DENDROMETRIC AND DENDROCLIMATOLOGICAL ANALYSES OF BLACK LOCUST

Edward Feliksik¹, Stanisław Orzeł², Sławomir Wilczyński^1
1 Department of Forest Protection and Forest Climatology, Agricultural University of Cracow, Poland
² Department of Forest Mensuration, Agricultural University of Cracow, Poland

ABSTRACT

The study was carried out in the 78-year-old black locust (Robinia pseudoacacia L.) stand growing in the Krosno Odrzańskie Forest District in western Poland. Its aims were to evaluate biometric characteristics of trees and to analyze climatic factors which condition radial increment of wood. On the basis of dendroclimatic analyses it was found that frosty winters and low temperatures in early spring months (January – April) as well as low precipitation during summer, especially in June and July, were among important factors limiting diameter increment of black locust trees. Also low precipitation in autumn of the previous year and low precipitation of February before the current growing season had a negative effect on diameter increment of this species. In spite of these limitations a dendrometric estimation of the stand showed that its characteristics exceeded values given in available volume tables [24] which are a deterministic model of growth of a pure even-aged black locust stand.

Key words: Robinia pseudoacacia, dendroclimatology, dendroecology, tree-ring, d.b.h., b.h..

INTRODUCTION

Black locust is a native tree of North America, with the Appalachian Mountains as the central area of its natural range. The second compact area of its occurrence, separated from the first one by the Mississippi valley, includes southern Missouri, eastern Oklahoma, and the Ouachita Mountains. Outside these two main ranges, the separate black locust populations are scattered over southeastern States. On eastern slopes of the Appalachian Mountains black locust grows up to 1040 m in elevation, while in the Great Smoky Mountains it may be found even at 1620 m [17].

Widely planted and naturalized black locust now occurs in almost the entire area of the United States as well as in southern Canada, Asia, and Europe.

Black locust grows on sites much diversified in respect of moisture and fertility, which is largely due to symbiotic bacteria assimilating free nitrogen from the atmosphere. However, it prefers fertile and moist calcareous soils, deep and permeable. Black locust is very tolerant as far as climatic conditions are concerned. Climate in its natural range is quite diversified. It is characterized by a high annual precipitation ranging from 1000 mm to 1800 mm. The mean temperature of January in some regions is -4°C while in other it reaches 7°C. In August, the warmest month, it ranges from 18°C to 27°C. The period without frosts lasts for 210 days in lowlands and 150 days in mountains.

Black locust was introduced to Europe in the 17^th century, and since that time it has spread widely, especially in the southern part of the continent. Because of a wide spectrum of site requirements black locust is being planted on waste lands, burns, and in lands degraded by industry and mining. As an ornamental tree with abundant inflorescence it is planted in parks, gardens, and along roads. It is also a valuable honey-bearing species [7]. In forestry black locust is used in plantations as a fast growing species and also as a nurse species. In some countries it has been introduced to forest stands as one of the main species [1]. In Poland it is growing mainly in gardens, parks and other wooded areas outside the forest, while in forest stands it occurs sporadically in small groups, mainly in the western part of the country.

In Poland the productive capacity of black locust is little recognized, and also the effect of climatic factors on its growth is little documented. For these reasons a study was carried out in a pure black locust stand of a high productive capacity [19] growing in the Krosno Odrzańskie Forest District. The objectives of the study were to analyze the effect of thermal and pluvial conditions on radial increment of black locust trees, and to estimate the value and structure of their basic biometric characteristics, and also to determine whether black locust is worth to be introduced to forest plantations on a wider scale.

MATERIAL AND METHODS

There was a pure 78-year-old black locust stand, 3.18 ha in area, growing in compartment 232i of the Osieczna forest range in the Krosno Odrzańskie Forest District (Phot. 1). It was growing on a fresh mixed broadleaf forest site (dry variant) with a brown podzolic soil formed from post-glacial sands, and built of coarse sand over loose sand. There were numerous black locust sprouts, black elder, blackberry, raspberry, and sporadic black cherry in the underbrush. The forest floor vegetation was composed of nettle, celandine, reed grass, and soft grass.

The Krosno Odrzańskie Forest District is located in the Great Poland-Pomerania natural forest region [18]. In respect of climate this area belongs to one of the warmest regions in Poland – Lower Silesia [25] (Fig. 1). The mean annual temperature there is 8.5 °C. However, in winter, especially in February, the mean monthly temperature frequently drops below -10 °C, while minimum temperatures are sometimes below this value. The total annual precipitation reaches 600 mm. Precipitation of summer months prevail with maximum in July. But in any month the precipitation deficit causing a long-term drought may occur (Fig. 1).

Dendrometric measurements were carried out in six 5-are circular plots systematically distributed in the black locust stand. Their centers were located in nodes of the network of squares. All trees growing in plots were measured. The following measurements were taken: d.b.h. at two perpendicular directions (N-S and W-E), exact to 0.1 cm; height using Vertex hypsometer, exact to 0.1m; bark thickness at b.h., twice, on northern and alternately eastern or western side of the stem, exact to 0.1 cm. All trees were cored to determine d.b.h. increment. Tree-ring widths for the last 5-year period (2001–2005) were measured using the BIOTRONIC BEPD-5 increment gauge in the increment laboratory of the Department of Forest Mensuration, Agricultural University of Cracow.

On the basis of measurements the parameters of distribution of selected biometric characteristics of black locust trees and characteristics of the stand were determined. Statistical analyses were carried out according to procedures of STATISTICA software system [21]. Volume of individual trees was computed according to the formula based on data from volume tables for standing trees [9]. From among different function forms [25] the following formula of a general form was chosen:

v = d^a · h^b · exp(c)

where: d – d.b.h. in cm, h – height in m, a, b, c – function parameters.

Values of parameters of the selected function for analyzed data were: a=1.84849; b=0.92634; and c = –9.43388, while the explained variance was as much as 99.95% (R² = 0.9995).

The elaboration of the formula was necessary in order to determine volume of individuals of greater dimensions. Maximum d.b.h. (36 cm) and height (26 m) values in the tables [9] were considerably smaller than the measured ones (43.0 cm and 35.3 m respectively), and there were no other model solutions for determination of tree volume.

Thirty dominant trees without any disease symptoms and injuries were selected for dendroclimatic investigations. Trees were cored 130 cm above the ground level using the Pressler’s increment borer. On the cores, the widths of annual wood increments (tree-ring widths) were measured, thus obtaining 30 tree-ring series i.e. chronologic series of data. The accuracy of tree-ring dating was verified using the computer program COFECHA [14] and the analysis of series convergence [16]. The tree-ring series were submitted to the process of indexing and autoregressive modeling using the program ARSTAN [8]. This removed trends, long-term fluctuations, and autocorrelation from tree-ring series. While the direction of changes in tree-ring size from year to year, mainly determined by meteorological conditions, was retained [12, 20] next step was to average, on the basis of actual and residual series, the values of tree-ring width during succeeding years. Thus the tree-ring chronology and residual chronology were worked out.

In searching for factors affecting the every year variation of tree-ring widths of black locust the principal components analysis [14] correlation function and convergence function [16, 10] were used. In a direct estimation of climate-tree growth relationships the method of the correlation and response function was used according to the program RESPO [15], and in addition the bootstrapped method for a single interval (1946–2005) and the variant with moving intervals [13, 4, 5] were used. The moving intervals essentially iterate the bootstrapped correlation and response functions for different time periods. Each bootstrapped estimate is obtained by generating 1000 samples selected at random with replacement, then running numerical computations on each sample. Computations involve linear correlation, Jacobean rotations for eigenvalues, singular value decomposition and solutions of linear systems accompanied by principle component regression. For each interval, the median coefficients estimated from the 1000 bootstrap samples are reported in the output, and are plotted only if they are significantly different from 0 at the 0.05 probability level. For this purpose the computer program DENDROCLIM2002 was used [5]. In these analyses the increment indexes of residual chronology were the predictands, while the mean monthly air temperatures and mean monthly total precipitation for the period 1945–2006 were the predictors. In computations the period from July of the previous year to September of the current year was taken into account.

The meteorological data were obtained from the near meteorological station of the Institute of Meteorology and Water Management in Zielona Góra.

RESULTS

There were 323 black locust trees growing in the 78-year-old stand analyzed in this study. Their d.b.h. ranged from 18.5 cm to 43.0 cm (mean 32.5 cm) (Table 1).

The variation of this characteristic described by the coefficient of variation amounted to 14.8%, and its distribution with slightly left skewness (–0.25) and positive kurtosis (0.32) at the level of α=0.05 did not differ significantly from the function of normal distribution density (Fig. 2).

The 5-year diameter increment was twice as variable as d.b.h. During the period 2001–2005 its mean value was 1.65 (0.60–2.97) cm, and its distribution did not differ significantly from normal distribution.

On the other hand the height distribution was significantly different from normal distribution. Its skewness was left-sided (coefficient of skewness was – 0.44) and its density was smaller (kurtosis was – 0.45) than the density of Gauss’s distribution. The height ranged from 17.7 m to 35.3 m (mean 27.4 m), and height variation did not differ from that of d.b.h. Black locust is a tree species producing thick bark. The mean bark thickness at 1.3 m above the ground level was 5.0 cm and it constituted 15.6% of the d.b.h. on the average. The bark thickness at b.h. varied from 3.1 cm (11.1%) to 7.1 cm (22.8%), and it was more variable than d.b.h. (coefficient of variation was 16.4%, while its distribution did not differ from normal distribution at the assumed significance level of 5%. The bark percentage in basal area of individual trees varied from 21.0% to 40.3%, assuming the mean value of 28.8%.

Generally the black locust trees were quite slender. In the case of 67% of them the coefficient of slenderness exceeded the value of 0.8 commonly considered to be a threshold value determined for trees (especially confers) accepted as stable ones.

The volume of large timber of individual trees varied from 0.2240 m³ to 2.0185 m³, and its mean value slightly exceeded 1.0 m³. The 5-year volume increment was on the average 0.1464 m³, and its dynamics described by the Pressler’s method [6] was 3.1%.

The volume of the black locust stand analyzed during this study was over 330m³ per hectare, and its average annual increment was about 9.5 m³ per hectare (Table 2).

Generally, values of stand characteristics presented in Table 2 exceeded values given in volume tables worked out for black locust by Erteld [24]. In general, black locust trees developed straight-grown stems (Phot. 2).

The series of tree-ring widths of black locust were highly similar to one another (Fig. 3). In some years almost all trees responded with the same decrease or increase of increment in relation to the previous year. This was the case in 1947, 1953, 1954, 1961, 1963, 1969, 1984, 1992, 1993, 2002, and 2003. This showed a high homogeneity of growth responses of the investigated trees to environmental factors during those years. In the remaining years this homogeneity was not as high (Fig. 4 – black dots).

The tree-ring width during the period from 1946 to 2006 varied from 1.10 mm to 5.05 mm, with mean width of 2.24 mm. The tree-ring chronology was characterized by a high autocorrelation of the first order (0.634) and the mean sensitivity equal to 0.159. After indexing the residual chronology had the mean index equal to 1.00, a lower autocorrelation equal to -0.145, and a higher sensitivity of 0.191 (Fig. 4).

As a result of the principal components analysis of indexed tree-ring series three principal components were separated (PC1, PC2, PC3). The first one explained 43.2% of total indexed tree-ring series variation, the second 7.9%, and the third 6.2%. Thus the factor described by the first principal component had the strongest effect on variation of radial increments of black locust trees, while the role of factors described by the second and third principal components was much smaller.

The analysis of values of factorial loadings showed that the first principal component integrated the indexed tree-ring series and it was strongly positively correlated with them. Correlations of the remaining two principal components with variables were considerably weaker. However they divided the indexed tree-ring series into two subsets (Fig. 5).

It was assumed that thermal and pluvial conditions, changing during successive years, were the determinants affecting radial increment of trees. Therefore, in order to identify the climatic elements influencing variation of radial increment of black locust the values of respective principal components were compared with mean values of air temperature and total precipitation in different seasons of the year. It was found that values of the first principal component were most strongly correlated with mean air temperature of the period February – April (r = 0.461, P < 0.01), and showed a highest convergence with this climatic element (GL = 79.6%, P < 0.01) (Fig. 6). Elements conditioning the wood increment and described by the second and third principal components were not identified during this study. Probably they were associated with other non-climatic causes.

The results of the analysis of the correlation and response function concerning tree increments during the period 1946–2002 positively verified the results obtained by the principal components analysis (Fig. 7). It was found that temperatures of winter and early spring months (January – April) before the growing season were the basic factors affecting variation of the annual radial increment of black locust. Besides, too high temperatures of autumn and early winter months (September, November and December) had a negative effect on the next year wood increment. While a high precipitation in September and February had a significantly positive effect (Fig. 7).

The analysis of climate-wood increment relationships for moving intervals confirmed a significantly positive effect of high temperatures of late winter and early spring (February – April), and also of precipitation of September and February (Fig. 8).

At the same time it indicated that in some years the precipitation deficit in June and July had a negative effect on increment of black locust. Moreover precipitation at the end of the previous growing season, i.e. from August to October, had a positive effect on the current year diameter increment. Precipitation in February before the growing was a substantial supplement to soil moisture required by trees (Fig. 8). Its importance was clearly shown by the analysis of the response function (Fig. 7).

DISCUSSION

In Poland black locust is a typical tree species grown in wooded areas outside the forest. Seldom pure dense stands of this species may be found. For these reasons publications concerning its productivity are very scarce in forestry literature. Therefore the results of this study, in spite of the fact that they concern only a single stand, are of significant importance for knowledge on productive capacity of black locust in western Poland. Values of dendrometric characteristics of the analyzed stand must be considered to be impressive. In general, they exceed the values given in accessible volume tables elaborated for this species by Erteld [24] and being a deterministic model of a pure even-aged black locust stand. In respect of productivity the investigated stand does not differ from stands mainly composed of native tree species growing in lowlands of Poland [23]. Its volume (333.1 m³·ha-¹) as well as increment (9.47m³·ha^-1·year^-1) are not the highest found so far for black locust. For example volume of over-ground biomass of the 49-year- old stand analyzed by Benčat’ [3] was as much as 538.0 m³·ha^-1, while its increment was 10.98 m³·ha^-1·year^-1. In the same paper volume of the 29-year-old stand determined by Benčat’ was 304.8 m³·ha^-1, and increment 10.51 m³·ha^-1·year-1. In another paper (1987) this author presented the 27-year-old black locust stand with volume of 287.1 m³·ha^-1 and increment of 10.6 m³·ha^-1·year^-1. The values stated above represent the total over-ground biomass of stands including leaves, branches and fructifications. They should be reduced by about 15% and then it would be possible to compare them with volume of the 78-year-old black locust stand analyzed in this paper, and for which only volume of large timber was determined.

The analysis of every year changes in tree-ring width and the history of their expressive characteristics permit to recognize the nature and dynamics of relationships occurring between climatic conditions and metabolism of trees [11, 27]. The results of Schmitt et al. [22] on the process of formation of annual diameter increment of European beech and black locust growing in temperate climate of western Europe showed that the highest activity of vascular cambium in black locust occurs at the beginning of the growing season At that time the production of xylem cells rapidly increases, and after reaching its maximum it decreases to cease in mid-September.

The outlined strategy well justifies the results of analyses of climatic factors conditioning the radial wood increment in black locust. As it seems, the thermo-pluvial conditions of the second half of the previous year as well as of late winter and early spring of the current year prepare black locust trees to put in motion the metabolic processes resulting in a dynamic increment of wood. This is favored by rainy and cool autumn, mild winter, February rich in precipitation, and warm early spring. However, it should be pointed out that under conditions of the Great Poland - Pomerania natural forest region frequently a low summer precipitation, especially in June and July, is the factor limiting the radial increment of black locust wood.

CONCLUSIONS

1. The rainy and cool autumn, mild winter, abundant precipitation in February, and warm early spring favor the diameter wood increment of black locust in western Poland. While drought occurring in summer months may be a factor limiting this increment.

2. Biological properties of black locust indicate that under a quite uniform climate in Poland this tree species may form pure stands (plantations) which are not much different from stands of native tree species in respect of productivity. Under proper management of black locust stands the trees may form straight-grown stems thus producing useful building and fuel wood. Besides this species may form a valuable admixture, biocenotic as well as productive, in stands composed of many species.

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Accepted for print: 20.03.2007

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