Predicting aboveground biomass of woody encroacher species in semi-arid rangelands,Ethiopia

Species-specific allometric models were developed to predict aboveground biomass (AGB) of eight woody species in the Borana rangelands, Ethiopia. The 23 equations developed (8 species; three biomass components: total aboveground, stem and branches) fit the data well to predict total AGB and by components for each of the species (r² > 0.70; p < 0.001). The AGB of tree shaped species (e.g., Acacia bussei and Acacia etabaica) were significantly predicted from a single predictor (circumference of the stem at ankle height), with a high coefficient of determination (r² > 0.95; p < 0.001). In contrast, the AGB of bushy shrubs (e.g., Acacia oerfota) was more effectively predicted by using the canopy volume (r² = 0.84; p < 0.001). Shrubs with a tall stem and an umbrella-like canopy structure (e.g., Acacia mellifera) were most accurately predicted by a combination of both circumference of the stem at ankle height and canopy volume (r² = 0.95; p < 0.001). Hence, our species-specific allometric models could accurately estimate their woody aboveground biomass in a semi-arid savanna ecosystem of southern Ethiopia. These equations will help in future carbon-trade discussions in times of climate change and CO₂ emission concerns and mitigation strategies.     

The impact of shade on establishment of shrubs adapted to the high light irradiation of semi-arid environments

Light may be an important limiting resource that influences community structure of chenopod shrublands. As part of a larger study that aimed to determine the factors that influence chenopod community structure, the focus of this study was the influence of plant canopy on the growth and establishment of smaller plants. We therefore measured the height and cover of three chenopods (Enchylaena tomentosa, Maireana brevifolia and Maireana georgei) when growing within and outside of the canopy of Atriplex bunburyana under field conditions. All three chenopods had lower cover and E. tomentosa was taller when growing within the canopy of A. bunburyana in comparison to those growing outside of the canopy. The chenopods were then grown under three artificial shade regimes. Plant height, cover, biomass, relative leaf area and photosynthetic surface area measurements showed that each species responded differently to shade. E. tomentosa biomass was facilitated by shade. This was inferred by an increase in total plant biomass. M. brevifolia, in contrast, tolerated shade by increasing above-ground biomass allocation. M. georgei was adversely affected by the shade regimes: root biomass decreased in response to shade. Competition for light is, therefore, likely to influence chenopod community structure of semi-arid and arid environments.     

Shrubs promote nucleation in the Brazilian semi-arid region

Anthropogenic environmental degradation transforms mature vegetation into sites in succession, and actions to restore these altered environments must be based on ecological theories. Nucleation, promoted by facilitation, is an ecological process that can be applied to the restoration of altered environments. The original vegetation of many semi-arid regions has been profoundly altered, and is difficult to recuperate due to rigorous climates. Observations of secondary succession sites raise the following question: do some semi-arid plant species promote nucleation processes and can they therefore be considered nurse species? To address this question, vegetation surveys were undertaken in different environments: under the canopy of the shrub Combretum leprosum and in adjacent open areas. Shrubs in different stages were classified by canopy size: small, intermediate and large. Diversity and number of seedlings increased as the size of the C. leprosum canopies increased. Some of the environmental variables examined supported the role of C. leprosum as a facilitator species, such as the improvement in soil conditions under its canopy. Thus C. leprosum could be of significant importance in restoring degraded areas of the semi-arid region where it is present, by allowing the establishment of other plant species.     

Regional aboveground biomass equations for North American arid and semi-arid forests

Calculations of dry tree, plot and regional aboveground biomass, M, and assessments of complex diverse forests remain key challenges in the implementation of conventional and sustainable projects. This study reports a set of robust regional equations and the application of several allometric tree and plot M equations in Mexican arid and semi-arid forest communities. The data were collected from 1084 tree M fitting models, and 195 arid and semi-arid and 206 mesquite trees and 55 plot datasets were used to validate the models. Theoretical, semi-empirical and empirical models with exogenous variables, as well as wood specific gravity, diameter, top height, and form-factor values, were used to evaluate aboveground biomass. Empirical computer-based equations projected more consistently similar tree M assessments than did semi-empirical or theoretical models. Therefore, empirical models are recommended for assessments of tree and plot M, and future allometry research should properly address independent variables in more theoretical models. The large sample size, model richness, economic and ecological importance, and wide coverage of this dataset make it useful for estimating several biogeochemical and bioenergy density parameters and contribute to the body of knowledge regarding heterogeneity in dry forest stand structures.     

The impact of African elephants on Acacia tortilis woodland in northern Gonarezhou National Park, Zimbabwe

We investigated the impact of African elephants (Loxodonta africana) on the structure and composition of Acacia tortilis woodland in northern Gonarezhou National Park, southeast Zimbabwe. A. tortilis woodland was stratified into high, medium and low elephant utilisation categories based on evidence of elephant habitat use as determined through dung-count surveys in relation to distance of woodland patches from perennial and natural surface water sources. The following variables were recorded in each study plot: tree height, species name, number of species, plant damage, basal circumference and number of stems per plant. A total of 824 woody plants and 26 woody species were recorded from the sampled A. tortilis woodland patches. Mean tree densities, basal areas, tree heights and species diversity were lower in areas with medium and high elephant utilisation as compared to low elephant utilisation areas. Plants damaged by elephants increased with increasing elephant utilisation. The study findings suggest that A. tortilis woodland is gradually being transformed into an open woodland. We recommended that protected area management in arid and semi-arid areas should consider (i) formulating clear thresholds of potential concern to allow for the conservation of sensitive woodlands such as A. tortilis woodlands and (ii) establishing long-term vegetation monitoring programmes.     

The effects of exclosures in restoring degraded semi-arid vegetation in communal grazing lands in northern Ethiopia

Rangeland degradation is a widespread problem throughout sub Saharan Africa and its restoration is a challenge for the management of many semi-arid areas. This study assessed the effectiveness of exclosures that have been protected from livestock from 5 to 15 years in restoring vegetation in northern Ethiopia. The species composition and diversity of herbaceous and woody plants were higher in the exclosures than in the grazed areas. Species richness responded positively to an increase in herbaceous productivity. The mean aboveground biomass measured inside the exclosures was more than twice that of the adjacent grazed areas and more biomass was produced from the young than the old exclosures. Stem height, canopy height, canopy cover, and browsing capacity of woody species were higher in the exclosures than in the grazed areas. Our study shows that degraded semi-arid vegetation is able to recover in a relatively short time when protected. Extended protection, beyond 8–15 years, reduces herbaceous species diversity and in one of the sites also the herbaceous biomass. Therefore, we suggest a slight shift in management where exclosures protected for longer periods may be moderately used by livestock.     

Spatiotemporal heterogeneity of soil fertility in the Central Monte desert (Argentina)

In arid environments, soil fertility exhibits a high degree of spatial and temporal heterogeneity, which results from high climatic variability seasonally and heterogeneous plant distribution. However, because most desert areas have been altered by human activities, heterogeneous fertility would originate from grazing or logging activities. We evaluated spatial and temporal heterogeneity of soil fertility in cattle-excluded sites under and outside woody plant cover (Prosopis flexuosa and Larrea divaricata), and in sites disturbed by tree removal during wet and dry season in Ñacuñán Biosphere Reserve (Central Monte desert of Argentina). Soil organic matter, fulvic acids, bioavailable organic matter, and nitrate were lower outside plant canopy (8.9 mg g^?1, 0.03 mg g^?1, 8.2 mg g^?1, and 4.17 mg kg^?1, respectively). Total N, humic acids, and abundance of microbial functional groups did not show differences among sites. Most parameters differed between seasons, tending to be higher in the wet season. Overall soils of Ñacuñán Reserve are characterized by: a) more homogenous spatial pattern than expected from woody plant presence; b) very heterogeneous temporal pattern; and c) after two years, tree removal does not seem to induce infertile soil formation.     

Effect of canopy cover and canopy background variables on spectral profiles of savanna rangeland bush encroachment species based on selected Acacia species (mellifera,tortilis, karroo) and Dichrostachys cinerea at Mokopane,South Africa

The proliferation of woody plant species on savanna rangelands (i.e. bush encroachment) degrades rangeland quality, thereby threatening the biodiversity conservation effort as well as pastoral farming. Hyperspectral remote sensing offers possibilities for discriminating encroaching bush species in support of management of semi-arid savanna rangelands. As a preliminary step towards establishing a spectral library of common encroaching species on savanna rangelands, the effect of canopy leaf cover, background dry soil and grass on the spectral profiles of the common encroaching species Acacia karroo, Acacia mellifera, Acacia tortilis and Dichrostachys cinerea was analysed. A sample of healthy mature plants in prime, full leaf condition was utilised at an encroached rangeland in Mokopane, South Africa. The spectral signatures were collected in-situ, using a field spectrometer pointed above the sample specimen canopies. The canopy and canopy background variables tended to modify the reflectance of the encroaching bush species in the near infrared (800–1300 nm) in which they were spectrally most separable. Canopy background dry grass tended to increase near infrared reflectance, while dry soil tended to reduce the spectral contrast among the species. These effects were reduced by high leaf content. In a thicket canopy structure, the overall reflectance tended towards the spectral profile of the more dominant species.     

Three California annual forbs show little response to neighbor removal

In semi-arid systems, competition among plants has limited effects on performance compared to facilitative interactions and abiotic factors. However, the balance between competition and facilitation can change over the course of plant development. Determining when each interaction is important for plant productivity allows us to identify factors limiting plant growth. In this field study, neighbor removal mid-way through the growing season did not result in a size change for any species (Clarkia purpurea, Plantago erecta, and Micropus californicus) in the first year. In the second year, two species had greater biomass when neighbors were removed very early in the growing season, indicating a competitive effect. The third species neither benefited nor suffered from neighbor removal. In some years, P. erecta and M. californicus may be negatively affected by competition at early growth stages and are later unable to capitalize on newly-available resources after removals of competitors. This study shows that early-stage competition can drive biomass production in some annual plants, even in semi-arid systems.     

Quantifying soil salinity in areas invaded by Tamarix spp.

Elevated soil salinity is often associated with Tamarix invasion; however, it's unclear whether soils are more saline because of Tamarix or other environmental factors. Surface soil salinity was investigated along a flow-regulated, arid river with dense Tamarix of varying age to determine which factors best explain soil salinity. Flooding was the most important predictor, reducing salinity by nearly 70%. Soils under Tamarix had lower salinity than adjacent areas without woody cover in non-flooded areas suggesting that evaporation in arid environments may contribute more surface salts than Tamarix or may exacerbate plant inputs. Under most conditions, higher salinities were found under Tamarix than natives. An exception to this pattern was that soils under the smallest trees were more saline for natives. Relationships between soil salinity and stem size suggest that salts increase over time under Tamarix unless they are removed by flooding. However, the most mature stands had lower salinity than expected, reflecting some additional mechanism. Soil texture and distance from the river were important, but interacted strongly with other factors. The observed relationships between surface soil salinity and Tamarix stem size, a predictor of aboveground age, suggest Tamarix plays an active role in floodplain salinization within the sampled area.     

Wood productivity of Prosopis flexuosa D.C. woodlands in the central Monte: Influence of population structure and tree-growth habit

A balance between forest production and protection is hard to achieve in arid zones due to their low potential for wood production. Prosopis flexuosa woodlands are the major woody formations in the Monte desert and are currently in a degraded state due to intense use. The main degradation factors in the study area are overgrazing and firewood extraction. We developed allometric models to estimate the aerial biomass of P. flexuosa, compared annual growth rates of one- and multi-stemmed individuals through dendrochronological methods, and estimated the productivity of four structurally different woodlands in the central Monte. Total dry weight was best estimated by power equations. Annual increments in basal area and dry weight were initially larger for multi- than one-stemmed individuals. However, whereas multi-stemmed individuals rapidly decreased their growth rates after 60 years of age, one-stemmed trees maintained steady growth rates during the first 100 years. Depending on woodland density and tree size, total woodland biomass varied between 4000 and 15 000 kg ha⁻¹. Wood productivity was similar in all four woodlands studied (121.6-173.7 kg ha⁻¹ year⁻¹). Our results reveal the importance of tree growth habit to productivity, and suggest that regulated extraction of firewood and poles from old multi-stemmed individuals could optimize wood productivity and contribute to the sustainable use and conservation of these woodlands.     

Soil hydrological and erosional responses in areas of woody encroachment, pasture and woodland in semi-arid Australia

In arid and semi-arid areas, woody encroachment is the increase in density, cover, extent and/or biomass of woody plants. Woody encroachment is often associated with increased runoff and soil erosion. Hydrological and erosional responses of woody encroachment and of pastures established after management of encroachment in semi-arid Australia are not well understood. This study compared the hydrological and erosional responses across vegetation states comprising woody plant encroachment (>1200 stems ha⁻¹), recently established pastures (<23 years of age), long-established pasture (50-100 years of age) and open woodland (<330 stems ha⁻¹) in semi-arid eastern Australia. Responses were measured using rainfall simulation with intensity of 35 mm h⁻¹ for 30 min applied on 1 -m² plots. Runoff and sediment production did not differ significantly between vegetation states. Average runoff in woody encroachment was 9.0 mm h⁻¹, followed by recent pasture (8.2 mm h⁻¹), long-established pasture (5.9 mm h⁻¹) and open woodland (4.2 mm h⁻¹). Total sediment production in recent pasture was 11.6 g m⁻², followed by woody encroachment (9.0 g m⁻²), long-established pasture (7.3 g m⁻²) and open woodland (4.3 g m⁻²). Runoff and sediment production were significantly lower at one pasture site (0.9 mm h⁻¹ and 1.3 g m⁻²) where rotational grazing and minimum tillage had been implemented than in the adjacent paired woody encroachment site (10.3 mm h⁻¹and 6.5 g m⁻², respectively). This example of a pasture that had been managed to increase ground cover illustrated the effect of pasture management on reducing runoff and sediment production. Across all vegetation states, small scale runoff and sediment production were minimal or zero where total ground cover was 73% or higher.     

Chronic wind and plant communities in dunes: Total biomass,inter-specific competition,and species abundance

Chronic wind is an important ecological factor, but its direct roles in shaping plant communities remain poorly understood. We hypothesized that chronic wind can modulate community productivity, inter-specific competition, and species abundance in inland dunes. We conducted an experiment with three shrubs (Artemisia ordosica, Caragana intermedia, and Hedysarum laeve) common to semi-arid sandlands, set up seven kinds of plant communities (i.e. Artemisia monoculture, Caragana monoculture, Hedysarum monoculture, Artemisia–Caragana mixture, Artemisia–Hedysarum mixture, Caragana–Hedysarum mixture, and Artemisia–Caragana–Hedysarum mixture), and communities subjected to two levels of wind exposure: shielded (by means of fencing) or exposed (no fencing). We measured total biomass per plot, competitive effects, and relative species abundance. Wind exposure did not significantly affect the total biomass of monocultures but increased their root weight ratio. However, wind exposure enhanced the total biomass of three-species mixtures but not two-species mixtures, and had no effects on root weight ratio of all mixtures. Wind exposed condition increased the competitive ability and relative abundance of Artemisia, decreased the competitive ability of Hedysarum but had no effects on its abundance, and did not affect the competitive ability of Caragana but decreased its abundance. These results suggest that chronic wind, as an environmental filter, can directly modulate plant communities through altering competitive outcomes and thus affect community functioning.     

Allometric relationships and peak-season community biomass stocks of native shrubs in Senegal's Peanut Basin

Allometric equations and community biomass stocks are presented for Guiera senegalensis J.F. Gmel (Gs) and Piliostigma reticulatum (DC.) Hochst (Pr) – two native shrub species in the Sahel. These shrubs are of interest because they dominate semi-arid sub-Sahalien Africa but have been largely overlooked as a key biomass component and regulator of ecosystem composition and function in this landscape. In Year 1, best predictors of aboveground biomass were height and number of stems (Gs) and crown diameter (Pr); and for belowground biomass were height and basal diameter (Gs) and basal diameter (Pr). In Year 2, height and crown diameter were the best predictors of aboveground biomass (R² = 0.90 for Gs and 0.87 for Pr), whereas basal diameter and number of stems (Gs) and basal diameter (Pr) were best predictors of belowground biomass. Peak-season biomass estimates ranged from 0.44 to 4.58 ton ha^?1 (mean = 2.38 ton ha^?1) in the Gs sites and from 0.33 to 7.38 ton ha^?1 (mean = 3.71 ton ha^?1) in the Pr communities. Both species exhibited unusually large root:shoot ratios (4.5:1 for Gs and 10.2:1 for Pr). Although models differ between years, allometric relationships provide reasonable biomass estimates for Gs and Pr.     

Effects of the abandonment of the burrowing mounds of fat sand rat (Psammomys obesus cretzschamar 1828) on vegetation and soil surface attributes along the coastal dunes of North Sinai,Egypt

Fat sand rats (Psammomys obesus) are strictly herbivorous and live in densely complex burrows in the desert of North Africa and the Middle East. Little is known however about the effect of their foraging and burrowing activities on surface morphology and plant community attributes. This study evaluated such effects by comparing burrow and mound surface morphology, canopy of the main host chenopod shrub Anabasis articulata, vegetation cover, and plant abundance and species richness on and off both active and abandoned colonies in the semi-stabilized sand dunes of the Northern Sinai. In general, active burrow systems were characterized by reduced A. articulata canopy area, and more soil disturbance, with higher and larger burrow mounds dominated by bare ground, dung, and dead and fresh litter. The abandonment of mounds for five years has resulted in significant increases of plant cover, canopy height, abundance and species richness. Vegetation structure and plant species assemblages differed between mound and non-mound patches of both active and abandoned sites. The results suggest that fat sand rats can have significant direct and indirect, short-term and long-term effects on vegetation dynamics and structure through their mound building and foraging.     

Wood biomass functions for Acacia abyssinica trees and shrubs and implications for provision of ecosystem services in a community managed exclosure in Tigray,Ethiopia

In the Ethiopian highlands, remarkable recovery of vegetation has been achieved using exclosures, protecting vegetation against livestock browsing and firewood harvesting. But these emerging forest resources require tools for sustainable use, implying knowledge on biomass stocks and growth. In this study we developed biomass functions estimating total, stem and branch biomass from diameter at stump height (DSH) and tree height (H) for an 11-year old exclosure in Tigray, Ethiopia. In a systematic grid of 55 plots, DSH and H of all trees and shrubs were recorded. 40 Acacia abyssinica trees were selected for destructive sampling. Allometric relationships using a natural log–log model were established between aboveground biomass, DSH and H. Models with only DSH were found best with R² between 0.95 and 0.98. The functions were 10 fold cross-validated and R²_{_}cv ranged from 0.94 to 0.97, indicating good model performance. The models were found well in range with those of other seasonal forests in East Africa. Total aboveground biomass was estimated 25.4 ton ha⁻¹ with an annual production of 2.3 ton ha⁻¹, allowing sustainable wood fuel use for 4 persons ha⁻¹. The presented predictive functions help to harmonize between ecological and societal objectives and are as such a first step towards an integrated planning tool for exclosures.     

Global greenhouse gas implications of land conversion to biofuel crop cultivation in arid and semi-arid lands – Lessons learned from Jatropha

Biofuels are considered as a climate-friendly energy alternative. However, their environmental sustainability is increasingly debated because of land competition with food production, negative carbon balances and impacts on biodiversity. Arid and semi-arid lands have been proposed as a more sustainable alternative without such impacts. In that context this paper evaluates the carbon balance of potential land conversion to Jatropha cultivation, biofuel production and use in arid and semi-arid areas. This evaluation includes the calculation of carbon debt created by these land conversions and calculation of the minimum Jatropha yield necessary to repay the respective carbon debts within 15 or 30 years.The carbon debts caused by conversion of arid and semi-arid lands to Jatropha vary largely as a function of the biomass carbon stocks of the land use types in these regions. Based on global ecosystem carbon mapping, cultivated lands and marginal areas (sparse shrubs, herbaceous and bare areas) show to have similar biomass carbon stocks (on average 4–8 t C ha⁻¹) and together cover a total of 1.79 billion ha. Conversion of these lands might not cause a carbon debt, but still might have a negative impact on other sustainability dimensions (e.g. biodiversity or socio-economics). Jatropha establishment in shrubland (0.75 billion ha) would cause a carbon debt of 24–28 t C ha⁻¹ on average (repayable within 30 year with yield of 3.5–3.9 t seed ha⁻¹ yr⁻¹). Land use change in the 1.15 billion ha of forested area under arid and semi-arid climates could cause a carbon debt between 70 and 118 t C ha⁻¹. This debt requires 8.6–13.9 t seed production ha⁻¹ yr⁻¹ for repayment within 30 years. If repayment is required within 15 years, the necessary minimum yields almost double. Considering that 5 t seed ha⁻¹ yr⁻¹ is the current maximum Jatropha yield, conversion of forests cannot be repaid within one human generation. Repayment of carbon debt from shrubland conversions in 30 years is challenging, but feasible. Repayment in 15 year is currently not attainable.Based on this analysis the paper discusses the carbon mitigation potential of biofuels in arid and semi-arid environments.     

Light responses of Carex planostachys from various microsites in a Juniperus community

Juniperus communities are found on over 50 × 10⁶ ha in arid and semiarid habitats in southwestern North America. The drought tolerant sedge Carex planostachys occurs below the canopy in some of these communities. Cover and biomass of C. planostachys are high below the canopy and low in associated gaps. The purposes of this study were to investigate the temporal and spatial physiologic response of C. planostachys to abiotic changes, and determine it's light response characteristics from four contiguous microsites. Net photosynthesis was highest in spring when temperature was cooler and soil water higher, but low carbon uptake continued during summer drought. In addition, C. planostachys demonstrates a capacity to recover from extreme drought, despite water potential measured below ?9.0 MPa. Based on physiological light response curves and gas-exchange measurements, C. planostachys appears tolerant of shaded and full sun habitats. Light levels below the canopy were reduced compared to the gaps, but light saturation of C. planostachys did not change and net CO₂ uptake was only reduced slightly. Carbon uptake was coupled to light levels and not soil moisture. Observed differences in physiological attributes and variation in C. planostachys cover and biomass correspond to the presence or absence of the canopy. Low light compensation points, coupled with reduced respiratory demand, maximize photosynthetic gain in low light microsites. C. planostachys appears to acclimate across a range of light regimes, suggesting photosynthetic plasticity, allowing growth and survival in diverse light microhabitats. C. planostachys, tolerant of drought, appears anisohydric and demonstrates a capacity to acclimate to sun and shaded habitats, which could allow it to occur across a wider range of arid areas.     

Carbon pools in an arid shrubland in Chile under natural and afforested conditions

The pattern of carbon (C) allocation among the different pools is an important ecosystem structural feature, which can be modified as a result of changes in environmental conditions that can occur gradually (e.g., climatic change) or abruptly (e.g., management practices). This study quantified the C pools of plant biomass, litter and soil in an arid shrubland in Chile, comparing the natural condition (moderately disturbed by grazing) vs. the afforested condition (two-year-old plantation with Acacia saligna (Labill.) H.L. Wendl.), each represented by a 60 ha plot. To estimate plant biomass, allometric functions were constructed for the four dominant woody species, based on the volume according to their shape, which showed high correlation (R² > 0.73). The soil was the largest C pool in both natural and afforested conditions (89% and 94%, respectively) and was significantly lower in the afforested than natural condition at all five soil depths. The natural condition had in total 36.5 ton (t) C ha⁻¹ compared to 21.1 t C ha⁻¹ in the afforested condition, mainly due to C loss during soil preparation, prior to plantation of A. saligna. These measurements serve as an important baseline to assess long-term effects of afforestation on ecosystem C pools.     

Effects of artificial vegetation arrangement and structure on the colonization and development of biological soil crusts

The colonization and development of biological soil crusts (BSCs) are rarely discussed when investigating vegetation restoration with difference arrangement and structure of anthropogenically damaged areas in semi-arid regions. The present study analyzes the relationships among coverage, height and density of woody vegetation and coverage and thickness of BSCs on the surface mine dumpsite in Heidaigou, China. Results showed that PR (Prunus sibirica L.), PT (Pinus tabulaeformis Carr.) and PPr (P. tabulaeformis Carr., P. sibirica L.) types had the highest coverage of total BSCs, which were 76.8%, 75.9% and 78.9%, respectively and PR showed the thickest BSCs of 4.41 mm. There was a significant correlation between coverage and thickness of BSCs and coverage and height of woody vegetation as a unimodal curve. Our findings suggest that a single woody plant species and low level coverage and height (no more than 30% and 300 cm, respectively) of woody plants may be able to create suitable conditions for facilitating BSCs restoration on the surface of mine dumpsites. The effects of vegetation arrangement and structure on BSCs colonization and development should be considered in reconstructing and managing woody vegetation in disturbed environments, such as surface mine dumpsites in semi-arid areas.