Conversely, the lower TNP observed in Dangs forest may be attributed to the low-efficiency photosynthetic plants under high moisture conditions. The forested areas with high humidity and also some low temperature (cooler) localities promote adaptation of plants which are photosynthetically not very efficient. The growth of most Western Indian grazing lands seems not to be limited by any mineral nutrients but rather by the availability of soil water. Other workers have distinguished soil-moisture ecotypes in such weeds as Euphorbia hirta, Echinochloa colonum and Setaria glauca (Ramakrishnan, 1959). Cenchrus ciliaris and C.
setigerus are two important forage grasses in Western India. These species have several ecotypes growing in the grazing lands of Gujarat, Rajasthan and other states. A survey of these grasses by Pandeya et al. (1977) revealed considerable population variability. It has also been found that these species prefer those grazing lands which are situated on fresh and old alluvia in arid and semiarid tracts.
The active growth period of these grasses varies from about 55 days in arid localities to 120 days in semiarid places. Whereas no large variations were detected in peak above- ground biomass, such variations were evident in peak belowground biomass, with very low values in arid locations and very high values in the semiarid Gujarat Plains. The corresponding rate of total annual primary production for the arid location (Bikaner desert) was lowest in contrast to the highest rate for Baroda (semiarid). In contrast, turnover rate was highest in arid locality and lowest in semiarid. Regression analyses indicated that total net primary production (TNP) is the integrated function of precipitation, evapotranspiration, water content, air temperature, exchangeable minerals, and water holding capacity.
Out of various species of fodder grasses analyzed, the Ecotypes No. 4, 5 and 6 of Cenchrus ciliaris have been reported to have the highest protein content especially in July just before flowering. Kaul (1967, 1971) studied the photoperiodic and thermoperiodic mechanism of ecotypic differentiation and followed the migration of Xanthium strumarium from the U.S.A.
to India. He has also achieved some success in controlling the flowering of this notorious weed, thereby partially controlling its propagation. Kaul and his associates have recently contributed significantly to our knowledge of the Dal Lake in Kashmir (Kaul and Kaul, 1988; Ishaq and Kaul, 1988; Ishaq and Kaul, 1989). More is known about the structure of the producer stratum (primary producers) of Indian grasslands than about the higher trophic levels.
Certain grasslands in various ecoclimatic zones of India, mainly north, west and central India have been studied recently, mainly from the viewpoint of the primary producers. In some cases the influence of soil characteristics, nutrient status, abiotic and climatic factors on the grasslands has also been investigated. A few studies of secondary producers and energetics have, however, been made and are briefly summarized in Coupland (1979). Most of the Indian grasslands studied fall in the monsoonic belt and the amount and duration of rainfall is the major factor influencing the structure and composition of the plant communities in grasslands which are generally very rich in species numbers. Many of the Indian grasslands intergrade into savannas (i.e.
, annual and perennial grasses, non-graminoid herbs, and a few scattered trees). The dominant grasses in these grasslands include Cynodon, Dichanthium, Heteropogon, Cenchrus, Aristida, and Cymhopogon (Misra, 1979) and the more common trees include Acacia, Prosopis, and Zizyphus. The microbial elements of these grasslands are equally rich and diverse, including various kinds of blue-green and green algae; fungi, bacteria, protozoa and other protists, but these components are much poorly known as compared to the angiospenns.
The diversity of the vertebrate and invertebrate animals is higher in the tropical (including Indian) grasslands than in temperate areas, and the Indian grasslands harbour several kinds of insects, termites, grasshoppers, ants, molluscs, herbivores, and carnivores. In addition, the grazing pressure by cattle, sheep and goats is very intense in most cases even though few, if any, managed pastures, meadows or rangelands exist in India (see however, Ahuja, 1972). Being profoundly influenced by the strongly periodical climate and the intense biotic pressure, the flora of the Indian grasslands tends to be dominated by therophytes (about 60 per cent). Three important characteristics of these grasslands are: (1) their seral nature, i.e.
, origin either from deforestation or abandoned cultivation; (2) their marked phenological diversity (Singh, 1967; Gill, 1975); and (3) the fairly rapid turnover of underground plant material (Kumar and Joshi, 1972; Singh and Yadava, 1974). Some recent studies of a number of Indian and foreign grasslands have indicated that net primary production at least in the above-ground parts may depend not on the amount of rainfall but rather on the species composition or the plants concerned (see Singh and Joshi, 1979). The efficiency of energy capture is also generally higher in tropical as compared to temperate grasslands.
Grazing and burning are important causal agents involved in the origin and maintenance of Indian grasslands. Grazing pressure is generally quite high and exerts a profound influence on the community structure and diversity as well as soil characteristics (e.g., erosion) of the grasslands.
As expected, energy fixation is quite low in the grasslands of the semiarid zone and is quite high in dry sub humid zone. The proportion of incident sunlight energy fixed varies from 0.05 per cent in the semiarid zone to 0.5 per cent in the dry sub humid. According to Singh et al.
(1979), the grasslands of semiarid and dry sub humid regions need less nitrogen to support the same magnitude of energy flow, as compared to those grasslands which are located in humid regions. In contrast, the humid zone grasslands cycle smaller quantities of phosphorus in fixing comparable amount of energy. Singh et al., have also pointed out that, in general, greater amounts of nutrients are absorbed per unit of energy captured than the amount released from equivalent energy dissipated. This pattern points to some conservation of nutrients in the carry-over herbage as a consequence of protection from grazing. In recent years, some workers have studied grasslands with a view to artificial management as rangelands or pastures for raising animals. When grazing is controlled, herbage production can increase significantly, depending on the amount of the rainfall (Ahuja, 1972). In some desert and arid areas, sheep and goats are allowed to graze on range dining the rainy season only, being maintained on irrigated, seeded grassland during the dry season.
This results in a more efficient range use and also in increased growth and production in the animals (Gupta and Ambasht, 1979). R.S. Ambasht (personal communication) has studied certain grasslands on alluvial plains and hill slopes analyzing their sociology and the rate of primary production. Paired stands, open to the prevailing level of grazing and fenced to exclude cattle, were studied. These stands are dominated by Dichanthium annulatum, Heteropogon contortus, Vetivieria zizanioides, Aristida cyanantha, and in savanna by Heteropogon-Bothriochloa pertusa and Desmostachya bipinnata with shrubs or Zizyphus jujuba. Under favourable soil conditions Heteropogon-dominated grasslands showed a net primary productivity as high as 42 tonnes/ha/yr. On restoration of grazing, this grassland showed about 65 per cent decline in net available production whereas another hardy species Aristida cyanantha showed only about 15 per cent decline under similar conditions (i.
e., from 23 tonnes/ha/yr to about 20 tonnes/ha/yr). Except for the Heteropogon protected stands (energy conserving efficiency, ECE=3.2 per cent), the energy conserving efficiency is 0.7 per cent or a little less in all these grasslands. Grasslands in hilly areas seem more productive than those of alluvial plains despite the better edaphic condition of the latter. Ambasht has also observed that Dichanthium annulatum, a palatable fodder grass, shows increased vegetative propagation following heavy grazing.
He has quantified conservation values of plant communities against soil erosion, water and nutrient runoff, on the watersheds and river slopes of the Ganga, Gomati and Rihand rivers. Sharma and Ambasht (1986, 1987) have made field measurements of nitrogen fixation in forest plantations of the actinorhizal Alnus nepalensis of different age classes. S.S. Pandeya has made detailed studies of grasslands of Saugar.
Three types of grasses have been recognized, viz., (1) species having high ecological amplitude and wide distribution (2) annual species that are poor competitors and have limited distribution, and (3) species which are good competitors but grow on fertile substrata.