Allometric equations relating trees’ vascular system and other stem metrics with foliage area and mass are important to estimate their growth, carbon stocks and interactions with abiotic environment in terms of carbon and water balance. In this study we focused on
In a broad sense, allometry describes how the characteristics of living organisms change with size (
In addition, allometric models for estimating canopy leaf area are vital, given the direct connection of leaf area to photosynthesis, transpiration and respiration and, thus, to the productivity and water balance of forest stands (
Black locust is a hardwood tree species, native to North America, which appeared in Europe in the beginning of the 17th century where is considered as alien and invasive (
Within the present study we aimed to establish and evaluate different relationships of tree foliage area and dry mass with xylem, sapwood and other stem metrics in
The study took place at the restoration plantations of
The climatic conditions are quite similar at both sites, according to the HPPC’s meteorological stations in Ptolemaida (40° 28′ 03.8″ N, 21° 44′ 48.7″ E; distance from the plantations 800-7000 m) and Amyntaio (40° 36′ 23.1″ N, 21° 39′ 00.9″ E; distance from the plantations 1200-5000 m). The mean annual air temperature is 13 °C, while mean annual precipitation is 521 mm in Ptolemaida and 528 mm in Amyntaio. According to the ombrothermic diagrams of both sites, a xerothermic period is observed from July to August (see Fig. S1 in Supplementary material).
Given the similar characteristics of the two restoration plantations of HPPC (climate, vegetation, planting history, substrate, tree height and DBH distribution), black locust trees from both sites were destructively sampled (10 from Amyntaio and 15 from Ptolemaida, considering the larger area of the latter) and pooled together. In total 25
For each selected tree, measurements of DBH (with a diameter measuring tape) and height/length (with Spencer loggers tape) were performed. After harvest, the determined biometrical traits comprised diameter at stump height (at 0.3 m height), length and diameter at the middle of the stem (mid-bole), the base of the live crown, at 1/3 and 2/3 of the crown. Stem disks, about 10 cm thick, were sampled at all above-mentioned stem sections for the estimation of cross-sectional areas (see below). The fresh weight of the whole crown, separated into thirds was determined in the field using an electronic balance with 100 g accuracy. Dead and epicormic branches were weighed separately from the live crown. The whole crown was taken to the laboratory, foliage was separated from branches and pods and all parts’ dry weight was determined after they were oven-dried at 80 °C until constant weight was reached. In order to estimate foliage leaf area, 30 leaves from the upper, middle and lower crown (90 leaves in total) were collected in the field, put in plastic bags in a cool box and transferred to the lab for leaf area and dry weight determination. Based on the collected leaves’ ratio of dry weight to leaf area, the respective area of foliage at the three parts of the crown, as well as of the total crown, were calculated.
The collected leaves and stem discs were scanned (Epson 11000XL®, resolution 200 dpi) to calculate the leaf area (LA), the area of sapwood (SAPA), current sapwood (CSA) and total cross-sectional area (STA) of the stem. The current sapwood area was defined as the cross-sectional area of the penultimate growth ring and the early wood portion of the current year’s growth ring, according to
For all variables, the Levene test was used to confirm the homogeneity of variances and the suitability of parametric tests to identify the homogeneous groups between the different height sections along the stem profile. An ANOVA was performed to check for statistically significant differences between the height sections along the stem profile for each of the independent variables (sapwood area, current sapwood area, total cross-sectional area of the stem). Tukey’s HSD test was used to investigate homogeneous groups. Simple linear regression was applied to identify the relationships between either leaf area (LA) or foliage dry weight (FDW) and the independent variables. Multiple linear regression analyses were also run to test whether the inclusion of additional variables, such as age at the breast height, total tree height, crown length (CrLen) and crown ratio (CR) in the linear models increased the accuracy of estimation of LA and FDW. Statistical analysis was conducted with R programming language (
Mean age of the harvested trees was 14 years, ranging from 4 to 25 years, reflecting the tree age variation of the restoration plantations of HPPC. Mean height and DBH of the trees were 11.93 m (range: 6.10-17.80) and 11.33 cm (range: 4.30-22.10), respectively. The distribution of the DBH and height classes of the felled trees is shown in Fig. S2 (Supplementary material).
The mean sapwood area, current sapwood area and total cross-sectional area were 30 cm2 (10.28-81.64), 9.07 cm2 (2.71-24.20) and 83.81 cm2 (10.28-272.68), respectively. The analysis of the crown showed that mean LA was 25.20 m2 (7.8-101) and mean FDW was 1.87 Kg (0.6-8.1). The average CrLen was 5.15 m (3.35-8.10), while the mean CR was 0.45 (0.30-0.76).
The multiple linear regression analyses for the estimation of foliage area and mass, based on the combination of several independent factors, resulted in models of lower accuracy than the simple linear regression ones (data not shown). Thus, we focused on simple linear regression analyses.
The regression models predicting LA and FDW from the different stem cross-sectional areas along the tree profile are presented in Tab. S2 (Supplementary material). Among these, the strongest models, having the higher R2 coefficients and smaller MSE values, are depicted in
LA/STA differed significantly among the different section heights across the tree profile (F = 7.39, p<0.001) and it presented a constant decreasing pattern from the crown to stump height (
The square of DBH was the best estimator of LA having the highest R2 (0.72) and the lowest SE (11.7 -
The estimation of forest trees’ leaf area is important for the assessment of their productivity and interaction with the environment, as foliage is actively involved in tree and ecosystem carbon and water balance (
We detected significant relationships between foliage area and sapwood area at breast height (Tab. S2 in Supplementary material), in line with literature. Foliage area has been commonly expressed as a function of sapwood area at breast height for several broadleaf forest trees, like cherrybark oak and green ash (
Total cross-sectional area was strongly related to leaf area and dry mass also at the stump height (
Following total cross-sectional area, current sapwood at stump height was also a reliable indicator of foliage area and dry mass (
The well-established relationships between sapwood area and crown dimensions were the least strong in our study, compared to total stem cross-sectional area and current sapwood area. Several studies reviewed by
The Pipe Model Theory predicts an increasing sapwood area downwards following the increase in foliage biomass/area towards the crown base and, subsequently, a constant sapwood area between crown and stem base (
Apart from the reliability of stem cross-sectional areas for the estimation of foliage mass and area, we also evaluated the easily determined DBH as a predictor of the leaf area of black locust. A strong regression model was built to predict leaf area based on DBH (
Another important relationship between stem and crown dimensions is the one connecting DBH and diameter at the base of the live crown. According to the Pipe Model Theory (
The estimation of trees’ foliage area and biomass is laborious, but crucial to assess forest ecosystems productivity and potential for climate change mitigation. A range of stem metrics have been evaluated for their strength in the prediction of foliage area and dry mass in black locust restoration plantations in Greece. Contrary to the often-used sapwood area, the total cross-sectional area and current sapwood area produced more accurate models for the estimation of these foliage traits. Diameter at breast height was also proven to be a reliable predictor of leaf area and diameter at the base of the live crown. The calibrated allometric models are species-specific and their replicability may be restricted by the particular edaphic conditions of the former lignite mines where the restoration plantations are established. In addition, the accuracy of the functions would benefit from a larger sample size which, on the other hand, is rather time and effort demanding. Future research should focus on the evaluation of the calibrated allometric relationships to other black locust plantations which are commonly established for the rehabilitation of regions that are heavily degraded by anthropogenic activities.
PMT: Pipe Model Theory; LA: Foliage Leaf Area (m2); FDW: Foliage Dry Weight (Kg); SAPA: Sapwood cross-sectional area (cm2); CSA: Current sapwood cross-sectional area (cm2); STA: Total stem (sapwood and heartwood) cross-sectional area (cm2); LA/SAPA: ratio of LA to SAPA; LA/CSA: ratio of LA to CSA; LA/STA: ratio of LA to STA; DBH: Diameter at breast height; Dhlc: Diameter at the base of the live crown; CrLen: Crown length; CR: Crown ratio.
We kindly acknowledge the Hellenic Public Power Corporation (HPPC) and, in particular Melina Andreadi, Marina Tentsoglidou, Aris Azas, Christos Papadopoulos and their teams, for the support with personnel and equipment during the field campaigns. We are also thankful to Konstantinos Tyraides for his assistance in field work and Georgios Xanthopoulos for contributing to lab processing of collected plant material.
GS and KR conceived and designed the study; SRT performed field and laboratory measurements; SRT and GS performed the statistical analyses; SRT, GS, MF, KR were involved in original draft preparation; MF, GS and SRT conducted review and editing; KR is responsible for funding acquisition and project administration.
This research was performed within the COFORMIT project “Contribution of the tree planted land of West Macedonia lignite center to protection of environment and to mitigation of climate change” (T1EDK-02521), which was financially supported by the Single RTDI state Aid Action Research - Create - Innovate funded by the Operational Program Competitiveness, Entrepreneurship and Innovation 2014-2020 (EPAnEK).
Location of the study area (lignite center of the Hellenic Public Power Corporation) in NW Greece, comprising the two black locust restoration plantations in Amyntaio and Ptolemaida (inner plots).
Tree diameter at breast height (DBH - a) and height (b) distribution of the black locust plantations inventory for Amyntaio (left), Ptolemaida (center) and both sites together (right). Data are obtained from 214 inventory plots. The dashed blue lines indicate the mean DBH and height of the trees.
Linear relationships between leaf area (LA) with STA at mid bole (A), CSA at stump height (B) and SAPA at stump height (C). The regression models, their coefficients (adjusted R2) and standard errors (RMSE) are presented (n=25).
Linear relationships between foliage dry weight (FDW) and STA at mid bole (A), CSA at stump height (B) and SAPA at 1/3 of crown height (C). The regression models, their coefficients (R2) and standard errors (RMSE) are presented (n=25).
Ratios of LA to SAPA (A), to CSA (B) and to STA (C) at different height sections of stem and crown. The points and bars represent the mean ± standard error (n=25). Two means are significantly different (p<0.05) when they share no common letter.
Linear relationships between (A) LA and DBH² and (B) Dhlc2 and DBH² (n=25).
Tab. S1 - Substrate characteristics at the two restoration plantations.
Tab. S2 - Characteristics of simple linear regression models of LA and FDW to SAPA, CSA and STA.
Tab. S3 - Characteristics of simple linear regression models of LA and FDW with DBH2 and Dhlc2, as well as of Dhlc2 with DBH2.
Fig. S1 - Ombrothermic diagrams of the two restoration plantations for the period 2008-2018.
Fig. S2 - Distribution of the diameter at breast height (DBH) and height classes of the 25 felled black locust trees.