FORESTRY COMMISSION BULLETIN No. 31 CODE OF SAMPLE PLOT PROCEDURE By F. C. HUMMEL, M.A., D.Phil., G. M. L. LOCKE, B.Sc., J. N. R. JEFFERS, A.I.S., and J. M. CHRISTIE FORESTRY COMMISSION LONDON: HER MAJESTY’S STATIONERY OFFICE FIFTEEN SHILLINGS NET Forestry Commission ARCHIVE FORESTRY COMMISSION BULLETIN No. 31 CODE OF SAMPLE PLOT PROCEDURE B y F. C. HUMMEL, M.A., D.Phil., G. M. L. LOCKE, B.Sc., J. N. R. JEFFERS, A.I.S., and J. M. CHRISTIE FORESTRY COMMISSION LONDON: HER MAJESTY’S STATIONERY OFFICE 1959 FOREWORD T he systematic measurement of sample plots in stands of growing timber provides the data for the compilation of yield tables and other estimates of increment which are essential to the economic management of woodlands. In order that such measurements shall be strictly comparable, both as between one sample plot and another, and within the same plot as measured on different occasions, a precise method must always be followed. This bulletin sets out in detail the procedure that has been developed by the Forestry Commission’s research officers since such measurements were begun in Britain, under the Board of Agriculture, forty-five years ago. Though intended primarily for the staff engaged on such work, it is believed that it contains much information of value to all who are concerned with the raising and accurate measurement of timber trees, both at home and abroad. FORESTRY COMMISSION, 25 Savile Row, London, W.l June, 1958 ACKNOWLEDGMENTS The Commission wishes to express its thanks to the Editor of the Incorporated Statistician, and to Miss Joan Keen and Mr. D. J. Page, for permission to reproduce copyright material in Appendix VI. Also to the Honorary Editor of the Indian Forester, and to Mr. Roy, for permission to reproduce copyright material in Appendix X ; and to Mr. F. G. Wilson, who kindly consented to the use of the logarithmic stand density sheet in Appendix III. CONTENTS PAGE Introduction ............................................................... .. 1 C h a pte r 1. Selec tio n a n d G en er a l M a na g em en t . 4 C h a pte r 2. L a y o u t o f P l o t a n d M a r k in g o f T h in n in g s 4 C h a pte r 3. D escriptions . 7 C h a pt e r 4. M easurements a n d M a in t en a n c e .. 10 C h a pte r 5. C omputations a n d R eco rd s . 16 Appendices I. L ist of E q u ipm e n t 23 II. T ree C lassification 24 III. G rades of T h in n in g .. .. .. .. 28 IV . U se of D en d rom eter fo r M ea su r in g V olum es o f Sta n d in g T rees . 31 V. C a l c u l a t io n o f V o lu m e of P a r tia lly C lim bed Sa m ple T rees 34 V I. K e e n ’s M et h o d o f E stim a t in g L in ea r R eg ressio n .... 37 V II. D etermination o f V olum e o f P revious T h in n in g s from St u m p D iameters . 39 VIII. Sum m aries of W o r k 43 IX. W o r k e d E x am ples from S a m ple P lo t F iles 49 X. F o rm u la e a n d T ables . 92 XI. G en era l N otes o n C o m pu t a t io n .... 102 XII. M a in D ifferen ces betw een th e R evised C o d e of 1958 a n d th e P r o c ed u r e of 1931 104 X III. S um m ary o f D uties of t h e S a m ple P lo t F o rester 107 XIV. D efin itio n s 110 v INTRODUCTION T h e Forestry Commission now has more than 700 ment did not undergo any important changes between permanent sample plots, the oldest of which were 1931 and 1955, sufficient minor modifications had established in 1913 by the Forestry Branch of the been introduced to warrant the preparation of an then Board of Agriculture and taken over by the up-to-date description of methods. The second Forestry Commission when it was formed in 1919. reason was that the expansion of sample plot work Most of these plots are in the Commission’s own and its execution by a larger number of research forests, but some, particularly the older ones, are on foresters made it necessary to replace James private estates. We are greatly indebted to the Macdonald’s general description of methods by a private woodland owners who have given us facilities more detailed code of instructions. In 1931 there to establish and measure these plots, for without were about 180 permanent sample plots and there was their co-operation we would have had few data in the only one sample plot party whose sole duty it was to older age classes. Most of the permanent sample measure and thin these plots. The observance of plots are in pure, even-aged stands of the commonly strictly uniform standards thus presented no diffi­ grown conifers. There are, however, about 100 plots culties. Now, however, the position has changed ; in broadleaved species and about 70 in the less on the 30th September 1958 there were 774 plots common, but promising, coniferous species, and and the responsibility for these is divided between these numbers are likely to increase. Some plots 12 different field parties, each of which performs have also been established in mixtures, in stands a considerable range of other duties in addition to which have been underplanted, and in stands which the measurement of sample plots. Under these are under conversion from coppice-with-standards to changed conditions, strict uniformity of procedure high forest. can only be ensured by a detailed code of instructions. The objects of sample plot work and the procedure The code is intended to help in the training of new used were described by James Macdonald (1931). staff and to serve as a guide and source of reference The objects as set out in that paper still apply. to those who are already trained. They are : Before preparing the new code, the whole of the (a) The study of the growth, production and existing procedure was subjected to a thorough increment of different species under definite review. The object was to make the procedure more and continuous systems of management. objective and to simplify it where possible without (b) The comparison of the effect of different loss of accuracy. At a meeting in May 1954, the methods of thinning on the growth of any senior members of the Mensuration Section, including given species in order to determine as far as the leaders of all the field parties, were invited to possible the method of treatment best suited criticise the existing procedure and to suggest to that species. improvements. A committee, consisting of F. C. (c) The investigation of silvicultural problems such Hummel (Mensuration Officer), G. M. I.. Locke as underplanting, conversion of coppice-with- (Census Officer), J. N. R. Jeffers (Statistician) and standards to high forest, treatment of mixtures, J. M. Christie (Forester responsible for sample plot etc. records), considered the suggestions made, arranged (d) To provide areas where different methods of for testing the various proposals and then, having thinning can be demonstrated. reached decisions on all major points, prepared a In addition to serving the above main objects, sample preliminary draft of the new code. The circulation plots are now used : of that draft to the field parties for further trials and (e) to study the degradation or improvement of a criticism led to several further amendments. The site over several rotations ; code as it now stands, although drafted by the above (/) to study the effect of espacement, thinning committee, is thus very much the result of team work treatment and provenance on the properties by the Mensuration Section as a whole. of the timber produced. The more important changes in procedure since There are two main reasons why it has become 1931 are discussed below, and are summarized in necessary to prepare a code of sample plot procedure. Appendix XII where the new procedure is compared In the first place, although the methods of measure­ with the procedure described in 1931. 1 2 SAMPLE PLOT PROCEDURE Before the 1st of March 1947 all volumes and basal described in Appendix V. (This method areas were expressed in terms of true measure. Since replaces the form quotient method formerly th is date, the Britishhoppus or quarter girth system for used.) determining the volumes and basal areas of trees has been used, in order to bring measurements taken by Previously, in order to calculate the volume of the the Research Branch of the Forestry Commission plot, the volumes of the sample trees were plotted into line with those in general use among foresters over girth, and a modification of Block’s system of and timber merchants throughout Great Britain. grouping was used. The volume of the plot is now Volumes continue to refer to stem-wood measured determined from the regression of the sample tree to a top diameter of 3 inches over bark, but they are volumes on their basal areas ; this relationship is now measured and recorded over bark instead of assumed to be linear (Hummel 1955) and the method under bark and only in certain circumstances are described in Appendix VI is used to determine the under bark volumes calculated as well.