Department of Botany, University of Wisconsin, Madison, Wisconsin 53706 USA.
Am J Bot. 2008 Nov;95(11):1395-407. doi: 10.3732/ajb.0800104. Epub 2008 Oct 8.
Leaf phenology varies markedly across tree species of temperate deciduous forests. Early leafing in spring may increase light capture and carbon gain prior to canopy closure, allowing saplings to survive in understory sites deeply shaded in midsummer. We quantified sapling leaf phenology for 18 tree species and seasonal variation in understory light availability at three sites along a ridge-slope-cove landform gradient in the Great Smoky Mountains National Park. Early leafing species (e.g., Aesculus flava, Carpinus caroliniana) broke bud an average of 24 d before late leafers (e.g., Magnolia fraseri, Nyssa sylvatica). Canopy closure occurred 14-18 d earlier and summer understory light was on average 63-74% lower on intermediate and mesic sites than on the xeric site. Early leafing species intercepted 45-80% of their growing season photon flux before canopy closure vs. 8-15% for late leafers. However, earlier leafing increased exposure to freezing temperatures by 5.5% per week near the mean time of bud break. Early leafing is strongly correlated with midsummer shade, risk of freezing temperatures, and distribution on mesic sites across a "main spectrum" of 15 deciduous species. Differences in leaf phenology and resultant impacts on spring carbon gain may help determine tree shade tolerance and distribution in southern Appalachian forests.
温带落叶林的树种间叶物候差异显著。春季提前展叶可在林冠郁闭前增加光捕获和碳增益，使幼树在仲夏时处于林下层深处仍能存活。我们在大烟山国家公园沿脊-坡-谷地形梯度的三个地点量化了 18 个树种的幼树叶物候和林下层光可用性的季节性变化。早叶物种（如美国七叶树、美国山核桃）比晚叶物种（如弗吉尼亚紫荆、美国椴）平均早 24 天展叶。林冠郁闭提前了 14-18 天，中湿和湿润地点的夏季林下层光照平均比干旱地点低 63-74%。早叶物种在林冠郁闭前截获了其生长季光子通量的 45-80%，而晚叶物种仅截获 8-15%。然而，早叶使幼树在芽期平均提前 5.5%暴露在冰点温度下。早叶与仲夏遮荫、受冻风险和中湿地点的分布密切相关，在 15 种落叶物种的“主谱”中。叶物候的差异及其对春季碳增益的影响可能有助于确定南阿巴拉契亚森林中树木的耐荫性和分布。
