Woody encroachment slows decomposition and termite activity in an African savanna

Woody encroachment can lead to a complete switch from open habitats to dense thickets, and has the potential to greatly alter the biodiversity and ecological functioning of grassy ecosystems across the globe. Plant litter decomposition is a critical ecosystem process fundamental to nutrient cycling and global carbon dynamics, yet little is known about how woody encroachment might alter this process. We compared grass decay rates of heavily encroached areas with adjacent nonencroached open areas in a semi‐arid South African savanna using litterbags that allowed or excluded invertebrates. We also assessed the effect of woody encroachment on the activity of termites— dominant decomposer organisms in savanna systems. We found a significant reduction in decomposition rates within encroached areas, with litter taking twice as long to decay compared with open savanna areas. Moreover, invertebrates were more influential on grass decomposition in open areas and termite activity was substantially lower in encroached areas, particularly during the dry season when activity levels were reduced to almost zero. Our results suggest that woody encroachment created an unfavourable environment for invertebrates, and termites in particular, leading to decreased decomposition rates in these areas. We provide the first quantification of woody encroachment altering the functioning of African savanna ecosystems through the slowing of aboveground plant decomposition. Woody encroachment is intensifying across the globe, and our results suggest that substantial changes to the carbon balance and biodiversity of grassy biomes could occur.     

Multi‐scale patterns and bush encroachment in an arid savanna with a shallow soil layer

Abstract. Question: Bush encroachment (i.e. an increase in density of woody plants often unpalatable to domestic livestock) is a serious problem in many savannas and threatens the livelihood of many pastoralists. Can we derive a better understanding of the factors causing bush encroachment by investigating the scale dependency of patterns and processes in savannas? Location: An arid savanna in the Khomas Hochland, Namibia. Methods: Patterns of bush, grass, and soil nutrient distribution were surveyed on several scales along a rainfall gradient, with emphasis on intraspecific interactions within the dominant woody species, Acacia reficiens. Results: Savannas can be interpreted as patch‐dynamic systems where landscapes are composed of many patches (a few ha in size) in different states of transition between grassy and woody dominance. Conclusions: In arid savannas, this patchiness is driven both by rainfall that is highly variable in space and time and by inter‐tree competition. Within the paradigm of patch‐dynamic savannas, bush encroachment is part of a cyclical succession between open savanna and woody dominance. The conversion from a patch of open savanna to a bush‐encroached area is initiated by the spatial and temporal overlap of several (localized) rainfall events sufficient for Acacia germination and establishment. With time, growth and self‐thinning will transform the bush‐encroached area into a mature Acacia stand and eventually into open savanna again. Patchiness is sustained due to the local rarity (and patchiness) of rainfall sufficient for germination of woody plants as well as by plant‐soil interactions.     

Managing Semi-Arid Rangelands for Carbon Storage: Grazing and Woody Encroachment Effects on Soil Carbon and Nitrogen

High grazing intensity and wide-spread woody encroachment may strongly alter soil carbon (C) and nitrogen (N) pools. However, the direction and quantity of these changes have rarely been quantified in East African savanna ecosystem. As shifts in soil C and N pools might further potentially influence climate change mitigation, we quantified and compared soil organic carbon (SOC) and total soil nitrogen (TSN) content in enclosures and communal grazing lands across varying woody cover i.e. woody encroachment levels. Estimated mean SOC and TSN stocks at 0–40 cm depth varied across grazing regimes and among woody encroachment levels. The open grazing land at the heavily encroached site on sandy loam soil contained the least SOC (30 ± 2.1 Mg ha^-1) and TSN (5 ± 0.57 Mg ha^-1) while the enclosure at the least encroached site on sandy clay soil had the greatest mean SOC (81.0 ± 10.6 Mg ha^-1) and TSN (9.2 ± 1.48 Mg ha^-1). Soil OC and TSN did not differ with grazing exclusion at heavily encroached sites, but were twice as high inside enclosure compared to open grazing soils at low encroached sites. Mean SOC and TSN in soils of 0–20 cm depth were up to 120% higher than that of the 21–40 cm soil layer. Soil OC was positively related to TSN, cation exchange capacity (CEC), but negatively related to sand content. Our results show that soil OC and TSN stocks are affected by grazing, but the magnitude is largely influenced by woody encroachment and soil texture. We suggest that improving the herbaceous layer cover through a reduction in grazing and woody encroachment restriction are the key strategies for reducing SOC and TSN losses and, hence, for climate change mitigation in semi-arid rangelands.     

Soil phosphorus does not keep pace with soil carbon and nitrogen accumulation following woody encroachment

Soil carbon, nitrogen, and phosphorus cycles are strongly interlinked and controlled through biological processes, and the phosphorus cycle is further controlled through geochemical processes. In dryland ecosystems, woody encroachment often modifies soil carbon, nitrogen, and phosphorus stores, although it remains unknown if these three elements change proportionally in response to this vegetation change. We evaluated proportional changes and spatial patterns of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) concentrations following woody encroachment by taking spatially explicit soil cores to a depth of 1.2 m across a subtropical savanna landscape which has undergone encroachment by Prosopis glandulosa (an N₂ fixer) and other woody species during the past century in southern Texas, USA. SOC and TN were coupled with respect to increasing magnitudes and spatial patterns throughout the soil profile following woody encroachment, while TP increased slower than SOC and TN in topmost surface soils (0–5 cm) but faster in subsurface soils (15–120 cm). Spatial patterns of TP strongly resembled those of vegetation cover throughout the soil profile, but differed from those of SOC and TN, especially in subsurface soils. The encroachment of woody species dominated by N₂‐fixing trees into this P‐limited ecosystem resulted in the accumulation of proportionally less soil P compared to C and N in surface soils; however, proportionally more P accrued in deeper portions of the soil profile beneath woody patches where alkaline soil pH and high carbonate concentrations would favor precipitation of P as relatively insoluble calcium phosphates. This imbalanced relationship highlights that the relative importance of biotic vs. abiotic mechanisms controlling C and N vs. P accumulation following vegetation change may vary with depth. Our findings suggest that efforts to incorporate effects of land cover changes into coupled climate–biogeochemical models should attempt to represent C‐N‐P imbalances that may arise following vegetation change.     

Woody encroachment reduces nutrient limitation and promotes soil carbon sequestration

During the past century, the biomass of woody species has increased in many grassland and savanna ecosystems. As many of these species fix nitrogen symbiotically, they may alter not only soil nitrogen (N) conditions but also those of phosphorus (P). We studied the N‐fixing shrub Dichrostachys cinerea in a mesic savanna in Zambia, quantifying its effects upon pools of soil N, P, and carbon (C), and availabilities of N and P. We also evaluated whether these effects induced feedbacks upon the growth of understory vegetation and encroaching shrubs. Dichrostachys cinerea shrubs increased total N and P pools, as well as resin‐adsorbed N and soil extractable P in the top 10‐cm soil. Shrubs and understory grasses differed in their foliar N and P concentrations along gradients of increasing encroachment, suggesting that they obtained these nutrients in different ways. Thus, grasses probably obtained them mainly from the surface upper soil layers, whereas the shrubs may acquire N through symbiotic fixation and probably obtain some of their P from deeper soil layers. The storage of soil C increased significantly under D. cinerea and was apparently not limited by shortages of either N or P. We conclude that the shrub D. cinerea does not create a negative feedback loop by inducing P‐limiting conditions, probably because it can obtain P from deeper soil layers. Furthermore, C sequestration is not limited by a shortage of N, so that mesic savanna encroached by this species could represent a C sink for several decades.     

Vegetation dynamics in a Quercus‐Juniperus savanna: An isotopic assessment

Abstract. Woody plants are increasing in many grassland and savanna ecosystems around the world. As a case in point, the Edwards Plateau of Texas, USA, is a vast region (93 000 km²) in which rapid woody encroachment appears to be occurring. The native vegetation (prior to the Anglo‐European settlement 150–200 yr ago) and the biogeochemical consequences of woody encroachment in this region, however, are poorly understood. To assess these matters we measured plant and soil δ¹³C, soil organic C and soil N content from grasslands and two important woody patch types (mature Quercus virginiana clusters and Juniperus ashei woodlands) in this region. Soil δ¹³C values showed that relative productivity of C₃ species has increased in grassland and both woody habitats in recent times. δ¹³C of SOC in grasslands and Q. virginiana clusters increased with depth from the litter layer to 30 cm (grasslands =?21 to ?13‰Q. virginiana clusters =?27 to ?17‰) and were significantly different between habitats at all depths, indicating that Q. virginiana has been a long‐term component of the landscape. In J. ashei woodlands, soil δ¹³C values (at 20–30 cm depth) near the woodland edge (‐13‰) converged with those of an adjacent grassland (‐13‰) while those from the woodland interior (‐15‰) remained distinct, indicating that the woodland has been present for many years but has recently expanded. Concentrations and densities of SOC and total N were generally greater in woody patches than in grasslands. However, differences in the amount of SOC and N stored beneath the two woody patch types indicates that C and N sequestration potentials are species dependent.     

Ecosystem responses to woody plant encroachment in a semiarid savanna rangeland

Woody plant encroachment alters the structure and function of rangeland ecosystems. The objective of this study was to explore the association between woody plant encroachment and various ecosystem properties (i.e. vascular plant species diversity, richness, evenness, soil organic matter, herbaceous biomass, leaf litter and bare ground cover) in a semiarid savanna rangeland, and also to test whether the relationships were influenced by woody species composition, elevation and site. We carried out a vegetation survey in four rangeland sites in the lower Omo region of southwestern Ethiopia, and regressed each one of the ecosystem properties, separately, against woody plant density, elevation and site using multiple linear regressions. We found that vascular plant species diversity, richness and evenness increased with woody plant density, most likely due to increased spatial heterogeneity and soil microclimate improvement. Bare ground cover increased significantly, whereas herbaceous biomass and soil organic matter did not respond to woody encroachment. In a subsequent investigation, we used a redundancy analysis to assess whether ecosystem properties were influenced by the identity of encroaching woody plant species. Species diversity and richness responded positively to Lannea triphylla, whereas leaf litter responded positively to Grewia tenax and G. villosa. Our findings suggest that woody plant encroachment in a semiarid rangeland does alter ecosystem properties. However, its impact is highly variable, influenced by a set of factors including the level of encroachment and identity of encroaching woody species.     

Effects of Herbivory,Fire and N<Subscript>2</Subscript>-fixation on Nutrient Limitation in a Humid African Savanna

The quantities and spatial distribution of nutrients in savanna ecosystems are affected by many factors, of which fire, herbivory and symbiotic N₂-fixation are particularly important. We measured soil nitrogen (N) pools and the relative abundance of N and phosphorus (P) in herbaceous vegetation in five vegetation types in a humid savanna in Tanzania. We also performed a factorial fertilization experiment to investigate which nutrients most limit herbaceous production. N pools in the top 10 cm of soil were low at sites where fires were frequent, and higher in areas with woody legume encroachment, or high herbivore excretion. Biomass production was co-limited by N and P at sites that were frequently burnt or heavily grazed by native herbivores. In contrast, aboveground production was limited by N in areas receiving large amounts of excreta from livestock. N₂-fixation by woody legumes did not lead to P-limitation, but did increase the availability of N relative to P. We conclude that the effects of fire, herbivory and N₂-fixation upon soil N pools and N:P-stoichiometry in savanna ecosystems are, to a large extent, predictable. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Author Contributions  P.C., H.O.V. and P.E. designed the study and wrote the paper. P.C. and T.K. performed the research and analyzed the data.     

Impacts of a mineral lick‐centred land use system on woody vegetation cover in an East African Savannah

Evaluation of woody vegetation changes with distance from a salt crater was conducted in the semi‐arid rangelands of southern Ethiopia. Data on live woody plants were collected over three seasons at 0, 1, 4, 6, 9 and 12 km from the salt crater. The density and diversity of woody plants differed significantly (p < .01) along the distance gradient. Six woody plant families were identified of which Fabaceae and Burseraceae were the dominant families. Acacia drepanolobium, Acacia nilotica, Commiphora africana and Acacia mellifera were among the severely encroaching woody species. There were high proportions of seedlings and saplings recorded closer to the salt crater showing a vigorous recruitment by woody plants. Woody plant encroachment along the 12‐km transect ranged from a low to severe encroachment, which could be translated into poor rangeland condition. Changes in soil characteristics increased grazing pressure and sedentary settlement around the salt crater, and the breakdown of traditional institutions seems to be major contributing factors to these vegetation changes. We suggest that severely encroached areas could be improved through a combination of methods such as bush clearing, prescribed fire, browsing animals and proper grazing management.     

Vegetation,fire and soil feedbacks of dynamic boundaries between rainforest,savanna and grassland

At fine spatial scales, savanna‐rainforest‐grassland boundary dynamics are thought to be mediated by the interplay between fire, vegetation and soil feedbacks. These processes were investigated by quantifying tree species composition, the light environment, quantities and flammability of fuels, bark thickness, and soil conditions across stable and dynamic rainforest boundaries that adjoin grassland and eucalypt savanna in the highlands of the Bunya Mountains, southeast Queensland, Australia. The size class distribution of savanna and rainforest stems was indicative of the encroachment of rainforest species into savanna and grassland. Increasing dominance of rainforest trees corresponds to an increase in woody canopy cover, the dominance of litter fuels (woody debris and leaf), and decline in grass occurrence. There is marked difference in litter and grass fuel flammability and this result is largely an influence of strongly dissimilar fuel bulk densities. Relative bark thickness, a measure of stem fire resistance, was found to be generally greater in savanna species when compared to that of rainforest species, with notable exceptions being the conifers Araucaria bidwillii and Araucaria cunninghamii. A transect study of soil nutrients across one dynamic rainforest – grassland boundary indicated the mass of carbon and nitrogen, but not phosphorus, increased across the successional gradient. Soil carbon turnover time is shortest in stable rainforest, intermediate in dynamic rainforest and longest in grassland highlighting nutrient cycling differentiation. We conclude that the general absence of fire in the Bunya Mountains, due to a divergence from traditional Aboriginal burning practices, has allowed for the encroachment of fire‐sensitive rainforest species into the flammable biomes of this landscape. Rainforest invasion is likely to have reduced fire risk via changes to fuel composition and microclimatic conditions, and this feedback will be reinforced by altered nutrient cycling. The mechanics of the feedbacks here identified are discussed in terms of landscape change theory.     

The ecological impact of bush encroachment on the yield of grasses in Borana rangeland ecosystem

The ecological impact of woody encroachment and the responses of herbage yield to encroachment were assessed at three locations in Borana rangeland at the end of the growing season. The study was carried out in two communal grazing areas (Medhecho and Dubluk) and one Government ranch (Dida‐Tuyura) in bush and/or shrub encroached and non‐encroached sites. In each area, three altitude ranges were distinguished and in each altitude range one transect, covering both encroached and non‐encroached rangeland, was selected. The assessment was based on the yield and botanical composition of the herbaceous layer. The grasses Cenchrus ciliaris, Chrysopogon aucheri and Panicum coloratum were common or dominant in both encroached and non‐encroached sites. Pennisetum mezianum and Pennise‐tum stramineum were typically found in encroached vegetation. The relative yield increased with non‐encroached sites and varied at different altitude ranges from about 106% to about 150%, thus increases ranged from 75% in Medhecho to 350% in Dubluk as determined from the lower values of the ranges. The encroached vegetation had a significantly lower score for herbage yield than the non‐encroached vegetation for most of the sites, although the differences were small. Differences based on altitude range were also significant for Eragrostis papposa and Pennisetum stramineum, while the three areas showed a significant difference for the mean yield of Aristida adscensionis, Cenchrus ciliaris and Eragrostis papposa.     

Bush encroachment under three contrasting land-use practices in a mesic South African savanna

This study determined the effects of land-use practice had on the rate and extent of bush encroachment in a mesic savanna in KwaZulu-Natal, South Africa. Changes in woody cover were measured for 1 km² sites in areas under communal, commercial and conservation land-use systems for the period between 1937 and 2000. Land users from each area were interviewed to gain the histories of each area and to determine how the changes in woody cover had impacted them and whether anything was being done to counteract the spread of trees and shrubs on their land. Bush encroachment occurred across all three of the land-use types in the 67-year period between 1937 and 2000. The results showed that land-use practice had enormous impacts on the process of bush encroachment. The communal site showed a decrease in grass (21%) and tree (5%) cover and an increase in shrub cover (13%). At the commercial site, there was a considerable decrease in grass cover (46%) and moderate increase in shrub cover (10%) and a massive increase in tree cover (36%). The area under conservation showed a substantial decrease in grass cover (47%), a slight decrease in shrub cover (19%) and a massive increase in tree cover (66%). The perceived causes of these changes were fairly similar amongst the different land users. The changes were mostly not perceived to be a problem for the communal land users. The main advantages mentioned were increased woody resources for building and firewood and increased browse availability. The commercial and conservation land users perceived the changes to have significant negative connotations including the loss of grazing land and biodiversity and secondary invasion of encroached areas by alien plant species. Despite these perceptions, very little has been done to combat bush encroachment in the commercial and conservation land use systems.     

Soil C and N dynamics in a Mediterranean oak woodland with shrub encroachment

Background and aims

Intensive land use has led to degradation and abandonment of Portuguese oak woodlands, and subsequent shrub encroachment may have altered the spatial heterogeneity of soil C and N pools. The aim of this study was to evaluate the effects of shrub invasion on soil C and N dynamics in an oak woodland in Southern Portugal.

Methods

Soil was sampled beneath and outside scattered Quercus suber L. canopies, considering non-encroached areas and areas encroached by shrubs (Cistus ladanifer L. or Cistus salviifolius L.).

Results

The spatial heterogeneity of soil C and N contents was mainly associated with tree presence. Outside tree canopies, the labile C pools were larger (mainly beneath C. ladanifer) and C cycling was faster in encroached areas than in non-encroached areas. Net and gross N mineralization and urease and protease activities were also higher in encroached than in non-encroached areas; however, the metabolic quotient and the Cmicrobial/Corganic ratio were not significantly affected. Beneath the tree canopy, significant effects of encroachment included a small increase in soil labile C and the enzymatic activity beneath C. ladanifer.

Conclusions

The results indicate the potential capacity of shrub encroachment to accumulate soil organic C in the long term. The rate of soil C and N turnover promoted by shrub encroachment may depend on the Cistus species present.     

Trade‐offs between savanna woody plant diversity and carbon storage in the Brazilian Cerrado

Incentivizing carbon storage can be a win‐win pathway to conserving biodiversity and mitigating climate change. In savannas, however, the situation is more complex. Promoting carbon storage through woody encroachment may reduce plant diversity of savanna endemics, even as the diversity of encroaching forest species increases. This trade‐off has important implications for the management of biodiversity and carbon in savanna habitats, but has rarely been evaluated empirically. We quantified the nature of carbon‐diversity relationships in the Brazilian Cerrado by analyzing how woody plant species richness changed with carbon storage in 206 sites across the 2.2 million km² region at two spatial scales. We show that total woody plant species diversity increases with carbon storage, as expected, but that the richness of endemic savanna woody plant species declines with carbon storage both at the local scale, as woody biomass accumulates within plots, and at the landscape scale, as forest replaces savanna. The sharpest trade‐offs between carbon storage and savanna diversity occurred at the early stages of carbon accumulation at the local scale but the final stages of forest encroachment at the landscape scale. Furthermore, the loss of savanna species quickens in the final stages of forest encroachment, and beyond a point, savanna species losses outpace forest species gains with increasing carbon accumulation. Our results suggest that although woody encroachment in savanna ecosystems may provide substantial carbon benefits, it comes at the rapidly accruing cost of woody plant species adapted to the open savanna environment. Moreover, the dependence of carbon‐diversity trade‐offs on the amount of savanna area remaining requires land managers to carefully consider local conditions. Widespread woody encroachment in both Australian and African savannas and grasslands may present similar threats to biodiversity.     

Edge influence on plant litter biomass in forest and savanna in the Brazilian cerrado

Edge influence, characterized by differences in ecosystem characteristics between the edge and the interior of remnants in fragmented landscapes, affects a variety of organisms and ecosystem processes. An important feature that may be affected by edges is the amount of plant litter, which provides important habitat for a large variety of organisms and influences ecological processes such as fire dynamics. We studied edge influence on plant litter and fine woody debris in the cerrado of São Paulo state, south‐eastern Brazil. We collected, sorted, dried and weighed plant litter along 180 m‐long transects perpendicular to three savanna and eleven forest edges adjacent to different anthropogenic land uses, with four to five transect per edge. There tended to be less biomass of the finer portions of fine woody debris at both savanna and forest edges. Graminoid litter at savanna edges was greater than in the corresponding interior areas, whereas other litter portions were either unaffected by edges or did not show consistent patterns in either savanna or forest. Edge influence was usually restricted to the first 20 m from the edge, was not influenced by edge characteristics and exhibited no clear differences between savanna and forest areas. Several mechanisms may have led to the variable patterns observed including variation in the plant community, plant architecture, and invasive species. The edge‐related variation in plant litter may putatively lead to, for example, increased fire frequency and intensity at the savanna edges and altered trophic dynamics at forest edges; the mechanisms and consequences of this edge influence should be addressed in future studies.     

Yellow Bush Lupine Invasion in Northern California Coastal Dunes I. Ecological Impacts and Manual Restoration Techniques

We studied the ecological effects of the invasion of coastal dunes by Lupinus arboreus (yellow bush lupine), an introduced species, and used the results to develop manual restoration techniques on the North Spit of Humboldt Bay. Vegetation and soil data were collected in five vegetation types representing points along a continuum of bush lupine's invasive influence. We collected data on the number and size of shrubs, vegetation cover, and soil nutrients. One set of plots was subjected to two restoration treatments: removal of lupine shrubs only, or removal of all nonnative vegetation and removal of litter and duff. Treatments were repeated annually for four years, and emerging lupine seedlings were monitored for three years. Prior to treatment, ammonium and nitrate were found to increase along the lupine continuum, but organic matter decreased at the extreme lupine end. Yellow bush lupine was not the most significant variable affecting variation in soil nutrients. After four years, nonnative grasses, including Vulpia bromoides, Holcus lanatus (velvet grass), Bromus spp. (brome), and Aira spp. (European hairgrass), were significantly reduced in those restoration plots from which litter and duff was removed. Native species increased significantly in vegetation types that were less influenced by lupine. By the third year, soil variables differed among vegetation types but not by treatment. Bush lupine seedling emergence was higher, however, in plots receiving the litter and duff removal treatment. Based on these results, we conclude that bush lupine invasion results in both direct soil enrichment and indirect enrichment as a result of the associated encroachment of other nonnative species, particularly grasses. Although treatment did not affect soil nutrients during the period of this study, it did reduce establishment of nonnative grasses and recruitment of new bush lupine seedlings. Restoration should therefore include litter and duff removal. In areas that are heavily influenced by lupine and contain few native propagules, revegetation is also required.     

Grass invasion causes rapid increases in ecosystem carbon and nitrogen storage in a semiarid shrubland

Accurately predicting terrestrial carbon (C) and nitrogen (N) storage requires understanding how plant invasions alter cycling and storage. A common, highly successful type of plant invasion occurs when the invasive species is of a distinctly different functional type than the native dominant plant, such as shrub encroachment throughout the western United States and annual grass invasions in Mediterranean shrublands, as studied here. Such invasions can dramatically transform landscapes and have large potential to alter C and N cycling by influencing storage in multiple pools. We used a manipulation of non‐native annual grass litter within a shrub‐dominated habitat in southern California (coastal sage scrub, CSS) to study how grass invasion alters ecosystem C and N storage. We added, removed, or left unchanged grass litter in areas of high and low invasion, then followed soil and vegetation changes. Grass litter greatly increased C and N storage in soil, aboveground native and non‐native biomass. Aboveground litter storage increased due to the greater inputs and slower decomposition of grass litter relative to shrub litter; shading by grass litter further reduced decomposition of both non‐native and native litter, which may be due to reduced photodegradation. Soil C and N pools in areas of high litter increased ～20% relative to low litter areas in the two years following manipulation and were generally sinks for C and N, while areas with low litter were sources. We synthesize our results into a C cycle of invaded and uninvaded areas of CSS and link changes in storage to increases in the soil fungi : bacteria ratio, increased plant inputs, and decreased litter loss. Overall, we show that grasses, especially through their litter, control important abiotic and biotic mechanisms governing C and N storage, with widespread implications for C sequestration and N storage in semiarid systems undergoing grass or shrub invasions.     

Shrub expansion stimulates soil C and N storage along a coastal soil chronosequence

Expansion of woody vegetation in grasslands is a worldwide phenomenon with implications for C and N cycling at local, regional and global scales. Although woody encroachment is often accompanied by increased annual net primary production (ANPP) and increased inputs of litter, mesic ecosystems may become sources for C after woody encroachment because stimulation of soil CO₂ efflux releases stored soil carbon. Our objective was to determine if young, sandy soils on a barrier island became a sink for C after encroachment of the nitrogen‐fixing shrub Morella cerifera, or if associated stimulation of soil CO₂ efflux mitigated increased litterfall. We monitored variations in litterfall in shrub thickets across a chronosequence of shrub expansion and compared those data to previous measurements of ANPP in adjacent grasslands. In the final year, we quantified standing litter C and N pools in shrub thickets and soil organic matter (SOM), soil organic carbon (SOC), soil total nitrogen (TN) and soil CO₂ efflux in shrub thickets and adjacent grasslands. Heavy litterfall resulted in a dense litter layer storing an average of 809 g C m^?2 and 36 g N m^?2. Although soil CO₂ efflux was stimulated by shrub encroachment in younger soils, soil CO₂ efflux did not vary between shrub thickets and grasslands in the oldest soils and increases in CO₂ efflux in shrub thickets did not offset contributions of increased litterfall to SOC. SOC was 3.6–9.8 times higher beneath shrub thickets than in grassland soils and soil TN was 2.5–7.7 times higher under shrub thickets. Accumulation rates of soil and litter C were highest in the youngest thicket at 101 g m^?2 yr^?1 and declined with increasing thicket age. Expansion of shrubs on barrier islands, which have low levels of soil carbon and high potential for ANPP, has the potential to significantly increase ecosystem C sequestration.     

Patch and soil characteristics of pastures with water‐spreading banks and woody encroachment in semi‐arid Australia

Water‐spreading banks are used in semi‐arid areas such as the Cobar pediplain in western New South Wales, Australia to encourage pasture growth, often after removal of woody encroachment. We studied the arrangement of bare inter‐patches and vegetated patches, and associated surface soil variables, in three pastures following installation of water‐spreading banks (2, 15, 38 years ago) and in an area of woody encroachment near Cobar. The aims of the study were as follows: (i) to determine the number and percent area of inter‐patches and vegetated patches, and associated surface soil variables at the three pasture sites and at the woody encroachment site and (ii) by inference, explore effects of establishing water‐spreading banks and pasture following removal of woody encroachment on these factors, to understand the role of water‐spreading banks as a management tool. The percent area of inter‐patches in pasture with 38‐year‐old water‐spreading banks was much lower, and the percent area of medium‐vegetated patches (but not of well‐vegetated patches) was substantially higher, than in the woody encroachment. Differences in soil carbon and nitrogen between the sites were related to their percent areas of inter‐patches and vegetated patches. The results suggest that the mosaic of bare inter‐patches and vegetated patches changes over time after clearing of woody encroachment and establishment of pasture with water‐spreading banks, from many large inter‐patches to a few small inter‐patches, and from small to large medium‐vegetated patches. Water‐spreading banks are a useful management tool in these landscapes because of their benefits for landscape function, that is, bare areas become less connected, the percent area of moderately vegetated patches increases, and soil carbon builds up with time following their installation.     

Cyanobacterial soil crusts and woody shrub canopies in Kalahari rangelands

Intensive grazing of Kalahari rangelands has led to woody plant encroachment, notably of Acacia mellifera and Grewia flava. The mechanisms causing this process, and the ecological stability of woody plant encroached ecosystems, remain uncertain. Past studies suggest that canopy–soil relations may enhance woody plant competitive dominance. This study aims to investigate one element of this ecological change by examining the spatial distribution of cyanobacterial soil crusts in two vegetation sub‐habitats at sites of different disturbance. Crust burial by litter was also assessed to analyse the dynamics of canopy–crust relations. Our results show there is enhanced cyanobacterial crust cover under A. mellifera canopies and that unlike G. flava canopies, the crust cover remains under A. mellifera even at highly disturbed sites. This canopy–crust association suggests A. mellifera encroachment exhibits intrinsic resilience because of the crusts ability to stabilize the soil surface and increase nutrient retention. Crust burial by litter that accumulates under larger woody plants restricts crust development under canopies. Disturbance restricts crust development in plant interspaces and under G. flava. These two mechanisms combine to restrict crust development to an observed 40% threshold, with nonlinear models required to explain spatial patterns of crust dynamics.