Vegetation Resilience Research Articles

Temporal persistence in vegetation cover changes observed from satellite: Development of an estimation procedure in the test site of the Mediterranean Italy

We present a procedure for estimating temporal persistence in Advanced Very High Resolution Radiometer (AVHRR)-Normalized Difference Vegetation Index (NDVI) data. Our algorithm provides for the estimations of the tendencies of NDVI to increase or decrease starting from selected reference conditions by means of linear fits that cover time periods increasing at a 1-year rate. The times at which such trends change their sign (first return times) are estimated, and the distribution that best fits their histogram is determined. Such a distribution reflects the character, stationary or not, of NDVI changes and allows for estimating persistence probability and for quantifying possible characteristic time scales. In order to check the algorithm's performances, we estimated persistence probability at a 1.1-km resolution over the test site of Southern Italy and selected subareas. Histograms of return times aggregated according to vegetation classes were also analyzed. In all of the cases, we found the exponential decay law that is typical of processes that attain to long-term stationary patterns. Estimated mean life times of decreasing processes were generally shorter than those characterizing the increasing ones. Our findings express a good ability of vegetation to recover from disturbances and to efficiently exploit resources. Overall, rather stable conditions seem to characterize the test site and local exceptions can be easily explained with independent ground data. The good agreement between our inferences and results from independent studies suggests that dynamics estimated from AVHRR data can provide interesting information on vegetation resilience and response to human/climate forcing. In particular, quantitative information can be a key tool for the characterization and parameterization of vegetation changes and for the development and validation of dynamical models.

The Sensitivity and Response of Terrestrial South American Vegetation to Interannual Climatic Variability Induced by the ENSO

The sensitivity of vegetation to climatic variations operates over a range of spatio-temporal scales. Whilst comprehensive and extensive studies that involve the spatial changes in terrestrial systems using remotely sensed data have been undertaken, only a few investigations have focused on temporal change in these systems. In this study econometric time-series modelling techniques were applied to National Oceanographic and Atmospheric Administration Advanced Very High Resolution Radiometer Normalized Difference Vegetation Index data sets in order to evaluate the resistance and resilience of terrestrial South American vegetation to the interannual El Nino Southern Oscillation perturbations. Lags between vegetation response and the El Nino Southern Oscillation perturbations are identified and quantified. The results indicate that the terrestrial vegetation loses its sensitivity to El Nino Southern Oscillation perturbations in the post 1993 period. This hypothesis is investigated further using stochastic Auto Regressive Integrative Moving Average model simulation techniques.

Some determinants of the spatio-temporal fire cycle in a mediterranean landscape (Corsica, France)

Based on recent needs to accurately understand fire regimes and post-fire vegetation resilience at a supra-level for carbon cycle studies, this article focusses on the coupled history of fire and vegetation pattern for 40 years on a fire-prone area in central Corsica (France). This area has been submitted since the beginning of the 20th century to land abandonment and the remaining land management has been largely controlled by frequent fires. Our objectives were to rebuild vegetation and fire maps in order to determine the factors which have driven the spatial and temporal distribution of fires on the area, what were the feed backs on the vegetation dynamics, and the long-term consequences of this inter-relationship. The results show a stable but high frequency of small fires, coupled with forest expansion over the study period. The results particularly illustrate the spatial distribution of fires according to topography and vegetation, leading to a strong contrast between areas never burnt and areas which have been burnt up to 7 times. Fires, when occuring, affect on average 9 to 12% of the S, SE and SW facing slopes (compared to only 2 to 5% for the N facing slopes), spread recurrently over ridge tops, affect all the vegetation types but reburn preferentially shrublands and grasslands. As these fire-proning parameters have also been shown to decrease the regeneration capacity of forests, this study highlights the needs in spatial studies (both in terms of fire spread and vegetation dynamic) to accurately apprehend vegetation dynamic and functionning in fire-prone areas.

Native American impacts on fire regimes of the California coastal ranges

AimNative American burning impacts on California shrubland dominated landscapes are evaluated relative to the natural lightning fire potential for affecting landscape patterns.LocationFocus was on the coastal ranges of central and southern California.MethodsPotential patterns of Indian burning were evaluated based upon historical documents, ethnographic accounts, archaeological records and consideration of contemporary land management tactics. Patterns of vegetation distribution in this region were evaluated relative to environmental factors and the resilience of the dominant shrub vegetation to different fire frequencies.ResultsLightning fire frequency in this region is one of the lowest in North America and the density of pre‐Columbian populations was one of the highest. Shrublands dominate the landscape throughout most of the region. These woody communities have weak resilience to high fire frequency and are readily displaced by annual grasses and forbs under high fire frequency. Intact shrublands provided limited resources for native Americans and thus there was ample motivation for using fire to degrade this vegetation to an open mosaic of shrubland/grassland, not unlike the agropastoral modification of ecologically related shrublands by Holocene peoples in the Mediterranean Basin. Alien‐dominated grasslands currently cover approximately one‐quarter of the landscape and less than 1% of these grasslands have a significant native grass presence. Ecological studies in the Californian coastal ranges have failed to uncover any clear soil or climate factors explaining grassland and shrubland distribution patterns.Main conclusionsCoastal ranges of California were regions of high Indian density and low frequency of lightning fires. The natural vegetation dominants on this landscape are shrubland vegetation that often form dense impenetrable stands with limited resources for Native Americans. Natural fire frequencies are not high enough to maintain these landscapes in habitable mixtures of shrublands and grasslands but such landscape mosaics are readily produced with additional human subsidy of ignitions. It is hypothesized that a substantial fraction of the landscape was type converted from shrubland to grassland and much of the landscape that underwent such type conversion has either been maintained by Euro‐American land management practices or resisted recolonization of native shrublands. It appears that these patterns are disturbance dependent and result from anthropogenic alteration of landscapes initiated by Native Americans and sustained and expanded upon by Euro‐American settlers.

Monitoring land-cover changes in semi-arid regions: remote sensing data and field observations in the Ferlo, Senegal

Dryland degradation rarely translates into linear, declining trends in vegetation cover due to interannual climatic variability. Appropriate indicators of land-cover modifications need to be defined for semi-arid regions. Our hypothesis is that degradation can be measured by: (1) a decrease in the resilience of vegetation to droughts; (2) a decrease in rain-use efficiency; and (3) a modification of floristic composition. The objective of this paper is to test the relationships between a remotely sensed indicator of vegetation, rainfall data and field measurements of biomass and floristic composition. The study was based on field measurements of vegetation conditions covering a period of 10 years, in the semi-arid region of the Ferlo in Senegal. Our results indicate that land-cover modifications in the Ferlo are best measured by changes in rain-use efficiency. No consistent trend in the relative abundance of grass species was visible at the scale of the decade, even on the two sites affected by degradation. Just after a drought, a given increase in rainfall results in less biomass production than is the case for normal years.

IVth International Bracken Conference, 20?23 July 1999

Raised and transitional bogs are one of the most threatened types of ecosystem, due to high specialisation of biota, associated with adaptations to severe environmental conditions. The aim of the study was to characterize the relationships between functional diversity (reflecting ecosystem-shaping processes) of raised bog plant communities and successional gradients (expressed as tree dimensions) and to show how impacts of former clear cuts may alter these relationships in two raised bogs in ‘Bory Tucholskie’ National Park (N Poland). Herbaceous layers of the plant communities were examined by floristic relevés (25 m2) on systematically established transects. We also assessed patterns of tree ring widths. There were no relationships between vegetation functional diversity components and successional progress: only functional dispersion was negatively, but weakly, correlated with median DBH. Lack of these relationships may be connected with lack of prevalence of habitat filtering and low level of competition over all the successional phases. Former clear cuts, indicated by peaks of tree ring width, influenced the growth of trees in the bogs studied. In the bog with more intensive clear cuts we found more species with higher trophic requirements, which may indicate nutrient influx. However, we did not observe differences in vegetation patterns, functional traits or functional diversity indices between the two bogs studied. We also did not find an influence of clear cut intensity on relationships between functional diversity indices and successional progress. Thus, we found that alteration of the ecosystems studied by neighbourhood clear cuts did not affect the bogs strongly, as the vegetation was resilient to these impacts. Knowledge of vegetation resilience after clear cuts may be crucial for conservation planning in raised bog ecosystems.

FOREST RESPONSE TO CATASTROPHIC WIND: RESULTS FROM AN EXPERIMENTAL HURRICANE

Infrequent, intense wind disturbance is an important factor in northeastern U.S. forests, yet little is known about the early stages of vegetation reorganization, or the processes that facilitate biotic regulation of ecosystem function after such storms. We designed an experiment, based on a simulated hurricane blowdown, to examine the relationship of tree damage patterns to mortality and regeneration, community dynamics, vegetation recovery, and ecosystem processes. In October 1990, selected canopy trees in a 50 × 160 m area within a 75-yr-old Quercus rubra–Acer rubrum forest in central Massachusetts were pulled over by a winch, using records from the 1938 hurricane to determine the number of trees and direction of fall. The resulting damage to 65% of trees closely approximated effects of the 1938 storm on New England forests. Damage and mortality varied by tree species and size, indicating the importance of pre-disturbance forest structure and composition in determining the range and severity of impact. Measurements of vegetation and environment in the experimental area and control indicated that, although the manipulated stand sustained dramatic damage and structural reorganization, resilience of trees and understory vegetation provided tight biotic control of ecosystem processes, including nutrient cycling. Continued leaf-out and induced sprouting by damaged trees, increased growth by saplings and understory plants, and seedling establishment on disturbed microsites stabilized the microenvironment. Our findings are in contrast to studies of disturbances in which mortality was higher when damaged trees were removed from the site. This suggests that salvage logging following wind disturbance may have serious long-term implications.

Ecological diversity and resilience of Mediterranean vegetation to disturbance

SummaryIn the Mediterranean region, plant communities are highly diverse with high total and local species richness, and a high spatial heterogeneity. They are also highly resilient to disturbance, which has been related to their evolutionary history of disturbance, and conceptually, although without formal proof, to their high ecological diversity. These ideas take on a new importance in the context of global change. In order to predict potential effects of future land use change, mechanisms underlying the resilience of ecosystems need to be understood. However, a review of the recent literature shows that confusion abounds as to the nature and measurement of resilience. In this paper we present a brief conceptual and methodological clarification and propose avenues for the analysis of mechanisms underlying the relationships between diversity and resilience, using Mediterranean ecosystems as an example.In Mediterranean ecosystems, rapid recovery from fire, soil disturbance and grazing has been attributed to two complementary causes. Firstly, disturbances often modify species relative abundances rather than composition, and recovery only involves the return to initial abundances. Secondly, life histories of dominant life forms in Mediterranean communities include efficient regeneration strategies that allow for recovery from dormant seeds or buds. In this paper, I use experimental results obtained in abandoned agricultural fields of southern France to investigate the mechanisms underlying the resilience of Mediterranean herbaceous communities to soil disturbance. These suggest that ecological diversity contributes to resilience in at least three ways.1.Post‐disturbance dynamics involve shifts in abundances of response groups to disturbance.2.Within response groups, processes of lottery competition account for the dynamic maintenance of numerous species.3.At the landscape scale, the fine grained distribution of communities at different successional stages allows for patch dynamics to operate with dispersal of species between patches.Further progress in the understanding of the relationships between ecological diversity and resilience of Mediterranean ecosystems can be achieved along two main avenues. Firstly, resilience of experimentally manipulated communities with differing levels of diversity needs to be assessed. Secondly, comparisons of those experiments across mediterranean‐climate regions with different evolutionary disturbance histories need to be coordinated.

Exotic weed control treatments for conservation of fescue grassland in Montana

An analytical framework is described to evaluate exotic weed control treatments applied to native vegetation for conservation purposes. The analyses were used to assess the following responses to herbicide applications for control of an exotic forb Centaurea maculosa in fescue grassland: (1) efficacy on the target weed species, (2) similarity to reference stands defining the potential natural communities, (3) resistance and resilience of the nontarget vegetation, and (4) community structure in terms of life form proportions. Vegetation data were collected from replicated plots one year before spraying and for the first three years after spraying at two sites in western Montana. Low rates of selected herbicides effectively suppressed the target weed for three years after treatment. Grasses responded positively to release from exotic forb competition, while total nontarget forb standing crops were similar between treated plots and untreated control plots three years after spraying. The site with lower pretreatment cover of exotic species and higher pretreatment composition of native species had the largest positive response to spraying in terms of similarity to the potential natural vegetation. Although some treatments caused depressions after one year, all treatments had the same or higher per cent of pretreatment species present compared to the untreated control plots in the third year after spraying. Herbicide-sensitive taxa were identified by multivariate analysis of canopy cover data. Implications of the results for conservation management of high priority grassland sites are discussed.

Resilience of soils and vegetation subjected to different grazing intensities in a semi‐arid rangeland of Kenya

The resilience of rangeland soils and vegetation to different levels of grazing is still poorly understood. A study was conducted to determine the recovery of a rangeland grazed at different intensities and allowed a two‐year rest period. The following treatments were applied to 0.5 hectare plots: 0, 4, 8 and 16 heifers per hectare, hereafter referred to as CL, X, 2X and 4X respectively. At the end of the grazing period, the highest stocked treatments (2X and 4X) had lower herbage biomass, higher soil bulk density, lower soil moisture and lower herbaceous cover than the lower stocked treatments (CL and X). Drought in the rest period caused an increase in bulk density and decline in soil moisture in all the treatments. Even after the two‐year rest period, the more heavily grazed treatments had higher bulk density and lower soil moisture than the more lightly grazed treatments. Similarly, the herbaceous biomass in the 2X and 4X treatments did not recover after the two‐year rest period and was lower (P < 0.05) than the CL and X treatments. At the end of the recovery period a trend of declining herbaceous cover with stocking density was still evident. The relative cover of forbs in the 4X treatments increased more than in the other treatments, while the cover of perennial grasses did not recover in the 4X treatments after the rest period. Thus, stocking above 2X produced negative soil and vegetation responses which did not recover during the two‐year rest period. This study also indicated that drought can cause vegetation and soil responses similar to those of overgrazing.

Estimating Landscape Resilience From Satellite Data and Its Application to Pastoral Land Management.

Episodic rainfall provides infrequent opportunities for vegetation recovery from grazing in the arid rangelands. The magnitude of resultant vegetation growth, or resilience, varies widely in space and is not simply controlled by rainfall amount or past grazing practices. The non predictable nature of vegetation response fits within the model of nonequilibrium rangeland behaviour and we propose use of the Resilience Method as a means of spatially assessing vegetation resilience, or the ability of herbage to respond to infrequent rainfall. Information required to calculate an index of resilience is obtained from remotely-sensed satellite data, is plant cover-based and is processed to produce landscape-scale maps of vegetation resilience. In this paper, we demonstrate and evaluate products developed to transfer the Resilience Method to a pastoralist family to assist in their future property management. These products include maps of scaled herbage response to rainfall, herbage response in conjunction with dry period vegetation cover and herbage biomass derived from vegetation cover. The cover-based products effectively showed herbage response following a large rainfall. Much of the variation in response was natural and was related to timber density and soil factors. Some areas with below average herbage response were attributable to damage caused by high rabbit populations. Herbage response on much of the productive grazing country was average to above average indicating good resilience and potential for continued beef production. The resilience approach to landscape assessment provided a useful pictorial representation of herbage response across the whole station following one rainfall. The participating pastoralists consider that the Resilience Method will have greater validity when repeated following further significant rainfalls. Some insights were gained into future property development. However, the technology was difficult to understand and requires a close liaison between the technician and client. Confusion arose where below average herbage response could occur in areas of both high and low initial cover when the two areas appear vastly different and require separate management for beef production and rehabilitation. Both the nonequilibrium approach to understanding vegetation dynamics and the Resilience Method need further explanation and demonstration before they are accepted as being useful for pastoral land management.

Fire and the dynamics of allochthonous detritus in a South African mountain stream

SUMMARY. Input of allochthonous material, standing stocks of benthic organic matter (BOM) and suspended paniculate organic matter (POM) were measured in a south‐western Cape mountain stream from March 1986 to February 1988. The surrounding fynbos‐dominated catch‐ment was subjected to a prescribed burn in March 1987. Litter‐fall in the pre‐burn year exhibited a distinct seasonal pattern, with peak falls during the early summer. Although the riparian canopy was not directly affected by the fire, in that it did not burn, a heavy, aseasonal leaf‐fall occurred shortly afterwards. The following summer, litter‐fall was less than half that of the pre‐burn summer. Standing stocks of BOM were significantly higher in autumn than in winter in the pre‐burn year and were inversely related to discharge. Despite the heavy post‐burn leaf‐fall and low litter‐fall during the post‐burn summer, there was no significant difference between pre‐ and post‐burn BOM standing stocks. Proportions and quantities of fine benthic organic matter (FBOM) in the soft BOM fraction were significantly higher in the post‐burn spring, and monthly accumulation of ultra‐fine benthic organic matter (UBOM) was also significantly higher in the post‐burn spring and summer. These results may reflect accelerated decay rates of BOM in response to enhanced post‐burn nitrate concentrations in stream water. Export of CPOM was low in comparison to FPOM and particularly to UPOM, and the stream appears to be highly retentive of CPOM. The natural resilience of the riparian vegetation minimizes the potentially disturbing effects of fire on the stream environment. As a result, the prescribed burn had a less than expected effect on both standing stocks of BOM and the stream environment in general.

Sahelian woody vegetation in S�n�gal

Structure and composition of woody vegetation in Sahelian forest types (not including riparian and riverine types) were studied in Senegal. The application of classification and ordination resulted in the recognition of seven distinctive woody community types, related to soil and precipitation gradients. The described vegetation types only contained four strictly Saharan species, the rest being of Sahelian and Soudanean affinity, so a floristic change from savanna to desert vegetation after two decades of drought could not be confirmed. Tree density was lower in the northern part than earlier, possibly an indication of degradation. Rejuvenation was greatest at the northernmost arenosolic stations, but low in the southern, ferrouginous areas. This presumably demonstrates the resilience of North-Sahelian vegetation, at the moment being in a dynamic rejuvenation phase, after a period of adverse conditions.