Study area

Arunachal Pradesh (26°28′–29°31′N; 91°30′–97°30′E) and Assam (24°09′–27°58′N; 89°42′–96°01′E) are located at the eastern end of the Himalayan region at altitudes between 100 and 7000 m. There are 3 seasons per year: a warm and dry summer (March–May), monsoon (June–September), and a cool and dry winter (October–February). About 85% of annual rainfall (3000 mm) occurs during the monsoon season. Mean annual maximum and minimum temperatures are 29.3°C and 19.2°C in Itanagar (200 m), 27°C and 16°C in Along (300 m), 22°C and 12°C in Ziro (1600 m), and 19°C and 5°C in Tawang (3000 m), respectively (Purkayastha 2008).

Vegetation types vary with altitude and climatic conditions, from tropical (below 900 m), subtropical (900–1800 m), temperate (1800–3500 m) to subalpine and alpine (above 3500 m). Rice is cultivated below 2000 m on the plain and valley floor, whereas shifting cultivation is conducted on hillsides up to 2500 m. Livestock, such as yak, yak-cattle hybrids, and sheep, are bred by highlanders and pastured above 3000 m.

There has been regional trade between Tibet and Arunachal Himalaya for centuries (Choudhury 1981; Choudhury 1996). However, Arunachal Himalaya was called the “hidden land,” because access for outside visitors was difficult (Blackburn 2003). After independence in 1947, the Indian government began a development program to construct roads, increase agricultural production, and improve public health (Elwin 1959). Nowadays, tourism is a significant economic activity, and many domestic and foreign tourists visit the region.

Material and methods

A total of 38 roadside plots at 5–30 km intervals were selected (Figure 1) along the Assam national highway (75–250 m) and a state major road in Arunachal Pradesh (200– 4200 m). Twenty-eight plots were in the tropical zone, 6 in subtropical, 3 in temperate, and 1 in subalpine and alpine zones. Strip-shaped plots alongside and parallel to the roads were sampled. Plot sizes varied from 50 m × 5 m to 200 m × 10 m because of variable land use, topography, and roadside vegetation. Between 2 and 5 replicates of a 50-m transect were sampled on each plot.

FIGURE 1

Map of 38 roadside plots in Arunachal Himalaya. (Map by Y. Kosaka)

The abundance of alien species was recorded on a qualitative scale (Pauchard and Alaback 2004): A, abundant (present in >25% of the transect length); C, common (10–25%); I, intermittent (< 10%); and R, rare when only 1 group of individuals was present. Species abundance was determined by the maximum value recorded along the transects within the plot. A subjective percentage cover value was assigned to each of the qualitative abundance classes (Pauchard and Alaback 2004): A  =  50, C  =  25, I  =  10 and R  =  1. The location and altitude of the plots were recorded by global positioning system (GPS; Garmin GPSmap 60Cx).

Additional habitats of invasive alien plants were recorded (arable field, grassland, settlement area, water body) through direct observation and field interviews. In North Lakhimpur, Along, and Ziro, 3–5 knowledgeable local residents were asked when they first detected invasive alien plants, the habitat in which they were detected, the mode of introduction, harmful effects, and beneficial uses. A literature survey of known harmful effects or beneficial uses was completed for each plant identified. Data on species of Cyperaceae and Poaceae were not collected, because many individuals of these species were not in flower during the survey period, and accurate identification requires inflorescence.

Relative abundance and frequency were calculated for recorded species, and data were analyzed by detrended correspondence analysis (DCA) (Hill and Gauch 1980). Species compositional gradients among the plots were detected by PC-ORD version 5. The ordination matrix contained 37 plots (1 plot lacked alien species) and 18 species. Species with frequencies of ≤5% were excluded from analysis because minor species may cause unnecessary noise. The correlation between plot score in DCA axis 1 and elevation was tested by linear regression.

Alien plant species were determined as those not native to the eastern Himalayan region. The nomenclature, origin, growth form (Khuroo et al 2007), mode of reproduction, and mode of dispersal were recorded by following Harada et al (1993), Takematsu and Ichizen (1987, 1993, 1997), and Weber (2003).

Characteristics of invasive alien plants in Arunachal Himalaya

A total of 18 invasive alien plants (frequency >5%) were recorded on 38 plots (Table 1). The dominant family was Asteraceae (9 species) followed by Fabaceae (3) and Verbenaceae (2). Most species originated in South America (13); others originated in North America (2), Europe (2), and Africa (1). Nine species also grew in arable fields, 3 in grassland, 3 in settlement areas, and 2 in water bodies. Ten species only reproduced by seed, 1 species by vegetative reproduction, and 7 species by both seed and vegetative reproduction. Aside from direct or indirect anthropogenic intervention, common dispersal modes included anemochory, endozoochory, and ombrohydrochory. Bidens pilosa also depends on epizoochory and Ipomoea carnea and Eichhornia crassipes are dispersed by hydrochory. Although most species were unintentionally introduced into this region, 4 species were intentionally introduced for beneficial uses (Table 2).

TABLE 1

Summary of the invasive alien plants in Arunachal Himalaya. (Table extended on next page.)

TABLE 2

Harmful effects and beneficial uses of invasive alien plants recorded in Arunachal Himalaya and other regions.^a)

Species composition by altitude

The 37 plots were arranged along DCA axis 1 (eigen value [EIG]  =  0.85) in order of altitude (R  =  0.92, P < .001), whereas DCA axis 2 (EIG  =  0.36) was slightly related to record location, Arunachal or Assam (Figure 2).

FIGURE 2

DCA diagram of plots classified by altitude and location in Arunachal Himalaya. Five areas in 4 altitude zones correspond with vegetation zones.

The number of species per plot varied from 2 to 8 in the tropical zone, from 3 to 8 in the subtropical zone, from 3 to 6 in the temperate zone, and only 1 species in the subalpine zone (Figure 3). The number of species per plot had no significant correlation with altitude (R  =  0.36) (Figure 3).

FIGURE 3

Number of species per plot at different altitudes in Arunachal Himalaya.

The highest number of species (13) grew in the tropical zone, followed by the subtropical (10), temperate (6), and subalpine zones (1) (Figure 4). Parthenium hysterophorus, Ipomoea carnea, Crotalaria pallida, Mimosa pudica, Cuphea carthagenensis, and Stachytarpheta dichotoma grew only in the tropical zone. Ageratum conyzoides, Ambrosia artemisiifolia, Bidens pilosa, and Solanum carolinense grew in the widest range of habitats, from tropical to temperate zones. Except for these 4, species growing in the subtropical zone were recorded up to 1600 m, whereas species found in the temperate zone occurred above 1600 m. Few temperate zone species grew above 3000 m. Only 1 species, Taraxacum officinale, was recorded on the mountain pass at 4200 m, the highest plot in the present study (Figure 4).

FIGURE 4

Altitudinal range of invasive alien plants found in Arunachal Himalaya.

Harmful effects and beneficial uses of invasive alien plants

Among 18 alien plants recorded, harmful effects were identified on crop production (14 species), livestock grazing (7), human health (4), water drainage (2), forest regeneration (1), and native vegetation (1), whereas beneficial uses were recorded as medicine (12), vegetable (4), fodder (2), hedge plant (2), fish growth (1), fallow plant (1), fish poison (1), manure (1), soil protection from erosion (1), and ornament (1).

Distribution patterns of invasive alien plants

Most (83.3%) of the invasive alien plants in the Arunachal Himalaya region originated in North and South America (Table 1). A. conyzoides, B. pilosa, Galinsoga quadriradiata, and P. hysterophorus have been reported from Nepal (Manandhar 2002) and Kashmir Himalaya (Khuroo et al 2007), and these species may be spreading in the Himalayan region.

Although species composition changed with the altitudinal gradient (Figure 2), the number of species per plot was not related to altitude (Figure 3). The altitudinal range of species (Figure 4) corresponded with known biogeographical affinities and environmental tolerances. Temperate species were limited to highlands (above 1600 m), whereas 10 species of tropical origin (62.5%) were limited to lowlands (below 1600 m). Apart from T. officinale, alien species were not recorded above 3000 m where the temperature dropped to below the freezing point in the winter (Figure 4). Low temperature and snowfall in the highlands may filter nonadapted species from tropical regions (Pauchard and Alaback 2004; Arevalo et al 2005).

Plant invasions are influenced by climate; Ageratina adenophora, Chromolaena odorata, Mikania micrantha, E. crassipes, and Lantana camara were regarded as invasive in tropical Asia but not in temperate Asia (Weber 2003).

Plant invasion through recent road construction

Some alien plants were introduced into Himalayan regions by historical trading (Khuroo et al 2007), but Pickering and Hill (2007) suggest that recent road construction and road use facilitate the establishment of invasive alien plants in mountain regions. This view was supported by the recollections of elderly residents at the study sites. In Along (270 m), residents reported that S. carolinense was found after a road was built to Assam before 1990. M. micrantha and S. dichotoma have expanded their range since 2000. In Ziro (1600 m), A. artemisiifolia and S. carolinense spread after the road was connected from Assam in the 1950s. The spread of S. carolinense was related to the introduction of cattle from Assam that disseminated its seed. E. crassipes was introduced in 1970s, followed by G. quadriradiata after 2000.

Unintentionally introduced alien plants may subsequently disseminate seed or vegetative propagates by wind, water, or animals (Table 1). Burning in shifting cultivation also promotes the establishment of A. adenophora and C. odorata (Tripathi and Yadav 1987), as well as M. micrantha (Swamy and Ramakrishnan 1987).

Perception of local residents

Local informants reported harmful effects of invasive alien plants (Table 2). I. carnea impedes drainage and E. crassipes prostrates rice plants when plants drift onto wet fields during seasonal floods in North Lakhimpur. M. micrantha and S. dichotoma were reported to damage agricultural production by overhanging crops in Along. The spines on S. carolinense disturbed agriculture in Yazali. A. conyzoides and G. quadriradiata were regarded as major weeds, and downy fruits of A. adenophora and C. odorata were reported to cause breathing problems in Dirang.

Beneficial uses of invasive alien plants were also reported (Table 2). The leaf of A. conyzoides was squeezed and applied to wounds as styptic. E. crassipes was introduced to improve fish growth in aquaculture in Ziro. The young leaf of G. quadriradiata was eaten as a vegetable, and the fruit of S. carolinense was used as medicine against toothache in Dirang.