Optimized Silviculture Mats Hagner

Transkript

1 1 Optimized Silviculture Mats Hagner From a financial perspective, tall trees will hamper the growth of smaller trees. UBICON Report 1, 2017 ISSN UBICON, Blåbärsvägen 19, Umeå, Sweden. Tel mats.hagner@allt2.se. Org.no:

2 2 Abstract An array of scientific reports describing water, minerals, soil biology, photosynthesis, tree growth and competition, laser scanning, lighter-than-air techniques and economics suggests, that the local structure among trees that share the same growth resources, could be used to determine which silvicultural treatment will optimize long-term profits. The growth of trees with potential high value can be promoted by removing mature trees in the local neighborhood. Small trees with low value or vitality are also removed. There should only be one dominant tree in each group of competing trees because the growth resources will then be focused toward this single tree, allowing it to mature in a shorter time. In each group, the number of recruits should not be higher than what is needed to provide a new dominant tree after liberation thinning. These recruits will obtain an ideal phenology, resulting in a low number of small twigs in the lower part of the stem. However, this silvicultural system, named Liberich, cannot be introduced in Sweden because, first, the Forest Act handles stands instead of groups and, second, the Act wrongly assumes that standing volume is positively correlated with productivity. A cadre of professionals certified in the Liberich system is available in Sweden, and they help forest owners maximize ecosystem services. These certified professionals perform manual tree marking after a detailed interview with the owner. They first ask for all the locations where aspects other than lumber profit should be maximized or maintained, after which they evaluate the owner s personal economy, harvesting technique, and to whom lumber will be sold. On this basis, they register the structure of each tree-group and mark trees > 8 cm in diameter, which will be removed to optimize long-term profits. In problematic cases, they utilize two models in their field computer to determine the best structure for each group. At the end of the process, they will recommend that the owner performs enrichment planting in the first summer after harvesting. This system does have certain economic drawbacks, such as a laborious tree harvest, as well as the costs of enrichment planting and felling damaged trees. However, practical experience and scientific tests have shown that the Liberich system will at least double the long-term profits per hectare associated with a clear-cutting system. We are fortunate enough to have an efficient solution for the problematic increase in atmospheric CO 2 levels. A switch from clear-cutting can increase the ability of forest soils to trap CO 2. Furthermore, the combination of modern technology and the Liberich system can increase the production of various forest products, and simultaneously improve both biodiversity and springwater quality. The preference of pyrolysis over burning can restore the productivity of the soils used for farming and silviculture, and also trap carbon. Keywords: Liberich, selective harvest, water, minerals, soil biology, tree growth, present net value, economy in broad sense, biodiversity, climate, air-ship.

3 3 Background My experience working in forests around the world has given me the impression that a forest with a natural structure provides the greatest benefit to society. Furthermore, my knowledge about statistical methods convinced me that a new statistical expression for the local structure, which is typical for a natural forest, was needed. This is why I worked together with the statistician Hans Nyqvist to develop the Dissimilarity coefficient (Hagner and Nyqvist 1998, Hagner 2015c), an index that was later found to function well in both tropical rain forests (Hagner 2001) and in Sweden (Ekelund 1999). Figure 1. Interest for alternatives to the clear-cutting system is increasing on a global level. The easiest alternative, and most profitable in the short-term, is high-grading, i.e. harvesting the tallest and most valuable trees. The economic draw-backs of such a system are well known, and illustrated in this figure. The large trees (numbers 1 and 2) were left by the lumber buyer because they have no value. These trees will now dominate the forest and consume most of the growth resources. Without compulsory demand for regeneration in gaps, the forest will become too open (Hagner 2008, 2016d). Figure 2. This is an example of an old tradition in Northern Sweden that functions very well. The worker sits on the stump of a valuable large pine tree that was recently removed. The forest has been open enough to permit younger pines to survive, but their growth was hampered by competition with the large pine. The number 1 and 3 trees will remain and produce knotfree timber of high value. Number 2 is downed as it has too many branches. Number 4 is harvested now, because it has wounded bark and would compete with number 1. The forest owner is satisfied because he obtains a high interest rate on the working capital through pines numbers 1 and 3 (Hagner 2016b). Many forest owners are not aware of the risks associated with high grading, and will be tempted to accept favorable offers from timber buyers. The economic drawbacks of this situation can be avoided by switching to manual tree marking, which is carried out by someone who has been educated in the Liberich system. In this scenario, the tree marker communicates with the forest owner and begins

4 4 the process by registering every spot where benefits other than lumber profit are preferred. The interview also reveals in which harvesting techniques will be employed and to whom the lumber will be sold. The tree marker can then estimate the value of each tree. When in the forest, the tree marker registers the features of local groups of trees that share the same growth resources. These features include site index, tree species, storm risk, range of competition, stem form, bark injuries, and diseases, among others (Hagner 2004, 2016a). In fertile site, one group has a diameter of 7 m, which expands to 20 m when a site has very low fertility. Figure 3. An educated tree marker has great knowledge about wood quality. Based on the form of the stem and its branches they are able to determine the potential quality and its effect on timber price. The lowest 5 meters of the stem normally represents 50 % of the value of a full-size tree in Sweden. The graphs are reproduced from Thörnqvist (1993). Small trees with no commercial value (< 8 cm diameter) are ignored. By considering the potential quality and value of all the taller trees in the group, the tree marker is able to regulate the structure of the group in a way that will maximize long-term profits from lumber. If any uncertainties about the best structure arise, the tree marker can solve the problem with two programs, TREE and GROUP, in his field computer (Hagner 1999 and 2000). Once mature trees, which are trees that have a lower interest rate than alternative investments, are removed, the certified tree marker uses the site index to define compreach, which is the distance within which a tree is able to compete with another tree. The diameter (d) of a tree group is equal to 2 * compreach. The tallest residual tree with acceptable quality is appointed dominant and represents the center of the group. The position of each tree within the group, along with its potential monetary net value, can then be used to determine which trees in the group should be retained to maximize the group s long-term present net value (Figures 4 a-c). Immature trees are left only if they do not compete too much with the dominant tree. Larger trees, those with diameters more than 75 % of that of the dominant tree, are named main competitors and those that are within the d of the dominant are removed. Smaller trees that remain in the group are named recruits, and their number should be restricted to the lowest amount needed to replace the dominant tree after the next thinning.

5 5 Figure 4a. A forest where tree-size is represented by the diameter (black dots). If tree-size and position are the only variables to consider, tree marking could be performed with a computer in the manner described in the following figure. Figure 4b. The computer program only considers commercially-viable trees ( > 8 cm diameter at breast height). It starts by identifying one dominant (D4, filled dot with white center) and the group that lies within diameter = d (big circle). Next, it removes the Main competitors from within d (indicated by open dots, which have a diameter > 75 % of that of the dominant). The number of Recruits that are needed to replace the dominant after the next harvest (filled circles) is then determined. After this, the process continues by the identification of the next dominant (C9) that is at least d away from the previous dominant. Figure 4c. The forest after computer-guided liberation thinning. The biological effect of this type of thinning is that the dominant trees acquire most of the available growth resources. Each dominant is surrounded by smaller trees. Figure 5. The result of optimized thinning is an open forest with a distance between dominants = 2r = d (see figure 4b) Figure 6 from Elfving (2005). The production of stem-wood over ten years after thinning from below (clear-cutting system) and from above (Liberich system) in experiments from a multi-storied spruce forest in Northern Sweden (Chrimes et al. 2007). The results show that the removal of smaller trees is detrimental for productivity.

6 6 Figure 7. Swedish pine and spruce are able to react to a release from competition with strong growth irrespective of age. This pine was 92 years old and had a diameter of 6 cm when it was released. It had reached a diameter of 15 cm thirteen years later. The broadest annual ring was 5 mm wide (Hagner 2009a). Figure 8. It is now technologically possible, and financially viable, to scan forests with lasers from an air-borne vessel (Wulder et al., 2012). This method generates 3D-maps of the ground, and displays the coordinates, height and crown form of taller trees. In addition, the vitality of each tree can be estimated using aerial infrared images (Johansson 2012). Nye data laser og foto Material and methods My experience stems from scientific work in Europe, Canada, USA, Indonesia, Malaysia, and British Guyana since Due to strong interest in tree-harvesting techniques, I spent multiple years studying Reduced Impact Logging in the Borneo Rain Forest by comparing tractor and helicopter approaches for harvesting and transporting timber. In Canada and the USA I studied balloon logging and was able to pilot a huge air-ship (Hagner 2002, Hagner 2013). Figure 9. Experimental set-up. Contr = Clear cutting which is compared to three levels of selective cutting. In 1990, I was fortunate enough to arrange 12 large-scale scientific tests in Sweden to compare the clear-cutting system with the Liberich system. Recently, we have been able to register what happened during the first decades (Hagner 2009b, 2015a). On my private forest estate in northern Sweden, I have studied the practical difficulties and economic effects of the Liberich system (Hagner 2007). In late 1990s I also wrote two computer programs, TREE and GROUP, which are now used by tree markers to carry out their work while in the forest.

7 7 Figure 10. TREE handles a single tree and illustrates the diameter at which the tree is economically mature. This program makes it easy to show forest owners that there is no economic basis for harvesting a half-grown tree by clear-cutting and then paying for the following artificial regeneration (Hagner 1999). GROUP recommends adjustment of the structure inside a group of 1-10 trees, with the objective to maximize the present net value of the group (Hagner 2000). Results Forest productivity 7 Annual Volume Increment, m3 / ha Well Wrong Figure 11. Statistician Sören Holm and I performed multidimensional analyses using data from 11 scientific test plots treated with selective cutting over many decades (Hagner and Holm 2003). We found that productivity declined over time when thinning decreased the variation in tree size Years from start of treatment Figure 12. When we investigated the correlation between standing volume and productivity in the long-term selective cutting tests, we found that productivity was at its highest when standing volume was at a minimum. Annual Volume Increment, m3 / ha Standing volume, m3 / ha Figure 13. An interview with two professors, Linder and Hällgren, revealed the physiological explanation for the negative correlation in figure 12, see attachment 1 (Hagner et al 2016). The stem-wood of the forest on the left-hand side has the highest productivity. The reason is that it has the same leaf-area as the forest on the right-hand side, but a smaller amount of stem-wood that uses up photosynthetic products. Education of tree markers After my retirement in 1990, I have, via the internet, educated people who want to help forest owners use the Liberich system. Students who pass one theoretical and one practical exam are given a Certificate in Liberich. The certified students organize their consultation work through the Naturkulturförmedlingen webpage. In this educational program I refer, among other literature, to my textbook Naturkultur pp 218 and to the TREE and GROUP computer models, all of which are available for free on the Föreningen Naturkultur website (Hagner 1999, 2000, 2016a).

8 8 Productivity of coniferous stem wood. Figure 14. Studies of Swedish Pine and Spruce have revealed that the long-term productivity of the stem-wood in area A is equal to that in area B (Elfving 1990, 2009, Elfving and Jakobsson 2006, Jakobsson and Elfving 2004). This means that a continuous silvicultural system, which results in a natural mixture of tree sizes, will be characterized by the same long-term productivity as a system in which the generations are separated. Long-term forest profits The published results of my practical and scientific tests have shown that the Liberich system can double the profits of the clear-cutting system (Hagner et al 2001, Hagner 2007). In Germany, Hanewinkel (2001) compared the registered net profits from forest estates using either the Liberich or clear-cutting system over 14 years. The estates using the Liberich system reported incomes per hectare that were 3.6 times higher than those of estates that employed clear-cutting. In northern Germany, Janssen (2000) reported that net profits had doubled ten years after Angela Merkel, the Minister of environment, had forced the governmental foresters to abandon clearcutting. Figure 15. In Sweden, Jakobsson and Nilsson (2005) studied dynamics in the zone of competition along the border of clear cuts. They found that the wood volume that was lacking in the zone was found as extra wood in the trees competing with the new regeneration. They drew the following economic conclusion: On the plus account the increased production in the old stand will be ready for cut a half rotation earlier than the lost volume in the new stand would have been. In other words, competition between trees of different sizes improves the present net value. I have also witnessed the immediate economic gains that arise from using the Liberich system instead of clear-cutting on my private forest estate in northern Sweden (Hagner 2007). Impacts on biodiversity A silviculture that is characterized by continuously covered forest ground and a forest structure that closely resembles a natural forest will have a higher degree of biodiversity than a plantation forest. Additional benefits. The Liberich system includes an interview with the forest owner at the start of the tree marking process, covering subjects such as hunting, picking of mushrooms and viewpoints. Therefore, the tree marker will understand the preferences of the forest owners and treat their forest accordingly. Climate, CO 2 emissions and optimized use of wood

9 9 Figure 16. This robot has been placed in the top of a tree by an auto-piloted SkyLifter air-ship. The robot first cuts the branch 50 cm from the stem to avoid splitting the stem and squeezing the saw, and then makes a second cut closer to the stem. Figure 17. The modern lighter-thanair (LTA) techniques, which have evolved from old air-ships, will make it possible to grab standing tree-stems from the top, cut them at stump height, lift them straight up, and finally, accumulate these stems under the air-ship (Hagner 2015b). Figure 18. The stems could be transported directly to the saw-mill by a SkyLifter air-ship. The harvest and transportation system described has been estimated to reduce CO 2 emissions by 99% (Hagner and Ancker 2012). Furthermore, a shift from transportation by tractors to LTA-based approaches will save the top soil from compaction and erosion. Figure 19. At the saw-mill, stems undergo x-ray analysis before cross-cutting. This approach enables the cuts to be determined in a way that optimizes the current customer demands (Hagner 2016a).

10 10 Figure 20. LTA-techniques could also open the door for the harvesting of medical plants, fruits and flowers from above. A net, rolled out on top of the trees, would support walking and light buildings. Openings in the net could be used for harvesting and scientific studies of the upper part of the ecosystem (Hagner 2015b). Figure 21. Trees can withstand a high degree of natural browsing, which would enable the LTA-technique to be used for harvesting small twigs and leaves. These products could be used as forage (Hagner 2016a). Figure 22. The low costs and reduced CO 2 -emissions associated with LTA-techniques will enable us to spread charcoal over forests and farmland. Pyrolysis, instead of burning, provides 80 % of the initial energy but results in charcoal rather than ash. Charcoal cannot rot away, enabling a long-term deposit of carbon. In addition, it restores the fertility that was lost by artificial manure, clear-cutting and mechanical soil treatment. Discussion The negative correlation between standing volume and productivity contradicts the conclusions drawn by Lundqvist (1989) and many other scientists. Unfortunately, the Swedish computer model Heureka, which is used for forest prognoses, includes a deleterious growth function that demonstrates a positive correlation with standing volume. The Swedish Forest Act forces forest owners to preserve dense forests, a tenet that effectively reduces productivity and the interest rate on working capital.

11 11 Conclusions We are fortunate enough to have an efficient solution for the increasing levels of CO 2 in the atmosphere. A shift away from using clear-cutting in silviculture could increase the ability of forest soils to trap CO 2 (Lindroth 2007). Furthermore, the application of modern technology, combined with the Liberich system, can increase the production of all types of forest products, and simultaneously restore both biodiversity and spring-water quality. The replacement of burning with pyrolysis can restore the productivity of the soils used for farming and silviculture, as well as help to trap carbon. Scientific results show that the most favorable silviculture, with respect to the environment, forest owner and forest industry, is a combination of liberation thinning and enrichment planting. Currently, hiring someone certified in the Liberich system to perform a manual marking of the trees that should be removed is one way to optimize silviculture. However, in the future this optimization could be achieved by using vertical and horizontal laser scanning to describe each tree in a forest (Wulder et al. 2012). When a computer can use these data to determine the position, size and potential quality of each tree > 8 cm diameter, it will become possible to replace the manual marking with a computer model. The combination of robots and lighter-than-air techniques could have applications in the debranching of standing trees. Once the stems have been cut, they can then be lifted up to an airship, thus avoiding the destruction that can result from felling. Direct air-ship transportation of the stems to the saw-mill does not require the building of roads, and the process will not cause any soil compaction because tractors are not required. The crosscutting of stems can then be optimized through X-ray analyses of the whole stem. This new system would dramatically reduce CO2 emissions when compared to traditional timber harvesting and transportation. In addition, it would leave the humus layer untouched and water in the creeks clear. References Clemensen K.E., Bahr A., Ovaskainen O., Dahlberg A., Ekblad A., Wallander H., Stenlid J., Finlay R.D., Wardle D.A. and Lindahl B.D. (2013) Roots and Associated Fungi Drive Long-Term Carbon Sequestration in Boreal Forest. Science 339, Chrimes D., Elfving B., Lundqvist L., Mörling T. and Valinger E. (2007) Stand development after different thinnings in two uneven-aged Pices abies forests in Sweden. Forest Ecology and Management 238, Ekelund M. (1999) Wind- and snow damage in an uneven-sized conifer forest in Sweden thinned from above. Sveriges Lantbruksuniversitet, Skogsskötsel, Examensarbete 2, Elfving B. (1990) Granplantering under gles högskärm i fjällskog. Sveriges Skogsvårdsförbunds Tidskrift 5, 1-8. Elfving, B. (2005) Kalhyggesfritt skogsbruk - hur fungerar det? SLU, Institutionen skogsskötsel, Stencil 1-7. Elfving B. (2009) Influence of retained trees on growth of the new stand. PM for Heureka, Appendix 18, 1. Elfving B. and Jakobsson R. (2006) Effects of retained trees on tree growth and field vegetation in Pinus sylvestris stands in Sweden. Scandinavian Journal of Forest Research 21(7), Hagner M. (1999) TREE01. A description of a computer model for choice of tree. Sveriges Lantbruksuniversitet, Institutionen skogsskötsel, Arbetsrapport 144, 1-4. Hagner M. (2000) Group02. Present value of a group of trees. Description of a computer model. The Swedish University of Agricultural Sciences, Department of Silviculture, Working paper 155, 1-4.

12 Hagner M. (2001) Differences in dimensional structure of a virgin and a selectively logged tropical rain forest. Swedish University of Agricultural Sciences, Department of Silviculture, Working Paper 163, Hagner M. (2002) Propelled balloons for harvesting and transporting timber. Forestry 75(4), Hagner M. (2004) Naturkultur: Ekonomiskt skogsbruk kännetecknat av befriande gallring och berikande plantering ( Mats Hagners bokförlag, Umeå, ISBN Hagner M. (2007) Tillväxtreaktion och ekonomi efter gallring enligt principen Naturkultur. ISSN UBICON Rapport 8, Hagner M. (2008) Berikande plantering i försök med Naturkultur. Överlevnad och tillväxt, med och utan markberedning. ISSN UBICON Rapport 7, 1-9. Hagner M. (2009a) Lönsam naturvård Skörda mogna träd. Befria gamla småträd. Plantera i luckor, ISSN UBICON Rapport 4, Hagner M. (2009b) Naturkultur i Piellovare. Tillstånd och utveckling i försöket 15 år efter avverkningen. ISSN UBICON Rapport 5, Hagner M. (2013) LTA Timber Transport System. ISSN UBICON Rapport 8, 6. Hagner M. (2015a) Naturkultur i Piellovare. Tillstånd och utveckling i försöket 15, 19 och 22 år efter avverkningen. ISSN UBICON Rapport 3, Hagner M. (2015b) Time for giant leaps in forest management. ISSN UBICON Report 1, Hagner M. (2015c) A new index describing the degree of virginity of forest structure. ISSN UBICON Report 7, 3. Hagner M. (2016a) Naturkultur, lärobok andra upplagan. Ett skogsbruk kännetecknat av befriande gallring och berikande plantering. Tillgänglig på internet: Mats Hagners Bokförlag, Umeå. pp 214 Hagner M. (2016b) Lönsamt skogsbruk. ISSN UBICON Rapport 9, 1-6. Hagner M. (2016c) Träkol via pyrolys till skogsmarken räddar klimatet. ISSN UBICON Rapport 1, 1-5. Hagner M. (2016d) Skogsstyrelsens form av hyggesfritt är detsamma som att skumma grädden och lämna blåmjölken till skogsägaren. ISSN UBICON Rapport 10, 1-8. Hagner M. and Jonsson C. (1995) Survival after planting without soil preparation for pine and spruce seedlings protected from Hylobius abietis L. by physical and chemical shelters. Scandinavian Journal of Forest Research 10, Hagner M. and Nyqvist H. (1998) A coefficient for describing size variation among neighbouring trees. JABES (Journ Agric Biol Environm Statistics) 3(1), Hagner M., Lohmander P. and Lundgren M. (2001) Computer-aided choice of trees for felling. Forest Ecology and Management 151, Hagner M. and Ancker K. (2012) Utsläpp av CO 2 vid transport av virke minskar kraftigt om heliumfyllda ballonger används. ISSN UBICON Rapport 3, Hagner M. and Holm S. (2003) Effects of standing volume, harvest intensity, and stand structure on volume increment in plots managed with single tree selection. In Proceedings with Abstracts: Uneven-aged Forest Management. IUFRO Conference in Helsinki Finland June 8-17 Finnish Forest Research Institute METLA 260. Hagner M., Hällgren J.-E. and Linder S. (2016) Virkesproduktionen minskar med ökande kubikmassa, ISSN UBICON Rapport 6, 1-5. Hanewinkel M. (2001) Financial results of selection forest enterprises with high proportions of valuable timber. Results of an empirical study and their application. Schweizische Zeitung fur Forstwesen 8, Jakobsson R. and Elfving B. (2004) Development of an 80-year-old mixed stand with retained Pinus sylvestris in Northern Sweden. Forest Ecology and Management 194,

13 Jakobsson R. and Nilsson M. (2005) Effect of border zones on volume production in Scots pine stands. Paper IV in Ph D thesis: Growth of Retained Scots Pines and Their influence on the New Stand. Acta Universitatis Agriculturae Sueciae 34, Janssen G. (2000) From forest devastation to close-to-nature managed forest, a precept of rational and economically sound forestry. In: Sustainability in Time and Space. Congress Report, Pro Silva Europe, Fallingbostel, Germany Johansson K. (2012) Utfordringer och muligheter med nye data. Oslo kommune Power point Lindroth A. (2007) Låt skogen göra jobbet. Sydsvenskan se 26-Nov Thörnqvist T. (1993) Properties of timber from Southern Sweden. Södra Paper 1, Wulder M.A., White J.C., Nelson R.F., Naesset E., Orka H.O., Coops N.C., Hilker T., Bater C.W. and Gobakken T. (2012) Lidar sampling for large-area forest characterization: A review. Remote Sensing of Environment 121,

14 14 Attachment 1 Virkesproduktionen minskar med ökande kubikmassa Intervju med Jan-Erik Hällgren som är professor i skogsträden fysiologi och Sune Linder som är professor i skogsekologi. Båda vid SLU. Mats Hagner Figur 4. Finns det tätt med plantor och träd skapar de tillsammans en maximal bladyta, oavsett trädens storlek. I en naturligt skiktad skog, med maximal bladyta, är överskottet av socker, som kan användas för tillväxt, störst när träden är små. Produktionen av virke minskar i så fall när mängden virke ökar. UBICON Rapport 6, 2016 ISSN UBICON, Blåbärsvägen 19, Umeå, Sweden. Tel mats.hagner@allt2.se Org. Nr

15 15 Sammanfattning Mats: Med ökande storlek hos de största träden i en fullskiktad skog upprätthålls maximal bladyta av allt färre träd. Mängden socker förblir konstant medan mängden stamved som förbrukar socker ökar. Detta får mig att tro, att överskottet för tillväxt minskar. Håller Du med mig? Sune: Ja Mats: Skogsvårdslagens virkesförrådsdiagram tvingar en skogsägare att upprätthålla en viss mängd stamved per hektar. Anser Du att diagrammet i stället borde föreskriva en viss minimal bladyta, eftersom det är tätheten av bladyta som avgör tillväxten och inte mängden stamved. Sune: Ja cccccccccccccccccccccccc ---Original Message--- From: Jan-Erik Hällgren To: Mats Hagner Sent: Monday, March 28, :07 PM Subject: SV: Skogsvårdslagen Bäste Mats Hagner, Jag har granskat och studerat intervjun med professor Sune Linder och instämmer helt i hans bedömningar. Jag anser att svaren är, med dagens vetenskapliga kunskaper, den bästa bedömning man kan göra om trädens tillväxt och vad som är de viktigaste faktorerna. Hoppas det kan vara till hjälp att reda ut problem med bedömningar av hur man bör sköta skogen. Hälsningar Jan-Erik Hällgren Prof. i skogsträdens fysiologi, SLU Cccccccccccccccccccccccccccc Intervju med Sune Linder, prof. em. i skogsekologi. Mats Hagner Mats: Finns det ett samband mellan mängden stamved och produktion av virke? Sune: Sambandet är indirekt. Produktionen är direkt kopplad till bladytan hos beståndet, vilken i sin tur beror av bördigheten (temperatur och tillgång på näring och vatten). När träden är små och står glest har de tillsammans en låg produktion beroende på att bladytan som fångar solljuset inte hunnit bli maximal. När träden blivit stora nog för att ha uppnått den maximala bladytan är produktionen av socker i beståndet maximal. Vid fortsatt tillväxt lyfts den redan maximala bladytan allt högre upp. På magra marker uppnås inte full kronslutenhet, dvs. allt solljus kan inte tas upp i trädkronorna. Figur 1. Energi från solen omvandlas i trädens blad till socker. Detta använder träden till att hålla liv i alla levande celler i krona, stam och rötter. Det kallas underhållsandning. Om det finns överskott på socker kan trädet använda det för tillverkning av ny ved, dvs. för tillväxt. Bladytan, som fångar solens strålar, är minst i

16 16 ungskogen till vänster. Bladytan hos beståndet i mitten är tillräcklig för att fånga allt ljus. Bladytan kan inte bli större. När träden blir högre lyfts bladytan allt högre upp utan att produktionen av socker ökar. När stammarna blivit så långa, som i beståndet till höger, förbrukas en allt högre andel av sockret till andning i stammar, grenar och rötter. Då avtar tillväxten över tiden. Stammar, grenar och rötter är en tärande del av skogsekosystemet. Den mängd socker som bildas i bladen ökar fram till den tidpunkt då maximal bladyta uppnås. Den del av sockret som finns tillgänglig för tillväxt minskar sedan när träden blir större och en allt större andel av sockret förbrukas av underhållsandningen. I en skog minskar därför tillväxten av virke efter att maximal bladyta har uppnåtts. Ju tätare ungskogen är, desto tidigare uppnås maximal bladyta. Gallring reducerar bladytan och minskar mängden producerat socker. Om gallringsstyrkan är låg och beståndet har maximal bladyta före gallringen, blir återhämtningen snabb, eftersom det fortfarande finns många blad som fångar upp det ljus som skulle ha fallit på det bortgallrade trädets blad. Figur 2. I en naturligt skiktad skog kan ett stort träd skördas utan att någon stor tillväxtnedsättning uppstår. Det beror på att bladen hos de mindre träden fångar det ljus som tidigare föll på det stora trädets blad. Mats: Har Du studerat skillnaden i dessa avseenden mellan en enskiktad skog och en fullskiktad skog? Sune: Nej Mats: Har Du anledning att tro att en fullskiktad skog skulle ha lägre maximal bladyta och lägre bruttoproduktion än en enskiktad skog? Sune: Nej Mats: Vi tänker oss två mycket olika bestånd men båda har maximal bladyta. Det första är ett fullskiktat bestånd med några få stora träd och många mindre träd av alla storlekar. Det andra är ett enskiktat bestånd med fullstora träd. Har det fullskiktade beståndet en lägre mängd stamved i kubikmeter per hektar, än det enskiktade? Sune: Ja Figur 3. Ett fullskiktat bestånd, med samma bladyta som ett enskiktat bestånd, har troligen en mindre mängd stamved. Mats: Vi har två bestånd med maximal bladyta, men med väldigt olika skiktning. Är det möjligt att det fullskiktade beståndet, som dras med mindre underhåll av levande stamved, har större överskott av socker tillgängligt för produktion av virke. Sune: Ja

17 17 Mats: Den ovan beskrivna fullskiktade skogen har inte har några fullstora träd men trots detta maximal bladyta. Är det troligt att denna fullskiktade skog, trots sin lägre mängd stamved per hektar, har full produktion av socker? Sune: Ja Figur 4. Finns det tätt med plantor och träd skapar de tillsammans en maximal bladyta, oavsett trädens storlek. I en naturligt skiktad skog, med maximal bladyta, är överskottet av socker, som kan användas för tillväxt, troligen störst när träden är små. Produktionen av virke minskar i så fall när mängden virke ökar. Mats: Med ökande storlek hos de största träden i en fullskiktad skog upprätthålls maximal bladyta av allt färre träd. Mängden socker förblir konstant medan mängden stamved som förbrukar socker ökar. Detta får mig att tro, att överskottet för tillväxt minskar. Håller Du med mig? Sune: Ja Mats: Skogsvårdslagens virkesförrådsdiagram tvingar en skogsägare att upprätthålla en viss mängd stamved per hektar. Anser Du att diagrammet i stället borde föreskriva en viss minimal bladyta, eftersom det är tätheten av bladyta som avgör tillväxten och inte mängden stamved.