Abstract
Plants acclimate to nitrogen (N) or moisture stress by respectively increasing photosynthetic N use efficiency (PNUE) or water use efficiency (WUE), in order to maximize their relative growth rate (RGR). These two phenotypic adaptations have opposite effects on specific leaf area (SLA). Thus, intraspecific variations in the SLA-RGR relationship should reflect the relative importance of N vs. moisture stress in plants. In this study, we measured needle gas exchanges and N concentrations in order to derive PNUE and WUE, as well as SLA and RGR of black spruce (Picea mariana) seedlings growing on a rapidly drained site in the presence or absence of Kalmia angustifolia. The eradication of Kalmia had resulted in a ~140% increase in seedling growth over a 6 year period. We found a negative SLA-RGR relationship where Kalmia had been eradicated, and a positive one where Kalmia had been maintained. Kalmia eradication resulted in higher WUE when measurements were made directly on the seedlings, and in lower PNUE when twigs were rehydrated prior to gas exchange measurements. Our data suggest that the bigger seedlings on Kalmia-eradicated plots increase RGR by decreasing SLA, as a means of coping with moisture stress. By contrast, increasing SLA on noneradicated plots may be a means of coping with nutrient stress exerted by Kalmia. The SLA-RGR relationship could potentially be used to identify the limiting resource for black spruce seedlings in different environments.
Highlights
Variations in a given plant trait can be attributed to interspecific differences arising from natural selection, or to intraspecific variations arising from phenotypic plasticity in response to environmental conditions
Our data show that the presence of Kalmia will significantly affect the relationship between specific leaf area (SLA) and relative growth rate (RGR) for black spruce seedlings
The eradication of Kalmia resulted in bigger black spruce seedlings, perhaps as a result of lower interspecific competition for soil nutrients
Summary
Variations in a given plant trait can be attributed to interspecific differences arising from natural selection, or to intraspecific variations arising from phenotypic plasticity in response to environmental conditions. Species that compete well in resource-poor environments have lower relative growth rates (RGR) than those that grow in resource-rich environments [1]. This is partly because leaf attributes that favour leaf longevity and nutrient conservation tend to diminish specific leaf area (SLA). The relationship between RGR and SLA is not as robust, as studies have found these two traits to correlate either negatively [4], positively [5], or not at all [6]. Elucidating the mechanisms that control intraspecific differences in the SLA-RGR relationship is important for understanding how species acclimate across environmental gradients
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