Swiss Method - Control of the Growing Stock

(based on Ipsden Estate experience - Oxfordshire Chilterns)
by Michael G Reade

The basic philosophy of this system of uneven aged woodland management is control of the growing stock (G.S.). If this is kept in suitable balance, according to site and tree species, continuous cover should result more or less automatically. The "control" consists in repeated checking of the actual growing stock for the time being, by enumerating all stems larger than 5" breast height quarter girth, typically at intervals of 6-10 years (ideally, perhaps, at the interval during which each tree can be expected to put on 1" Q.G., though the actual interval is not itself at all critical and we do not lay any stress on always keeping to the same interval). A reasonable enumeration target for a team of two measurers (with girthing tapes) and one recorder (with pencil and clip board) is 1 hectare per hour (less, if the stem count exceeds 450-500 per ha; more, if the going is easy and the number of stems is low, but a low stem count is usually associated with difficult going and obstructive weed growth).


The current Ipsden assessment is that a maintained average G.S. of 200 m3/acre should prove near to the optimum in our circumstances (mixed hardwood-conifer stocking, predominantly hardwood). This can only be a preliminary estimate but the few compartments which are approaching this level of G.S. are confirming that it is a reasonable one; the level should actually increase with time, as and when selective thinning improves the average quality of the remaining stems. The current expectation is that, at equilibrium, individual compartment growing stocks will fluctuate periodically between about 180 and 260 m3/Ha. (to convert m3/Ha to Hoppus ft3/acre, multiply by 11.22).



Woodland Heritage Trustees at Ipsden.

The Ipsden Estate woodland first became dedicated in 1952 and the first whole compartment enumeration (24.8 acres) was made in May 1953. As yet (2002), Ipsden is still in a building-up (transitional) phase. Actual total G.S. is assessed at 10 year intervals and we have now completed five such 10 year periods. The average G.S. at the end of each period has been 89, 88, 98.3, 123 and 150 m3/Ha, which incidentally confirms my reading of both my own and continental European experience, namely that if one tinkers with a hardwood management plan more than once in 80 years (or thereabouts), the end result is likely to be at least partial failure of the whole.


It is especially distressing that the Forestry Commission keeps changing the rules at shorter intervals than this, the abandonment of Dedication having caused us considerable trouble (e.g. the modern WGS rules appear to require a higher priority to be given to individual conservation aims than to the health of the woodland as a whole).


The rate of increase of G.S. at Ipsden is still accelerating but it will sooner or later start to fall off (as and when individual compartments come to contain enough or too much G.S. - though the same effect could be produced by over-cutting or by excessive losses from windfall or disease - not a very major consideration, incidentally, as we did experience a loss of about 4 years’ growth, 33,000 ft3, in the gales of January 1990, but the effect of this loss on the rate of increase of G.S. has not proved at all conspicuous, partly perhaps because so many of the lost trees were not themselves very vigorous ones).


It would almost certainly be unwise to attempt to introduce such a system into a whole forest overnight. It is clearly preferable to start with a few selected compartments and then gradually expand the controlled area until the whole becomes routinely enumerated. At the end of the first 10 year period, we had 131 acres which had been enumerated at least once. By the end of the second period this had risen to 166 acres and by the end of the third period it was 237 acres. Today, 55 years after the first tentative attempts to re-introduce regular management at Ipsden, the area routinely enumerated is 330 acres (about 92% of the whole). The whole has increased by about 120 acres in the course of the 50 year period and is still being expanded, but the grand total does include some coppice of nominal zero G.S. which slightly distorts the statistical picture as a whole.


Extraction rates (timber + thinnings) have been consistently 5000-7000 ft3 per year throughout the period that we have been operating the system. In the first 20 years, there was a negligible yield of pole thinnings, only timber and firewood. In the third period, 26.3% of the yield was pole thinnings, rising to 47% in the fourth and 63% in the fifth (the pole thinnings coming very largely from plantations established since 1951, there having been no significant planting on the Estate during the previous 75 years; the G.S. had also become severely depleted in the course of the two World Wars).



A beautiful Ipsden compartment of mixed species.

The most revealing statistic is probably the yield of timber + thinnings per acre of new plantation established. In a "perfect" selection forest, this figure would be infinitely large, as there would be no formal new planting at all. Our actual ratios have been 670 ft3 felling per acre planted in the first 10 year period, 963 ft3/acre in the second period, 1919 ft3/acre in the third, 3756 ft3/acre in the fourth and 5525 ft3/acre in the fifth. Provided that the total G.S. is either steady or increasing and that gaps in the stocking are not being deliberately left unplanted, this measure appears to present a good indicator of progress toward true "continuous cover".


No single statistic can be relied upon absolutely, but the Swiss method does comprise quite a number of cross-checks within itself and the obvious self-consistency of the figures which it generates gives one considerable confidence, despite the apparent crudity of the measuring system itself (there is no differentiation between trees of different heights, use of the same volume table for all species). It has also proved more stable in practice than one might at first sight expect. For instance, we have accidentally omitted enumeration of an acre or so in a 24 acre compartment on at least one occasion but the omission was not at all obvious from the results and the management plan would not have been significantly affected if this omission had remained undetected. There have also been other and more minor lapses, shown up by cross-checks, but their end effect has always proved more academic than otherwise.


One does need to be careful of misinterpretation of statistics, however - for instance, one small compartment showed a near ideal composition at its first enumeration, even having the much recommended "inverse-J" stem distribution curve, but it took us some years to discover that virtually all the smaller stems in that first enumeration were actually suppressed ones which were dying rather than growing on into the larger sizes ! The same effect can apply to almost any compartment but regular silvicultural thinning eventually eliminates it. Obviously, one must be continuously on one’s guard against placing too much reliance on the seeming infallibility of the system.












Last modified onFriday, 19 August 2011 11:09

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