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

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166 THE LICHENOLOGIST Vol.24 factor for epiphytes. Barkman (1958) divided the whole forest stand vertically into a number <strong>of</strong> microclimatic zones through which trees grow and which govern the composition <strong>of</strong> the epiphytic vegetation in different parts <strong>of</strong> the tree, while Kershaw (1964) indicated that the distribution <strong>of</strong> epiphytes on young trees has the same kind <strong>of</strong> zonation as on older ones but in a more compressed form. Yarranton (1972) observed that the epiphytic species on the black spruce {Picea mariana (Mill.) BSP) fall into four main distributional types: (1) crown and tree base; (2) base only; (3) lower part <strong>of</strong> crown; (4) branchless bole and base. Generally the structure <strong>of</strong> the branches and the shape <strong>of</strong> the crown have a considerable influence on the distribution <strong>of</strong> epiphytic lichens. The aim <strong>of</strong> the present work was to determine the major environmental factors influencing the distribution <strong>of</strong> epiphytic lichen vegetation on the trunks <strong>of</strong> the two commonest conifers in the northern European boreal zone: the Scots pine (Pinus sylvestris L.) and Norway spruce {Picea abies (L.) Karst.). Special attention was paid to differences between' near natural' and' managed' forests and to changes attributable to ageing <strong>of</strong> the forests. Materials and Methods Area studied The research was carried out at Muhos in the central part <strong>of</strong> Finland, about 30 km SE <strong>of</strong> Oulu and the same distance from the coast <strong>of</strong> the Gulf <strong>of</strong> Bothnia (64°45'N, 26°00'E, Fig. 1). The area is located in the middle boreal subzone <strong>of</strong> Finland (see Ahti et al. 1968), where Scots pine dominates the drier heath forests and Norway spruce is the main tree species in the mesic and moist forest types. According to statistics supplied by the Meteorological Institute <strong>of</strong> Finland, the mean annual temperature in Muhos for the decade 1980-1989 was 1 -7°C (range —0- TC in 1985 to 40°C in 1989) and mean annual precipitation 549 mm, <strong>of</strong> which 311 mm fell during the growing season (for higher plants) in May-September. The following climatic parameters are also considered important for the ecology <strong>of</strong> epiphytic lichens: duration <strong>of</strong> the growing season with mean daily temperatures exceeding 5°C (147 days), annual number <strong>of</strong> days with precipitation >01 mm (160 days), and mean maximum snow depth in forests (60 cm). The climate <strong>of</strong> the area is oceanic in terms <strong>of</strong> temperature, but less so regarding humidity conditions. Sample plots The 45 sample plots (see Table 1) make up four age series and were classified as open, managed (i.e. thinned effectively) pine and spruce stands, and denser, near natural state pine and spruce stands. The pine-dominated sites were mainly <strong>of</strong> the Empetrum-V'actinium type (EVT), two <strong>of</strong> them belonging to the Empetrum-Calluna type (ECT) and the spruce sites to the Vaccinium- Myrtillus type (VMT) or the intermediate type <strong>of</strong> EVT and VMT in the Finnish classification (Kalela 1961). The main criterion for choice was sufficient homogeneity <strong>of</strong> the stand and its surroundings (exposure, etc.). Furthermore, the managed stands selected were required to have remained undisturbed for a period assessed to be sufficient for the epiphytic lichens to respond to the microclimatic conditions <strong>of</strong> the environment (about 15-20 years). The following variables were determined for each site: forest site type, canopy cover (scale: 1=0- 10%;2 = ll-20%;. .. 10 = 91-100°,,), basal area(m 2 ha ')<strong>of</strong> the trees (with a relascope, Bitterlich 1948), age <strong>of</strong> trees (with an increment borer), average height <strong>of</strong> trees (with a hypsometer) and the diameter <strong>of</strong> the sample trees. The acidity <strong>of</strong> the bark at breast height on the trunks was analysed by a method in which powdered bark was suspended in distilled water (1/10 v/v) for 24h, filtered and measured with a pH meter. The pH value used was the average <strong>of</strong> the trees studied at each plot. Five Scots pine trunks <strong>of</strong> medium diameter or three trunks <strong>of</strong> Norway spruce were examined at each plot. The more heterogeneous epiphytic vegetation <strong>of</strong> pine was considered to need a bigger sample intensity than spruce to reduce sample variance between the plots. The percentage cover <strong>of</strong> the lichen species around the trunk was estimated on a five-pointscale (1 = < 3",,; 2 = 3-10° o ; 3 = 11-25%; 4 = 26-50%; 5= >50%) separately in two height classes, 0-0-7 m and 0-7-20 m. The
1992 Epiphytic lichens—Hyvdrinen et al. 167 26° N 64 °45' KILOMETRES FIG. 1. Map <strong>of</strong> the area studied. The pine stands are indicated by dots and the spruce stands by filled squares. medians <strong>of</strong> the cover values for the various species on the trunks were used as values for the whole plot, except in the case <strong>of</strong> a species occurring with the lowest degree <strong>of</strong> cover on less than half <strong>of</strong> the trunks, when it was given a cover value <strong>of</strong> 0-5. The following cover percentages corresponding to the estimates obtained in the fieldwere used for the ordination: 0-5 = 0-5; 1 = 1-5; 2 = 7; 3= 18; 4 = 38; 5 = 76. Species occurring on only one sample plot were omitted from the data matrix. The cover <strong>of</strong> all the macrolichens {Cladonia at genus level only) were determined and also <strong>of</strong> those microlichens that can be easily identified in the field. Consequently, some crustose groups and related species, e.g. the order Caliciales, are treated in the ordination as if they were one species. The species identifications (e.g. for the genus Usnea) were complemented with chemical methods such as TLC in the laboratory (Culberson & Kristinsson 1970). Data analysis The modified data were analysed by canonical correspondence analysis (CCA), a technique for direct gradient analysis proposed by ter Braak (1986,1987), which allows one to test whether all the species simultaneously are linearly related to the environmental variables supplied. Consequently, the direct use <strong>of</strong> environmental data makes it possible to combine the ordination and direct gradient
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