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Nội dung Text: Báo cáo khoa học: "Discussion about the articles by E Lahde et al in Ann Sci For (1994) 51, 97-120: "Structure and yield of all-sized and even-sized conifer-dominated stands on fertile sites" and "Structure and yield of all-sized and even-sized Scots pine-dominated stands" Pierre"

Discussion Discussion about the articles by E Lahde et al in Ann Sci For (1994) 51, 97-120: "Structure and yield of all-sized and even-sized conifer-dominated stands on fertile sites" and "Structure and yield of all-sized and even-sized Scots pine-dominated stands" Pierre Duplat (Office national des forêts, département des recherches techniques, 77306 Fontainebleau, France) plot is considered as a stand, with Each I have some reservations about 2 papers itsinventory, its total stem volume (tariff by E Lähde et al in Annales des Sciences function(s)?), its dominant height and its Forestières (1994) 51, 97-120, entitled: total stem current increment. "Structure and yield of all-sized and even- sized conifer-dominated stands on fertile Precise criterions, applied to the all- sites" and "Structure and yield of all-sized species diameter histogram of each stand, and even-sized Scots pine-dominated make it possible to decide whether it belongs stands". or the other of 2 groups of "structure": to 1 "all-sized" and "even-sized"; 83% of the I will express the most important ones stands may thus be classified in 1 or the referring to the first paper, the second one other group, and constitute the material for using the same methodology. the study. The stands belonging to each large group of structure are classified into 2 groups of INTRODUCTION "composition", depending on whether they include less or more than 120 broad-leaved The material used comes from the third trees per hectare: "coniferous", "mixed". national forest inventory 1951-1953 in The "yield" of each group structure x com- southwestern Finland, on Myrtillus type sites the basis of 2 results: is position judged on or more fertile ones: temporary plots, 1 000 of current increments for stands the mean 2 m in size, inventory by species and diam- - given standing volume (137 < V ≤ with a eter class starting from the 2-6 cm class, /ha) 3 167 m measure of dominant height (100 largest trees/ha), and estimation of current volume the of relative current increments mean - (increment/volume) of all stands in the increment over the 5 preceding years (by boring every tree?). group.
explicit objective of An the paper is to For even-aged stands, dominant height is conclusion, in this region and on to a not a site index by itself, as it depends on come these sites, about which structure, which age at a given site quality. composition, or which combinations, are uneven-aged stands, it could roughly For far better volume is concerned. yield as as by assumed, if the 100 largest trees/ha were In my opinion, the method used and cur- more or less at the same stage of maturity in sorily described does not fulfill this expec- every stand, but nothing ensures that this tation;I will explain why. here. Given the criterions used to is the case classify the stands into groups of structure, Current increment of a stand, at a given it is even likely that the "all-sized" group time, mainly depends: includes part of the young even-aged i) on the quality of the site where it grows stands. ii) on its specific composition So, site quality cannot be controlled by a iii) on its age, if it is an even-sized (even- consistent index in the present study, where aged) stand, or on its intimate structure (in the problem is still further complicated by ages, dimensions, spatial distribution) if it the presence of several species. Stating is an uneven-aged stand that dominant height alone "may give a good iv) incidentally and the above conditions base for comparisons of site quality" being fixed, on its standing volume if it falls between stands, whether even-aged or far below the biological maximum attainable uneven-aged, is not satisfactory. under those conditions. As this factor is not controlled, correla- A first remark is essential: the current tions may happen between site quality and volume increment of a stand at a given time other factors: it is even likely for the "com- is not its long-term yield, especially if it is position" factor, with broad-leaved trees an even-aged stand. I will come back to that being noticeably present upon certain site point when examining factor 3. types rather than upon other ones. Being not too demanding about the def- inition of the factors and their levels, one Factor 2: composition of the stand can say that factor 2 (composition) and fac- tor 3 partly (large types of structure) are tested factors in the present paper. The Composition is explicitly tested as a factor in other ones - factor 1 (site quality) factor 3 the paper, considering only 2 levels: "conif- partly (age or intimate structure), factor 4 erous" and "mixed". Therefore, it should not (volume) - are therefore to be controlled. pose any special problem (apart from the Let us look at that. varying proportions of spruce and pine). However, one reservation must be made: stands are classified into "coniferous" or Factor 1: site quality "mixed" according to a criterion which is an absolute number of stems per hectare; it is Comparisons between types of structure or less demanding for young (and "all-sized") composition make sense only for a given stands than for ageing ones. So, young site quality. In the present study, the mate- stands showing a small proportion (5-10%) rial used spreads over a large range of site of broad leaves will be classified as "mixed", quality (Myrtillus site type or more fertile); whereas similar but ageing ones, of whose but site quality of the stands is not known, increment is smaller, will be classified as only their dominant height is. "coniferous".
Factor 4: standing volume Factor 3: structure and age It is not the value of standing volume per Factor 3 is a composite, hierarchical factor se that has an effect, even less a linear with an upper stage: "structure" ("even- effect, upon current increment. It acts notice- sized"/"all-sized") and a lower one: age for ably, everything else (site quality, compo- "even-sized" stands, intimate structure for "all-sized" stands. sition, age or structure) being fixed, only as it may fall markedly below the correspond- At the upper stage (the 2 groups of struc- ing maximum attainable volume. The rele- ture), it is a tested factor in the paper. But, vant control variable would be rather the there too, a reservation must be made. Cri- ratio of standing volume to that maximum, terions used by the authors lead to classify but this one is unknown here. young even-aged stands (eg 3 000 stems/ha So, would even site quality and age or with a mean diameter of 8 cm) as "all-sized"; the average histogram of stands classified intimate structure be properly controlled in as "even-sized" shows a lack of young other respects, there would be little sense to work at a fixed absolute volume (152 m /ha). 2 stands. A certain number of such young stands, whose increment is larger than that There is no more sense to consider the rel- of aged ones’ for the same levels of other ative volume increment rate, as though factors, reinforced the "all-sized" group. standing volume were a capital proportion- ally yielding its annual interest. One may At the lower stage (age or intimate struc- refer to Assmann (The principles of forest ture), it is obviously a factor to be controlled. yield study, 1970, Pergamon Press, pp 229 Let us be more accurate: firstly, the "yield" of sq) in this respect. an even-aged stand is not its current incre- ment which strongly depends on age, but standing volume is therefore This of use its increment over a rotation. More- when the other factors are not pertinent mean over, this one depends on the rotation’s properly controlled. One cannot a fortiori duration, which must therefore be fixed, for hope it becomes relevant when these other instance at the average duration imple- factors are not controlled. mented actually or at the duration leading to the maximum mean increment. If this CONCLUSION mean increment is to be derived from the average of current increments observed at a given date on even-aged stands of differ- "Yield" indicators obtained, with the method ent ages, all other factors being fixed, the and material used in this study, do not seem age distribution of these stands must be uni- to be relevant indicators for the reasons form over the chosen duration. This is not expounded here, among whichI summa- controlled in the present paper, the age of rize the main ones: even-sized stands being unknown. confusion between yield and current incre- - As for uneven-aged stands, the ideal case ment, lack of control of age is a stand whose intimate structure is kept lack of control of site quality - stationary with enough standing material to standing volume that is not per- control of - avoid any noticeable loss of production, and tinent whose current increment is constant over criterions, used to classify stands into time: then it is equal to the mean increment. - groups, such as the appellations of these In general, it is very difficult to assess how far groups are not strictly consistent with the one stands from this ideal case, and the nature of the stands included. issue is not considered in this paper.
Therefore, these indicators do not allow, agement records. Studied units were not in my opinion, a conclusion about the plots, but whole forests, planned and man- respective yields of all-sized or even-sized aged for a long time according to one or and coniferous mixed stands. the other structure (330 forests, 64 000 or ha). He found that annual volume incre- Recently, a colleague from our depart- (Demolis, 1993, "Étude de l’influence ment, during the 10 or 20 years preceding ment the last revision of the management plan, de la fertilité du sol sur le choix du traite- was distinctly larger in the even-aged high ment sylvicole des peuplements résineux forest group. He wisely did not conclude jurassiens", document interne, départe- that the even-aged system is better than ment des recherches techniques ONF) the uneven-aged one, but that forests so conducted a comparative study about even-aged and uneven-aged high forest treated are likely to be standing on more in the Jura range, on the basis of man- fertile sites on average.
Reply note to P Duplat’s letter: comparisons among structurally all-sized "Yield and even-sized forest stands" Lähde, O Laiho, Y Norokorpi, T Saksa by E parameters for comparing structurally dif- A long tradition in the Finnish forestry and ferent stands. The Finnish forest site type National Forest Inventories is to apply the system is not influenced by these prob- forest site type system when classifying for- lems. est sites (ilvessalo, 1951).This classification is based on the ground cover vegetation The group of the Myrtillus type and more and it is independent of the growing stock, fertile sites (Lähde et al, 1994a; Ann Sci For its age, structure and species composition 51, 97-109) in the study material did not (Cajander, 1909, 1949; Vuokila, 1980). demonstrate any bias in favour of all-sized When comparing structurally all-sized and stands; instead, the distribution slightly even-sized (all-aged and even-aged) favoured even-sized stands. The distribu- stands, the forest site type system is more tion of the study stands by site type and reliable than the site index (H 100) classifi- structure class was in order of site fertility cation. The reason is that the harvesting of as follows (percent of sample plots): timber in all-sized stands takes place as single-tree selection, which also focuses on bigger trees. The low-thinning in even- sized stands focuses primarily on smaller trees. When comparing dominant height after different logging operations on a par- ticular site type, it is shorter in the all-sized stand than in the even-sized stand (eg Sterba and Monserud, 1993). The same The similarity of the stands’ tree species applies to tree age when comparing stands composition was another reason for com- with the same history of establishment bining the fertile site types to form1 group. (Lähde, 1995). The material on pine-dominated stands (Lähde et al, 1994b; Ann Sci For 51, 111- 120) represented only 1 forest site type INCREMENT CORRELATE (Vaccinium type). It should be noted that WITH SIZE, NOT WITH AGE southwestern Finland is a relatively uniform region in terms of climate and topography, and thus differs considerably from the Increment in all-sized stands shows bet- mountain regions of Central Europe, for ter correlation with size than with age instance. (Cajander, 1934; Sarvas, 1944; Schütz, The dominant heights were presented in 1969; Indermühle, 1978). After releasing, the publications for describing stands, but the undergrowth trees generally attain the not for classifying site quality due to the same size as trees which have always aforementioned limitations. Nearly all sam- been released to grow in (Sarvas, 1951; ple plots also included tree age measure- Vaartaja, 1951; Vuokila, 1956; Klensme- ments, but these parameters represented den, 1984). Therefore, the age and height only the average age of the dominant trees. reliable of the dominant trees are not
The ages of dominant trees in the BROAD-LEAVED TREES mean were as follows (fertile sites stands study AND STRUCTURE INFLUENCE STAND INCREMENT example): as an Each sample plot was sampled for 10-20 sample trees and these were measured for their 5- and 10-year increments. The publi- cations set out only the 5-year increment data because the differences for the 10- year period were similar to the 5-year dif- ferences. The sample trees were used in determining the tree increment for each sample plot as a whole. This is often not differ any Thus, the material did more expressed using the concept yield, but more than might be expected due to the different precisely, it is the increment and yield for a felling methods because all-sized stands particular period. should be harvested before reaching matu- The criterion used in the publications for rity. The general understanding appears to out pure coniferous stands and sorting be that more research needs to be done on mixed ones was the number of broad-leaved the subject of site index (H 100) classifica- trees. Instead, we could have used, for tion for all-sized stands (Andreassen, 1994; example, the proportion of the basal area Keller, 1994). or of volume as recommended by Dr Duplat. Both methods have their weaknesses, how- ever. In further studies, we have examined EACH SAMPLE PLOT REPRESENTED the dependence of yield on the proportion of AN INDIVIDUAL STAND broad-leaved trees (in terms of both stem tally and basal area) separately in each structural group. According to these results, It is necessary to underscore the fact that it appears that when the proportion of broad- each sample plot represented an individual leaves increases to about 50% of the grow- stand. They were classified according to ing stock, stand increment in both structural development stage, site type and structure groups increases and then begins to class in the said publications. Adhering to decrease. inventory instructions, a sample plot was moved if it happened to fall on the boundary Using the number of broad-leaved trees between 2 different stands (Ilvessalo, 1951). criterion in classifying mixed stands as a Thus, it was not possible for a particular may have caused some bias in favour of sample plot to contain growing stocks dif- the even-sized stands and not the all-sized fering in age or otherwise, as is suspected stands. The explanations for this are as fol- by Dr Duplat. The growing stock on each lows: 1) the relative proportion of broad- sample plot was classified separately. The leaved trees in the even-sized mixed stand material included only advanced, primarily (because of its smaller overall stem tally) well- or satisfactorily-managed stands with was greater than in the structurally all-sized normal growing stocks; even-sized stands mixed stand and 2) the broad-leaved trees were not classified as being all-sized. The in the even-sized stand were of greater aver- stands also identical in terms of their age size. The same stem tally as the limi- were of establishment - ie they were of history tation for a pure coniferous stand in both natural origin. structural groups of advanced forests was
Cajander AK (1909) Über Waldtypen. Acta For Fenn 1, justified; eg due to differences in the stem 1-175 diameter distribution, ecological-biological Cajander AK (1949) Forest types and their significance. factors (litter etc). Acta For Fenn 56, 1-71 One of the results in our studies was that Cajander EK (1934) Kuusen taimistojen vapauttamisen all-sized stands grew better (with statistical jälkeisestä pituuskasvusta. Commun Inst For Fenn 19, 1-53 significance) than equal even-sized stands with the equal growing stock volumes. In (1951) III valtakunnan metsien arviointi. Ilvessalo Y Suunnitelma ja maastotyön ohjeet. Summary: Third the material as a whole, the increment of national forest survey of Finland. Plan and instruc- even-sized stands was slightly better than tions for field work. Commun Inst For Fenn 39, 1- that of all-sized stands when examining 67 stands dominated by spruce; however, this Indermühle MP (1978) Struktur-, Alter-, und Zuwachs- difference was not statistically significant untersuchungen in einem Fichten-Plenterwald de subalpinen Stufe. Beiheft zu den Zeitschriften des despite the extensive material used. The Schweizerischen Forstvereins 60, 98 p results covered, of course, only that stage in (1994) Standörtliche Inhomogenität von Keller W stand development that the sampling data ertragskundlichen Versuchsflächen - eine Ursache represented. It should be noted that the Fehlschlüssen. Schweiz Z Forstwes 145, 1031- von growing of even-sized stands (during the 1038 rotation) includes regeneration and a fairly Klensmeden U (1984) Stamvis blädning. Några studier på två försöksytor i Dalarna. Examensarbete long period after regeneration when the 1984-1986. Sveriges lantbruksuniversitet. Institu- annual volume increment is below average. tionen för skogsskötsel. Umeâ, 38 p Our subsequent studies dealing with other (1995) Metsää puilta. Vihreä Eämänsuojelun Lähde E research material, and also arranged com- liitto. Hakapaino Helsinki, 112 p parative field trials, support the results pre- Lähde E, Laiho O, Norokorpi Y, Saksa T (1994a) Struc- sented in the publications. ture and yield of all-sized and even-sized conifer- fertile sites. Ann Sci For 51, dominated stands on In conclusion, there is increasing an 97-109 necessity for scientific debate on the sub- Lähde E, Laiho O, Norokorpi Y, Saksa T (1994b) Struc- ject of stand structure and species compo- ture and yield of all-sized and even-sized Scots pine- sition. This demand involves the interna- dominated stands. Ann Sci For 51, 111-120 tional conventions on forest biodiversity and Sarvas R (1951) Tutkimuksia puolukkatyypin kuusikoista. Summary: Investigations into the spruce ecological sustainability. The requirement stands of Vaccinium type. Commun Inst For Fenn of biodiversity will most probably be 39, 1-82 extended to the within-stand level, too. Sterba H, Monserud RA The maximum (1993) density When this happens, there will be an even concept applied to uneven-aged mixed-species greater demand for scientific knowledge on stands. For Sci 39, 432-452 the functioning of structurally all-sized mixed Vaartaja Y (1951) Alikasvosasemasta vapautettujen stands. We wish to express our warmest männyn taimistojen toipumisesta ja merkityksestä metsänhoidossa. Summary: On the recovery of appreciation to Dr Duplat for the active inter- released pine advance growth and its silvicultural est afforded to our research work. importance. Acta For Fenn 58, 1-133 Vuokila Y (1956) Etelä-Suomen hoidettujen kuusikoiden kehityksestä. Summary: On the development of man- REFERENCES aged spruce stands in southern Finland. Commun Inst For Fenn 48, 1-138 Vuokila Y (1980) Metsänkasvatuksen perusteet ja Andreassen K (1994) Development and yield in selection menetelmat. WSOY, Helsinki-Porvoo, 256 p forest. Commun Skogsforsk 47, 1-37