Effects of Stand Structure on Stem and Crown Biomass
Ana Cristina Goncalves*
Department of Rural Engineering, Evora University, Portugal
Submission: January 02, 2018; Published: May 14, 2018
*Corresponding author: Ana Cristina Gonsalves, Department of Rural Engineering, School of Sciences and Technology, Institute of Agrarian and Environmental Environmental Sciences (ICAAM), Institute of Advanced Research and Training, University of Evora, Apartado 94, 7002-544 Evora, Portugal, Tel: +351266760800; Fax: +351266760911, E-mail: acag@uevora.pt
How to cite this article: Gonçalves A C. Effects of Stand Structure on Stem and Crown Biomass. JOJ Hortic Arboric. 2018; 1(3): 555565.
Abstract
Above ground biomass can be divided in two broad classes, according to its potential use: stem biomass used mainly for timber; and crown biomass used either for bio energy or to maintain and improve ecosystem services, in particular stand and site sustainability and fertility. The aim of this study is to evaluate the differences and similarities of stem and crown biomass in forest stands as function of species, composition (pure vs mixed), structure (even-aged vs multiaged) and the stage of development (young vs adult). The analysis was carried out for eight species in 255 plots, in Portugal. The results revealed stem and crown biomass proportion depends on the tree species ecological characteristics, especially if trees are in free growth. Stem biomass proportion tends to be higher in stands managed for timber regardless composition or structure while crown biomass is higher in stands managed as agro forestry systems.
Keywords: Composition; Stage of development; Species; Statistical analysis
Abbreviations: Qr: Quercus Rotundifolia; Qs: Quercus Suber; Pp: Pinus Pinea; Ppi: Pinus Pinaster; Cs: Castanea Sativa; Qru: Quercus Rubra; Bc: Betula Celtiberica; PE: Pure Even-Aged; PM: Pure Multi Aged; ME: Mixed Even-Aged; MM: Mixed Multi Aged; QRS: Quercus Suber; SP: Mixed of Pinus Pinea; SPP: Pinus Pinaster; PCR: Quercus Robur; RRB: Betula Celtiberica
Introduction
From late 201,2] the direct and the indirect. The indirect method, the most frequently used, is mathematical functions, with one or more dendrometric variables at tree level (diameter at breast height and total height) as explanatory variables. The functions are species and site-specific, due to the development behaviour of each species, which is also related with the site quality. This results in a wide number of functions [3-6]. Most of the allometric biomass functions per species are developed per component. While functions are always developed for stem, in which regards the other components some are developed for branches or leaves, whereas others aggregate the latter components in a class, the crown. Above-ground biomass is the sum of the biomass of all components. Thus due to their formulation it is possible to divide it in two broad classes; stem and crown. These classes can be related to of each component utilisation; stem for timber and crown either for bioenergy or to remain in the stand to maintain and/or improve the stand and site productivity. The maintenance of biomass in the forest stand, especially the crown components, is suggested in the frame of sustainability of the site, stand and productivity [7], especially in the poorer sites, due to the amounts of nutrients in the crown [8]. Inversely stems are poor in nutrients thus their removal has less impact on the overall productivity and sustainability of the system [9].
Stand structure dynamics are determined by the interactions between the individual trees in a stand, which are also result of composition (pure 10]. As a result, above ground biomass as well as the stem and crown biomass are influenced by the aforementioned tree and stand characteristics.
The goal of this study is the evaluation of the effect of species and stand structure on the partitioning of above-ground biomass in two broad classes, stem and crown. The specific objectives are the analysis of species ecological characteristics, stand composition (pure vs mixed), structure (even-aged vs multiaged) and stage of development (young vs adult) on the proportion of stem and crown biomass.
Material and Methods
A set of 255 plots representative of the Portuguese forest area were selected (Table 1), to enable the characterisation of stem and crown biomass of different species and stand structures. In the field surveys, tree species were recorded and diameter at breast height, total height and height of the beginning of the crown were measured for all individuals with diameter at breast height larger than 5 cm [11]. Plots' composition (pure 12], and structure (even-aged vs multiaged) with diameter distributions using 2.5cm classes. Two stages of development were considered, evaluated visually in the field, young, pure even-aged plots (24 plots) and adult (231 plots). Many plots included young and adult trees, though in a small number in the even-aged plots and larger in the multiaged plots.
Stem and crown biomass were calculated with the allometric functions at tree level of Paulo and Tomé for 13] for 14] for Table 2) . Other species than the aforementioned with a very small number of individuals were not considered in this analysis, as the inclusion could originate bias in the analysis [15]
The stem and crown biomass was analysed considering species, structure and composition classes to enable further detail in the analysis. The species include
Results and Discussion
The stem and crown biomass proportion show a wide variability. It is derived, at least partially, from the differences between species (Table 3), decreasing in general from species managed as agro forestry systems (Qr, Qs and Pp) to those managed for timber (Cs, Qro, Qru, Be). This is probably related with the spatial arrangement of the trees. In the agro forestry systems stands have usually low density and trees are frequently isolated. However, the horizontal spatial distribution can be rather irregular with some trees isolated and others in clusters, which derives in a larger variability when compared with stands managed for timber where the horizontal spatial arrangement tends to be more regular. The proportion of stem biomass for adult trees is in average 64%, varying between 17% and 94%, with a coefficient of variation (CV) of 26%, while young trees is 63%, varying between 31% and 91%, with CV of 23%. The crown biomass for adult and young individuals has an average of 36% and 37%, CV of 47% and 38%, and ranges between 6% and 83%, and 9 and 70%, respectively. Noteworthy is the large variability of Table 3).
For all species crown biomass proportion when compared with stem biomass has a larger coefficient of variation (Table 3). Crown dimensions and consequently biomass are related to the species genetic characteristics, stage of development, epinastic control and shade tolerance. Different species have a wide range of crown shapes; 16,17]. The photo assimilates in young trees are primarily allocated to height growth while in adult trees are used predominantly for diameter growth, both stem and crown. Nevertheless, when available growing space is limiting, individual trees are not able to express their genetic characteristics due to the influence of the constellations of neighbours. Another determinant feature is the shade tolerance. Except for 18-20], consequently stand structure and aerial growing space, determine their lateral development. Also, shading results in the death of the inferior part of the crown, especially in the shade intolerant species, reducing their length. The differences between species are corroborated by the significant differences among species by Wilcoxon test Figure 1.
The analysis per composition, structure and development stage classes of the plots show wider range for crown than for stem biomass proportion (Table 4). In general, variability of biomass increases from pure to mixed plots and from even-aged to multiaged plots, except for stem biomass that has a larger CV for pure than for mixed plots, reflecting the difference in the stage of development of the plots. When discriminating plots per structure classes (Figure 2a,c) variability increases from even-aged (PE, ME) to multiaged plots (PM, MM), explained by the number of species and their proportions per cohort [21]. PE plots dispersion is due to the stage of development of the plots.
Further details can be attained when plots are analysed per composition classes. In general, stem biomass increases from agro forestry systems (QR, QS, PP, QRS, SP, SPP) to stands managed for timber (PPi, PCR, RRB), while the inverse is observed for crown biomass (Table 5 and Figure 2b,c). This is mainly due to stand management options. Agro forestry systems focus their management on bark and/or fruit production, thus promoting stem and crown diameter growth and trees with relatively short stems, frequently forked [22]. Contrariwise, in stands managed for timber the management focus is attaining a high straight stems, free of branches that optimise timber volume. The structure indices such as hd ratio, linear crown ratio and crown ratio [23,24] can bring some insights towards the understanding of the differences between the two stand types. Stands managed for timber when compared with agro forestry systems have higher hd ratio and crown ratio and lower linear crown ratio [25], in general resulting in a lower proportion of crown biomass in the former. These differences are corroborated by the significant differences of stem and crown biomass between the stands managed as agro forestry (QR, QS, PP, QRS, SP, SPP) and those managed for timber (PPi, PCR, RRB) 24-31].
Conclusion
Forest trees ecological characteristics determine the proportion of stem and crown biomass, particularly if they are in free growth. However, stand structure plays a key role on the above-ground biomass proportions. Composition, structure, stage of development and spatial arrangement, both horizontal and vertical, encompass a suite of interactions between individuals in a stand resulting in a wide range of variation of stem and crown biomass proportions per stand.
1n general, crown biomass proportion is larger for species with wider crowns, weak epinastic control, shade tolerant and in free growth. At stand level crown biomass is larger for stands managed as agro forestry systems, especially those in the adult stage of development, while for stands managed for timber stem biomass proportion is larger. Overall stem biomass increases from pure to mixed, from even-aged to multiaged and from young to adult stands, while for crown biomass the opposite is observed.
Acknowledgment
The work was financed by National Funds through FCT- Foundation for Science and Technology under the Project UID/ AGR/00115/2013.
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