Riparian forests (RF) growing along streams, rivers and lakes have special functions in the landscape as the interface between the terrestrial and the aquatic ecosystem (Malanson 1993). They are distinctly different from the surrounding lands because of unique soil and vegetation characteristics that are strongly influenced by free or unbound water in the soil. Riparian zones are usually a diverse mosaic of landforms, communities and environments within landscapes and they serve as a framework for understanding the dynamics of communities associated with fluvial ecosystems (Gregory et al., 1991; Naiman et al., 1993; Naiman et al., 2005). Being a transition zone between aquatic and terrestrial area where structural and functional properties change with space and time, discontinuously. Typical examples of riparian zones would include flood plains, stream banks and lake shores. The interfaces in riparian zones possess physical and chemical attributes, biotic processes, material flow processes, but they are unique in their interactions with adjacent ecological systems.. Riparian zones are habitats of critical conservation concern worldwide, as they are known to filter agricultural contaminants, buffer landscapes against erosion, and provide habitat for high numbers of species (John et al., 2005). The Riparian forests are habitats for a large number of forest species including many of the rare species that depend on water and as such serve as important areas for biodiversity (Gundersen et al., 2010; Darveau et al. 1995; Hylander 2006).
Riparian lands can also include intermittent streams gullies and dips which sometimes run with water. The vegetation ranges from emergent aquatic and semi-aquatic plants through to terrestrial understorey and canopy species (Parsons 1991). Further, the zone can be seen as an interface between terrestrial and aquatic systems and is described as a series of ecotones between these systems (Risser 1990). Riparian vegetation plays an important role in the maintenance of stream and foreshore stability. Streams and rivers are essentially dynamic systems, their path and flow constantly changes with the time (Warner 1983). The presence of vegetation in riparian areas acts to reduce the rate of change and therefore maintain a level of stability.
2. Ecology and biodiversity in riparian forest
Plant communities in large river flood plains are amongst the most productive and diverse in the world and frequently support higher number of plant species arranged in vegetation associations of greater complexity than surrounding landscaping units (Menges and Waller, 1983; Tockner and Stanford, 2002). Water level patterns are critical for the successful establishment of new plants (both exotic and native species) following dispersal of seeds or other propagules by water, wind, animal vectors or other dispersal agents. Flow has been determined as primary factors for determining plant community composition and structure along the riparian zone (Blom et al., 1990; Ferreira, 1997). Many plant species depended particularly on the flow for dispersal of their propagules a process referred to as 'hydrochory' (Nilsson et al., 1991). Types of propagules include sexually derived seeds as well as vegetative fragments (mechanically sheared or physiologically abscised branch or root segments) that can re-sprout to result in asexual propagation. Propagative dispersal typically occurs in a downstream direction along streams but may be wind-aided along lakes or reservoirs. Thus, hydrochory may occur in multiple directions along relatively stationary water bodies. Propagative dispersal by water is an effective adaptation of native plants but also provides a major mechanism for invasion by exotic weeds, of which noxious species can have severe ecological and economic impacts (Braatne et al., 2002).
Riparian vegetation changes continuously from the beginning of a river in the mountains up to the river mouth with the changing environmental parameters like altitude, humidity, soil conditions and also in the conditions of water like quantum and flow, temperature, pH, salinity. In a tropical countries, the riparian vegetation in a first order stream in the mountain may be ferns and other associated herbaceous plants in the rock crevices. When coming down, evergreen forest samples can be observed in the riparian zone as a quantum and the lateral influence of the water increases. Further going down the bed conditions of the river changes from rocky to sandy especially in the floodplains. Here the soil becomes looser, sedimentation rate will be high, and a good amount of alluvium can be found. In these areas the water influence on the vegetation may be more. Herbaceous, grass and hydrophytic plant communities will be abundant in these zones (Amitha, 2003).
Ripairan areas acts as a migratory corridor and routes for many wildlife as it has been used for regular daily movements and seasonal migration. Riparian zones offers an three critical resources for wildlife: cover, food and water in one space. The undisturbed stands of age old woody species provide habitat for nesting birds resided in the forests. Riparian zones are utilized by wildlife as a sort of "natural highway". They are important to mammals and birds as they journey up and down the river during daily movements besides seasonal migrations. Much wildlife is found to be associated on floodplains than in any other landscape unit in most regions of the world (Klement and Stanford, 2002). In the Pacific coastal ecoregion (USA), for example, approximately 29% of wildlife species found in riparian forests are riparian obligates (Kelsey & West 1998). It provides habitat for more species of breeding birds than any other vegetation association. For example, of all bird species breeding in northern Colorado, 82% occur in riparian vegetation, and about half of south-western species depend upon riparian vegetation (Knopf & Samson 1994). Riparian areas in semiarid zones are critical in providing stopover areas for en route migrants (acting as ‘dispersal filters’), and therefore affect the breeding success of northern bird populations (Skagen et al. 1998). In Europe, 30% of threatened bird species are inland wetland-dependent species and 69% of the important breeding areas for birds contain wetland habitats, primarily flood plains (Tiker & Evans 1997). In Switzerland, 10% of the entire fauna is restricted in its occurrence to riverine flood plains, although flood plains only cover 0.26% of the country’s surface. Among 10%, 28% of the fauna frequently uses flood plains and about 44% is occasionally found in flood plains. A high proportion of the riparian obligates (47%) is listed as endangered, compared to 28% for the entire fauna (Walter et al. 1998).
3. Ecosystem services of riparian vegetation
Riparian forests performs an array of functions in its buffer are which are beneficial to regional ecosystem to meet the some of their essential needs for their survival in the ecosystem. Some specific species stand unique in portraying their services in the particular ecosystem due to its morphological and phenological nature where their life cycle influences to protect stability of several flora and fauna in the ecosystem. Besides these functions, several species of riparian vegetation render services to the humans, as they provide several direct and indirect economic supports to run their livelihoods.
4. Ecological significance
The riparian plant species improves the microclimatic condition thereby allowing the other associated species to to grow in the community. The forks of old trees in the riparian zone provide wantage points to epiphytes.
Riparian species develops typical root modifications to withstand during the flood events. Such typical modifications of plant root systems are called as buttressed root systems. The buttressed root systems provide the strength to the tree species and to facilitates a suitable site to other riparian species to grow. Rivers combined with such root systems in conjunction with other herbaceous vegetation dissipate stream energy, resulting in less erosion and a reduction in flood damage. A 5 cm deep root system resists erosion up to 20,000 times better than bare soil stream banks. A woody root mat is the "re-bat' of stream banks. The riparian canopy provides organic matter via litter fall; surfaces of submerged leaves are sites of primary and secondary production by micro algae and bacteria, which can rival that of phytoplankton and bactereophils in water column. The Logs of riparian vegetation play an important role in the dynamics of stream morphology and serve as substrates for biological activity by microbial and invertebrate organisms. On land the riparian stream ecosystem is the single most productive type of wildlife habitat. The Riparian areas act as a corridor for big game migratory animals between summer and winter range.
5. Social significance
Past civilizations came up on river banks, the followed generations used rivers as a source of water and food. The flood plains of the Indus, the Nile delta, and the fertile crescent of the Tigris and Euphrates rivers provided man with all his basic necessities. They can be considered the pillars of human civilization as they have formed the nuclei for human settlements from the very origins of mankind. Fishing is a major means of livelihood for the people who resided in and around the riparian zones. Many of the tribal’s depend upon the river for fishing. The riparian vegetation decrease soil erosion and support silt thereby avoiding the pollutant input to the river. The shade, fruits and flowers offered by the riparian vegetations promotes the fish abundance in the aquatic ecosystems. The riparian vegetation provides Non Wood Forest Products for the dependent communities especially tribals who use the riparian forest to make their huts (Mainly Bamboo and Ochlandra), honey collections, timber, manure for farming and medicinal plants etc.
6. General overview of Cauvery riverine ecosystem
The Cauvery river originates at Talakaveri (12° 25' N, 75° 34' E) in the Western Ghats at an altitude of 1341m. It is the 8th largest river in the subcontinent and ranks as a medium river on a global scale. The Cauvery River basin is estimated to occupy 81155 km2 area occuping nearly 2.5% of the total geographical area of the country. The Cauvery river basin areas have a large floristic wealth enough to constitute as a separate phyto-geographic unit. The vegetation of the entire peninsular India excluding Western Ghats is adequately represented in this tract alone (Jayaram, 2000). The known flora of the basin comprises 2037 species from 990 genera belonging to 180 families. The Cauvery river system harbors 1050 species belonging 128 families. 504 herbs (48%), 270 shrubs (25.7%), 170 trees (16.2%) other plant forms like climber, twinners etc constitutes 10%. The river basin is in human use since the beginning of the human civilization. As increase in the population growth intensified demands keep putting pressure on these riparian areas for agricultural development, recreational uses, commercial development, housing development and others.
The Cauvery river basin from headwater reaches to outlet exhibits remarkable habitat heterogeneity. The river is reserved by guilds of fish species. Headwater support more endangered fish which is confined to rock stream types having high gradients and predominantly bedrock substrates (Smakhtin et al., 2006; Lakra et al., 2010). The riparian zone in the sacred landscape provides habitat for wildlife such as Asian elephants (Elephas maximus), Otter species (Amblonyx cinereus) (near threatened) (Shenoy, 2005), Endangered Nilgiri languar (Trachypithecus johnii) (Sunderraj and Johnsingh 2001), Indian civet (Viverricula indica), Lion-tailed Macaque (Macaca silenus) and so on. The forest landscapes here act as corridors for wildlife, as they are in contiguous with large protected areas such as Nagarahole National Park, Talacauvery, Brahmagiri and Pushpagiri Wildlife Sanctuaries.
The river bordering the Cauvery Wildlife Sanctuary in lower reaches of the river has a population of otters, crocodiles and many varieties of fishes along with the famous Masheer. This area is the breeding ground for a number of reptilian species like crocodiles, turtles, python, cobra, russell’s viper, banded krait and masheer fish besides wild boar, barking deer, four-horned antelope, green-billed malkoha, white-browed bulbul, pigmy woodpecker. Around 1000 elephants (Elepha maximus) graze through these riparian areas, as it also provides connectivity to Biligiri Rangan Hills Temple (BRT) wildlife Sanctuary and Mudumalai Tiger Reserve, which are in conjunction with Mysore – Nilgiri corridor (largest population of Asian elephants is found here).
7. Terminalia arjuna as a keystone species in Cauvery riverine ecosystem
Distributed throughout moist deciduous places of southern India, frequenting the banks of the water courses. Identified by thick grey smooth bark, exfoliating in large thin irregular sheets and buttressed trunk. It thrives best on loose moist, fertile alluvial loams and light deep sandy soils, often overlying more or less imprevious rock. The soil should have ample water supplies but should normally be well-drained. The soil under this tree becomes rich in calcium as the leaves are rich in this element. Terminalia arjuna species is deciduous, dominant canopy species and a representative riparian elements in riparian forests in lower reaches of Cauvery river. It can live grows to approximately 30m-45 in height, with a diameter at breast height (DBH) ranging from 300 cm – 600 cm. The Terminalia arjuna species is well adapted in the riparian zone by developing the buttressed type of root system to withstand the flood events.
Terminalia arjuna scattered along the lower of stretch of riparian forest is identified as a Keystone species. These scattered trees will acts as keystone structures as it supports wide array of species groups (e.g. arthropods, birds or mammals) for food resource and as shelter or nesting site (Munzbergova and Ward 2002; Plieninger et al., 2003; Tews et al., 2004).
8. Ecological significance of Terminalia arjuna in Cauvery river
8.1. Ecosystem engineers
Terminalia arjuna in the Cauvery riverine ecosystem can be referred as ‘Ecosystem Emgineers” as it modifies the physical environment by releasing resources to be used by other species. The activities of many organisms provide habitat that would not otherwise be available, often by means of disturbance to the physical habitat. Because of structural alterations they support many organisms and are often referred to as ecosystem engineers (Jones et al., 1994). Terminalia arjuna stabilizes river banks, trap sediments, increases nutrient availability in the top soil so as to provide a competitive advantage for adventitive forbs and grasses with higher nutrient requirements than their native counter parts.
The interlocking root system of this tree reduces the efficiency of rivers to withstand flood events and the butresses roots of this species are effective soil binders. Thus play a significant role in modifying the physical environment in ways that release resources for other species. Flood is a regular event in the downstream of River Cauvery, Terminalia arjuna act as barrier against erosion and stabilizes river bank in the riparian forest. It is the lone species along the riparian corridor acting as an emergent layer with good amount of canopy contributing to maintenance of micro climatic conditions viz., soil moisture and nutrirents. It is also necessary for the survival of the other evergreen species such as Olea dioca, Syzygium sp, Madhuca neriifolia, Madhuca latifolia etc during the seedling and sapling stage in the lower riparian stretch. The laden and gravels retained between the roots of T.arjuna retains soil moisture required for vegetation establishment and also provides a new substrates for the colonization of riparian plants. This species with good canopy cover limits the establishment or invasive from the adjacent scrub and dry land harboring Canthium sp, Alangium salviforum, Acacia catechu etc., as potential dominants. Thereby competition with semi-evergreen species is avoided. Hence absence of this species along the riparian corridor might cause a major change in the riparian vegetation structure and composition.
9. Resource providers
Terminalia arjuna acts as resource provider, as the leaves and flowers of this species falling into the water form diet for a number of fishes. The tree-lined river bank also provides shelter and shade to fish. Shade also keeps the growth of water weeds in balance, and regulates the temperature of water. The smooth coated otter (Lutra perspicillata) categorized as ‘vulnerable’ by 2004 IUCN Red List in the Cauvery Wildlife Sanctuary (CWS), needs a healthy aquatic ecosystem with plenty of fish. The shade provided by trees along the water's edge help to promote fish abundance with obvious benefits for the otter. Besides, gaint trees of Terminalia arjuna in the riparian zone act as a good potential nesting sites for bees and numerous bats which roost during day time. The bats play an important role as pollinators and seed dispersal agents. The riparian vegetation in the middle reaches of the Cauvery river is fragmented by various types of anthropogenic pressures resulted in shrinkage of several endemic species in riparian zone (Sunil et al., 2010). As the larger forks and branches of Terminalia arjuna provides a habitat for natural pollinators like honey bees, bats etc., influences the chance of recovery of native species in the fragmented patches of the riparian buffer in the middle reaches of the river. The huge canopy offered by T.arjuna species forms a thick patches in the riparian zone serving as important buffers in the semi-arid ecosystem, enable to provide a vital links to sensitive wildlife species such as Ratufa macroura (grizzled giant squirrel), an IUCN Red listed –near threatened species (Baskaran et al., 2011) which demands thick canopy cover along the riparian zones for breeding and feeding purpose (Joshua and Johnsingh, 1994).
10. Control of Invasive species in riparian zones
Riparian habitats are more susceptible to exotic species invasion due to the nutrient rich laden sediments and periodic flooding followed by hydrochory (Pyse and Prach 1994; Gregory and Naiman 2000). Invasion of non-native species in the riparian zone constitutes most serious threats to the biodiversity through the displacement of native plants (Shigenari and Izumi 2004). The Cauvery river in the lower reaches is is surrounded by dry deciduous to scrub type forests, and moist deciduous to semi-evergreen type trees along the river bank. Since the riparian zone stands distinctly here by harboring moist deciduous to semi-evergreen type vegetation, during dry season they assumes a very significant place for wildlife (Natta et al. 2003) particularly to the otters and wide elephant herds found in the sanctuary. But, the riparian vegetation here stands in high risk areas, as there is a chance of invasion of several pioneer species resided in the adjoining dry deciduous and scrub type vegetation into the riparian areas (Manjunath, 2001). Some of the fragmented corridors in riparian forest has already witnessed the invasion of scrub type species by lessening the native riparian species (Sunil et al., 2011).
Riparian species demands shade and moisture in soil in the early stages of their germinations. Huge canopy offered by Terminalia arjuna provide sufficient shade and holds soil moisture during the germination stage of riparian tree species. Some native species which supports avifaunal abundance such as Ixora bracheata, Syzygium cumini, Syzygium jambose, Diospyros melanoxylon and Madhuca latifolia resembles healthy association to the keystone species Terminalia arjuna. Also, it checks the growth of pioneer species in the riparian zone, thereby competition with riparian and semi-evergreen species harbored in riparian zone is avoided. Decline in native species such as Syzygium cumini, Syzygium jambose, Madhuca sp along the river bank might lead to the decline of natural source of leaves, twigs, fruit and insects that underpins the aquatic food web (Lovett et al., 2007). Hence, canopy species like T.arjuna is much inevitable in this region where their absence might cause a major change in the riparian vegetation structure and composition which inturn affects the aquatic ecosystem in the region.
11. Social significance of Terminalia arjuna in Cauvery riverine ecosystem
The primary uses of Cauvery river are providing water for irrigation, household consumption, industries and the generation of electricity (Varunprasath and Daniel, 2010). Over 90% of the river water is abstracted for irrigation. Population density in Cauvery is perhaps among the highest in the world (350 people/ km2; Smakhtin et al., 2006) indicating that potential for human disturbance is inevitable along the basin. The watershed regions of the Cauvery river is strongly affected by water stress in recent years (Ferdin, et al., 2010). Besides meeting industrial and agricultural needs, drinking water demands from the two major urban centres namely Bangalore (6th largest city in India) and Mysore with a millions population is increasing at an faster rate. The river being completely dependent on the monsoon for replenishment, the amount of water the Cauvery can provide to the various users varies with the fluctuating strength of the monsoon rainfall (Ferdin, et al., 2010). Providing clean water and improving the chemical quality of waters for both human consumption needs and ecosystem health have become important policy goals in the worldwide. Management of riparian vegetation is one strategy to achieve these goals. Terminalia arjuna is one of the key species in the Cauvery river to fulfill the strategy to maintain the river quality healthier. The widespread rootmat of this species protect the waterway from erosion and pollutants entering the river. It acts has a natural wall along the river bank resists soil erosion during flooding thereby avoiding the water loss due to the bank widening. Keeping increasing water scarcity and flood disaster in the lower reaches during monsoon, conservation and management of Terminalia arjuna in the upper reaches helps to reduce flood velocities and increase the further flow towards lower reaches, thereby maintaining the river water healthier.
We thank University Grant Commission for providing financial assistance, Karnataka State Forest Department in for extending the permission to carry out the studies and helping in field work.