Nutrient use efficiency of current-yr seedlings by using stable isotope (15N)

1)落叶阔叶树种当年生苗木氮素利用效率研究结果表明：不同母株、不同土壤养分条件下的板栗种子含氮量差异不显著，不同质量的种子含氮量差异不显著。在贫氮条件下，幼苗生长量与种子大小呈显著正相关，而在富氮条件下，幼苗生长量与种子大小呈显著负相关，这是由于在富氮条件下，种子大小对幼苗生长的影响减弱了，其中，小种子幼苗的生物量在叶片中的分配大于大种子幼苗，叶片中的氮含量也高于大种子幼苗，导致更大的RGR在小种子幼苗。小种子幼苗叶片氮含量的增加是由于对土壤氮（15 N）的吸收增加，而非种子氮（14）。2）在蒙古栎（Quercus mongolica）林下，冬蛾（Operophtera brumata）发生时，氮和单宁作为影响食草动物落叶的因子，对树冠和上生枝的落叶率分别为54%和81%。这种差异表明，橡树分配更多的防御化学物质，如单宁，冠层叶片比上分支叶。冠层叶片含氮量高，光合活性高。这种分配的防御化学品被认为具有选择优势，特别是在K-战略家，或橡树物种。

1) Nitrogen use efficiency of current-yr seedlings of deciduous broad-leaved tree species In Castanea crenata, percent nitrogen in seeds were not different among seeds from different mother trees of different soil nutrient conditions and not different among seeds of different mass. Seedling mass increment was positively correlated with seed size in nitrogen poor conditions (NPC) but not in nitrogen rich conditions (NRC) , due to that negative relationship between RGR and seed size diminished the effects of seed size on seedling growth in NRC.In which, seedlings from small-seeds showed greater allocation of biomass to the leaves, and greater nitrogen contents in the leaves than the seedlings from large-seeds, resulted in greater RGR in the small-seeded seedlings. Greater leaf nitrogen contents in small-seeded seedlings was due to greater nitrogen absorption from soil (15N) not from seeds (14) .2) Nitrogen and tannin as factors affecting defoliation by herbivores Percentages of defoliation in tree crowns and epicormic branchs were 54% and 81%, respectively, in an oak stands (Quercus mongolica) in the outbreak of the winter moth (Operophtera brumata) . This difference indicated that oaks allocated more defense chemicals, such as tannin, to canopy leaves than to epicormic branch leaves. Canopy leaves had higher nitrogen contents and had higher photosynthetic activity. Such allocation of defense chemicals is thought to have selection advantage particularly in K-strategists, or oak species.