Scholarly article on topic 'Invasion establishment and habitat suitability of Chromolaena odorata (L.) King and Robinson over time and space in the western Himalayan forests of India'

Invasion establishment and habitat suitability of Chromolaena odorata (L.) King and Robinson over time and space in the western Himalayan forests of India Academic research paper on "Biological sciences"

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Abstract of research paper on Biological sciences, author of scientific article — Gautam Mandal, Shambhu Prasad Joshi

Abstract Habitat suitability assessment of the invasive species Chromolaena odorata (L.) King and Robinson from Himalayan forests reveals some interesting findings and conclusions. At different study sites, 29 of 72 species were exotic and invasive and comprised 21 genera and eight families. Indigenous species accounted for 59% of the total species and comprised 26 genera and 11 families. Perennials outnumbered the annuals in all study sites. Chromolaena odorata and Lantana camara L. were the only invasive species that were common to all sites with high importance value index values. The present work reveals that sites with high biotic pressure, maximum temperature variation, open forest canopy, and free from herbivory are the most suitable habitat for the growth of C. odorata. An elevated level of phosphorus, potassium, magnesium, soil organic matter, and nitrogen and acidic soil in all invaded sites are possible reasons for further invasion of C. odorata.

Academic research paper on topic "Invasion establishment and habitat suitability of Chromolaena odorata (L.) King and Robinson over time and space in the western Himalayan forests of India"

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Original article

Invasion establishment and habitat suitability of Chromolaena odorata (L.) King and Robinson over time and space in the western Himalayan forests of India

Gautam Mandal*, Shambhu Prasad Joshi

Ecology Research Laboratory, Department of Botany, DAV (PG) College, Dehradun, Uttarakhand, India

ARTICLE INFO

ABSTRACT

Article history:

Received 29 July 2014

Received in revised form

25 August 2014

Accepted 17 September 2014

Available online 23 September 2014

Keywords: Habitat suitability Importance value index Invasive alien species Soil analysis

Habitat suitability assessment of the invasive species Chromolaena odorata (L.) King and Robinson from Himalayan forests reveals some interesting findings and conclusions. At different study sites, 29 of 72 species were exotic and invasive and comprised 21 genera and eight families. Indigenous species accounted for 59% of the total species and comprised 26 genera and 11 families. Perennials outnumbered the annuals in all study sites. Chromolaena odorata and Lantana camara L. were the only invasive species that were common to all sites with high importance value index values. The present work reveals that sites with high biotic pressure, maximum temperature variation, open forest canopy, and free from herbivory are the most suitable habitat for the growth of C. odorata. An elevated level of phosphorus, potassium, magnesium, soil organic matter, and nitrogen and acidic soil in all invaded sites are possible reasons for further invasion of C. odorata.

Copyright © 2014, National Science Museum of Korea (NSMK) and Korea National Arboretum (KNA).

Production and hosting by Elsevier. All rights reserved.

Introduction

Chromolaena odorata (L.) King and Robinson (henceforth C. odorata), a species of the Asteraceae family—also known as "Christmas bush", "bitter bush", "Siam weed", "baby tea", "car-iaquillo", "Santa María", and "fleurit Noël"—is a scrambling shrub (Howard, 1989; Liogier, 1997). It may reach 1 m or more as a free standing shrub and 4 m or more when climbing into trees or shrubs. Stems reach 2 cm in diameter (Figure 1). The plants are maintained by a system of abundant, yellow, fine lateral roots. Individual branches are long with relatively little branching. The seeds are a brown-gray to black achenes that are 4 mm long with a pale brown pappus 5—6 mm long (Howard, 1989; Liogier, 1997).

Chromolaena odorata is native plant from Florida to the West Indies and from Texas through Central America and through South America to Argentina (Howard, 1989; Liogier, 1997). It is found accidentally or is deliberately introduced. It is naturalized

* Corresponding author. Tel.: +91 9459695429. E-mail address: gautam231@gmail.com (G. Mandal). Peer review under responsibility of National Science Museum of Korea (NSMK) and Korea National Arboretum (KNA).

throughout much of the tropics such as in Guam and Hawaii (Pacific Island Ecosystems at Risk, 2001). It is reportedly one of the world's most invasive weeds, and is a serious weed in central and western Africa, India, Australia, the Pacific Islands, and Southeast Asia (McFadyen, 2003). This species has a wide tolerance to various climates and has invaded five continents (i.e. Asia, North and South America, and North and South Africa; Kriticos et al., 2005). It can become quickly established and smother plant crops, forestry, and native vegetation (McFadyen and Skarratt, 1996). It is unpalatable and noxious and may cause death if domesticated animals ingest it (Aterrado and Bachiller, 2002). Chromolaena odorata distribution areas are roughly divided into three types: (1) areas where Chro-molaena is not yet reported; (2) areas where Chromolaena is introduced; and (3) areas where Chromolaena is native (Zachariades et al., 1999) (Figure 2). The geographical distribution of C. odorata is limited to regions within the latitudes of 30° N and 30° S and in areas with a rainfall of > 200 cm and where air temperature ranges 20—37°C (Timbilla and Braimah, 2002).

The weed grows in areas that are near sea level to areas > 1000 m in elevation (Binggeli, 1999). It thrives in all types of well-drained soil and can grow on soils that are relatively low in fertility. Disturbance is required before a site can be colonized (Pacific Island Ecosystems at Risk, 2001). Once established, this weed competes aggressively with herbs, grass, and shrubs in open areas. In its

http://dx.doi.org/10.1016/j.japb.2014.09.002

2287-884X/Copyright © 2014, National Science Museum of Korea (NSMK) and Korea National Arboretum (KNA). Production and hosting by Elsevier. All rights reserved.

Figure 1. A sketch of Chromolaena odorata (L.) King and Robinson. (Image courtesy of http://www.tramiLnet/fototeca/imageDisplay.php?id_elem=169&lang=en; Reprinted with permission).

native range, it frequently grows on roadsides, riverbanks, vacant lots, abandoned farmland, and neglected pastures. According to Ohtsuka (1999) this weed can be found in a particular niche in the slash-and-burn agriculture cycle, and in Borneo this weed along with other perennial grasses and shrubs are invaded within 3 years of abandonment and are gradually replaced by trees. The species is shade intolerant and will not grow under a closed forest stand. It is also intolerant of frost (Binggeli, 1999) and is limited by drought (i.e. approximately < 900 mm of mean annual precipitation).

The invasion of natural communities by these introduced species constitutes a major threat to biodiversity globally (Lodge 1993; Adair and Groves, 1998). Invasive plants adversely affect ecosystem structure and function in habitats throughout the world by reducing native species richness, altering water or fire regimes, changing soil nutrient status, and altering geomorphological processes (Macdonald et al., 1989; Cronk and Fuller, 1995; Rose, 1997). A major challenge for invasive plant research is developing the ability to predict the invasiveness of species and invisibility of habitats (Kareiva, 1996). Human activities have transported the plants worldwide, yet only approximately 10% of introduced species become established, and 10% of these become invasive (Groves, 1991).

For predicting invasiveness, several generalizations have been proposed such as the degree of similarity between the new climate and the native climate (Cronk and Fuller, 1995; Crawley et al., 1997)

and invasiveness in other new habitats (Scott and Panetta, 1993; Reichard and Hamilton, 1997). Many studies have focused on identifying plant traits that confer invasiveness. Early work by Baker, (1974) identified the attributes of an ideal weed, including fast vegetative growth to flowering, production of large quantities of seed, vegetative propagation, and nonspecialized pollination system, and germination requirements. This approach is useful as a checklist of potential warning signs, although it is too broad to be of much predictive value (Noble, 1989). Some studies have been too broad in their scope by using too wide a range of habitats [e.g. worldwide (Binggeli, 1996) or North America (Reichard and Hamilton, 1997)] or by combining agricultural and environmental weeds (Newsome and Noble, 1986; Williamson and Fitter, 1996a). The lack of clear patterns has led to the suggestion that predicting invasiveness is impossible (Williamson and Fitter, 1996b).

Such studies have lacked environmental specificity (Newsome and Noble, 1986; Reichard and Hamilton, 1997). The most successful attempts to predict invasiveness based on species attributes have been confined to the genus Pinus (Rejmánek and Richardson, 1996; Grottkopp et al., 2002). An alternative hypothesis to explain the success of exotic species is that they are released from coevolved natural pests and predators in the new environment. Release from natural pests may increase plant fitness by several potential mechanisms: (1) a direct effect, the so-called "predator-release effect" (Newsome and Noble, 1986); (2) reallocation of resources from defense to growth (Crawley et al., 1997); or (3) as a result of selection of genotypes with increased allocation to growth and decreased allocation to defense [i.e. the evolution of increased competitive ability hypothesis (Blossey and Notzold, 1995)].

Landscape transformation by humans has been rapid, widespread, and extraordinarily thorough in many cases (Whitney, 1994). It is therefore no coincidence that anthropogenic disturbances resulting in habitat destruction and fragmentation are viewed as the leading threats to biodiversity, followed by the threat posed by invasive species (Wilcove et al., 1998). Fragmentation is characterized as "landscape level" disturbance and the disturbance is nearly unanimously acknowledged to influence invasive spread (Fox and Fox, 1986). Thus, habitat loss and fragmentation may facilitate the spread of invasive species. Other studies have shown that, in dry regions, increasing the water supply (whether by natural rainfall or by experimental additions) increases the invisibility of vegetation by a direct effect of the water supply or by improved access to mineral nutrients (Burgess et al., 1991; Dukes and Mooney, 1999).

In this study, we hypothesized that, after the Doon Valley became the capital of the state Uttarakhand, there has been a considerable change in economic possibilities available to local and outside people; hence to gain maximum benefit, there is a significant increase in the migration of people from within the state and outside the state. This increasing human population of the state has decreased land cover and forest size and has increased the number of people per forest area. Thus, increasing pressure on remaining forest areas may have led to changes such as forest canopy gaps, reduced understory biomass, and soil nutrient alteration, which further intensify the invasion of introduced species such as C. odorata, Lantana camara L., and Parthenium hysterophorus L. In particular, we try to disentangle the factors influencing the establishment of invasive exotic species in different altitudes and disturbance types. In this study, we attempted to assess the distribution, frequency, and abundance of C. odorata from different locations in the Doon Valley, based on different plant species' importance value index calculation (a simple yet powerful tool for assessing biodiversity). The scope of present study does not restrict itself to this alone but also throws light on the other dominant plant

Figure 2. International Physiographic distribution of Chromolaena odorata (L.) King and Robinson.

species growing along with C. odorata. Soil sampling was performed to calculate different physicochemical parameters so as to identify their role in the invasion success of C. odorata and other invasive species. Pertaining to this we also wanted to determine whether altitude, topography, and human-disturbed areas increase invasiveness and whether exotic species richness increases with increased availability of limiting resources.

Materials and methods

Study sites and experimental design

The present study site was the Doon Valley, which is situated in western Himalaya and is a major part of Uttarakhand, India. The state of Uttarakhand is in the northern part of India and shares an international boundary with China in the north and Nepal in the east. It has an area of 53,483 km2 and lies between latitude 28°43'N and 31°28'N and longitude 77°34'E and 81°03'E. The state has a temperate climate, except in the plain areas where the climate is tropical (Figure 3). The average annual rainfall of the state is 1550 mm and temperatures range from 0°C to 43°C (Forest Survey of India, 2013). Approximately 19% of the total geographical area of the state is under permanent snow cover, glaciers, and steep slopes where tree growth is impossible because of climatic and physical limitations (Forest Survey of India 2013). According to 2013 forest survey of India assessment, the total forest cover of the country is 697,898 km2, which amounts to 21.23% of the country's total geographic area. The recorded forest area of the state is 24,508 km2, which constitutes 45.82% of the state of Uttarakhand geographical area (Forest Survey of India, 2013). The whole valley has presumably been at one time or another in the condition of a single bed, but a large proportion of the area is now covered with Shorea robusta Roxb. ex Gaertner f. or with sal forest. The sal is a tree of an exacting

nature and requires good fertile moist soil with good drainage. It is also a species that is so severely damaged by frost when young that it cannot grow in open places, except under the protection of other trees. Therefore, sal forests cannot have come directly into existence on open exposed alluvial single soils. It is not possible to actually observe the successive stages in the development of the sal forests in the same area because the changes are gradual and may occupy a long period; however, an examination of the existing types of forests and what is taking place leaves little doubt as to the stages through which the sal forests is ultimately developed.

In the years 2011—2013, phytosociological studies of the selected sites were conducted constantly during rainy seasons for herbaceous vegetation and once for trees and shrubs. The vegetation was analyzed by random sampling to give the most representative composition of vegetation. The vegetation survey was performed by the nested quadrat method. Twenty quadrats, each 10 x 10 m2 or 5 x 5 m2, were laid on each site for studying trees or shrubs, respectively, whereas 30 quadrats, each 1 x 1 m2, were laid for herbaceous vegetation. The percentage abundance of all plant species, including C. odorata, was computed as the length (in meters) intercepted by each species within a transect line. The biomass (kg dry weight/m2) of large colonies of C. odorata was recorded. The location of each site was recorded using a global positioning system (GPS) device (Garmin 72; Garmin, Olathe, KS, USA). Fresh samples and photographs of each plant species were obtained and an herbarium was made for identification purposes. Plants were identified according to the standard protocol and with the help of experts from the Archives of Forest Research Institute in Dehradun, India. Table 1 shows the detailed description of the study sites.

Site 1

These sites grow understory vegetation or forest periphery vegetation at an altitude above 1000 m. The forest canopy is dense

and dominated by Quercus leucotrichophora A. Camus trees. The area is mountainous with well-developed soil.

Site 2

These sites are typically in the swampy localities of the Doon Valley in some areas in which the dominant tree taxa include Shorea robusta Roxb. ex Gaertner f., Diospyros malabarica (Des-rousseaux) Kosteletsky, Toona ciliata Roemer, and Trewia nudiflora L. The area has well-developed soil with wet to moist conditions.

Site 3

This study site is at an altitude of 750 m on the north-facing slopes in the northern part of the Doon Valley. The soil is dry and the temperature is relatively cool. The dominant tree taxon is Shorea robusta Roxb. ex Gaertner f. and Syzygium cumini (L.) Skeels. There is relatively little biotic interference in these sites because they are away from human settlement.

Site 4

The site is heavily disturbed because of a heavy influx of year-round tourists. The basis of the selection of the sites was to create variation in the habitat, aspects, average top soil depth, and biotic interference level. The major plant species are Adhatoda vesica

Nees., Carex nubigena D. Don ex Tilloch and Taylor, Barleria cristata L., Ageratum conyzoides L., Barberis aristata DC. Bidens pilosa L., and Boehmeria platyphylla D. Don.

The dominance of the plant species was determined by the species' importance value index (IVI). Vegetation composition was evaluated by analyzing the frequency, density, abundance, and IVI, according to Mishra (1968) and Curtis and McIntosh (1950), as follows:

Frequency

Total no. of quadrats in which the species occurred

Total no. of quadrats studied

Relative Frequency

Frequency of a species Frequency of all species

Density

Total no. of individuals of a species

Total no. of quadrats studied

Relative Density (

No. of individuals of a species No. of individuals of all species

Table 1

Site characteristics of the Doon Valley in the western Himalayan part of India.

Site Elevation Aspects (m)

Location in the Doon Average depth Relative soil, moisture, and Dominant tree and shrub taxa Valley of the top soil temperature condition

Biotic interference

1 1200 South North (Kolhu Khet) 0-30 Moist and cool

2 550 Open/north South (Mothronwala) 0-30 Wet and warm

3 750 North North (Rajpur Forest 0-30 Dry and cool

peripheries)

4 700 Northeast North (Sahastradhara) 0-30 Dry and cool

Quercus leucotrichophora, Lantana Grazing, lopping camara

Shorea robusta, Lantana camara Heavy grazing, camping

and lopping

Shorea robusta, Lantana camara Moderate biotic interference

Mangifera indica, Lantana camara High tourist activity zone,

gravelly substratum

Abundance

Total no. of individuals of a species Total No. of quadrats in which the species occurred

Relative Dominance (%)

Basal area of a species Basal area of all the species

Importance Value Index (IVI)

relative frequency + relative density + relative dominance

Basal cover is the portion of ground surface occupied by a species (Greig-Smith, 1983). The basal area measurement was based on the following formula:

Total basal cover (TBC) = mean basal area of a species

x density of that species

Mean basal area MBA

(MBA) = pr2 (in cm2; r = C/2 x 3.14)

4 x p2

in which C = average circumference of one individual of that species and MBA is expressed as cm-2 plant-1 (Mishra, 1968).

Diversity indices and evenness

The Shannon—Wiener diversity index (H') (Shannon and Wiener, 1963) was calculated from the IVI values using the formula provided by Magurran (1988).

H' = pi In pi i = 1

in which s = the number of species; pi = the proportion of individuals or abundance of the ith species expressed as a proportion of total cover; and ln = log base n.

The beta diversity was computed to measure the rate of species change across the sites using the following formula (Whittaker, 1975):

Beta diversity i

Equitability or evenness was calculated as described by Pielou (1969):

Equitability (J)

Ef= i Pi In Pi

in which s = the number of species; pi = the proportion of individuals or abundance of the ith species expressed as a proportion of total cover; and ln = log base n.

Soil chemical analysis

Air-dried 2-mm sieve soil samples collected from the two studies were subjected to routine chemical analysis. Total nitrogen was determined by the micro-Kjeldahl approach and available phosphorus was determined by molybdenum blue colorimetry. Exchangeable potassium, calcium, and magnesium were extracted using ammonium acetate; potassium was determined by flame

photometer; and calcium and magnesium were determined by atomic absorption spectrophotometer (Okalebo et al., 1993). The pH and electrical conductivity of the soil (soil:water, 1:5) was determined by a water analysis kit (Systronics Ltd., Gujarat, India). The organic carbon of the soil was determined by the rapid titration method of Walkley and Black (1934), as described by Piper (1944). Statistical analysis was performed using XLSTAT version 2011 software (Addinsoft, Rue Damremont, Paris, France) for Microsoft Windows.

Results

Chromolaena odorata was present in open well-drained ground (e.g. dry and exposed slopes, abandoned fields, and pastures) and in the peripheries and deep forests throughout the study sites. All sites had fundamentally been heavily invaded by C. odorata. Our dry weight biomass estimates ranged 0.58—1.88 k/m2 for large colonies of the species. Seventy-one plant species were recorded as associates of C. odorata (Table 2). This value does not include all species present in these sites because many species did not occur within the quadrat sampled. Of the total 72 species, 29 were exotic and comprised 21 genera and eight families. Indigenous species accounted for 59% of the total species (Figure 4). Most species were dicots rather than monocots and perennials were outnumbered in all sites. Chromolaena odorata was the only common species in all four sites. Eight species were present in two sites, whereas the remaining species were exclusive to a particular site.

Our studies have shown that the vegetation of various sites was well dominated by several families. The varied colored plant species reflect colorful vegetation; however, the flowers of these species are unattractive and large. Most species were perennial and perennated either by dormant aerial buds or by root stocks. As regards plant density, C. odorata outnumbered all other species from the beginning. In general, standard errors of the plant numbers were high, which indicated a patchy distribution over the plots. The density of C. odorata, shrubs, and other grasses decreased with increasing duration of fallow age. The soil cover was patchy, partly because C. odorata seedlings were numerous while growing vigorously around the stumps of the same species. Chromolaena odorata was the dominant species in sites 1 and 3 with maximum IVIs of 88.04 and 52.62, respectively. The codominants include Lantana camara L. Boenninghausenia albiflora (Hook) Reichb ex Meissn, Myrisine africana L., Randia tetrasperma Benth. and Hook. f., Daphne cannabina Wall., and Lonicera quinquelocularis Hardw.

The sites were represented by 22 commonly growing plant species of which 10 plant species alone contributed 2/3 of the IVI values (Table 2). Canopy cover of the understory vegetation is 56% for the dominant invasive C. odorata.

In site 2, Adenostemma lavenia (L.) O. Kuntze was the dominant species with maximum IVI (76.10). The codominants included Lantana camara L. (IVI, 28.77) Rorippa nasturtium-aquaticum (L.) Hayek. (IVI, 65.35), Perilla frutescens (L.) Britt. (IVI, 32.96), and Ageratum conyzoides L. (IVI, 32.40). In this site, the presence of the invasive species C. odorata was surprisingly strong (IVI, 21.54), despite the fact that warm temperature conditions persist in these areas.

Site 4 was heavily disturbed because of the heavy influx of year-round tourists. This site was dominated by three invasive species [i.e. Lantana camara L. (IVI, 34.90), C. odorata (IVI, 22.70), and Par-thenium hysterophorus L. (IVI, 17.60)]. The site had 32 species, yet C. odorata had the maximum canopy cover in this area. This area had no dominant tree taxon; however, the sporadic distribution of Azadirachta spp. and Mangifera spp. was apparent.

Chromolaena odorata contributed the highest IVI values (88.04 and 52.62 at Site 1 and Site 3, respectively). In Site 2, Adenostemma lavenia (L.) O. Kuntze contributed the highest IVI value (76.10); in

Table 2

Importance value index of various plant species in different sites.

Plant species

Importance value index

Site 1 Site 2 Site 3 Site 4

Acer oblongum (Wall.) S. and S. 8.88 - - -

Adenostoma lavenia (L.) O. Kuntze - 76.10 - -

Adhatoda vasica Nees - - - 12.90

Barleria cristata L. - - - 3.20

Ageratum conyzoides L. - 32.40 - 5.30

Amaranthus spinosus L. - - - 14.70

Berberís aristata DC. - - - 8.20

Berberís asiatica Roxb. 10.21 - - -

Bidens pilosa L. - - - 14.60

Boehmeria platyphylla D. Don - - - 12.30

Boenninghausenia albiflora (Hook.) 22.70 - - -

Reichb ex Meissn

Carissa opaca Stapf. Ex Haines - - 14.99 -

Carpinus faginea Lindl. 1.05 - - -

Cassia tora L. - - 5.62 4.90

Carex nubigena D. Don ex Tilloch and Taylor - - - 13.80

Cestrum aurantiacum Lindl. 7.96 - - -

Chromolaena odorata (L.) King and Rob. 88.04 21.54 52.62 22.70

Clerodendrum infortunatum Ven. Jard. Malm. - - 16.22 -

Cocculus laurifolius DC. - - 5.53 -

Colebrookia opposoitifolia Smith - - - 8.00

Cyanotis cristata L. - - - 4.30

Daphne cannabina Wall. 17.31 - - -

Debregeasia hypoleuca Hochs. - - - 15.90

Desmodium podocarpum DC. - - 17.78 -

Discorea elata L. - - - 3.20

Eleusine indica L. - - - 8.50

Eriophorum comosum Wallich. - - - 5.80

Ficus heterophylla L.f. - - - 4.70

Floscopa scandens Lour. - 23.60 - -

Geniosporum coloratum (D. Don) Kuntze - - 3.17 -

Hypericum oblongifolium Choisy. - - - 7.00

Ilex dipyrena Wall. 7.56 - - -

Lantana camara L. 18.88 28.77 17.99 34.90

Leea alata L. - - 24.38 -

Lepidogathus cuspidata L. - - 32.29 11.20

Lonicera quinquelocularis Hardw. 16.20 - - -

Machilus odoratissima Wall. ex Nees 6.33 - - -

Mahonia acanthifolia D. Don 9.64 - - -

Mallotus philippensis (Lam.) Muell.-Arg. - - 25.51 -

Mangifera indica L. - - - 5.90

Millettia auriculata Bak. ex Bran - - 5.38 -

Murraya koenigii L. - - 30.59 -

Myrsine africana L. 22.44 - - -

Oplismenus compositus L. - - - 16.70

Oxalis coniculata L. - - - 4.00

Parthenium hysterophorus L. - - - 17.60

Perilla frutescens (L.) Britt. - 32.96 - -

Phyllanthus parvifolius Ham. 3.61 - - 2.40

Polygonum hydropiper L. - 28.41 - -

Pouzolzia pentandra (Roxb.) Benn. - 10.00 - -

Pycerus sanguinolentus (Vahl.) Nees - 9.64 - -

Pyrus foliolosa Wall. - - 8.08 -

Quercus leucotrichophora A. Camus 10.50 - - -

Randia tetrasperma Benth. and Hook. f. 18.16 - 13.41

Reinwardtia indica Dum. 7.76 - 5.54 -

Rhamnus virgata Roxb. 15.42 - - -

Rorippa nasturtium-aquaticum (L.) Hayek - 65.35 - -

Rosa moscahta Mill. 3.96 - - -

Rubus ellipticus Smith 1.31 - - 8.90

Rumex hestatus D. Don - - - 6.80

Sarcococca pruniformis Lindl. 13.91 - - -

Shorea robusta Roxb. ex Gaertner f. - - 9.75 -

Sida cordifolia L. - - - 2.60

Smilax aspera L. 3.95 - - -

Solanum hispidum L. - - - 7.10

Syzygium cumini (L.) Skeels 4.08

Tabernaemontana coronaria Willd. - - 3.04 -

Thalictrum foliolosum DC. - - - 2.60

Urtica dioca L. - - - 16.60

Viburnum cotinifolium D. Don 3.10 - - -

Woodfordia floribunda L. - - - 4.50

Xylosma longifolium Clos. - - 8.11 -

70% 60% 50% 40% 30% 20% 10% 0%

Exotic

■ Iiuligeims

Families Genus Species

42% 45% 41%

58% 55% 59%

Figure 4. Status of exotic and indigenous species in all sites.

Site 4, Lantana camara L. and C. odorata were dominant with IVI values of 34.90 and 22.70, respectively (Table 2). Parthenium hys-terophorus L. was the major codominant species in Site 4. Lantana camara L. was the only species found with C. odorata in all four study sites, making them an equally dominant species in the Doon Valley. A significant difference in the Standard Error (SE) of C. odorata density was recorded between Sites 2 and 3 (p < 0.05). The difference between Sites 1, 3, and 4 was not apparent and was less prominent than the patterns observed among the different study sites (Table 3). At most study sites, the largest proportion was recorded at the 2.5—3.0 height class, whereas the lowest proportion was recorded at the 0—1.0 height class.

The rooting system of C. odorata was superficial in all plots; approximately 80% of roots were in the upper 15—20 cm. Most roots spread laterally with only a few roots penetrating into deeper soil. Soil characteristics varied between sampling areas (Table 4). The mean soil pH in the sampling sites was acidic and ranged from 5.88 to 6.33. Mean organic matter was low (2.66—3.01%) in all study sites. Total nitrogen was within the range of 0.31% to 0.54%. The total phosphorus was lowest in Site 4 and highest in Site 1. The total potassium was also lowest in Site 4 and highest in Site 1. The heavy infestation of C. odorata in all four sites of the Doon Valley was favored by its acidic soil and high potassium and phosphorus content. Soil pH was not a limiting factor in the germination and growth of C. odorata because it can grow in a wide range of pH (4—8). It adapted to a wide range of soil conditions. The moisture content was high in Sites 1 and 2, thus giving an indication that a high moisture content favors the growth of this invasive species.

Diversity indices

The values of alpha diversity ranged between 9 and 32. Alpha diversity was recorded as least for Site 2 and as highest for Site 4. In Sites 1 and 3, richness was recorded at 22 and 19, respectively. The Shannon—Wiener Diversity Index or the species diversity index did not vary significantly between Sites 1 and 4 and Sites 2 and 3. It was least for Site 2 (2.27) and highest for Site 4 (3.21). Species turnover was high in Site 4.

The values of evenness varied between sites. It was 4.56, 2.09, 4.21, and 5.84 in Sites 1—4, respectively. Pielou's evenness values (Table 5) clearly show that evenness was similar in all sites within the range of 0.92 (Site 4) and 0.98 (Site 2).

Table 3

The density and plant height class of Chromolaena odorata (mean shrub per hectare) from all four study sites. SE = standard error, mean shrub density with different subscripts are statistically different (p < 0.05).

Plant height class (m, %)

Site name Shrub density (mean ± SE) 0-1 1 -1.5 1.5-2.0 2.0-2.5 2.5-3.0 3.0-3.5 3.5-4.0 Soil type

North (Kolhu Khet) 23890 ±1225a 3.18 3 10.11 12.01 25.88 17.94 27.88 Silt gravel

South (Mothronwala) 19589 ± 2189a 4.87 9.55 13.01 12.66 33.01 23.86 3.04 Silt clay

North (Rajpur Forest Peripheries) 22795 ± 1342b 2.01 3.77 5.39 10.98 14.33 18.57 44.95 Coarse grained

North (Sahastradhara) 17675 ± 2329a 4.33 8.48 16.24 15 38.99 12.97 3.99 Organic mix soil

a and b are Standard Error (SE) values. All 'a' values are statistically similar and 'b' is statistically different from other values.

Discussion

Mountain systems provide a variety of often very pronounced environmental gradients and may therefore be particularly suitable study areas for a limiting conditions approach to investigating the mechanisms of plant invasions. They also represent valuable but endangered ecosystems that have not been sufficiently studied from the point of view of biological invasions. Furthermore, because there are many different factors that could, singly or in combination, limit the ability of an alien plant to invade, it would be desirable to study plant invasions along environmental gradients that include changes in most potential limiting factors at different degrees of independence. The IVI calculation is essential in demonstrating resource apportionment and niche space among species over the span of time. Therefore, it is always a simple, strong, and powerful tool for assessing biodiversity. Many alien species are introduced deliberately. A lot of risk is associated with the introduction of these new species.

The introduction of C. odorata, Lantana camara, Parthenium hysterophorus, Cassia tora, and Datura stramonium has become problematic because of their invasiveness on natural areas and urban ecosystem such as the Doon Valley. Biological control should be undertaken for such species, but only after critical evaluation of the risks involved. Because most invasive alien species in India are of neotropical origin (e.g. South America), they cause loss of biodiversity, which includes species extinction, changes in water cycle, and smooth ecosystem functioning. The introduction of invasive species and their strong competition with native species for resources may change the soil structure, its profile, decomposition, nutrient content of soil, moisture availability, etc. Invasive species are thus a serious encumbrance to conservation and sustainable use of biodiversity.

Biological invasions now operate on a global scale and will undergo a rapid increase in this century because of interaction with other changes such as increased globalization of markets and the rise in global trade, travel, and tourism. For effective management of invasive species, it is essential to know their ecology, morphology, phenology, reproductive biology, physiology, and phytochemistry (Raghubanshi et al., 2005). Keeping these facts in

mind, the present study attempted to assess the habitat suitability of an invasive species, C. odorata, in the Doon Valley in the Himalayan foothill. The use of IVI to assess the biodiversity of plant species is well advocated by many workers (Hazarika et al., 2006; Sahu et al., 2010; Biswas et al., 2014; Surendra et al., 2013; Awodoyin et al., 2013; Sarma and Deka, 2014; Mandal, 2014); therefore, it was chosen as the main tool to assess the habitat suitability of C. odorata.

In the present study, 72 plant species from 20 families were recorded. The plant species of an area are largely determined by moisture and length of the growing season. Bliss (1963), Douglas and Bliss (1977), Billings (1979), Mandal and Joshi (2014) have already reported in their studies that the vegetation of any area results from the interaction between many factors: the meso-topographic gradient, the elevation, soil, slope, nearness to existing glaciers, species composition, and biotic interferences. In contrast with the aforementioned studies, the present study demonstrated that biotic interference and soil nutrients are the major factors that determine the community structure of the area; however, the microclimatic conditions also had a significant role in study Sites 1 and 2. Individual C. odorata plants invade various sites when their seeds are readily available. Before the invasion occurs, physical disturbance of the habitat due to various biotic stresses make the factors in a particular site favorable. In the present study, available soil resources for Site 4 and moist to mesic condition in Sites 1 and 2 appear to be most suitable for the invasion of C. odorata. These findings are in agreement with those of Tilman (1997), Belnap and Phillips (2001), Safford and Harrison (2001), Williamson and Harrison (2006), and Kandwal et al. (2007). The present work was well supported by the IVI and the canopy cover of the C. odorata in all four sites.

Based on the present results, Sites 1 and 3 were the most suitable sites for this invasive species. However, in Site 4, which was the most disturbed site because of heavy human activity, the dominance of C. odorata was shared by two other invasive species: Lantana camara L. and Parthenium hysterophorus L.

Elevation by itself may not have strong direct effects on the distribution of species, but most likely a complex gradient composed of several different variables such as temperature, light, moisture,

Table 4

Soil parameters (n = 15) and soil type at four sampling sites in the Doon Valley.

Soil factors (depth 0e30 cm) Site 1 Site 2 Site 3 Site 4

pH 5.88 (±0.17)a 6.11 (±0.11)b 5.98 (±0.10)a 6.33 (±0.07)

Available potassium (cmol/kg) 0.44 (±0.01)a 0.42 (±0.17)a 0.32 (±0.09)a 0.29 (±0.10)

Total nitrogen (%) 0.55 (±0.20)c 0.54 (±0.91)c 0.49 (±0.16) 0.31 (±0.09)

Soil organic matter (%) 2.66 (±0.14)b 2.99 (±0.15)a 2.78 (±0.05)b 3.01 (±0.13)

Available phosphorus (mg/kg) 9.1 (±0.14) 8.5 (±0.07) 7.5 (±0.19) 6.7 (±0.61)

Calcium (cmol/kg) 4.3 (±0.32)a 3.8 (±0.63) 3.5 (±0.08) 4.1 (±0.19)

Magnesium (cmol/kg) 1.39 (±0.03)c 0.98 (±0.19)b 1.25 (±0.08)c 1.22 (±0.37)

Bulk density (g/cm3) 1.4 (±0.27) 1.2 (±0.17) 1.8 (±0.29) 1.2 (±0.23)

Moisture content (%) 29.1 (±0.41)a 28.5 (±0.31)a 28.4 (±0.34)a 27.9 (±0.32)

Total porosity (%) 55 (±0.33)a 57 (±0.02)a 61 (±0.09) 58 (±0.27)

The data are presented as the mean (± the standard deviation) and by percentage (%), unless otherwise denoted. Values with same superscript letters are not significantly different, based on Tukey's test; p > 0.05.

Table 5

Diversity indices of various sites in the present study.

Sites Alpha Beta diversity Shannon-Weiner Pielou's

diversity Index index

Site 1 22 4.56 3.01 0.97

Site 2 09 2.09 2.27 0.98

Site 3 19 4.21 2.85 0.96

Site 4 32 5.84 3.21 0.92

and nutrients have some direct effects (Stevens, 1992). Exotic alien plants in the valley occurred sporadically at higher elevations—most were found below 1500 m, except for C. odorata—and with far greater cover estimates, compared to higher elevation areas.

In the Old World, C. odorata is an invasive transformer species (Richardson et al., 2000), at least partly because it lacks natural enemies. It grows rapidly and often forms a dense scrambling thicket that grows through and over existing vegetation. It most readily invades areas of natural disturbance or human-induced disturbance, but it can invade undisturbed land. Chromolaena odorata affects subsistence and commercial agriculture, including crops and plantations, grazing lands, and silviculture. Site 1 had a significantly greater density of C. odorata (23,890 plants per hectare), compared to the other study sites. This may be because of the high altitude, a high rate of grazing, high lopping, and past management of the area. The normal plant height ranged up to 3 m in open land and in fallow land; however, at the 3.5-4.0 m height class, a large proportion was recorded from Site 1 (27.88%) and Site 3 (44.95%). This may be explained by its highly competitive nature with trees to avail of the resources and light. These findings are in line with previous findings that show that C. odorata invades a wide range of natural vegetation types (from grassland through savannah bush, open woodland, and forest margins and gaps) and that it is highly competitive with indigenous species (Goodall and Erasmus, 1996; Prasad et al., 1996).

Natural forests are not usually invaded by C. odorata because of its high light requirements, but forest degradation generally allows this weed to establish itself by suppressing the recruitment of trees. Forest gaps that naturally develop through tree fall are colonized rapidly by C. odorata (Epp, 1987; Goodall and Erasmus, 1996). The plant scrambles up through the surrounding trees and emerges on top of the canopy, eventually causing its collapse (Goodall and Erasmus, 1996). However, removing C. odorata allows rapid regeneration of the indigenous forest (Honu and Dang, 2000). Thus, we conclude from our findings that suitability of C. odorata depends on physical disturbance of the site and on the overall site characteristics. The present study reveals that the peripheries of forests and fallow lands are fully occupied by C. odorata; these findings are also supported by Ambika (2007). The observation of this study partly supports the statement of Awanyo et al. (2011) who mentioned that the highly invasive C. odorata grows aggressively and suppresses other vegetation by easily forming a thick cover in a very short time. In another study, the high allelopathic properties of this weed (Sahid and Sugau, 1993) support its gaining dominance in vegetation and in replacing other aggressive invaders such as Lantana camara L. (Verbenaceae) and Imperata cylindrica (L.) Beauv. of Poaceae (Eussen and de Groot, 1974; Ivens, 1974) in Asia and Africa. The present study shows that C. odorata forms extensive infestation in slightly moist frost-free areas, but it has also started appearing in warm places and will become invasive in medium to arid land use types that are not water stressed during summer season; this is in line with the findings of Goodall and Erasmus (1996).

It is evident from past and foregoing discussions that soils, their texture, quality, and nature are vital for the germination and

growth of native species and exotic species. Soils are considered living systems and, like any other organism, they develop and decay, degrade, and respond to proper treatment if it is administered in time. These have serious repercussions on other components of the system of which they themselves are important parts. In our research we aimed to understand how this invasive alien species influences soil processes. The significant increases of Soil Organic Carbon (SOC) and total nitrogen under the fallow within the 0- to 10-cm layer, which further revealed the potential of C. odorata in all fallow lands, was recorded by Tondoh et al. (2013). This was well supported by the previous findings of Goyal et al. (1999) and Manama et al. (2000) in which increased SOC and nitrogen level are recognized as a key attribute of soil quality and a deciding factor for C. odorata invasion.

The present study also supported previous findings by recording an elevated level of soil nutrients, including SOM and nitrogen, from all invaded sites. In contrast to the aforementioned studies, the current study overtly suggested that Chromolaena has high fertilizing and sound potentials for building SOM to adequate levels that will surely meet the nutritional needs of different herbs and shrub species and will surely improve the nutrient element status of the Doon Valley forests. This further supports the fact that when understory shrub bushes such as Lantana camara are dominated by such organic resources that are available because of the invasion of C. odorata, instead of suppressing the weed growth, the fallow fields and forest peripheries will further be taken over by these invasive species.

In a similar study, C. odorata increased soil nutrients such as nitrogen, phosphorus, potassium, magnesium, calcium, and organic matter of fallow land, and thereby further favored the invasion of other opportunistic species (Openly et al., 2012). Because of the ability of the weeds to shield the soil, propagate the surface soil with their roots, increase biomass and organic matter, the fallow lands were able to improve soil construction and porosity, reduce bulk density, and increase soil fertility and growth of other invasive plants. Our study findings are also in line with those of the study by Openly et al. (2012), and showed that the SOM and other soil properties such as the levels of nitrogen, phosphorus, potassium, and the acidity of the soil significantly increased in all invaded sites. In her review, Ehrenfield (2003) similarly reported elevated levels of these nutrients. These properties were associated with further invasion of plants. Different theories and concepts have been given in the plant invasion process, but the most likely reason that the search has largely failed is that invasiveness depends more on the interaction between the characteristics of the nonnative species and their potential new habitats than on the characteristics of the nonnative species alone. It is probably no accident that the best general predictors of invasiveness across habitats—native range and rapid dispersal—are both traits likely to affect the probability of the initial introduction of a species, the phase of invasion, which is most independent of habitat. Species that occur more widely and produce more propagules should have a better chance of being picked up and transported. However, the reason these traits appear to explain only a small part of the variation between species in invasiveness may be because the second phase of invasion (i.e. spread in new habitats) is habitat-specific. Habitat specificity of invasiveness is consistent with the observation that different growth forms tend to be invasive in different habitats. It may therefore be necessary to predict invasiveness separately for different habitat types.

In conclusion, the study of gradual changes in population patterns at the landscape scale, which may occur near a hypothetical invasion front, could help to elucidate the processes underlying an invasion. In most cases, however, there is no clearly defined invasion front and the scales over which gradual changes may occur are

too large to be manageable. Hence, it is often difficult to find gradients in the population structure of an invasive plant at the landscape scale that reveals the conditions limiting the invasion. Furthermore, because many different factors could, singly or in combination, limit the ability of an alien plant to invade, it would be desirable to study plant invasions along environmental gradients that include changes in most potential limiting factors at different degrees of independence. This may allow researchers to disentangle the various factors influencing a plant's invasion.

The following are the conclusions of the present study. (1) Sites 1 and 3, which are situated at higher altitude with moist to mesic and cold climatic conditions, were the most suitable habitat of C. odorata, but the presence of this species in all four study sites with significant IVI values show that it can very well grow in cold, moist, and unconstrained areas, and in mesic, humid, warm, and human-disturbed areas. This property is giving wide amplitude to the growth of C. odorata in a wide variety of habitats. (2) The success of C. odorata can therefore be dependent on the degradation of sal forests in which large canopy gaps are created because of heavy human activity and because the availability of light facilitates the invasion establishment of C. odorata populations. (3) Chromolaena odorata population increases soil fertility by enhancing the soil nutrient content of potassium, phosphorus, magnesium, nitrogen, and SOM, and by reducing bulk density. This further provided a corridor to invasion by other noxious weeds such as Lantana camara L., Parthenium hysterophorus L. and Ageratum conyzoides L. (4) The soil nutrient was high in all study sites where C. odorata had grown. This soil suitability can support optimal growth of different crops and can thereby reduce the overdependence on inorganic fertilizers, which usually increases the cost of food production. (5) The pH was acidic in all study sites. This suggests that C. odorata is more adapted for acidic soil than for nonacidic soil. However, this factor was not the limiting factor in determining the adaptation of C. odorata to a particular acidic nature of soil because it can grow on a wide range of acidic soil (pH 4—8). (6) Chromolaena odorata aggressively grows on fallow lands. Therefore, C. odorata is a "good" fallow plant. Because of its fast growth and development during the cropping phase and from abandoned forest peripheries where light availability is greater, it can provide a protective cover and allow better weed suppression, compared to fallow lands that are not dominated by C. odorata. The results of the present study show that only Lantana camara L. managed to grow alongside C. odorata in all four study sites, but other species were only present in selected sites. However, we cannot fully neglect the possibility of further plant invasion owing to the increase in the nutrient value of the soil. Therefore, scientists need to consider the broad amplitude and wide range of invasion spread for the proper management of C. odorata in the Himalayan foothills.

Conflicts of interest

The authors declare no conflicts of interest. Acknowledgments

We thank the Department of Botany, DAV (PG) College in Deh-radun, India and the Archives of Forest Research Institute in Deh-radun, India for providing necessary help in the identification of plant species during the work. We also thank APEX Laboratory (Dehradun, India) for assisting in soil testing and analysis.

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