Dynamics of southern beech (Nothofagaceae) stands in the lowland North Island of New Zealand

In south-central Chile, the dynamics of southern beech (Nothofagaceae) stands vary predictably along climatic gradients. Beeches form persistent stands at high elevations, but their establishment on lowland sites depends mainly on catastrophic disturbances. In New Zealand, nearly all studies of beech forest dynamics to date have reported all-sized, persistent populations. However, previous work has focused largely on the South Island or on North Island mountains, where beeches are widespread and often dominant. Little is known about beech dynamics in the warm North Island lowlands, where beeches are less common. We aimed to clarify the circumstances that enable beeches to establish on lowland sites in the North Island. We measured population structures of beeches and associated species on 11 plots distributed across a range of climate, lithology and topography. On sites where the most shade-tolerant broadleaved trees were absent or uncommon, all-sized populations showed that beech regeneration did not depend on major disturbances. These stands were located mostly on steep faces or ridges with soil carbon:nitrogen ratios &gt; 18, usually with ericaceous understoreys. On sites with higher N availability, where shade-tolerant broadleaved trees were abundant, beech populations were generally sparser, with restricted age ranges. One of these ± even-aged beech stands appeared to have established after windthrow, the other four after fires. Recent dieback of <i>Weinmannia racemosa</i> associated with insect attack had resulted in another wave of beech regeneration in one of the post-fire stands. Establishment on landslides appears to only rarely enable beeches to capture new sites in the lowland North Island. We conclude that beeches form stable self-replacing stands on lowland North Island sites with low nitrogen availability, but that exogenous disturbances occasionally enable beeches to establish as pioneers on sites with higher nitrogen availability. The early successionalrole commonly seen in lowland south-central Chile thus appears less widespread in New Zealand.

智利中南部地区，南部山毛榉（南青冈科，Nothofagaceae）林分的动态沿气候梯度呈现出可预测的变化规律。高海拔区域内，山毛榉可形成稳定持久的林分；但在低地生境中，其定植主要依赖灾难性干扰事件。迄今为止，新西兰几乎所有关于山毛榉林动态的研究均报道了全龄级、稳定存续的种群。然而，过往研究多聚焦于南岛或北岛的山地生境——在这些区域中山毛榉分布广泛且通常为优势种。但对于山毛榉在北岛温暖低地的动态变化则知之甚少，毕竟该区域中山毛榉较为少见。本研究旨在阐明北岛低地生境中山毛榉得以定植的具体条件。我们在覆盖不同气候、岩性与地形条件的11个样地中，调查了山毛榉及其伴生种的种群结构。在耐荫阔叶树种缺失或较为稀少的生境中，全龄级种群数据表明，山毛榉的更新并不依赖大规模干扰事件。这类林分多分布于坡度陡峭的坡面或山脊，其土壤碳氮比大于18，且通常伴生有杜鹃花科（Ericaceae）灌丛下层植被。在氮素有效性较高、耐荫阔叶树种丰富的生境中，山毛榉种群通常较为稀疏，且年龄结构受限。其中1处近似同龄级的山毛榉林分似乎是在风倒事件后形成的，其余4处则是在火灾后建立的。近期受虫害影响的总状威曼木（*Weinmannia racemosa*）枯梢现象，使得其中一处火烧后生境中出现了新一轮的山毛榉更新浪潮。在北岛低地，山毛榉似乎极少能通过滑坡生境实现新位点的定植。本研究结论表明，在北岛低地氮素有效性较低的生境中，山毛榉可形成稳定的自我更新林分；而在氮素有效性较高的生境中，外源干扰偶尔可促使山毛榉作为先锋种完成定植。由此可见，智利中南部低地常见的山毛榉早期演替先锋种角色，在新西兰的分布范围似乎更为有限。