Composition of understory vegetation in tree species of Cholistan desert , Pakistan

In the present study the understory vegetation or communities under the canopy of tree species of Cholistan desert (27° 42’ and 29° 45’ North and longitudes 69° 52’ and 75° 24’ East) and the effect of canopy cover on the soil chemistry were studied. Quadrats of 1 m 2 were laid to record the different plant species underside the canopy cover of tree species and then frequency, density and plant cover were recorded, whereas relative frequency, relative density, relative cover and IVI for each of the plant species were computed following the standard methods. The study was repeated for three times and field guides were used to identify the plant species. Soil analysis showed that tree species greatly modified the soil chemistry beneath the canopy cover. Stipagrostis plumosa, Salsola baryosma and Cenchrus cilaris were the major plant species that form the understory communities of tree species due to their close association with each other in the Cholistan desert. Other plant species also take part in forming the understory vegetation of tree species but to some less extent as they fail to form any type of association with each other.


INTRODUCTION
The Cholistan desert, a stretch of about 26,000 km 2 , situated in the Southern part of the Punjab province, Pakistan.The vegetation is a typical of arid regions and comprises of xerophytic plant species, adapted to extreme seasonal temperature, moisture fluctuation and a wide variety of edaphic conditions (Arshad et al., 2008).Vegetation cover is comparatively better in eastern region (200 mm rainfall zone) than the hyper arid southern region (100 mm rainfall zone) (Nisar et al., 2010).The soil topography and chemical composition is playing an important role in plant distribution in the area.The association of certain plant species to certain soils at different places is very common (Arshad et al., 2008).
It has been recognized that understory biomass increases with an increase in incident solar energy (Zavitkovski, 1976).In fact, the understory layer is affected by a reduction of light, when canopy is closed and stratified and by nutrient availability (Gilliam and Turrill, 1993).The different nutrient requirements of the species in a stand and their relative contributions to the chemistry litter affect soil nutrient availability and other soil properties such as pH and net nitrification (Ste-Marie and Pare, 1999).The relationship between overstory and understory is complex and also dependant on permanent site factors such as surface deposit.It has been demonstrated that canopy type exert its influence on nutrient availability, affects understory composition (Legare et al., 2001).Communities in fact are the mirrors of landmass or indicators of land's biological resources and based upon this information.The plant communities are considered as reliable indicators of environments and economic potentialities of the area (Arshad and Akbar, 2002).The association of certain plant species to certain soils at different places is very common and different plant communities at different soil types have been identified (Arshad et al., 2007(Arshad et al., , 2008)).Arshad (2003) has reported the major parameters that, are responsible for vegetation distribution in Cholistan desert are the salinity and pH.While Rao et al. (1989) are of the view that phytosociological groups are determinant of the soil types as the edaphic factors influence the vegetation more than any other factor.By reviewing the above reports the present study was conducted.The principal objective of this study was to determine the impacts of tree species on composition of understory vegetation and the chemistry of soil under it.

MATERIALS AND METHODS
The study was conducted in Cholistan desert (27° 42' and 29° 45' North and longitudes 69° 52' and 75° 24' East) on 10 different sites to determine the understory vegetation composition of five plant species namely Acacia jacquemontii, Prosopis cineraria, Prosopis juliflora, Tamarix aphylla and Acacia nilotica.Quadrat method was used to record understory vegetation (Oosting, 1957;Chul and Moody, 1983).Three quadrats measuring 1 x 1 m 2 were randomly taken from underside of the canopy of each plant species.The study was repeated for three times.The plant species were identified in the field with field guide (Rao et al., 1989).Frequency, density and plant cover were recorded, whereas relative frequency, relative density and relative cover for each of the plant species was computed following the methods described by Hussain (1989).Importance value index (IVI) was calculated by the direct summation of relative frequency, relative density and relative cover of each plant species (Curtis and Mclntosh, 1951;Muller-Dombois and Ellenberg, 1974;Chul and Moody, 1983).The species having the highest IVI were considered as the leading dominant plant species of understory vegetation.Soil samples from underside of each plant were also taken and were transferred to Soil Testing Laboratory, for complete soil assay to judge the effect of plant canopy on the soil enrichment.Chemical analysis conducted included soil pH, organic matter (Walkley-Black method), Phosphorus (Bray-2 method), and exchangeable cations, namely: sodium (Na), potassium (K), calcium (Ca) and magnesium (Mg).The exchangeable cations were extracted by leaching the sample soil with normal ammonium acetate and analyzed spectrophotometrically.The soil chemical analyses were done in triplicate to verify the results.

RESULTS AND DISCUSSION
The results of soil samples collected from underside the canopies of P. cineraria, P. juliflora, T. aphylla and A. nilotica are presented in Table 1.The texture of the soil was sandy and the soil was found to be moderately alkaline with the highest pH 8.6 in the soil collected from underside the canopy of T. aphylla, while minimum pH 8.2 was recorded in the soil samples collected from underside the canopy cover of P. juliflora (Table 1).The pH of the soil samples collected from underside the canopies of A. nilotica and P. cineraria were 8.4 and 8.3, respectively.
The electrical conductivity (E.C.) was maximum (4.57ds/m) in the soil samples from underside the canopy of T. aphylla.This increase of E.C. may be due to addition of salt accumulated leaves of this plant.Minimum E.C. 1.21 ds/m was recorded in the soils from underside the canopy of A. nilotica.The concentration of E.C. in the soils collected from underside the canopy P. juliflora and P. cineraria was 1.31 ds/m and 1.26 ds/m.
The soil samples collected from underside the canopy of trees in Cholistan desert were analyzed for soluble ions (Table 1).The maximum phosphorus contents (6.2 ppm) were recorded in the soil collected from underside the canopy cover of P. juliflora, while minimum phosphorus contents (4.9 ppm) was noted in the soil sample from underside the canopy of P. cineraria.Phosphorus contents in the soil underside the canopy of T. aphylla and A. nilotica was 5.03 ppm and 5.2 ppm, respectively.Similar results were observed for Potassium contents in soil samples.Maximum potassium contents (123.3 ppm) were recorded in soil samples underside the canopy of P. juliflora and minimum 105.6 ppm underside the canopy of P. cineraria.While potassium contents in soil samples collected from underside the canopy cover of T. aphylla and A. nilotica was 117.3 ppm and 113.0 ppm.
The results of the exchangeable cations (Na, Mg, Ca) measured in the different soil samples collected from underside the canopy cover of P. juliflora, P. cineraria, T. aphylla and A. nilotica varied greatly (Table 1).The maximum amount of these exchangeable cations was noted in the soil underside the canopy of T. aphylla namely: Ca (1.02 meq/L), Mg (3.09 meq/L) and Na (0.596 meq/L), while minimum amount of Ca (0.69 meq/L) was recorded in the soils from A. nilotica and P. cineraria.Calcium contents in the soils from the underside canopy cover of P. juliflora was 0.98 meq/L.Magnesium ions in soil samples from underside the canopy cover of P. cineraria were 0.38 meq/L, P. juliflora 0.45 meq/L and A. nilotica 0.47 meq/L.Minimum amount of Na (0.05 meq/L) was recorded in soil sample from underside the canopy of A. nilotica while soil samples from underside the canopy cover of P. juliflora and P. cineraria showed 0.19 and 0.23 meq/L of Na ions, respectively.
The accumulation of organic matter was meager in all the soil samples collected underside the canopies of studied tree species (Table 1).Highest organic matter (0.34 %) was recorded in the soil from underside the canopy cover of P. juliflora and minimum (0.25%) under P. cineraria.In the soil from A. nilotica and T. aphylla organic matter was 0.30 and 0.28%, respectively.

Plant density
Number of plant species recorded under the canopy of P. juliflora, P. cineraria, T. aphylla and A. nilotica varied greatly from one plant species to other species (Figure 1).Under the canopy cover of P. juliflora nine plant spe-cies were recorded.S. plumosa appeared as the domi-nant plant species having maximum plant density (23.55), whereas minimum plant density (0.11) was recorded by Panicum antidotale, Lasiurus scindicus and Leptadenia pyrotechnica.The density of Cenchrus ciliaris was 2.22, Haloxylon salicornicum 2.11, Salsola baryosma 1.88, Tribulus terristris 0.44 and Suaeda fruiticosa 0.22.Eleven plant species were recorded under the canopy of P. cineraria.S. plumosa again appeared as dominant plant having maximum density (15.00), while the lowest plant density was attained by Launea procumbens (0.11).Plant density of C. ciliaris and S. baryosma was 10.77 and 9.33.However the density in other plant species ranged between 0.22 to 1.00.

Plant frequency percentage
Plant frequency recorded under the canopy of P. juliflora, P. cineraria, T. aphylla and A. nilotica varied from one species to the other (Figure 2).Under the canopy of P.
The tree species in the present study were of the same height and age but this introduces a bias while comparing the effect of the tree canopies on the understory vegetation.The composition of understory vegetation underside the canopy cover may differ from one tree species to another one as the canopy of tree species had a great influence on shrub and herb cover underside it (Klinka et al., 1996).This may be due the fact that there occurs competition among understory plant species for soil moisture, soil nutrients or may be to escape high solar radiation and heat stress to avoid excessive transpiration as there is already scarcity of water in deserts.
Soil characteristics were the most striking factors which best explained the composition of ground flora underside the tree canopy.More precisely the organic matter availability of soil best explained the vegetation composition (Missin et al., 2001;Hutchinson et al., 1999).P. cineraria had high amount of soil organic matter under its canopy as compared to other tree species and this was due the reason that the leaves of P. cineraria are smaller and can easily be decomposed, as the result much litter was produced (Gower and Son, 1992).Top soil pH and other earth-alkaline cations (Na, Mg, Ca) were highest in the soil samples underside the canopy of T. aphylla.It was explained that T. aphylla was a highly salt tolerant and incorporate these salts in its metabolism and finally these salts ooze out through salt glands present in the leaves.
The highest phosphorus and potassium contents were observed under P. cineraria while all other tree species were intermediate.The species richness and diversity was much higher underside the canopy cover of A. nilotica (13 plant species) and T. aphylla (13 plant species) very closely followed by P. cineraria (11 plant spp.) and 9 plant species were recorded underside the canopy of P. juliflora.S. plumosa, S. baryosma and C. ciliaris appeared as the most frequently and abundantly occurring plant species forming the understory composition of vegetation.Some scientists reported the same canopy cover effects on understory vegetation (Callaway et al., 1991) while many others reported that the effect of tree canopy cover on understory vegetation diversity was much low (Whitney and Foster, 1988;Hong et al., 1997).The variation in the understory flora was may be due to the presence of thick litter layers or some plant species were sensitive to thick litter layers (Sydes and Grime, 1981 a, b;Holderegger, 1996).
It is concluded from the present study that tree species greatly modified the soil chemistry beneath the canopy cover.S. plumosa, S. baryosma and C. cilaris were the major plant species that form the understory flora of tree species due to their close association with each other in the Cholistan desert.Other plant species also take part in forming the understory vegetation of tree species but to some less extent as they fail to form any type of association with each other.Arshad M, Akbar G (2002).Benchmark of plant communities of Cholistan desert.Pak.J. Biol. Sci. 5: 1110-1113. Arshad M (2003)

Table 1 .
Soil analysis collected from underside the canopies of tree species of Cholistan desert, Pakistan.

Plant species under Tamarix aphylla
. Cholistan desert ecosystem monitoring for future management.Annual Technical report of a project sponsored by WWF-Pakistan.(Unpublished)