Abstract
Botanical gardens, as one of the most important urban forests to any region, play an important role in the ecology of human habitats in many ways (e.g., air filtering). The National Botanical Garden of Iran, with an area of 145 ha, includes various woody species with a predominance of oaks (Quercus spp.). The size of acorns, fruits of oaks, varies in different species, which can affect their biomass. The biomass and carbon content of acorn components (endocarp, pericarp, and cupule) of four native oak species (Q. castaneifolia C.A. Mey., Q. libani Oliv., Q. infectoria Oliv., and Q. brantii Lindl.) and one exotic oak species (Q. ilex L.), planted in the National Botanical Garden, were studied to obtain detailed comparative results. Regarding the biomass of acorn components, Q. libani and Q. ilex showed the highest and lowest values among the study species, respectively. The ranges of carbon content of acorn components were 53.5% (pericarp of Q. brantii) to 58% (cupule of Q. castaneifolia). These results confirm the variation of biomass and carbon content of acorn components among the oak species studied.
Urban forests are ecosystems characterized by the presence of trees and other vegetation in association with human developments (Miller 1997; Dwyer et al. 2000). There are many positive incentives or rationales for having urban forest ecosystems within cities, including environmental, social, and economic values. From an ecological function point of view, urban forests play an important role in many diverse aspects: reducing carbon dioxide, reducing air temperature, increasing air humidity, reducing wind speed, absorbing air pollutants, reducing noise levels, and providing shelter to animals and recreational areas for people (Streiling and Matzarakis 2003). The National Botanical Garden of Iran, as one of the most important urban forests in Tehran, Iran, is a valuable collection of woody and herbal species. This garden plays an important role in the ecology of human habitats.
Carbon taken up by the forest canopy is allocated to tree organs for biomass production and respiration (Campioli et al. 2010). On the other hand, carbon allocation within a plant depends on complex rules linking carbon source organs (mainly leaves) and carbon sink organs (mainly the sapwood of stems, branches and roots, and fruits) (Génard et al. 2008). Hence, biomass-related studies have become important for studying ecosystem productivity and carbon budgets (Ryu et al. 2004; Thomas and Malczewski 2007; Saatchi et al. 2011).
Aboveground biomass consists of the biomass of all living vegetation above the soil. The carbon stored in the aboveground living biomass of trees is typically the largest pool. Thus, estimating aboveground forest biomass is the most critical step in quantifying carbon stocks.
Carbon (C) allocation among plant processes (e.g., respiration, biomass production) and organs (e.g., leaves, reproductive organs, and stem) is a key process in the C cycle because it determines the residence time and location of C in the ecosystem (Campioli et al. 2008). For example, C allocated to structural biomass of organs with high turnover and decomposition rate, such as deciduous leaves, returns to the atmosphere within few months to years, whereas C allocated to organs with lower turnover and decomposition rate, such as stem wood, returns to the atmosphere only after decades or centuries (Campioli et al. 2008).
In comparison to other components of forest ecosystems, the biomass of fruits is considered to be small (Mälkönen 1974; Muukkonen 2006) and is sometimes dismissed as negligible. However, the biomass of fruits may play an important role in many ecosystem processes, such as in the nutrient and carbon cycle, due to rapid turnover rate (Muukkonen 2006) at the biomass level.
Members of the genus Quercus L. (Fagaceae), including evergreen and deciduous shrubs and trees, have a wide geographical range, occupying vast territories of the Northern Hemisphere in North America, Europe, and Asia (Denk and Grimm 2010; Johnson et al. 2010). Quercus is the most common genus of Fagaceae in forests of Iran (Sabeti 1994). Several species of oaks grow abundantly in the Zagros, Arasbaran, and Hyrcanian forests (Sagheb Talebi et al. 2014). During four past decades, native oak species of Iran as well as some exotic oak species, have been planted in the National Botanical Garden of Iran. Today, oak species are one of the most important elements of the garden, distributed throughout the garden.
Fruits of oaks, acorns, are usually associated with an involucre forming a cup around the mature fruit. Acorns are hard, one-seeded, and dry. Due to the woody structure, acorns have a considerable role in biomass production and carbon stock in oakdominated forests. Acorn production varies from year to year and among species (Lashley et al. 2009; Koenig and Knops 2014).
The biomass and carbon content of acorns may differ because of high variation in morphology among oak species. Some researchers have mentioned the variation in acorn biomass of oak species (e.g., Callahan et al. 2008; Steen et al. 2009; Sánchez-Humanes et al. 2011), but the evidence on acorn carbon content is limited (e.g., Sun et al. 2012). On the other hand, only two reports on acorn biomass (Panahi et al. 2009; Iranmanesh et al. 2013) and one report on carbon stock (Iranmanesh et al. 2013) of native oak species of Iran have been published. The present study gives an account on the mature acorn biomass and carbon stock of five oak species, cultivated in the National Botanical Garden of Iran.
The aim of this study was to determine the site-specific acorn biomass, carbon stock, and carbon content of the target species, and to examine if they are significantly different.
MATERIALS AND METHODS
Study Area
The study was conducted at the National Botanical Garden of Iran (35°41′N, 51°19′E) in 2016. The garden was founded in 1968. An area of 145 ha was allocated to the garden in Tehran at an altitude of 1,320 m (Figure 1). The area is flat and slopes gently to the south. The climate is dry with an average annual precipitation of 257 mm, falling between November and May. Temperature reaches as much as 42°C–43°C during July and August. During winter, the temperature may fall to −10°C or lower. The garden consists of some native and exotic collections with different areas. The Hyrcanian and Zagros collections, as symbols of the Hyrcanian and Zagros forests of Iran, are the most important forest collections of the garden. Most of the oak trees are found in these collections. The other individual oak trees are distributed throughout the garden with more density in the Systematic collection.
Map of the National Botanical Garden of Iran (Tehran, Iran), including native, exotic, and subjective collections.
Study Species
Five oak species were chosen for this study as follows. Four deciduous native oaks of Iran, including chestnut-leaved oak (Q. castaneifolia), Lebanon oak (Q. libani), Aleppo oak (Q. infectoria), and Brant’s oak (Q. brantii), as well as one evergreen exotic oak species (Q. ilex). All sample oak trees have been planted since 1988. The chestnut-leaved oak is a light-demanding tree and is one of the most productive, valuable, and precious species in the Hyrcanian forests of Iran. Other three native oaks naturally grow in Zagros forests in the west of Iran (Sagheb Talebi et al. 2014). In contrast to the native oaks of Iran studied, Q. ilex is an evergreen oak, native to the Mediterranean region. Quercus ilex is planted in a number of garden collections, such as the systematic collection.
Sampling Procedure
After a garden survey, 15 sample trees were randomly selected for each species. The selected trees were mature and in dominant crown positions. Ten mature and undamaged acorns were selected at random for each sample tree (totally, 150 acorns per species) from different aspects of the crowns or from freshly fallen acorns without regard to acorn size. Samples were placed in plastic bags, and transported to the Forest Research Division laboratory at Research Institute of Forests and Rangelands of Iran. The laboratory is located in the vicinity of the garden.
Laboratory Measurements
Owing to the fact that the C content of acorn components is different, the measurement was done separately. Acorns were divided into three components, including endocarp, pericarp, and cupule (Olson 1974; Figure 2) and labeled. The fresh weight of different components of the acorns was separately weighed by a digital scale with an accuracy of 0.001 g. Then, different components of the acorns were dried in an oven at 80°C to obtain the constant dry weight (Ketterings et al. 2001; Losi et al. 2003). The carbon content of dry biomass samples was obtained by combustion method (Allen et al. 1986).
Acorn components of the oak species studied (left column: pericarps; center: endocarps; right: cupules). Species from top to bottom: Q. castaneifolia, Q. libani, Q. infectoria, Q. brantii, and Q. ilex).
The normality distribution of variables was assessed by test of Shapiro-Wilk (P < 0.01), and one-way ANOVA with Duncan test was used to compare the mean values of variables at the 0.05 probability level.
RESULTS
The normality of data was tested and confirmed. Descriptive statistics of fresh and dry mass of acorn components are given in Table 1. Based on one-way ANOVA analysis, all variables in three acorn components showed significant differences (endocarp fresh mass: df = 4, P < 0.000, F = 649.527; pericarp fresh mass: df = 4, P < 0.000, F = 331.074; cupule fresh mass: df = 4, P < 0.000, F = 381.429; endocarp dry mass: df = 4, P < 0.000, F = 509.614; pericarp dry mass: df = 4, P < 0.000, F = 544.949; cupule dry mass: df = 4, P < 0.000, F = 406.649). In Q. libani, the mean values of fresh and dry mass of all acorn components were considerably more than those of the others, so that it was categorized in a separate group. Quercus ilex was on the opposite side of Q. libani. Other species had an intermediate situation.
Descriptive statistics (mean ± SD and range) of acorn components fresh and dry mass. Significant differences in mean fresh and dry mass are denoted by different letters among species.
The percent of carbon content was calculated for three acorn components (Table. 2). The carbon content of endocarp ranged between 53.8% in Q. libani to 57.7% in Q. ilex. Also, the carbon content of pericarp ranged between 53.5% in Q. brantii to 57.8% in Q. infectoria, and for cupule it ranged between 54.2% in Q. brantii to 57.9% in Q. ilex.
Regarding the carbon content of acorn components, Q. libani and Q. ilex had the highest and lowest mean values, respectively (Figure 3).
Carbon content (g) of acorn components. Significant differences in mean carbon content are denoted by different letters among species.
DISCUSSION
This research focused on acorn biomass and carbon content of different oak species, which usually receives less attention. In other words, researchers considered one of the main ecological roles of botanical gardens as one of the most important urban forests. The National Botanical Garden of Iran occupies a vast area in one of the most pollutant cities of the world. In this situation, the ecological role of gardens is more valuable. The garden includes a lot of trees with different sizes and ages, and oaks are the dominant species. This healthy collection is normally considered as a big source for absorbing carbon dioxide from the atmosphere.
Since the forest trees produce seeds with different sizes, their role in carbon sequestration differ significantly. Among tree species, some of them produce large seeds, which could be seen in conifers and some broadleaved species such as oaks. Regarding the biomass production, acorns have two advantages in comparison with other seeds: large size and woody components. Besides, an oak tree produces considerable amount of acorns, especially in mast years. The number of acorns produced per tree ranges from zero to more than 7,000 (Christisen and Kearby 1984; Sork 1993; Pulido 1999; Pourhashemi et al. 2011). In a given area, the number of acorns produced per year may range from zero to more than 650,000/ha, (Auchmoody et al. 1993; Healy et al. 1999; Guariguata and Sáenz 2002). Some researchers have mentioned that the acorn biomass could reach more than 15 kg per tree (Panahi et al. 2009; García-Mozo et al. 2012) and 800 kg/ha in bumper and good years (Christisen and Kearby 1984; Drake 1991).
Regarding the biomass of acorn components, Q. libani had the highest values among the study species (Table 1). Acorns of Q. libani are bigger than the others (Djavanchir Khoie 1967; Sabeti 1994; Figure 2). On the other hand, biomass of acorn components was in the lowest values in Q. ilex because of the small size of the acorns (de Rigo and Caudullo 2016).
Most researchers estimate carbon by assuming the carbon content of dry biomass to be a constant 50% by weight (e.g., Montagnini and Porras 1998; Das et al. 2016). However, other authors have used a carbon concentration of 45% by weight (Whittaker and Likens 1973). Limited research (e.g., Cañellas et al. 2008; Li et al. 2015) has used other rates. Occasionally, carbon is measured directly by combustion method or a carbon analyzer (e.g., Kraenzel et al. 2003; Iranmanesh et al. 2013). In the current study, researchers calculated the carbon content of acorns directly by the combustion method, which is a very accurate method. Based on the results, carbon stock (percent) was different among the study species and acorn components (Table 2). The maximum and minimum mean values of carbon content were observed in the pericarp of Q. infectoria (57.7%) and the cupule of Q. libani (55.5%), respectively. In addition, the ranges of this variable were 53.5% (pericarp of Q. brantii) to 58% (cupule of Q. castaneifolia) among acorn components of the study species. In a same research in Zagros forests of Iran, the acorn biomass of the seed-originated Q. brantii trees was six times more than that of coppice ones, and the carbon content was 40% of its dry weight (Iranmanesh et al. 2013). The mean value of carbon content of 21 oak (Q. variabilis) populations in eastern China was calculated to be 43.6% (Sun et al. 2012) as well.
Carbon content (mean ± SD and range) of acorn components.
Different values of carbon content have been reported for the fruits of other species (e.g., Geiger et al. 1989; Ogawa and Takano 1997; Campioli et al. 2010; Priyadi et al. 2014; Li et al. 2015) or other organs of oak species (e.g., Thomas and Malczewski 2007), but it is difficult to compare the results with those of previous studies because of the differences in studied species or organs. This study focuses exclusively on carbon content related to acorns.
These results present a detailed comparative analysis of biomass and carbon content of acorn components among five oak species. Based on the grouping of the variables, a clear differentiation was observed in the study characteristics among the species studied, especially in Q. libani and Q. ilex. Finally, the study authors propose to repeat the same research in other sites and on other oak species to complete the results.
Acknowledgments
The authors wish to thank the authorities of Research Institute of Forests and Rangelands for providing the facilities for this research. We also thank the authorities of Forest Laboratory, Research Institute of Forests and Rangelands. Thanks are due to Dr. Jamzad, Dr. Sadeqzade, Mr. Vahdatian, and Mr. Azarsa for their collaboration.
- © 2018, International Society of Arboriculture. All rights reserved.
LITERATURE CITED
Jump to section
Related Articles
Cited By...
- No citing articles found.