Hammond David S, Brown Valerie K, Zagt Roderick
Iwokrama INT'L centre for Rain forest conservation and development, 67 Bel Air, Georgetown, Guyana, , , , , , GY.
CABI Bioscience Environment, Silwood Park, Ascot, Berks SL7 5PY, UK, , , , , , GB.
Oecologia. 1999 May;119(2):208-218. doi: 10.1007/s004420050778.
It has long been argued that seed dispersal enhances recruitment in tropical trees by allowing offspring to escape' strong density/distance-dependent attack by insects, pathogens and rodents. Here we examined the effects of canopy openness and parent-offspring distance upon the frequency and timing of Chlorocardium rodiei seed attack and germination within a 15-ha plot of Guyanan tropical rain forest. Seeds were artificially dispersed beneath parent trees, in the understorey away from trees and in gaps. Analysing our data from an 85-week period of regular monitoring, we found that the main spatial gradients, canopy openness and distance to nearest adult conspecific, do not lead to differences in the final number of seeds attacked by infesting scolytid beetles or rodents. The timing of beetle attack, however, varied along the distance gradient and this difference affords seeds at further distances a window' in which to germinate and produce a seedling before attack. Canopy openness was not a good predictor of rooting success, but distance was strongly associated with root and shoot formation success and the mean time to shoot formation. There was a strong negative effect of distance on the likelihood of a seed being colonised by scolytid beetles prior to removal by rodents and shoot failure was strongly associated with prior infestation. We believe these results bring a key point to bear on the well-established notion of distance-dependent attack on seeds in tropical rainforests, viz. that seed characteristics (size, germination syndrome) and the timing of attack may be more important in explaining patterns of early seedling recruitment than distance. Our studies suggest that advantages accrued through dispersal in species like Chlorocardium will depend heavily on the race' between seed germination and attack. In the case of Chlorocardium, the race' can be lost at considerable distances due to its prolonged dormancy and the temporal fluctuations in fruitfall and rainfall which influence attack and germination. The results presented here suggest that the lag between seed attack and germination in tropical trees can regulate the influence of parent-offspring distance on cohort recruitment at this life history stage.
长期以来，人们一直认为种子传播有助于热带树木的更新，因为它能让后代“逃离”昆虫、病原体和啮齿动物对其密度/距离依赖性的强烈攻击。在此，我们在圭亚那热带雨林的一块15公顷的样地中，研究了林冠开阔度和亲子距离对绿心樟种子被攻击频率和时间以及种子萌发的影响。种子被人工播撒在母树下、远离树木的林下以及林中空地。通过分析我们在85周定期监测期间获得的数据，我们发现主要的空间梯度，即林冠开阔度和到最近成年同种个体的距离，并不会导致被蛀干小蠹甲虫或啮齿动物攻击的种子最终数量出现差异。然而，甲虫攻击的时间沿着距离梯度有所变化，这种差异为距离较远的种子提供了一个“窗口”，使其能够在被攻击之前发芽并长成幼苗。林冠开阔度并非生根成功的良好预测指标，但距离与根和芽的形成成功以及芽形成的平均时间密切相关。距离对种子在被啮齿动物移除之前被蛀干小蠹甲虫侵染的可能性有很强的负面影响，并且芽失败与先前的侵染密切相关。我们认为这些结果为热带雨林中种子距离依赖性攻击这一既定概念带来了一个关键要点，即种子特征（大小、萌发综合征）和攻击时间在解释早期幼苗更新模式方面可能比距离更为重要。我们的研究表明，像绿心樟这样的物种通过传播所获得的优势将在很大程度上取决于种子萌发和攻击之间的“竞赛”。就绿心樟而言，由于其休眠期延长以及影响攻击和萌发的果实掉落和降雨的时间波动，在相当远的距离“竞赛”可能会失败。此处呈现的结果表明，热带树木种子被攻击和萌发之间的时间间隔可以在这个生活史阶段调节亲子距离对种群更新的影响。
