Changes In Vegetation Patterns Research Articles

Reconstructing a century of Spartina alterniflora invasion with historical records and contemporary remote sensing

:Remote sensing is becoming a vital tool for understanding the changing vegetation patterns that are associated with broad-scale plant invasions. The establishment of North American east coast native Spartina alterniflora (smooth cordgrass) in Willapa Bay, Washington is a specific example of broad-scale invasion following local introduction. In this study, we examined a 120-y historical record of aerial photographs, oral histories, museum records, and publications to reconstruct the spatial, temporal, and historical elements of this invasion. We conclude that the most likely means for S. alterniflora to have reached Willapa Bay was the transport and translocation of oysters from New York harbour. Our data and analysis suggest that multiple areas were colonized between 1894 and 1920 coincident with sustained import of oysters from the Atlantic seaboard. We have evidence that S. alterniflora had been long established and growing in multiple widely spread locations by 1945, which is in contrast to a widely reported single introduction. Multiple foci would not only explain the colonization patterns we have observed, but could also increase the heterogeneity of the founding populations, helping to overcome barriers to reproduction that may have initially slowed the colonization in isolated populations during the first 50–70 y.

Mountain Mediterranean landscape evolution caused by the abandonment of traditional primary activities: a study of the Spanish Central Pyrenees

This paper analyses changes in plant cover in a valley of the Spanish Central Pyrenees during the second half of the 20th century. The role played by the extensification of farming activities in this process has been examined. The method used was based on evolutionary mapping of the plant cover, application of landscape indices and spatial analysis of livestock pressure. Throughout the century the stocking rate in the valley has decreased dramatically and sheep have been replaced by cattle. Most crop areas have been abandoned, and these areas have gone through a revegetation process (involving both natural vegetation and forest plantation). Nowadays the forest is the most abundant element. The study shows that both agricultural set-aside and the stocking rate decrease have led to a more natural landscape. Finally, the positive and negative effects of extensification and the changes in vegetation patterns associated with it are debated.

Did Paleogene North Atlantic rift-related eruptions drive early Eocene climate cooling?

The delivery of volcanogenic sulphur into the upper atmosphere by explosive eruptions is known to cause significant temporary climate cooling. Therefore, phreatomagmatic and phreatoplinian eruptions occurring during the final rifting stages of active flood basalt provinces provide a potent mechanism for triggering climate change. During the early Eocene, the northeast Atlantic margin was subjected to repeated ashfall for 0.5 m.y. This was the result of extensive phreatomagmatic activity along 3000 km of the opening northeast Atlantic rift. These widespread, predominantly basaltic ashes are now preserved in marine sediments of the Balder Formation and its equivalents, and occur over an area extending from the Faroe Islands to Denmark and southern England. These ash-bearing sediments also contain pollen and spore floras derived from low diversity forests that grew in cooler, drier climates than were experienced either before or after these highly explosive eruptions. In addition, coeval plant macrofossil evidence from the Bighorn Basin, Wyoming, USA, also shows a comparable pattern of vegetation change. The coincidence of the ashes and cooler climate pollen and spore floras in northwest Europe identifies volcanism as the primary cause of climate cooling. Estimates show that whilst relatively few phreatomagmatic eruptive centres along the 3000 km opening rift system could readily generate 0.5–1 °C cooling, on an annual basis, only persistent or repeated volcanic phases would have been able to achieve the long-term cooling effect observed in the floral record. We propose that the cumulative effect of repeated Balder Formation eruptions initiated a biodiversity crisis in the northeast Atlantic margin forests. Only the decline of this persistent volcanic activity, and the subsequent climatic warming at the start of the Eocene Thermal Maximum allowed the growth of subtropical forests to develop across the region.

Evidence of Hierarchical Patch Dynamics in an East African Savanna?

The Hierarchical Patch Dynamics Paradigm provides a conceptual framework for linking pattern, process and scale in ecosystems, but there have been few attempts to test this theory because most ecological studies focus on only one spatial scale, or are limited in their temporal scope. Here I use palaeoecological techniques (analysis of fossil pollen and stable carbon isotopes) to compare vegetation heterogeneity in an east African savanna at three spatial scales, over hundreds of years. The data show that patterns of vegetation change are different at the three spatial scales of observation, and suggest that different ecological processes dominate tree abundance at micro, local and landscape scales. Interactions between plants, disturbance (e.g., by fire and herbivores), climate and soil type may influence tree density at differing spatial and temporal scales. This hierarchical explanation of savanna vegetation dynamics could inform future biodiversity conservation and management in savannas.

Vegetation patterns as biological indicators for identifying runoff and sediment source and sink areas for semi-arid landscapes in Spain

The rationale behind this research concerns the need to better understand relationships between vegetation characteristics and hydrological processes. Changes in vegetation patterns could provide sensitive indicators of both desertification and water availability. This paper presents a methodology to use vegetation-bare soil patterns as indicators for identifying the extent, distribution and connectivity of runoff and sediment source and sink areas. During field studies it has been found that biological indicators can be used as evidence that areas are functioning as a source or sink for runoff and sediment. It has also been found that the locations of such source and sink areas are tightly linked to the spatial distribution of vegetation patches within the vegetation-bare soil mosaic. Therefore, patterns in vegetation and soil can be important indicators for ecosystem health and hillslope hydrology. Four pattern indices are introduced to describe the degree of contagion and bare area distribution within the vegetation mosaic and the degree of connectivity between different source areas. Binary pattern maps retrieved from a digital aerial photograph are used as examples to illustrate the behaviour of the indices. It is concluded that the indices yield important information on the spatial structure of the patterns studied and can be used to understand the interactions between pattern and its underlying processes.

Testing non-equilibrium theories in savannas: 1400 years of vegetation change in Tsavo National Park, Kenya

Non-equilibrium processes in savannas are poorly understood because little data is available on how tree abundance has changed over long periods of time. Here I analyse fossil pollen and charcoal from Kanderi Swamp in Tsavo National Park, Kenya in order to investigate patterns of vegetation change over the past 1400 years. I compare the results with the predictions of cyclical, phase and transition and stochastic non-equilibrium models. Fourier analysis of the pollen data provides evidence of cyclical changes in tree abundance with a periodicity of 250–500 years, but the pollen data also suggests that the vegetation surrounding Kanderi swamp remained in a prolonged woodland phase, as predicted by phase and transition models, from approximately 770–1520 a. d. In addition, the results provided evidence of much shorter term changes in tree/grass abundance, perhaps reflecting stochastic variations in rainfall. The charcoal data suggests a link between vegetation change and fire history, possibly mediated through the effects of rainfall variation on the accumulation of combustible biomass in the herb layer. There was also evidence of extreme fires occurring twice in the past 1400 years. The results suggest that at the local scale, the vegetation near Kanderi Swamp has been mainly influenced by localised disturbance, rather than climatic shifts or the Rinderpest epidemic, and that elements of cyclical, phase and transition and stochastic models are needed in order to fully understand the observed pattern of vegetation change.

Global change and the boreal forest: thresholds, shifting states or gradual change?

Changes in boreal climate of the magnitude projected for the 21st century have always caused vegetation changes large enough to be societally important. However, the rates and patterns of vegetation change are difficult to predict. We review evidence suggesting that these vegetation changes may be gradual at the northern forest limit or where seed dispersal limits species distribution. However, forest composition may be quite resilient to climate change in the central portions of a species range until some threshold is surpassed. At this point, changes can be rapid and unexpected, often causing a switch to very different ecosystem types. Many of these triggers for change are amenable to management, suggesting that our choice of policies in the coming decades will substantially influence the ecological and societal consequences of current climatic change.

Changes in vegetation patterns on the margins of a constructed wetland after 10 years

Summary A constructed urban wetland in Adelaide was surveyed 18 months and 10 years after construction to see how shoreline vegetation, soil electrical conductivity (EC), texture and pH changed over time and to provide data for future site management. Multivariate analysis detected four plant associations at 18 months: salt‐tolerant taxa on conductive clays; a weed‐dominated community on lower EC soil; and two smaller waterlogged, low EC clusters dominated by Common Reed (Phragmites australis) and Sea Club‐Rush (Bolboschoenus caldwellii), respectively. At 10 years, site cover and heterogeneity was higher, with the margins dominated by Phragmites and salt‐tolerant species. EC was much lower and more uniform, and the soils were heavier and more alkaline. Managed storm water flushing apparently lowered soil EC, but possibly also disturbed the shoreline. However, weeds were still common, and the potential for domination by Phragmites at the expense of other native shoreline species means that ongoing monitoring and hydrological and vegetation management are essential to maintain site habitat diversity.

Lateglacial and Holocene vegetation change on the Isle of Skye: new data from three coastal locations

Detailed lithostratigraphical and biostratigraphical studies from three contrasting coastal sites on the Isle of Skye were undertaken to investigate spatial variations in vegetation development on the island since the Lateglacial. The pollen profiles were then compared with the published pollen data for Skye. The new data reveal broadly similar trends for the sites studied and show that these correspond well to the previously established chronology. A date of ca. 10110 radiocarbon years B.P. has been obtained for the rational rise in Corylus, a date of ca. 8850 uncal B.P. for the appearance of Ulmus and a date of ca. 6600 uncal B.P. for the rise in Alnus. However, significant variations were evident and these were due to (a) coastal proximity, which directly affects vegetation composition through the low but persistent presence of Armeria and Chenopodiaceae, (b) altitude, which determines the effect of rising groundwater that may be related to relative sea level rise, and (c) aspect and position in relation to central Cuillin mountain range, through exposure to SW winds that restrict the development of thermophilous woodland and encourage the acidification of heathlands and development of herb-rich grasslands from ca. 4000 B.P.. This study reveals that the range of variation caused by these local factors is as great as the variation found throughout the island. This has negative implications for the reconstruction of regional patterns of vegetation change but highlights the range of local habitats and wild resources available to early inhabitants of the island.

The use of sequential vegetation maps for monitoring in coastal areas

A method is presented and discussed that minimises uncertainties in change analysis of sequential vegetation maps due tot subjectivity in boundary interpretation. This previous boundary method is based upon the concept of „map changes”, which implies that sequential maps are not made independently but are based upon a previous map: only changes in the old maps boundaries (both in space and vegetation typology) are interpreted. It was shown that the map changes concept increased the reliability of change analysis results in a coastal salt marsh in the Netherlands. The method may be applied easily in other studies in which photo-interpretation is used for monitoring purposes. The method is especially recommended in low dynamic areas, where slow changes in vegetation patterns take place and the effects of photo-interpretation uncertainty are relatively high.

Combining remotely sensed spectral data and digital surface models for fine-scale modelling of mire ecosystems

The detection and evaluation of changes in vegetation patterns is a prerequisite for monitoring programs. The Swiss mire monitoring program aims to assess the changes in mire vegetation in order to examine the efficiency of the management measures. A promising way to explore and detect vegetation structure and vegetation change is the application of predictive vegetation mapping that combines image classification and predictive habitat distribution models. These models deal with predictor variables derived from remotely sensed spectral data and from environmental variables such as a digital surface model (DSM). Low accuracy of environmental data to predict vegetation at the local scale is due to the difficulties to capture dominant fine-scale enironmental gradients. Using high resolution spectral and topographical data sets of 50 cm pixel size and below, the study presented here aims to improve the simulation of local-scale vegetation properties.

Vegetation change associated with land-use in tropical dry forest areas of Western Mexico

Patterns of vegetation changes associated with land-use and successional pathways in areas occupied by tropical dry forest in Western Mexico were described. A diagram of vegetation states and transitions (events that promote the change) was constructed using Petri nets, a tool for graphic and mathematical modelling. This conceptual model was verified by analysing the perception of 28 settlers, using an environmental narrative method. Three main pathways of vegetation changes associated with land-use were detected: (i) forest replaced by agriculture in flatlands, (ii) pasture established on slopes, and (iii) wood extraction carried out without slash-and-burn on hill crests. If cultivated areas in flatlands and pasture fields on slopes are not continuously maintained by farmers, thorny vegetation develops within one to 3 years. If left untouched, this secondary vegetation becomes a low forest dominated by Acacia and Mimosa sp. persisting for at least 20 years.

Are meadows with high ecological and patrimonial value endangered by heathland invasion in the French central Pyrenees?

Due to their high ecological and patrimonial value Festuca eskia and Nardus stricta meadows and Rhododendron ferrugineum heathlands need to be locally preserved in the Pyrenees. This could pose dilemmas to conservation managers if heathlands invade and replace the meadows. However, studies showing that the heathlands do actually invade meadows are lacking in the Pyrenees and the invasion hypothesis therefore remains to be proved. So, vegetation pattern changes in a particularly vulnerable site from the central Pyrenees was investigated by two means: (i) interpretation of stereo pairs of aerial photographs and (ii) analysis of the demography of Rhododendron populations in ten 150 m 2 experimental plots. The main results from photo interpretation indicated that, during the period between 1954 and 1995, 15% of a 17 ha meadow area shifted to heathland. This corresponded with a 592 m 2 year −1 colonization rate and to an overall front progression of heathland over meadows of 3–60 m. Demographic data showed that meadow colonization by heathland passed through successive phases but appeared to be a rather slow process that can take as long as 120–320 years. During the process the cover rate over the 150 m 2 plots varied between 1 and 0.4 m 2 year −1. Heathland extension does not seem to have been favored by the decrease in grazing pressure. Despite this, the progression of heathlands over meadows seems endangering the meadows. Finally we propose practices locally to limit the progression of Rhododendron.

Vegetation Effects on Atmospheric Pollution

ATMOSPHERIC SCIENCE How changes in the composition of Earth's atmosphere may affect climate has received much attention, but how climate change may influence the composition of the atmosphere is not so clear. In particular, it is important to know how global climate change may affect pollutants such as surface ozone. This is a complicated task, however, because pollutant concentrations depend on a complex web of chemical reactions and species. For example, surface ozone concentrations are sensitive to the abundance of isoprene, which is produced by vegetation and thereby is a function of climate. Sanderson et al . use a global atmospheric general circulation model coupled to dynamic vegetation and chemistry models to predict how climate change will affect vegetation, isoprene, and ozone over the 21st century. Although changing patterns of vegetation will reduce the emission of isoprene, background ozone levels are expected to rise by 10 to 20 parts per billion by volume (ppbv) and to cause parts of China, Korea, and the eastern USA to exceed the health safety limit of 60 ppbv. — HJS Geophys. Res. Lett . 30 , 1936 (2003).

Landscape patterns of vegetation change indicated by soil carbon isotope composition

Vegetation change, particularly from the grass to shrub life form, is a critical issue on the world's semiarid rangelands. Stable carbon isotope (δ 13C) values and associated radiocarbon ages from soil organic matter (SOM) were used to evaluate vegetation change across five landscape positions at a small enclosed basin in southeastern Arizona. Light and dense SOM fractions were separated to distinguish recent vegetation changes. The direction and timing of vegetation change differed with landscape position along a gentle elevation gradient from the basin outlet to a nearby volcanic ridge top. C 4 perennial grasses have dominated the basin outlet, center, and toe slope landscape positions since at least 5000–6000 years BP, except for the dominance of C 3 plants at the bottom of the outlet excavation at 5000 years BP. This isotopic change is associated with rounded cobbles that may have been a stream channel, suggesting the presence of C 3 herbaceous or woody riparian vegetation. On mid-slope and ridge top landscape positions, where semidesert shrubs now dominate, the proportion of plants with C 4 metabolism calculated from mass balance mixing formulas decreased from approximately 60% as recently as 400 years BP to only 1.5% observed today. The light SOM fraction from mid-slope and ridge top surface soil horizons was approximately 30% C 4 and had a post-bomb date, suggesting that the conversion from grass to shrub occurred over the last several decades.

The irreversible cattle‐driven transformation of a seasonally flooded Australian savanna

AbstractAim Anecdotal historical and photographic evidence suggests that woody vegetation is increasing dramatically in some northern Australian savanna habitats. Vegetation change in savannas has important implications for pastoral land‐use, conservation management, and landscape‐scale carbon storage, and informs theoretical debates about ecosystem function. This study seeks to determine the nature, extent and cause(s) of woody vegetation change in a seasonally flooded alluvial savanna habitat.Location The study area is located within the seasonally inundated alluvial zone of the tidal portion of the Victoria River, Northern Territory, Australia. The study area has been grazed by domestic stock since c. 1900, prior to which the area was inhabited and more likely regularly burnt by Aboriginal people for thousands of years.Methods Digital georeferenced aerial photographic coverages were used to examine and quantify woody vegetation change between 1948 and 1993. Transect surveys of woody and herbaceous vegetation were carried out to ground‐truth air‐photo results and determine the nature and causes of observed vegetation changes.Results There has been a dramatic increase in woody vegetation cover throughout the study area. Vegetation change patterns are roughly uniform across the full range of edaphic habitat variation and are unrelated to the depositional age of fluvial sediments. Two woody species, Eucalyptus microtheca and Excoecaria parvifolia, are predominantly responsible for observed increases. Demographic analyses reveal that woody invasions have been episodic and indicate that in most locations peak woody species establishment occurred in the mid‐1970s. Grasses are almost absent in a majority of habitats within the study area. Instead, large areas are covered by scalded soil, dense invasive weed populations, and unpalatable forbs and sedges. What grasses do occur are predominantly of very low value for grazing. The condition of the herbaceous layer renders most of the study area almost completely non‐flammable; what fires do burn are small and of low intensity.Main conclusions Multiple working hypotheses explaining observed patterns of woody vegetation increase were considered and rejected in turn. The only hypothesis consistent with the evidence is as follows: (1) observed changes are a direct consequence of extreme overgrazing by cattle, most likely when stocking rates peaked in the mid‐1970s; (2) prolonged heavy grazing effected the complete transformation of much of the herbaceous vegetation to a new state that is not flammable; and (3) in the absence of regular fire mortality, woody vegetation increased rapidly. The relatively treeless system that existed in 1948 was apparently stable and resilient to moderate grazing levels, and perhaps also to episodic heavy grazing events. However, grazing intensity in excess of a sustainable threshold has forced a transition that is irreversible in the foreseeable future. Stable‐state transitions such as this one inform debates at the heart of ecological theory, such as the nature of stability, resilience, equilibrium and carrying capacity in dynamic savanna ecosystems.

Eighty years of change: vegetation in the montane ecoregion of Jasper National Park, Alberta, Canada

Repeat ground photographs (taken in 1915 and 1997) from a series of topographical survey stations and repeat aerial photographs (flown in 1949 and 1991) were analysed to assess changes in vegetation composition and distribution in the montane ecoregion of Jasper National Park, in the Rocky Mountains of Alberta, Canada. A quantitative approach for assessing relative vegetation change in repeat ground photographs was developed and tested. The results indicated a shift towards late-successional vegetation types and an increase in crown closure in coniferous stands. Grasslands, shrub, juvenile forest, and open forests decreased in extent, and closed-canopy forests became more prevalent. The majority of forest stands succeeded to dominance by coniferous species. Changes in vegetation patterns were likely largely attributable to shifts in the fire regime over the last century, although climatic conditions and human activity may also have been contributing factors. Implications of observed changes include decreased habitat diversity, increased possibility of insect outbreaks, and potential for future high-intensity fire events. Results of the study increase knowledge of historical reference conditions and may help to establish restoration goals for the montane ecoregion of the park.

Variability in an edaphic indicator in alpine tundra

Spatial patterns of soil in alpine tundra just above timberline may determine patterns of advance by woody species. Patterns of advance since the Little Ice Age show spatial aggregation. If soil resources have similar patterns of aggregation, they might determine the pattern of vegetation change. Effective soil depth (ESD), which takes into account stoniness, was measured in tundra just above present timberline on regular grids at two scales and across solifluction treads and risers. No non-random spatial pattern was found, nor was there any difference related to relict solifluction patterns. Current patterns of woody vegetation could not have developed in direct response to the pattern of effective soil depth found in tundra. These patterns may develop in response to other soil factors, but may also include positive feedback.

Changing vegetation patterns in the Endinger Bruch area (Vorpommern, NE Germany) during the Weichselian Lateglacial and Early Holocene

Comparison of the pollen record of several cores at different distances to palaeo-vegetation units enabled reconstruction of changing vegetation patterns during the Weichselian Lateglacial and Early Holocene in and around the Endinger Bruch (Vorpommern, NE Germany). Various vegetation phases of the complete Lateglacial are discussed, which are introduced to avoid a widespread scientific confusion about stratigraphic terminology. During Open vegetation phase I, the upland was inhabited mainly by herbs, in which during the Hippophaë phase, Hippophaë shrubs expanded. Open vegetation phase II was mainly dominated by herbs and (dwarf)shrubs. During the Lateglacial Betula/Pinus forest phase, the upland vegetation closed, and subsequently opened-up again in Open vegetation phase III. A new expansion of forests took place during the Early Holocene Betula/Pinus forest phase. A comparison with pollen diagrams from southern regions shows that Vorpommern, Germany, can be considered as a separate vegetation region during the Lateglacial with respect to the openness of the vegetation during Open vegetation phases II and III, and with respect to Pinus expansion during the Lateglacial Betula/Pinus forest phase.

Patterns of vegetation change and the recovery potential of degraded areas in a coastal marsh system of the Hudson Bay lowlands

Summary In recent decades, foraging by increasing numbers of lesser snow geese has led to loss of vegetation and changes in soil conditions in marshes on the Hudson Bay coast. Changes in species composition were recorded in areas unprotected from goose foraging and in exclosures of varying age (5–15 years) erected in intact swards and on bare sediments where foraging had occurred at La Pérouse Bay, Manitoba. In the supratidal marsh, plants failed to establish naturally in either open or exclosed (15 years) plots in bare areas. In moist intertidal soils, vegetative fragments of the asexual species Puccinellia phryganodes readily established and formed a mat in exclosures (5 years). Changes in species assemblages occurred over 11 years in exclosed and adjacent open plots in intertidal and supratidal marshes. Loss of vegetation cover and species richness, particularly dicotyledonous species, and the reversion of later successional plant assemblages to earlier successional assemblages occurred in open plots. In the absence of foraging, late successional graminoids and willow species replaced early successional graminoids. Late successional grasses of the upper intertidal marsh died when transplanted into degraded soils but still survived after one season in control plots, suggesting that an early successional template is needed for establishment. In the absence of goose foraging, natural re‐vegetation by clonal propagation can occur only where edaphic conditions are suitable. Within exclosures, vegetation changes resemble those in undamaged areas where goose foraging pressure is still moderate. We propose a state and transition model for vegetation change in the system based on succession patterns, alternative vegetation states and geomorphological events.