Montgomery R, Chazdon R
University of Connecticut, Box 3043, Storrs, CT, 06269, USA.
, P.O. Box 11, Hawaii National Park, HI, 96718, USA.
Oecologia. 2002 Apr;131(2):165-174. doi: 10.1007/s00442-002-0872-1. Epub 2002 Apr 1.
To explore the importance of light availability for seedling growth in low light environments, we examined light-dependent growth, biomass allocation and mortality of tree seedlings growing in sites with 0.2-6.5% full sun, the range of light commonly encountered in the understory of closed canopy, lowland tropical forests. We transplanted seedlings of the canopy tree species, Dipteryx panamensis, Virola koschnyii, and Brosimum alicastrum into second-growth forest and native tree plantations at La Selva Biological Station, Costa Rica. We assessed seedling survival, growth, and seedling light environments bimonthly for 14 months. Plants were harvested at the end of the study to assess leaf area, total biomass, biomass partitioning and root growth. Survivorship of all species exceeded 60% across all microsites, although both D. panamensis and B. alicastrum had lower probabilities of survival in the darkest microsites. All species showed a strong positive relationship between light availability and growth, increasing in total biomass as light increased. However, the strength of the growth response differed among species causing a change in the rank order of species growth rates as light availability increased. Although D. panamensis showed the lowest growth rates in the darkest microsites, a strong response to increasing light led to a cross-over in performance, such that D. panamensis had the highest growth rate at the highest light levels studied. These data suggest that resource gradient partitioning could occur even in low light environments (0.2-6.5%). Given the limited range of light regimes sampled (i.e., non-gap microsites), our data demonstrate that growth of tropical tree seedlings beneath closed canopies is highly sensitive to light availability and that shade-tolerant species vary in these responses. Our results show that understory light heterogeneity, in the absence of canopy gaps, can significantly affect recruitment processes for shade-tolerant tree species.
为探究光照可利用性对低光照环境下幼苗生长的重要性，我们研究了在全日照0.2%-6.5%光照条件下生长的树木幼苗的光依赖生长、生物量分配和死亡率，该光照范围常见于封闭树冠的低地热带森林林下。我们将冠层树种巴拿马二翅豆、科氏油桃木和多香果移植到哥斯达黎加拉塞尔瓦生物站的次生林和原生树木种植园中。我们每两个月评估一次幼苗存活、生长及幼苗光照环境，持续14个月。在研究结束时收获植株，以评估叶面积、总生物量、生物量分配和根系生长。所有物种在所有微生境中的存活率均超过60%，尽管巴拿马二翅豆和多香果在最阴暗的微生境中存活概率较低。所有物种的生长与光照可利用性之间均呈现出强烈的正相关关系，随着光照增加，总生物量也增加。然而，不同物种的生长响应强度有所不同，导致随着光照可利用性增加，物种生长速率的排名顺序发生变化。尽管巴拿马二翅豆在最阴暗的微生境中生长速率最低，但对光照增加的强烈响应导致其生长表现出现交叉，即在研究的最高光照水平下，巴拿马二翅豆的生长速率最高。这些数据表明，即使在低光照环境（0.2%-6.5%）中也可能发生资源梯度分配。鉴于所采样的光照条件范围有限（即非林窗微生境），我们的数据表明，封闭树冠下热带树木幼苗的生长对光照可利用性高度敏感，且耐荫物种在这些响应方面存在差异。我们的结果表明，在没有林冠空隙的情况下，林下光照异质性会显著影响耐荫树种的更新过程。
