THE CALCIUM AND MAGNESIUM CONTENTS OF TREE SPECIES CROWN IN CLOSE STANDS

Transcription

1 THE CALCIUM AND MAGNESIUM CONTENTS OF TREE SPECIES CROWN IN CLOSE STANDS BY J. D. The Nature Conservancv, Grange-over-Sands, Lancashire {Received 6 March 1958) (With 4 figures in the text) SUMMARY The percentages of calcium and magnesium at different positions in the boles and canopies of a number of trees from three experimental areas are given. The total weights of calcium and magnesium in the individual trees and the crops are determined. The calcium and magnesium contents of the trees and other components of the woodland ecosystems are compared. The balanceandcirculationof calcium and magnesium within the woodland ecosystems are discussed. INTRODUCTION During the last quarter of a century, large areas of Britain have been afforested, using both indigenous and exotic tree species. The volume of timber produced by a woodland depends upon a variety of factors and silviculturalists have set up experimental forest plots in order to compare the productivities of pure stands of different tree species growing under identical conditions of soil and climate. Using results obtained from species trials and growth data from sample plots, detailed volume production tables have been prepared for the main species of trees planted in Britain, The tables show both the volume of usable wood produced at successive stages of forest development and the effects of site quality on tree growth (Hummel and Christie, 1953), In comparison, little is known of the uptake of plant nutrients by woodlands, and therefore determinations are being made of the amounts of nutrients contained in twenty-nine forest plantations at three series of experimental plots. The experimental areas are located at West Tofts, Bedgebury and Abbotswood. Data have been given in earlier papers (Ovington, i956fl, i957rt; Ovington and Madgwick, 1958) of the dry weights and the quantities of carbon, nitrogen, sodium, potassium and phosphorus in sample trees from the plots. This account is concerned with the amounts of calcium and magnesium in the individual trees, plantations and woodland ecosystems. METHODS The tree, or pair of trees, corresponding most closely in linear dimensions to the calculated average tree of each plantation, was felled and its oven-dry weight determined. Bole and crown samples of the trees were taken and ground down in a Christy and 164

2 Mineral content of tree species 165 Norris mill to pass through a sieve having holes of.4 mm diameter. Samples of the ground material were digested in a mixture of nitric, sulphuric and perchloric acids (Piper, 195). The calcium content of the acid solution was determined by the versenate technique and magnesium by the titan yellow method. By combining the analytical results with the dry-weight data, the quantities of calcium and magnesium in the individual trees and plantations have been estimated. RESULTS Chemical composition on a dry weight basis The distribution of calcium and magnesium in some of the older trees at Abbotswood are illustrated in Fig. i and the general variations shown by these Abbotswood trees are fairly typical of all the trees that were analysed. The chemical composition of both bole and canopy samples varies considerably depending upon the position of the sample in the tree, the location of the plots and the species of tree sampled. In general, the proportions of both elements in the stem samples increase with greater height up the trees, the increase, particularly for magnesium, often being most marked over the top 4 m of bole. The maximum amounts of calcium and magnesium recorded for individual bole samples are 655 and 79 mg per 1 g of oven-dry matter respectively. Due to the uneven distribution of weight along the tree stems, the larger percentages of the two elements in the apical lengths of bole are representative of only a small proportion of the total trunk weight. Consequently, average values calculated for whole boles are relatively small, when compared with the results for the apical lengths, and vary between 45 to 355 mg of calcium and 1 to 39 mg of magnesium per 1 g of ovendry material (Table i). Table i. The range of calcium and magnesium for the complete canopies and boles of sample trees as mgj 1 g of the O.D. weight WEST TOFTS (Trees 22 yrs old) Canopv Bole BEDGEBURY (Trees ys old) Canopv Bole ABBOTSWOOD (Trees yrs old) Canopv Bole Min Calcium Max Av Min Magnesium Max. The chemical composition of the tree boles seems to be related to the nutrient status of the soil. For example, the mineral soils at West Tofts contain between ten to thirty times the amount of exchangeable calcium present in the soils of the other two areas and the tree boles from West Tofts have the greatest percentages of calcium. Similarly, the Bedgebury soils, compared with those of West Tofts and Abbotswood, are relatively rich in magnesium and the tree stems from Bedgebury give the highest percentage values of magnesium. At each locality, the boles of the various tree species differ greatly in average percentages of calcium and magnesium. When the trees are arranged in order according to the magnitude of percentage calcium, the species sequence is fairly consistent for all three \v

3 i66 J. D. OVINGTON experimental areas. Thus, hardwood stems, particularly oak, tend to be richer in calcium than conifer stems for the same plot series, although some exceptions to this general rule occur. For instance, the bole of Thuja plicata gives a greater average percentage calcium value than any other sample stem at Bedgebury and it is interesting that the leaves and litter of Thuja are also relatively rich in calcium (Ovington, 1956*). The percentages of calcium in the conifer boles decrease in the following order: spruce, pine, larch and Douglas flr. The tree boles have a wide range of average magnesium values but the differences between species are not very consistent when all three localities are considered together, although spruce and pine boles have greater magnesium averages than those of larch and Douglas flr. The canopy samples are composed mainly of leaf and branch material and the proportions of the two vary throughout the tree crowns. Because of the complex structure of the canopies it is difficult to ensure that the samples are truly representative of the tree crowns but the results for each pair of trees at West Tofts are sufficiently close to suggest that the sampling technique is reasonably satisfactory. The percentages of calcium and magnesium in the canopy tend to increase towards the top of the tree (Fig. i), the maximum values of calcium and magnesium recorded for canopy samples being 1393 and 186 mg per 1 g of oven-dry matter respectively. The average percentage values of calcium and magnesium have been determined for each canopy and the canopy average is always greater than the corresponding bole average of the same tree. No consistent order of tree species based on the calcium or magnesium contents of the tree crowns can be recognized for all three series of plots. The composition of the tree crowns, like that of the boles, seems to be influenced by soil properties, since the relatively large values of calcium in the canopies of some trees at West Tofts and of magnesium at Bedgebury apparently reflect the large amount of exchangeable calcium in the West Tofts soils and of exchangeable magnesium in the Bedgebury soils. Increasing percentages of calcium and magnesium are associated with a greater ash content, so that, when calcium or magnesium is expressed as percentages of the ash, the values for each element in the stem or canopy become more constant (Fig. i). Calcium accounts for about a quarter of the tree ash whilst magnesium represents approximately 4% ofthe ash weight. The nutrient content of sample trees Even though the sample trees may be of identical age and soil conditions are fairly uniform over each experimental area, the trees differ enormously in the amounts of calcium and magnesium which they contain (Fig. 2). The maximum quantities of the two elements recorded per tree are 1139 g of calcium and 29 g of magnesium (Table 2). At West Tofts and Bedgebury, where the trees are relatively immature, the tree crowns usually contain more calcium and magnesium than the boles; at Abbotswood where the trees are older, the boles tend to contain larger amounts than the canopies. Occasionally, hardwoods contain greater quantities of nutrients than conifers but on the whole the reverse is more common. Differences in the calcium and magnesium contents of the individual trees primarly reflect variations in the growth of the different tree species, rather than differences in percentage composition. There is a linear relationship, between the logarithms of the dry weights and the logarithms of the weights of calcium or magnesium in the trees (Fig. 3), which can be expressed for the trees from West Tofts, Bedgebury and Abbotswood by the formulae:

4 Mineral content of tree species 167 =,74 logiow^+.82 (S.E. of estimate =.17) =.82 logio 1^.15 (S.E. of estimate = o.io) where Ca and Mg are the weights of calcium and magnesium respectively per tree in grams and W is the oven dry weight of the tree in kilograms. 3- Picea abies (8) Pinus sylvestris Pseudotsuga taxifolia Quercus robur Castanea sativa Height of sample up tree Calcium mg./ioog. O.D. 3-1 Height of sample up tree o Magnesium mg./ioog. O.D n Height of sample up tree o o* Calcium as percentage of the ash 3-1 Q, Height of sample up tree o o o Magnesium as percentage of [he ash 14 Fig, I, The calcium and magnesium contents of some of the sample trees at Abbotswood as mg/ioo g O.D, matter and as percentages of the ash, # = bole; O = canopy.

5 l68 J. D. OVINGTON When the equation for calcium is recalculated to include additional data published by numerous other workers it is modified slightly and becomes logiocrt =.66 log,jt+i.o8 (S.E. of estimate =.27). WEST TOFTS (Trees 22 yrs old) Canopv Bole Total BEDGEBURY (Trees yrs old) Canopv Bole Total.ABBOTSWOOD (Trees yrs old) Canopy Bole Total Table 2. The range of calcium and magnesium in sample trees (g) Min Calcium Max "39 AY '23 2=; Min Magnesium Max SS Av. II The nutrient content of all the trees in the plantations The total amounts of calcium or magnesium contained in the trees growing in the various plots all occur within the following limits: calcium 119 to 674 kg per ha and magnesium 19 to 113 kg per ha. The nutrient content of the tree stock does not give a complete picture of the total uptake of nutrients from the soil by the forest crops, since plant nutrients are returned annually to the soil in the litter fall and tree stems have been removed from some plots. For instance, the plantations of Tsiiga heterophylla, Larix eurolepis and Nothofagtis obliqua at Bedgebury have had different intensities of thinning whilst other tree stands such as those of Quercus petraea or Thuja plicata have not been thinned at all. Although the trees at Abbotswood are individually larger than those at Bedgebury or West Tofts thinning has been so heavy at Abbotswood that the growing trees there contain smaller amounts of calcium and magnesium per hectare than the trees of some younger plantations at the other two areas. Occasionally, thinning has been so heavy that the quantities of calcium and magnesium in the extracted boles are almost as great as, or are greater than, the amounts in the remaining trees. The nutrient content of the ecosystem The nutrient capital of woodland ecosystems is distributed in the trees, ground flora, surface litter, fauna and mineral soil, the actual distribution depending on the type of woodland and the location of the plots (Table 3). The standing trees contain a relatively large proportion of the calcium and magnesium in the woodland ecosystems, even though large amounts of the two elements may have been removed in tree stems already harvested. The rate at which nutrients are permanently accumulated by the growing stock must depend largely upon the current increment and consequently will vary as the plantations mature, but the overall effect of this accumulation is that large amounts of calcium and magnesium become immobilized in the trees and diverted from the active nutrient cycle. Normally, only the tree boles are harvested and the crowns are left to decompose on the forest fioor so that the nutrients present in the tree canopies are not taken from the

6 Mineral content of tree species 169 woodland area. If all the trees were clear felled now and the boles extracted, up to 392 kg of calcium and 73 kg of magnesium per ha would have been removed as bole material. The average annual removal of calcium in the boles as kg per ha from the time of planting to clear felling would range from 4.2 to 14.9 at West Tofts, 2.8 to 12. at Bedgebury and 2.5 to 8.3 at Abbotswood. The corresponding removal of magnesium in the tree boles, as kg per ha per annum, would be.32 to.91 at West Tofts,.33 to 2.19 at Bedgebury and.61 to 1.59 at Abbotswood. In practice, the plantations are not usually thinned until about 2 years after planting and thinning may be on a 3 to 5 year cycle so that the removal of nutrients at any one thinning is usually much greater than the calculated average annual removal of calcium and magnesium in the boles. Pseudotsuga caxifolia A O. Mg Pseudotsuga taxjfolia B 4-r 8 Pinus nigra A WEST TOFTS Pinus Larix AInus nigra B leptolepis B incana B Larix Ainus leptolepis A incana A BeCula alba A Betula alba B Picea omorika Pinus nigra Picea abies Chamaecyparis lawsoniana Tsuga Thuja Pseudotsuga Larix Abies heterophyila plicata taxifolia eurolepis grandis Qtjercus Quercus Castanca Mothofagus petraea rubra sativa obliqui ABBOTSWOOD Larix Picea Picea Pinus Pinus Abies Pseudotsuga Fagus Quercus Quercus Castanea decidua abies (8) abies (5) nigra sylvestris grandis taxifolia sylvatica sp. robur sativa Ca. Mg. 8 e- 8- -i6 Fig 2 The calcium (shaded) and magnesium (unshaded) contents of sample trees. The area above the zero line represents the amounts present in the canopy and that below, the amounts present m the boles. On the whole, smaller amounts of calcium and magnesium are contained m the ground flora than in the trees but, where the plantations are fairly mature and the canopy is very open, the herb layer may contain significant weights of the two elements. For example, the dense growth of bracken in the two pine stands at Abbotswood, the bramble-bracken association under larch at Bedgebury and the herbaceous vegetation of the alder plot at West Tofts all contain large amounts of calcium and magnesium. The nutrient content of the ground vegetation of some Abbotswood plots almost equals or exceeds that of the tree canopies. The luxuriance of the ground flora in woodlands is closely related to the

7 17 J. D. OVINGTON Weight of calciuiti in individual i oqo trees g. lit. V TT^ I ' I ' '"I , Oven dry weight of individual trees Kg. Weight of QO magnesiuin in individual frees g ' ' ' I " " I -I I r "TTTT , Oven di-y weight of individual trees Kg. Fig. 3. The calcium, magnesium and dry weights of individual trees. Data obtamed at West Tofts = >, at Bedgebury = O, at Abbotswood = % and by other workers = Zi.

8 Mineral content of tree species 171 intensity of the tree cover and will change as a plantation matures. The gradual shading out and decomposition of the vegetation in the early thicket stages of plantation development must release nutrients for tree growth, but later, as a new ground flora forms with the opening up of the canopy, strong competition for nutrients between the trees and herbaceous layer may occur. Fortunately the history of the Abbotswood plots is fairly well documented (Ovington, 1955) and it is known, for instance, that the dense growth of bracken in the plot of Pinus nigra has developed within the last 23 years. The average, annual increase of calcium and magnesium in the ground vegetation of the plot of Corsican pine during this build-up period amounts to just over 2 kg of calcium per ha and about.4 kg of magnesium per ha. When a ground flora is well developed, the annual turnover of plant nutrients resulting from the growth and decomposition of the herbaceous vegetation may well exceed that associated with the fall of leaves from the trees and the breakdown of the tree litter. Localitj' Calcium Canopy Boles Thinnings Ground flora Litter Total Soil (Exch.) Magnesium Canopv Boles ' Thinnings Ground flora Litter Total Soil (Exch,) Table 3. Calcium and magnesium in the woodland ecosystems (kg I ha) West Min, Tofts Max, , Bedgeburv Min. 'Max, I :; II Abbotswood Mm, Max, A layer of organic matter, derived mainly from litter falling from the trees and ground flora, covers the mineral soils of the forest plots. The rate at which this organic matter forms will depend on the balance between litter fall and decomposition. Where a thick layer of surface humus has developed, as for instance in old stands of conifers, it may contain about a tenth of the total calcium and magnesium in the ecosystem but the proportion is usually much smaller than this. The litter in the plantation of Douglas fir at West Tofts contains the largest recorded weight of calcium (23 kg per ha) whilst the litters of Norway spruce and larch at Abbotswood have the greatest amount of magnesium (27 kg per ha). As the layer of surface humus becomes thicker, it accumulates increasing quantities of calcium and magnesium and the average annual accumulation of calcium per year for all the plots amounts to 3.4 kg per ha and of magnesium to,4 kg per ha. The most rapid build-up of calcium in the surface humus (1,5 kg per ha per annum) has occurred under Pseudotsuga taxifolia at West Tofts whilst for magnesium the greatest rate of accumulation (i,2 kg per ha per annum) was attained where Picea abies had been planted at Bedgebury. If a balance could be achieved between the nutrient content of the litter fall and the release of nutrients by decomposition, the loss of calcium and magnesium from the nutrient cycle through the formation of surface humus would cease. From a long-term point of view the nutrient content of the litter represents only a temporary storage and the calcium and magnesium retained in the organic layers are likely to become available for tree growth at some future date. The collection of tree litter r

9 172 J. D. OVINGTON for animal bedding would result in the removal of substantial amounts of calcium and magnesium from the forest ecosystem but this is rarely, if ever, done in Britain. The fauna and microfiora were not sampled as separate components of the ecosystem and the bulk samples of the litter and mineral soil would inevitably contain part of the soil microfiora and microfauna. The characteristic fauna and microfiora appear to differ from plot to plot but it seems likely that their total nutrient content will be small compared to that of the trees. The \\ eight of nutrients contained in the invertebrate canopy fauna and the main bird species for plantations of Scots pine near to West Tofts was determined and calcium did not exceed.5 g per ha whilst magnesium was less than.5 g per ha (Grimshaw, Ovington, Betts and Gibb, 1958). The mineral soil contains relatively large amounts of exchangeable calcium and magnesium but the total calcium and magnesium contents of the soil must be much greater than the exchangeable values. In Britain, soil may vary considerably over a small area and in selecting the location of a series of experimental plots foresters would try to obtain an area having fairly uniform soil conditions. Unfortunately, little is known of the original variation in soil properties prior to planting but the present soil differences between the plots of each series probably result from both initial site variations and the differential effects of tree growth after planting. DISCUSSION Despite great technical difficulties, detailed balance sheets have been prepared to show the production of organic matter and the circulation of mineral elements in aquatic habitats such as rivers, estuaries, lakes and seas (Odum, 1956, 1957; Steele, 1956). In contrast, few comprehensive studies have been made of the circulation of nutrients within terrestrial communities. Woodland ecologists, in particular, have tended to concentrate on some individual aspect of the ecosystem such as litter formation or vegetation type rather than considering the community as a whole. A broad assessment can now be made of the relative importance of the various dynamic processes characteristic of the woodland associations at the three experimental areas. In the forest plots, the trees, litter and the associated fiora and fauna contain a large proportion of the total nutrient capital, even after a short period of afforestation. The rates at which calcium and magnesium have been accumulated in the organic matter depend upon the type and age of the plantations as well as site conditions but may average up to 2 kg of calcium and 3 kg of magnesium per ha per annum for the first 5 years of afforestation. The annual uptake of calcium and magnesium from the soil must greatly exceed the build-up of the two elements in the organic matter since there is a considerable interchange of nutrients between woodland plants and the soil. Nutrients circulate through woodland ecosystems by very different routes and at different rates (Fig. 4). For example, in deciduous, hardwood plantations all the tree leaves are shed in the autumn and have almost decomposed completely by the following autumn so that the return of nutrients to the mineral soil involves a 2-year cycle. The same nutrient cycle in a plantation of evergreen conifers may take at least 6 years, since the tree leaves persist in the canopy for 3 or 4 years before falling and, if a thick surface layer of humus has formed, the breakdown of the leaves may not be completed for a further 2 or more years. The circulation of calcium and magnesium by the decomposition of branch material is usually slower than that for leaves since branches tend to persist longer in the tree canopies and are more resistant to decomposition. The passage of

10 Mineral content of tree species 173 nutrients through other circulatory systems is relatively rapid compared with that associated with litter fall. Preliminary investigations using radioactive isotopes have shown that plant nutrients absorbed by the tree roots are translocated within a few hours to the tree canopies from which they may be leached out and returned to the soil in rainwater. Distribution of calcium in the ecosystem Tree Crop Iree Boles Tree Branches Tree Leaves Litter Mineral Soil Ground Flora = 25 Kg./ha. Annual turnover of calcium Tree Tree Leaves Precipitation Soil Drainage 11=5 Kg./ha./annum Fig. 4. The distribution and circulation of calcium in the plantation of" Larix eurolepis at Bedgebury, 23 years after planting. The area of the squares represents the calcium content whilst the breadth of the arrows represents the rate of circulation of calcium. Although only small amounts of nutrients are contained in the canopy fauna, chemical elements circulate through the animals and are returned to the soil either in the frass or in animal remains. When woodlands are defoliated by invertebrate pests eating the leaves, the animal cycle becomes the major route for the return of nutrients to the soil.

11 174 J- ^- OVINGTON Woodlands are not closed systems since nutrients are removed in the tree crop and drainage water whilst additional amounts of calcium and magnesium are supplied in the precipitation or are released by weathering. Only a small part of the forest production is harvested and the loss of nutrients through the removal of the tree boles is relatively small compared with the loss in agricultural crops. Russell (195) gives the annual removal of plant nutrients by some typical agricultural crops grown in England and for calcium the average removal is 33,1 kg per ha with minimum and maximum values of 7,4 and 59.2 kg per ha respectively. The corresponding removal of magnesium in the agricultural crops averages 11,6 with a minimum of 4.8 and a maxim of 28.7 kg per ha per annum. Nevertheless, when the nutrient content of the boles is accumulated, the total loss of nutrients, if all the tree boles were extracted after 5 years, would be considerable (maximum for calcium 392 and magnesium 73 kg per ha). Analysis of the precipitation at Bedgebury over a 2-year period has shown that about 11 kg of calcium and less than 4 kg of magnesium per ha are contained in the annual precipitation. The nutrients supplied in the rainwater are broadly equivalent to the build-up of nutrients in the tree boles. About three-quarters of the precipitation falling in the forest plots will be evaporated but the remaining 25 o drains away and may contain significant amounts of calcium and magnesium (Ovington, 1958). Few data are available of the rate at which nutrients are released by the decomposition of the mineral soil under forest conditions but it would appear that soil weathering would be greater in rapidly growing woodlands with high biological activity. In forestry, as in agriculture, the maintenance of soil fertility is important and the effects of forest plantations on the soil and ultimately on the growth of succeeding crops needs to be carefully considered. The accumulation of calcium and magnesium within the organic matter formed in woodland ecosystems could represent a considerable drain on the soil reserves, particularly since woodlands are rarely manured and the soils available for afforestation are usually poor in nutrients. However, there is no evidence of a close relationship between the build-up of the two elements in the trees, ground flora and litter, and the loss of exchangeable calcium or magnesium from the whole soil profile. In some plots exchangeable calcium has been redistributed within the soil profile and the general effect of afforestation has been to reduce the amount of exchangeable calcium in the uppermost soil. The breakdown of the litter layer and the tree canopies left on the forest floor after clear felling would tend to increase the calcium content of the surface soil. In addition, once the trees are felled the roots would begin to decay and make available the nutrients which they contain. In this investigation, the tree roots have not been sampled but the roots of Scots pine may represent 3% of the total tree weight (Ovington, 19576), Fast-growing conifers with a heavy litter fall, etc., may have a larger gross annual turnover of nutrients than neighbouring hardwood stands, even though the time interval between nutrient absorption and release is greater in the coniferous plantations. Long-term changes in soil conditions resulting from the growth of woodlands must depend on the balance attained between the internal circulation of nutrients as well as the external ffow of nutrients into and out of the system. ACKNOWLEDGMENTS It is a pleasure to acknowledge the assistance of the Forestry Commission who are responsible for the management ofthe experimental areas. In the course of this investiga-

12 Mineral content of tree species 175 tion a great deal of primary data of the nutrient composition of trees has been collected. Unfortunately, not all of this supporting data could be given in this account but it will be supplied on application to the author or to the Nature Conservancy, London. REFERENCES GRIMSHAW, H. M., OVINGTON, J. D., BETTS, H. M. & GIBB, J. A. (1958). The mineral content of birds and insects in plantations of Pinus sylvestris L. Oikos, 9, 26. HUMMEL, F. C. & CHRISTIE, J. (1953). Revised yield tables for conifers in Great Britain. For. Comm. For. Rec, 24, 23 pp. ODUM, H. T. (1956). Primary production in flowing waters. Limnology and Oceanographv, i, 12. ODUM, H. T. (1957). Trophic structure and productivity of Silver Springs, Florida. EcoL Monogr., 27, 55. OVINGTON, J. D. (1955). Studies of the development of woodland conditions under different trees. 3. The ground flora, y. EcoL, 43, i. OVINGTON, J. D. (1956a). The form, weights and productivity of tree species grown in close stands. Nezc PhytoL, 55, 289. OVINGTON, J. D. (19566). The composition of tree leaves. Forestry, 29, 22. OVINGTON, J. D. (19573). The volatile matter, organic carbon and nitrogen contents of tree species grown in close stands. New PhytoL, 56, i. OVINGTON, J. D. (19576). Dry matter production by Pinus sylvestris L. Ann. Bot. N.S., 21, 287. OVINGTON, J. D. & MADGWICK, H. A. I. (1958). The sodium, potassium and phosphorus contents of tree species grown in close stands. New PhytoL, 57, 273. OVINGTON, J. D. (1958). Vegetation and water conservation. Nezo BioL Penguin Books, 25, 47. PIPER, C. S. (195). Soil and plant analysis. Monograph from the Waite Agricultural Research Institute, Adelaide, Australia. RUSSELL, E. W. (195). Soil Conditions and Plant Growth. Longmans, London. STEELE, J. H. (1956). Plant production on the Fladen Ground, jf. Mar. BioL Ass. U.K., 35, i.

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