Long-term Vegetation Changes Research Articles

Invasive forest defoliator contributes to the impending downward trend of oak dominance in eastern North America

Disturbanceby non-native insect species can be an important ecological driver shaping long-term changes in vegetation and plant species composition. While impacts of gypsy moth (Lymatria dispar L.) outbreaks in North American forests have been extensively studied, the results are quite inconsistent, particularly with respect to the amount of tree mortality associated with defoliation. In this study, we integrate geographical data describing historical gypsy moth defoliation with forest inventory data collected by a national forest inventory programme to quantify regional impacts across several million hectares of forest land in the northeastern US. While observed increases in host tree mortality rates and decreases in growth rates associated with defoliation were expected, the study also indicates that this overstory mortality, coupled with ongoing declines in oak regeneration, will result in a long-term reduction of oak density in defoliated areas. Eventually, these impacts will likely contribute to regional shifts in tree species composition and forest succession pathways. Gypsy moth outbreaks thus appear to exacerbate ongoing declines in young oak age classes in the region.

Stand-scale reconstruction of late Holocene forest succession on the Gdańsk Upland (N. Poland) based on integrated palynological and macrofossil data from paired sites

This study concerns the stand-scale palaeoecological reconstruction of the subsequent stages of late Holocene vegetation development on habitats recently covered by beech-dominated woodland in the southern Baltic region. The data, based on pollen, non-pollen palynomorphs, macrofossil and charcoal analyses from two close-lying sites, demonstrated that each of the subsequent late Holocene shifts in dominating forest communities took place because of human impact coupled with climatic events or episodic human-made disturbances. Shifts from the Tilia dominated forest to the Quercus-Corylus plant community at around 3300 bc was most probably driven by the coupled effect of climate change and human activity. Human impact was the primary driver of the final Corylus decline and concurrent major Carpinus expansion at c. 460 bc, and the Carpinus decline followed by Fagus expansion at c. ad 900. Carpinus had been lingering on the edge of a major expansion in the local forest for at least 2,500 years while Fagus had persisted in a small admixture for no fewer than 1,500 years before becoming important forest constituents. Our data illustrate the role of episodic disturbances as turning points that initiate long-term vegetation changes.

Vegetation responses to the last glacial and early Holocene environmental changes in the northern Leizhou Peninsula, south China

A well-dated palynological record spanning the interval ~40,500–7060calyr BP, retrieved from a peatland on the Leizhou Peninsula in south China, clearly shows regional vegetation and climate changes during the last glacial period. Pollen data showed that the study region was mainly covered by subtropical evergreen trees during Marine Isotope Stage 3 (MIS 3), indicating a subtropical climate with relatively high temperature and precipitation. During MIS 2, subtropical evergreen-deciduous forest with large areas of grassland occurred, implying cooler and drier conditions. Some tropical forest elements increased during the early Holocene, indicating a warm and wet trend. Several millennial-scale oscillations of the pollen records appeared to correlate with the cold anomalies in the North Atlantic region. Our records agree well with many records from other regions, but they are a bit different than that inferred from the neighboring Huguang Maar Lake. Furthermore, our results suggest that the vegetation surrounding Xialu peatland was strongly influenced by the migration of the intertropical convergence zone (ITCZ) and variability in the East Asian summer monsoon (EASM). Changes of atmospheric CO2 concentration (pCO2) levels may have also affected the long-term vegetation changes in the study region.

Integrating precipitation, grazing, past effects and interactions in long-term vegetation change

Determining the causes of vegetation change in arid and semi-arid environments can be difficult and may involve multiple factors, including disturbance, inter-annual climatic variation, soils, effects from years past and interactions between these factors. Theoretical models describing vegetation change in these systems have generally focused on a single aspect as the primary driver. The integration of these factors into a single model may be what is required to fully understand the drivers of vegetation change in desert systems. To test the contributions of these various factors, we analyzed a long-term (1979–2011) vegetation dataset using multiple linear regression.While precipitation and livestock density were important variables for explaining vegetation change, the consistency with which past effects and interactions significantly improved the models underscores their importance. Past effects were included in every model except for shrub diversity, and included both precipitation and livestock density effects. A novel approach to addressing the interaction between grazing and precipitation was included by dividing precipitation by stocking density. Grass density had a high positive correlation with this metric, while shrub cover had a small negative correlation. These results support the integration of multiple factors to explain vegetation change.

A Temporal-Spatial Iteration Method to Reconstruct NDVI Time Series Datasets

Reconstructing normalized difference vegetation index (NDVI) time series datasets is essential for monitoring long-term changes in terrestrial vegetation. Here, a temporal–spatial iteration (TSI) method was developed to estimate the NDVIs of contaminated pixels, based on reliable data. The NDVIs of contaminated pixels were first computed through linear interpolation of adjacent high-quality pixels in the time series. Then, the NDVIs of remaining contaminated pixels were determined based on the NDVI of a high-quality pixel located in the same ecological zone, showing the most similar NDVI change trajectories. These two steps were repeated iteratively, using the estimated NDVIs as high-quality pixels to predict undetermined NDVIs of contaminated pixels until the NDVIs of all contaminated pixels were estimated. A case study was conducted in Inner Mongolia, China. The accuracies of estimated NDVIs using TSI were higher than the asymmetric Gaussian, Savitzky–Golay, and window-regression methods. Root mean square error (RMSE) and mean absolute percent error (MAPE) decreased by 16.7%–86.6% and 18.3%–33.0%, respectively. The TSI method performed better over a range of environmental conditions, the variation of performance by the compared methods was 1.4–5 times that of the TSI method. The TSI method will be most applicable when large numbers of contaminated pixels exist.

Vegetation and climate change, fire-regime shifts and volcanic disturbance in Chiloé Continental (43°S) during the last 10,000 years

Disentangling the roles of paleofires and explosive volcanism from climatic drivers of past vegetation change is a subject insufficiently addressed in the paleoecological literature. The coastal region of the Chiloé Continental sector of northwestern Patagonia is ideal in this regard considering its proximity to active eruptive centers and the possibility of establishing comparisons with more distal, upwind sites where volcanic influence is minimal. Here we present a fine-resolution pollen and macroscopic charcoal record from Lago Teo with the aim of documenting the local vegetation and climate history, and assessing the role of disturbance regimes as drivers of vegetation change during the last ∼10,000 years.The Lago Teo record shows a conspicuous warm/dry interval between ∼7500 and 10,000 cal yrs BP followed by a cooling trend and increase in precipitation that has persisted until the present, in agreement with previous studies in the region and interpretations of past southern westerly wind activity at multi-millennial scales. The presence of 26 tephras throughout the record allows examination of the relationship between explosive volcanism and vegetation change under contrasting climatic states of the Holocene. We found consistent statistically significant increases in Tepualia stipularis after tephra deposition over the last 10,000 years, in Eucryphia/Caldcluvia between 7500 and 10,000 cal yrs BP and in Hydrangea over the last 7500 years. Our results indicate a primary role of climate change as driver of long-term vegetation change and as a modulator of vegetation responses to volcanic disturbance at multidecadal and centennial timescales.

Biome stability and long-term vegetation change in the semi-arid, south-eastern interior of South Africa: A synthesis of repeat photo-monitoring studies

We synthesize recent literature concerned with the nature, extent and rate of vegetation change in the Albany thicket, Grassland and Nama-karoo biomes of the semi-arid, south-eastern interior of South Africa at a range of spatial and temporal scales in relation to local and global drivers. The change in cover of three main growth forms (grasses, dwarf shrubs [<1m] and tall shrubs [>1m]) at the community, ecotone and biome levels was analysed using field surveys and repeat photography spanning 27 to >100years. There has been an increase in grass and tall shrub cover and a decrease in dwarf shrub cover across the region over time. Vegetation composition and cover has been most stable in the Albany Thicket biome while Azonal habitats across the region have changed the most. The Grassland biome has experienced a significant increase in the cover of grasses and tall shrubs and a significant decline in cover of dwarf shrubs. Total cover and the cover of grasses have increased significantly in the Nama-karoo biome. We interpret these patterns as evidence for a south westwards shift in the Grassland/Nama-karoo biome boundary in the second half of the 20th century. Although there has been relatively little change in the boundaries between the Albany Thicket and adjacent biomes, several species with subtropical biogeographic affinities have increased in cover and abundance over this period in all biomes investigated. Drivers of the observed changes include rainfall amount and seasonality, temperature, land use and elevated CO2 concentrations in the atmosphere.

From pollen percentage to vegetation cover: evaluation of the Landscape Reconstruction Algorithm in western Norway

The Landscape Reconstruction Algorithm (LRA) with the two models REVEALS and LOVE is developed to transform pollen percentage data to vegetation cover. This paper presents the first study to evaluate LRA in a region with large topographic variations within a short distances. The REVEALS model estimates regional vegetation abundance based on pollen assemblages from large lakes (100–500 ha). Pollen surface samples from one large and 28 small lakes are used together with a combination of regionally derived pollen productivity estimates and available estimates from other regions of Europe. The results show a good relationship between REVEALS-estimated forest cover and vegetation abundance based on the CORINE land-cover data. The REVEALS results using various sets of pollen assemblages from small lakes were comparable to those using one large lake. Local vegetation abundance using the LOVE model was estimated around 26 lakes. For common taxa, such as Pinus and Poaceae, the LOVE-based estimates of plant abundance match well with the distance-weighted plant abundances based on vegetation maps. Our results indicate that the LRA approach is effective for reconstruction of long-term vegetation changes in western Norway and other regions with high topographic relief when no major gradients exist in the pollen data.

Compound-specific carbon isotope records of vegetation and hydrologic change in central Sulawesi, Indonesia, since 53,000 yr BP

Lake Matano (2°29.1′S 121°20′E) in central Sulawesi, the deepest and one of the largest lakes in Indonesia, provides an excellent opportunity to reconstruct long-term hydrological and vegetation changes at the heart of the Maritime Continent within the tropical Western Pacific Warm Pool. We measured the carbon isotopic composition of terrestrial leaf waxes (δ 13 C wax ) in new sediment cores from Lake Matano to understand regional variations in vegetation and their interactions with local and high latitude climate forcings during the past 53 kyr. Variations in δ 13 C wax record the expansion and contraction of C 3 versus C 4 vegetation as well as changes in plants' carbon and water use efficiency and, by inference, regional hydroclimate. During Marine Isotope Stages 1 and 3, more negative δ 13 C wax values indicate that closed-canopy C 3 rainforests predominated in Sulawesi, reflecting a wetter, less seasonal climate. In contrast, more positive δ 13 C wax values record a terrestrial ecosystem with more abundant open canopy vegetation and possible expansion of C 4 grasses between 29 and 14 ka BP, indicating that during Marine Isotope Stage 2, including the Last Glacial Maximum, the climate was more arid, likely with increased precipitation seasonality, in central Sulawesi. Comparison of our record with other paleovegetation and paleohydrological records from Sulawesi and nearby islands suggests coherent long-term hydrological changes in central Indonesia that appear to be sensitive to high latitude climate across glacial-interglacial boundaries. However, variations in the amplitude of the response of vegetation to these glacial-interglacial forcings highlight the importance of regional topographic control on climate and vegetation and suggest an important role for higher elevations in maintaining rainforest refugia during regionally arid time intervals when C 4 savannas and grasslands expanded at lower elevations. • Vegetation and hydrologic changes in central Sulawesi are sensitive to high latitude climate forcings across glacial-interglacial boundaries since 53 kyr BP. • Altitudinal gradients within the interior of Indonesian islands influence local manifestations of hydrological and vegetation changes at orbital timescales. • Higher elevations providing rainforest refugia during arid and more seasonal interval.

Long-term warming and vegetation change

Climate Change Ecology How climate change specifically affects ecosystems can be difficult to sort out. To determine cause and effect, Harte et al. examined changes to vegetation and soil in plots of land in the U.S. Rocky Mountains that they either warmed experimentally or left untouched (ambient) over 23 years. They found that both experimental warming and the natural warming of the Rockies over that time period led to a shift from herbaceous meadow to woody shrub vegetation. Soil carbon declined, too (although in both cases more slowly in the ambient plots). These results demonstrate that climate change caused the observed vegetation and soil changes. Global Change Biol. 10.1111/gcb.12831 (2015).

Long-term vegetation changes in the four mega-sandy lands in Inner Mongolia, China

Context Desertification in China has become one of the most serious ecological and social problems. The four mega-sandy lands (Hulunbeir, Horqin, Otindag, and Mu Us) in Inner Mongolia are reported to be the most widespread and seriously desertified areas in China.

Impact of late-Holocene aridification trend, climate variability and geodynamic control on the environment from a coastal area in SW Spain

A detailed pollen analysis has been carried out on two sediment cores taken from a marsh area located in the Doñana National Park, southwestern Spain. The studied sedimentary sequences contain a similar late Holocene record of vegetation and climate and show a progressive aridification trend since at least 5000 cal. yr BP, through a decrease in forest cover in this area. Long-term vegetation changes shown here (semi-desert expansion and Mediterranean forest decline) paralleled declining summer insolation. Decreasing summer insolation most likely impacted negatively on tree growing season as well as on winter precipitation in the area. Superimposed on the long-term aridification trend were multi-centennial scale periods characterized by forest reductions or increases in arid and halophytic plants that can be interpreted as produced by enhanced droughts and/or by local geodynamic processes. These are centered at ca. 4000, 3000–2500, and 1000 cal. yr BP, coinciding in timing and duration with well-known dry events in the western Mediterranean and other areas but could have also been generated by local sedimentary or geodynamic processes such as a marine transgression in a subsidence context and extreme wave events (EWEs). The alternation of persistent North Atlantic Oscillation modes probably played an important role in controlling these relatively humid–arid cycles.

An improved species distribution model for Scots pine and downy oak under future climate change in the NW Italian Alps

Warming will induce an upward displacement of Scots pine, but this can be partially mitigated by maintaining a more intense land use. Scots pine is currently declining in most inner alpine sectors of southern Europe. The relative contribution of climate, land use change, and disturbances on the decline is poorly understood. What will be the future distribution of the species? Is vegetation shifting toward oak-dominated forests? What is the role of extreme drought years? The aims of the study were to determine drivers of current distribution of Scots pine and downy oak in Aosta valley (SW Alps), to extrapolate species distribution models to year 2080 (Special Report on Emissions Scenarios (SRES) A1B), and to assess the correlation between pine vitality after the extreme droughts of 2003 and 2006, and modeled longterm vegetation changes. Ensemble distribution models were created using climate, topography, soil, competition, natural disturbances, and land use. Species presence was derived from a regional forest inventory. Pine response to drought of 2003–2006 was assessed by Normalized Difference Vegetation Index (NDVI) differencing and correlated to modeled cover change between 2080 and present. Scots pine and downy oak were more likely to occur under higher climatic aridity. Scots pine was also associated to higher wildfire frequency, land use intensity, and lack of competition. In a warming scenario, pine experienced an elevational displacement. This was partially counteracted if no land abandonment was hypothesized. Downy oak cover increased in all scenarios. Short- and long-term drought responses of pine were unrelated. Warming will induce an upward displacement of pine, but this can be partially mitigated by maintaining a more intense land use. The drought-induced decline in pine vitality after extreme years did not overlap to the modeled species response under climate warming; responses to short-term drought must be more thoroughly understood in order to predict community shifts.

Long-term environmental change and geographical patterns of violence in Darfur, 2003–2005

This paper investigates spatial associations between environmental change and violence in Darfur. Long-term variations in the geographical distribution of water and vegetative resources can foster migration from areas with decreasing levels of resource availability to areas with increasing levels. Rising ethnic diversity and resource competition can, in turn, escalate the risk of violence in areas of high in-migration. This paper employs a multimethod approach to investigate this hypothesis. Qualitative evidence is used to demonstrate the plausibility of the argument for the case of Darfur. The quantitative analysis is based on information retrieved from satellite imagery on long-term vegetation change and the spatial distribution of attacks on villages in the early phase of the civil war (2003–2005). The findings indicate that violence has been more likely and intense in areas that experienced increasing availability of water and vegetative resources during the 20 years prior to the civil war.

Satellite-indicated long-term vegetation changes and their drivers on the Mongolian Plateau

The Mongolian Plateau, comprising the nation of Mongolia and the Inner Mongolia Autonomous Region of China, has been influenced by significant climatic changes and intensive human activities. Previous satellite-based analyses have suggested an increasing tendency in the vegetation cover over recent decades. However, several ground-based observations have indicated a decline in vegetation production. This study aimed to explore long-term changes in vegetation greenness and land surface phenology in relation to changes in temperature and precipitation on the Plateau between 1982 and 2011 using the normalized difference vegetation index (NDVI). Across the Plateau, a significantly positive trend in the growing season (May–September) NDVI was observed from 1982 to 1998, but since that time, the NDVI has not shown a persistent increase, thus causing an insignificant trend over the entire study period. For the steppe vegetation (a major vegetation type on the Plateau), the NDVI increased significantly in spring but decreased in summer. Precipitation was the dominant factor related to changes in steppe vegetation. Warming in spring contributed to earlier vegetation green-up only in meadow steppe vegetation, implying that water deficiency in typical and desert steppe vegetation may eliminate the effect of warming. Our results also suggest a combined effect of climatic and non-climatic factors and highlight the need to examine the role of regional human activities in the control of vegetation dynamics.

Monitoring Cumulative Long-Term Vegetation Changes Over the Athabasca Oil Sands Region

This study uses two remotely sensed vegetation indices to investigate cumulative long-term changes of undisturbed vegetation in the Athabasca Oil Sands region of Alberta, Canada, between 1984 and 2012. The Normalized Difference Vegetation Index (NDVI) and the Normalized Difference Wetness Index (NDWI) were derived from both Landsat and MODIS time series, for comparative purposes and to increase confidence in detected trends. Trend analysis of undisturbed forest areas, i.e., area without abrupt changes revealed a consistent decrease in vegetation condition, quantified by an average reduction of 18.6% (SD=5.02% ) in NDVI and of 31.0% (SD=10.06%) in NDWI, over the 28-year period. The study does not conclusively associate the trends with any single stressor, but seeks to quantify the spatial and temporal distribution of cumulative effects resulting from a variety of natural and anthropogenic causes. Examination of the temporal pattern of trends showed an increase in the occurrence of decreasing trends in the last 10 years. The decreasing trends were more frequent closer to mining developments for both the Landsat and MODIS time series. Climate change was not considered a major causal factor as climate normalized trends had little effect on the results. The trend analysis undertaken can be used to enhance in situ monitoring programs for site selection of additional monitoring facilities particularly regarding potential cumulative effects, provide an indication of likely future short-term changes in the region, and to aid in the development of mitigation measures.

Response of Bird Populations To Long-Term Changes In Local Vegetation and Regional Forest Cover

We analyzed data from a woodland site for a 59-year period to determine whether changes in bird populations are related to changes in the diversity and relative abundance of woody plant species even when vegetation structure, degree of forest fragmentation in the surrounding landscape, and regional changes in bird populations are taken into account. Principal component analyses generated vegetation factors encompassing variables such as total basal area, shrub density, basal area of common tree species, and measures of tree and shrub species diversity. We also calculated a forest edge/forest area index based on GIS analysis of the landscape within 2 km of the study site. Poisson regression models revealed relationships between these covariates and population changes for 19 bird species and for seven groups of species characterized by similar migration strategies or habitat requirements. All groups of habitat specialists showed a positive relationship with the first vegetation factor, which indicates that they declined as total basal area and dominance of oaks and maples increased and as tree and shrub diversity decreased. This suggests that floristic diversity may be important for determining habitat quality. Bird species associated with the shrub layer and with hemlock stands showed positive relationships with the second vegetation factor, suggesting that the recent decline in eastern hemlocks (Tsuga canadensis) because of hemlock woolly adelgid (Adelges tsugae) had an adverse impact on these species. Forest migrants, shrub-layer specialists, long-distance migrants and permanent residents showed negative relationships with the forest edge/forest interior index, indicating that conservation efforts to protect bird communities should take the wider landscape into account. The strongest relationship for most species and species groups was with the first vegetation factor, which suggests that species composition and diversity of trees and shrubs may be especially important in determining abundance of many forest bird species.

Closing the gap between plant ecology and Quaternary palaeoecology.

Ecology and Quaternary palaeoecology have largely developed as parallel disciplines. Although both pursue related questions, information exchange is often hampered by particularities of the palaeoecological data and a communicational gap has been perceived between the disciplines. Based on selected topics and developments mainly in Quaternary palaeoecology, we show that both disciplines have converged somewhat during recent years, while we still see untapped potential for closer interactions. Macroecology is probably the discipline that most easily combines different time-scales and where co-operations between palaeoecologists, geneticists and vegetation modellers have been inspiring. Quantitative vegetation reconstructions provide robust estimates of tree composition and land cover at different spatial scales, suitable for testing hypotheses about long-term vegetation changes or as quantitative background data in studies on contemporary vegetation patterns. Palaeo-data also hold yet unexplored potential to study the drivers of long-term diversity and aspects of functional diversity may facilitate comparisons between continents and over glacial-interglacial cycles.

Long-term changes in calcareous grassland vegetation in North-western Germany – No decline in species richness, but a shift in species composition

We aimed to answer the question of whether the species richness and composition of calcareous grasslands in North-western Germany had changed over the last 70years as a result of atmospheric nitrogen (N) deposition. In total, 1186 plots of Festuco-Brometea (alliance Bromion erecti) grasslands from the sub-oceanic regions of the country were compiled (1061 plots from literature sources spanning a time period from 1936 to 1996, 125 new plots from 2008). Environmental descriptors recorded for each plot included geographic coordinates, altitude, heat index (combining slope and aspect), mean Ellenberg indicator values for light, soil moisture, soil pH and soil N, and cumulative N deposition (the latter being highly positively correlated with the year of sampling).In a Detrended Correspondence Analysis, the sample plot scores along axis one were highly correlated with the mean Ellenberg N-values, those along axis two were significantly affected by the year of sampling. In a general linear model, species richness of vascular plants showed a markedly hump-shaped relationship with mean Ellenberg N-value, whereas it was weakly affected by year (cumulative N load). Species with a significant negative trend over time were more often (than expected by chance) habitat specialists of dry grasslands, small, light-demanding and winter-green or evergreen with smaller seeds and scleromorphic leaves. In contrast to what has been found for acidic grasslands, N deposition in calcareous grasslands did not result in a decline in species richness, most likely because calcareous grasslands are water- and phosphorus-limited, and are well-buffered in terms of soil pH. To prevent a further change in species composition towards more mesophytic communities, grassland management by the site managers needs to be intensified.

Beavers (Castor fiber and Castor canadensis), the founders of habitats and phytophages

A review of the literature on the influence of beavers on the environment has been presented with regard to following aspects: (1) specific features of the ecology of beavers crucial for understanding their effects on the environment: (2) changes in the physical characteristics of habitats due to the activity of beavers (beavers as engineers); (3) the role of the beaver as a phytophage; (4) long-term changes of vegetation in beavers’ habitats and the possible consequences of these changes for beavers.