WHOLE-PLANT NITROGEN-RELATION AND WATER-RELATION TRAITS, AND THEIR ASSOCIATED TRADE-OFFS, IN ADJACENT MUSKEG AND UPLAND BOREAL SPRUCE SPECIES

Black and white spruce (Picea mariana and P. glauca) exhibit a strikin g micro-geographic distribution pattern at the southern edge of the bo real forest. Black spruce grows in flooded nutrient-poor muskegs, whil e white spruce is found primarily on drier upland sites, and the two r arely form mixed stands. In an attempt to characterize the physiologic al and, hence, mechanistic basis of this pattern, we sampled five adja cent populations of black and white spruce from northern British Colum bia and measured a suite of physiological and allocative characteristi cs, and associated trade-offs. that may be important to survival in ha bitats limited in nutrient or water availability. Two laboratory exper iments were conducted: a greenhouse dry-down experiment to assess rela tive degree of drought tolerance; and a 2x2 nested factorial experimen t in which seedlings were subjected to varying water and nitrogen regi mes for approximately 16 weeks. White spruce was more drought-tolerant (i.e., maintained positive net photosynthesis at lower shoot water po tential) and more efficient in water-use (as indicated by carbon isoto pic composition) than black spruce. Black spruce was found to be signi ficantly less sensitive to nitrogen stress, exhibited greater plastici ty in nitrogen-use efficiency (measured as the carbon-to-nitrogen rati o in total plant tissue), and had a greater specific N absorption rate under high-N conditions than white spruce. Trade-offs hypothesized to be associated with these nitrogen and water relations traits were exa mined, but few were confirmed. Water-use efficiency and nitrogen-use e fficiency did not trade-off between species, but did trade-off plastic ally (i.e., across treatments) within species. When exposed to simulta neous limitations of N and water both species were forced to utilize e ach resource with suboptimal efficiency. The change in isotopic compos ition per unit change in C/N ratio was not the same in the two species . This difference may reflect optimization of the trade-off, whereby e ach species maximizes the use efficiency of the most limiting resource (respective to its habitat), while minimizing the concomitant reducti on in the use efficiency of the other resource.