Biomass and mortmass of woody vegetation in metal-contaminated areas (Southern Urals, Russia)

Study Extent

The study was carried out in vicinities of the Karabash Copper Smelter (55.469 N, 60.209 E) located within the city of Karabash (90 km northwest of Chelyabinsk, Southern Urals). A total of 41 sampling plots were established in seven types of habitats: pine, birch, and floodplain forests, reed swamp, sparse birch stand, marshy meadow, and dump of household waste. All habitats were surveyed using six sampling plots, except for the reed swamp (five plots). The study was completed in July 2012.

Sampling

The survey of forest woody vegetation is the leading and only method of accounting for forest resources, making possible the quantitative and qualitative evaluation of forest health. During the survey, we identified the following forest elements: 1. the forest stand – living trees with a diameter at breast height (DBH) of more than 5 cm; 2. the subcanopy – all living trees with DBH of less than 5 cm and height of more than 10 cm; this forest element includes the undergrowth (trees, capable of forming the forest stand) and underwood (bushes, not capable of forming the forest stand); 3. the half-dead tree – the main trunk of which died, but branches with leaves emerge from the dormant buds preserved at the trunk. All detected trees were assigned to the forest stand; 4. the standing dead tree – dead but not fallen trees with a DBH no less than 5 cm and height more than 2 m; 5. the stump – lower part of the dead tree trunk, less than 2 m in height. The diameter was usually measured at the point of breaking of a tree trunk; 6. the – a fallen/hung tree trunk (or part of it), which is entirely within the sampling plot; 7. the fragment of a downed bole – a fallen tree trunk (or part of it), which is partially within the sampling plot (only the part within the plot was taken into account). For representatives of these forest elements, the diameters (at breast height or root collar level) were measured, and phytomass (for living trees) or mortmass (for coarse woody debris) were calculated. Diameter measurements were taken in two mutually perpendicular directions using a caliper (or a pole caliper for subcanopy), with the subsequent calculation of the arithmetic mean diameter. Phytomass and mortmass were calculated using our detailed characterization of the model trees, obtained in August 2010 outside the sampling plots under consideration and published earlier (Usoltsev et al., 2012). A detailed description of the methods for determining the aboveground phytomass of model trees is given below. The sample of model trees of each species was formed following the series of tree diameter distributions. The model trees were fallen in August when the foliage/needles of the current year were fully formed. After felling, the tree’s length was measured. First, the trunk was divided into ten sections. Then, in the middle of each section, measuring from the butt, discs were cut out made it possible to define the diameters of the trunk in the bark and without bark. These measurements were used to calculate the volumes of the tree’s wood and bark. Next, the bark was removed from the disks taken at relative heights of 20, 50, and 80% of the total trunk height, the wood and bark were weighed separately with an accuracy of 0.1 g, their volume was determined, and dried to constant weight in an oven at 110°C. Then the oven-dry mass of the wood and bark were used to calculate a wood/bark proportion and the oven-dry phytomass of the wood and bark of the entire tree trunk. The phytomass of tree crowns and their structural parts was determined after dividing the crowns into three sections of the same length since crowns are heterogeneous in the vertical direction regarding the age and thickness of branches, branch coverage, and qualitative composition of needles. After weighing each section of the crown (with an accuracy of 50 g), they were divided into leafy/needled and non-leafy/non-needled branches. Then, a sample (about 0.5 kg) was taken from each section’s leafy/needled part to establish the ratio of needles and skeletal parts. For this purpose, we separate needles from the branches and then separately weigh these components’ mass for each sample (with an accuracy of 1 g). The phytomass of needles and woody parts was determined according to the established ratios for each section and the entire crown. To determine the moisture content and oven-dry weight of needles and branches, samples were taken from each part of the crown and then immediately weighed with an accuracy of 0.01 g. Samples of branches were taken separately from leafy/needled and non-leafy/non-needled branches. The obtained values were used to calculate the oven-dry weight of the needles and tree branches. Weighed portions of needles and branches were dried to constant weight in thermostats at a temperature of 100–105 ºC. To determine the phytomass of subcanopy trees, they were subdivided into two groups. First, in height, trees less than 0.5 m were fractionated (divided into a trunk, branches, and foliage). After that, they were weighed, dried at 110°C to constant weight, and the aboveground oven-dry phytomass was determined. From trees more than 0.5 m in height, leafy/needled shoots were cut off with pruning shears, a sample of 100–500 g was taken and weighed; then leaves/needles were removed from the sample, and it was re-weighed. Then the leaves/needles and the rest of the sample were dried separately to constant weight, weighed again, and the oven-dry matter content in both fractions was calculated. Their values were used to determine the oven-dry weight of the crown of the entire plant. Together with non-leafy/non-needled shoots, the trunk mass was weighed in total, dried at 110°C to constant weight, summarized with the crown’s phytomass, and then determined the aboveground oven-dry phytomass.

Quality Control

Plant species identification was carried out mainly in the field; specimens with controversial species affiliation were photographed or herborized and identified later in a laboratory by specialists from the Institute of Plant and Animal Ecology of the Ural Branch of the Russian Academy of Sciences (IPAE UB RAS). In the following cases, occurrences were identified only to taxonomic ranks of high level; first, the forest elements related to coarse woody debris (downed boles, fragments of downed boles, and stumps). As a rule, these elements are destructed, and reliable determination of their taxonomic affiliation is difficult even down to the family level. Second, forest elements that were absent at the study sites and added to the dataset to record the very fact of their absence. In this case, we found it unnecessary to detail more than to the level of the type.

Method steps

1. Fieldwork. The sampling plots were tried to be founded so that the anthropogenic impact (recreation, grazing, felling, haymaking) was minimal. Three sampling plots with a size of 25×25 m were founded within each type of habitat, except for the reed swamp (two plots at the non-polluted area). When the habitat configuration did not allow to arrange a rectangular sampling plot, its size could vary within 471–691 m². Complete estimation of forest woody vegetation was performed at each site in both the subcanopy and forest stand. The trees in the forest stand were estimated throughout the entire sampling plot. Estimation of trees in the subcanopy was carried out at 1–10 subplots of 1–34 m2 selected within the sampling plot, depending on the biotope. The distance between the sampling plots in the birch, pine, and floodplain forests and the sparse birch stand was 15–40 m. Therefore, subplots in these habitats were arranged randomly: the site was divided into equal squares 5×5 m (1×1 m for the sparse birch stand), each square was assigned a number. Then, three cards (except for sparse birch stand (10 cards) and marshy meadow (5 cards)) with a number were pulled out of the pocket, and the corresponding subplots within the sampling plot were approved. The reed swamp and marshy meadow were of limited size: in these habitats, the distance between sampling plots was several meters; in some cases, plots were adjacent to each other. The location of subplots within the sampling plot was uniform. For the waste dump, only one subplot within each sampling plot was founded at places where aboveground vegetation was detected. In the forest stand, a caliper with a scale up to 1 cm was used to measure the trunk diameters. Tree specimens with a diameter at breast height (1.3 m) equal to 5 cm or more were taken into account. Diameter measurements were taken in two mutually perpendicular directions, with the subsequent calculation of the arithmetic mean diameter. The diameters of the standing dead trees and half-dead trees were measured in the same way. The stump diameters were measured using a caliper at the base of the stumps in two mutually perpendicular directions. Stumps with a base diameter of more than 5 cm were taken into account. Next, the diameter of the subcanopy trees was measured at the level of the root collar using a pole caliper with an accuracy of 0.1 cm. Finally, the downed boles and fragment of downed boles were measured using a caliper: the base and the top diameters (cm) and its length (m) were measured.
2. Biomass calculation. Based on the data of measurements of the forest elements, we calculate the volume of downed bole (V_dbole): V_dbole = 1/3 × π × H(R^2_base + R_base × R_top + R^2_top) where H – length of downed bole, m; R_base – base radius of downed bole, m; R_top – top radius of downed bole, m. The volume (m3) of each downed bole that falls within the sampling plot (a whole or a fragment) was calculated. To calculate the aboveground phytomass of trees of the forest stand, subcanopy, and standing dead trees in each habitat type, we used our data obtained and published earlier, which represent a detailed characterization of the model trees (Usoltsev et al., 2012). A detailed description of the methods for determining the aboveground phytomass of model trees is given in the Sampling section. These data served as the basis for constructing regression equations and subsequent evaluation of the aboveground phytomass of each tree specimen in each habitat type. For tree species of the forest stand, subcanopy, and standing dead trees, the calculation of the parameters of the regression equations were performed by the formula: P_phytomass = a1 × D^a2 where P_phytomass – oven-dry aboveground phytomass of the plant, kg; D – diameter of the trunk, cm (for forest stand and standing dead trees measured at the height of 1.3 m, for subcanopy measured at root collar level); a1 and a2 – constants of the equation. This type of regression is considered to be the most biologically determined (Kofman, 1986). Equation constants are selected by the nonlinear estimation method (Levenberg-Macwardt algorithm, calculated in STATISTICA v.8.0); the formulas for different tree species are presented below. Tree species as part of the forest stand: P=0.098613D^2.53413 (Betula sp.); P=0.12004D^2.38449 (Pinus sylvestris); P=0.321051D^2.04809 (Picea obovata); P=0.14492D^2.30065 (Populus tremula); P=0.065389D^2.4907 (Alnus incana); P=0.050513D^2.69275 (Sorbus aucuparia). Tree species as part of the subcanopy: P=0.040593D^2.17829 (Chamaecytisus ruthenicus); P=0.025749D^3.28474 (Sorbus aucuparia); P=0.01467D^2.00523 (Rubus idaeus); P=0.060674D^2.78288 (Rosa majalis); P=0.017878D^2.92874 (Pinus sylvestris); P=0.023576D^3.15147 (Betula sp.); P=0.03445D^3.08644 (Populus tremula); P=0.017657D^3.27133 (Alnus incana); P=0.052977D^2.40134 (Abies sibirica); P=0.065709D^1.90958 (Prunus padus); P=0.041403D^2.73328 (Cotoneaster lucidus); P=0.033553D^2.67063 (Salix caprea). Tree species as standing dead trees of the forest stand: P=0.12954D^2.39547 (Betula sp.); P=0.130432D^2.31176 (Pinus sylvestris); P=0.068865*D^2.47813 (Alnus incana). The coefficients of determination (R^2) for all equations vary within 0.71–0.99. All constants are statistically significant (p<0.05), except for Rubus idaeus (p<0.08). Due to restrictions from the forestry, the following tree species were prohibited for felling: in the forest stand – Acer negundo, Alnus incana, Crataegus sanguinea, Larix sibirica, Malus baccata, Populus tremula, Prunus padus, all species of Salix and Ulmus, and all half-dead trees; in the subcanopy – Acer negundo, Larix sibirica, all species of Salix (except for Salix caprea), and Viburnum opulus. Biomass for Alnus incana (including the mortmass of downed bole of this species) and Populus tremula was estimated using literature data (Usoltsev et al., 2018). The mortmass of the downed bole (P_mort) was calculated based on the volume estimations: P_mort = V_mort × p_mort where P_mort – oven-dry mortmass, kg; V_mort – the volume of downed bole, m3; p_mort – average density of moderately decomposed downed bole, kg/m3. The data on downed bole densities were taken as constants from literature source (Klimchenko et al., 2011) for dead pine (constant for downed bole of coniferous trees, 0.307 kg/m3) and birch (constant for downed bole of deciduous trees, 0.428 kg/m3), as these species predominate in the territory of our study.

General taxonomic coverage is 1 phylum, 2 classes, 7 orders, 8 families, 19 genera, 23 species of woody vegetation.

1. Magnoliopsida
   rank: class
2. Dipsacales
   rank: order
3. Adoxaceae
   rank: family
4. Fabales
   rank: order
5. Fabaceae
   rank: family
6. Fagales
   rank: order
7. Betulaceae
   rank: family
8. Malpighiales
   rank: order
9. Salicaceae
   rank: family
10. Rosales
    rank: order
11. Rosaceae
    rank: family
12. Ulmaceae
    rank: family
13. Sapindales
    rank: order
14. Sapindaceae
    rank: family
15. Pinopsida
    rank: class
16. Pinales
    rank: order
17. Pinaceae
    rank: family

The studied areas are located in the southern taiga subzone of the Southern Urals, in the vicinity of Karabash (polluted sites) and 20 km south of Karabash (sites with a background level of pollution). The same set of habitats represents both polluted and non-polluted areas: pine, birch, and floodplain forests, reed swamp, sparse birch stand, marshy meadow, and dump of household waste.