First attempt to measure rainfall canopy interception ...

Int. J. Water, Vol. X, No. Y, xxxx

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First attempt to measure rainfall canopy interception loss, throughfall, and stemflow in Juglans regia Linn and Cup. Sempervirens L. Var. fastigiata in the north of Iran Mohammad S. Lazerjan Laurier Institute for Water Sciences, Cold Region Research Center and Department of Geography and Environmental Studies, Wilfrid Laurier University, 75 University Avenue West, Waterloo, Ontario, N2L 3C5 Canada E-mail: [email protected] Abstract: Each individual tree significantly alters the growth of surrounding vegetation by partitioning of rainfall and nutrients to the rooting zone. The Iranian Hyrcanian forests are among one of the most fragile ecosystems of the country due to large industrial activities for logging and extensive urbanisation. These may accelerate disturbing of this unique type of forest. Of concern currently is an increase in plantations of Juglans regia Linn and a Cup. Sempervirens L. Var. fastigiata species, which has taken place over the past few decades for agro-forestry and ornamental purposes. As such it has the potential to affect the growth of associated vegetation by altering the hydrological balance within this region. The results indicate that walnut trees are at the range of interception of temperate forest trees whereas cypress trees have a very high amount of interception. Thus, care should be taken in planting cypress trees over a vast area. Keywords: cypress; Hyrcanian forest; interception; rainfall; stemflow; throughfall; walnut; Iran. Reference to this paper should be made as follows: Lazerjan, M.S. (xxxx) ‘First attempt to measure rainfall canopy interception loss, throughfall, and stemflow in Juglans regia Linn and Cup. Sempervirens L. Var. fastigiata in the north of Iran’, Int. J. Water, Vol. X, No. Y, pp.000–000. Biographical notes: Mohammad S. Lazerjan is an MSc student in Ecohydrology in the Department of Geography at Wilfrid Laurier University. He works on the ecohydrological processes in the Western Boreal Forest on both natural and reclaimed landscapes. His research interests include ecosystem functioning, nutrient cycling, measuring interception, evapotranspiration, leaf area index and isotope hydrology. He also works on evaluation of reclamation process for success in Fort MC Murray, Alberta, Canada. He has experience to work on both temperate and boreal forests.

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Introduction

Temperate forested areas cover approximately 4 billion hectares of land surface of the world, of which 75% are located in the northern hemisphere (FAO-UN, 2005). Temperate forests in North America, Europe and East Asia are very well documented

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(Walter, 1984; Archibold, 1995). However, there is little information available on the temperate deciduous forests growing under the same climates in the Middle East and central Asia in English (Salehi et al., 2007). The Hyrcanian ecozone is a green belt stretching over the northern slopes of Alborz Mountain Range and covers the southern coasts of the Caspian Sea (Sabeti, 1976) is an example of such hardwood forests. This area stretches from Astara in the northwest to Gorgan vicinity in the northeast of Iran (Sabeti, 1976) and is adjacent to a larger forest block extending across eastern Turkey and Caucasia (Salehi et al., 2007). This ecozone is approximately 800 km long and 110 km wide and has a total area of 1.85 million ha of forests comprising 15% of the total Iranian forests and 1.1% of the country’s area (Sabeti, 1976). Moreover, 60% of these forests are managed for commercial purposes and the remainders are degraded to varying degrees (Forest and Rangelands Organization, 1997). This ecozone is rich in biodiversity, which includes about 8,000 plant species that represent a variety of life forms such as grass, herb, shrub and trees (Heshmati, 2007). However, the vegetation of the Hyrcanian temperate forests is not very similar to the Mediterranean flora and comprises of a large number of central European species (Dawan and Famouri, 1964). Among the vegetation in this area Fagus orientalis, Carpinus betulus, Tilia rubra, Taxus baccata, Ulmus glabra, Quercus castanefolia, Parotia percisa, Alnus glutinosa, and Punica granatum are the important tree species (Heshmati, 2007). Additionally, vegetation types and soil conditions of this area are mainly affected by the heterogeneous topographical features, in particular altitude and slope (Salehi et al., 2007). Therefore, the temperate forest of the Hyrcanian zone is classified into different vegetation belts based on altitude (Dawan and Famouri, 1964; Sabeti, 1993). This zone is located in the humid climatic region of the north of Iran. The average annual rainfall ranges between 530 mm in the east and 1,350 mm in the west reaching up to an occasional record of 2,000 mm in the west (Akhani et al., 2010). In general, the Hyrcanian climate is warm Mediterranean in the east and temperate and semi-temperate Mediterranean and occasionally temperate xeric in the central and western areas (Akhani et al., 2010). The primary function of these forests other than wood production is supportive and environmental and their vital role in soil and water sources conservation (Akhani et al., 2010). Forests play an important role in the water balance, which can control the growth and productivity of an ecosystem. Each plant has its own specific characteristics that alter the hydrological process of a forest such as physical properties, i.e., height, canopy density, canopy structure, vegetation types, root systems, orientation of leaves and morphological properties that include the length of growing season and seasonal foliar changes. The hydrological processes in forest ecosystems can be influenced by the interactions of vegetation-atmosphere and soil. This process is rather complex, requiring lots of field work and data collection to be accurately understood (Chang, 2006). However, one of the main hydrological processes in an ecosystem is canopy interception loss, which is the evaporation of rainfall during and after a rainfall event (Toba and Ohta, 2005). Interception is one of the most important parameters in water cycle because the amount of interception from the canopies of vegetation is a significant proportion of precipitation (Toba and Ohta, 2005). Moreover, Asdak et al. (1998), Calder and Rosier (1976), Gash and Stewart (1977) and Scatena (1990) in an article by Crockford and Richardson (2000) argue that interception is the major component of the water balance in a forested area. According to Link et al. (2004), interception is the main component of the surface water balance. For example, annual net interception losses of temperate forests ranges between 11% to 36% of gross precipitation (PG) in hardwood canopies and 9% to 48% of PG in

First attempt to measure rainfall canopy interception loss

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softwood species (Hörmann et al., 1996). According to Hubert (2004) the interception amount from rough canopies can be as large as 35% of PG in upland forests of England in areas with 1,000 mm of precipitation yearly. Additionally, Pidwirny (2006) stated that plants can intercept up to 50% of the PG, which falls on their canopies. The leaves of deciduous trees normally intercept anywhere from 20%–30% of the falling precipitation (Pidwirny, 2006). However, hydrological modellers ignore the amount of interception, as they believe that interception is a minor ratio of the total evaporation rate (Hubert, 2004), but Beven (2001) stated that interception amount is not small, but it is the main proportion of the intercepted water on the leaf surfaces in rough canopies and is a major part of the total water balance in ecosystems. Moreover, interception can sometimes reach to 20%–40% of the precipitation (Gerrits et al., 2006). Interception plays a crucial role in the hydrological cycle of forested areas (Toba and Ohta, 2005) and has two main applications within a watershed. First, it is an important component of the water balance, which serves as both a loss and a gain of water to the watershed (Tate, 1996). Secondly, interception can decrease the soil erosion as well as protecting the structure of the soil and its penetration capacity (Tate, 1996). The reduction in infiltration as a result of interception losses may also decrease the risk of landslides (Miller and Sias, 1998). Interception also can affect the biological processes within an ecosystem by spreading of plant pathogens (Huber and Gillespie, 1992), and can also reduce the soil water content increasing the risk of drought stress (Link et al., 2004). One of the components of interception is throughfall, which is the proportion of precipitation that reaches the forest floor by passing the canopy gaps or by canopy drip (Tate, 1996). The amount of throughfall is spatially different either under a single tree or under a forest canopy, and is influenced mainly by vegetation types, canopy structure and density, seasonal factors and wind (Tate, 1996). However, there is a weak correlation between canopy coverage and throughfall (Tobon Marin et al., 2000; Loescher et al., 2002), which is arguable. Additionally, neighbouring canopies of trees can receive different amount of precipitation. Consequently, there is a variable pattern of throughfall due to inclined rainfall and shading effects of nearby trees (Herwitz and Slye, 1992). Further, the temporal persistence of spatial patterns of throughfall were found to be stable for three forest stands with different canopy structures in the Pacific Northwest, USA (Keim et al., 2000). The final component of interception is stemflow, which is an important part of water balance as it can be used for plant growth and productivity (Tate, 1996). Stemflow is the proportion of precipitation that flows down the tree trunk; often piping water directly into the forest floor (Tate, 1996). The value of stemflow is very important, especially in semi-arid areas where water availability can increase plant yields (Elewijck, 1989). It is rather hard to measure stemflow accurately, and in many studies a few trees, based on the convenience and accessibility, are chosen to measure stemflow (Crockford and Richardson, 2000). However, this is not always the case. In some studies stemflow is large enough to be measured. According to Crockford and Richardson (2000), stemflow was 4.1%, and 8.9% of PG for eucalyptus and pines respectively. Lorens et al. (1997) reported that stemflow was 1.3% of the PG in pinus sylvestris and Singh (1987) found that stemflow was 2.7% of PG in a Pinus wallinchiana plantation in India. Furthermore, the amount of stemflow varies in harvested and non-harvested sites, for example, According to Asdak et al. (1998), stemflow was 1.4% of PG in an unlogged site in central Kalimantan, Indonesia, whereas it was 0.4% for a harvested site. Moreover, Kelliher

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et al. (1992) in a study in New Zealand found an increase of 1.3% in the amount of stemflow after pruning the trees from 5–6 m to 2–3 m. This rise is mainly due to a better access of rain to the trunks (Crockford and Richardson, 2000). The value of stemflow in an oak forest in the Netherlands was very small during foliated periods, but it was 5% of PG in non-foliated periods (Dolman, 1987). Table 1 presents the important factors that can alter rainfall canopy interception loss and its components. Table 1

Presents the important factors that can influence rainfall canopy interception loss

Physical properties of trees

Meteorological characteristics

Phenology and age of trees

Rainfall intensity

Crown size

Rainfall duration

Height of trees and diameter at the breast height (DBH)

Rainfall type

Leaf shape, orientation and branch angle

Rainfall angle

Leaf area index (LAI), gap in the canopy

Temperature, humidity

Bark type and secured flow path

Wind speed, direction

Source: Adapted from Crockford and Richardson (2000), Asdak et al. (1998), Lloyd and Marques (1988) and Herwitz (1987)

Studies of forest hydrology in temperate regions have shown that interception loss is an important component of the water balance in forests (Gash and Morton, 1978). However, such data was unavailable regarding the importance of interception process by different tree canopies in different ecosystems of Iran. Particularly lacking data are Hyrcanian forests, which are the main industrial forests of the country. The broad goal of this research was to address the gaps in the forest hydrology in the northern part of Iran as there has not been any research on this aspect of eco-hydrology in this area. Consequently, this project was carried out as one of the first interception studies in Iran, which would be the basis to develop the knowledge of eco-hydrology in this region. The results of this study would be useful in understanding the catchment water balance and forecasting effects of land use and artificial plantations. In particular, it could help resource managers to be aware of the effects of land use change on water yields. This is necessary due to an increase in plantations of walnut and cypress trees over the last decades for agro-forestry and ornamental purposes planted without any consideration for how they affect the water balance of this fragile ecozone. This has the potential to create a problem with the water balance of this region in the future. This paper discusses the results of the interception data from Juglans regia Linn and Cup. Sempervirens L. Var. Fastigiata in the Hyrcanian ecozone. In the temporal analysis data that was collected from 2005–2006 (November–March) from relatively young species of Juglans regia Linn and mature Juglans regia Linn as well as from a tall and a very dense canopy of Cup. Sempervirens L. Var. Fastigiata tree in Chaboksar area in Gilan. The specific aims of this research were to: 1

examine intra-storm interception loss, throughfall, and stemflow

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compare the interception results between coniferous and deciduous trees over the course of the research.

Moreover, a part of this research focused on the hydrochemistry of rainfall and stemflow of walnut and cypress trees, which has been published separately (Lazerjan, 2012).


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Study area and climate conditions of the site

The research site was located in Chaboksar region (36° 57’ N, 50° 35’ E) in the north of Iran. A plot of 1,234 m2 was selected at an elevation of –26 m above the sea level (Figure 1). The main reason for selecting this particular area was accessibility to the site and logistical issues like feasibility, cost of stay and easy availability of transportation. There were three trees selected at the site, two walnut trees, one mature and one young and a cypress tree were randomly selected which had DBH of 25 cm, 15 cm and 32.5 cm respectively. In terms of height, the walnut trees were 8 m and 4.5 m and the cypress tree was 13.5 m. The canopy areas of the trees were 55.5 m2, 28 m2 and 4.5 m2 respectively. The canopy heights were 4 m and 3 m in mature and young walnut trees respectively and were 11 m in cypress tree. Figure 1 shows the study site and locations of instruments at the site. The mean annual precipitation and temperature of the site were approximately 1,164 mm and 19°C, respectively based on Ramsar Meteorological Station (data from 1995 to 2005). Figure 1

Shows the location of study area and instruments at the site (see online version for colours)

Source: Adapted from Lazerjan (2012)

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Figure 2

Annual mean precipitation of Hyrcanian zone (from west to east direction) (see online version for colours)

Annual mean

precipitation (mm)

1400 1200 1000 800 600

G or g an

N os ha hr G ha em sh ah r

R am sa r

R as ht

A st ar a

400

City name

Source: Akhani et al. (2010) Figure 3

Annual mean temperature of Hyrcanian zone (from west to east direction) (see online version for colours)

temprecher (c)

Annual mean

18 17 16 15

G or ga n

R am sa r N os ha hr G ha em sh ah r

R as ht

As ta ra

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City

Source: Akhani et al. (2010)

There are several components that control the climate of south Caspian Sea such as local atmospheric circulation which is modulated by compound topography as well as the maritime effect of the Caspian Sea (Kendrew, 1961; Alijani and Harman, 1985). In addition, there is also westerly winds that carry large amount of moisture from the north of Atlantic Ocean, Mediterranean Sea and Black Sea into the Middle East which is an important contributor of precipitation in the south of Caspian Sea (Kendrew, 1961; Alijani and Harman, 1985). The Hyrcanian zone is a humid zone in the north of Iran. The average annual rainfall ranges between 530 mm in the east and 1,350 mm in the west reaching up to 2,000 mm in the west. Relative humidity is also continually high with an


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average value fluctuating from 74.6% in the east to 84.6% in the west (hardly ever dropping below 60% at the hottest hours). There is a big difference in the amount of precipitation from the western to the eastern areas of this region, the amount of rainfall in the western Hyrcanian zone is higher in autumn due to the location of this region at the head of north-easterly winds that originates from the Siberian anticyclone (Akhani et al., 2010). The following figures (Figure 2 and Figure 3) show the annual mean precipitation and temperature from different cities in the north of Iran (Akhani et al., 2010).

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Floristic diversity

This area is rich in flora and consists of 3,234 species that belong to 856 genera and 148 families of vascular vegetation (Akhani et al., 2010). The indicator trees of this area are shown in Table 2. Table 2

The indicator trees of Hyrcanian zone Cupressus sempervirens

Pterocarya fraxinifolia

Acer velutinum

Diospyrus lotus

Quercus castaneifolia

Albizia julibrissin

Fagus orientalis

Sorbus torminalis

Acer cappadocicum

Alnus subcordata

Fraxinus excelsior

Taxus baccata

Buxus hyrcana

Gleditschia caspica

Tilia platyphyllus

Carpinus betulus

Parrotia persica

Ulmus glabra

Cerasus avium

Popolus caspica

Zelkova carpinifolia

Source: Adapted from Akhani et al. (2010)

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Research methods

4.1 Canopy interception loss Interception is usually measured directly as the difference between gross (above the canopy) and net (below the canopy) rainfall. This latter is calculated by summing the total throughfall (canopy drip) and stemflow (Voigt, 1960; Mitscherlich, 1971; Heuveldope et al., 1972; Weihe, 1974; Jackson, 1975; Bultot et al., 1972; Tanaka et al., 1984; Loustau et al., 1992). This relationship was also used in this research to identify rainfall canopy interception loss. The interception loss by the forest canopy, which includes stems, and branches, leaves can be calculated by the following water balance equation: I = PG − (TF + SF )( mm)

(1)

where I is rainfall canopy interception, PG is gross precipitation, TF is throughfall, SF is stemflow (Toba and Ohta, 2005).

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4.2 Measurement of gross precipitation Precipitation is the main source of water for the biosphere, and is simply measured with an open container (gauge) placed under a plastic or metal cone of known area. However, location of the rainguage at the site is very important, and should be placed in an open area. To accurately account for spatial variability more than one rainguage in a given catchment required. This gauge is calibrated to read the depth of rainfall usually recorded daily or over short intervals. Two bulk precipitation samplers were located in the opening at the site for measuring precipitation and the average value was counted as the precipitation amount of the site. These rainguages are placed according to the Iranian Meteorological Standards (Yatagi et al., 2008).

4.3 Measurement of throughfall The amount of throughfall is spatially different either under a single tree or under a forest canopy, and is influenced mainly by vegetation types, canopy structure, seasonal factors, wind, and canopy density. Measurement of throughfall is required to determine what proportion of precipitation passes through canopy gaps of aspen stand. Throughfall collectors or troughs are used and they can be connected to automatic data loggers. In this study throughfall collectors were used to measure throughfall. They were placed at 20 cm above the ground level (Figure 4). Each 8 m2 was covered by a throughfall collector under the canopy of walnut trees. There are 12 and 8 throughfall collectors placed under the canopy of mature and young walnut trees respectively. There are also 6 throughfall collectors under the canopy of cypress tree due to its dense canopy. The average value of the throughfall has been calculated for each tree separately. Figure 4

Throughfall collector (see online version for colours)

First attempt to measure rainfall canopy interception loss Figure 5

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Stemflow collar on the bole of walnut tree (see online version for colours)

4.4 Measurement of stemflow It is rather hard to measure stemflow accurately and in many studies only a few trees, based on the convenience and accessibility, are chosen to measure the stemflow (Crockford and Richardson, 2000). However, as aforementioned, stemflow is the proportion of precipitation that flows down of the tree trunk, often piping water directly into the forest floor (Tate, 1996). This is an important part of water balance as it can be used for plant growth and productivity. There is a lack of standard agreement on the number of gauges to accurately sample stemflow volume. However, it is often measured by fixing flexible tubing (Figure 5) around the bole of trees (Levia and Frost, 2003). The basic technique to measure stemflow involves the placement of a collar around the main trunk of the plant at a certain angle so that the volume of stemflow can be transferred to a collector. Moreover, Toba and Ohta (2005) stated that stemflow can be collected by a tube, which can be fixed onto the bole of the trees at a height of 1.2 m. Crockford and Richardson (2000) explained that stemflow is measured by splitting a plastic hose, and wrapping and fixing it around the bole of a tree by galvanised iron staples then sealed silicone. In this study, three trees, two walnut trees and a cypress tree were randomly selected to represent a range of tree diameters as well as canopy areas within the stands. In this method, a garden hose was used, of which one third of the circumference was removed, and then was fixed to the bole of the trees. The hose was then wrapped two times around the bole of the tree using silicon mastic and stainless steel screws and nails. The end of the hose was directed into 20 litre jugs. If the volume of stemflow happened to be more than 20 litres, another 10 litre jug was added. The material of stemflow bin is an opaque graduated polyethylene bins (20 and 10 litres) to prevent any chemical

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reactions for chemical analysis (Lazerjan, 2012). Three trees were equipped with three stemflow collars on them, which were connected to jugs. After each rainfall event the total amount of water in collectors was considered as the volume of stemflow, which was determined volumetrically using measuring cylinders. The measurements were recorded every 24 hours in accordance to rainfall. However, if the rain had continued it would be counted as one event.

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Results

This study highlights the complex interactions and feedbacks between precipitation and rainfall canopy interception of walnut and cypress trees. It was identified that each tree species reacts differently as rainfall occurs on their canopies. These reactions mainly depend on rainfall characteristics and physiological properties of vegetation. This can change the hydrological responses within the selected trees. As it is shown in Figures 6 and 7, the amount of interception is reduced in the walnut trees from November to March due to senescence. However, there is a big difference in the interception amount of the mature versus the young walnut. The mature walnut has more canopy area, therefore, has more ability to intercept rainfall in compared to the young walnut. In addition, the amount of stemflow in young walnut is higher as its canopy requires less amount of rainfall to produce stemflow, so the rainfall canopy capacity is smaller in compared to the older walnut tree, which requires more rainfall to produce stemflow. There is a positive correlation between LAI and interception loss, but interception significantly varies due to different leaf properties in the canopies with the same LAI (Hall, 2003). However, the aerodynamic properties of temperate forests have an important role in the initialising of interface for atmosphere-biosphere interaction within these ecosystems (Levia and Frost, 2003). Furthermore, the foliar and woody components of temperate forests can significantly alter the spatial patterning of atmospheric inputs significantly (Ford and Deans, 1978; Staelens et al., 2008) by intercepting incident precipitation (Levia and Frost, 2003). The cypress tree had more canopy interception loss and less stemflow and throughfall in compared to walnut trees from November to March (Figures 8 and 9) due to its canopy density and canopy height. Therefore, these parameters can strongly affect the throughfall and stemflow values. A dense canopy can capture more precipitation and can increase the interception amount resulting in a reduction of rainfall reaching the rooting zone. This can have an impact on the growth of associated ground vegetation of this ecozone. Moreover, the amount of stemflow depends on the age of the tree. For example, a young walnut tree (Juglans regia linn) can produce more stemflow in a short period of rainfall than a mature walnut tree. There are two main reasons for this; first a young tree has a smaller canopy area and therefore a smaller canopy saturation value in compared by the mature walnut tree. Also, a young tree has a very smooth bark that could route the water on the bark easily resulting in a larger stemflow. The second reason is that a mature tree has grown horizontally and vertically, therefore, it intercepts more rainfall and it reduces the amount of net precipitation reaching the ground to some degree. In addition, the number of furrows (grooves) on the bark of mature trees can also be a reason for a lesser stemflow amount. Additionally, a mature tree can be a host to moss and lichen on the bark, which can act as an obstruction of stemflow. However, if the amount of precipitation exceeds the amount of canopy saturation, then a mature tree can produce


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larger amount of stemflow. For example in this study, the mature walnut tree had to capture at least 2.7 mm of rainfall to produce stemflow and could produce throughfall with a small amount of rainfall whereas the young walnut tree required 0.75 mm of rainfall to produce stemflow and throughfall. Conversely, the situation is completely different in cypress trees as 10 mm of rainfall was required to produce stemflow and 2.4 mm of rainfall had to be captured by the canopy to produce throughfall. The amount of stemflow increases as a tree grows because the bark will be thicker and well structured as a result the DBH will increase (Toba and Ohta, 2005). Crockford and Richardson (2000) also stated that the amount of stemflow can be related to DBH or basal area. Interestingly, over the course of this research 172 litres of stemflow was generated in young walnut tree, 151 litres in old walnut tree, and the cypress tree produced 80 litres of stemflow (data is not presented here). It is known that deciduous trees have greater ability to produce stemflow than coniferous vegetation (Pidwirny, 2006), which was also found in this study. The amount of throughfall is spatially different for either under a single tree or under a forest canopy and is influenced mainly by vegetation types, canopy structure, seasonal factors, canopy density, and wind. The throughfall amount in this study was higher in young walnut in comparison to mature walnut because it has developed less of its horizontal and vertical foliage whereas the amount of throughfall in cypress depends upon precipitation value. This means the higher the precipitation, the greater the throughfall because of cypress’ canopy density and canopy height, requiring the canopy interception capacity is reached before throughfall and stemflow will commence. In fact, physical features and a secured flow path were the main reasons for this huge amount of stemflow (Lloyd and Marques, 1988). Figure 6

Variations in interception components from November–December in the young walnut (see online version for colours)

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Figure 7

Variations in interception components from November–December in the old walnut (see online version for colours)

Figure 8

Variations of average values of interception components from November–December in walnut tress (see online version for colours)

First attempt to measure rainfall canopy interception loss Figure 9

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Variations in interception components from November–December in the cypress tree (see online version for colours)

Results and discussion

Based on the results of this study, it was identified that vegetation type, canopy structure, canopy architecture, canopy height and canopy density play important roles in the partitioning of rainfall in this ecosystem. The outcomes of this study demonstrated that the amount of interception in the mature walnut tree was 27% of gross precipitation and the values of throughfall and stemflow were 72% and 1% respectively (Figure 10) whereas the young walnut tree intercepted 2% of gross precipitation and the value of throughfall and stemflow were 93% and 5% respectively (Figure 10). However, the cypress tree intercepted 82% of gross precipitation, which is very high and the amount of throughfall and stemflow were 15% and 3% respectively (Figure 10). Figure 11 presents a comparison between the interception components of walnut and cypress trees in this study. As it is shown in the figure, the amount of interception in cypress is far greater than walnut, which is almost equivalent to the amount of throughfall that has been produced by the walnut trees. The amount of stemflow in cypress is less than walnut due to the dense canopy structure of cypress. These differences can influence the growth and productivity of the other Hyrcanian associated plants. Trees in forest and agroforestry systems redistribute rainfall and nutrient fluxes to the rooting zone. This can strongly influence the productivity and growth of associated plants within an ecosystem. Even though the results of this study appear promising, long term and inclusive research is required to link each of the factors affecting the process of interception, such as plants physiological properties and meteorological parameters, in order to accurately assess water balance of walnut and cypress trees in this area. As it was discussed earlier, such data was unavailable regarding the importance of interception process, which is the main component of water balance in the Hyrcanian ecozone. Therefore, the main purpose of this study was to highlight the importance of this process

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by carrying out research on the comparison of rainfall canopy interception loss of walnut and cypress trees because they are in high demand for agroforestry and ornamental purposes in this area yet are planted without any consideration to how they may affect the water and nutrient fluxes of the rooting zone (Lazerjan, 2012). The author acknowledges the limitations in this research because it was carried out as the first interception study in this area, but at the same time hopes that that the outcomes of this research would be considered as a basic step to further the knowledge of water balance, which has ecological and biological roles in temperate regions. Moreover, this research aims to help forest managers to have a better understanding of interception values of different species and the implications on the productivity of vegetation in this area. Finally, the results will be useful in forecasting effects of land use on water yields of different ecosystems to understand how the diversity of species can affect water yields and vegetation distribution in this ecosystem. Figure 10 Overall interception components of walnut and cypress trees over the course of the study (see online version for colours)


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Figure 11 A comparisons between the interception components of walnut and cypress from November to March (based on the results of a Tukey HSD test, there was a significant difference between the young (M = 0.0658, SD = 0.1471) F = (2.93) = 346.10, P < 0.05, the old walnut (M = 0.255, SD = 0.135) and the cypress tree (M = 0.885, SD = 0.1320) (see online version for colours)

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Summary and conclusions

In summary, different vegetation types can significantly alter the growth of surrounding vegetation through partitioning of rainfall and nutrients to the rooting zone. The Iranian Hyrcanian forests are among one of the most fragile ecosystems of the country due to large industrial activities for logging and extensive urbanisation. Of current concern is an increase in plantations of Juglans regia Linn and a Cup. Sempervirens L. Var. fastigiata species, which has taken place over the last few decades for agro-forestry and ornamental purposes. This practice has become normal in this area where these species are planted without any considerations on how they affect the water balance and the growth of associated vegetation within this unique ecozone in the future. The results of this study suggest that some of the most important factors altering the process of interception are vegetation type, canopy structure, canopy architecture, canopy height and canopy density. The outcomes of this study pointed out that the amount of interception in the mature walnut tree was 27% of gross precipitation and the values of throughfall and stemflow were 72% and 1% respectively whereas the young walnut tree intercepted 2% of gross precipitation and the value of throughfall and stemflow were 93% and 5% respectively. On the other hand, the cypress tree intercepted 82% of gross precipitation, which is very high, and the amount of throughfall and stemflow were 15% and 3% respectively. Moreover, the walnut trees were at the range of interception of temperate forest trees whereas the cypress tree had a high amount of interception due to its dense canopy structure and coniferous nature. As a result, cypress trees can reduce the partitioning of rainfall to the rooting zone of surrounding vegetation and limit their growth. Thus, care should be taken in planting cypress trees over a vast area. Further research is required on different vegetation type of this region in order to understand how different vegetation contributes to the sustainability and productivity of this ecozone.

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Acknowledgements The author would like to thank Farhang Assadollahi and Mohammad Reza Azarnosh for supervising this project at the Department of Forestry, Noshahr-Chalous University. Some sources of funding were provided by the Department of Forestry, Noshahr-Chalous University, which is appreciated. Many thanks to the staff at Tehran and Ramsar meteorological stations for their advice on designing some of the instruments. For the field assistance, the author would like to thank Anosh Pakravan who helped him with designing the required practical instruments. For site preparation, the author would like to thank Parvin Irannejad and also Kara Schimmelfing who helped him in editing this paper. Finally, the author would also like to thank the anonymous reviewers for their insightful comments.

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