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Some notes on the regeneration of Norway spruce on six permanent plots managed with single-tree selection
Forest Ecology and Management, 46 ( 1991 ) 49-57
49
Elsevier Science Publishers B.V., Amsterdam
Some notes on the regeneration of Norway spruce on six permanent plots managed with single-tree selection Lars Lundqvist Department of Silviculture, Factdty of Forestry, Swedish Universityof Agricultural Sciences, S-901 83 Ume~, Sweden (Accepted 4 October 1990)
ABSTRACT Lundqvist, L., 1991. Some notes on the regeneration of Norway spruce on six permanent plots managed with single-tree selection. For. Ecol. Manage., 46: 49-57. Interest in single-tree selection is today increasing in Sweden, but Swedish foresters believe that the harsh climate usually makes it impossible to achieve sufficient regeneration and ingrowth after selection cuttings. In this paper, data on abundance, height increment and ingrowth of Norway spruce seedlings on six permanent plots are presented. Methods to estimate seedling mortality and future ingrowth are also suggested. Four plots are situated in central Sweden, and two plots in southern Sweden. All plots are dominated by Norway spruce (Picea abies (L.) Karst.). The soil is glacial till on all plots, soil moisture is mesic and the ground cover is dominated by bilberry ( Vaccinium myrtillus L. ). The results show that it is possible to obtain abundant regeneration and high ingrowth rates in Norway spruce selection stands in Sweden, with high levels of standing volume and bilberry as the dominating ground vegetation. They also indicate that regeneration and ingrowth into the tree layer may not be as serious a problem as Swedish foresters have traditionally assumed.
INTRODUCTION
The use of single-tree selection has been very limited in Sweden, and since the 1950s the use of the method has been restricted by the Swedish Forestry Act. Today, interest in single-tree selection is increasing. There is, however, resistance to the method among Swedish foresters in general. They believe that, as a result of the harsh climate, it is usually not possible to obtain sufficient regeneration after selection cuttings, except in the southernmost parts of Sweden (see Wellander, 1938, S~derstr6m, 1971; Lundberg, 1973; H~iggstr6m, 1982). Several of the early studies, conducted in the northern half of Sweden, showed that ground conditions in high-graded uneven-aged Norway spruce stands are unfavourable for germination and establishment of spruce seed© 1991 Elsevier Science Publishers B.V. All rights reserved 0378-1127/92/$03.50
50
L. LUNDQVIST
lings, especially if bilberry ( Vaccinium myrtillus L. ) dominates the ground vegetation (Holmgren, 1914; Hesselman, 1917, 1937; Kallin, 1926; Teikmanis, 1952, 1954). Furthermore, seedling mortality is very high, 87-90%, during the first few years after seed germination in uneven-aged Norway spruce stands that have been selection cut or high-graded (Arnborg, 1947; Nilsen, 1986). It should be noted that none of these studies was conducted in stands managed with single-tree selection. Before single-tree selection is reintroduced in Swedish forestry it should, therefore, be established whether the above-related results are applicable in stands managed with this system. In this paper, data on abundance, height increment and ingrowth of Norway spruce seedlings are presented for six permanent plots managed with single-tree selection. Methods to estimate seedling mortality and future ingrowth into the tree layer from survey data are also suggested.
MATERIALAND METHODS The six plots have been established and managed independently of each other, and the cuttings have not followed a predefined plan. All cuttings have, however, been carefully recorded. Four plots ( S I - $ 4 ) are situated in central Sweden at Siljansfors experimental forest ( 6 0 ° 5 0 ' N ) . The other plots are situated in southern Sweden, in Gammelstorp ( G I ) and Glim'~kra (G2) ( 5 6 ° 2 0 ' N for both plots). At all plots, Norway spruce (Picea abies (L.) Karst) constitutes more than 80% of standing volume. The soil is a glacial till, soil moisture is mesic and the ground cover is dominated by bilberry (Vaccinium myrtillus L.) at all plots; however, site productivities differ (Table 1 ). On plots S I and $2, small gaps, 15-25 m in diameter, were created in the TABLE I
Site indexand site productivityfor the experimentalplots, estimatedfrom site factorsaccordingto H/igglundand Lundmark ( 1981 ) Plot
Officialname
Site index (HI00) a
Site productivity (m3ha-~ year-i )
S! $2 $3 $4 Gl G2
Sf22.3 Sf 82 Sf 56.! Sf 56.2 Gammelstorp Glimfikra2
G 22 G 24 G 24 G 24 G 30 G 30
5.3 6. ! 6.1 6. i 10.I !0. I
~Dominantheightin even-agedNorwaysprucestandsat age 100 years.
REGENERATION OF NORWAYSPRUCE
5[
1950s in addition to the single-tree selection. Plots $3 and $4 were subjected to heavy cuttings in 1982, which changed them into shelterwood stands. Plot G 1 has not been managed at all for several decades. Plot G2 was cut from above down to 5 in ( 12.7 cm) diameter at breast height (DBH) during the 1920s. In 1945 and 1958 selection cuttings were made. On plots S 1, $2, G 1 and G2 regeneration of Norway spruce was surveyed in autumn 1988. At each permanent plot, 25 circular sub-plots of 12.5 m 2 were distributed systematically. All Norway spruce seedlings in the height interval 0.1-1.29 m were counted, and the total height (cm) and the length of the leading shoot ( m m ) of each seedling were measured. For plots $3 and S4, data on regeneration were included from a pilot study, performed on these two plots in the spring of 1982, before the cuttings that changed the stand structure. In that survey, 20 and 25 sub-plots, respectively, were randomly distributed on each plot. The circular sub-plots were each of 25 m 2. The accumulated length of the leading shoots for the last 3 years was measured in centimetres on the four seedlings nearest to the centre of each circular sub-plot. For all calculations the seedlings are divided into four 0.3-m height classes (0.1-0.39, 0.4-0.69, 0.7-0.99. 1.0-1.29 m). Although seedling mortality could not be measured in the regeneration survey, an attempt has been made to estimate it theoretically. For this purpose, an equation was developed from Prodan's (1949) equation for computing selection equilibrium diameter distributions. According to Prodan, the numbers of trees (n) in two adjacent diameter classes (j, j + 1 ) should be inversely proportional to the arithmetic mean annual diameter increment (i) of trees in these diameter classes
nj/n~+, = ij+l ~it
( 1)
Equation ( 1 ) is here applied for the height classes and height increment of seedlings. Prodan's equation is valid only under the assumptions that there is (1) a constant input of new seedlings into the lowest height class, (2) no seedling mortality, and (3) a stable relation between seedling height and height increment. • A transformation of Eqn. ( 1 ) shows that the sum of the annual height increments ( n • i) within each height class should be equal for all height classes. With seedling mortality (m) the sum decreases over the height range. The annual mortality rate in each height class can be estimated as
m = 1 - ( (nj+, *6+,)/(nj*iJ* ))I/t,
(2)
where tj is the time in years needed by seedlings to grow through the height class j, calculated as
tj=w/ij
(3)
52
L. LUNDQVIST
where w is the width of the height class and ij is the mean annual height increment of the seedlings in the class. For plots $2-$4, records make it possible to calculate the historical ingrowth rates past 1.3 m height during recent decades. Annual ingrowth (g) is calculated from the records as
g=(Ne-Nb +Nc)/y
(4)
where Arc is the number of trees at the end of the period, Nb the number of trees at the beginning of the period, Nc the number of trees removed through cutting or mortality during the period, and y the length of the period in years. The potential annual ingrowth (gp) into the tree layer in the near future has been calculated from the regeneration survey data as
gp=nHs/t
(5)
where net5 is the number of seedlings in the height class 1.0-1.29 m and t is the average time needed by seedlings to grow through this height class, calculated with Eqn. (3). Stand density is expressed as standing volume, i.e. total stem volume over bark for trees with DBH >I 4.5 cm. Standing volumes are calculated for each plot from diameter distributions and mean stem volume in each diameter class. Mean stem volumes are estimated with N/islund's ( 1940, 1947 ) functions for tree volume, using the mean height, diameter and length of living crown of sample trees in each diameter class. As stand inventory data were not available for the year of the regeneration survey for plots S 1 and $2, the standing volume in 1985 (S l ) and 1979 ($2) was projected to the year of the inventory, using the current annual volume increment for each plot. RESULTS AND DISCUSSION
The estimated numbers of seedlings on the plots range from 1570 to 9140 seedlings ha-~ (Table 2). Although the values in Table 2 indicate a positive correlation between standing volume and number of seedlings, such a conclusion should not be drawn from this material. The S- and G-plots are located in different parts of the country and have different site productivities. Furthermore, the results for plots $3 and $4 refer to a situation seven growing seasons earlier than for the other four plots. From studies conducted in Norway spruce stands regenerated with the shelterwood system, it is known that increased stand densities usually reduce the amount of spruce advance growth (Amilon, 1929; Cajander, 1934; Hagner, 1962a,b; Zybura, 1983). Nilsen (1988), however, found no relationship between stand density, expressed as basal area, and number of seedlings in uneven-aged sub-alpine Norway spruce stands in Norway that had been subjected to heavy selection cuttings. His results agree well with those of studies
REGENERATIONOF NORWAYSPRUCE
53
TABLE 2 Estimated number of seedlings in the regeneration surveys, and standing volume at the time of the surveys Plot
SI $2 $3 $4 G! G2
No. of seedlings (stems h a - ~) Mean
Standard error
1570 3070 7460 9140 6530 3490
242 546 1375 1512 1748 1046
Standing volume (m3ha -j )
i05 197 244 238 278 187
in high-graded stands in Finland (Sarvas, 1944), experimental plots managed with group selection in Norway (B~ihmer, 1957), selection stands in Austria (Kammerlander, 1978), and experimental plots treated with singletree selection in Finland (Valtanen, 1988). The spatial distribution of seedlings was not measured in the surveys, but the general impression ofthe surveys was that most seedlings were growing in the vicinity of larger trees and not in canopy openings. It is therefore interesting to note that the lowest number of seedlings were found on plots S 1 and $2, which have had very low levels of standing volume for a long time, and, in addition, have had gaps created on them. The annual height increment of seedlings is positively correlated with seedling height on all plots except GI (Fig. 1 ). Before the cutting in 1980, plot Gl had been unmanaged for several decades, and this can possibly ,~xplain the deviating results for this plot. Plot G2 has the highest value in all cla~ses, but there are small differences in absolute levels between the other plots. This indicates that standing volume has little influence on the height increment of seedlings in uneven-aged Norway spruce stands, but the weaknesses in the material do not permit such a conclusion. It should be noted, however, that plots $2-$4, which are situated close to each other and have similar site conditions, have similar height increment for the seedlings in spite of different levels of standing volume. The impression gained during the surveys was also that there is no obvious difference in height increment between seedlings growing close to trees and those growing in the open. The annual mortality rates, calculated with Eqn. (2), vary considerably, but are mainly below 10% (Table 3). Also, very few dead seedlings were observed during the regeneration surveys. It should be noted that the high mortality rates reported by Arnborg ( 1947 ) and Nilsen (1986) refer to the first few years after germination. The potential ingrowths past breast height calculated with Eqn. (5) are tel-
54
L LUNDQVIST "r.
Mean annual height increment (mm year - l )
i00 80
Us1
60
I~s3
Hs2 ~s4
40
I~IG1 I-I G2
20
0 0.25
0.55
0.85
1.15
Seedling height (m) Fig. 1. Mean annual height increment for seedlings in 0.3 m height classes. For plots $3 and $4 the value for each seedling is the mean annual increment during the last 3 years, measured on a subsample of seedlings. Seedling height refers to start height, i.e. exclusively the leading shoot. TABLE 3
Annual mortality rates for seedlings in 0.3 m height classes, calculated with Eqn. (2), and accumulated mortality from 0. i to 1.0 m height Plot
SI $2 $3 $4 GI G2
Annual mortality rate (%)
Accumulated mortality 0.1-1.0 m height (%)
0.1-0.39 m
0.4-0.69 m
0.7-0.99 m
-4.55 2.20 2.62 8.75 6.53 7.21
9.73 3.43 0.64 3.52 8.92 5.33
16.50 - 7.28 11.44 4.42 3.73 ! 1.21
71.3 ! 9.8 74.2 69. ! 86.7 79.2
atively high for all plots except plot S 1, and exceed the historical values for plots $2-$4 (Table 4). Whether these ingrowth rate.s will be sufficient to maintain the stand structure on the plots in the future ~epends on the number of trees that will be harvested and lost through mortality. To maintain these levels of ingrowth it is also necessary that a sufficient number of new seedlings grow past 10 cm height. If we assume 70% accumulated mortality from 0.1 to 1.0 m height (see Table 3), 5% annual mortality from 1.0 to 1.3 m height and a mean annual height increment of 50 mm in the height interval 1.0-1.29 m (see Fig. 1 ), fewer than 200 seedlings ha- ~ are required annually to grow past
REGENERATIONOF NORWAYSPRUCE
55
TABLE 4 Historical ingrowth, calculated with Eqn. (4) from plot records, and potential ingrowth, calculated with Fqn. (5) from regeneration survey data Plot
Period
Mean standing
Ingrowth (stems h a - ~year- ~)
volumein period (ms ha- ~)
Historical
-
Sl
-
$2
1959-79
143
49.0
84.2
$3
1972-82
227
37.2
51.3
$4 GI
1972-82 -
232 -
59.5 -
88.8 46.1
G2
-
-
49.1
-
-
Potential future 7.9
10 cm height, to maintain an annual ingrowth past breast height of 40 seedlings ha- t. The calculations of mortality rates and potential ingrowths are based on the assumption that the observed relationship between seedling height and seedling height increment is stable and can be regarded as a description of the dynamic development of the seedlings. The mortality calculations further require that the number of seedlings annually growing into the lowest height class is more or less constant. These assumptions should be fairly safe for plots $3, $4, GI and G2, as the stand density and the stand structure have changed very little during recent decades on these plots. On plots S1 and $2, however, there is negative mortality for one of the height classes. This can be explained by variations over time in the relationship between seedling height ana height increment, and in the number of new seedlings growing past 0.1 m height during recent decades. On both plots gaps have been created. Furthermore, on plot Sl the standing volume was very low (40-80 m 3 ha -I ) for several decades as a result of heavy cuttings, and it has only recently passed 100 m 3 h a - i. CONCLUSIONS
The results of this study show that it is possible to obtain abundant regeneration and high ingrowth rates in Norway spruce selection stands in Sweden, with high levels of standing volume and bilberry as the dominating ground vegetation. As the plots have been established and managed independently of each other, and without an explicit objective, the results do not permit very detailed conclusions. They do indicate, however, that regeneration and ingrowth into the tree layer may not be as serious a problem as Swedish foresters have traditionally assumed.
56
L. LUNDQVIST
ACKNOWLEDGEMENTS I t h a n k I n g v a r S v e n s s o n , OstersliSv, w h o g r a c i o u s l y g a v e m e access to all n e c e s s a r y d a t a o n his p r i v a t e s e l e c t i o n p l o t in G l i m f i k r a ( G 2 ) . T h e s t u d y w a s financed by the National Swedish Environmental Protection Board (Statens Naturvfirdsverk).
REFERENCES Amilon, J.A., 1929. HyggesskiStselnoch f'6ryngringen inom mossrika skogar tillhSrande Vacciniumtypen inom Orli revir. Norrlands Skogsvfirdsf'6rbunds Tidskr., 78-137. Arnborg, T., 1947. FOryngringsunders/Skningar i mellersta Norrland. Norrlands Skogsvfirdsf6rbunds Tidskr., 247-293. BOhmer, J.G, 1957. Bledningsskog II. Tidsskr. Skogbruk, 65: 203-247. Cajander, E.K., 1934. Kuusen taimistojen vapauttamisen j~ilkeisest[ipituuskasvusta. Commun. Inst. For. Fenn., 19.5. H~igglund, B. and Lundmark, J.-E., 1981. Handledning i bonitering med SkogshOgskolans boniteringssystem, Del 2 Diagram och tabeller. Skogsstyrelsen, JOnkOping, Sweden, 70 pp. HiiggstriSm, B., 1982. Om de biologiska f'6rutsiittningarna fOr skogens fOryngring i Norrlands hiijdl~igen. Sveriges Skogsv~rdsf6rb. Tidskr., No. 6: 25-33. Hagner, S., 1962a. Ett exempel pfi bestfindstiithetens betydelse for den naturliga f6ryngringens uppkomst och utveckling pfi god granmark i Skfine. Statens Skogsforskningsinstitut, Uppsatser, 85: 1-6. Hagner, S., 1962b. Naturlig f6ryngring under sk~irm. Medd. Statens Skogsforskningsinst., 52.4, 263 pp. Hesselman, H., 19 ! 7. Om vfira skogsf6ryngringsfitg/irders inverkan pfi salpeterbildningen i marken och dess betydelse f6r barrskogens f6ryngring. Medd. Statens Skogsf6rs6ksanstalt, 14: 923-1076. Hesselman, H., 1937. Om humust~ickets beroende av bestfindets alder och sammansiittning i den nordiska granskogdn av blfibiirsrik Vacciniumtyp och dess inverkan p~ skogens f6ryngring och tillviixt. Medd. Statens Skogsf6rs/Sksanstalt, 34:529-716. Holmgren, A., 1914. Bl~idning och trakthuggning i norrlandsskogar. Norrlands Skogsv~rdsf6rbunds Tidskr., 266-323. Kailin, K.E., 1926. F~ryngringsstudier i Norrlandsskogar. Svenska Skogsvfirdsf'Oren. Tidskr., 253-261. Kammerlander, H., 1978. Aufbau, Verjiingung und Verbissgef~ihrdung der Plenterwiiider im Raum Kufstein/Tirol. Schweiz. Z. Forstwes., 129:7 i 1-726. Lundberg, H., 1973. Skogsbrukets ekonomi och teknik. In: Svenska Naturskyddsf6reningens firsbok 1973. Sveriges Natur 64: 43-56. N~islund, M., 1940. Funktioner och tabeller f6r kubering av stfiende trgd - Tall, gran och bj~rk i norra Sverige. Medd. Statens Skogsf'6rsbksanstalt, 32: 87-i 42. N~islund, M., 1947. Funktioner och tabeiler f6r kubering av stfiende tr~id - Tall, gran och bj6rk i sOdra Sverige saint i hela lander. Medd. Statens Skogsforskningsinst., 36 ( 3 ): i - 8 1 . Nilsen, P., 1986. Tap av fro og spireplanter av gran (Picea abies (L.) Karst) etter sfiing lafi forskjellige vegetasjonstyper i t~ellskogog lavlandsskog. Medd. Norsk Inst., Skogforsk., 39 (8): 129-145.
REGENERATIONOF NORWAYSPRUCE
57
Nilsen, P., 1988. Fjeliskoghogst i granskog - gienvekst og produksjon etter tidligere hogster. Rapp. Norsk Inst. Skogforsk., 2/88: 1-26. Prodan, M., 1949. Die theoretische Bestimmung des Gleichgewichtszustandes im Plentero waldes. Schweiz. Z. Forstwes., 100: 81-99. Sarvas, R.. 1944. Tukkipuun harsintojen vaikutus etel~-Suomen yksityismetsiin. Commun. Inst. For. Fenn., 33.1,268 pp. S0derstr6m, V., 197 i. Va~6r hyggen? Ymer, 161- ! 75. Teikmanis, A., 1952. Om markvegetationens inflytande p~ uppkomsten av naturlig f'6ryngring i J~imtlands 6rtrika granskogar. Norrlands Skogsvfirdsf6rbunds Tidskr., 1-44. Teikmanis, A., 1954. N~gra studier 0vet de mossrika granskogarna i Norrland ocb deras f6ryngringsproblem. Norrlands Skogsv~rdsf6rbunds Tidskr., 307-434. Valtanen, J., 1988. Korkeiden maiden metsien uudistaminen oulun l~i~iniss~i.Folia For., 718, 41 PP. WeUander, P.O., 1938. J~imn~ldriga och olik~ldriga best~ndsformer. Norrlands Skogsvfirdsf6rbunds Tidskr., bilaga till 3:dje h~iftet. Zybura, H., 1983. Wplyw drzewostanu oslaniajacego na dynamike odnowien podokapowych swierka w drzewostanach z udzialem sosny i swierka w polnocno-wschodniej czesci Polski. Sylwan, No. 9,10: 41-52.
49
Elsevier Science Publishers B.V., Amsterdam
Some notes on the regeneration of Norway spruce on six permanent plots managed with single-tree selection Lars Lundqvist Department of Silviculture, Factdty of Forestry, Swedish Universityof Agricultural Sciences, S-901 83 Ume~, Sweden (Accepted 4 October 1990)
ABSTRACT Lundqvist, L., 1991. Some notes on the regeneration of Norway spruce on six permanent plots managed with single-tree selection. For. Ecol. Manage., 46: 49-57. Interest in single-tree selection is today increasing in Sweden, but Swedish foresters believe that the harsh climate usually makes it impossible to achieve sufficient regeneration and ingrowth after selection cuttings. In this paper, data on abundance, height increment and ingrowth of Norway spruce seedlings on six permanent plots are presented. Methods to estimate seedling mortality and future ingrowth are also suggested. Four plots are situated in central Sweden, and two plots in southern Sweden. All plots are dominated by Norway spruce (Picea abies (L.) Karst.). The soil is glacial till on all plots, soil moisture is mesic and the ground cover is dominated by bilberry ( Vaccinium myrtillus L. ). The results show that it is possible to obtain abundant regeneration and high ingrowth rates in Norway spruce selection stands in Sweden, with high levels of standing volume and bilberry as the dominating ground vegetation. They also indicate that regeneration and ingrowth into the tree layer may not be as serious a problem as Swedish foresters have traditionally assumed.
INTRODUCTION
The use of single-tree selection has been very limited in Sweden, and since the 1950s the use of the method has been restricted by the Swedish Forestry Act. Today, interest in single-tree selection is increasing. There is, however, resistance to the method among Swedish foresters in general. They believe that, as a result of the harsh climate, it is usually not possible to obtain sufficient regeneration after selection cuttings, except in the southernmost parts of Sweden (see Wellander, 1938, S~derstr6m, 1971; Lundberg, 1973; H~iggstr6m, 1982). Several of the early studies, conducted in the northern half of Sweden, showed that ground conditions in high-graded uneven-aged Norway spruce stands are unfavourable for germination and establishment of spruce seed© 1991 Elsevier Science Publishers B.V. All rights reserved 0378-1127/92/$03.50
50
L. LUNDQVIST
lings, especially if bilberry ( Vaccinium myrtillus L. ) dominates the ground vegetation (Holmgren, 1914; Hesselman, 1917, 1937; Kallin, 1926; Teikmanis, 1952, 1954). Furthermore, seedling mortality is very high, 87-90%, during the first few years after seed germination in uneven-aged Norway spruce stands that have been selection cut or high-graded (Arnborg, 1947; Nilsen, 1986). It should be noted that none of these studies was conducted in stands managed with single-tree selection. Before single-tree selection is reintroduced in Swedish forestry it should, therefore, be established whether the above-related results are applicable in stands managed with this system. In this paper, data on abundance, height increment and ingrowth of Norway spruce seedlings are presented for six permanent plots managed with single-tree selection. Methods to estimate seedling mortality and future ingrowth into the tree layer from survey data are also suggested.
MATERIALAND METHODS The six plots have been established and managed independently of each other, and the cuttings have not followed a predefined plan. All cuttings have, however, been carefully recorded. Four plots ( S I - $ 4 ) are situated in central Sweden at Siljansfors experimental forest ( 6 0 ° 5 0 ' N ) . The other plots are situated in southern Sweden, in Gammelstorp ( G I ) and Glim'~kra (G2) ( 5 6 ° 2 0 ' N for both plots). At all plots, Norway spruce (Picea abies (L.) Karst) constitutes more than 80% of standing volume. The soil is a glacial till, soil moisture is mesic and the ground cover is dominated by bilberry (Vaccinium myrtillus L.) at all plots; however, site productivities differ (Table 1 ). On plots S I and $2, small gaps, 15-25 m in diameter, were created in the TABLE I
Site indexand site productivityfor the experimentalplots, estimatedfrom site factorsaccordingto H/igglundand Lundmark ( 1981 ) Plot
Officialname
Site index (HI00) a
Site productivity (m3ha-~ year-i )
S! $2 $3 $4 Gl G2
Sf22.3 Sf 82 Sf 56.! Sf 56.2 Gammelstorp Glimfikra2
G 22 G 24 G 24 G 24 G 30 G 30
5.3 6. ! 6.1 6. i 10.I !0. I
~Dominantheightin even-agedNorwaysprucestandsat age 100 years.
REGENERATION OF NORWAYSPRUCE
5[
1950s in addition to the single-tree selection. Plots $3 and $4 were subjected to heavy cuttings in 1982, which changed them into shelterwood stands. Plot G 1 has not been managed at all for several decades. Plot G2 was cut from above down to 5 in ( 12.7 cm) diameter at breast height (DBH) during the 1920s. In 1945 and 1958 selection cuttings were made. On plots S 1, $2, G 1 and G2 regeneration of Norway spruce was surveyed in autumn 1988. At each permanent plot, 25 circular sub-plots of 12.5 m 2 were distributed systematically. All Norway spruce seedlings in the height interval 0.1-1.29 m were counted, and the total height (cm) and the length of the leading shoot ( m m ) of each seedling were measured. For plots $3 and S4, data on regeneration were included from a pilot study, performed on these two plots in the spring of 1982, before the cuttings that changed the stand structure. In that survey, 20 and 25 sub-plots, respectively, were randomly distributed on each plot. The circular sub-plots were each of 25 m 2. The accumulated length of the leading shoots for the last 3 years was measured in centimetres on the four seedlings nearest to the centre of each circular sub-plot. For all calculations the seedlings are divided into four 0.3-m height classes (0.1-0.39, 0.4-0.69, 0.7-0.99. 1.0-1.29 m). Although seedling mortality could not be measured in the regeneration survey, an attempt has been made to estimate it theoretically. For this purpose, an equation was developed from Prodan's (1949) equation for computing selection equilibrium diameter distributions. According to Prodan, the numbers of trees (n) in two adjacent diameter classes (j, j + 1 ) should be inversely proportional to the arithmetic mean annual diameter increment (i) of trees in these diameter classes
nj/n~+, = ij+l ~it
( 1)
Equation ( 1 ) is here applied for the height classes and height increment of seedlings. Prodan's equation is valid only under the assumptions that there is (1) a constant input of new seedlings into the lowest height class, (2) no seedling mortality, and (3) a stable relation between seedling height and height increment. • A transformation of Eqn. ( 1 ) shows that the sum of the annual height increments ( n • i) within each height class should be equal for all height classes. With seedling mortality (m) the sum decreases over the height range. The annual mortality rate in each height class can be estimated as
m = 1 - ( (nj+, *6+,)/(nj*iJ* ))I/t,
(2)
where tj is the time in years needed by seedlings to grow through the height class j, calculated as
tj=w/ij
(3)
52
L. LUNDQVIST
where w is the width of the height class and ij is the mean annual height increment of the seedlings in the class. For plots $2-$4, records make it possible to calculate the historical ingrowth rates past 1.3 m height during recent decades. Annual ingrowth (g) is calculated from the records as
g=(Ne-Nb +Nc)/y
(4)
where Arc is the number of trees at the end of the period, Nb the number of trees at the beginning of the period, Nc the number of trees removed through cutting or mortality during the period, and y the length of the period in years. The potential annual ingrowth (gp) into the tree layer in the near future has been calculated from the regeneration survey data as
gp=nHs/t
(5)
where net5 is the number of seedlings in the height class 1.0-1.29 m and t is the average time needed by seedlings to grow through this height class, calculated with Eqn. (3). Stand density is expressed as standing volume, i.e. total stem volume over bark for trees with DBH >I 4.5 cm. Standing volumes are calculated for each plot from diameter distributions and mean stem volume in each diameter class. Mean stem volumes are estimated with N/islund's ( 1940, 1947 ) functions for tree volume, using the mean height, diameter and length of living crown of sample trees in each diameter class. As stand inventory data were not available for the year of the regeneration survey for plots S 1 and $2, the standing volume in 1985 (S l ) and 1979 ($2) was projected to the year of the inventory, using the current annual volume increment for each plot. RESULTS AND DISCUSSION
The estimated numbers of seedlings on the plots range from 1570 to 9140 seedlings ha-~ (Table 2). Although the values in Table 2 indicate a positive correlation between standing volume and number of seedlings, such a conclusion should not be drawn from this material. The S- and G-plots are located in different parts of the country and have different site productivities. Furthermore, the results for plots $3 and $4 refer to a situation seven growing seasons earlier than for the other four plots. From studies conducted in Norway spruce stands regenerated with the shelterwood system, it is known that increased stand densities usually reduce the amount of spruce advance growth (Amilon, 1929; Cajander, 1934; Hagner, 1962a,b; Zybura, 1983). Nilsen (1988), however, found no relationship between stand density, expressed as basal area, and number of seedlings in uneven-aged sub-alpine Norway spruce stands in Norway that had been subjected to heavy selection cuttings. His results agree well with those of studies
REGENERATIONOF NORWAYSPRUCE
53
TABLE 2 Estimated number of seedlings in the regeneration surveys, and standing volume at the time of the surveys Plot
SI $2 $3 $4 G! G2
No. of seedlings (stems h a - ~) Mean
Standard error
1570 3070 7460 9140 6530 3490
242 546 1375 1512 1748 1046
Standing volume (m3ha -j )
i05 197 244 238 278 187
in high-graded stands in Finland (Sarvas, 1944), experimental plots managed with group selection in Norway (B~ihmer, 1957), selection stands in Austria (Kammerlander, 1978), and experimental plots treated with singletree selection in Finland (Valtanen, 1988). The spatial distribution of seedlings was not measured in the surveys, but the general impression ofthe surveys was that most seedlings were growing in the vicinity of larger trees and not in canopy openings. It is therefore interesting to note that the lowest number of seedlings were found on plots S 1 and $2, which have had very low levels of standing volume for a long time, and, in addition, have had gaps created on them. The annual height increment of seedlings is positively correlated with seedling height on all plots except GI (Fig. 1 ). Before the cutting in 1980, plot Gl had been unmanaged for several decades, and this can possibly ,~xplain the deviating results for this plot. Plot G2 has the highest value in all cla~ses, but there are small differences in absolute levels between the other plots. This indicates that standing volume has little influence on the height increment of seedlings in uneven-aged Norway spruce stands, but the weaknesses in the material do not permit such a conclusion. It should be noted, however, that plots $2-$4, which are situated close to each other and have similar site conditions, have similar height increment for the seedlings in spite of different levels of standing volume. The impression gained during the surveys was also that there is no obvious difference in height increment between seedlings growing close to trees and those growing in the open. The annual mortality rates, calculated with Eqn. (2), vary considerably, but are mainly below 10% (Table 3). Also, very few dead seedlings were observed during the regeneration surveys. It should be noted that the high mortality rates reported by Arnborg ( 1947 ) and Nilsen (1986) refer to the first few years after germination. The potential ingrowths past breast height calculated with Eqn. (5) are tel-
54
L LUNDQVIST "r.
Mean annual height increment (mm year - l )
i00 80
Us1
60
I~s3
Hs2 ~s4
40
I~IG1 I-I G2
20
0 0.25
0.55
0.85
1.15
Seedling height (m) Fig. 1. Mean annual height increment for seedlings in 0.3 m height classes. For plots $3 and $4 the value for each seedling is the mean annual increment during the last 3 years, measured on a subsample of seedlings. Seedling height refers to start height, i.e. exclusively the leading shoot. TABLE 3
Annual mortality rates for seedlings in 0.3 m height classes, calculated with Eqn. (2), and accumulated mortality from 0. i to 1.0 m height Plot
SI $2 $3 $4 GI G2
Annual mortality rate (%)
Accumulated mortality 0.1-1.0 m height (%)
0.1-0.39 m
0.4-0.69 m
0.7-0.99 m
-4.55 2.20 2.62 8.75 6.53 7.21
9.73 3.43 0.64 3.52 8.92 5.33
16.50 - 7.28 11.44 4.42 3.73 ! 1.21
71.3 ! 9.8 74.2 69. ! 86.7 79.2
atively high for all plots except plot S 1, and exceed the historical values for plots $2-$4 (Table 4). Whether these ingrowth rate.s will be sufficient to maintain the stand structure on the plots in the future ~epends on the number of trees that will be harvested and lost through mortality. To maintain these levels of ingrowth it is also necessary that a sufficient number of new seedlings grow past 10 cm height. If we assume 70% accumulated mortality from 0.1 to 1.0 m height (see Table 3), 5% annual mortality from 1.0 to 1.3 m height and a mean annual height increment of 50 mm in the height interval 1.0-1.29 m (see Fig. 1 ), fewer than 200 seedlings ha- ~ are required annually to grow past
REGENERATIONOF NORWAYSPRUCE
55
TABLE 4 Historical ingrowth, calculated with Eqn. (4) from plot records, and potential ingrowth, calculated with Fqn. (5) from regeneration survey data Plot
Period
Mean standing
Ingrowth (stems h a - ~year- ~)
volumein period (ms ha- ~)
Historical
-
Sl
-
$2
1959-79
143
49.0
84.2
$3
1972-82
227
37.2
51.3
$4 GI
1972-82 -
232 -
59.5 -
88.8 46.1
G2
-
-
49.1
-
-
Potential future 7.9
10 cm height, to maintain an annual ingrowth past breast height of 40 seedlings ha- t. The calculations of mortality rates and potential ingrowths are based on the assumption that the observed relationship between seedling height and seedling height increment is stable and can be regarded as a description of the dynamic development of the seedlings. The mortality calculations further require that the number of seedlings annually growing into the lowest height class is more or less constant. These assumptions should be fairly safe for plots $3, $4, GI and G2, as the stand density and the stand structure have changed very little during recent decades on these plots. On plots S1 and $2, however, there is negative mortality for one of the height classes. This can be explained by variations over time in the relationship between seedling height ana height increment, and in the number of new seedlings growing past 0.1 m height during recent decades. On both plots gaps have been created. Furthermore, on plot Sl the standing volume was very low (40-80 m 3 ha -I ) for several decades as a result of heavy cuttings, and it has only recently passed 100 m 3 h a - i. CONCLUSIONS
The results of this study show that it is possible to obtain abundant regeneration and high ingrowth rates in Norway spruce selection stands in Sweden, with high levels of standing volume and bilberry as the dominating ground vegetation. As the plots have been established and managed independently of each other, and without an explicit objective, the results do not permit very detailed conclusions. They do indicate, however, that regeneration and ingrowth into the tree layer may not be as serious a problem as Swedish foresters have traditionally assumed.
56
L. LUNDQVIST
ACKNOWLEDGEMENTS I t h a n k I n g v a r S v e n s s o n , OstersliSv, w h o g r a c i o u s l y g a v e m e access to all n e c e s s a r y d a t a o n his p r i v a t e s e l e c t i o n p l o t in G l i m f i k r a ( G 2 ) . T h e s t u d y w a s financed by the National Swedish Environmental Protection Board (Statens Naturvfirdsverk).
REFERENCES Amilon, J.A., 1929. HyggesskiStselnoch f'6ryngringen inom mossrika skogar tillhSrande Vacciniumtypen inom Orli revir. Norrlands Skogsvfirdsf'6rbunds Tidskr., 78-137. Arnborg, T., 1947. FOryngringsunders/Skningar i mellersta Norrland. Norrlands Skogsvfirdsf6rbunds Tidskr., 247-293. BOhmer, J.G, 1957. Bledningsskog II. Tidsskr. Skogbruk, 65: 203-247. Cajander, E.K., 1934. Kuusen taimistojen vapauttamisen j~ilkeisest[ipituuskasvusta. Commun. Inst. For. Fenn., 19.5. H~igglund, B. and Lundmark, J.-E., 1981. Handledning i bonitering med SkogshOgskolans boniteringssystem, Del 2 Diagram och tabeller. Skogsstyrelsen, JOnkOping, Sweden, 70 pp. HiiggstriSm, B., 1982. Om de biologiska f'6rutsiittningarna fOr skogens fOryngring i Norrlands hiijdl~igen. Sveriges Skogsv~rdsf6rb. Tidskr., No. 6: 25-33. Hagner, S., 1962a. Ett exempel pfi bestfindstiithetens betydelse for den naturliga f6ryngringens uppkomst och utveckling pfi god granmark i Skfine. Statens Skogsforskningsinstitut, Uppsatser, 85: 1-6. Hagner, S., 1962b. Naturlig f6ryngring under sk~irm. Medd. Statens Skogsforskningsinst., 52.4, 263 pp. Hesselman, H., 19 ! 7. Om vfira skogsf6ryngringsfitg/irders inverkan pfi salpeterbildningen i marken och dess betydelse f6r barrskogens f6ryngring. Medd. Statens Skogsf6rs6ksanstalt, 14: 923-1076. Hesselman, H., 1937. Om humust~ickets beroende av bestfindets alder och sammansiittning i den nordiska granskogdn av blfibiirsrik Vacciniumtyp och dess inverkan p~ skogens f6ryngring och tillviixt. Medd. Statens Skogsf6rs/Sksanstalt, 34:529-716. Holmgren, A., 1914. Bl~idning och trakthuggning i norrlandsskogar. Norrlands Skogsv~rdsf6rbunds Tidskr., 266-323. Kailin, K.E., 1926. F~ryngringsstudier i Norrlandsskogar. Svenska Skogsvfirdsf'Oren. Tidskr., 253-261. Kammerlander, H., 1978. Aufbau, Verjiingung und Verbissgef~ihrdung der Plenterwiiider im Raum Kufstein/Tirol. Schweiz. Z. Forstwes., 129:7 i 1-726. Lundberg, H., 1973. Skogsbrukets ekonomi och teknik. In: Svenska Naturskyddsf6reningens firsbok 1973. Sveriges Natur 64: 43-56. N~islund, M., 1940. Funktioner och tabeller f6r kubering av stfiende trgd - Tall, gran och bj~rk i norra Sverige. Medd. Statens Skogsf'6rsbksanstalt, 32: 87-i 42. N~islund, M., 1947. Funktioner och tabeiler f6r kubering av stfiende tr~id - Tall, gran och bj6rk i sOdra Sverige saint i hela lander. Medd. Statens Skogsforskningsinst., 36 ( 3 ): i - 8 1 . Nilsen, P., 1986. Tap av fro og spireplanter av gran (Picea abies (L.) Karst) etter sfiing lafi forskjellige vegetasjonstyper i t~ellskogog lavlandsskog. Medd. Norsk Inst., Skogforsk., 39 (8): 129-145.
REGENERATIONOF NORWAYSPRUCE
57
Nilsen, P., 1988. Fjeliskoghogst i granskog - gienvekst og produksjon etter tidligere hogster. Rapp. Norsk Inst. Skogforsk., 2/88: 1-26. Prodan, M., 1949. Die theoretische Bestimmung des Gleichgewichtszustandes im Plentero waldes. Schweiz. Z. Forstwes., 100: 81-99. Sarvas, R.. 1944. Tukkipuun harsintojen vaikutus etel~-Suomen yksityismetsiin. Commun. Inst. For. Fenn., 33.1,268 pp. S0derstr6m, V., 197 i. Va~6r hyggen? Ymer, 161- ! 75. Teikmanis, A., 1952. Om markvegetationens inflytande p~ uppkomsten av naturlig f'6ryngring i J~imtlands 6rtrika granskogar. Norrlands Skogsvfirdsf6rbunds Tidskr., 1-44. Teikmanis, A., 1954. N~gra studier 0vet de mossrika granskogarna i Norrland ocb deras f6ryngringsproblem. Norrlands Skogsv~rdsf6rbunds Tidskr., 307-434. Valtanen, J., 1988. Korkeiden maiden metsien uudistaminen oulun l~i~iniss~i.Folia For., 718, 41 PP. WeUander, P.O., 1938. J~imn~ldriga och olik~ldriga best~ndsformer. Norrlands Skogsvfirdsf6rbunds Tidskr., bilaga till 3:dje h~iftet. Zybura, H., 1983. Wplyw drzewostanu oslaniajacego na dynamike odnowien podokapowych swierka w drzewostanach z udzialem sosny i swierka w polnocno-wschodniej czesci Polski. Sylwan, No. 9,10: 41-52.