Influence of Stand Age And Structure on the Epiphytic Lichen ...

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168 THE LICHENOLOGIST Vol. 24 TABLE 1. Characteristics <strong>of</strong> the sample plots Pine stands Spruce stands Variable Managed (» = 12) Near natural (71=13) Managed (71=10) Near natural (71=10) <strong>Age</strong> <strong>of</strong> trees (years) (range) Canopy cover (%) (range) Average height (ra) (range) Bark pH Basal area (m 2 ha~') 67 + 27-1* (20-110) 58+16-4 (40-80) 13-7 + 41 (7-19) 3-73 + 010 25-3 + 6-9 91 + 36-9 (35-150) 73 + 8-5 (50-80) 15-5 + 4-2 (6-20) 3-75 + 0-13 28-9 + 5-9 75 + 33-7 (30-130) 65+14-3 (50-90) 16-1+5-5 (11-19) 3-48 + 014 23-1+8-3 103 + 41-4 (40-170) 80 + 9-9 (70-100) 15-3 + 5-7 (6-22) 3-36 + 0-17 23-8 + 9-3 *The figures are averages with standard deviations for the plots in each group. analysis, and to observe the overall influence <strong>of</strong> the environment by approximating the fitting Gaussian response surfaces to the species (ter Braak 1986). In the present work certain environmental variables (Table 1) were chosen a priori according to their assumed importance as factors influencing the epiphytic lichen vegetation. The multiple correlations between environmental variables (i.e. multicollinearity) were controlled by a Variance Inflation Factor given by CCA [VIF = (1—R 2 ,)" 1 , where R ; = multiple correlation between explanatory variable j and other variables in the analysis]. Large values <strong>of</strong> VIF (>20) indicate near linear dependences amongst variables and instability <strong>of</strong> canonical coefficients (for details see Montgomery & Peck 1982; ter Braak 1988). The regression coefficients for the environmental variables were tested by means <strong>of</strong> Student's r-test, for which purpose the data matrix was also analysed by reciprocal averaging (Hill 1973), in which the regression is calculated after extraction <strong>of</strong> the species and sample scores and not within the iterative ordination algorithm as in CCA (ter Braak 1988). The direct use <strong>of</strong> Student's r-test with canonical coefficients is not recommended because they have larger variances than regression coefficients. The CANOCO computer programme produces ordination plots in which the environmental variables can be combined with species or study sites (ter Braak 1988). Continuous variables (e.g. tree age) are shown as arrows, the length <strong>of</strong> which indicates the correlation between the variable and the ordination axes, and the direction <strong>of</strong> which points to the maximum change in the variable. Nominal variables (e.g. forest site type) are presented in terms <strong>of</strong> centroids showing the weighted average for the species or sites belonging to that class in relation to the ordination axes. Results Sample plots The ages <strong>of</strong> the trees in the stands varied from 20 to 170 years (Table 1). The age gradient thus included stages from saplings to old forests. The variation in stand age is likely to obscure much <strong>of</strong> the variation in other factors measured between the pine and spruce stands, and to some extent between the classes ' managed ' and ' near natural'. The basal area was high in young stands (the mean for the pine stands under 70 years was 29-9, n = 8), for example, even if they had been subject to management. Canopy cover shows better the idea <strong>of</strong> the original division into classes. The pH <strong>of</strong> the bark, which fitted into a normal distribution (Wilk-Shapiro test for pH <strong>of</strong> pines W=0-953, P>0-31 and spruces H^=0-936, P>0-22), varied to some extent between plots, but the difference between the Scots pines (x = 3-74, S.D. 011) and Norway spruces
1992 Epiphytic lichens—Hyvdrinen et al. 169 TABLE 2. Pearson correlation coefficients for the environmental variables measured in pine and spruce stands Variable <strong>Age</strong> <strong>of</strong> trees Canopy cover Average height BarkpH Basal area Spruce stands <strong>Age</strong> <strong>of</strong> trees (years) Canopy cover (° 0 ) Average height (m) Bark pH Basal area (ra'ha" 1 ) -0042 0-805 -0-426 -0-333 ^-0-114 -0-203 0062 0-683 0-601 -0-286 -0-315 -0173 -0-773 -0050 -0-463 0-265 -0076 0-534 0130 -0-208 Pine stands (x = 3-42, S.D. 017) as a whole was rather small, albeit significant (ANOVA, F, 44 = 61-099, P< < 0-001). Furthermore, the pH <strong>of</strong> the bark <strong>of</strong> the spruces showed a close negative correlation with stand age (r= —0-773, F 219 = 25-741, P= 0001) while that <strong>of</strong> the bark <strong>of</strong> the pines did so to a lesser extent (r = — 0-426, F 424 = 5095, P = 0034, see Table 2). Accordingly, the environment <strong>of</strong> the sample plots is heterogeneous within the framework <strong>of</strong> a few forest types. Distribution <strong>of</strong> the species A total <strong>of</strong> 58 lichen species was found on the bark <strong>of</strong> the conifers, 48 <strong>of</strong> them occurring on pine and 38 on spruce (Table 3). The macrolichen vegetation on the trunks <strong>of</strong> the pines and spruces was relatively uniform, while the crustose lichens were found to be more phorophyte-specific, since ten out <strong>of</strong> the 13 pinespecific species (excluding Cladonia species) and nine out <strong>of</strong> the ten sprucespecific species were crustose. This is partly a consequence <strong>of</strong> the relatively low frequency <strong>of</strong> microlichens present. Foliose and fruticose species were more frequent than crustose species on both phorophytes, although some <strong>of</strong> them (e.g. Cladonia and Cladina) represent epigeic lichens generally occurring only on the bases <strong>of</strong> trunks. Some <strong>of</strong> the species are rare in the area and are more likely to occur on pines, for example Alectoria sarmentosa, a common species on both conifers in eastern Finland (Halonen et al. 1991). Species completely absent from spruce were Evernia mesomorpha, Calicium parvum, Haematomma elatinum, Hypocenomyce scalaris, and some species <strong>of</strong> the genus Lecanora, whereas E. prunastri and the Caliciaceae, except for C. glaucellum and C. parvum, were exclusive to spruce. The macrolichens tended to occur more densely on pines. The frequencies <strong>of</strong> occurrence on boles were used to analyse the vertical distribution <strong>of</strong> species on different parts <strong>of</strong> the tree bole by testing for chisquare goodness <strong>of</strong> fit. The null hypothesis was equal rate <strong>of</strong> occurrence on both parts <strong>of</strong> the trunk, although the upper trunk was 1 1 times larger than the lower, assuming that the tree has a conical shape. The shape <strong>of</strong> the lowest part <strong>of</strong> base, however, equalizes the areas perceptibly. Hypogymnia tubulosa was more frequent on the upper trunk <strong>of</strong> pine (x 2 = 4-45, P
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