Banko Janakari, Vol 28 No. 1, 2018 11 Litter production and nutrient return to soil through litterfall is important pathway for the regulation of nutrient cycling and primary production of the forest. Litterfall dynamics is generally influenced by phenology of tree species, seasons and altitude of the forest stand. As most of the information on litter production are from temperate and dry tropical region. A comparative study on litter production and nutrient return were conducted in Terai Sal forest (TSF) and Hill Sal forest (HSF) located in moist tropical region of eastern Nepal. Litter samples were collected from the litter traps (1m × 1m size) placed randomly in the forest. Collection was done at two months interval for one year. Annual litterfall in TSF (8.82 Mg ha-1y-1) was significantly (p < 0.001) higher than in HSF (7.18 Mg ha-1y-1).There was distinct seasonality in litter production. In TSF and HSF, litterfall was maximum in the summer (6.57 Mg ha-1 and 5.05 Mg ha-1, respectively) and minimum in winter season (0.86 Mg ha-1 and 0.72Mg ha-1, respectively). Amount of nutrient return to forest soil through litterfall (kg ha-1 y-1) was higher in TSF (72.44 N, 6.80 P and 33.23 K) than HSF (54.31 N, 4.84 P and 22.23 K). The difference in litter production between these two forests was influenced by the phenology of dominant tree species, variation in altitude and seasons. Nutrient return through litterfall is a great input of nutrients in soil which is required for production process. Thus, litter constitutes a significant role in forest management. Key words: Hill Sal forest, litterfall, nutrient return, seasonal variation, Tarai Sal forest Comparative study on litter production and nutrient return to soil in Tarai and Hill Sal (Shorea robusta Gaertn.) forests of eastern Nepal K. P. Bhattarai1* and T. N. Mandal2 Plant litter production and its decay are the two important processes which provide the main input of organic matter in soil and regulate the patterns of nutrient cycling in forest ecosystems. Litterfall reflects primary productivity which represents approximately 30% of annual production and characterizes a major proportion of forest carbon fluxes (Macinnis-Ng and Schwendenmann, 2014). It is a central nutrient resource in tropical forest ecosystems where soils are generally nutrient poor and highly weathered (Maritus et al., 2004). So, litterfall is an important pathway of nutrient succession which preserves soil fertility in forest ecosystems (Bellingham et al., 2013). Litterfall compilation is a standard non-destructive method for assessing the productivity and turnover of organic matter in a forest. Therefore, determining the dynamics of litterfall and nutrient return to the soil through it with time is a fundamental aspect of functioning of terrestrial ecosystem (Maritus et al., 2004). Litterfall dynamics in the natural forest ecosystems is strongly influenced by species composition (Singh and Kushwaha, 2006), age structure (Stonhlgren, 1988), seasons (Sundharapandian and Swamy, 1999), altitude (Garkoti and Singh, 1995) and latitude (Bray and Gorham, 1964). Precipitation, temperature, radiation and soil features are also major controlling factors of litterfall in tropical forests where it occurs during the dry season (Zhang et al., 2014). Litterfall exhibits distinct seasonality in different forest ecosystems and depends mainly upon the location and nature of plant species. Many deciduous species shed their leaves during 1. Department of Botany, Mechi Multiple Campus, TU, Bhadrapur, Nepal * E-mail: krishnaprbhattarau@gmail.com 2. Department of Botany, Post Graduate Campus, TU, Biratnagar, Nepal Banko Janakari, Vol 28 No. 1, 2018 12 the dry season (Elliott et al., 2006). Distinct seasonality occurs in sub-tropical mixed oak forest of northeastern India, where maximum litterfall takes place during November to March (Devi and Yadava, 2010). Zhang et al. (2014) documented the seasonal pattern of litterfall in forest ecosystems by collecting data from existing literature and concluded that the peak of litterfall in tropical forest was in drought season corresponding to spring or winter season. However, peak of litterfall could occur at various seasons in temperate broadleaved and needle- leaved evergreen forests, and peak of litterfall was observed in autumn in temperate deciduous broadleaved and boreal evergreen needle-leaved forests. From a large number of published and unpublished datasets across South American tropical forests, it is concluded that seasonality in litterfall was significantly correlated with the rainfall (Chave et al., 2010). As the litter is the above ground source of nutrients, it helps to manage the nutrient cycling process for better forest production. Regarding the litterfall and its seasonality mostly the information are from dry tropics. Here, an attempt has been made to document the information from moist tropical region. The present study was carried out to answer the following questions. (I) What is the status of litter production in Tarai and Hill Sal forests ? (ii) Does seasonality affect the litter production in these forests ? (iii) What is the contribution of litter in providing the nutrients (N, P, K) to the soil in Sal forests? Materials and methods Study area The study was carried out in Sal forest located in Tarai and Hilly regions of eastern Nepal. Sal forest of Tarai region is addressed as Tarai Sal forest (TSF) and Hilly region as Hill Sal forest (HSF). TSF is located at Jalthal near Kechana (extreme low land of Nepal) of Jhapa district. It occupies an area of 6300 ha. of land and lies in between 87o 55’ and 88o 03’ E longitude and 26o 27’ and 26o 32’ N latitude. The forest floor has uneven surface and topographical variation ranges from 62 to 129 m msl. HSF is located at Kiteni of Kolbung, Ilam district. The forest lies in sub-himalayan tract (Shiwaliks) at an altitudinal range of 500 to 850 m msl. The HSF is situated in between 88o 02’ and 88o 04’ E longitude and 26o 44’ and 26o 47’ N latitude (Fig. 1). Fig. 1: Location of study area of Hill Sal forest at Kiteni, Ilam district and Tarai Sal forest at Jalthal, Jhapa district in eastern Nepal The climate of the study area is tropical monsoon type. Based on the data pertain to the period, 2001— 2014, the mean monthly minimum temperature of TSF ranged from 10oC to 24oC and maximum temperature ranged from 23.9oC to 33.4oC (Fig. 2a). Likewise, the mean monthly minimum temperature of HSF ranged between 9.4oC to 19.9oC and maximum temperature between 16.4oC to 25.9oC (Fig. 2b). The average annual rainfall of TSF was 2130.4 mm and HSF was 1776.07 mm in which maximum rainfall (80 — 85%) occurred during rainy season. Fig. 2a: Ombrothermic representation of the climate the climate in Tarai Sal forest Bhattarai and Mandal Banko Janakari, Vol 28 No. 1, 2018 13 Fig. 2b: Ombrothermic representation of in Hill Sal forest The temperature ( ̶○̶ ; mean monthly mimimum and ̶●̶ ; mean monthly maximum) and ̶∆̶ ; rainfall data pertain to the period, 2001—2014. Both TSF and HSF (tropical moist forest according to the life zone classification of Holdridge et al., 1971) are dominated by Shorea robusta Gaertn. The main associated species like Lagerstroemia parviflora Roxb., Dillenia pentagyna Roxb., and Schima wallichii D. C Korth are common in both forests. All these species are summer deciduous (Gautam, 2015). However, TSF is peculiar in containing Artocarpus chaplasa Roxb. and sub-tropical species like Castanopsis indica (Roxb.) Miq, Michelia champaca Land Madhuca longifolia (Koenig) Mac. Soil of TSF is Sandy loam Mollisols which has dark top soil. In the HSF, soil is Sandy loam Entisols with much gravel, stones and rock fragment (Jackson, 1994). Estimation of litter fall The inner core area in each forest stand (TSF and HSF) was divided into 100 grids each having 100m ×100m size. Among them 30 grids were selected randomly for the study purpose. Selection of grids was done alternately in clockwise direction for periphery to centre. Within each selected grid a permanent plot of 20m × 20m was fixed. Sampling plot was fixed in three ways e.g. at upper, middle and lower portion of the grids in each forest. Litter fall samples were collected from the litter traps. One litter trap (1m × 1m size) was fixed in each plot. Within the thirty plots in each forest the litter traps were located near the trees, far from the trees and between the trees. Collection was done at an interval of two months for one year from March 2013 to February 2014. The collected samples were brought to the laboratory and separated into leaf and non-leaf (small branches, reproductive parts and miscellaneous) components. Litter samples were oven dried at 80oC for 24 hours and the mean bi-monthly dry weight value for each forest was estimated. For the purpose of the chemical analysis, litter samples were mixed and pooled separately component wise in proportion to their volume to represent annual sample for each forest site. Pooled samples were stored in dried form in air- tight polythene bags for chemical analysis. Chemical analysis of litter The oven dried form of pooled samples of each litter component were ground separately and passed through 1mm mesh screen. Chemical analysis was done in triplicates for each litter component. The total nitrogen concentration was determined by micro-Kjeldahl method (Peach and Tracey, 1956). Using the method of Allen et al. (1974), 200 mg oven dried plant material was digested in 7 ml triacid mixture (5:1:1, nitric acid: sulphuric acid: perchloric acid), cooled and transferred on hot plate till the material changed to pink color and diluted to 100 ml by using triple distilled water. Using 5ml aliquot, ammonium molybdate and SnCl2, the total P was determined by developing blue colour and with the help of spectrophotometer. Potassium was determined by atomic absorption spectrophotometer. Statistical analysis Statistical tests were carried out in SPSS (IBM Statistics, ver. 20) packages. The data were checked for normality (Kolmogorov-Smirnov test) before statistical analysis. Two ways ANOVA was used to test the significant difference in the amount of litterfall due to forest types (TSF and HSF) and seasons. Bhattarai and Mandal Banko Janakari, Vol 28 No. 1, 2018 14 Results and discussion Litter production in TSF and HSF Annual litterfall in TSF (8.82 Mg ha-1y-1) was higher than in HSF (7.18 Mg ha-1y-1) (Table 1). ANOVA suggested that the variation in litterfall was significantly (p < 0.001) different for forest types (Table 2). Table 2: Effect of forest sites, seasons and forest sites × seasons interaction in the litterfall in TSF and HSF as indicated by ANOVA. Source of Variation df F Significance Forest 1 113.76 P<0.001 Seasons 2 4038.84 P<0.001 Forest × Seasons 2 96.45 P<0.001 Contribution of leaf litter was always higher (70%) than non-leaf litter (30%) in both forests. Leaves comprised the most important part of litterfall, as has been found for most of the forest ecosystems (Paudel et al., 2015; Wang et al., 2007; Yang, 2005; Martius et al., 2004; Arunachalam et al., 1998). In the present study the higher litter production in TSF than HSF could mainly be due to differences in microclimate and soil properties which affect the productivity (Vitousek, 1984). As temperature declines with increasing altitude (Girardin et al., 2010), the decomposition process and nutrient supply became retarded due to which above- ground net production including litter production declined (Belligham et al., 2013; Kitayama and Aiba, 2002; Garkoti and Singh,1995). Comparative account of litter production in some tropical and sub-tropical forests is presented in table 3. The value estimated for TSF was Table 1: Annual litter fall (Mg ha-1 y-1± 1 SE) in Tarai Sal forest and Hill Sal forest Forests Leaf litter % of total Non-leaf litter % of total Total Tarai Sal forest 6.16±0.06 70 2.66±0.05 30 8.82±0.06 Hill Sal forest 5.01±0.12 70 2.17±0.08 30 7.18±0.19 Table 3: Comparative account of litter production (Mg ha-1 y-1) in some tropical and sub-tropical forests Location Forest types Litter production References Nepal Jalthal, Jhapa Tarai Sal forest 8.82 Present study Kiteni, Ilam Hill Sal forest 7.18 Present study Charkoshe, Sunsari Tropical moist Sal forest 11.8 Gautam, 2015 Panchakanya, Sunsari Plateau Sal forest 10.3 Mandal, 1999 India Manipur Sub- tropical Oak forest 10.94 Devi andYadaba, 2010 Kodiyar, Tamilnadu Deciduous forest 5.76 - 8.65 Sundarapandian and Swamy, 1999 Kodiyar, Tamilnadu Evergreen forest 5.63 - 7.84 Sundarapandianand Swamy, 1999 Vidhyan plateau Dry tropical savannahs 2.8 - 5.9 Tripathi and Singh, 1995 Nanda Devi Reserve Forests of Central Himalaya 4.22 Garkotiand Singh, 1995 Thrissur, Kerala Moist deciduous 12.2 - 14.4 Kumar and Deepu, 1992 Mornihills, Haryana Moist deciduous 10.4 Gupta and Raut, 1992 Sal and mixed Sal forest 2.8 - 7 Sharma et al., 1990a, b Deciduous forest 1 - 6.2 Singh, 1968 Other counties China Evergreen broad-leaved forest 3.28 - 11.26 Paudel et al., 2015 China Global pattern 3.0 - 11 Zhang et al., 2014 New Zealand Evergreen montane rain forest 2.81 Bellingham et al., 2013 South America Tropical forests (n= 81) 8.61 Chave et al., 2010 China Sub tropical forest 4.89 - 10.61 Zhou et al., 2007 China Evergreen broad-leaved forest 4.63 - 8.85 Yang et al., 2005 Bhattarai and Mandal Banko Janakari, Vol 28 No. 1, 2018 15 comparable to Plateau Sal forest of Nepal (10.3 Mg ha-1y-1; Mandal, 1999) and moist deciduous forest of Moni Hills, Haryana, India (10.4 Mg ha- 1y-1; Gupta and Raut, 1992). On the other hand the value obtained for HSF was comparable to tropical forests of South America (8.61 Mg ha- 1y-1; Chave et al., 2010). Seasonal variation in litter production There was distinct seasonality in the pattern of litter production in both forests (Table 2). In TSF and HSF, it was higher in the summer season (6.57 Mg ha-1and 5.05 Mg ha-1) followed by rainy (1.39 Mg ha-1 and 1.41Mg ha-1) and winter season (0.86 Mg ha-1 and 0.72 Mg ha-1), respectively (Fig. 3). Environmental variables like temperature and rainfall greatly influence the seasonal pattern of litter fall in tropical forests (Zhang et al., 2014; Chave et al., 2010). High temperature during summer season reduces the humidity and increases the rate of transpiration which enhances the rate of litter fall (Twilley et al., 1986). 3 a 3 b Fig. 3 a & 3 b: Seasonal variation in litterfall (Mg ha-1) in Tarai Sal forest and Hill Sal forest of eastern Nepal. Seasonal representations are as: Summer (March-June), Rainy (July- October) and Winter (November-February). Nutrient return through litter fall Nutrient concentration of different components of litter was slightly higher in TSF than HSF as summarized in table 4. Concentration of nutrients in different litter components in diminishing order was: N>K>P in both forests. Nutrient concentration in leaf litter was nearly 1.5 times higher than the non-leaf litter in each forest. Generally, the woody litter has lower N and P concentration than the foliage (Arunachalam et al., 1998) because woody litter has high amount of sclerophyllous tissue which contains less amount of nutrient (Vitousek and Sanford, 1986). The total amount of nutrient return to forest soil through litterfall is mentioned in table 5. N, P, K return through litterfall (kg ha-1y-1) was higher in TSF than HSF due to higher amount of litterfall. Along with, it also depends upon the nutrient Table 4: Concentration (% ± SE) of nutrients in litterfall in Tarai Sal forest and Hill Sal forest Forests/Components Nutrients N P K Tarai Sal forest Leaf litter 0.93±0.023 0.086±0.001 0.41±0.01 Non leaf litter 0.57±0.024 0.057±0.002 0.30±0.02 Hill Sal forest Leaf litter 0.85±0.023 0.077±0.001 0.34±0.01 Non leaf litter 0.53±0.025 0.046±0.002 0.24±0.02 Bhattarai and Mandal Banko Janakari, Vol 28 No. 1, 2018 16 concentration of litter of tree species involved (Yang et al., 2005). Generally, montane forest leaves have lower nutrient concentration than those of fertile lowland forest (Vitousek and Sanford, 1986). The nutrient contribution to the forest floor through the litterfall was comparable to Wanmulin Nature Reserve, China (Yang et al., 2005), forest of central Himalaya, India (Garkoti and Singh, 1995) and Plateau Sal forest, Nepal (Mandal, 1999). However, in Sal forest and mixed Sal forest of Bidhyan plateau, India (Sharma et al., 1990a) and in deciduous forest, India (Singh, 1968) the nutrient return from litterfall was relatively lower than the present study. Litterfall is only the above ground source of soil organic matter which enriches the soil with nutrients essential for forest production. Removal or reduction of litter from forest floor can directly reduce the soil nutrients which ultimately affect the forest productivity. Hence, litter stands as an essential factor for better forest management. Conclusion It is concluded that litter production is influenced by the phenology of dominant tree species, variation in altitude and seasons, due to which low land Tarai Sal forest showed greater litter production and nutrient return to the soil. As TSF is rich in litter production and nutrient input in soil, it may have high production potential. Further, to manage the soil fertility, litter should remain undisturbed on the forest floor to regulate the nutrient cycling process. As the litter and litter mediated soil nutrients are essential for biomass production, it serves as a pronounced factor for the forest management. Acknowledgements We are grateful to the Head, Department of Botany and to the Campus Chief of Post Graduate Campus, T. U., Biratnagar, Nepal for providing laboratory and library facilities. The first author is grateful to the Institute of Science and Technology, Tribhuvan University, Kathmandu for study leave and to the University Grants Commission, Nepal for the research fellowship. References Allen, S.E., Grimshaw, H.M., Parkinson, J.A. and Quarmby, C. 1974. Chemical Analysis of Ecological Materials. Blackwell Scientific Publication, Oxford, UK. Arunachalam, A., Arunachalam, K., Pandey, H. N. and Tripathi, R.S. 1998. Fine litterfall and nutrient dynamics during forest regrowth in the humid sub-tropics of north-eastern India. Forest Ecology and Management 110: 209 — 219. Table 5: Amount of nutrient return (kg ha-1 y-1 ± SE) through litterfall in Tarai Sal forest and Hill Sal forest Forests/Components Nutrients N P K Tarai Sal forest Leaf litter 57.28±2.15 5.29±0.09 25.25±0.7 Non leaf litter 15.16±0.63 1.51±0.03 7.98±0.38 Total 72.44±1.22 6.80±0.11 33.23±1.09 Hill Sal forest Leaf litter 42.81±0.28 3.85±0.04 17.03±0.72 Non leaf litter 11.50±0.52 0.99±0.07 5.20±0.37 Total 54.31±0.92 4.84±0.04 22.23±1.1 Bhattarai and Mandal Banko Janakari, Vol 28 No. 1, 2018 17 Bellingham, P. J., Morse, C. W., Buxton, R. P., Bonner, K. I., Mason, N. W. H. and Wardle, D. A. 2013. Litterfall, nutrient concentrations and decomposability of litter in a New Zealand temperate montane rain forest. New Zealand Journal of Ecology 37 (2): 162—171. 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