Patch Dynamics and Clonal Expansion of Reeds in the Letaba River, Kruger National Park

A multiscale approach was used to study the dynamics of reed patches in a section of the Letaba River flowing through the Kruger National Park (KNP), South Africa's largest conservation area. The valley floor consists of a mosaic of patches of reeds, water, sand and rock. Reed vegetation change in space and time is closely linked to geomorphic change in the rivers of the KNP, because of reciprocal interactions between reeds, flow and sediment. Reeds are thus considered a potentially important species around which to formulate management goals for these rivers. A study of clonal growth characteristics at the scale of individual reedbeds was combined with a study of reed patch dynamics at the river reach scale. Mechanisms affecting clonal expansion across reedbed boundaries were combined with short-term patterns of reed persistence, loss and gain at the reach scale, and longer-term trends in reed cover, to produce a conceptual model of proposed relationships between flow conditions and reed cover in the Letaba River. Reed areal cover increased between 1988 and 1999. Despite this, reed persistence was low, and reeds were a dynamic element of the river landscape in the short term. Turnover of water and reed patches was high, within a matrix of sand. Complex patterns of reed gain and loss occurred at the boundaries of reed patches, with gain exceeding loss throughout the period of study. During a drier period (1988-1996), clonal expansion of patches and establishment of new patches contributed equally (50% each) to the increase in reed cover. During a wetter period (1996-1999), the relative contribution of clonal expansion increased to 90%. Reed persistence was higher in the wetter period due to less reed loss, and reed cover increased more rapidly. The relative proportions of reed, water and sand patches did not remain constant over 3 years or over 8 years in the alluvial section of the river studied. At the time and space scales of this study, therefore, the Letaba River did not display the kind of compositional stability described in other river systems. This points to the difficulty of applying equilibrium concepts of either geomorphology or vegetation dynamics to semi-arid rivers. A clear spatial patchiness was present in the distribution of sediment and nutrient resources on the macro-channel floor. Reed patches were associated with `current shadow' features, the characteristic raised elevation of the central, denser parts of reedbeds suggesting accumulation of sediment within reedbeds. Silt and clay particles accumulated preferentially within reedbeds. The results suggest that this accumulation was achieved primarily by selective retention of nes within reedbeds, and to a lesser extent by selective deposition of stones within reedbeds. Total carbon, nitrogen and phosphorus were all higher beneath reedbeds than in uncolonised sediment. The distributions of total N and total P were not correlated with that of total C, suggesting that nutrient accumulation does not simply reflect breakdown of reed organic matter. It was proposed that the nutrient accumulation recorded in this study was due primarily to interactions between reedbeds and flow, and to a lesser extent to the input of reed organic matter. The fact that surface resource distribution patterns did not always correspond to subsurface patterns illustrates the importance of viewing reed patches as three-dimensional entities. A high stem density and limited plasticity in spacer characteristics of Phragmites mauritianus in the Letaba River were consistent with a consolidation strategy of clonal growth. Physiological integration within within the clone was, however, limited—a notable departure from the consolidation strategy. While Phragmites mauritianus showed little evidence of plasticity in the horizontal spacer characteristics traditionally measured in studies of clonal growth patterns (internode length, branching pattern and bud activity), it was proposed that plasticity in rhizome characteristics related to regenerative capacity is important for reed persistence and clonal expansion in this habitat. The added vertical dimension of the rhizome system in species such as P. mauritianus limits the usefulness of horizontal spacer characteristics in explaining patterns of clonal expansion. Progressive accumulation of sediment above the rhizomes and upgrowth of reeds to the surface was proposed as a mechanism whereby the depth of the rhizome layer is increased, thus increasing the density of reed stems and enhancing the persistence of the reedbed. Rhizomes at reedbed boundaries did not show particular adaptations for rapid expansion into available space. Clonal expansion and consolidation of the space aroud established reedbeds was limited both by features of the rhizomes themselves (high proportion of dormant buds, low branching probability and shallow depth), and by external factors (flow disturbance). Management concerns over reed `encroachment' in the Letaba River appear unfounded. While rapid reed expansion may occur during favourable periods, conditions suitable for large increases in reed cover occur relatively infrequently and persist for short periods of time. It was proposed that wet periods with a high frequency of medium flows and no large floods are most likely to be associated with large increases in reed cover.本研究采用多尺度方法，对流经南非最大保护区克鲁格国家公园（Kruger National Park, KNP）的莱塔巴河（Letaba River）某河段的芦苇斑块动态展开探究。河谷滩涂呈现出芦苇、水体、沙地与岩石斑块镶嵌分布的格局。由于芦苇、水流与沉积物之间存在双向交互作用，芦苇植被的时空变化与克鲁格国家公园内河流的地貌变化紧密相关。因此，芦苇被视为制定该类河流管理目标的潜在关键物种。 本研究将单芦苇床尺度下的克隆生长特性研究与河段尺度的芦苇斑块动态研究相结合。将影响芦苇床边界克隆扩张的机制、河段尺度下芦苇留存、消失与新增的短期格局，以及芦苇盖度的长期变化趋势加以整合，构建了莱塔巴河水流条件与芦苇盖度之间潜在关联的概念模型。 1988年至1999年间，芦苇的面积盖度呈上升趋势。尽管如此，芦苇的短期留存率较低，且在短期内芦苇是河流景观中的动态组分。在沙地基质中，水体与芦苇斑块的周转速率较高。芦苇斑块边界处存在复杂的新增与消失格局，且在整个研究周期内，芦苇新增量始终高于消失量。 在干旱时段（1988-1996年），斑块克隆扩张与新斑块定植对芦苇盖度提升的贡献相当（各占50%）。而在湿润时段（1996-1999年），克隆扩张的相对占比提升至90%。由于湿润时段芦苇消失量更少，芦苇留存率更高，芦苇盖度的增长也更为迅速。 在所研究的冲积河段中，芦苇、水体与沙地斑块的相对占比在3年与8年的观测周期内均未保持恒定。因此，从本研究的时空尺度来看，莱塔巴河并未呈现其他河流系统中所观测到的组分稳定性。这表明，将地貌学或植被动力学的平衡概念应用于半干旱河流存在一定难度。 大型河道滩涂的沉积物与营养盐资源分布呈现显著的空间斑块性。芦苇斑块与‘水流阴影区’特征相关联，芦苇床中部密集区域特有的抬升高程表明沉积物在芦苇床内发生了积累。粉砂与黏土颗粒优先在芦苇床内聚集。研究结果表明，这种积累主要源于芦苇床对沉积物的选择性留存，其次源于芦苇床对石块的选择性沉积。 芦苇床下方的总碳、总氮与总磷含量均高于未被定植的沉积物。总氮与总磷的分布与总碳的分布并无相关性，这表明营养盐积累并非仅仅由芦苇有机质的分解所导致。本研究认为，观测到的营养盐积累主要源于芦苇床与水流的交互作用，其次源于芦苇有机质的输入。地表资源分布格局并不总是与地下分布格局相一致，这一现象凸显了将芦苇斑块视为三维实体的重要性。 莱塔巴河中的马氏芦苇（Phragmites mauritianus）具有较高的茎秆密度，且其间隔构件（spacer）特性的可塑性有限，这符合克隆生长的巩固策略。然而，克隆内部的生理整合性较弱，这与巩固策略存在显著差异。 尽管马氏芦苇（Phragmites mauritianus）在克隆生长格局研究中传统测定的水平间隔构件特性（节间长度、分枝模式与芽活性）方面几乎未表现出可塑性，但本研究认为，与再生能力相关的根状茎特性可塑性，对于该生境中芦苇的留存与克隆扩张至关重要。马氏芦苇等物种的根状茎系统具有额外的垂直维度，这使得水平间隔构件特性在解释克隆扩张格局时的适用性受限。本研究提出，根状茎上方沉积物的逐步积累以及芦苇向上生长至地表，是增加根状茎层深度、进而提高茎秆密度并增强芦苇床留存能力的机制。 芦苇床边界处的根状茎并未表现出向可利用空间快速扩张的特殊适应性。已定植芦苇床的克隆扩张与空间巩固过程同时受到根状茎自身特性（高比例休眠芽、低分枝概率与较浅深度）与外部因素（水流扰动）的限制。 针对莱塔巴河芦苇‘入侵’的管理担忧似乎并无依据。尽管在适宜条件下芦苇可能快速扩张，但能够促使芦苇盖度大幅提升的环境条件相对罕见，且持续时间较短。本研究认为，中等流量发生频率高且无大规模洪水的湿润时段，最有可能与芦苇盖度的大幅增长相关联。