Vegetation-climate Feedbacks Research Articles

Dispersal distances and migration rates at the arctic treeline in Siberia – a genetic and simulation-based study

Abstract. A strong temperature increase in the Arctic is expected to lead to latitudinal treeline shift. This tundra–taiga turnover would cause a positive vegetation–climate feedback due to albedo decrease. However, reliable estimates of tree migration rates are currently lacking due to the complex processes involved in forest establishment, which depend strongly on seed dispersal. We aim to fill this gap using LAVESI, an individual-based and spatially explicit Larix vegetation simulator. LAVESI was designed to simulate plots within homogeneous forests. Here, we improve the implementation of the seed dispersal function via field-based investigations. We inferred the effective seed dispersal distances of a typical open-forest stand on the southern Taymyr Peninsula (northern central Siberia) from genetic parentage analysis using eight nuclear microsatellite markers. The parentage analysis gives effective seed dispersal distances (median ∼10 m) close to the seed parents. A comparison between simulated and observed effective seed dispersal distances reveals an overestimation of recruits close to the releasing tree and a shorter dispersal distance generally. We thus adapted our model and used the newly parameterised version to simulate south-to-north transects; a slow-moving treeline front was revealed. The colonisation of the tundra areas was assisted by occasional long-distance seed dispersal events beyond the treeline area. The treeline (∼1 tree ha−1) advanced by ∼1.6 m yr−1, whereas the forest line (∼100 trees ha−1) advanced by only ∼0.6 m yr−1. We conclude that the treeline in northern central Siberia currently lags behind the current strong warming and will continue to lag in the near future.

Disentangling the multi-faceted growth patterns of primary Picea abies forests in the Carpathian arc

A tree’s radial growth sequence can be thought of as an aggregate of different growth components such as age and size limitations, presence or absence of disturbance events, continuous impact of climate variability and variance induced by unknown origin. The potentially very complex growth patterns with prominent temporal and spatial variability imply that our understanding of climate-vegetation feedbacks essentially benefits from the expansion of large tree ring networks into data-poor regions, and our ability to disentangle growth constraints by comparing ring series at multiple scales. In this study, we analyze Central-Eastern Europe’s most substantial assemblage of primary Norway spruce forests found in the Carpathian arc. The vast data set, >10,000 tree-ring series, is stratified along a prominent gradient in climate response space over four separate landscapes. We integrated curve intervention detection and dendroclimatic standardization to decompose tree growth variance into climatic, disturbance and residual components to explore the behavior of the components over increasingly larger spatial hierarchies. We show that the residual variance of unknown origin is the most prominent variance in individual Carpathian spruce trees, but at larger spatial hierarchies, climate variance dominates. The variance induced by climate was further explored with common correlation analyses, growth response to extreme climate years and forward modeling of tree growth to identify leading modes of climate response, and potentially non-linear and mixed climate response patterns. We find that the climatic response of the different forest landscapes overall can be described as an asymptotic response to June and July temperatures, most likely intermixed with influence from winter precipitation. In the collection of landscapes, Southern Romania stands out as being the least temperature sensitive and most likely exhibiting the most complicated mixed temperature and moisture limitation.

Effects of vegetation fraction variation on regional climate simulation over Eastern China

Abstract Vegetation-atmosphere interaction is thought to have significant impacts on modulating regional climate. The main purpose of this paper is to reveal the impact of vegetation change on regional climate simulation over Eastern China. Therefore, we have performed three 30-year simulations from 1982 to 2011 with the WRF model coupled with the Noah land surface model as follows: a control experiment with climatology green vegetation fraction (GVF) (CTL), a sensitive experiment with WRF default climatology GVF (CLMV), and a sensitive experiment with dynamic vegetation (DYNV). The climatology difference between the CTL and CLMV experiment points out that during 1982–2011 over Eastern China, the increase of vegetation causes the decrease of temperature; on the other hand, the increase of temperature often corresponds to the reduction of vegetation. In the meantime, the increase/decrease in GVF is quite consistent with the simulated increase/decrease in precipitation, which is caused by both circulations and water vapor distributions at lower levels. The difference in the temperature trend between the CTL and DYNV experiment is almost opposite to that between the DYNV and GVF trend, which means that the decreased/increased vegetation will increase/decrease the surface air temperature. However, the trend of precipitation variation shows no clear relation with that of GVF. In response to GVF change, the results show that in the vegetation-climate feedback the moisture feedback is dominant over the radiation and momentum feedback. Significant decrease in GVF and LAI is detected across the East China, which leads to the decrease in the latent heat flux and increase in the sensible heat flux. For extremes and heat waves, it is shown that DYNV leads to better stimulation on the maximum of daily maximum temperature (TXx), warm spell days (WSDI), heat wave days (HWNY) and the averaged heat wave duration (HWDU) than CTL does over most of the Southeast China, although it may introduce some new biases in the Eastern coastal regions as well as the Northeastern China.

Correspondence: Uncertainty in Climate-Vegetation Feedbacks on Fire Regimes Challenges Reliable Long-Term Projections of Burnt Area from Correlative Models

Recent studies have explored the use of simple correlative models to project changes in future burnt areas (BAs) around the globe. However, estimates of future fire danger suffer from the critical shortcoming that feedbacks on climate change effects on vegetation are not explicitly included in purely correlative approaches causing potential major unknown biases on BA projections. In a recent application of this approach led by Marco Turco and co-workers in the journal Nature Communications (doi:10.1038/s41467-018-06358-z), a simple correlative model was used to project an increase in future burnt areas for the Mediterranean region. The authors related BAs to regional estimates of cumulative drought surrogates, and later used this relationship to infer changes derived from future climate data. To account for negative climate-vegetation feedback on fire regimes, they used regional variability in the BA–drought relationship. The main assumption behind the approach used was that fire–drought relationships currently measured under a given productivity gradient (i.e., sensitivity of fire activity to dry periods is stronger in cooler/productive sites) can be consistently used to infer new relationships arising in the future. While representing a step forward in acknowledging the pitfalls of current projections of BAs, this short-cut falls short in allowing to account for the key process behind climate–vegetation-fire feedbacks. We argue that a series of mechanisms, ranging from the dynamic nature of fire–drought relationships to the human influences they experience, do not ensure that these relationships are to be maintained in the future with major, overall still unknown, consequences on future fire danger projections. Resolving this challenge will greatly benefit from the development of mechanistic approaches that explicitly consider the processes by which vegetation changes derived from climate influence fire regimes.

Temporal and spatial patterns of mitochondrial haplotype and species distributions in Siberian larches inferred from ancient environmental DNA and modeling

Changes in species’ distributions are classically projected based on their climate envelopes. For Siberian forests, which have a tremendous significance for vegetation-climate feedbacks, this implies future shifts of each of the forest-forming larch (Larix) species to the north-east. However, in addition to abiotic factors, reliable projections must assess the role of historical biogeography and biotic interactions. Here, we use sedimentary ancient DNA and individual-based modelling to investigate the distribution of larch species and mitochondrial haplotypes through space and time across the treeline ecotone on the southern Taymyr peninsula, which at the same time presents a boundary area of two larch species. We find spatial and temporal patterns, which suggest that forest density is the most influential driver determining the precise distribution of species and mitochondrial haplotypes. This suggests a strong influence of competition on the species’ range shifts. These findings imply possible climate change outcomes that are directly opposed to projections based purely on climate envelopes. Investigations of such fine-scale processes of biodiversity change through time are possible using paleoenvironmental DNA, which is available much more readily than visible fossils and can provide information at a level of resolution that is not reached in classical palaeoecology.

Simulated Long-Term Vegetation–Climate Feedbacks in the Tibetan Plateau

The Tibetan Plateau (TP) is an important region of land–atmosphere interactions with high climate variability. In this study, an atmosphere–vegetation interaction model was applied to explore the possible responses of vegetation to climate warming, and to assess the impacts of land cover change on the land surface physical processes across the TP. Results showed that long-term warming over the TP could influence vegetation growth via different mechanisms. Most likely, increased temperature would enhance the physiological activity in most high cold areas on the TP, whereas high temperature would inhibit vegetation growth by increasing respiration in areas with favorable water and temperature conditions. In addition, for areas where the climate is warmer but not wetter, higher temperature could influence photosynthesis via the moisture condition of the vegetation rather than by modulating respiration. Numerical simulations demonstrated that vegetation could control the land surface–atmosphere energy balance effectively. The change of land cover from vegetated land to desert steppe decreased the net radiation absorbed by the surface, weakening the surface thermal effects, and reducing sensible and latent heat fluxes. Furthermore, sensitivity simulations also revealed that global warming would likely accelerate vegetation growth in most areas of the TP, resulting in increased surface heat flux.

Mapping the Leaf Economic Spectrum across West African Tropical Forests Using UAV-Acquired Hyperspectral Imagery

The leaf economic spectrum (LES) describes a set of universal trade-offs between leaf mass per area (LMA), leaf nitrogen (N), leaf phosphorus (P) and leaf photosynthesis that influence patterns of primary productivity and nutrient cycling. Many questions regarding vegetation-climate feedbacks can be addressed with a better understanding of LES traits and their controls. Remote sensing offers enormous potential for generating large-scale LES trait data. Yet so far, canopy studies have been limited to imaging spectrometers onboard aircraft, which are rare, expensive to deploy and lack fine-scale resolution. In this study, we measured VNIR (visible-near infrared (400–1050 nm)) reflectance of individual sun and shade leaves in 7 one-ha tropical forest plots located along a 1200–2000 mm precipitation gradient in West Africa. We collected hyperspectral imaging data from 3 of the 7 plots, using an octocopter-based unmanned aerial vehicle (UAV), mounted with a hyperspectral mapping system (450–950 nm, 9 nm FWHM). Using partial least squares regression (PLSR), we found that the spectra of individual sun leaves demonstrated significant (p < 0.01) correlations with LMA and leaf chemical traits: r2 = 0.42 (LMA), r2 = 0.43 (N), r2 = 0.21 (P), r2 = 0.20 (leaf potassium (K)), r2 = 0.23 (leaf calcium (Ca)) and r2 = 0.14 (leaf magnesium (Mg)). Shade leaf spectra displayed stronger relationships with all leaf traits. At the airborne level, four of the six leaf traits demonstrated weak (p < 0.10) correlations with the UAV-collected spectra of 58 tree crowns: r2 = 0.25 (LMA), r2 = 0.22 (N), r2 = 0.22 (P), and r2 = 0.25 (Ca). From the airborne imaging data, we used LMA, N and P values to map the LES across the three plots, revealing precipitation and substrate as co-dominant drivers of trait distributions and relationships. Positive N-P correlations and LMA-P anticorrelations followed typical LES theory, but we found no classic trade-offs between LMA and N. Overall, this study demonstrates the application of UAVs to generating LES information and advancing the study and monitoring tropical forest functional diversity.

Rapid increase in simulated North Atlantic dust deposition due to fast change of northwest African landscape during the Holocene

Abstract. Marine sediment records from a series of core sites along the northwest African margin show a sudden increase in North Atlantic dust deposition at about 5 ka BP that has been associated with an abrupt end of the African Humid Period (AHP). To assess the causes of the abrupt shift in North Atlantic dust deposition, we explore changes in the Holocene dust cycle and in North African climate and landscape by performing several time slice simulations from 8 ka BP until the preindustrial era. To do this, we use the coupled aerosol–climate model ECHAM6–HAM2 including dynamic vegetation and interactive dust, wherein ocean conditions and lake surface area are prescribed for each time slice. We find a rapid increase in simulated dust deposition between 6 and 4 ka BP that is fairly consistent with the abrupt change in marine sediment records at around 20∘ N close to the northwest African margin. At more northern and more remote cores, a significant change in dust deposition is noticeable roughly between 6 and 2 ka BP in the simulations as well as in the records, but the change is less sharp compared to the near-margin core sites. The rapid change in simulated dust deposition is caused by a rapid increase in simulated dust emissions in the western Sahara, where the main dust sources for dust transport towards the North Atlantic are located. The sudden increase in dust emissions in the western Sahara is according to our simulations a consequence of a fast decline of vegetation cover from 22 to 18∘ N that might occur due to vegetation–climate feedbacks or due to the existence of a precipitation threshold on vegetation growth. Additionally, the prescribed gradual reduction of lake area enforces accelerated dust release as highly productive dust sources are uncovered. Concurrently with the continental drying, surface winds in the western Sahara are accelerated. Changes in the Saharan landscape and dust emissions south of 18∘ N and in the eastern Sahara play a minor role in driving the dynamics of North Atlantic dust deposition at the core sites. Our study identifies spatial and temporal heterogeneity in the transition of the North African landscape. As a consequence, implications from local data records on large-scale climate have to be treated with caution.

Long-term recruitment dynamics of arctic dwarf shrub communities in coastal east Greenland

Warming-induced biological and ecological responses have been reported from high-northern latitude sites, where changes in dwarf shrub communities translate into complex vegetation-climate feedbacks. Most of the available Arctic tree-ring evidence is, however, restricted to a limited number of species and locations. A combination of wood anatomical and ‘dendro’-ecological techniques provides insights into past growth rates, recruitment dynamics and even community assemblages of Arctic vegetation. Here, we use thin sectioning and ring counting of 1432 dwarf shrub samples from eight species and two tundra regions in coastal east Greenland to assess community recruitment history and its relation to climate. Site and species-specific annual stem increments, as well as estimated plant ages, range from 0.013-0.720 mm and from 4 to 204 years, respectively. The mean ring width is 0.086 mm, with a mean age of 50 years. Decadal-scale recruitment dynamics of the studied vegetation cover respond to Greenlandic summer temperature variations back to the late 19th century (r = 0.7; 1881–2000).

Noise-induced transitions and shifts in a climate-vegetation feedback model.

Motivated by the extremely important role of the Earth’s vegetation dynamics in climate changes, we study the stochastic variability of a simple climate–vegetation system. In the case of deterministic dynamics, the system has one stable equilibrium and limit cycle or two stable equilibria corresponding to two opposite (cold and warm) climate–vegetation states. These states are divided by a separatrix going across a point of unstable equilibrium. Some possible stochastic scenarios caused by different externally induced natural and anthropogenic processes inherit properties of deterministic behaviour and drastically change the system dynamics. We demonstrate that the system transitions across its separatrix occur with increasing noise intensity. The climate–vegetation system therewith fluctuates, transits and localizes in the vicinity of its attractor. We show that this phenomenon occurs within some critical range of noise intensities. A noise-induced shift into the range of smaller global average temperatures corresponding to substantial oscillations of the Earth’s vegetation cover is revealed. Our analysis demonstrates that the climate–vegetation interactions essentially contribute to climate dynamics and should be taken into account in more precise and complex models of climate variability.

High gene flow and complex treeline dynamics of Larix Mill. stands on the Taymyr Peninsula (north-central Siberia) revealed by nuclear microsatellites

Arctic treelines are facing a strong temperature increase as a result of recent global warming, causing possible changes in forest extent, which will alter vegetation-climate feedbacks. However, the mode and strength of the response is rather unclear, as potential changes are happening in areas that are very remote and difficult to access, and empirical data are still largely lacking. Here, we assessed the current population structure and genetic differentiation of Larix Mill. tree stands within the northernmost latitudinal treeline reaching ~ 72° N in the southern lowlands of the Taymyr Peninsula (~ 100° E). We sampled 743 individuals belonging to different height classes (seedlings, saplings, trees) at 11 locations along a gradient from ‘single tree’ tundra over ‘forest line’ to ‘dense forest’ stands and conducted investigations applying eight highly polymorphic nuclear microsatellites. Results suggest a high diversity within sub-populations (HE = 0.826–0.893), coupled, however, with heterozygote deficits in all sub-populations, but pronounced in ‘forest line’ stands. Overall, genetic differentiation of sub-populations is low (FST = 0.005), indicating a region-wide high gene flow, although ‘forest line’ stands harbour few rare and private alleles, likely indicating greater local reproduction. ‘Single tree’ stands, located beyond the northern forest line, are currently not involved in treeline expansion, but show signs of a long-term refuge, namely asexual reproduction and change of growth-form from erect to creeping growth, possibly having persisted for thousands of years. The lack of differentiation between the sub-populations points to a sufficiently high dispersal potential, and thus a rapid northward migration of the Siberian arctic treeline under recent global warming seems potentially unconstrained, but observations show it to be unexpectedly slow.

Towards a more detailed representation of high-latitude vegetation in the global land surface model ORCHIDEE (ORC-HL-VEGv1.0)

Abstract. Simulation of vegetation–climate feedbacks in high latitudes in the ORCHIDEE land surface model was improved by the addition of three new circumpolar plant functional types (PFTs), namely non-vascular plants representing bryophytes and lichens, Arctic shrubs and Arctic C3 grasses. Non-vascular plants are assigned no stomatal conductance, very shallow roots, and can desiccate during dry episodes and become active again during wet periods, which gives them a larger phenological plasticity (i.e. adaptability and resilience to severe climatic constraints) compared to grasses and shrubs. Shrubs have a specific carbon allocation scheme, and differ from trees by their larger survival rates in winter, due to protection by snow. Arctic C3 grasses have the same equations as in the original ORCHIDEE version, but different parameter values, optimised from in situ observations of biomass and net primary productivity (NPP) in Siberia. In situ observations of living biomass and productivity from Siberia were used to calibrate the parameters of the new PFTs using a Bayesian optimisation procedure. With the new PFTs, we obtain a lower NPP by 31 % (from 55° N), as well as a lower roughness length (−41 %), transpiration (−33 %) and a higher winter albedo (by +3.6 %) due to increased snow cover. A simulation of the water balance and runoff and drainage in the high northern latitudes using the new PFTs results in an increase of fresh water discharge in the Arctic ocean by 11 % (+140 km3 yr−1), owing to less evapotranspiration. Future developments should focus on the competition between these three PFTs and boreal tree PFTs, in order to simulate their area changes in response to climate change, and the effect of carbon–nitrogen interactions.

Dissimilar responses of larch stands in northern Siberia to increasing temperatures-a field and simulation based study.

Arctic and alpine treelines worldwide differ in their reactions to climate change. A northward advance of or densification within the treeline ecotone will likely influence climate-vegetation feedback mechanisms. In our study, which was conducted in the Taimyr Depression in the North Siberian Lowlands, w present a combined field- and model-based approach helping us to better understand the population processes involved in the responses of the whole treeline ecotone, spanning from closed forest to single-tree tundra, to climate warming. Using information on stand structure, tree age, and seed quality and quantity from seven sites, we investigate effects of intra-specific competition and seed availability on the specific impact of recent climate warming on larch stands. Field data show that tree density is highest in the forest-tundra, and average tree size decreases from closed forest to single-tree tundra. Age-structure analyses indicate that the trees in the closed forest and forest-tundra have been present for at least ~240yr. At all sites except the most southerly ones, past establishment is positively correlated with regional temperature increase. In the single-tree tundra, however, a change in growth form from krummholz to erect trees, beginning ~130yr ago, rather than establishment date has been recorded. Seed mass decreases from south to north, while seed quantity increases. Simulations with LAVESI (Larix Vegetation Simulator) further suggest that relative density changes strongly in response to a warming signal in the forest-tundra while intra-specific competition limits densification in the closed forest and seed limitation hinders densification in the single-tree tundra. We find striking differences in strength and timing of responses to recent climate warming. While forest-tundra stands recently densified, recruitment is almost non-existent at the southern and northern end of the ecotone due to autecological processes. Palaeo-treelines may therefore be inappropriate to infer past temperature changes at a fine scale. Moreover, a lagged treeline response to past warming will, via feedback mechanisms, influence climate change in the future.

East African weathering dynamics controlled by vegetation-climate feedbacks

Tropical weathering has important linkages to global biogeochemistry and landscape evolution in the East African rift. We disentangle the influences of climate and terrestrial vegetation on chemical weathering intensity and erosion at Lake Malawi using a long sediment record. Fossil pollen, microcharcoal, particle size, and mineralogy data affirm that the detrital clays accumulating in deep water within the lake are controlled by feedbacks between climate and hinterland forest composition. Particle-size patterns are also best explained by vegetation, through feedbacks with lake levels, wildfires, and erosion. We develop a new source-to-sink framework that links lacustrine sedimentation to hinterland vegetation in tropical rifts. Our analysis suggests that climate-vegetation interactions and their coupling to weathering/erosion could threaten future food security and has implications for accurately predicting petroleum play elements in continental rift basins.

Dynamic Carboniferous tropical forests: new views of plant function and potential for physiological forcing of climate.

Contents 1333 I. 1334 II. 1335 III. 1339 IV. 1344 V. 1347 VI. 1347 1348 1348 References 1348 SUMMARY: The Carboniferous, the time of Earth's penultimate icehouse and widespread coal formation, was dominated by extinct lineages of early-diverging vascular plants. Studies of nearest living relatives of key Carboniferous plants suggest that their physiologies and growth forms differed substantially from most types of modern vegetation, particularly forests. It remains a matter of debate precisely how differently and to what degree these long-extinct plants influenced the environment. Integrating biophysical analysis of stomatal and vascular conductivity with geochemical analysis of fossilized tissues and process-based ecosystem-scale modeling yields a dynamic and unique perspective on these paleoforests. This integrated approach indicates that key Carboniferous plants were capable of growth and transpiration rates that approach values found in extant crown-group angiosperms, differing greatly from comparatively modest rates found in their closest living relatives. Ecosystem modeling suggests that divergent stomatal conductance, leaf sizes and stem life span between dominant clades would have shifted the balance of soil-atmosphere water fluxes, and thus surface runoff flux, during repeated, climate-driven, vegetation turnovers. This synthesis highlights the importance of 'whole plant' physiological reconstruction of extinct plants and the potential of vascular plants to have influenced the Earth system hundreds of millions of years ago through vegetation-climate feedbacks.

Intensity of African Humid Periods Estimated from Saharan Dust Fluxes.

North Africa experienced dramatic changes in hydrology and vegetation during the late Quaternary driven by insolation-induced shifts of the tropical rain belt and further modulated by millennial-scale droughts and vegetation-climate feedbacks. While most past proxy and modelling studies concentrated on the temporal and spatial dynamics of the last African humid period, little is known about the intensities and characteristics of pre-Holocene humid periods. Here we present a high-resolution record of fine-grained eastern Saharan dust from the Eastern Mediterranean Sea spanning the last 180 kyr, which is based on the clay mineral composition of the marine sediments, especially the kaolinite/chlorite ratio. Minimum aeolian kaolinite transport occurred during the African Humid Periods because kaolinite deflation was hampered by increased humidity and vegetation cover. Instead, kaolinite weathering from kaolinite-bearing Cenozoic rocks was stored in lake basins, river beds and soils during these periods. During the subsequent dry phases, fine-grained dust was mobilised from the desiccated lakes, rivers and soils resulting in maximum aeolian uptake and transport of kaolinite. The kaolinite transport decreased again when these sediment sources exhausted. We conclude that the amount of clay-sized dust blown out of the Sahara into the Eastern Mediterranean Sea is proportional to the intensity of the kaolinite weathering and accumulation in soils and lake sediments, and thus to the strength of the preceding humid period. These humid periods provided the windows for the migration of modern humans out of Africa, as postulated previously. The strongest humid period occurred during the Eemian and was followed by two weaker phases centred at ca. 100 ka and ca. 80 ka.

Climate–vegetation–fire interactions and feedbacks: trivial detail or major barrier to projecting the future of the Earth system?

Fire is a complex process involving interactions and feedbacks between biological, socioeconomic, and physical drivers across multiple spatial and temporal scales. This complexity limits our ability to incorporate fire into Earth system models and project future fire activity under climate change. Conceptual, empirical, and process models have identified the mechanisms and processes driving fire regimes, and provide a useful basis to consider future fire activity. However, these models generally deal with only one component of fire regimes, fire frequency, and do not incorporate feedbacks between fire, vegetation, and climate. They are thus unable to predict the location, severity or timing of fires, the socioecological impacts of fire regime change, or potential non‐linear responses such as biome shifts into alternative stable states. Dynamic modeling experiments may facilitate more thorough investigations of fire–vegetation–climate feedbacks and interactions, but their success will depend on the development of dynamic global vegetation models (DGVMs) that more accurately represent biological drivers. This requires improvements in the representation of current vegetation, plant responses to fire, ecological dynamics, and land management to capture the mechanisms behind fire frequency, intensity, and timing. DGVMs with fire modules are promising tools to develop a globally consistent analysis of fire activity, but projecting future fire activity will ultimately require a transdisciplinary synthesis of the biological, atmospheric, and socioeconomic drivers of fire. This is an important goal because fire causes substantial economic disruption and contributes to future climate change through its influence on albedo and the capacity of the biosphere to store carbon. WIREs Clim Change 2016, 7:910–931. doi: 10.1002/wcc.428This article is categorized under: Climate Models and Modeling > Model Components Assessing Impacts of Climate Change > Evaluating Future Impacts of Climate Change Climate, Ecology, and Conservation > Observed Ecological Changes

Vegetation–climate feedbacks modulate rainfall patterns in Africa under future climate change

Abstract. Africa has been undergoing significant changes in climate and vegetation in recent decades, and continued changes may be expected over this century. Vegetation cover and composition impose important influences on the regional climate in Africa. Climate-driven changes in vegetation structure and the distribution of forests versus savannah and grassland may feed back to climate via shifts in the surface energy balance, hydrological cycle and resultant effects on surface pressure and larger-scale atmospheric circulation. We used a regional Earth system model incorporating interactive vegetation–atmosphere coupling to investigate the potential role of vegetation-mediated biophysical feedbacks on climate dynamics in Africa in an RCP8.5-based future climate scenario. The model was applied at high resolution (0.44 × 0.44°) for the CORDEX-Africa domain with boundary conditions from the CanESM2 general circulation model. We found that increased tree cover and leaf-area index (LAI) associated with a CO2 and climate-driven increase in net primary productivity, particularly over subtropical savannah areas, not only imposed important local effect on the regional climate by altering surface energy fluxes but also resulted in remote effects over central Africa by modulating the land–ocean temperature contrast, Atlantic Walker circulation and moisture inflow feeding the central African tropical rainforest region with precipitation. The vegetation-mediated feedbacks were in general negative with respect to temperature, dampening the warming trend simulated in the absence of feedbacks, and positive with respect to precipitation, enhancing rainfall reduction over the rainforest areas. Our results highlight the importance of accounting for vegetation–atmosphere interactions in climate projections for tropical and subtropical Africa.

Effects of changes in vegetation on precipitation in the northern Tianshan Mountains evaluated using multiple time scales

This study used a combination of the wavelet cross-correlation technique and numerical analysis of vegetative feedback to study the role of climate–vegetation feedback from 1981 to 2009 in the northern Tianshan Mountains, Xinjiang Province, China. The study area included the Irtysh River, the Bortala and Ili River valleys, the northern slopes of the Tianshan Mountains, and the western Junggar Basin. The feedback effects of changes in vegetation on precipitation appeared to vary in these five regions when different time scales are used to examine them. The most useful time scale was generally found to be 4–6 months. Time lag was another characteristic of this process, and the optimal time lag was 3–4 months. Nevertheless, optimal time scale and time lag did not differ significantly in these five regions. In this way, the correct time scale of the effects of variations in vegetation on precipitation in this cold, arid area was found. This time scale and time lag can be assessed through wavelet cross-correlation analysis. Then numerical analysis can be used to improve the accuracy of the analysis.

Dispersal limitation drives successional pathways in Central Siberian forests under current and intensified fire regimes.

Fire is a primary driver of boreal forest dynamics. Intensifying fire regimes due to climate change may cause a shift in boreal forest composition toward reduced dominance of conifers and greater abundance of deciduous hardwoods, with potential biogeochemical and biophysical feedbacks to regional and global climate. This shift has already been observed in some North American boreal forests and has been attributed to changes in site conditions. However, it is unknown if the mechanisms controlling fire-induced changes in deciduous hardwood cover are similar among different boreal forests, which differ in the ecological traits of the dominant tree species. To better understand the consequences of intensifying fire regimes in boreal forests, we studied postfire regeneration in five burns in the Central Siberian dark taiga, a vast but poorly studied boreal region. We combined field measurements, dendrochronological analysis, and seed-source maps derived from high-resolution satellite images to quantify the importance of site conditions (e.g., organic layer depth) vs. seed availability in shaping postfire regeneration. We show that dispersal limitation of evergreen conifers was the main factor determining postfire regeneration composition and density. Site conditions had significant but weaker effects. We used information on postfire regeneration to develop a classification scheme for successional pathways, representing the dominance of deciduous hardwoods vs. evergreen conifers at different successional stages. We estimated the spatial distribution of different successional pathways under alternative fire regime scenarios. Under intensified fire regimes, dispersal limitation of evergreen conifers is predicted to become more severe, primarily due to reduced abundance of surviving seed sources within burned areas. Increased dispersal limitation of evergreen conifers, in turn, is predicted to increase the prevalence of successional pathways dominated by deciduous hardwoods. The likely fire-induced shift toward greater deciduous hardwood cover may affect climate-vegetation feedbacks via surface albedo, Bowen ratio, and carbon cycling.