Vegetation Structure Patterns Research Articles

Using fractal dimension to correct clumping effect in leaf area index measurement by digital cover photography

The leaf area index (LAI), defined as one-half of the total green leaf area per unit horizontal ground surface area, is a key parameter in agriculture, forestry, ecology, and other fields. The clumping index (CI) accounts for the nonrandom spatial distribution of the foliage elements in the canopy, thereby considerably influencing the accuracy of LAI estimation with optical field-based instruments. Most of the traditional clumping effect correction methods for LAI measurements are based on the theory developed for one-dimensional (1D) data of the vegetation canopy. The development of new methods remains necessary for LAI measurement with two-dimensional (2D) data, including images from digital cover photography (DCP). We found that the fractal dimension (FD) of a 2D ground-based DCP image is an effective tool for correcting the clumping effect and estimating the LAI. The universal formula was derived to describe the relationship between the LAI and FD for randomly distributed leaves using the box-counting method (BCM) and the Boolean model. For the clumped leaves, the universal formula is related to the FD, LAI, and CI. The LAI and CI can be calculated with the FD and gap probability derived from DCP images. Eighteen simulated scenes of different vegetation structure patterns, three realistic canopy scenes of the fourth phase of the radiative transfer model intercomparison (RAMI), and field-measured data acquired from 5 plots in 4 temporal phases were provided to validate the method. The results showed good agreement with the reference (R2 = 0.93 and RMSE = 0.46 for simulated data; uncertainty from 0.31 to 1.05 for realistic canopy; R2 = 0.85 and RMSE = 0.34 for field-measured data). This validation with downward DCP images shows that the proposed FD method assesses the clumping effect more thoroughly compared to the clumping effect correction methods originally designed for 1D data. The FD method is expected to improve the measurement accuracy of LAI with DCP images, especially for heterogeneous canopies.

Spatial variation and drivers of vegetation structure and composition in coastal freshwater wetlands of subtropical Australia

Coastal freshwater wetlands (CFWs) are among the most understudied wetlands globally and are highly vulnerable to projected climate changes. To address CFW knowledge gaps in south-east Queensland, Australia, we surveyed the floristic composition and structure of wooded CFWs and explored variation in vegetation patterns in relation to selected environmental drivers. Understorey and shrub assemblages were surveyed using a cover-class scale and stem counts for tree species abundance. Vegetation structure attributes (stem density, basal area) were calculated from survey data. Redundancy analysis was used to investigate drivers of vegetation structure and the species composition of each stratum. Vegetation structure patterns were associated with gradients of rainfall, soil moisture, salinity and pH. Understorey species composition was associated with wallum wetland species, native perennial grass and herb species, and vegetation patterns of the canopy. Common CFW species, namely Melaleuca quinquenervia and Eucalyptus tereticornis, dominated tree assemblage variation. Overall, CFW vegetation exhibited strong associations with gradients of salinity, rainfall, groundwater dependence and disturbance. Alterations to key drivers of vegetation pattern with future climate changes are likely to markedly influence the composition, structure and function of CFW vegetation communities. Action is therefore required to maintain CFW vegetation communities and ecological function in these diverse and unique wetland systems.

Landscape-scale effects of geomorphological heterogeneity on variability of oak forest structure and composition in a monogenetic volcanic field

Background: Eruptive events in monogenetic volcanic fields create mosaics of lava fields that result in different successional times and stages, and environmental conditions. Such geomorphological heterogeneity may be related to patterns of vegetation structure, diversity, and composition across a landscape.Aims: To examine landscape-scale effects of geomorphological heterogeneity on the spatial variability of oak forest structure and composition in a monogenetic volcanic field.Methods: We sampled oak forests in six geomorphological units within a monogenetic volcanic field in El Tepozteco National Park and carried out redundancy analyses to relate environmental variables to forest composition and structure.Results: Geomorphological heterogeneity explained 28.8 and 25.7% of the variation in canopy structure and composition, respectively, and 21.6% of the variation in understorey composition. Exposed rock was the most important predictor of understorey composition and canopy structure, while elevation was a better predictor of canopy composition. The role of chronic human disturbance as a driver of forest composition and structure was minor compared to that of geomorphological heterogeneity.Conclusions: The geomorphological heterogeneity created by volcanic activity in this landscape drives oak forest variability; however, considerable unexplained variation in vegetation traits remains, which could be associated with unmeasured dimensions of environmental heterogeneity, neutral processes, and the effects of long-term human activity in the area.

Implications of accounting for land use in simulations of ecosystem carbon cycling in Africa

Abstract. Dynamic global vegetation models (DGVMs) are important tools for modelling impacts of global change on ecosystem services. However, most models do not take full account of human land management and land use and land cover changes (LULCCs). We integrated croplands and pasture and their management and natural vegetation recovery and succession following cropland abandonment into the LPJ-GUESS DGVM. The revised model was applied to Africa as a case study to investigate the implications of accounting for land use on net ecosystem carbon balance (NECB) and the skill of the model in describing agricultural production and reproducing trends and patterns in vegetation structure and function. The seasonality of modelled monthly fraction of absorbed photosynthetically active radiation (FPAR) was shown to agree well with satellite-inferred normalised difference vegetation index (NDVI). In regions with a large proportion of cropland, the managed land addition improved the FPAR vs. NDVI fit significantly. Modelled 1991–1995 average yields for the seven most important African crops, representing potential optimal yields limited only by climate forcings, were generally higher than reported FAO yields by a factor of 2–6, similar to previous yield gap estimates. Modelled inter-annual yield variations during 1971–2005 generally agreed well with FAO statistics, especially in regions with pronounced climate seasonality. Modelled land–atmosphere carbon fluxes for Africa associated with land use change (0.07 PgC yr−1 release to the atmosphere for the 1980s) agreed well with previous estimates. Cropland management options (residue removal, grass as cover crop) were shown to be important to the land–atmosphere carbon flux for the 20th century.

Detecting wind disturbance severity and canopy heterogeneity in boreal forest by coupling high-spatial resolution satellite imagery and field data

Wind disturbance events can impact spatially heterogeneous patterns in vegetation structure and disturbance severity in forested landscapes. Characterizing these patterns in forested ecosystems with remote sensing data has been a persistent challenge as variation in severity may be heterogeneous at fine spatial scales. Yet the degree and pattern of disturbance severity are an important influence on successional dynamics. This study explored how spectral and textural characteristics of high-spatial resolution IKONOS imagery reflected patterns of disturbance severity across a windstorm damaged, 121-km2 area of the Boundary Waters Canoe Area Wilderness (BWCAW) in northeastern Minnesota, USA. In this study, spectral and spatial features of high-spatial resolution (1-m panchromatic and 4-m multispectral) IKONOS satellite imagery from a single post-disturbance date are coupled with field observations of disturbance within 0.045-ha field plots to access the potential for empirically modeling disturbance severity across this heterogeneous landscape within the BWCAW. Combining textural and spectral features led to a multiple regression model that explained 68% of the variance, and predicted disturbance severity equally well for ground data not included in the model development. The results suggest the utility of combining spatial and spectral data for detecting differences in forest structure caused by ecological processes such as disturbance.

Altitudinal patterns of vegetation, flora, life forms, and environments in the alpine zone of the Fiord Ecological Region, New Zealand

The altitudinal zonation patterns of vegetation structure, vascular flora, and life/growth forms were comprehensively assessed in relation to temperature and soil factors from treeline (1040 m) to the high‐alpine summit of Mt Burns (1645 m) in southeastern Fiord Ecological Region. We tested Körner's hypothesis which stipulates that the physiognomic zonation pattern: treeline, shrub line, tussockline, and beyond, is driven mainly by increased decoupling between the ambient temperature and that experienced directly by plants in relation to proximity of their canopy to the ground. This hypothesis is generally supported, particularly with replacement of the tussock life form by dwarfed, mostly cushion species, at the low‐ to high‐alpine zone transition. The soil pattern appears to be more of a response to, rather than a driver of, the alpine vegetation pattern, including a localised area of frost‐active solifluc‐tion terraces. The Nothofagus menziesii treeline conformed to the “warmest month” model and also with a worldwide growing season mean (7.15°C) of 5.5–7.5°C. We stress the closer analogy in the overall alpine zonation pattern in this region of oceanic New Zealand to that of the tropical high mountains and other oceanic regions, than with the temperate Northern Hemisphere continental mountains.

Vegetation Structure of Ajlun Woodlands in Northern Jordan

Woodland vegetation in the Ajlun mountains of Northern Jordan was investigated and surveyed to determine the influences of altitude and topography on patterns of vegetation structure. Three tree species (Quercus calliprinos Webb, Q. ithaburensis Decne, and Q. aegilps) and seven shrubs (Styrax officinalis L., Cistus creticus L., Cistus salviifolius L., Atriplex halimus L., Calycotome villosa (Poir.) Link, Retema raetam L., and Sarcopoterium spinosum (L.) Spach) dominate the woody vegetation in the study area. Higher plant densities were found on the lower altitudes of southwest-facing slopes than on the higher altitudes of northeast-facing slopes. Species segregation occurred on the basis of three main investigated factors: (1) Community ecological importance value complex of the species which combines the relative density, the relative coverage, and the relative frequency of the species; (2) altitudes above sea level; and (3) northeast and southwest slopes. Regeneration was negatively affected by consequently continuous reduction in species diversity and tree-size distribution.

Spatial variation in vegetation structure coupled to plant available water determined by two-dimensional soil resistivity profiling in a Brazilian savanna

Tropical savannas commonly exhibit large spatial heterogeneity in vegetation structure. Fine-scale patterns of soil moisture, particularly in the deeper soil layers, have not been well investigated as factors possibly influencing vegetation patterns in savannas. Here we investigate the role of soil water availability and heterogeneity related to vegetation structure in an area of the Brazilian savanna (Cerrado). Our objective was to determine whether horizontal spatial variations of soil water are coupled with patterns of vegetation structure across tens of meters. We applied a novel methodological approach to convert soil electrical resistivity measurements along three 275-m transects to volumetric water content and then to estimates of plant available water (PAW). Structural attributes of the woody vegetation, including plant position, height, basal circumference, crown dimensions, and leaf area index, were surveyed within twenty-two 100-m(2) plots along the same transects, where no obvious vegetation gradients had been apparent. Spatial heterogeneity was evaluated through measurements of spatial autocorrelation in both PAW and vegetation structure. Comparisons with null models suggest that plants were randomly distributed over the transect with the greatest mean PAW and lowest PAW heterogeneity, and clustered in the driest and most heterogeneous transect. Plant density was positively related with PAW in the top 4 m of soil. The density-dependent vegetation attributes that are related to plot biomass, such as sum of tree heights per plot, exhibited spatial variation patterns that were remarkably similar to spatial variation of PAW in the top 4 m of soil. For PAW below 4 m depth, mean vegetation attributes, such as mean height, were negatively correlated with PAW, suggesting greater water uptake from the deep soil by plants of larger stature. These results are consistent with PAW heterogeneity being an important structuring factor in the plant distribution at the scale of tens of meters in this ecosystem.

Drainage Size, Stream Intermittency, and Ecosystem Function in a Sonoran Desert Landscape

Understanding the interactions between terrestrial and aquatic ecosystems remains an important research focus in ecology. In arid landscapes, catchments are drained by a channel continuum that represents a potentially important driver of ecological pattern and process in the surrounding terrestrial environment. To better understand the role of drainage networks in arid landscapes, we determined how stream size influences the structure and productivity of riparian vegetation, and the accumulation of organic matter (OM) in soils beneath plants in an upper Sonoran Desert basin. Canopy volume of velvet mesquite (Prosopis velutina), as well as overall plant cover, increased along lateral upland–riparian gradients, and among riparian zones adjacent to increasingly larger streams. Foliar δ13C signatures for P. velutina suggested that landscape patterns in vegetation structure reflect increases in water availability along this arid stream continuum. Leaf litter and annual grass biomass production both increased with canopy volume, and total aboveground litter production ranged from 137 g m−2 y−1 in upland habitat to 446 g m−2 y−1 in the riparian zone of the perennial stream. OM accumulation in soils beneath P. velutina increased with canopy volume across a broad range of drainage sizes; however, in the riparian zone of larger streams, flooding further modified patterns of OM storage. Drainage networks represent important determinants of vegetation structure and function in upper Sonoran Desert basins, and the extent to which streams act as sources of plant-available water and/or agents of fluvial disturbance has implications for material storage in arid soils.

Relationship between topographic heterogeneity and vegetation patterns in a Californian salt marsh

Abstract:Questions: Are species richness and species abundances higher in the presence of tidal creeks? Do species richness and species abundances vary with plot size?Location: Intertidal plain of Volcano Marsh, Bahia de San Quintin, Mexico.Methods: We analysed vegetation patterns in large areas (cells) with tidal creeks (+creek) and without (‐creek). We surveyed vegetation cover, microtopography, habitat type, and distance to creeks in nested plots of five sizes, 0.1, 0.25, 1, 2.5, and 10 m2.Results: Species richness, frequency, cover, and assemblages differed between ±creek cells. Richness tended to be higher in +creek cells, and cover and frequency of individual species differed significantly between ±creek cells. We found consistent patterns in vegetation structure across plot sizes. We encountered 13 species that occurred in 188 unique assemblages. The most common assemblage had six species: Batis maritima, Frankenia salina, Salicornia bigelovii, S. virginica, Salicornia spec. and Triglochin concinna. This assemblage occurred in ±creek cells and at all spatial scales. Of the most common assemblages all but one were composed of multiple species (3–9 species/plot).Conclusions: The persistence of vegetation patterns across a 100‐fold range in spatial scale suggests that similar environmental factors operate broadly to determine species establishment and persistence. Differences in assemblage composition result from variation of frequency and cover of marsh plain species, particularly Suaeda esteroa and Monanthochloe littoralis. The recommendation for restoration of Californian salt marshes is to target (and plant) multi‐species assemblages, not monocultures.

Forest Understory Vegetation along a Productivity Gradient

these communities. We examined vegetation structure and species distribution patterns in the forest understory community of two topographically similar watersheds to evaluate the predicted roles of abiotic limitation and competition along a productivity gradient. Canonical correspondence analysis identified a dominant compositional gradient that was strongly correlated with moisture, light, and herbaceous-layer cover. Given these relationships, the first axis of the ordination was taken as a reliable index of productivity. Contrary to expectation, species richness increased monotonically along the gradient rather than reaching a maximum and then declining which suggests that species are not excluded by light competition at the high-productivity end of the gradient. Other vegetation patterns, however, indicated both abiotic limitation in regions of low productivity and competition at high productivity. Patterns in species' distributions also suggested resource limitation at low productivity, but they showed mixed evidence of competition at high productivity. Taken together, these patterns in vegetation structure and species' distributions imply that while competition may occur among herbaceous-layer species, particularly at high productivity, competitive exclusion remains low. This may be because the traits that permit survival under tree canopy shading reduce the effects of light competition with other understory species, or because understory productivity is not high enough to induce intense light competition among herbaceouslayer species.

Relationship between topographic heterogeneity and vegetation patterns in a Californian salt marsh

Abstract Questions: Are species richness and species abundances higher in the presence of tidal creeks? Do species richness and species abundances vary with plot size? Location: Intertidal plain of Volcano Marsh, Bahia de San Quintin, Mexico. Methods: We analysed vegetation patterns in large areas (cells) with tidal creeks (+creek) and without (−creek). We surveyed vegetation cover, microtopography, habitat type, and distance to creeks in nested plots of five sizes, 0.1, 0.25, 1, 2.5, and 10 m2. Results: Species richness, frequency, cover, and assemblages differed between ±creek cells. Richness tended to be higher in +creek cells, and cover and frequency of individual species differed significantly between ±creek cells. We found consistent patterns in vegetation structure across plot sizes. We encountered 13 species that occurred in 188 unique assemblages. The most common assemblage had six species: Batis maritima, Frankenia salina, Salicornia bigelovii, S. virginica, Salicornia spec. and Triglochin conci...

The influence of soil texture on the soil water dynamics and vegetation structure of a shortgrass steppe ecosystem

We analyzed soil water data from three sites with different soil textures in the shortgrass steppe of northeastern Colorado, USA. Our objective was to evaluate the relationship between the occurrence of plant functional types and the effect of soil texture on soil water availability. Soil water availability was greatest in the upper soil layers at all three sites, but the loamy sand site had significantly greater soil water availability than the sandy clay loam and sandy clay sites in wetter years at depths below 60 cm. Calculations of proportional water availability by layer using both field data and fifty-year soil water model simulations, showed that the sandy clay loam and sandy clay soils on average had greater water availability in layers 30 cm and above, but that the loamy sand had the greatest water availability in layers beneath this, particularly at 105 cm. This observation can be linked to the occurrence of a fine textured subsoil at this site. The textural pattern in the loamy sand profile effectively creates two water resources: a shallow pool accessible to all plants; and a deep pool accessible only to deep-rooted plants. This is offered as an explanation for the co-dominance of the two main plant functional types at the loamy sand site. At the other two sites, shallow-rooted shortgrass vegetation dominated, being more consistent with the general pattern for the area. Thus the patterns of vegetation structure at the three sites were consistent with the hypothesis. Aboveground net primary production data for the three sites, along with transpiration estimates from the model simulations, indicated that the additional water availability in the coarse textured soil was associated with higher overall plant productivity. Nomenclature: Taxonomic nomenclature follows R. L. McGregor & T. M. Barkley (1986) Flora of the Great Plains. Great Plains Flora Association. University Press of Kansas, Lawrence.