© Institute of Chartered Foresters, 2005. All rights reserved. Forestry, Vol. 78, No. 5, 2005. doi:10.1093/forestry/cpi053For Permissions, please email: journals.permissions@oxfordjournals.org Advance Access publication date 12 October 2005

The stability of different silvicultural systems: a wind-tunnel investigation BARRY GARDINER 1* , BRYAN MARSHALL 2,3 , ALEXIS ACHIM 1 , REX BELCHER 2 and COLIN WOOD 2

1 Forestry Research, Northern Research Station, Roslin, Midlothian EH25 9SY, Scotland 2 Wind Engineering Research Group, Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ 3 MathEngine Ltd, Mill Street, Oxford OX2 OXJ * Corresponding author. E-mail: barry.gardiner@forestry.gsi.gov.uk

Summary

We report on a wind-tunnel study with 1 : 75 scale model trees designed to examine the infl uence of canopy structure on the formation of turbulent gusts above forests. This was to test the hypothesis that more irregular canopy structures produce less intense gusts because the change in wind speed with height at canopy top is less severe. Measurements were made of wind speeds and turbulence within and above the model forests and of the wind loading on model trees in four different silvicultural systems. The systems tested were even-aged, single-tree selection, shelterwood/group selection and strip felling. The measurements showed that the profi les of different mean wind and turbulence characteristics above the forests are remarkably similar when vertical heights are normalized by the height of the tallest tree but differences do exist within the canopy. The wind loading measurements indicated no difference between the systems in terms of stability except possibly for the shelterwood/group selection. In the shelterwood/group selection system the presence of smaller sub-canopy trees appears to reduce the loading on the main canopy trees either by providing support and increasing damping or by absorption of energy from the canopy-penetrating gusts.

Introduction

There is increased interest in Britain in the use of continuous cover forestry to meet a range of man-agement objectives, including improvements in landscape quality and biodiversity ( Mason et al ., 1999 ). However, wind damage is a serious limit-ing factor to commercial forestry in many parts of Great Britain, particularly in the exposed north and west ( Quine, 1995 ) and, therefore, it is

important to understand the vulnerability of such systems to the wind.

Continuous cover forestry uses a number of silvicultural systems that will promote irregular structures such as the use of single-tree and group selection ( Cameron et al ., 2001 ) and there have been claims that irregular-structured forests are more stable than uniform forests (for a com-prehensive discussion on the subject, see Mason, 2002 ). The possible benefi ts of irregular forests

472 FORESTRY

are a spreading of the risk within a single area and the encouragement of adaptive growth of the dominants to withstand the wind ( Mattheck, 1991 ). Furthermore, there is evidence from stud-ies of turbulence over forest canopies that less uniform canopies will not produce as strong or frequent gusts as a very uniform canopy ( Finni-gan and Brunet, 1995 ). These gusts which form in the region of the infl exion in the wind speed profi le at the top of canopies are known to be responsible for wind damage in forests ( Gardiner, 1995 ). However, it needs to be remembered that predicting the vulnerability of particular types of forest to damage is extremely complicated because of the number of compounding factors ( Savill, 1983 ).

To investigate the hypothesis that less-regular forests will reduce the intensity of damaging gusts, a series of wind-tunnel studies have been carried out. The experiments were based on the wind-tunnel experiments on a uniform model forest previously carried out at Oxford University by Stacey et al . (1994) and Gardiner et al . (1997) , which was an extension of wind-tunnel work by Fraser (1964) and Papesch (1984) . By carrying out experiments in the wind tunnel it is possible to reduce the number of compounding factors and concentrate primarily on the infl uence of canopy structure.

Methods

Model trees

The model trees (1 : 75 scale) used in the experi-ments were of necessity a compromise because the same model trees were to be used in a variety

of silvicultural systems. The model trees were designed to represent Sitka spruce ( Picea sitchen-sis (Bong.) Carr.) growing at Yield Class 14 at an initial spacing of 1.7 m × 1.7 m (3460 trees ha − 1 ) and with intermediate thinning which represents a typical regime for upland forestry in Great Britain ( Edwards and Christie, 1981 ). This also allows for the increased spacing between trees growing in single-tree or shelterwood/group selec-tion forests. Five different model tree heights were used, representing 7.5 – 22.5 m at full scale ( tbl1 Table 1 ) with the canopy elements (i.e. foliage and branches distributed so as to be both physically realistic and to give the correct dynamic behaviour (appro-priate scaled natural frequency and drag) accord-ing to Gardiner (1989) and Stacey et al. (1994) . The compromise in model trees means that they will tend to be more like trees grown under even-aged conditions and consequently diameters will tend to be slightly smaller and crowns less deep than might be expected of trees grown in uneven-aged stands. The effects on the measurements are diffi cult to assess absolutely but are almost cer-tainly minor compared with the consequences of differences in tree height and between-tree spacing. Previous studies by Stacey et al . (1994) found minor differences in measured bending moments when additional lower crown elements were added to edge trees.

The model stems were injection moulded (Cade-muir Tooling, Selkirk, UK) in Nylon-66 and the branches in low-density polyethylene (LDPE). The model trees are illustrated in fi g1 Figure 1 . The can-opies for the 0.1 and 0.15 m trees used the same canopy elements as the original 0.2 m trees ( Stacey et al ., 1994 ). For the canopies of the 0.25 and 0.3 m trees the canopy mould was modifi ed to dou-ble the weight of the canopy elements. Reasonable

Table 1 : Model tree physical and dynamic characteristics (full-scale values based on Edwards and Christie, 1981 ; Gardiner, 1989 )

Tree height Tree height Diameter at Diameter at Sway Target sway Actual model (full scale) (model) breast height breast height frequency frequency sway (m) (m) (full scale) (cm) (model) (cm) (full scale) (Hz) (model) (Hz) frequency (Hz)

7.5 0.1 11.25 0.15 0.85 12.7 12.4 11.25 0.15 15.0 0.20 0.51 7.5 8.7 15.0 0.2 16.5 0.22 0.33 4.9 4.9 18.75 0.25 22.5 0.30 0.27 4.1 4.3 22.5 0.3 30.0 0.40 0.25 3.8 4.8

WIND TUNNEL INVESTIGATION ON FOREST STABILITY 473

agreement was found be tween the target and actual sway frequencies of the trees, although the 0.15 and 0.3 m model trees have frequencies 16 per cent and 26 per cent too high, respectively ( tbl1 Table 1 ).

Comparison of silvicultural systems

The different experiments reported in this paper were designed to simulate an even-aged system regenerated by clearfelling, a single-tree selection system, a two-storied shelterwood system, and a variant of the even-aged system, which is regener-ated by strip clearfelling. The regeneration phase of a group selection system was also simulated within the shelterwood system. These experiments were chosen to represent a range of generic silvi-cultural systems which are applicable to British commercial conifer forests. The following section describes each system individually:

1 Even-aged . Forest constructed out of only 0.2 m trees at a spacing of 0.0231 m (1874 trees m − 2 ) to simulate an even-aged stand of 15 m height and a spacing of 1.73 m. This is identical to the forest used in previous wind-tunnel experiments and therefore allows com-parison with previously obtained results. This will be referred to as ‘ EA ’ in the remainder of the text. Results for a similar forest from which 50 per cent of the trees were removed are also presented. These were measured and presented in Stacey et al . (1994) . It will be referred to as ‘ EA-50 per cent thinned ’ .

2 Single-tree selection . The forest was made up of a pseudo-random mixture of all fi ve tree sizes (0.1, 0.15, 0.2, 0.25 and 0.3 m) and gaps to represent seedlings. This arrangement was designed to simulate a forest managed under a single-tree selection system. In the ideal case, a forest managed under a single-tree selection system should have a classical reverse J distri-bution of tree sizes and each size class should occupy an equal area of ground ( Nyland, 1996 ). However, a limitation of the model forest was that the spacing between the trees could not be changed. Equal numbers of trees of each size had to be used in order to have equal areas of ground occupied by each tree size. This system will be referred to in the paper as ‘ ST ’ .

3 Shelterwood/group selection . The forest was constructed with equal numbers of trees of two heights (0.1 and 0.2 m) to simulate a canopy with an understorey, which will replace the high canopy after felling. The pattern was a regular repetition of both sizes. Although it may differ slightly from the true characteristics of a forest managed under this system, the sim-ulation was designed to represent a shelter-wood system where the forest canopy would eventually consist of trees from two different size classes. In practice, two-storied stands would often be expected to have a greater dif-ference between the heights of the two size classes ( Nyland, 1996 ). However, this arrange-ment was chosen to allow direct comparison with the EA-50 per cent thinned stand in order to determine any stability benefi t due to the presence of the understorey trees. The system will be referred to as ‘ SW ’ . To represent a group selection system, small gaps of different

Figure 1. Model trees (1/75th scale) used in wind tunnel simulations. From left to right: 0.3 m, 0.25 m, 0.2 m, 0.15 m, 0.1 m.

474 FORESTRY

sizes (equivalent to 0.01 – 1 ha) were then cre-ated within the forest by removing the over-storey to simulate different sizes of felling coupe.

4 Strip (narrow and wide) . The forest was con-structed of strips of even-sized trees with the strips increasing in height downwind. First there was an open gap (representing seedlings) followed by three heights of strip (0.1, 0.2 and 0.3 m) with the pattern repeating through the depth of the forest. Strips of 15 trees ( ‘ narrow ’ = 26 m) and 30 trees ( ‘ wide ’ = 52 m) width were tested. They will be referred to as ‘ NS ’ and ‘ WS ’ , respectively. Such systems have been attempted in New Zealand to reduce wind damage ( Somerville, 1989 ) and are typically at least 100 m wide. Such widths could not be accommodated in this study because of the room available in the wind tunnel. The WS system represents the closest to actual practice.

The wind tunnel was run at a uniform fl ow of 6 m s − 1 , which represents a 30 m s − 1 storm at full scale. Two different sets of conditions upwind of the model trees were simulated:

1 Infi nite forest . Roughness elements (bricks and plastic cups) were used to simulate an infi nite upwind stretch of even-aged forest. By replicating forest canopy fl ow upwind the fl ow over the model forest, which occupied an area of 2.4 m 2 (full size = 3.3 ha), adjusts more quickly and the measurements can be treated as occurring in an extensive forest of the particular type being simulated.

2 Forest margin . Smaller roughness elements were used to simulate an upwind stretch of open farmland/grassland. This allowed the investigation of edge effects.

Wind-tunnel measurements

A series of standard measurements were made on each of the forest designs:

1 Bending moments . Extreme and mean bend-ing moments experienced by each height of model tree were measured using the bending moment balance designed and constructed by Oxford University ( Stacey et al. , 1994 ).

2 Turbulence . Turbulence profi les were meas-ured within and above each model forest.

Measurements were made of the mean, vari-ance, skewness and kurtosis of the streamwise and vertical velocities. Measurements were made also of the streamwise turbulence inten-sity ( σ u / u– ) and the Reynolds stress (― u ′ ―w ′ ) , which is a measure of the drag exerted on the fl ow by the forest.

3 Gust frequency and structure . Identifi cation of the frequency and intensity of gusts at can-opy top was obtained with a laser Doppler anemometer which simultaneously measures horizontal and vertical velocities ( Marshall et al ., 1999 ).

Results from the bending moment balance are presented in this paper as resultant means and extremes. Extremes were calculated from 16 in dependent bending moment samples, each of periods of 40 s, and extracting the unfi ltered instantaneous maximum moment from each sample. The mode of the associated probability distribution was obtained by the procedure described in Lieblein (1974). All bending mo -ments are expressed in terms of a bending moment coeffi cient Q m , which is dimensionless:

Q m = Q/(1/2 ρ U 2 H 3 ) (1)

where Q is the bending moment measured at the base of the tree, ρ the density of air (1.226 kg m − 3 ) and U ref the mean reference velocity. Irrespective of the height of the model tree, all overturning moments are expressed with a height term H of 0.2 m for comparison to the previous wind-tunnel results. U ref is measured so as to represent the refer-ence velocity at a height of 0.38 m above ground level (28.5 m full scale) at a position deep within the forest (9 H back from edge). By making the bending moments dimensionless it is much more straightforward to compare different experimental results, including those carried out at full scale.

The bending moments on trees of different heights for the same wind speed will vary because the wind acts on moment arms of different lengths. Therefore, it is necessary fi rst to deter-mine the inherent differences in bending moment we would expect due simply to differences in tree height. In tbl2 Table 2 , the bending moments for indi-vidual trees of different heights placed in a uni-form non- turbulent fl ow are given as a ratio of the bending moment for a 0.2 m tree (e.g. for the 0.3 m tree the value in tbl2 Table 2 = Q m(0.3) / Q m(0.2) ).

ref

WIND TUNNEL INVESTIGATION ON FOREST STABILITY 475

The extreme bending moment experienced by a 0.2 m tree is slightly higher than the values obtained with the EA stand but is much less than in the EA-50 per cent thinned stand tested previ-ously. The presence of the 0.1 m trees between the taller trees appears to reduce the bending moment experienced by these taller trees. Within the ST model forest, the 0.1, 0.15, 0.2 and 0.25 m trees all benefi ted from substantial shelter pro-vided by the 0.3 m trees, but the 0.3 m trees were subjected to higher extreme loading than in an EA forest. Similar results were observed for mean bending moments, except for 0.1 m trees which experienced slightly higher mean bending mo -ments in the SW and ST systems than in the EA. The ratios between mean and extreme bending moments increased with increasing tree height in the simulation and were very similar for the 0.2 m trees in the EA and SW forests ( tbl3 Table 3 ).

Edges and gaps

The new measurements were able to reproduce the original measurements for the EA forest made by Stacey et al. (1994) for different positions relative to the forest edge ( fi g3 Figure 3 ; h tallest = height of the standard 0.2 m tree). This gave confi dence that the new data could be usefully compared with previous experiments.

The extreme and mean bending moments at different distances downwind from the forest edge for an SW forest are very similar to the values obtained from the EA forest for a tree height of 0.2 m, but the values for the 0.1 m trees are substantially less ( fi g4 Figure 4 ). The relative mag-nitude of bending moment coeffi cients for each tree size in the ST forest remains relatively con-sistent at any distance from the edge ( fi g5 Figure 5 ). There is a group of shorter sheltered trees (0.1, 0.15 and 0.2 m), whereas the 0.25 m trees and, particularly, the 0.3 m trees are more exposed.

For all forest structures, there is a measurable increase in extreme bending moment on the taller trees after fi ve tree heights back from the edge while the mean values continue to decline ( fi g3 Figures 3 and fi g4 4 ). This results in a sharp increase in gust factor (ratio of extreme to mean bending moments) for the tallest trees in the canopy as illustrated in fi g6 Figure 6 . Conversely, the shorter trees in the ST and SW simulations continue to receive shelter from the tallest trees ( fi g4 Figures 4

Table 2 : Comparison of model tree bending moments in uniform (non-turbulent) fl ow

Tree bending moment/0.2 m tree Tree size (m) bending moment

0.1 0.34 0.15 0.58 0.2 1.00 0.25 1.93 0.3 2.41

These ratios represent the differences we would expect in the mean and extreme bending moments for conventional forests (EA type stand) with trees of different heights. It is clear that small changes in height and canopy depth can make a big difference in bending moment so that the 0.25 m tree has almost double the bending moment for the same wind speed as the 0.2 m tree. It follows that if the bending moment ratio for a particular size of tree in a specifi c forest simulation is lower than the bending moment ratio in tbl2 Table 2 , the forest can be said to provide stability benefi ts to that size of tree compared with conventional (EA) forestry.

This paper concentrates on the results of the bending moment and turbulence profi le measure-ments and the consequences for the stability of different silvicultural systems. The results of the analysis of gust structure over the forest canopy were presented in a separate paper ( Marshall et al ., 2000 ). Full details of the experimental arrangements and measurement techniques can be found in Marshall (1998) .

Results

Bending moments

fi g2 Figure 2 presents the mean and extreme bending moments in SW, ST, EA and EA-50 per cent thinned. (Note that the results for the EA stands were measured for 0.2 m trees and derived from tbl2 Table 2 for other tree heights.) All bending moments are given as bending moment coeffi -cients as described in equation 1 . In the SW sys-tem, the extreme bending moment experienced by 0.1 m trees is lower than that of the EA stand, indicating a sheltering effect from the overstorey.

476 FORESTRY

and fi g5 5 ). This means that the gust factor for the 0.1, 0.15 and 0.2 m trees only increases slowly back from the edge in the ST forest, whereas the gust factor on the 0.25 and 0.3 m trees increases sharply at four to fi ve tree heights downwind from the forest edge ( fi g7 Figure 7 ).

The bending moments of the trees of each suc-cessively taller strip in the wide-strip (WS) system ( fi g8 Figure 8 ) are very similar to the values found for the same heights of trees back from the edge of the ST forest ( fi g5 Figure 5 ). The patterns of bending

moments back from the edge of each strip are almost identical to the patterns back from a com-pletely exposed edge including the rapid initial decrease in magnitude and subsequent increase a few tree heights back into the strip. Therefore, each strip appears to behave like an individual forest edge with no obvious stability benefi t arising from the presence of any upwind strips. An identical pattern was observed in the narrow (NS) system.

There was little change in the mean bending moment acting on the most-exposed 0.1 m tree at

Figure 2. Mean (a) and extreme (b) mid-forest bending moments for shelterwood/group selection (SW), single-tree selection (ST) and even-aged forests (EA).

(a)

Bend

ing

mom

ent c

oeffi

cien

t

0.000

0.001

0.002

0.003

0.004

(b)

Tree height0.1 m 0.15 m 0.2 m 0.25 m 0.3 m

0.00

0.01

0.02

0.03

0.04EAEA - Derived from Table 2 SWSTEA-50% thinned

WIND TUNNEL INVESTIGATION ON FOREST STABILITY 477

the downwind side of different-sized gaps created by removing the 0.2 m trees in the SW system ( fi g9 Figure 9 ). The sheltering effect of the overstorey rapidly diminishes so that the extreme bending moment reached a maximum at a gap size of 1.5 h tallest. This suggests that gusts were propagat-ing down to the smaller trees before the mean fl ow had increased at this height.

Airfl ow

Differences in the vertical profi les of mean wind speeds for the EA, SW and ST forests were ob -served ( fi g10 Figure 10a ). This indicates that the shear (gradient of wind speed with height) at canopy top was signifi cantly modifi ed by changes in forest

structure. The SW and EA forests had very similar profi les above the canopy. However, below the canopy ( z/h < 1), the wind speed in the SW forest was reduced by the presence of the 0.1 m trees. This results in a more gradual reduction in the wind speed near the top of the canopy ( z/h ≈ 0.75). In the ST forest, the most rapid change in the wind speed profi le occurs near the top of the 0.3 m trees ( z/h ≈ 1.5). The wind speed above the ST forest canopy is lower than for the other forests up to a height of 3.5 h and the shear at canopy top ( z/h = 1.5) is slightly lower than over the EA forest ( z/h = 1) ( fi g10 Figure 10a ). However, when the velocity pro-fi les were normalized by the height of the tallest trees (0.2 m for the EA and SW forests and 0.3 m for the ST forest) the profi les were found to be very similar, particularly in the vicinity of canopy top (0.75 < z/h tallest < 2) ( fi g10 Figure 10b ).

Similarly, vertical profi les of turbulence intensity and the Reynolds stress were very similar for all the forests when the data were normalized by the height of the tallest trees ( fi g11 Figure 11 ). Note that both the turbulence intensity and the Reynolds stress ( ―u ′ ―w ′ ) peak at canopy top for all three forests.

Discussion

The hypothesis tested in this study was that more irregular forests would reduce the intensity of

Table 3 : Ratio of extreme to mean bending moments

Model tree Extreme/mean bending System height (m) moment ratio

ST 0.1 4.11 ST 0.15 4.90 ST 0.2 5.79 ST 0.25 9.62 ST 0.3 12.79 SW 0.1 6.14 SW 0.2 11.48 EA 0.2 11.70

Figure 3. Comparison of original and current wind-tunnel measurements ( x = distance from edge and h tallest = tallest tree height = 0.2 m).

0

0 2 4 6 8 10 12

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02

Distance from leading edge x/htallest

Ben

din

g M

om

en

t C

oefficien

t

Extreme

Extreme-Stacey et al.(1994)

Mean

Mean-Stacey et al. (1994)

478 FORESTRY

Figure 4. Change of bending moments as function of distance from edge for shelterwood/group selection (SW) forest ( x = distance from edge and h tallest = tallest tree height = 0.2 m).

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02

Ben

din

g M

om

en

t C

oefficien

t

Mean 0.2 m tree

Extreme 0.2 m tree

Mean 0.1 m tree

Extreme 0.1 m tree

0 2 4 6 8 10 12

Distance from leading edge x/htallest

Figure 5. Change in extreme bending moments as a function of distance from edge for the single-tree selec-tion (ST) forest ( x = distance from edge and h tallest = tallest tree height = 0.3 m).

0

0.01

0.02

0.03

0.04

0.05

0.06

Ben

din

g M

om

en

t C

oefficien

t

0.1 m tree

0.15 m tree

0.2 m tree

0.25 m tree

0.3 m tree

0 2 4 6 8 10 12

Distance from leading edge x/htallest

shear at canopy top and reduce the frequency and severity of damaging gusts. The wind profi le data show that some modifi cation to the wind profi le below canopy top does occur, resulting in less intense shear but that the turbulence characteris-

tics over the different forests are similar if profi les are plotted against height normalized by the height of the tallest trees. Marshall (1998) and Marshall et al . (2000) have confi rmed that the canopies show extremely similar wind speed and

WIND TUNNEL INVESTIGATION ON FOREST STABILITY 479

turbulence profi les if normalized by canopy height, the height of the infl exion in the wind pro-fi le ( h i ) or a shear scale factor ( L s = u ( h i )/[ du ( h i )/ dz ]) suggested by Raupach et al. (1996) . This is supported by the fact that the tallest trees in all the simulations have very similar extreme to mean bending moment ratios. This suggests that gusts are produced close to canopy top but the inten-sity is hardly affected by the vertical distribution of the canopy. The ratio of extreme to mean

bending moments is an important determinant of windthrow risk because trees are damaged by extremes but are believed to grow in response to the mean climate ( Gardiner et al. , 1997 ; Nicoll and Dunn, 2000 ).

The bending moment data illustrate how trees of different sizes are affected by the canopy struc-ture. Trees fall into two basic categories, exposed or sheltered with the tallest trees in each simula-tion being exposed and the shorter trees being

Figure 6. Gust factor (extreme to mean bending moment ratio) as a function of the distance from the forest edge for the tallest trees in the single-tree selection (ST), even-aged (EA) and shelterwood/group selection (SW) simulations ( x = distance from edge and h tallest = tallest tree height = 0.2 m for EA and SW and 0.3 m for ST).

0

2

4

6

8

10

12

14

16

Ex

tre

me

/ m

ea

n b

en

din

g m

om

en

t ra

tio

0.3 m (ST)

0.2 m (EA)

0.2 m (SW)

0 2 4 6 8 10 12

Distance from leading edge (x/htallest

)

Figure 7. Gust factor as a function of the distance from the forest edge for all trees in the single-tree selection (ST) simulation ( x = distance from edge and h tallest = tallest tree height = 0.3 m).

0

2

4

6

8

10

12

14

16

18

0 1 2 3 4 5 6 7 8

Distance from leading edge (x/htallest

)

Extrem

e / m

ean

b

en

din

g m

om

en

t ratio

0.1 m

0.15 m

0.2 m

0.25 m

0.3 m

480 FORESTRY

sheltered. This shows up most clearly in the extreme to mean bending moment ratios which are consistently high (>11) for the tallest trees and consistently low (<8) for the shorter trees.

Only in the region between the forest edge and a distance of fi ve tree heights downwind are these gust ratios lower for the tallest trees. This suggests that no matter how the forest is manipulated the

Figure 9. Extreme and mean bending moments of the 0.1 m tree on the downwind side of different-sized gaps in the (SW) forest ( x = distance from edge and h tallest = tallest tree height = 0.2 m).

0

0.001

0.002

0.003

0.004

0 1 2 3

Size of Gap (htallest

)

Ben

din

g M

om

en

t C

oefficien

t

Extreme

Mean

Figure 8. Mean and extreme bending moments for wide-strip (WS) forest ( x = distance from edge and h mean = mean tree height = 0.2 m).

0

0 2 4 6 8 10 12

0.01

0.02

0.03

0.04

0.05

0.06

Distance from downwind edge (x/hmean

)

Ben

din

g M

om

en

t C

oefficien

t

0.1 m tree mean

0.1 m tree extreme

0.2 m tree mean

0.2 m tree extreme

0.3 m tree mean

0.3 m tree extreme

Edge of Strips

WIND TUNNEL INVESTIGATION ON FOREST STABILITY 481

tallest trees are subjected to very much the same conditions and only the shorter trees below the main canopy benefi t. This is contrary to what was expected because it was thought that gusts would still penetrate below the canopy top while at the same time the mean fl ow would be substantially reduced so that the gust factors for the shortest trees would be higher than for the tallest trees.

The strip system appears to offer no stability benefi ts with the wind loading on the trees in the belts of different heights being virtually identical to what they would experience at the edge of the forest. This confi rms suggestions that strip sys-tems simply increase the number of edges within the forest rather than defl ecting the wind over the top of the forest ( Somerville, 1989 ). The single-tree selection forest provides shelter for all the trees shorter than the tallest 0.3 m tree. However, in the course of normal management operations these tallest trees will eventually be removed. It would be useful to determine how the shelter benefi ts to the 0.25 m trees are affected if the 0.3 m trees were removed. The

risk of damage to these trees may increase fol-lowing such an operation because they would not be adapted to these changed conditions but rather to the more sheltered conditions they pre-viously experienced.

The shelterwood/group selection forest appears to offer the best possibilities for improved stabil-ity. Even though the bending moments acting on the 0.2 m trees were slightly higher than those experienced by trees of the same height in the even-aged forest, they were substantially less than those for an even-aged forest (EA 50 per cent thinned) of 0.2 m trees growing at the same density as the 0.2 m trees in the shelterwood/group selection forest. The presence of the 0.1 m trees appears to reduce the loading on the taller trees probably by increasing their damp-ing and reducing swaying. A potential draw-back of this strategy is that, as in the case of the single-tree selection forest, the tallest trees would eventually be removed in the course of normal forest management operations, leaving the 0.1 m trees subjected to more exposed con-ditions. The practical importance of this change

Figure 10. Mean wind speed profi les over even-aged (EA), shelterwood/group selection (SW) and single-tree selection (ST) forests (a) normalized by standard tree height ( h = 0.2 m) and (b) normalized by the height of the tallest tree ( h tallest ) in each forest. U ref is the wind speed at a height of 0.38 m.

0

0 0.5 1 1.5

1

2

3

4

5

6

7

(a) (b)

Umean

/Uref

0 0.5 1 1.5

Umean

/Uref

Heig

ht (z/h

)

EA

SW

ST

0

1

2

3

4

5

6

7

Heig

ht (z/h

tallest)

482 FORESTRY

on the risk of windthrow may depend on the timing of the operation. If, as in the simulations, there is a considerable size and age difference between the overstorey and understorey, the increased risk may be of little signifi cance. This is because shorter, younger, and hence more fl exible, trees are known to have the capacity to dissipate high levels of wind energy (e.g. Savill, 1983 ).

The wind loading on trees in different forests cannot be disassociated from the ability of the trees to resist breakage or overturning. Unlike normal engineering structures, trees modify their growth in response to their wind environment ( Mattheck, 1991 ; Telewski, 1995 ; Nicoll and Ray, 1996 ) and, therefore, the resistance of a tree growing in a windy environment will be greater than one growing under calm conditions, all other things being equal. Unfortunately at present we are unable to relate the magnitude of tree response to wind intensity exactly; we do not have the dose-response curves. If the mean

wind strength is the critical parameter then the ratio of extreme to mean bending moments becomes extremely important and those trees with low ratios will be more stable. Gardiner et al. (1997) showed that trees at wider spacings have lower bending moment ratios but they argued that wider spaced trees were more likely to be overturned by the wind. However, this was based on the extrapolation of relationships obtained from tree pulling data obtained at close spacings which may not be relevant to wide-spaced trees. Achim et al . (2005) used a winch and cable system to apply loads to trees growing at different spacings and found some benefi ts of wider spacing on resistance to windthrow. How-ever, more work is required to investigate, in detail, the growth response of trees to wind load-ing and the resistance of wide-spaced trees to breakage and overturning. In particular, the re -sponse of tree roots and consequent anchorage to the wind environment and changes in spacing require further work.

Figure 11. Profi les of (a) turbulence intensity σ U /U ref and (b) Reynolds stress ―u′―w′/ U ref for the even-aged (EA), shelterwood/group selection (SW) and single-tree selection (ST) forests. U ref is the wind speed at a height of 0.38 m.

0

0 0.1 0.2 0.3 0.4

1

2

3

4

5

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7

(a) (b)

Heig

ht (z/h

tallest)

Heig

ht (z/h

tallest)

EA

SW

ST

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1

2

3

4

5

6

7

-0.03 -0.02 -0.01 0

u'w'/Uref

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2

WIND TUNNEL INVESTIGATION ON FOREST STABILITY 483

Conclusions

All conclusions have to bear in mind that the experiments in this paper were conducted with model trees in a wind tunnel and of necessity are a simplifi cation of a real forest. In some instances, therefore, the simulations can only provide a rough approximation to reality. For example, we were unable to simulate fully the distribution of tree diameters in a single-tree selection forest. Further-more, this paper does not provide the answers to how to manage forests to maximize tree stability, but it does provide measurements of the differences in wind loading on trees of different sizes under different simulated silvicultural systems. Such meas-urements can be very valuable in developing and testing the modelling tools that are now becoming available (e.g. Ancelin et al ., 2004 ; Cucchi et al ., 2005 ) to evaluate the risk to the whole population of trees in stands of complex structure.

A key conclusion that we can make is that there is no simple solution which optimizes stand sta-bility. Any change to the forest structure due to clear-felling, thinning, group selection, etc. will initially have a negative impact on the stability of the stand. Understanding the change in wind loading on the stand following any adjustment in its structure is necessary to calculate the immedi-ate change in wind risk and how that risk will subsequently change as the trees adjust to their new circumstances. However, it is clear that changes which occur ‘ little and often ’ are better than those which occur ‘ a lot and infrequently ’ . The key is the magnitude of any change in the forest. The smaller the change the less the increase in risk and the more chance the forest will have to recover. No silvicultural system is ideal for stand stability but those that employ a more subtle approach are likely to be less vulnerable overall. At the same time it must be remembered that all forests are at risk if the wind is strong enough. Balancing the risk of wind damage over the life time of the stand and the time required to meet the objectives set out for the stand is key to devel-oping silvicultural strategies for different site and climatic conditions.

Acknowledgements

The authors wish to thank the Forestry Commission and the Scottish Forestry Trust for providing the fund-

ing to support this project. Particular thanks to all the people who helped put the model trees together, par-ticularly Mrs G. Favager. We would also like to thank the two anonymous reviewers who provided detailed and helpful reviews.

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Received 10 December 2004