Similarity of nutrient uptake and root dimensions of Engelmann spruce and subalpine fir at two contrasting sites in Colorado

Nutrient uptake capacity is an important parameter in modeling nutrient uptake by plants. Researchers commonly assume that uptake capacity measured for a species can be used across sites. We tested this assumption by measuring the nutrient uptake capacity of intact roots of Engelmann spruce (Picea engelmanni Parry) and subalpine fir (Abies lasiocarpa (Hook.) Nutt.) at Loch Vale Watershed and Fraser Experimental Forest in the Rocky Mountains of central Colorado. Roots still attached to the tree were exposed to one of three concentrations of nutrient solutions for time periods ranging from 1 to 96 h, and solutions were analyzed for ammonium, nitrate, calcium, magnesium, and potassium. Surprisingly, the two species were indistinguishable in nutrient uptake within site for all nutrients (P > 0.25), but uptake rates differed by site. In general, nutrient uptake was higher at Fraser (P = 0.01, 0.15, 0.03, and 0.18 for NH4+, NO3-, Ca2+, and K+, respectively), which is west of the Continental Divide and has lower atmospheric deposition of N than Loch Vale. Mean uptake rates by site for ambient solution concentrations were 0.12 mu mol NH4+ g(fwt)(-1) h(-1), 0.02 mu mol NO3- g(fwt)(-1) h(-1), 0.21 mu mol Ca2+ g(fwt)(-1) h(-1), and 0.01 mu mol Mg2+ g(fwt)(-1) h(-1) at Loch Vale, and 0.21 mu mol NH4+ g(fwt)(-1) h(-1), 0.04 mu mol NO3- g(fwt)(-1) h(-1), 0.51 mu mol Ca2+ g(fwt)(-1) h(-1), and 0.07 mu mol Mg2+ g(fwt)(-1) h(-1) at Fraser. The importance of site conditions in determining uptake capacity should not be overlooked when parameterizing nutrient uptake models. We also characterized the root morphology of these two species and compared them to other tree species we have measured at various sites in the northeastern USA. Engelmann spruce and subalpine fir were indistinguishable in specific root length and diameter distribution, while most of the other 10 species had statistically distinct diameter distributions across five diameter classes < 2 mm. Based on specific root length, subalpine fir and Engelmann spruce had significantly coarser roots than red pine (Pinus resinosa Soland), yellow birch (Betula allegheniensis Britt.). sugar maple (Acer saccharum Marsh.), chestnut oak (Quercus prinus L.), black cherry (Prunus serotina Ehrh.), and red spruce (Picea rubens Sarg.). White oak (Quercus alba L.), balsam fir (Abies balsamea (L.) Mill.), American beech (Fagus grandifolia Ehrh.) and loblolly pine (Pinus taeda L.) were intermediate in SRL (indistinguishable from Engelmann spruce and subalpine fir by ANOVA). Species that differ more in physiology and morphology than the two species we compared would likely show dissimilar uptake characteristics even at the same site. (C) 2009 Elsevier B.V. All rights reserved.