Recovery Pattern of Vegetation during Succession Following Slash and Burn Agriculture in Mizoram, North-East India

The recovery of forest after shifting cultivation (locally known as ‘jhum’) has multiple complications as observed in various forest types, disturbed regimes, climate conditions and so forth. Rate of recovery and species composition are of major concern to understand the process of succession. The present study was carried out in Champhai district of Mizoram, India with the aim to comprehend the recovery of vegetation at different stages of succession after abandonment. Jhum fields of 5, 10, 15 and 25 fallow ages were located from the satellite imagery and the plot was navigated using GPS. Quadrats were randomly laid within those plots and the plants were enumerated and analysed. The species were contiguously distributed. Ecological indices indicated high diversity and species richness for tree and shrub components but low diversity and species richness for herbaceous component. The number of tree species declined by 5.1% from 5 years fallow to 10 years fallow. However, it increases from 10 years to 15 years fallow by 8.1% and was further enhanced to 32.5% on 25 years fallow. It was also observed that the number of shrub species declined from 5 years to 10 years fallow by 4.9% which worsened to 15.4% on 15 years fallow. Nevertheless, there was a distinct rise in the number of species on the 25 years fallow by 54.6%. Interestingly, the number of herbaceous species was observed to decrease from 5 years to 10 years fallow by 6.8% and further declined by 2.4% and 17.5% on 15 years and 25 years fallow, respectively. Therefore, the percent recovery of tree and shrub species from 5 years to 25 years fallow was 35.9% and 24.4%, respectively, while 25.0% of herb species declined during the same fallow period. Introduction Slash and burn agriculture or shifting cultivation is the predominant and primitive agriculture in the hills of Northeast India, wherein settled cultivation constitutes only a small portion of the total cultivated land mostly confined to the valley areas. This agriculture involves the cutting of forest followed by burning of the plant material. When the soil loses its fertility and crop productivity becomes meagre the farmer shifts to another piece of virgin or secondary forest for cultivation, leaving the abandoned land to regenerate naturally during the fallow period. The farmer then returns back again to the same piece of land for cultivation after a certain period of cropping and fallow depending on ethnicity, population and land ratio [1]. A series of disturbances that the slash and burn agriculture constitute such as cutting, burning and weeding have dramatic effects on the mortality and regeneration of plant species [2]. The combine effect of each of these events may reduce or eliminate the potential for on-site species regeneration, leaving seed dispersal as the major way for establishing of species from the nearby forest. The rate of recovery in species diversity, species richness, basal area and species density are influenced by the climatic conditions. Studies have shown that the rate of recovery is higher in the humid and moist tropics, whereas regeneration to restore the structure of a mature forest is faster in the dry tropical forest [3]. Vegetation succession following shifting cultivation in tropical regions also indicated that the diversity of woody species gradually increases with fallow age [4-7]. The cropping period as well as jhum cycle affects the vegetation recovery process [2,7]. Studies from tropical rainforest of Madagascar have shown that higher frequency of jhum cycle decreased the richness and abundance of tree seedlings and sapling, and therefore suggested that reducing slash and burn intensity will increase the speed of tree recruitment and fallow recovery [8]. In present scenario, shifting cultivation is considered unsustainable due to decrease in the jhum cycle with the rise in population resulting in very low input-output ratio [1,9,10]. When the jhum cycle drops below a critical time period, the natural regeneration of vegetation is negatively affected [11-13]. The short jhum cycle in Northeast India has resulted in arrested succession where weedy species were not succeeded by pioneer woody species, and over time the soil seed bank is replaced with seeds of weedy shrubs [14]. Since the practice involves a continuous cycle of clear-felling and regeneration, shifting cultivation sites provide an opportunity to understand the secondary succession patterns. However, relatively few studies have examined the recovery pattern of vegetation following shifting cultivation in Northeast India in general and no attempt was done in Mizoram in particular. The present work is an attempt to study the recovery pattern of plant communities during succession following slash and burn agriculture. Materials and Methods Site description The study was undertaken in shifting cultivation areas in Dulte Pentile Thong1, Rocky Pebam2 and Uttam Kumar Sahoo1* 1Department of Forestry, Mizoram University, Tanhril, Aizawl, Mizoram, India 2North-Eastern Space Applications Centre, Umiam, Meghalaya, India Address for Correspondence Uttam Kumar Sahoo, Department of Forestry, Mizoram University, Tanhril, Aizawl, Mizoram 796004, India, Tel/Fax: 86-21-57762652; E-mail: uksahoo_2003@rediffmail.com Submission: 03 October, 2016 Accepted: 15 November, 2016 Published: 24 November, 2016 Copyright: © 2016 Thong P, et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Journal of Plant Biology & Soil Health Avens Publishing Group Inviting Innovations Research Article Open Access Avens Publishing Group Inviting Innovations Citation: Thong P, Pebam R, Sahoo UK. Recovery Pattern of Vegetation during Succession Following Slash and Burn Agriculture in Mizoram, North-East India. J Plant Biol Soil Health. 2016;3(2): 8. J Plant Biol Soil Health 3(2): 8 (2016) Page 02 ISSN: 2331-8996 village in Champhai district of the state Mizoram in North-east India located at 23.4454° N, 93.1780° E with an area of 3,185.83 square kilometers. Champhai has a moderate climate with average annual rainfall of 1,814 millimeters. In winter the mean temperature varies from 10 °C to 20 °C and in summer, it ranges from 15 °C to 30 °C. Data collection Vegetation analysis were carried out in two replicate sites for fallow aged 5 (J5), 10 (J10), 15 (J15) and 25 (J25) years during the month of January and February. The fallow age of the jhum fields was located from the satellite imagery and the plot was navigated using a GPS. Within each sample plot, ten quadrat of 10 × 10 m, fifteen quadrat of 3 × 3 m and twenty quadrat of 1 × 1 m were randomly laid for identifying trees, shrubs and herbs respectively. The diameter at breast height (dbh) of trees was measured at 1.3 m height but in case of shrubs and herbs the dbh was measured on the ground level. The heights of all individuals were also measured. Mature trees were defined as stems with > 30 cm girth at breast height (1.3 m) and > 5 m height, poles as individuals > 10 cm to < 30 cm girth and > 2 m height, saplings are individuals > 1 cm to < 10 cm collar girth and > 0.5 m to < 2 m height, seedlings as individuals > 1 cm collar girth and > 0.1 to < 0.5 m height and recruits as individuals > 1 cm collar girth and up to 0.1 m height. All plants were enumerated and analyzed in each sample plot. The species sampled in the five layers of vegetation were classified into the five growth forms, namely tree, pole, shrub, herb and recruit. All the emergent species were grouped under trees and poles. Shrubs included species that were found in sub-canopy, were shade-loving, prevalent in understory. Bamboo and woody climbers were also considered in shrub category. Herbs and recruits included shadeloving, annual, biennial or perennial, and herbaceous climbing species. Analysis of plant diversity Analysis of plant diversity was carried out by calculating Shannon Diversity Index (H’), Simpson Dominance Index (C), Pielou’s Evenness Index (J’) and Margalef Species Richness Index (Dmg). These indices were adopted for their low sensitivity to the sample size [15]. H’= -∑ pi ln pi where pi is the proportional abundance of species (i.e., number of species divided by total number in the community). C= ∑ pi2 where pi is same for Shannon’s index. Trees J5 J10 J15 J25 No. of species 39 37 40 53 No. of families 21 20 24 31 Density (haˉ1) 810 96