Tracing carbon sources in small urbanising streams: catchment‐scale stormwater drainage overwhelms the effects of reach‐scale riparian vegetation

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Landscape scale effects of disturbance on riparian vegetation

1. Differing responses in riparian species richness and composition to disturbance have been reported as a possible explanation for the differences along and between rivers. This paper explores the role of physical disturbance in shaping landscape‐scale patterns of species distribution in riparian vegetation along a free‐flowing river in northern Sweden. 2. To test whether sensitivity to disturbance varies across large landscapes, we experimentally disturbed riparian vegetation along an entire, free‐flowing river by scouring the soil and the vegetation turf, cutting vegetation, applying waterborne plant litter, and after a period of recovery we measured vegetation responses. The experiment was repeated for two consecutive years. 3. We found no significant effect of disturbance on species composition, but all three forms of disturbance significantly reduced species richness. There was no downstream variation in community responses to disturbance but morphological groups of species responded differently to different kinds of disturbance. Graminoids were most resistant, suppressed only by litter burial. All forms of disturbance except cutting reduced the density of herbaceous species, and species density of trees + shrubs and dwarf shrubs was negatively affected by both scouring and cutting. We also evaluated the effects of disturbance in relation to varying levels of species richness. In nearly all cases, responses were significantly negatively correlated with control plot species richness, and relative responses indicated that species‐rich plots were less resistant to scouring and cutting. 4. Our results suggest that although all disturbance treatments had an effect on species richness, variation in sensitivity to disturbance is not the most important factor shaping landscape‐scale patterns of riparian plant species richness along rivers.     

Litter input from riparian vegetation to streams: a case study of the Njoro River,Kenya

Allochthonous coarse particulate organic matter (CPOM) input into the Njoro River was measured between January and June 1998 at two contrasting sites: open-canopy and closed-canopy sites. Bank runoff and aerial drift traps were used for collecting CPOM inputs over periods of two weeks. Collected litter was sorted into four categories: leaves, fruits, wood and plant fragments. Monthly input ranged from 77 to 228 g ash free dry weight m^–1 for bank runoff input and from 64 to 129 g ash free dry weight m^–2 for aerial input. The highest input of 228 g ash free dry weight m^–1 was recorded in May at the closed-canopy site. Wood, fruits and plant fragments of particle size >100 m contributed a mean ± SE of 60±9% of the total inputs with the rest from leaf litter. The closed-canopy site had higher inputs (P<0.05) of bank and aerial input than the open canopy site. There was no relationship between total bank runoff input and rainfall (r _s = 0.08), however, total aerial input increased with decrease in rainfall (r _s = – 0.59). There were differences between inputs from different plant species (P<0.05) that ranked in the following order: Syzygium cordatum > Rhus natalensis > Pittosporum viridiflorum > Vangueria madagascariensis. Removal of riparian vegetation from the banks of the Njoro River would alter the quantity and quality of the litter and reduce CPOM inputs to the river and to a downstream lake with attendant consequences to the energy budget of biocoenoses in the two ecosystems.     

空间显式模型模拟河流岸边带植被在水库运行作用下的演替

岸边带是水-陆之间的过渡和缓冲地带,是河流生态系统的重要组成部分.岸边带对拦截径流中的固体颗粒、吸收营养盐、减少入河污染负荷有重要作用.受河流水位季节性波动的影响,岸边带生态系统的变化非常剧烈,而当水库等水工建筑的运行剧烈改变河流的水文情势时,水库下游的岸边带生态系统将受到长期的累积性影响.因此,研究复式河道岸边带植被动态对于受损河流生态修复以及河流开发运行的生态环境影响规避具有重要意义.通过原位样方观测和室内水槽模拟试验,开发了岸边带植被演替模型,该模型耦合了全局基于连续性模式的水动力模块和局部基于元胞自动机模式的植被演替模块,并以漓江中游的一段复式河道为例,通过模拟水库运行前后长序列的水文情势变化和3种岸边带植物(刺果酸模、水蓼和益母草)的生长演替,分析了为满足旅游航道需求上游水库补水运行对下游岸边带植被的影响.     

Salmon‐derived nutrient and organic matter fluxes from a coastal catchment in southeast Alaska

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Effects of river ice on riparian vegetation

1. Many rivers and streams experience pronounced ice dynamics caused by the formation of anchor and frazil ice, leading to flooding and disturbance of riparian and aquatic communities. However, the effects of dynamic ice conditions on riverine biota are little known. 2. We studied the formation of anchor ice in natural streams over 2 years and assessed the effects of anchor ice on riparian vegetation by comparing sites with frequent or abundant and little or no anchor ice formation. We also studied the direct impact of ice on riparian plants by experimentally creating ice in the riparian zone over three winters and by exposing plants of different life forms to ?18 °C cold ice in the laboratory. 3. Riparian species richness per 1‐m² plot was higher at sites affected by anchor ice than at sites where anchor ice was absent or rare, whereas dominance was lower, suggesting that disturbance by ice enhances species richness. Species composition was more homogenous among plots at anchor ice sites. By experimentally creating riparian ice, we corroborated the comparative results, with species richness increasing in ice‐treated plots compared to controls, irrespective of whether the sites showed natural anchor ice. 4. Because of human alterations of running waters, the natural effects of river ice on stream hydrology, geomorphology and ecology are little known. Global warming in northern streams is expected to lead to more dynamic ice conditions, offering new challenges for aquatic organisms and river management. Our results should stimulate new research, contributing to a better understanding of ecosystem function during winter.     

子午岭植被演替过程中土壤剖面有机质与持水性能变化

研究表明,自然植被正向演替对土壤剖面有机质积累有显著促进作用,表现为：由弃耕地、草地、灌木、乔灌群聚到乔木的植被演替过程中,0-25cm土层有机质含量逐步增加,且演替初期的增加速度较快,而后增加速度相对变缓。植被演替过程中土壤有机质含量变化的主要原因在于植物凋落物归还量的变化。在同一时间测定的土壤剖面水分含量以及0-5cm土层田间持水量、容重、总孔度等与土壤持水性能相关的指标都与有机质含量呈极显著或显著相关,表明随剖面有机质的积累,土壤持水性能得到改善。     

Flow extremes and benthic organic matter shape the metabolism of a headwater Mediterranean stream

1. Single‐station diel oxygen curves were used to monitor the oxygen metabolism of an intermittent, forested third‐order stream (Fuirosos) in the Mediterranean area, over a period of 22 months. Ecosystem respiration (ER) and gross primary production (GPP) were estimated and related to organic matter inputs and photosynthetically active radiation (PAR) in order to understand the effect of the riparian forest on stream metabolism. 2. Annual ER was 1690 g O₂ m^?2 year^?1 and annual GPP was 275 g O₂ m^?2 year^?1. Fuirosos was therefore a heterotrophic stream, with P : R ratios averaging 0.16. 3. GPP rates were relatively low, ranging from 0.05 to 1.9 g O₂ m^?2 day^?1. The maximum values of GPP occurred during a few weeks in spring, and ended when the riparian canopy was fully closed. The phenology of the riparian vegetation was an important determinant of light availability, and consequently, of GPP. 4. On a daily scale, light and temperature were the most important factors governing the shape of photosynthesis–irradiance (P–I) curves. Several patterns could be generalised in the P–I relationships. Hysteresis‐type curves were characteristic of late autumn and winter. Light saturation responses (that occurred at irradiances higher than 90 μE m^?2 s^?1) were characteristic of early spring. Linear responses occurred during late spring, summer and early autumn when there was no evidence of light saturation. 5. Rates of ER were high when compared with analogous streams, ranging from 0.4 to 32 g O₂ m^?2 day^?1. ER was highest in autumn 2001, when organic matter accumulations on the streambed were extremely high. By contrast, the higher discharge in autumn 2002 prevented these accumulations and caused lower ER. The Mediterranean climate, and in its effect the hydrological regime, were mainly responsible for the temporal variation in benthic organic matter, and consequently of ER.     

河岸带生态系统退化机制及其恢复研究进展

恢复和重建自然和人为干扰导致的退化河岸带生态系统是目前恢复生态学、流域生态学等学科研究的重要内容之一．对河岸带生态系统的干扰表现在河流水文特征改变、河岸带直接干扰和流域尺度干扰3个方面,分别具有不同的影响机制．河流水文特征改变通过改变河岸土壤湿度、氧化还原电位、生物生存环境以及沉积物传输规律对河岸带生态系统产生影响;河岸带直接干扰通过人类活动及外来物种入侵而直接影响河岸带植被多样性;流域尺度干扰则主要表现在河道刷深、河道淤积、河岸带地下水位降低和河流冲刷过程改变等．河岸带生态恢复评价对象包括河岸带生态系统各要素,评价指标已从单一的生态指标转向综合性指标．河岸带生态恢复应在景观或者流域尺度上进行考虑,识别对其影响的生物和物理过程以及导致其退化的干扰因子,通过植被重建与水文调控来进行．扩展研究尺度和研究对象及采用多学科的研究方法将是今后相关研究中的重要问题．     

Searching for effective indicators of ecosystem function in urban streams: assessing cellulose decomposition potential

1. Urbanisation and landuse changes threaten the ecological integrity of rivers and streams globally. A major challenge in such impacted environments is to develop functional indicators for use by waterway managers. We first reviewed cellulose decomposition potential (CDP) as one such indicator, and here present current understanding and highlight the knowledge gaps which hinder its widespread incorporation into programmes monitoring stream health. In a field study, we also evaluated two differing cellulose materials (Shirley soil burial test material and unbleached calico) and measurement techniques. 2. We also investigated the effects of urbanisation and riparian cover on CDP as a standardised indicator of ecosystem function. Cotton fabrics were deployed in paired open and closed reaches across six stream catchments varying in degree of urbanisation (in south‐eastern Australia). After 7, 14 and 21 days materials were retrieved and their decomposition assessed in conjunction with a number of physicochemical variables. 3. We observed a strong positive relationship between the decomposition of Shirley and calico materials, thus indicating unbleached calico is an effective substitute for Shirley test cloth, which is no longer available. 4. Across sites, CDP was positively correlated with ammonium concentrations and also, to a lesser extent, with filterable reactive phosphorus and sediment silt and clay content/carbon availability. Despite these relationships, cellulose decomposition was not correlated with urbanisation or riparian cover. 5. Cellulose decomposition is a simple, rapid and sensitive functional indicator of water quality and associated anthropogenic landuse changes and is thus a valuable tool for monitoring stream health.     

Storage and dynamics of organic matter in different springs of small floodplain streams

The flow of groundwater through the sediment layer (underflow or hyporheic zone) of streams and at the origin of streams can influence organic matter uptake dynamics of floodplain. The River Rhône floodplain has limestone foothills. Here we studied 2 karstic and 2 interstitial springs differing by aquifer geology. Organic matter, physico-chemical conditions were compared between these springs during two seasons (from March to September 1989) and at different depths (0, –20 cm, –40 cm).Temperatures indicated large differences in underflow between springs, in their relation to the surrounding environment, and between seasons. Springs are well oxygenated, with differences between layers. Cultivated fields supply interstitial springs with nitrates, and pools are nutrient traps. DOC was heterogeneous in space and time and correlates with VFPOC. Particulate nutrients were correlated with available surface area of sediment grains. Physical conditions of each spring were prominent in determining storage and turnover of organic matter. Each spring, by its own characteristics and dynamics regulating stability and turnover, had an effect or control on storage, transport and retention of organic matter (quality, quantity). These springs offer an example of the heterogeneity, and give a view of the diversity of patches within a floodplain. The data suggest that groundwater flow of springs may be a major factor in the functioning of floodplain tributaries.     

Retention of particulate organic matter in a tropical headstream

Retention of particulate organic matter was investigated over a range of water discharges in pool and riffle zones of a tropical stream (Itaúna Stream, São Paulo, Brazil). A closed-system leaf release and capture method was used to quantify leaf retention in a 50-m reach of the stream. Instantaneous retention rates (k) were calculated by a negative exponential model and specific retention rates (k _e) computed for each 1m interval. The mean k was 0.0161±0.0101 (S.E.) and ranged from 0.005 to 0.036 during the study period. This suggests a low retention of particulate matter. The evidence provided by this study shows that there was a relationship between retention and discharge. At low discharges, greater percentages of leaves were retained in a 3-h period while high discharges resulted in low retention percentages within the studied reach. In the Itaúna Stream, no significant difference (p<0.05) was found between the k _e values of pool and riffle zones.     

Impact of spruce forest and grass vegetation cover on soil micromorphology and hydraulic properties of organic matter horizon

Two organic matter horizons developed under a spruce forest and grass vegetation were chosen to demonstrate the impact of a different vegetation cover on the micromorphology, porous system and hydraulic properties of surface soils. Micromorphological studies showed that the decomposed organic material in the organic matter horizon under the grass vegetation was more compact compared to the decomposed organic material in the organic matter horizon under the spruce forest. The detected soil porous system in the organic matter horizon under the spruce forest consisted of two clusters of pores with different diameters that were highly connected within and between both clusters. The soil porous system in the organic matter horizon under the grass vegetation consisted of one cluster of pores with the larger diameters and isolated pores with the smaller diameter. The retention ability of the organic matter horizon under the grass vegetation was higher than the retention ability of the organic matter horizon under the spruce forest. Presented at the International Conference on Bioclimatology and Natural Hazards, Poľana nad Detvou, Slovakia, 17–20 September 2007.     

滨河湿地不同植被对农业非点源氮污染的控制效果

滨河湿地作为连接河流水体和陆地的一个功能过渡界面区,是河流生态系统与陆地生态系统进行物质、能量、信息交换的一个重要过渡带,也是保护河流水体的最后一道屏障,对水质净化和农业非点源污染控制起着非常重要的作用。以黄河湿地国家自然保护区孟津扣马段为研究对象,采取野外定位观测试验和稳定同位素示踪(人工富集15N源的同位素稀释法)相结合的方法,研究了滨河湿地土壤对农业非点源氮的持留作用、渗漏到地下水中的农业非点源氮和湿地不同植被对滞留在土壤中的农业非点源氮的吸收作用。结果显示:通过地表径流进入滨河湿地的农业非点源氮在3个实验样方的垂向和侧向都发生了渗漏。滨河湿地土壤对农业非点源氮的滞留作用主要发生在0—10cm,相当于一个过滤器的功能。3种受试植被土壤表层的滞留量为芦苇(0.045mg/g)藨草(0.036mg/g)水烛(0.032mg/g),分别占到土壤滞留氮的59.2%、56.3%和56.1%。滞留在土壤中的农业非点源氮污染存在一个相对较长的效应。滨河湿地特殊的氧化还原条件导致强烈的土壤微生物反硝化作用以及滨河湿地植被对氮素的吸收作用,使得0-10cm土层土壤外源氮变化速度最快,1个月后,滞留芦苇、水烛、藨草样方中15N下降了77.8%、68.8%和8.3%;3个月后,芦苇、藨草、水烛样方中的15N下降了93.3%、72.2%和37.5%。滨河湿地复杂的水文过程,使得滞留在土壤表层的农业非点源氮迁移转化更为复杂多变。监测数据显示,在实验设计的浓度和强度范围内农业非点源氮没有对地下水造成影响。不同植被和同一植物的不同生长时期对滞留在土壤中氮的吸收能力差别较大,吸收量依次为芦苇嫩芽(9.731mg/g)老芦苇(4.939mg/g)藨草(0.620mg/g)水烛(0.186mg/g)。通过对生物量计算得出滨河湿地芦苇、水烛和藨草对农业非点源氮的吸收能力分别为氮吸收量96.11、3.76、0.32kg/hm2。可见,滨河湿地作为连接河流与陆地的缓冲带,通过截留、过滤、植物吸收等过程能有效的削减农业非点源氮对临近地表水体污染,对农业非点源氮污染控制起着非常重要的作用。合理的滨河湿地保护与利用模式对河流水环境保护具有重要的意义。     

Seasonal dynamics of macroinvertebrate assemblages in the benthos and associated with detritus packs in two low-order streams with different riparian vegetation

• 1 The seasonal dynamics of the benthic macroinvertebrate assemblage, and the subset of this assemblage colonising naturally formed detritus accumulations, was investigated in two streams in south‐west Ireland, one draining a conifer plantation (Streamhill West) and the other with deciduous riparian vegetation (Glenfinish). The streams differed in the quantity, quality and diversity of allochthonous detritus and in hydrochemistry, the conifer stream being more acid at high discharge. We expected the macroinvertebrate assemblage colonising detritus to differ in the two streams, due to differences in the diversity and quantity of detrital inputs.
• 2 Benthic density and taxon richness did not differ between the two streams, but the density of shredders was greater in the conifer stream, where there was a greater mass of benthic detritus. There was a significant positive correlation between shredder density and detritus biomass in both streams over the study period.
• 3 Detritus packs in the deciduous stream were colonised by a greater number of macroinvertebrates and taxa than in the conifer stream, but packs in both streams had a similar abundance of shredders. The relative abundance of taxa colonising detritus packs was almost always significantly different to that found in the source pool of the benthos.
• 4 Correspondence analysis illustrated that there were distinct faunal differences between the two streams overall and seasonally within each stream. Differences between the streams were related to species tolerances to acid episodes in the conifer stream. Canonical correspondence analysis demonstrated a distinct seasonal pattern in the detrital composition of the packs and a corresponding seasonal pattern in the structure of the detritus pack macroinvertebrate assemblage.
• 5 Within‐stream seasonal variation both in benthic and detritus pack assemblages and in detrital inputs was of similar magnitude to the between‐stream variation. The conifer stream received less and poorer quality detritus than the deciduous stream, yet it retained more detritus and had more shredders in the benthos. This apparent contradiction may be explained by the influence of hydrochemistry (during spate events) on the shredder assemblage, by differences in riparian vegetation between the two streams, and possibly by the ability of some taxa to exhibit more generalist feeding habits and thus supplement their diets in the absence of high quality detritus.

     

陆地生态系统中水溶性有机物动态及其环境学意义

水溶性有机物(DOM)是陆地生态系统中最活跃的有机碳库,也是土壤圈层与相关圈层进行物质与能量交换的重要表现形式,它对重金属、养分元素和有机污染物的活化、迁移与生态毒性有较大影响,在农业土壤溶液中DOM浓度通常在10～80mgC·L^-1,湿地土壤中多数在25～50mgC·L^-1,与森林土壤剖面淋滤水中的DOM相近,但在某些微域土壤环境(如根际和有机肥施用点附近)中DOM浓度可高达200～1000mgC·L^-1,不同来源的DOM在土壤中的迁移性与降解性明显不同,含低分子量组分或亲水性组分较多的DOM不易被土壤吸持而易被微生物降解,pH值相对较高的土壤(如石灰性土壤)对DOM吸附较弱,但pH较低和含有大量氧化物的土壤(如红壤、赤红壤和砖红壤等)则对DOM的吸附较强,施用石灰、土壤淹水或干湿交替、温度升高等有利于土壤保持较高的DOM浓度,由于DOM-金属配合物的形成,DOM能明显促进土壤重金属活化和向下迁移,而且DOM中低分子量或亲水性组分所占比例越低活化作用越强,同样地,由于DOM具有两亲性质,也能明显提高疏水性有机污染物(如农药和持久难降解有机污染物)的水溶性,增加其对环境污染的风险,特别是含疏水性组分越多的DOM这种作用越强．可以认为,继续加强有关DOM在陆地生态系统中产生与消长规律,特别是DOM及其与污染物的配合物从陆地生态系统向水体迁移的机理及其通量的研究,对合理预测污染物的环境行为和科学地进行环境风险评估有重要意义。     

Effect of Dicranopteris dichotoma on spectroscopic characteristic of dissolved organic matter in red soil erosion area

Aims Dissolved organic matter (DOM) is the most active component of organic matters in soils, and plays an important role in carbon cycles. It is a mixed organic compound with varying molecular sizes and weights. We aimed to explore the impacts of Dicranopteris dichotoma coverage on quantity and structure of DOM after vegetation restoration in severely eroded red soil region. Methods A typical sequence of vegetation restoration (Y0, without ecological restoration; Y13, ecological restoration for 13 years; Y31, ecological restoration for 31 years) was selected as the research object in Hetian Town, Changting County, Fujian Province, China. At each experimental site, soils were subject to three treatments—NRd, not removed D. dichotoma; Rd, removed D. dichotoma; and CK, control, and the effects of D. dichotoma on the spectral characteristics of DOM were evaluated.Important findings The results indicated that the quantity of soil DOC under NRd treatment of the Y0, Y13 and Y31 was 7.61, 4.83, and 5.47 times higher than their CK treatment, respectively. The Rd treatment had significantly lower DOC than that under NRd treatments, and it was 1.84, 4.12, and 4.73 times higher than their CK treatments, respectively. Thus the D. dichotoma had exerted significant effects on the quantity of soil DOM. The Aromaticity index (AI), emission fluorescence spectrum humification index (HIX_em) and synchronous fluorescence spectrum humification index (HIX_syn) of DOM under the NRd treatment were significantly higher than those of the CK treatments in Y13 and Y31, respectively. However, the ratio of ultraviolet-visible light absorption photometric quantity at 250 nm wavelength to ultraviolet-visible light absorption photometric quantity at 365 nm wavelength (E₂:E₃) had an opposite trend. It showed that the DOM structure in soils covered by D. dichotoma contained more aromatic nucleus and had higher aromaticity and humification, and DOM molecular was larger. In addition, the AI and humification index (HIX) of DOM under the Rd treatment was significantly decreased compared with the NRd treatment. Similar results were observed by analysis of emission and synchronous fluorescence spectrum, and by the Fourier infrared transmission spectrum analysis. These results suggest that D. dichotoma had positive impacts on the complexity of DOM structure, but it was a long and slow process. The DOM spectral analysis showed that the soil DOM covered by D. dichotoma had a stable and complex structure and was easily adsorbed by soil colloid. As a result, Dicranopteris dichotoma had a positive effect on the accumulation of soil organic carbon.     

Effects of urban stream burial on organic matter dynamics and reach scale nitrate retention

Nitrogen (N) retention in streams is an important ecosystem service that may be affected by the widespread burial of streams in stormwater pipes in urban watersheds. We predicted that stream burial suppresses the capacity of streams to retain nitrate (NO₃ ^?) by eliminating primary production, reducing respiration rates and organic matter availability, and increasing specific discharge. We tested these predictions by measuring whole-stream NO₃ ^? removal rates using ¹⁵NO₃ ^? isotope tracer releases in paired buried and open reaches in three streams in Cincinnati, Ohio (USA) during four seasons. Nitrate uptake lengths were 29 times greater in buried than open reaches, indicating that buried reaches were less effective at retaining NO₃ ^? than open reaches. Burial suppressed NO₃ ^? retention through a combination of hydrological and biological processes. The channel shape of two of the buried reaches increased specific discharge which enhanced NO₃ ^? transport from the channel, highlighting the relationship between urban infrastructure and ecosystem function. Uptake lengths in the buried reaches were further lengthened by low stream biological NO₃ ^? demand, as indicated by NO₃ ^? uptake velocities 17-fold lower than that of the open reaches. We also observed differences in the periphyton enzyme activity between reaches, indicating that the effects of burial cascade from the microbial to the ecosystem scale. Our results suggest that stream restoration practices involving “daylighting” buried streams have the potential to increase N retention. Further work is needed to elucidate the impacts of stream burial on ecosystem functions at the larger stream network scale.     

喀斯特不同植被恢复阶段土壤有机酸季节变化与有效氮磷的关系

植物分泌有机酸在提高土壤养分有效性方面起到重要作用。为了解喀斯特地区不同植被恢复阶段土壤有机酸含量季节性变化与氮磷有效性的关系,在灌木林和原生林各选择3种优势植物,测定雨季和旱季两个季节根际土和非根际土的有机酸含量、碳氮磷含量和比值、有效性氮磷含量及微生物生物量碳。结果表明:原生林植物根际土的草酸含量高于灌木林,而苹果酸和乙酸含量则低于灌木林;根际土草酸含量均高于非根际土; 2个植被根际土和非根际土的草酸含量在雨季高于旱季,而苹果酸和乙酸含量则低于旱季;土壤草酸含量与有机碳、全氮、全磷和N∶P值呈显著正相关,与C∶N呈显著负相关;土壤有效氮和有效磷与草酸和微生物生物量碳呈显著正相关。上述结果表明,植物分泌有机酸的季节性变化与土壤养分状态和自身养分需求相关,而有机酸耦合微生物对养分有效性的提高具有积极的作用。因此,根际土的有机酸季节变化可能是喀斯特生态系统中植物适应土壤养分限制的一种重要机制。     

Development of a stable isotope index to assess decadal-scale vegetation change and application to woodlands of the Burdekin catchment, Australia

Forty-four study sites were established in remnant woodland in the Burdekin River catchment in tropical north-east Queensland, Australia, to assess recent (decadal) vegetation change. The aim of this study was further to evaluate whether wide-scale vegetation ‘thickening’ (proliferation of woody plants in formerly more open woodlands) had occurred during the last century, coinciding with significant changes in land management. Soil samples from several depth intervals were size separated into different soil organic carbon (SOC) fractions, which differed from one another by chemical composition and turnover times. Tropical (C₄) grasses dominate in the Burdekin catchment, and thus δ¹³C analyses of SOC fractions with different turnover times can be used to assess whether the relative proportion of trees (C₃) and grasses (C₄) had changed over time. However, a method was required to permit standardized assessment of the δ¹³C data for the individual sites within the 13 Mha catchment, which varied in soil and vegetation characteristics. Thus, an index was developed using data from three detailed study sites and global literature to standardize individual isotopic data from different soil depths and SOC fractions to reflect only the changed proportion of trees (C₃) to grasses (C₄) over decadal timescales. When applied to the 44 individual sites distributed throughout the Burdekin catchment, 64% of the sites were shown to have experienced decadal vegetation thickening, while 29% had remained stable and the remaining 7% had thinned. Thus, the development of this index enabled regional scale assessment and comparison of decadal vegetation patterns without having to rely on prior knowledge of vegetation changes or aerial photography.