University of Minnesota, Department of Forest Resources, 1530 Cleveland Ave. N., St. Paul, Minnesota 55108, USA.
Ecology. 2010 Dec;91(12):3641-55. doi: 10.1890/09-1663.1.
In ecological communities, the outcome of plant-plant interactions represents the net effect of positive and negative interactions occurring above and below ground. Untangling these complex relationships can provide a better understanding of mechanisms that underlie plant-plant interactions and enhance our ability to predict population, community, and ecosystem effects of biotic interactions. In forested ecosystems, tree seedlings interact with established vegetation, but the mechanisms and outcomes of these interactions are not well understood. To explore such mechanisms, we manipulated above- and belowground interactions among tree seedlings, shrubs, and trees and monitored seedling survival and growth of six species (Pinus banksiana, Betula papyrifera, P. resinosa, Quercus rubra, P. strobus, and Acer rubrum) in mature pine-dominated forest in northern Minnesota, USA. The forest had a moderately open canopy and sandy soils. Understory manipulations were implemented in the forest interior and in large gaps and included removal of shrubs (no interactions), tieback of shrubs (belowground), removal of shrubs with addition of shade (aboveground), and unmanipulated shrubs (both below- and aboveground). We found that shrubs either suppressed or facilitated seedling survival and growth depending on the seedling species, source of interaction (e.g., above- or belowground), and ecological context (e.g., gap or forest interior). In general, shrubs strongly influenced survival and growth in gaps, with more modest effects in the forest interior. In gaps, the presence of shrub roots markedly decreased seedling growth and survival, supporting the idea that belowground competition may be more important in dry, nutrient-poor sites. Shrub shade effects were neutral for three species and facilitative for the other three. Facilitation was more likely for shade-tolerant species. In the forest interior, shrub shade negatively affected seedling survival for the most shade-intolerant species. For several species the net effect of shrubs masked the existence of both positive and negative interactions above and below ground. Our results highlight the complexity of plant-plant interactions, demonstrate that outcomes of these interactions vary with the nature of resource limitation and the ecophysiology of the species involved, and suggest that ecological theory that rests on particular notions of plant-plant interactions (e.g., competition) should consider simultaneous positive and negative interactions occurring above and below ground.
在生态群落中，植物-植物相互作用的结果代表了地上和地下正相互作用和负相互作用的净效应。理清这些复杂的关系可以更好地理解植物-植物相互作用的机制，并提高我们预测生物相互作用对种群、群落和生态系统影响的能力。在森林生态系统中，幼苗与已建立的植被相互作用，但这些相互作用的机制和结果还不太清楚。为了探索这些机制，我们在树木幼苗、灌木和树木之间进行了地上和地下相互作用的操纵，并监测了美国明尼苏达州北部成熟的以松树为主的森林中六种物种（银松、纸皮桦、加拿大铁杉、红栎、北美红杉和红枫）幼苗的存活和生长。该森林树冠适中开阔，土壤为沙质。在森林内部和大型空隙中进行了下层植被的处理，包括去除灌木（无相互作用）、将灌木系绳（地下）、去除灌木并增加遮荫（地上）和未处理的灌木（地上和地下）。我们发现，灌木根据幼苗的物种、相互作用的来源（如地上或地下）和生态背景（如空隙或森林内部），要么抑制要么促进幼苗的存活和生长。一般来说，灌木在空隙中强烈影响幼苗的生长和存活，而在森林内部的影响则较小。在空隙中，灌木根的存在明显降低了幼苗的生长和存活，这支持了地下竞争在干燥、养分贫瘠的地方可能更为重要的观点。对于三种物种，灌木遮荫的影响是中性的，对于另外三种物种则是促进性的。耐荫性更强的物种更有可能受到促进。在森林内部，对于最不耐荫的物种，灌木遮荫会对幼苗的存活产生负面影响。对于几个物种来说，灌木的净效应掩盖了地上和地下正相互作用和负相互作用的存在。我们的结果突出了植物-植物相互作用的复杂性，表明这些相互作用的结果随资源限制的性质和所涉及物种的生态生理学而变化，并表明基于植物-植物相互作用的特定概念（例如竞争）的生态理论应该考虑同时发生的地上和地下的正相互作用和负相互作用。
