the role of revegetation for rehabilitation of sodic soils in semiarid subtropical forest, india

The Role of Revegetation for Rehabilitation ofSodic Soils in Semiarid Subtropical Forest, India

K. P. Tripathi1 and Bajrang Singh1,2

Abstract

A 40-year-old rehabilitated forest developed on a sodicwasteland at Banthra, Lucknow, north India, was studiedfor the performance of various species in different vegeta-tion strata as well as their overall impact in soil ameliora-tion. The plant communities of the three selected stands(S1, S2, and S3) of this forest were categorized into threevegetation strata: overstory trees, understory trees andshrubs, and a ground layer with scattered herbs and treeseedlings. The three stands contained 44, 19, and 8 speciesin each stratum, respectively, and three climber species.Importance value index (IVI) and basal area/cover didnot show a clear dominance for particular species, and thisis identified as a mixed forest with deciduous as well asevergreen species. Therefore, dominant species in eachlayer were categorized according to an IVI value of 10and greater than 10% relative basal area. Within eachstratum, species richness and plant population densitydecreased with an increase in plant size. Both speciesdiversity and productivity were relatively high comparedto the reference site because of protection from bioticdisturbances, which cannot be controlled on the reference

site. Creation of new forest on the barren land has contrib-uted significant soil amelioration in the degraded sodic soilof the Indogangetic plains. The soil properties of the threestands did not vary much, although different tree speciesdominated the stands. Maximum soil amelioration wasrecorded for total N, followed by mineralized N, availableN, and organic carbon contents for the nutritional prop-erties. With regard to chemical properties, exchangeablesodium was greatly reduced in comparison to other prop-erties viz pH, electrical conductivity, cation exchangecapacity, and exchangeable Ca content. During 40 yearsof growth and development of the diverse vegetation inthe revegetated forest, microbial C increased to about fivetimes that of the surrounding barren sodic soils. Therewere no significant changes in soil structure even thoughthe water-holding capacity of the soil improved to about53% of the once barren land due to a 7-fold increase inorganic carbon content.

Key words: basal area, biomass, diversity indices, forestcommunity, importance value index, sodic soil, soilamelioration, species richness, vegetation strata.

Introduction

Several million hectares of barren sodic soils occur in aridand semiarid regions of India, Pakistan, Australia, theUnited States, and Canada. In India, these soils are centu-ries old without any vegetation cover. The major con-straints against the establishment and growth of plants areattributed to structural, chemical, nutritional, hydrologi-cal, and microbial deterioration of soils (Gupta & Abrol1990; Naidu & Rengasamy 1993; Sumner 1993; Garg 1998).Management of these soils has been done largely for cropproduction, and there were only a few attempts to afforest(Yadav 1980). Creation of a new forest ecosystem on bar-ren sodic land accomplishes the growing needs of societyand maintains a novel habitat for survival of many organ-isms. In addition, it ameliorates the soil to various degreesthrough the additions of large amounts of organic matterand nutrients from litter and fine roots (Singh 1996, 1998).Brown and Lugo (1994) have illustrated several concepts

of ecosystem rehabilitation for developing and managingdegraded/derelict land, which are widely applicable in di-verse habitats. Even monoculture plantations, if establishedon degraded lands, improve the soil condition and enrichthe species diversity of the understory (Lugo 1997; Singh etal. 2002). Sodic wastelands of north India are degraded tosuch an extent that natural succession is arrested. Few trop-ical tree species have been established using appropriatesilvicultural technology, but this has resulted in the colo-nization of native salt-tolerant species. The knowledgeobtained from the performance of species and processesinvolved in enhancing natural succession is indispensablefor ecorestoration of barren lands. Restoration and man-agement of degraded lands to accommodate high speciesrichness and heterogeneity is thought to improve degradedsoil more efficiently (Nicholson & Monk 1974; Lugo 1995).

The ecological diversity of a particular ecosystem maybe viewed from three perspectives: compositional (speciesrichness), structural (spatial distribution), and functional(ecological processes) (Noss 1990; Heywood & Watson1995), which control the soil development. Sodic soils ofIndogangetic alluvium were formed by periodic submer-gence and evaporation due to natural as well as anthropo-genic reasons. The geological deposition of clay minerals

1National Botanical Research Institute, Rana Pratap Marg, Lucknow 226001,Uttar Pradesh, India.2Address correspondence to B. Singh, email [email protected]

� 2005 Society for Ecological Restoration International

MARCH 2005 Restoration Ecology Vol. 13, No. 1, pp. 29–38 29

(sedimentation) in undulating topography formed thesodic soils in depressions during recurrent submergenceand evaporation. The constructions of road, canal, dams,and railway line have further compounded the problem byimpeding natural drainage. In addition, the lateral move-ments of soil water from the erstwhile sodic patches to thecultivated fields have contributed to the extension of sodiclands in India. Although some studies on species structureand diversity of the natural forests in northern India havebeen done (Singh & Misra 1979; Singh & Singh 1991),these were not collated with the soil properties. The objec-tives of the present study were (1) to quantify the variousvegetation characteristics that have influenced the landrenewal; (2) to determine the species composition ofplanted and colonizing species, both native and exotic, ona site rehabilitated some 40 years ago on highly sodic soil;and (3) to examine the soil chemical and physical proper-ties in rehabilitated and unrevegetated soils to determinethe long-term effects of vegetation on soil characteristics.

Materials and Methods

Site

The forest was established on abandoned sodic soil duringthe 1960s at Banthra, Lucknow, in a subtropical, semiaridregion of north India (lat 26�450N, long 80�530E). Geo-graphically this region is classified as Gangetic alluvialplains of the Uttar Pradesh state due to transported depo-sition of minerals from the Himalayan rocks by the GangaRiver. A large tract of this region consists of cultivatedland (17 million hectares) interspersed with barren sodicland (1.3 million hectares). The prehistoric natural forestsin this region were sparse and, where still present, werereplaced by Sal (Shorea robusta Gaertn. f.) and Teak(Tectona grandis L. f.) forests in the middle of the nine-teenth century. Therefore, the natural dry tropical forestsof Varanasi (lat 24�550N, long 83�30E) and Mirzapur(lat 24�550N, long 82�320E) in India were considered asreference sites, which are about 250–300 km northeast ofLucknow. The dominant species of these forests wereAnogeissus latifolia Bedd., Diospyros melanoxylon Roxb.,Buchnania lanzan Spr., Pterocarpus marsupium Roxb.,Emblica officinalis Gaertn., Boswellia serata Roxb., Aca-cia catechu (L. f.) Wild., Lagerstroemia parviflora Roxb.(among trees) and Holarrhena antidysenterica (Roth)A. Dc., Nyctanthes arbortristris L., Ziziphus glaberrima(Sedgw.) Santap., and Mimosa himalayana Gamble (amongshrubs). See Table1 for family names.

Average annual rainfall at Lucknow ranged from 840to 980 mm during the past 10 years, which is slightly lessthan that at the reference site (1,057 mm/year). More than80% of the precipitation occurs in the monsoon season(July–September), with the remainder of the year verydry. Average minimum and maximum temperature differsignificantly from winter (8�C, night; 20�C, day) to summer

(27�C, night; 40�C, day), indicating a seasonally distinct cli-mate. Average relative humidity was 63% during the year.

The site soil is an Inseptisol (Typic Natrustalf) with siltyclay loam texture. A compact layer of indurated pan com-prising CaCO3 gravels and iron granules exists between0.4- and 0.8-m depths in these sodic soils. Structural degra-dation of the heavy (high density) and impervious soilsleads to crusting in winters and waterlogging conditionduring rainy seasons. During summer, efflorescence ofNaCO3 salt occurs as a powdery layer on the soil surface.Consequently, suspended particulate matter is quite highin the atmosphere during the day. The soil was character-ized by a high pH (>10) and exchangeable sodium per-centage (ESP) (>50) and low electrical conductivity (EC)(<0.4 dS/m) and organic matter content (<1%) prior toplanting (Garg 1987). Carbonate and bicarbonate of Naand Ca were the dominant ions. When the content of solu-ble salts (EC) is low and exchangeable Na high, the physi-cal condition of the soil is usually unfavorable for tillageand the establishment and growth of desired plants. As aconsequence, only a few grasses, viz Sporobolus and Des-mostachia, are found sporadically under these natural con-ditions. Attempts were made to rehabilitate these barrenlands through afforestations as well as other land use sys-tems over an area of about 50 ha acquired during the 1960s.The entire area was demarcated with barbed wire fencingand designated as Banthra Research Station (BRS). Someof the native tree species commonly occurring in tropicalforests of north India (Acacia nilotica, Albizia lebbeck,Albizia procera, Bauhinia variegata, Ficus bengalensis,F. rumphii, Syzygium heyneanum, Syzygium cumini, Ter-minalia arjuna, Derris indica) were planted in plantationpits of 1 m3 that had been filled with a mixture of soil,compost manure, and decomposed leaf litter in 2:1:1 pro-portion on a 5-ha site (Fig. 1). They were also plantedalong the marked boundary of the BRS. The initial popu-lation density was around 1,000 trees/ha. A drainagechannel 1.5 m deep and 2 m wide was developed aroundthe plantation site to prevent waterlogging, which was com-monly observed during the rainy season. Mortality duringthe initial years was greater than 50%, and trees were re-planted in consecutive years. Several species invaded andcolonized this area over time due to changes in microre-lief. The process of natural succession was observed on the5-ha plot, so the site was fenced again to protect it fromherbivores. Seed dispersal and natural regeneration oftrees and many other species gradually extended to coverabout 17 ha of total forest area in the campus of BRS.

Three stands were selected in this forest according togross morphology and a basal area gradient within theoriginal 5-ha revegetated area. Sample plots, each 1 ha,were marked in all three stands denoted as S1, S2, and S3;herbivory (by rabbit, monkey, wild cow) was similar inall three sites. Vegetation analysis was carried out onbelt transects (10 m wide). The method and quadrat sizewere standardized using a species–area-curve relationship.Thirty-four quadrats of 10 3 10 m along three transects

Role of Revegetation for Rehabilitation of Sodic Soils

30 Restoration Ecology MARCH 2005

Table 1. IVI and relative basal area (% of the total) of the dominant species (>10 IVI and >10% BA) in a rehabilitated forest community at

Lucknow, India.

S1 S2 S3

Strata Species IVI BA/LA % IVI BA/LA % IVI BA/LA %

Overstory Albizia lebbeck L. Benth.a,b (Leguminosae) 26.9 16.9 28.5 17.1 35.8 19.0Albizia procera Benth.c (Leguminosae) — — — — 26.6 16.0Azadirachta indica A. Jussb,d (Meliaceae) 12.8 12.6 19.2 — 28.1 —Bauhinia variegata Linn.c (Leguminosae) 41.4 — 65.3 10.2 — —Cassia siamea Lam.c (Leguminosae) — — 10.3 — — —Callistemon lanceolatus Sweet.c,d (Myrtaceae) 10.4 — — — — —Derris indica Bennet.c (Leguminosae) — — — — 13.4 —Ficus bengalensis Linn.a,b (Moraceae) — — 11.9 10.3 — —Ficus glomerata Roxb.a,b (Moraceae) 10.1 — — — — —Ficus retusa Linn.c (Moraceae) 18.6 14.7 — — — —Ficus rumphii Blume.a,b (Moraceae) — — — — 16.0 10.3Leucaena leucocephala (Lam) de wit.b,e

(Leguminosae)20.8 — — — — —

Sterculia alata Roxb.b (Sterculiaceae) 10.1 — — — — —Streblus asper Lour.b,d (Moraceae) 11.4 — 11.6 — 11.5 —Syzygium cumini L. Skeels.c (Myrtaceae) 14.9 — 13.3 — 32.4 —Syzygium heyneanum Wall ex duthieb,d (Myrtaceae) 21.0 — 64.3 21.7 60.3 13.9Terminalia arjuna Wight & Arn.c (Combraetaceae) — — 20.8 12.6 — —

Understory Azadirachta indica A. Jussb,d (Meliaceae) — — — — 10.0 —Bauhinia variegata Linn.c (Leguminosae) — — 33.9 15.9 10.2 —Carissa opaca Stapf ex Hainesb (Apocynaceae) — — — — 11.9 —Clerodendrum vescosum Vent.b (Verbinaceae) — — 20.6 — — —Derris indica Bennet.c (Leguminosae) — — — — 31.7 13.0Diospyros cordifolia Roxb.a,b (Ebenaceae) — — 10.6 — 17.5 —Holoptelea integrifolia (Roxb.) Plancha,b (Ulmaceae) — — — — 10.4 —Ichnocarpus frutescense Linn.b,d (Apocynaceae) — — 11.3 — 12.5 —Lantana camara (L.) Mold.b,d (Verbenaceae) 10.0 — 10.9 — 15.10 —Leucaena leucocephala (Lam) dewitb,e

(Leguminosae)88.3 20.2 — — 18 —

Phoenix sylvestris (L.) Roxb.b (Palmaceae) 43.4 40.3 40.9 38.0 — —Putranjiva roxburghii Wall.b (Euphorbiaceae) 11.3 — — — 10.0 —Sterculia alata Roxb.b (Sterculiaceae) 18.2 — — — — —Streblus asper Lour.b,d (Moraceae) 16.7 — 10.4 — 20.6 —Syzygium cumini L. Skeels.c (Myrtaceae) 10.7 — 16.7 17.9 29.3 17.4Syzygium heyneanum Wall ex duthieb,d (Myrtaceae) 27.2 — 51.5 — 51.3 21.7Zizyphus nummularia W. & Arn.b,d (Rhamanaceae) — — — — 10.2 —

Groundlayerf

Achyranthes aspera Linn.b (Amaranthaceae) 36.9 — — — — —Alangium salvifolium (L .f.)a,b (Alangiaceae) 10.7 — — — — —Barleria prionitis Linn.a,b (Acanthaceae) 44.3 35.9 124.4 49.2 — —Blepharismaderaspatensis L. Heyneex Rotha,b

(Acanthaceae)— — 18.8 — — —

Celastrus paniculatus Willd.a,b (Celastraceae) 18.9 12.0 — — — —Cissampelos pareira Linn.b (Minispermaceae) — — — — 15.04 —Clerodendrum vescosum vent.b (Verbenaceae) — — 73.9 39.9 78.5 37.8Cocculus histutus Linn.b (Minispermaceae) 19.3 12.8 — — — —Ichnocarpus frutescens Linn.a,b (Apocynaceae) 56.0 13.7 — — 41.0 —Lantana camara (L.) Mold.b (Verbenaceae) — — — — 11.5 —Leucus bifolora R. Br.b (Labiaceae) 83.8 12.9 — — — —Malvastrum coromandalianum Linn.a,b

(Malvaceae)14.6 — — — — —

Pedilanthus tithymaloides Poit climberc

(Euphorbiaceae)— — — — 111.4 57.0

Phoenix sylvestris (L.) Roxb.b (Palmaceae) 10.5 — — — — —Putranjiva roxburghii Wall.b (Euphorbiaceae) 46.2 — — — — —

Family in parentheses.aLate successional species.bNaturally colonized species.cPlanted species.dSpecies not found in natural dry tropical forest, India (reference forest).eExotic species.fLeaf area (LA) measurements for ground layer.


MARCH 2005 Restoration Ecology 31

spaced 10 m apart were laid out contiguously in each stand.

Plants were enumerated and measured for growth parame-

ters. Ninety-five percent of the species were identified

through the use of the National Botanical Research Insti-

tute’s herbarium records. Girth of taller trees (>20 cm gbh)

was measured at 130 cm above the ground for overstory

species, whereas diameter of young trees and shrubs occu-

pying less than 10 cm dbh was measured 50 cm above the

ground, using an electronic vernier caliper (Mitutoyo Cor-

poration, Japan). These were classified as understory spe-

cies. Height of small seedlings less than 50 cm in height and

of herbaceous species were measured and represented the

ground layer community. Species structure (frequency, den-

sity, abundance, basal area/cover, importance value index

(IVI), etc.) was determined from the field data (Misra 1968).

The cross-sectional area of the stem at measured levels is

the basal area of all woody species; leaf area for seedlings

and herbs was measured using an area measurement system

(Delta-T Devices, Ltd., Burwell, Cambridge, U.K.). Leaf

area cover (basal cover) for the ground layer was computed

by specific leaf area ratio, which is defined as area per unit

weight of the leaf (Misra 1968). Species having greater than

10 IVI were considered dominant species in each stratum

(i.e., overstory, understory, and ground layer) because IVI

is the sum of relative frequency, relative density, and rela-

tive dominance of the species in a plant community. Basal

area for trees and shrubs and leaf area (basal cover) for

herbs were considered for the determination of dominance.Three composite soil samples were taken at 0–15, 15–30,

and 30–45 cm, from four randomly selected points within

0.3 ha of the middle area in a stand. Because major changes

were in the upper 45 cm, lower depths were not included.

Similarly, soil samples from surrounding barren land (con-trol) were collected and processed for the determinationof various properties. The biological properties, viz mic-robial biomass carbon (MBC), microbial biomass nitrogen(MBN), andmicrobial biomass phosphorus (MBP), were as-sessed by a chloroform fumigation and incubation method(Jenkinson & Powlson 1976). Bulk density (BD) was mea-sured by coring and weighing soil in a measured volume(100 cm3). Water-holding capacity (WHC) was determinedby the method described by Piper (1950). Soil pH and ECwere measured on a 1:2 soil:water suspension (Richards1954). Soil organic carbon was determined by the Walkleyand Black rapid titration method (Kalra & Maynard 1991);exchangeable cations (Ca,K,Na) by flamephotometer (Sys-tronics, New Delhi, India); and ESP by Richards (1954)method. Total Kjeldhal N was estimated by Kjeltech 1035N Autoanalyzer (Tecator AB Box 70, Hoganas, Sweden).Mineral N (NO3

2 and NH4+) was extracted in alkaline

potassium permanganate solution and estimated in thesame way as for total N. Air-dry soil samples (20 g) wereincubated for 2 weeks at room temperature (27�C). Thedifference in pre- and postincubation measurement ofmineral N was recorded as mineralized nitrogen. Total soilP was measured through tri-acid digestion followed bychlorostanus-reduced molybdo-phosphoric acid blue colordevelopment (Jackson 1967). Available P was extracted inmild H2SO4 (0.002 N) for 1 hr and also measured colori-metrically (Jackson 1967).

Results

The rehabilitated forest that developed on sodic waste-land during 40 years consisted of 74 species, compared to

Figure 1. An overview of rehabilitated forest after 40 years of development on once barren sodic wasteland.



37 species in the natural forest, belonging to 35 families,including 40% from the Leguminoceae. Of the 74 speciesidentified, trees were 62%, shrubs 15%, perennial herbs16%, and annual herbs 7%. These species were classifiedas overstory (44), understory (19), ground layer (8), andclimber (3) species. Several overstory species were alsofound in understory and ground layer vegetation, whichwere assumed to be seedlings/saplings of overstory spe-cies. The dominant species were identified based on anIVI, with a value greater than 10. Only a few species hadgreater than 10% of the total basal area (relative domi-nance), which generally indicates dominance (Table 1).Leucaena leucocephala was dominant in one of the threestands (S1), whereas Syzygium heyneanum was domi-nant in all three stands. Bauhinia variegata and Phoenixsylvestris were dominant in both the S1 and S2 stands, andin the S3 stand Albizia procera, Azadirachta indica, andSyzygium cumini were the most dominant species. In theground layer, dominance was shared by several speciesin the S1 stand, whereas in the S2 and S3 stands, it wasshared by a few species, mainly Barleria prionitis in S2 andPedilanthus tithymaloides in S3. Clerodendrum viscosum,an indicator of disturbed conditions, was dominant in two

(S2 and S3) of the three stands but replaced by severalother dominants in S1. About 16 (36%) species in over-story, 10 (53%) in the understory including climbers, and7 (88%) in ground layer invaded the area. The greatestnumbers of dominant species in the overstory and groundlayer were recorded in stand S1, whereas stand S3 hadmore dominant species in the understory vegetation. Thiswas an aggregate community structure over the 40 yearsof forest growth, and development of this rehabilitatedforest.

There were only five species in common between therehabilitated and natural forests in overall species compo-sition of both forests. The density of the overstory wasgreater in the natural forest compared to the rehabilitatedforest, whereas in the case of the understory it was lessthan in the natural forest. The ground layer consisted ofthe greatest number of individuals among all the strata inthe rehabilitated forest (Table 2). However, density onthe basis of number of individuals per unit area does notgive any additional weight to the size of individuals. Basalarea is a composite function of the number and size of theindividuals per unit area (1 ha), which shows the relativecontribution of the species in structure and function of the

Table 2. Population size and plant diversity in both a 40-year-old rehabilitated forest community developed on barren sodic land (mean of three

stands) and a natural forest.

Parameter LayerRehabilitated

Forest Mean ± SENatural

Forest a Mean ± SE

Density (no./ha) overstory 554 ± 28 1055 ± 68.7b

understory 3728 ± 913c 343 ± 121b

ground layer 6813 ± 456d —Basal area (m2/ha) overstory 29.9 ± 1.9e 16.53 ± 0.82b

understory 4.2 ± 1.2 1.34 ± 0.47b

Leaf area (m2/ha) ground layerf 112 ± 66.4 —Standing biomass (mg/ha) overstory 343 ± 27e 89.87 ± 8.62b

understory 3.8 ± 0.4c 4.28 ± 0.34b

ground layer 0.04 ± 0.02d —Species richness (no./ha) overstory 44 ± 3.10 9 ± 0.24g

understory 18 ± 1.60 8 ± 0.28g

ground layer 12 ± 0.72 —Richness index (d) overstory 2 ± 0.1 0.39 ± 0.11g

understory 1 ± 0.1 0.67 ± 0.09g

ground layer 0.8 ± 0.1 —Equitability (e) overstory 1.1 ± 0.07 0.72 ± 0.11g

understory 0.9 ± 0.08 1.06 ± 0.05g

ground layer 0.7 ± 0.08 —Shannon–Wiener’s index (H) overstory 3.6 ± 0.17 1.07 ± 0.32g

understory 3.3 ± 0.27 1.98 ± 0.09g

ground layer 1.8 ± 0.19 —Concentration of dominance(C)

overstory 0.14 ± 0.02 0.58 ± 1.07g

understory 0.23 ± 0.07 0.31 ± 0.04g

ground layer 0.40 ± 0.05 —

aNatural dry tropical forest, Varanasi and Mirzapur, India.bAfter Singh and Misra (1979).cGreatest in S1 stand.dGreatest in S2 stand.eGreatest in S3 stand.f Leaf area.gAfter Singh and Singh (1991).



forest ecosystems. The predominance of overstory andunderstory basal area in the rehabilitated forest indi-cated their healthy state compared to the natural forest(Table 2). As a consequence, their biomass was also ob-served to be more than three times that of the natural for-est. Similarly, species richness has also increased in therehabilitated forest. The overstory vegetation has rela-tively high species richness compared to understory andground layer because many tree species were plantedinitially.

Elements of both richness and abundance can be in-cluded in several mathematical indices computed for thisforest (Table 2). The species richness index or varietyindex (d) is defined as S/ON, where S = number of speciesand N = number of individuals (irrespective of species).This index decreased from overstory to understory by 50%,followed by ground layer in the rehabilitated forest,whereas in natural forest both the indices (e—equita-bility and d—richness) showed an opposite pattern. Thespecies appear to be less evenly distributed in the groundlayer (lowest e) than the overstory vegetation (highest e).Shannon–Wiener index (H = 2�pi 3 ln pi, where pi is theproportional abundance of the species) decreased from theoverstory to the ground layer in accordance with the rich-ness index, indicating that the diversity of tree species wasapparently higher in comparison with the ground flora.However, in the natural forest, most of the diversity indiceswere higher for understory vegetation in comparison to theoverstory. Concentration of dominance (C) = �pi

2 showedthat the dominance was more concentrated in the overstoryvegetation with fewer species in the ground layer, whereasin the natural forest, dominance was concentrated in theunderstory with fewer species in overstory vegetation.

The classification of species and their population inthree vegetation stands as percentage of the total speciesin corresponding vegetation strata revealed low evenness.Only a few species had thousands of individuals present inthe populations, and a high percentage of the total speciesof the respective strata was present as a few individuals oras tens of individuals per species. A general relationshipbetween number of species (S) and number of individualsper species (N/S) shows a common response in most ofthe natural plant communities, in which only a few speciesare represented by a large number of individuals. Therewere only three species, viz L. leucocephala, a fast-growingexotic in the overstory, B. prionitis, an indigenous medi-cinal plant in the understory, and C. vescosum, an indica-tor of disturbed conditions in the ground layer, that werefound in large numbers (thousands) in different strata,whereas a large number of species had relatively smallpopulations, such as 17% species in hundreds, 38% intens, and 41% as individuals. Thus, only 4% of the totalspecies were found in abundance. Percentage of the totalspecies belonging to the unit population (individualsbetween one and nine) decreased from the overstory toground layer, with an inverse pattern for the very large(thousands) populations.

Consequent upon the development of vegetation coveron the barren land, the degraded soil was ameliorated sig-nificantly. The maximum improvement was observed inbiological properties owing to the manifold increases inMBC, MBN, and MBP in forested soil during 40 years(Table 3). MBC and MBN might have contributed to anincrease in the relatively high basal area (biomass) ofthe overstory trees. BD was reduced in forested soil,whereas WHC increased in comparison to nonforestedbarren soil (Table 3). Textural analysis has further re-vealed that the clay proportions decreased in forested soiland percent sand increased in comparison to barren soil.Simultaneously, the porosity increased in forested soil.Soil pH and EC were reduced significantly in forested soilin comparison to the surrounding sodic soil as a result ofincreased organic carbon content in the forested soil.There were three main sources of C enrichment in the soil,viz leaf litter, fine roots, and herbs, which contributed tothe annual recycling of organic matter. The high carboncontent of the soil appears to support the development ofluxuriant ground vegetation in the tropics. As a result, Cand N status of the surface 15 cm of soil depth increasedsignificantly in comparison to barren land. Lower soildepths (45 cm) had C and N pools 1.5 and 3.4 times lessthan that of barren land compared to 7 and 8 times that ofbarren land in the surface soil (15 cm). The differentfractions of N have shown major increases in forested soil(gain of 120–259%) because of the inputs received fromthe mineralization of organic matter (Table 3). Therewas a great reduction in ESP of about 90% in the barrensodic soils.

Discussion

This was a case study to rehabilitate barren land undera forest ecosystem, which demonstrates that opportunitiesfor the restoration of sodic wastelands exist even withseverely degraded land where natural succession does notoccur without management practices. Many other repli-cated trials with suitable experimental designs failed dueto lack of proper attention, protection, funds, and provenafforestation technology (Yadav 1975, 1980). This site hasbeen successfully reforested because of intensive care,biotic protection, and repeated revegetation attempts withadapted tree species.

Although tree density of the natural dry tropical forest(reference site) was relatively high, the understory treesand shrubs were less abundant compared to the rehabili-tated forest. Tree density has been found to be as low as40 plants/ha in a subtropical wet forest and as high as2,005 plants/ha in evergreen forests of Brazil and evenhigher (3,310–3,167 plants/ha) in a mangrove forest ofGuiana (Rao et al. 1990; Gilliam et al. 1995; Haase 1999).In our forest, tree density of 554 plants/ha is intermedi-ate with respect to these subtropical and evergreen forests.



The average basal area (30 m2/ha) of the rehabilitatedforest was greater than that of other dry tropical forestsof India (Jha & Singh 1990; Singh & Singh 1991). Itappears that the carrying capacity for supporting the treestock on sodic soils differs by species, ranging from 12 to38 m2/ha basal areas in Acacia nilotica and Eucalyptuscamaldulensis plantations of the same age, respectively(Singh et al. 2000). These values are comparable to 17–40and 20–75 m2/ha for dry and wet forests of the world,respectively (Murphy & Lugo 1986a). Basal area in a sub-tropical dry forest of Puerto Rico was estimated to be 19.8m2/ha (Murphy & Lugo 1986b). Therefore, on averagea basal area value of 20–30 m2/ha may be appropriate insuch environments represented by most of the sites in ourstudy. The basal area of this rehabilitated forest is alsocomparable to that of deciduous and evergreen forests(16–33 m2/ha) of Brazil (Haase 1999).

IVI of this rehabilitated forest, in general, ranged from10 to 77 including 10 to 48 for overstory species. Thesevalues agree with the range of 11–52 for subtropical treespecies of a wet hill forest in India (Rao et al. 1990). TheIVI of tree species of a protected forest in Orissa (India)ranged from 12 to 55 (Verma et al. 1997), and trees ofa dry tropical forest of the Vindhyan region ranged from 3to 32 in IVI (Singh & Singh 1991). Thus, tree species inour rehabilitated forest represents a modest IVI comparedto these two estimates.

Because many tree species were planted in our rehabili-tated forest, species richness and tree diversity indexexceeded that observed in dry tropical forests of India(reference site), whereas ground layer diversity was al-most similar to that of other native tropical dry forests(Jha 1990). The Shannon–Wiener diversity index of thisforest was relatively low compared to tropical rainforests(3.8–4.8) of Silent Valley, India (Singh et al. 1984).

Successional patterns on plant species diversity duringrehabilitation of barren land in India are not known, andthus species recruitment/replacement rates with time fromthe initiation of the plantation are not understood. Natu-ral forests are being diminished because of high popula-tion pressure for timber, industrial pulp, and fuelwood,which have damaged the ecosystem diversity at variouslevels. For instance, a dry tropical forest of the Vindhyanregion in India, considered here as the reference site, con-sisted of a lower species diversity and basal area comparedto the rehabilitated forest due to several biotic disturban-ces (Singh & Singh 1991). The species compatibility alsodiffers substantially between the rehabilitated and naturalforests because many of the species planted in the rehabil-itated forest are early successional species and thereforedo not resemble the late-successional species of naturalforests. But at the same time when a forest is not disturbedduring growth and development, the species richness isreduced (Odum 1960). Therefore, a moderate disturbance

Table 3. Soil physical, chemical, and nutritional properties in a 40-year-old rehabilitated forest and a nonforested control site at Lucknow, India

(mean ± SE, 0–15 of cm soil depth).

PropertiesRehabilitated

Forest SoilControl

(Bare) Soil LSD 05Gain % onBare Soil

BD (g/cm3) 1.42 ± 0.05 1.85 ± 0.04 0.12 23WHC (%) 52.36 ± 0.32 34.25 ± 4.4 11.14 53Sand (%) 36.20 ± 2.4 29.20 ± 2.9 n.s. 24Silt (%) 28.30 ± 0.66 24.00 ± 2.31 n.s. 8Clay (%) 35.50 ± 1.76 46.60 ± 2.4 n.s. 24Particle density (g/cm3) 2.56 ± 0.06 2.73 ± 0.03 n.s. 6Porosity (%) 45.30 ± 0.93 28.80 ± 0.79 5.13 57pH 8.34 ± 0.03 10.13 ± 0.17 0.46 18EC (dS/m) 0.20 ± 0.01 0.96 ± 0.19 0.46 82Organic carbon (%) 1.06 ± 0.10 0.15 ± 0.05 0.45 86Cation exchange capacity (cmol/ kg) 11.33 ± 0.44 18.00 ± 0.33 3.1 37ESP (%) 6.66 ± 0.52 65 ± 7.75 18 90Total nitrogen (mg/g) 0.79 ± 0.05 0.22 ± 0.06 0.27 259Available nitrogen (lg/g) 59.19 ± 3.7 26.90 ± 9.74 8.57 120Mineralized nitrogen (lg/g) 18 ± 1.65 6.72 ± 1.71 8.7 168Total phosphorus (lg/g) 514 ± 48 474 ± 45 n.s. 8Available phosphorus (lg/g) 11 ± 0.72 12.90 ± 2.37 n.s. 15Exchangeable potassium (cmol/kg) 0.66 ± 0.02 0.39 ± 0.08 n.s. 69Exchangeable calcium (cmol/kg) 9.49 ± 0.33 6.88 ± 0.79 2.15 38Exchangeable sodium (cmol/kg) 1.01 ± 0.01 9.98 ± 1.92 4.44 90MBC (lg/g) 334 ± 35 86 ± 9.3 18 288MBN (lg/g) 55 ± 13.6 16 ± 4.2 9.6 244MBP (lg/ g) 25.5 ± 4.1 7.2 ± 2.14 3.1 254

LSD = least significant difference; n.s. = not significant.aThrough student t test.



may favor high species richness. Other studies on speciesdiversity with succession have reported these conflictingpatterns. Monk (1967) found that diversity increased withsuccession. Others noted highest diversity in the earlystages of succession (Habeck 1968; Long 1977; Peet 1978),whereas several others have depicted a polynomial in-crease followed by a decrease during succession (Margalef1963, 1968; Loucks 1970; Auclair & Goff 1971;Schoonmaker & Mackee 1988). In some cases, diversitymay show multiple peaks during succession as found byHalpern and Spies (1995).

In general, most of the diversity indices (richness, Shan-non–Wiener’s, and equitability) increased from groundlayer to overstory vegetation in our study due to introduc-tion of various tree species in the sample plots, whereasmost shrubs and herbs invaded naturally. As a conse-quence, species and density in the understory and groundlayer vegetation developed through interactions amongthemselves as well as resource competitions. The approxi-mate 50% decrease in overstory tree density since theirplanting may be attributed to either a decrease in theresource levels or their sensitivity to high ESP in sodicsoils, which has limited the density during the 40 years offorest development. This suggests that on degraded sites,introduction and nurturing suitable species provides betterresult for rehabilitation rather than promoting the growthof natural invaders; therefore, some of them, like the non-native Leucaena leucocephalla, which was abundant in S1,should be removed to reduce competition to facilitate thegrowth of other desirable species and provide space fornew native invaders.

The present status of diversity and productivity in thisforest has ameliorated the soil at various degrees. Ourfindings revealed that soil structure changed with the in-corporation of humus in the soil and leaching of clay par-ticles down to depth (reducing clay fraction in upper 0- to30-cm strata). Because a high clay fraction impedes watermovement, its reduction associated with the fine root pro-liferation must have improved the water infiltration rate.Penetration, expansion, and subsequent decay of the fineroots create channels and change the soil structure andhydrology. Various organic acids are released duringdecomposition and humification of litter and fine roots,including the carbonic acid formed from the CO2, evolvedfrom the respiration of live roots. These acids dissolve theCaCO3 granules and increase the soluble Ca++ in soil solu-tion. Na+ being a monovalent cation is easily replaced bydivalent Ca++ on clay particles. The free Na+ ions areleached down during infiltration. This process has resultedin the increase in cation exchange capacity and a decreasein ESP. Consequently Ca++ and K+ have moderatelyincreased in forested soils, and Na+ has decreased fromthe nonforested control soils. The different forms of nitro-gen (total, available, and mineralized) increased in for-ested sodic soil compared to the barren land. However,the changes in total and available P including K werenot significantly different from the control site. Because

minerals largely contribute P content in the soil, it did notdepend as much on minor differences of soil organic mat-ter built-up by the vegetation. Although microbial carbonincreased significantly in the barren site, it did not reachthe levels of 564 and 609 lg/g that are generally found inthe natural dry tropical forest (reference site) of thisregion (Raghubanshi 1991; Srivastava & Singh 1991).However, MBN and MBP levels compare fairly well withthe natural forests. If C and N are considered as the recla-mation indices to examine the status of the restoration ofdegraded lands, the surface 15 cm of soil depth in the pres-ent forest has almost approached the status of the naturaldry tropical forests found in this region (Singh & Misra1979; Singh & Singh 1991); however, the increases inthe soil organic matter in lower depth below 30 cm wererelatively low.

The self-regenerating mixed sand dunes of South Africahave exhibited a recovery of about 50% in C and N statusin 16–18 years in the top 10 cm of the soil (Van Arde et al.1998). In comparison to this index, the present recoveryin our study of 86% (C) and 259% (total N) in the surface15 cm of the soil during 40 years appears to be a goodprogress. A relatively high recovery of N has been associ-ated with improvement in the soil biological properties.Reductions in ESP to the level of about 22–65% havebeen measured in 8-year-old, high-density monocultureplantations with different species on sodic wastelands(Garg & Jain 1992; Jain & Garg 1996; Garg 1999). Thisdemonstrates that manipulation of population density ofadaptable species on a specific site may accelerate the soilrestoration process.

Several studies have suggested that soil C, N, P, andmoisture may influence the succession dynamics and com-munity structure of forests (Lloyd & Pigott 1967; Walkeret al. 1981; Tilman 1986). Studies on both primary and sec-ondary successions reveal N to be a major nutrient limita-tion that increases with successional age (Tilman 1987).On N-poor sites, N fixers colonize initially, which laterpromote the recruitment of other species as N level isenhanced. Litter decomposition and nutrient cyclingbecome more efficient in older, rehabilitating tree standsdue to feedback processes of soil–plant interactions(Vitousek &Walker 1989; Wedin & Tilman 1990). In addi-tion, pH decreases as vegetation development proceedsdue to leaching and accumulation of weak organic acids(Crawley 1986).

Rehabilitation and management of sodic soils throughafforestation have been intensified in northern Indiaduring the past two decades. In order to ensure the rightprescription, model development of desired communitystructure, periodic monitoring of species succession, andsoil rehabilitation processes are necessary. When a nitro-gen-poor sodic site is rehabilitated, legumes perform rela-tively well as initial colonizers (Garg 1999; Singh et al.2002). Many species that naturally regenerate in reha-bilitated forests are assumed to become acclimatized onthe once degraded site owing to the transformation in



microrelief. About 28 species were found naturally regen-erating in our rehabilitated forest, which was about 64%of the total species listed in the forest, and these may beconsidered adapted to sodic soils. Some of the importantones may be cited here for further trials on sodic lands, vizAegle marmelos, Alangium salvifolium, Albizia lebbeck,*Azadirachta indica,* Bauhinia variegata, Cassia siamea,Cassia fistula, Cordia dichotoma, Dalbergia sissoo,* Ficusglomerata, Holoptelea integrifolia, Pithecellobium dulce,Derris indica, Putranjiva roxburghii, Sterculia alata,Terminalia arjuna,* etc. Five of these (*) were previouslyestablished in replicated plot trials (15-year-old monocul-ture) in the Biomass Research Centre, at Lucknow, undera fuelwood production program on substandard sodic soils(Chaturvedi & Behl 1996).

The changes in soil properties are influenced by a largenumber of vegetation characters such as species diversity,population growth, density, basal area, productivity, litterand fine roots quantity, quality, and their decompositionrate affecting the nutrient-cycling efficiency. Diversity andproductivity both contribute equally to soil reclamationbecause some properties are more affected by diversity,whereas others are regulated by inputs resulting from pro-ductivity (Singh et al. 2004). Forest growth influences soilproperties and aids in the amelioration of undesirableproperties. Biorehabilitation of sodic soils is a very slowprocess. Reforestation of sodic soils could not offsetthe degraded conditions completely even in 40 years andhas made significant soil amelioration to a cultural depth(0–15 cm). Three vegetation layers of this forest playeda vital role in the ecorestoration process by efficient utili-zation of the available resources (carbon, energy, nutri-ents, water, etc.) from different aerial and subaerial strata.

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the role of revegetation for rehabilitation of sodic soils in semiarid subtropical forest, india

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