Forests and Their ManagementDr. Ankur AwadhiyaDepartment of BiotechnologyIndian Institute of Technology, KanpurModule – 08 Silvicultural Management – IILecture - 23 Shelter wood System – II[FL] In today’s lecture, we will carry forward our discussion on the Shelter wood Systemthat we left in the last lecture. So, this lecture is shelter wood system part II. (Refer Slide Time: 00:28)Now, as you remember in the last lecture, we discussed that how do we define a shelterwood system. A shelter wood system is a system in which we leave the mother trees toshelter for the young ones. This is especially used in the case of those species that areshade loving. And, in this case, if we go through this flow chart, the concentration of felling anddegeneration operations is there on part of the forest area; it is not there in the completeforest area. So, you do it in certain locations. And, you clear the old crop in successiveregeneration fellings. So, you do not clear the old crop in one area completely; you do itin part by part. So, that you are able to open up the canopy in a phased manner, so thatyour our seedlings are able to get light. And, at the same time, they are also protectedagainst very harsh light or against wind damaged or damaged by frost.So, this is how we define a shelter wood system. And, a shelter wood system is furtherlassified into three different categories. We have the uniform shelter wood, in which youdo the on the canopy clearing evenly over the over the compartment. Or, you can do it inscattered groups, in the case of a group shelter wood system. Or, you can do it in anirregular and gradual manner, in the case of an irregular shelter wood system. (Refer Slide Time: 02:07).Now, shelter wood system these variations can be shown in this slide. So, in the case of auniform shelter wood system, the shelter trees are more or less uniformly distributedthroughout the block.(Refer Slide Time: 02:29)So, what we are saying here is that, you have this block, and you are leaving the shelterwood trees more or less uniformly in the whole of the area. Or, you could have avariation that is the group shelter wood, in which case, the regeneration felling’s aredone in groups either due to the presence of advance growth, or to reduce or to induceregeneration with area enlarged centrifugal around the gaps.So, what we are saying here is that, in pace of doing it regularly, what you do is that inyour forest block. Suppose, you have an advance growth here; you have an advancegrowth here; and you do have an advance growth here. So, in this case, you begin byfelling these areas, in the first instance. So, in the first instance, you will fell it here. Inthe second instance, you will fell it in a centrifugal manner around these areas. In thethird instance, you will further increase your centrifugal areas and so on.So, this sort of a system will be known as a group shelter wood because you are doingthe felling in certain groups. Or, you can have a strip shelter wood system, in which youare doing harvesting in uniformly spaced linear strips with new strips addedprogressively into the wind.(Refer Slide Time: 03:57)So, what we are saying here is that, in the case of a strip shelter wood system, you do afelling here in the first instance. Then, you do a felling here in the second instance. Then,you do a felling here and so on. So, in this case, the trees that are remaining on the leftside they are providing the shelter. And, you are doing the felling progressively into thewind direction. Or, you can have an irregular shelter wood system, in which there is a very long periodbetween the initial cut and the final cut, which leads to an uneven aged crops, and this issomething that we will look into greater detail in the next lecture. Or, you can have anatural shelter wood system that is also known as an overstory removal or releasecutting. In which case, the overstorey is removed to create open growing conditions. So, what do we mean by that?(Refer Slide Time: 04:59) As we saw earlier, in the case of a forest, you have a canopy. But then, there are sometrees that go over the canopy and form an overstory. There are also some trees that formthe understory, and then you have the forest floor. Now, a natural shelter wood systemwould preferentially remove the over canopy. So, this over canopy will be removed.So, in this case, you are removing the over canopy or the overstory to create an opengrowing condition, but you are leaving the canopy trees to provide the shelter, so that is a natural shelter wood. Then, you can also have different combinations. You can use agroup system together with a strip system; you can use a uniform system together with agroup system, and so on. Now, when we are doing a felling operation, in the case of ashelter wood system, we define a periodic block.(Refer Slide Time: 06:09)Now, what is a periodic block? So, periodic block is defined as, “the part of the forestthat is set aside to be regenerated or otherwise treated during a specific period.” So, aswe saw in the earlier lecture, we have preparatory felling; we have the seeding felling;we have the secondary felling; and, we have the final felling. Now, those parts of theforest where we are doing one operation, say seeding felling. So, all those portions wherewe are doing the seeding felling, at one particular point of time, will be called as aperiodic block. So, the part of the forest that is set aside to be regenerated or otherwisetreated. So, you are doing any sort of a treatment, but you are doing it in the whole of theperiodic block during a specified period.Now, this period is not necessarily a single year, but it can be, say 10 years 20 years 30years or so on. But then, during one period, you will be doing the treatment in oneperiodic block. So, example the part undergoing preparatory felling is a periodic block. The part that is a undergoing seeding felling is another periodic block and so on.Now, we also have the regeneration period which is the period that is required toregenerate the whole of the periodic block.(Refer Slide Time: 07:39)So, for instance, what we are saying here is that, suppose this is your forest, and you aredividing it into four periodic blocks. So, this is your periodic block 1, periodic block 2,periodic block 3 and periodic block 4. Now, regeneration period - ask the question howmuch time do I need to regenerate the whole of a periodic block.Now, regeneration would mean that you that, you have the seeds that are coming intothat area that, those seeds are getting germinated, then they are establishing themselves.So, how much time does it take for this for all these operations to happen in one periodicblock, that is the regeneration period? How much time do you require to regenerate thearea in one periodic block, that is the regeneration period? Then, we had also defined the rotation period. So, rotation period is the time that atwhich your tree is considered mature for felling. Now, the number of periodic blocks isgiven by the rotation period divided by the regeneration period. So, suppose the rotationperiod is 120 years. So, what we are saying by a rotation period is 120 years is that, atthe age of 120 years, your tree is now fully developed. It has the maximum volume thatyou want to extract from this tree; it is completely mature. And so, now, it is economical to cut this tree and extract the timber out of it. So, we aresaying that the rotation period is 120 years. So, it takes 120 years for this for thisparticular species, at this particular site to attain. And, suppose the regeneration period is30 years; so, in this case, what we are saying is that, if you work on an area for 30 years,then it will, if you have worked on this area, then it will take 30 more years to regeneratethis area.So, you had cut your trees in one periodic block. And now, to regenerate this area foryour seeds to become germinated and then establish the seedlings, it is now taking 30years. So, in this example, your the rotation period is 120 years and the regenerationperiod is 30 years. So, the number of periodic blocks is given by the rotation perioddivided by the regeneration period, which is 120 by 30 is equal to 4.So, in this case, you will be having 4 periodic blocks, because every periodic block willtake 30 years to regenerate. And, by the time you are done regenerating the 4th periodicblock, the first one is now ready to be harvested and regenerated. So, this is what theperiodic block concept means. (Refer Slide Time: 10:46)So, in the case of a shelter wood system, we define the periodic blocks as these. So, inthis case for the maturity age is 120 years. So, the periodic block one 1 is that periodicblock on which you are doing the regeneration operation. So, in this regenerationoperation what you are doing is that you are extracting the timber. And, you are ensuringthat your next generation is established in this area. So, if you have to do extraction, thenyour timber has to be fully mature. And, you do not want to leave any timber after 120years.So, the PB 1, in this case of 30 years regeneration period, will have those trees that are ofthe age of 90 to 120 years. So, in the next 30 years; so, in this year, suppose you removeall your trees that are 120 years of age. Next year you will remove those trees that havebecome 120 years of age in the next year; then in the. So, those trees are now currently inand of the age of 119 year, the year after that you will be removing some trees. So, thosetrees will be probably be having the year the age of 118 years now. So, all those treesthat are in this age group of 90 to 120 years will form the periodic block 1. Now, in here,it is important to emphasize that it is nearly impractical to find out the exact age of everytree. And, to cut only those trees that are 120 years of age, because in practicality whatyou will find is that there might be a tree that is 120 years of age which is smaller. (Refer Slide Time: 12:38). Then another tree which is say 100 years of age and that is the larger tree. So, in the fieldsituations, you might remove those trees that are of a lesser age, and leave out those treesthat are of a greater age. But then, overall what we are saying is that we will be workingon those trees that are currently in the majority age; so, that is between 90 and 120 years. Now, periodic block 2 will comprise of those trees that have the age of 60 to 90 years. Periodic block 3 will have 30 to 60 years, and periodic block 4 will have 0 to 30 years.(Refer Slide Time: 13:25)Now, how does how do these periodic blocks cycle? So, currently we have this this areathe red area is PB 1, which has the crop age of 90 to 120 years. Now, once you haveharvested and you have regenerated this red area, what will it become? (Refer Slide Time: 13:44)So, it will become the periodic block 4, because in the case of periodic block 1, you werehaving 90 to 120 years. Now, if you have cut these trees, you have regenerated this area.So, now in this area, the trees of the next generation after 30 years will be having an ageof 0 to 30 years and so, this area will become PB 4; it will not become PB 2, it willbecome PB 4. And in the previous cycle where we had the trees of 60 to 90 years, whichwere PB 2 in that time, that will now become PB 1. So, if you look at any particular areathis is how it will move; so, from PB so, from PB 1, it will become PB 4. (Refer Slide Time: 14:32) And then, it will become PB 3, then it will become PB 2. (Refer Slide Time: 14:34) And then, again it will become PB 1 in the next cycle.So, essentially what we are seeing here is that, the these periodic blocks move as PB 1becoming PB 4 becoming PB 3 becoming PB 2 and finally, becoming PB 1. So, this ishow we are going to process these different periodic blocks. Now, we defined thenumber of periodic blocks as the rotation age divided by the regeneration age. Now,rotation age is easy to fix that is the age of maturity. So, when you have trees that are oflarge enough size, they have large enough volume of timber inside it that is the rotationperiod. How do we get to the regeneration period? (Refer Slide Time: 15:24)So, the regeneration period depends on a number of factors. It depends on the frequencyof seed years. Now, seed years as we had seen earlier if you have a species.(Refer Slide Time: 15:38) And, you are plotting the number of seeds with time in years, then probably you have aspecies that is giving out large number of seeds in this year, and then for next 2 years, itis not giving out large number of seeds. Then, it is giving out another large quantity ofseeds. Then, for another 2 years, it does not give larger number of seeds and so on.So, in this case, the seed year is coming as one in every 3 years, whereas you could behaving another species where you have a large number of seeds in this year, then lessnumber of seeds, then less number less number of seeds, then less number of seeds, thenless number of seeds and then a large number of seeds.So, in that case you will be having a curve like this. So, for the blue species thefrequency of the seed year is one in every 5 years, whereas, in the case of the red species,the frequency is one in every 3 years. So, if you have a species at a particular site that ishaving a more frequent seed year; so, in that case you will be having enough number ofseeds more quickly. And, in that case, the regeneration will be faster. So, theregeneration period is dependent on the frequency of the seed years. It is also dependenton the light requirement of your seeds to germinate. Now, because in the case of a shelter wood system, we have those species that are shadetolerant. So, we are providing them with a shelter, but then if this shelter reduces theamount of light and a few species probably required a bit more amount of light. So, itwill take a bit more time to germinate and to become established. So, the lightrequirement also determines what is the regeneration period of this particular species atyour particular site. It will also depend on the soil condition. So, suppose you have a soil that is very welldrained, which is able to retain adequate amount of moisture and that is full of nutrients.So, in that cas,e your seedlings will become established faster because they will be ableto have a faster rate of growth. And, they will be able to resist the adverse influences in abetter manner because they because they are getting sufficient amount of nutrition.So, in that case, your regeneration period will be lesser than in a site which has a poorsite quality and in where seeds take a bit longer to get established. It also depends on thepresence of grass, weeds and invasive species, because if you have an area that is full ofgrass or is full of invasive species or weeds; so, in that case, your seeding your seeds willtake longer to germinate. And, to get established, because they have to compete withthese other species and so, the regeneration period will be longer. It also depends on thebiotic pressure or the pressure of grazing and browsing. (Refer Slide Time: 19:16) Because you had seed; you had a seedling; and, when it reaches a height of say thismuch, then we will say that it is it has become established. But then, when it grew to thisheight, there was a cow that came and ate away the top portion.So, it will take some more time to grow up another shoot and reach the site, and then thisportion gets eaten up again. Then, it will try to grow up another shoot, and so, it will takea much longer period to become established.So, it will depend on whether it gets a chance to grow to the particular height to getestablished. So, if you have more amount of grazing and browsing, then you will takemore amount of time to regenerate your area. So, the regeneration period depends on thebiotic pressure as well. It also depends on forest fires, because in the case of some, species you have a good rootsystem that has developed. (Refer Slide Time: 20:20) And again, if your plant grows to this height, then we say that it has become established. But then when grew to this height and then there was a forest fire, and the whole of theshoot portion got destroyed. The root portion remains that gives out another shoot, andthen there is another fire, and then probably in the third year, it did not get any fire andso, it was able to reach.So, something that it could. So, the height or the establishment that it could reach in asingle year it took 3 years, because two years were fire years and so, the regenerationperiod also depends on the frequency of forest fires in that particular area. So, it dependson grazing, it depends on browsing, it depends on forest fires; it depends on invasives; itdepends on insects that might be eating up or devouring the young crop; it depends onthe site quality; it depends on the frequency of seed years; it depends on the lightrequirement. So, it depends on a number of factors.So, this is how we get to the regeneration period, and this is how we get to the number ofperiodic blocks, because we can fix up the rotation period as the maturity. Regenerationperiod will depend on all these factors and so, if you can control these factors, you caneven reduce the regeneration period. And, in that case, you will even be able to reducethe number of periodic blocks that will be required by you. (Refer Slide Time: 21:49)Now, next we look at the group system. Now, the shelter wood system has a number ofvarieties and one such variety is a group system. Now, group system was devised by KarlGayer. And, it was there to check the conversion of irregular forests into pureplantations. Now, an irregular forest is a forest in which different age classes do notoccupy roughly the same amount of areas.(Refer Slide Time: 22:25)So, what we are saying here is that, suppose you consider trees that are 0 to 30 years, 30to 60 years, 60 to 90 years and 90 to 120 years. Now, ideally in in a managerial sense, wewould want all of these four age classes to occupy 25 percent area of the land. But suppose, you have a forest in which 0 to 30 years is occupied by say 60 percent of theplant - 60 percent of the area; this is 15 percent of area; this is 10 percent of area and thisis 5 percent of area; fifteen15 percent of area.So, in this case, we will say that this forest is an irregular forest, because different ageclasses do not occupy the same amounts of area in this forest. Now, in the case of a clearfelling system, when you clear up this whole forest, you converted into a pure plantationand you have an even age class distribution. But then, suppose you have a forest that isan irregular forest and you want to maintain it as an irregular forest, because you do notwant to make a large number of changes into your system; so, that was the question thatKarl Gayer was addressing.So, he devised the group system to check the conversion of irregular forest into pureplantations. So, in this case, at the end of all the operations, you will still have theirregular forest. So, in the beginning, suppose you had that 15 percent of the area wasoccupied by 90 to 120 years of age trees. At the end of all your operations, you will stillhave 15 percent of the area that is occupied by these trees with the age of 90 to 120years.So, you maintain the forest in its natural condition, and that was the aim of the shelterwood system. So, in the group shelter wood system, regeneration fellings are done ingroups either due to the presence of advanced growth or to induce regeneration. So, whatdo you do here, you do your regeneration felling in groups and then the area is enlargedcentrifugal around the gaps. (Refer Slide Time: 25:08)So, as we saw before we begin with a preparatory felling, so that you are removing thedead, dying and diseased trees. And, you are inducing the seed production; this isfollowed by the seeding felling, in which case, you are trying to go for a good amount ofgermination followed by a secondary felling, in which you are opening up the canopy, sothat your plants are able to get a bit more amount of light, and are able to show thegrowth, followed by the final felling, in which you remove all the plants that are left ofthe previous generation.So, how does the gear system or the group system choose which areas do we begin withfor the preparatory felling?(Refer Slide Time: 25:52)So, the group system begins with the identification of advanced growth. So, suppose thisis your complete forest and there are these three locations; because this is an irregularforest. So, here you are saying that in these locations you are having trees that are of alarger age group. So, for instance, you have these 90 to 120 years age trees in these areas.(Refer Slide Time: 26:21)So, you do the preparatory felling in this area; in these areas that are showing theadvance growth. So, for the next 30 years, you are doing the preparatory felling in theseareas and the other areas you are not touching whatsoever.(Refer Slide Time: 26:37)Now, at the end of your 30 years, what you do is that you do the seeding felling in theseareas. And in the surrounding areas, you move in a centrifugal pattern and you do thepreparatory felling in the surrounding areas.So, what you have done now is that, those areas that were having the advance growth,you did the preparatory felling, and then in the next cycle, you did they see the seedingfelling. And in the surrounding areas, now, you have done the preparatory felling. Now,here again, you will choose these surrounding areas, looking at the age distribution oftrees; and so, these will not be concentric circles, but then the shape might differ. So, suppose in this area, you have more number of trees. So, it will look like this. But youare expanding it centrifugally.(Refer Slide Time: 27:31)So, once you have done the seeding and the preparatory felling, next you are doing thesecondary felling. So, the these areas that initially showed the advanced growth. Now,they have moved to secondary felling; these areas have now moved to the seeding fellingand the surrounding areas have moved into the preparatory felling phase.(Refer Slide Time: 27:49) And then, you do the final felling in these areas.So, now at the end of your cycle, what you have is that in these three areas that had anadvanced regeneration at the end of your of all your operations, you still have advancedregeneration in these areas. So, in this way, the structure or the special structure of theforest is maintained. So, in these areas, you have all the four fellings that have beendone. In these areas, you have had three fellings. In these areas, you have had twofellings, and in this pink area, you have had only one felling. And, this cycle would then continue because these areas where you had had your finalfellings. Now, with the advanced regeneration in the next cycle, you will again do thepreparatory felling in these areas. These areas where you were having the secondaryfelling, you will do the final fellings here; these areas with the seeding fellings, you willdo the secondary fellings. And, this these pink areas we had, you did the preparatoryfelling, would now move into the seeding felling.So, till perpetuity, we can maintain the spatial structure of this forest while extractingtimber out of it and by ensuring that there is adequate amount of regeneration through theshelter wood system.(Refer Slide Time: 29:18)So, the group system provides a number of advantages that you begin with the use ofadvanced growth areas. So, because you are using the advanced growth areas, theregeneration period is now less, because you did your preparatory felling in those areasthat were that were already showing some amount of advanced growth.So, there are some seedlings; there are some junk plants that have already establishedand so, the so, the total amount of time that you would we would to require to regenerateany periodic block would be considerably less than if you began with those areas that didnot have any advanced regeneration. Then, it provides adequate protection to the young crop because you are leaving trees toact as shelter trees. Then, there is also less wind damage, less wind damage because inthese cases you are moving at centrifugally. So, there will be a wind damage only in theouter areas.But then, the majordisadvantage is that this system is a lot more complicated than say a clear felling system. And so, you have to impart large amount of training skill, equipment to the people whoare working your forests. And, the second major disadvantage is that you have to work inevery area of the forest again and again.So, whenever you are doing your extract; your harvesting operations; your timberextraction, you are also damaging the young crop, because of which the system did notattain a huge amount of popularity. And then, in certain areas, if you say have anadvanced regeneration in suppose 70 percent of the area; so, when you create these hugegaps in 70 percent of your area, then there is a huge possibility that you will have winddamage in that area.So, because in this system you do not have 4 periodic blocks that occupy equal areas ofland. So, there is a big possibility that sometime in the whole of the operation, there willbe time, in which you will be clearing a large area of the forest. And, in that case, youwill be exposing your trees to wind damage, and also to desiccation, or drying up, or tofrost. So, these are the advantages and disadvantages of the group shelter wood system.So, that is all for today. Thank you for your attention [FL].