Needle parameter variation of mature black spruce families displaying a genetic x environment interaction in growth
To examine soil moisture stress, light, and genetic effects on individual needle parameters and investigate total needle contribution to productivity, individual and total needle parameter variation were quantified in 32-year-old black spruce from five crown positions from four full-sib families studied previously for drought tolerance and differential productivity on a dry and a wet site. The wet site had greater average needle length (NL), specific needle area (SNA), and needle N concentration (NN) than the dry site. Site differences in NN were most likely driven by soil moisture stress impairing N uptake as soil N was equal at both sites. Drought-tolerant families had greater average needle area (NA), but also greater needle dry mass (NDM), than drought-intolerant families. From the top to bottom crown position, needle parameters showing a linear or near linear increase were NL, SNA, and NN; needle parameters showing a linear decrease were NW, NA, NDM and C:N ratio. For total tree needle area, the wet and dry sites had 18.7 and 16.0 m2 tree-1 (leaf area index (LAI) 5.6 and 4.8), respectively, whereas total needle C mass was not significantly different between sites. Drought-tolerant and intolerant families had a total NA of 18.8 and16.0 m2 tree-1, respectively. However, the greater total NA of drought-tolerant families was driven by one family. Thus, the role of total foliage area at these high LAI values in genetic differences in productivity is inconsistent and most likely low. Therefore, based on a previous 3-year campaign of gas exchange measurements and confirmed independently with carbon isotope analyses, the rate of net photosynthesis is a more important component of site and genetic growth differential in mature black spruce than total needle area.