u n d a t i o n S r i L a n k a 2 0 0 0 28(1): [43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62] Abstract: The main ohjective of the present study was to investigate the effects of different levels of shade and water availability on tree seedling p o w t h and on some related physiological parameters. The five tree species were selected to represent two early successional sp-cies h e . AJsto~~ia nzacropl~~~lla andAcucia c~zrricu.liforn~.is), two late-successional

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... pecies (i.e. Artocurpzrs heteroplqtdlr~s and Termimalia oi-jz~~1.a) and one dry zone species (i.e. Azudiractu indicu). Even-aged seedlings of the tree species werr grown in pots in a plant house under three levels of artificial shade,i.e. open, medium (40% shade) a n d high (70%) shade, and two w a t e r regimes, i.e. well-watered and water-stressed u p to temporary early-morning wilting, clul.ing the period from 27 May to 10 Novemhel; 1998. Both absolute and relative biomass gain of tree species during t h e experimental period decreased significantly wit11 increasing shade. Alston,iu had the bighest relative growth rates under all shade x water regime cornhinations whe~,eas Az(.tdirtlctu had the lowest. Significant inter-.:pe:i-s variation was shown in leaf' water potential ('i'), leai'stornatal conductance ( g , ) , leat'chloropl~yll (LCC), nitrogen (LNC) and potassium (LPC) contents under all shade x water regime comhinatiuns. Water stress decreased biomass gain, LCC and g, and increased LNC. Increasing shade decreased g, and leaf'weidlt ratio, hut increased root weight ratio under. hoth water regimes. Howevel; there were significant shade x water regime ia1;eraction effects on LCC, LNC and Y . Both absolute hiomass gain and relative gi;owth rate (RGR) had significant positive correlations with Y', g, and LC!C hut were not correlated with LNC or LPC. Multiple regression analysis identified Y and g, as the factors which contrihuted most to the ohservedva~iation of absolute biomass gain and RGR.