Interactions with soil fungi alter density-dependence and neighborhood effects in a locally abundant dipterocarp species

Seedling recruitment can be strongly affected by the composition of nearby
plant species. At the neighborhood scale (on the order of tens of meters),
adult conspecifics can modify soil chemistry and presence of host microbes
(pathogens and mutualists) across their combined canopy area or rooting
zones. At local or small spatial scales (on the order of one to few
meters), conspecific seed or seedling density can influence the strength
of intraspecific light and resource competition and also modify the
density-dependent spread of natural enemies such as pathogens or
invertebrate predators. Intrinsic correlation between proximity to adult
conspecifics (i.e. recruitment neighborhood) and local seedling density,
arising from dispersal, make it difficult to separate the independent and
interactive factors that contribute to recruitment success. 
Here, we present a field experiment in which we manipulated both the
recruitment neighborhood and seedling density to explore how they interact
to influence the growth and survival of Dryobalanops aromatica, a dominant
ectomycorrhizal tree species in a Bornean tropical rainforest. First, we
found that both local seedling density and recruitment neighborhood had
effects on performance of D. aromatica seedlings, though the nature of
these impacts varied between growth and survival. Second, we did not find
strong evidence that the effect of density on seedling survival is
dependent on the presence of conspecific adult trees. However,
accumulation of mutualistic fungi beneath conspecifics adults does
facilitate establishment of D. aromatica seedlings. In total, our results
suggest that recruitment near adult conspecifics was not associated with a
performance cost and may have weakly benefitted recruiting seedlings.
Positive effects of conspecifics may be a factor facilitating the regional
hyperabundance of this species.  Synthesis: Our results provide
support for the idea that dominant species in diverse forests may escape
the localized recruitment suppression that limits abundance in rarer
species.

幼苗更新（Seedling recruitment）极易受到邻近植物物种组成的影响。在邻域尺度（neighborhood scale，数十米量级）下，同种成年个体（adult conspecifics）能够通过其冠幅（canopy area）或根系分布区（rooting zones）改变土壤化学性质（soil chemistry）以及宿主微生物（host microbes，包含病原菌与互利微生物）的存在情况。在局域或小空间尺度（local spatial scales，1至数米量级）下，同种种子或幼苗的密度会影响种内光照与资源竞争（intraspecific light and resource competition）的强度，同时还会改变天敌（natural enemies，如病原菌或无脊椎动物捕食者（invertebrate predators））的密度依赖扩散过程。由于扩散（dispersal）作用，与同种成年个体的距离（即更新邻域（recruitment neighborhood））与局域幼苗密度之间存在内在关联（intrinsic correlation），这使得区分影响更新成功的独立因子与交互因子变得困难。
本研究开展了一项野外实验（field experiment），同时操控更新邻域与幼苗密度，以探究二者的交互作用如何影响婆罗洲热带雨林（Bornean tropical rainforest）中的优势外生菌根树种（ectomycorrhizal tree species）——香坡垒（Dryobalanops aromatica）的幼苗生长与存活情况。
首先，我们发现局域幼苗密度与更新邻域均会对香坡垒幼苗的生长表现（performance）产生影响，不过这些影响的性质在生长与存活两个维度上存在差异。其次，我们未找到充分证据表明，幼苗密度对幼苗存活的效应依赖于同种成年植株的存在。但同种成年植株下方的互利真菌聚集确实能够促进香坡垒幼苗的定植。
总体而言，我们的研究结果显示，在同种成年植株附近进行的幼苗更新并未带来生长表现上的代价，反而可能对幼苗定植产生微弱的促进作用。同种植株的正向效应或许是促成该物种在区域内高度富集的关键因素之一。
综合分析：我们的研究结果支持这一假说——在物种多样的森林中，优势物种或许能够摆脱限制稀有物种种群丰度的局域更新抑制效应。