Day Michael E, Zazzaro Sarah, Perkins L Brian
School of Forest Resources, University of Maine, 5755 Nutting Hall, Orono, Maine 04469 USA.
Am J Bot. 2014 Jan;101(1):45-55. doi: 10.3732/ajb.1300253. Epub 2013 Dec 24.
Seedling success is determined by evolved strategies of intrinsic genetic programming and plasticity that are regulated by extrinsic pathways. We tested the relative importance of these mechanisms in red spruce (Picea rubens Sarg.) and balsam fir (Abies balsamea Lin.), which share understory regeneration niches in northeastern North America. Although its reproductive effort is adequate, spruce has decreased in abundance, in relation to fir, in seedling and sapling populations, even in forests that have a predominance of spruce in the overstory.
To understand the factors that regulate this phenomenon and their implications for tree populations, we compared intrinsic and plastic regulation of first- and second-year seedlings under steady understory irradiance levels and in response to increases in light environment.
Both species exhibited interactions of ontogenetic patterns and plasticity in first- and second-year seedlings. Physiologically, spruce had higher photosynthetic capacity, allocation to photoprotective xanthophylls, and greater plasticity in response to light treatments. Although both species demonstrated an inability to plastically increase photosynthetic capacity in the short term, spruce benefited from greater allocation to foliage under increased irradiance. Fir showed a conservative strategy in root-shoot allocation that may better equip seedlings to withstand drought adaptations and attributes associated with greater shade tolerance.
These attributes likely contribute to the relative success of fir seedlings in the current climate. By contrast, they indicate that spruce would be a superior competitor in cooler, moister climates, which suggests that future forest composition will be largely determined by an interaction of disturbance and moisture regimes.
幼苗的成功取决于内在遗传程序设定和可塑性所演化出的策略，这些策略由外在途径调控。我们测试了北美东北部林下更新生态位相同的红云杉（Picea rubens Sarg.）和香脂冷杉（Abies balsamea Lin.）中这些机制的相对重要性。尽管云杉的繁殖能力足够，但在幼苗和幼树种群中，相对于冷杉，其数量有所减少，即使在林冠层以云杉为主的森林中也是如此。
为了解调控这一现象的因素及其对树木种群的影响，我们比较了一年生和二年生幼苗在林下稳定光照水平下以及对光照环境增加的响应中的内在和可塑性调控。
两个物种在一年生和二年生幼苗中均表现出个体发育模式与可塑性的相互作用。生理上，云杉具有更高的光合能力、对光保护叶黄素的分配以及对光照处理更大的可塑性。尽管两个物种在短期内都无法通过可塑性增加光合能力，但云杉在光照增加时通过增加对叶片的分配而受益。冷杉在根茎分配上表现出保守策略，这可能使幼苗更能抵御干旱适应以及与更强耐荫性相关的特性。
这些特性可能有助于冷杉幼苗在当前气候下相对成功。相比之下，这表明云杉在更凉爽、湿润的气候中会是更具优势的竞争者，这意味着未来森林组成将在很大程度上由干扰和水分状况的相互作用决定。
