No abstract

Black and white spruce (Picea mariana and P. glauca) exhibit a striking micro-geographic distribution pattern at the southern edge of the boreal forest. Black spruce grows in flooded nutrient-poor muskegs, while white spruce is found primarily on drier upland sites, and the two rarely form mixed stands. In an attempt to characterize the physiological and, hence, mechanistic basis of this pattern, we sampled five adjacent populations of black and white spruce from northern British Columbia and measured a suite of physiological and allocative characteristics, and associated trade-offs, that may be important to survival in habitats limited in nutrient or water availability. Two laboratory experiments were conducted: a greenhouse dry-down experiment to assess relative degree of drought tolerance; and a 2×2 nested factorial experiment in which seedlings were subjected to varying water and nitrogen regimes for approximately 16 weeks. White spruce was more drought-tolerant (i.e., maintained positive net photosynthesis at lower shoot water potential) and more efficient in water-use (as indicated by carbon isotopic composition) than black spruce. Black spruce was found to be significantly less sensitive to nitrogen stress, exhibited greater plasticity in nitrogen-use efficiency (measured as the carbon-to-nitrogen ratio 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 examined, but few were confirmed. Water-use efficiency and nitrogen-use efficiency did not trade-off between species, but did trade-off plastically (i.e., across treatments) within species. When exposed to simultaneous limitations of N and water both species were forced to utilize each resource with suboptimal efficiency. The change in isotopic composition 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 each species maximizes the use efficiency of the most limiting resource (respective to its habitat), while minimizing the concomitant reduction in the use efficiency of the other resource.

Effects of shoot water potential (Psi) and leaf-to-atmosphere vapor pressure difference (VPD) on gas exchange of jack pine (Pinus banksiana Lamb.), black spruce (Picea mariana (Mill.) B.S.P.), and aspen (Populus tremuloides Michx.) were investigated at the northern edge of the boreal forest in Manitoba, Canada. Laboratory measurements on cut branches showed that net photosynthesis (A(n)) and mesophyll conductance (g(m)) of jack pine and g(m) of black spruce did not respond to Psi until a threshold Psi was reached below which they decreased linearly. Photosynthesis of black spruce decreased slowly with decreasing Psi above the threshold and declined more rapidly thereafter. The threshold Psi was lower in black spruce than in jack pine. However, stomatal conductance (g(s)) of black spruce decreased continuously with decreasing Psi, whereas g(s) of jack pine showed a threshold response. Mesophyll limitations were primarily responsible for the decline in A(n) at low Psi for jack pine and black spruce in the middle of the growing season, but stomatal limitations became more important later in the season. Field measurements on in situ branches on warm sunny days showed that both conifer species maintained Psi above the corresponding threshold and there was no evidence of Psi limitation on A(n) of jack pine, black spruce or aspen. Vapor pressure difference was important in regulating gas exchange in all three species. An empirical model was used to quantify the g(s) response to VPD. When parameterized with laboratory data for the conifers, the model also fit the corresponding field data. When parameterized with field data, the model showed that stomata of aspen were the most sensitive of the three species to VPD, and stomata of black spruce were the least sensitive. For jack pine and aspen, stomata of foliage in the upper canopy were significantly more sensitive than stomata of foliage in the lower canopy. Vapor pressure difference had a greater impact on A(n) of aspen than on A(n) of the conifers as a result of aspen's greater stomatal sensitivity to VPD and greater slope of the relationship between A(n) and intercellular CO(2) concentration (C(i)). During the 1994 growing season, VPD averaged 1.0 kPa, corresponding to ratios of C(i) to ambient CO(2) of 0.77, 0.71 and 0.81 for jack pine, black spruce and aspen, respectively. We conclude that increases in VPD at the leaf surface in response to climate change should affect the absolute CO(2) and H(2)O fluxes per unit leaf area of the aspen component of a boreal forest landscape more than those of the conifer component.