channel survey indicate that fallen trees influenced the stream channel by introducing woody debris from as far as 40 m from the channel in un-reentered forests and 70 m along buffer strips (fig. 4). About 90 percent of the instances of debris input occurred from falls within 35 m of the channel in un-reentered forests and within 50 m of the channel in buffer strips. The pattern for un-reentered forests approximately follows the distributions measured by McDade and others (1990) for mature and old-growth forests (fig. 1). However, the distribution predicted by VanSickle and Gregory (1990) for a mixed-age conifer forest (fig. 1) underrepresents the observed importance of trees falling from greater distances. This discrepancy is probably due to the observed tendency of Caspar Creek trees to fall downslope, whereas the modeling exercise assumed a random distribution of tree-fall directions (VanSickle and Gregory 1990). The distribution of source distances observed for buffer strips at Caspar Creek demonstrates higher rates of input at greater distances than measured or modeled previously, possibly reflecting the combined effects of high tree-fall rates in the selectively logged portion of the buffers and the predominantly downslope orientation of falls.
Figure 4 The distribution with respect to the channel edge of sources for woody debris inputs to North Fork Caspar Creek from buffer strips and un-reentered forest. The recalculated curve accounts for inputs from tree falls triggered by trees falling from farther upslope. An average canopy height of 55 m is assumed.
Tree falls were triggered by another falling tree in about 30 percent of the surveyed cases. Thus, even though the width of the zone of direct influence to the channel is a distance approximately equal to a tree's height, the importance of secondary tree falls expands the width of buffer strip required to maintain the physical integrity of the channel. The distribution of input sources was thus recalculated to account for the influence of trigger-trees (fig. 4).
Influences of tree fall on sediment inputs to the channel are of three types: direct input from sediment transport by uprooting, direct input from the impact of the falling tree on channel banks, and indirect input from modification of channel hydraulics by wood in the channel. At low to moderate rates of debris loading, sediment input from the latter source is expected to be proportional to the amount of woody debris introduced to the channel. Sediment input from root-throw depends strongly on the proximity of the tree-throw mound to the channel. In general, only those uprooted trees originally located within a few meters of the channel contributed sediment from rootwads. Observations after the 1995 storm suggest that 90 percent of the sediment introduced directly by tree fall in the un-reentered forested reaches originated from within 15 m of the channel, while in buffer strips, sediment was introduced by trees falling from considerably farther away (fig. 5). Rates of direct sediment input by tree fall during the storm were on the order of 0.1 to 1 m3 of sediment per kilometer of main-stem channel bank.
Figure 5 The distribution with respect to the channel edge of tree falls associated with sediment production to North Fork Caspar Creek from buffer strips and un-reentered forest. Sources are weighted by the approximate volume of sediment contributed. An average canopy height of 55 m is assumed.
The second focus of the study was to describe the distribution of fall rates as a function of distance from a clearcut edge and to compare rates of fall in buffer strips and un-reentered forest. Total rates of woody debris input from buffer strips and un-reentered forests reflect both inherent differences in the likelihood of failure for individual trees and differences in the population of trees capable of contributing woody debris: selectively cut buffer strips have fewer trees available to fall. Failure rates thus were calculated as the average probability of failure for a living tree 15 cm in diameter or greater.
Rates of tree fall were expected to be high immediately adjacent to channels, where high flows and bank erosion might also destabilize trees. Tree-fall rates thus were calculated as a function of distance from the channel bank for both buffer strips and un-reentered forest. Results show no statistically significant increase in fall rate near the channel in either setting. Mortality rates on the floodplain site are equivalent to those expected for other sites at similar distances from a clearcut edge.
Rates of tree fall were also expected to decrease as a function of distance from the outer edge of the buffer strip as the extent of exposure decreases. However, data indicate no clear pattern in fall distribution, and increased rates persist for the full widths of the buffer strips. In particular, excess fall rates do not consistently decrease through the width of the selectively cut portion of the buffer, suggesting that the fall rate may reflect the combined influence of the selective removal of trees and the presence of the newly exposed stand edge.
The distribution of fall rates within un-reentered stands was then examined to determine whether the influence of the clearcut boundary persisted even farther into the stands. In un-reentered stands (fig. 6, open symbols), average fall rates (R, percent downed per year) are correlated with the distance between the plot center and the nearest clearcut edge (x, meters):
R = 1.36e-0.00915x r2 = 0.66 (1)
Figure 6 Probability of failure of live trees as a function of the distance between the center of each study plot and the edge of the nearest clearcut. The regression line and 95 percent confidence band are calculated for un-reentered plots (open symbols); data from buffer-strip plots are not included in the regression.
That the relationship exists suggests that, of the measured rates, the most appropriate estimate of a background fall rate is that of 0.12 percent per year measured in the most isolated control plot for the 6-year study period, or about 0.4 trees per hectare per year.
However, the trend of the relation shown in figure 6 suggests that the background rate has not been fully achieved by 264 m into the stand, the distance represented by the most isolated of the study plots. The potential minimum background rate can be estimated using the assumption that the influence of a clearcut edge would not be felt beyond a ridge. Extrapolation of the relation to estimate the failure rate at a distance of 350 m from a clearcut edge (the maximum slope length for the watershed) provides an estimated minimum of 0.06 percent mortality per year for a completely un-reentered second-growth forest. The rate measured for the 264 m plot thus is no more than about twice the minimum possible rate, and the actual difference is likely to be considerably less. The measured value of 0.12 percent thus will be assumed to be representative of background fall rates in subsequent calculations.
In any case, tree falls were identified and counted consistently in each study plot, so tree falls in excess of the expected background rate can be attributed to the inherent spatial variability of rates and to the effects of logging. Assuming that spatial variability is randomly distributed, calculated excess tree falls for each plot can be divided by the number of years since logging to estimate the average annual rate of tree fall induced by nearby logging. Results for buffer strips (exclusive of the landslide and floodplain sites) show an average annual total fall rate (including both excess and background rates) of 1.9 ± 0.7 percent for the period since logging, or about 3 to 7 trees per hectare per year. Comparison of rates between buffer plots and the most isolated control plot suggests that the probability of failure for a tree in a buffer strip is approximately an order of magnitude higher than that for trees in the un-reentered second-growth redwood forest.
Equation (1) provides an expected annual fall rate of 1.1 percent for the average distance-to-edge of 21 m for the buffer-strip plots. The expected average rate is about 60 percent of the measured average, again suggesting that proximity to the boundary may not be the only factor influencing the fall rates in the selectively-cut buffer strips. It is possible, for example, that opening of the buffer-strip stand by selective logging may also contribute to destabilization of the remaining trees.
Rates of tree fall are expected to stabilize with time after logging. Weidman (1920), for example, reports that two-thirds of the excess tree fall in selectively logged western yellow pine stands occurs within 5 years of logging, and Steinblums and others (1984) note that most excess tree fall in 40 buffer strips of the Oregon Cascades occurred during the first few years. For Caspar Creek, data from the long-channel survey allow calculation of rates of failure during a single storm that occurred 4 to 6 years after clearcutting. By this time stands had already been partially depleted of unstable trees, and remaining trees had had an opportunity to increase their wind-firmness through modification of foliage and rooting patterns. Overall instances of woody-debris input per unit length of channel during the storm generally increase slightly upstream, and rates of input were similar along uncut and buffered reaches. However, stand density at the time of the storm was significantly lower along the buffered reaches, and the probability of failure for an individual tree remains higher in buffer zones even after 4 to 6 years after cutting when stand density (fig. 7) and long-channel location (fig. 8) are accounted for. This pattern suggests that the disparity in stocking rates between buffer strips and un-reentered forests is continuing to increase.
Figure 7 Distribution of tree falls along North Fork Caspar Creek during the storm of December 1995 in terms of probability of failure for a live tree. High rates at 1300 m and 3050 m represent localized concentrations of very intense blowdown, possibly reflecting the influence of localized wind eddies.
Figure 8 Tree-fall rates from the December 1995 storm in buffer strips and un-reentered forest stands along the south bank of North Fork Caspar Creek. Data from localized areas of extreme blowdown and from uncut forests opposite buffer strips are not included. The dummy variable, U, takes on a value of 0 for un-reentered forest and 1 for buffer strips.
Data from the post-storm survey also indicate that in-fall frequencies from the north bank were generally lower than those from the south bank. The pronounced tendency of trees on both north- and south-facing slopes to fall downslope suggests that the larger angle between buttressing roots and bole may provide less resistance to failure in a downslope direction. Failure may thus be most likely when strong winds blow downslope. In addition, any north-side trees that did fall in the direction of the major southerly storm winds would fall away from the channel and thus not be included in the post-storm survey.
In general, then, results suggest that the presence of a clearcut can at least double tree-fall rates for a distance of more than 150 m into a stand composed of 50- to 60-m-high second-growth trees (fig. 6). These results are consistent with results reported by Chen and others (1995), which show that wind speeds remain higher than expected for a distance of 240 m in from the edge of stands of 50- to 65-m-tall old-growth Douglas-fir forests in Washington and Oregon. The effect might be expected to be even stronger where a portion of the remaining stand has been selectively logged, as in the Caspar Creek case.
Results suggest that the strategy used for buffer-strip design along North Fork Caspar Creek produces an order-of-magnitude increase in the probability of failure for individual trees during the first 6 years after logging, and that a more modest increase in fall rate persists beyond 6 years. Failure rates throughout the width of the selectively logged portion of the buffers remain higher than predicted on the basis of proximity to the edge, suggesting that the increased rates in that portion may reflect both selective logging and the presence of a clearcut edge. In any case, the 30-m-wide selective cut does not protect the innermost 15 m of uncut buffer from accelerated rates of fall.
Results also suggest the need to expand the conceptual basis for defining a "core buffer" with a natural distribution of tree species and sizes, as required to sustain the physical integrity of the stream channel. Although 96 percent of the in-falling wood is derived from within one tree-height's distance of the channel, about 30 percent of these falls resulted from trees being hit by another falling tree. Because the triggering trees could have been located at even greater distances from the channel, this pattern indicates that an additional increment of width is required to sustain the appropriate fall rate of potential trigger trees. If this additional width is not considered during the design of a fringe buffer, accelerated fall rates of marginal trigger trees would increase rates of secondary tree fall within the core zone.
How wide a buffer is wide enough? In this case, preliminary results suggest that a one-tree-height width of uncut forest that is allowed to sustain appropriate fall rates would include 96 percent of the potential woody debris sources for the channel system. Combining the pattern of source trees with the distribution of triggering tree-fall distances indicates that an additional 0.1-tree-height's width would be needed to preserve the fall rate of trigger-trees that is needed to sustain the 96 percent input rate (fig. 4). Beyond this, an uncut fringe-zone of 3 to 4 tree-height's width would be necessary to ensure that the fall rate within the core zone is within a factor of 2 of background rates (fig. 6). Thus, a total no-cut zone of at least 4 to 5 tree-heights' width would appear to be necessary if woody debris inputs are to be maintained at rates similar to those for undisturbed forested channels. Such provisions might be necessary also along property boundaries if neighboring landowners are to be protected from excessive wind damage.
However, the utility of the fringe zone is highest during the years immediately after logging because fall rates will eventually decrease as neighboring stands regrow, as marginal trees become more wind-firm, and as the most susceptible trees topple. If the early increase in fall rates attenuates rapidly and growth rates within the depleted stand increase because of the "thinning," the long-term influence of the pulse of tree fall may be relatively small. Longer-term monitoring of fall rates and stand development is necessary if the long-term significance of accelerated fall ratesand thus the level of protection needed from a fringe buffer zoneis to be assessed adequately.
At this point, results of the study are preliminary, and they reflect conditions in a single watershed. Relationships such as that shown in figure 6 will need to be tested in other areas. Sites will also need to be monitored over a longer period. Because the partially cut buffers and nearby uncut stands now have significantly fewer standing trees than the more remote un-reentered stands, it is likely that the disturbed stands will eventually start producing less woody debris. Additional information concerning debris mobility, decay rates, stand development, and stand-age-dependent mortality could be used to model future changes in debris input, allowing assessment of future influences on channel processes.
This study's results are based on measurements made in a 50- to 60-m-tall, second-growth redwood forest. Not only do these sites not reflect the canopy height in which the local stream ecosystems developed, but they do not reflect the background rates or characteristics of tree fall appropriate for the setting. Under natural conditions, woody debris inputs would have included pieces far larger and more decay-resistant than the current stand is capable of producing. Additional information about debris input and decay rates in natural settings could be used to compare predicted future debris regimes in the recovering system with those appropriate for the natural system in the area.
Thembi Borras, Annie Breitenstein, Dina Ederer, Jen Feola, Amanda Jameson, Lindsey Johnston, Lex Rohn, Diane Sutherland, Chris Surfleet, and especially Jay Arnold provided indispensable assistance with fieldwork. Liz Keppeler profoundly influenced the study through her insights into the workings of the Caspar Creek environment.
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