Spatial heterogeneity in water velocity drives leaf litter dynamics in streams

1. Stream hydro-morphology refers to the heterogeneous distribution of hydrologic conditions that occur above a complex benthic surface such as a streambed.
2. We hypothesised that hydro-morphological conditions will influence the retention, re-distribution, and microbial-driven decomposition of leaf litter inputs in stream ecosystems because each process varies with overlying water velocity.
3. We tested this hypothesis using: (1) the spatial distribution of water velocity within a stream reach as a surrogate of stream hydro-morphology; (2) leaf tracer (i.e. Ginkgo biloba L.) additions with serial recovery to examine the relationship between benthic retention and overlying velocity; and (3) measurements of leaf litter decomposition (i.e. Alnus glutinosa [L.] Gaertn.) under different water velocity conditions.
4. Results demonstrate that water velocity exerts a significant influence on the retention and re-distribution of leaf litter inputs within the reach. The observed range of water velocity (from c. 0 to 0.92 cm/s) also strongly influences the range of leaf litter decomposition rates (0.0076–0.0222/day).
5. Our findings illustrate that water velocity influences leaf litter dynamics in streams by controlling leaf litter transport, retention and re-distribution as well as how leaves decompose within recipient stream reaches. Ultimately, the results show that the efficiency of leaf litter inputs in supporting stream ecosystem function is dependent on the hydro-morphological characteristics of the receiving stream ecosystems.

     

云冷杉针阔混交林叶面积指数的空间异质性

叶面积指数(Leaf area index,LAI)是森林生态过程的关键参数和描述森林冠层结构的重要指标。用半球摄影技术对吉林省汪清林业局金沟岭林场的10块1 hm2云冷杉针阔混交林的LAI进行测定,采用地统计学的半变异函数和普通克里格法对研究区的LAI的空间异质性进行了分析。结果表明:10块样地的10 m×10 m小样方内以及样地间的LAI离散程度较小,但分布有一定的规律。LAI的空间相关性存在不确定性,可由线性、孔状、高斯3种模型有效的描述,空间相关性存在的尺度范围变异大,中等和强空间相关性的影响范围在15—155m之间,10块样地LAI的空间相关性的影响范围平均为65.637m。普通克里格插值结果显示,LAI的空间分布呈明显的条带状和斑块状的梯度变化。LAI与林分平均胸径、林分平均高和林分平均冠长呈显著负相关,与林分平均冠幅、林分密度以及树种个数呈显著正相关。研究结果可为不同尺度云冷杉针阔混交林LAI的估计提供依据。     

Spatial and seasonal heterogeneity in understory light conditions caused by differential leaf flushing of deciduous overstory trees

In a deciduous broad-leaved forest, we investigated the seasonality and heterogeneity of understory light conditions in relation to the leaf phenology of overstory trees. Fisheye photographs were taken from spring to autumn to estimate direct and indirect light conditions above the understory. Spatial variation in daily direct photosynthetic photon flux density (PPFD) was highest in late May, when the early and the intermediate-flushing trees had finished flushing and the late-flushing trees had just started flushing. After whole canopy closure, spatial variation in direct PPFD became low. Thus, asynchronous overstory leaf flushing in spring resulted in spatial heterogeneity in understory light conditions. These results suggest that the leaf phenological patterns of overstory trees are an important factor in the formation of the understory community.     

森林更新与空间异质性

森林更换是一个重要的生态学过程,一直是森林生态系统动态研究中的主要领域之一。森林更新受物理环境、自然干扰、人为干扰、更新树种特性、树种对干扰的反应等因素及其相互作用的影响。这些生物和非生物的因素随空间和时间而不断变化,构成了森林的空间异质性和时间异质性,使森林更新具有空间和时间上的变化特点,表现在异质性的格局和过程中。探索森林更新与空间异质性的内在规律,可揭示空间格局对更新的生态学过程的潜在作用机制。本文主要综述了近年来有关森林更新与空间异质性研究的主要内容和一些观点,分析了更新中空间异质性的来源,着重评述了空间异质性的生境及更新树种的反应、小尺度的空间异质性与更新动态、林分中光有效性的空间异质性与更新格局以及土壤和更新的空间异质性尺度的关联性等方面的研究。     

生态学系统的空间异质性

空间异质性是生态学系统的一个普遍牧场生,生态学家对它在生态学中的重要性已取得了比以往更深刻的认识。试图从空间异质性的含义,空间异质性与尺度和等级的关系,空间异质性的定量描述,空间异质性对生物和非生物过程的影响,以及空间异质性的动态等5个方面综述了有关空间异质性的生态学研究的新进展。     

Spatial heterogeneity and interspecific competition

A model of two competing species is presented in which each species is able to disperse over a single spatial axis. The spatial axis is composed of two intervals with different carrying capacities. We ask the question: If species one is alone and at population dynamic equilibrium, then when can species two successfully invade when rare? We say that an interval is “suitable” if the interval can be invaded by species two in the absence of dispersal by both species, and we say an interval is “unsuitable” if the interval cannot be invaded by species two in the absence of dispersal by both species. We offer three findings: (I) If one interval is suitable and the other is unsuitable, then the success of invasion depends upon the length of the suitable interval. Invasion succeeds if the suitable interval is larger than a threshold minimum and fails otherwise. (II) It is possible for species two to invade even though both intervals are unsuitable. (III) It is possible for species two to fail to invade even though both intervals are suitable.     

土地利用强度变化的空间异质性研究

以晋江市为例,以系统网格采样法获取研究样本,以地统计学理论和半方差函数为工具,探讨晋江市1989～2001年人类对土地景观干扰活动的空间异质性.结果表明,研究时段人类干扰活动存在明显的空间异质性,尤其是NE-SW向的方向性结构突出,主要原因在于交通干线的影响;各级行政中心成为其生长极点;交通干线与行政中心联合作用,使人类干扰呈现明显的点-轴格局.     

长白落叶松冠层光合作用的空间异质性

以2014年黑龙江省帽儿山林场14年生人工长白落叶松为研究对象,对比分析了各项光合指标、环境因子及光合生理参数在冠层内的空间差异性,并探讨了净光合速率(P_n)与其他指标的关系.结果表明: 在树冠垂直方向,上层P_n、气孔导度(g_s)和蒸腾速率(T_r)显著高于中层和下层,胞间CO₂浓度(C_i)表现为下层>中层>上层;光合有效辐射(PAR)从上层外部到下层内部呈显著降低趋势,水汽压差(VPD)和叶片温度(T_l)表现为上层显著高于中层和下层,相对湿度(RH)则无显著差异;最大净光合速率(P_{n max})、暗呼吸速率(R_d)、光补偿点(LCP)和光饱和点(LSP)均表现为上层>中层>下层,下层比上层分别降低32.7%、55.8%、80.2%和51.6%,表观量子效率(AQY)表现为下层>中层>上层,下层分别是中层和上层的1.2和1.3倍.水平方向,光合指标和环境因子的差异性主要体现在树冠上层,P_n、g_s、T_r、PAR和VPD表现为树冠外部显著高于树冠内部,而C_i和RH差异不显著;P_{n max}、R_d、LCP和LSP表现为外部>内部,内部比外部分别降低0.4%、37.7%、42.0%和16.4%,而AQY在内部比外部高0.7%.C_i是限制P_n的主要生理因子,PAR是影响P_n的主要环境因子,尤其在弱光区域PAR对P_n的影响十分明显.因此,在模拟和预估树木冠层光合作用时,考虑空间异质性有一定的必要性.     

干旱荒漠区煤矸石山覆土区土壤水分物理性质的空间异质性

基于网格取样(20m×20m),采用经典统计学和地统计学相结合的方法,研究了干旱荒漠区煤矸石山表层(0～5 cm)土壤水分物理性质的空间异质性和分布格局.结果表明:研究区土壤容重、毛管孔隙度、毛管最大持水量、总孔隙度、饱和含水量表现为弱变异,土壤含水率表现为中等变异.除土壤容重的最佳拟合模型为高斯模型外,其余指标的最佳...     

Spatial heterogeneity of daphniid parasitism within lakes

Spatially explicit models show that local interactions of hosts and parasites can strongly influence invasion and persistence of parasites and can create lasting spatial patchiness of parasite distributions. These predictions have been supported by experiments conducted in two-dimensional landscapes. Yet, three-dimensional systems, such as lakes, ponds, and oceans, have received comparatively little attention from epidemiologists. Freshwater zooplankton hosts often aggregate horizontally and vertically in lakes, potentially leading to local host–parasite interactions in one-, two-, or three-dimensions. To evaluate the potential spatial component of daphniid parasitism driven by these local interactions (patchiness), we surveyed vertical and horizontal heterogeneity of pelagic Daphnia infected with multiple microparasites in several north temperate lakes. These surveys uncovered little evidence for persistent vertical patchiness of parasitism, since the prevalence of two parasites showed little consistent trend with depth in four lakes (but more heterogeneity during day than at night). On a horizontal scale of tens of meters, we found little systematic evidence of strong aggregation and spatial patterning of daphniid hosts and parasites. Yet, we observed broad-scale, basin-wide patterns of parasite prevalence. These patterns suggest that nearshore offshore gradients, rather than local-scale interactions, could play a role in governing epidemiology of this open water host–parasite system. Electronic Supplementary Material Supplementary material is available for this article at     

Spatial and functional heterogeneity of sphingolipid-rich membrane domains

Little is known about the organization of lipids in biomembranes. Lipid rafts are defined as sphingolipid- and cholesterol-rich clusters in the membrane. Details of the lipid distribution of lipid rafts are not well characterized mainly because of a lack of appropriate probes. Ganglioside GM1-specific protein, cholera toxin, has long been the only lipid probe of lipid rafts. Recently it was shown that earthworm toxin, lysenin, specifically recognizes sphingomyelin-rich membrane domains. Binding of lysenin to sphingomyelin is accompanied by the oligomerization of the toxin that leads to pore formation in the target membrane. In this study, we generated a truncated lysenin mutant that does not oligomerize and thus is non-toxic. Using this mutant lysenin, we showed that plasma membrane sphingomyelin-rich domains are spatially distinct from ganglioside GM1-rich membrane domains in Jurkat T cells. Like T cell receptor activation and cross-linking of GM1, cross-linking of sphingomyelin induced calcium influx and ERK phosphorylation in the cell. However, unlike CD3 or GM1, cross-linking of sphingomyelin did not induce significant protein tyrosine phosphorylation. Combination of lysenin and sphingomyelinase treatment suggested the involvement of G-protein-coupled receptor in sphingomyelin-mediated signal transduction. These results thus suggest that the sphingomyelin-rich domain provides a functional signal cascade platform that is distinct from those provided by T cell receptor or GM1. Our study therefore elucidates the spatial and functional heterogeneity of lipid rafts.     

Spatial heterogeneity and the stability of host-parasite coexistence

Spatially heterogeneous environments can theoretically promote more stable coexistence of hosts and parasites by reducing the risk of parasite attack either through providing permanent spatial refuges or through providing ephemeral refuges by reducing dispersal. In experimental populations of Pseudomonas aeruginosa and the bacteriophage PP7, spatial heterogeneity promoted stable coexistence of host and parasite, while coexistence was significantly less stable in the homogeneous environment. Phage populations were found to be persisting on subpopulations of sensitive bacteria. Transferring populations to fresh microcosms every 24 h prevented the development of permanent spatial refuges. However, the lower dispersal rates in the heterogeneous environment were found to reduce parasite transmission thereby creating ephemeral refuges from phage attack. These results suggest that spatial heterogeneity can stabilize an otherwise unstable host-parasite interaction even in the absence of permanent spatial refuges.     

Spatial heterogeneity and the design of immunization programs

Conventional epidemiological models usually assume homogeneous mixing, with susceptible and infected individuals mingling like the molecules in an ideal gas. It has recently been noted, however, that variability in transmission rates—arising, for example, from some hosts being in dense aggregates while others are in small or remote groups—can result in the intrinsic reproductive rate, R₀, of a microparasitic infection being greater than would be estimated under the usual assumption of homogeneous mixing; this implies the infection may be harder to eradicate under a homogeneously applied immunization programme (that is, a larger proportion of the population must be vaccinated) than simple estimates might suggest. In this paper we consider a spatially heterogeneous population arbitrarily subdivided into n groups, with one transmission rate among individuals within any one group, and another, lower transmission rate between groups. We define an optimum eradication program as that which—treating different groups differently—achieves its aim by immunizing the smallest overall number in each cohort of newborns, and we show this optimum program requires fewer immunizations than would be estimated under the (false) assumption that the population is homogeneously mixed. We prove this result in general form, and illustrate it for some special examples (in particular, for a population subdivided into one large ”city“ and several small ”villages“).     

额济纳绿洲土壤养分的空间异质性

利用地统计学方法研究了额济纳绿洲土壤有机质、全氮、全磷、全钾的空间异质性.结果表明:土壤养分含量分布均有较大的空间异质性,土壤有机质的理论模型为指数模型,全氮、全磷和全钾的理论模型均为球状模型;额济纳绿洲土壤有机质、全氮、全钾主要受结构性因子的影响,全磷受结构性因子和随机因子的共同影响;全氮、全磷和全钾的变程在7.68～7.86 km,土壤有机质的变程较小,为5.4 km.土壤养分的空间分布表现为南北高,中间低的分布格局.     

Spatial heterogeneity enhance robustness of large multi-species ecosystems

Understanding ecosystem stability and functioning is a long-standing goal in theoretical ecology, with one of the main tools being dynamical modelling of species abundances. With the help of spatially unresolved (well-mixed) population models and equilibrium dynamics, limits to stability and regions of various ecosystem robustness have been extensively mapped in terms of diversity (number of species), types of interactions, interaction strengths, varying interaction networks (for example plant-pollinator, food-web) and varying structures of these networks. Although many insights have been gained, the impact of spatial extension is not included in this body of knowledge. Recent studies of spatially explicit modelling on the other hand have shown that stability limits can be crossed and diversity increased for systems with spatial heterogeneity in species interactions and/or chaotic dynamics. Here we show that such crossing and diversity increase can appear under less strict conditions. We find that the mere possibility of varying species abundances at different spatial locations make possible the preservation or increase in diversity across previous boundaries thought to mark catastrophic transitions. In addition, we introduce and make explicit a multitude of different dynamics a spatially extended complex system can use to stabilise. This expanded stabilising repertoire of dynamics is largest at intermediate levels of dispersal. Thus we find that spatially extended systems with intermediate dispersal are more robust, in general have higher diversity and can stabilise beyond previous stability boundaries, in contrast to well-mixed systems.     

Evaluation of models of leaf water 18O enrichment using measurements of spatial patterns of vein xylem water, leaf water and dry matter in maize leaves

The effectiveness of several leaf water models (‘string‐of‐lakes’, ‘desert river’ and the Farquhar–Gan model) are evaluated in predicting the enrichment of leaf water along a maize leaf at different humidities. Progressive enrichment of both vein xylem water and leaf water was observed along the blade. At the tip, the maximum observed enrichment for the vein water was 17.6‰ at 50% relative humidity (RH) whereas that for the leaf water was 50‰ at 34% RH and 19‰ at 75% RH. The observed leaf water maximum was a fraction (0.5–0.6) of the theoretically possible maximum. The ‘string‐of‐lakes’ and ‘desert river’ models predict well the variation of leaf water enrichment pattern with humidity but overestimate the average enrichment of bulk leaf water. However, the Farquhar–Gan model gives good prediction for these two aspects of leaf water enrichment. Using the anatomical dimensions of vein xylem overestimates the effective longitudinal Péclet number (P_l). Possible explanations for this discrepancy between the effective and the xylem‐based estimate of P_l are discussed. The need to characterize the heterogeneity of transpiration rate over the leaf surface in studies of leaf water enrichment is emphasized. The possibility that past atmospheric humidity can be predicted from the slope of the Δ¹⁸O spatial variation of leaf macrofossils found in middens is proposed.