Michigan State University, Department of Forestry, 126 Natural Resources, East Lansing, Michigan 48824, USA.
Ecology. 2011 Sep;92(9):1828-38. doi: 10.1890/10-1697.1.
Plant growth responses to resources may be an important mechanism that influences species' distributions, coexistence, and community structure. Irradiance is considered the most important resource for seedling growth in the understory of wet tropical forests, but multiple soil nutrients and species have yet to be examined simultaneously with irradiance under field conditions. To identify potentially limiting resources, we modeled tree seedling growth as a function of irradiance and soil nutrients across five sites, spanning a soil fertility gradient in old-growth, wet tropical forests at La Selva Biological Station, Costa Rica. We measured an array of soil nutrients including total nitrogen (total N), inorganic N (nitrate [NO3-] and ammonium [NH4+]), phosphate (PO4-), and sum of base cations (SBC; potassium, magnesium, and calcium). Shade in the forest understory did not preclude seedling growth correlations with soil nutrients. Irradiance was a significant predictor of growth in 52% of the species, inorganic N in 54% (NO3- in 32%; NH4+ in 34%), total N in 47%, SBC in 39%, and PO4- in 29%. Overall, growth was correlated with both irradiance and soil nutrients in 45% of species and with soil nutrients only in an additional 48%; rarely was irradiance alone correlated with growth. Contrary to expectations, the magnitudes of growth effects, assessed as the maximum growth response to significant resources for each species, were similar for irradiance and most soil nutrients. Among species whose growth correlated with soil nutrients, the rank importance of nutrient effects was SBC, followed by N (total N, NO3-, and/or NH4+) and PO4-. Species' growth responsiveness (i.e., magnitudes of effect) to irradiance and soil nutrients was negatively correlated with species' shade tolerance (survival under 1% full sun). In this broad survey of species and resources, the nearly ubiquitous effects of soil nutrients on seedling growth challenge the idea that soil nutrients are less important than irradiance in the light-limited understory of wet tropical forests.
植物对资源的生长反应可能是影响物种分布、共存和群落结构的重要机制。光照被认为是湿热带森林林下幼苗生长最重要的资源，但在野外条件下，还需要同时考虑多种土壤养分和物种，而不仅仅是光照。为了确定潜在的限制资源，我们模拟了树木幼苗的生长，将其作为光照和土壤养分的函数，跨越五个地点，这些地点跨越了哥斯达黎加拉塞尔瓦生物站的古老、潮湿的热带森林的土壤肥力梯度。我们测量了一系列土壤养分，包括总氮（总 N）、无机氮（硝酸盐[NO3-]和铵[NH4+]）、磷酸盐（PO4-）和碱金属阳离子总和（SBC；钾、镁和钙）。森林林下的阴凉处并没有阻止幼苗生长与土壤养分的相关性。光照是 52%物种生长的重要预测因子，无机氮是 54%（硝酸盐[NO3-]占 32%；铵[NH4+]占 34%），总氮占 47%，SBC 占 39%，PO4-占 29%。总的来说，在 45%的物种中，生长与光照和土壤养分都相关，在另外 48%的物种中，仅与土壤养分相关；光照单独与生长相关的情况很少见。与预期相反，以每个物种对显著资源的最大生长响应来评估生长效应的幅度，在光照和大多数土壤养分方面是相似的。在与土壤养分相关的生长的物种中，养分效应的等级重要性是 SBC，其次是 N（总 N、硝酸盐[NO3-]和/或铵[NH4+]）和 PO4-。物种对光照和土壤养分的生长反应性（即效应幅度）与物种对遮荫的耐受性（在 1%全日照下的生存能力）呈负相关。在对物种和资源的广泛调查中，土壤养分对幼苗生长的几乎普遍影响挑战了土壤养分在潮湿热带森林光照有限的林下比光照不重要的观点。
