Characteristics of hollows and hollow‐bearing trees in semi‐arid river red gum woodland and potential limitations for hollow‐dependent wildlife

Up to 37 species of the birds and microbats inhabiting inland Australia are dependent on tree cavities for breeding or roosting. The river red gum (Eucalyptus camaldulensis), a well‐known hollow‐bearing tree species, occurs in linear semi‐arid woodland along thousands of kilometres of ephemeral river channels and is the only tree species that provides widespread, aggregated hollow resources across a landscape otherwise dominated by shrublands. Here we assess the type and quantity of hollows available along ephemeral rivers of the MacDonnell Ranges bioregion in central Australia and determine which characteristics of river red gums best predict the abundance and characteristics of different tree hollows, as first steps towards assessing the current availability of hollows in the region. Approximately a third of all river red gums sampled were hollow‐bearing, but individual trees with abundant hollows were rare. Further, 36% of hollows had an entrance ≤ 5 cm, and 37% had entrances which were 6–10 cm in diameter, whereas only 13% of hollows had an entrance diameter > 20 cm suitable for larger hollow‐using species. Large and high hollows only occurred on trees that did not display post‐disturbance resprouting. Trees with multiple and diverse hollows were rare and tended to be in advanced stages of senescence and had larger stems (82.3 ± 3.33 cm) and were taller (14.4 ± 0.53 m) compared to non‐hollow‐bearing trees (23.44 ± 1.68 cm, 8.0 ± 0.34 m). Further research is required to establish whether the current abundance of hollows and diversity of hollow types are limiting to cavity‐dependent wildlife, and to identify any threats to availability of hollows.     

Fourteen months of seed rain in three Australian semi‐arid communities

The abundance and composition of seed rain was measured over 14 months (February 2004 to March 2005) in Currawinya National Park, western Queensland. The experimental design included four measurement periods, three vegetation communities and two grazing regimes. A total of 12 586 seeds from 104 species were captured. There were significantly more seeds and species captured during the measurement period with the least rainfall, although no significant correlation was found between the amount of rainfall and the number of seeds or species captured. More seeds and species were captured where native and feral grazing pressure was removed, but this was only significant for the number of species. The above‐ground vegetation showed no significant difference between grazing treatments over the study period and exhibited far fewer species than the seed rain. However, the majority of species found in the above‐ground vegetation were represented in the seed rain. Hypotheses are explored as an attempt to understand the apparent lack of a relationship between seed rain and rainfall. The effects of grazing and seed movement and storage are also discussed.     

Mistletoe (Tapinanthus bangwensis Reichenbach) infestation of indigenous and non‐indigenous trees at Amani Nature Reserve,Tanzania

A survey of Tapinanthus bangwensis was carried out in a plantation in the botanic garden at Amani Nature Reserve, which forms part of the East Usambara Mountain, located in the north‐east of Tanzania. A total of 169 trees were examined on four transects of 100 × 50 m (5000 m²) each, located about 700 m apart. Three treatment transects were established in open woodland with a control transect in the closed canopy. Out of 101 trees (81 non‐indigenous and 20 indigenous) examined in the open canopy, T. bangwensis was present on 24; eighteen non‐indigenous and six indigenous. Of all the infested trees, a non‐indigenous species, silky oak (Grevillea robusta) was the most common (37.4%), followed by an indigenous species, mzindanguruwe (Blighia unijugata) with 16.6%. Chi‐squared tests showed that there was no significant difference in frequency of infestation between non‐indigenous and indigenous species (χ² = 0.715, P = 0.5826). t‐Testing showed that T. bangwensis preferred taller and larger trees (t = ? 3.930, P = 0.0002 and t = ? 2.416, P = 0.0175, respectively). No T. bangwensis was found on the 68 trees examined in the closed canopy.     

Rapid warming accelerates tree growth decline in semi‐arid forests of Inner Asia

Forests around the world are subject to risk of high rates of tree growth decline and increased tree mortality from combinations of climate warming and drought, notably in semi‐arid settings. Here, we assess how climate warming has affected tree growth in one of the world's most extensive zones of semi‐arid forests, in Inner Asia, a region where lack of data limits our understanding of how climate change may impact forests. We show that pervasive tree growth declines since 1994 in Inner Asia have been confined to semi‐arid forests, where growing season water stress has been rising due to warming‐induced increases in atmospheric moisture demand. A causal link between increasing drought and declining growth at semi‐arid sites is corroborated by correlation analyses comparing annual climate data to records of tree‐ring widths. These ring‐width records tend to be substantially more sensitive to drought variability at semi‐arid sites than at semi‐humid sites. Fire occurrence and insect/pathogen attacks have increased in tandem with the most recent (2007–2009) documented episode of tree mortality. If warming in Inner Asia continues, further increases in forest stress and tree mortality could be expected, potentially driving the eventual regional loss of current semi‐arid forests.     

Impacts of altered precipitation regimes on soil communities and biogeochemistry in arid and semi‐arid ecosystems

Altered precipitation patterns resulting from climate change will have particularly significant consequences in water‐limited ecosystems, such as arid to semi‐arid ecosystems, where discontinuous inputs of water control biological processes. Given that these ecosystems cover more than a third of Earth's terrestrial surface, it is important to understand how they respond to such alterations. Altered water availability may impact both aboveground and belowground communities and the interactions between these, with potential impacts on ecosystem functioning; however, most studies to date have focused exclusively on vegetation responses to altered precipitation regimes. To synthesize our understanding of potential climate change impacts on dryland ecosystems, we present here a review of current literature that reports the effects of precipitation events and altered precipitation regimes on belowground biota and biogeochemical cycling. Increased precipitation generally increases microbial biomass and fungal:bacterial ratio. Few studies report responses to reduced precipitation but the effects likely counter those of increased precipitation. Altered precipitation regimes have also been found to alter microbial community composition but broader generalizations are difficult to make. Changes in event size and frequency influences invertebrate activity and density with cascading impacts on the soil food web, which will likely impact carbon and nutrient pools. The long‐term implications for biogeochemical cycling are inconclusive but several studies suggest that increased aridity may cause decoupling of carbon and nutrient cycling. We propose a new conceptual framework that incorporates hierarchical biotic responses to individual precipitation events more explicitly, including moderation of microbial activity and biomass by invertebrate grazing, and use this framework to make some predictions on impacts of altered precipitation regimes in terms of event size and frequency as well as mean annual precipitation. While our understanding of dryland ecosystems is improving, there is still a great need for longer term in situ manipulations of precipitation regime to test our model.     

Carbon cycle responses of semi‐arid ecosystems to positive asymmetry in rainfall

Recent evidence shows that warm semi‐arid ecosystems are playing a disproportionate role in the interannual variability and greening trend of the global carbon cycle given their mean lower productivity when compared with other biomes (Ahlström et al. 2015 Science, 348, 895). Using multiple observations (land‐atmosphere fluxes, biomass, streamflow and remotely sensed vegetation cover) and two state‐of‐the‐art biospheric models, we show that climate variability and extremes lead to positive or negative responses in the biosphere, depending on vegetation type. We find Australia to be a global hot spot for variability, with semi‐arid ecosystems in that country exhibiting increased carbon uptake due to both asymmetry in the interannual distribution of rainfall (extrinsic forcing), and asymmetry in the response of gross primary production (GPP) to rainfall change (intrinsic response). The latter is attributable to the pulse‐response behaviour of the drought‐adapted biota of these systems, a response that is estimated to be as much as half of that from the CO₂ fertilization effect during 1990–2013. Mesic ecosystems, lacking drought‐adapted species, did not show an intrinsic asymmetric response. Our findings suggest that a future more variable climate will induce large but contrasting ecosystem responses, differing among biomes globally, independent of changes in mean precipitation alone. The most significant changes are occurring in the extensive arid and semi‐arid regions, and we suggest that the reported increased carbon uptake in response to asymmetric responses might be contributing to the observed greening trends there.     

Species limits and phylogenetic relationships of red‐finned cryptic species of the seasonal killifish genus Hypsolebias from the Brazilian semi‐arid Caatinga (Teleostei: Cyprinodontiformes: Rivulidae)

Members of the Hypsolebias antenori species group comprise a diverse clade of morphologically similar seasonal killifishes occurring in a vast region of the semi‐arid savannah of northeastern Brazil. The present paper focuses on an assemblage of three allopatric cryptic species (H. antenori from isolated coastal river drainages, Hypsolebias igneus from the São Francisco River basin and Hypsolebias coamazonicus sp. nov. from the Parnaíba River basin) sharing almost identical colour patterns, including the presence of an orangish red anal fin in males, thus herein named as the red‐finned assemblage. A tree‐based approach using mt‐DNA (cytochrome b) supports delimitation of all three species, but indicates that the red‐finned assemblage is paraphyletic – H. igneus and H. coamazonicus are closely related to Hypsolebias nudiorbitatus, whereas H. antenori is the sister group to a clade comprising all 13 species of the H. antenori group included in the analysis. Morphological characters are useful to diagnose species, but are not informative for most clades highly supported by molecular data. H. coamazonicus is distinguished from all other congeners by the possession of a dark grey or black stripe on the dorsal fin in males. The basal position of H. antenori is related to uplift episodes involving the Araripe‐Borborema plateau during the Miocene, which isolated the coastal area inhabited by H. antenori from the remaining areas of the Caatinga. The sister group relationship between H. igneus and H. coamazonicus is attributed to a past connection between the São Francisco and Paranaíba River until the Tertiary.     

Carbon sequestration and soil restoration potential of grazing lands under exclosure management in a semi‐arid environment of northern Ethiopia

Exclosures are used to regenerate native vegetation as a way to reduce soil erosion, increase rain water infiltration and provide fodder and woody biomass in degraded grazing lands. Therefore, this study assessed the impact of grazing exclosure on carbon sequestration and soil nutrients under 5 and 10 years of grazing exclosures and freely grazed areas in Tigray, northern Ethiopia. Carbon stocks and soil nutrients increased with increasing grazing exclusion. However, open grazing lands and 5 years of grazing exclosure did not differ in above‐ and belowground carbon stocks. Moreover, 10 years of grazing exclosure had a higher (p < 0.01) grass, herb and litter carbon stocks compared to 5 years exclosure and open grazing lands. The total carbon stock was higher for 10 years exclosure (75.65 t C ha^‐1) than the 5 years exclosure (55.06 t C ha^‐1) and in open grazing areas (51.98 t C ha^‐1). Grazing lands closed for 10 years had a higher SOC, organic matter, total N, available P, and exchangeable K + and Na + compared to 5 year's exclosure and open grazing lands. Therefore, establishment of grazing exclosures had a positive effect in restoring degraded grazing lands, thus improving carbon sequestration potentials and soil nutrients.     

Influence of Colophospermum mopane canopy cover on litter decomposition and nutrient dynamics in a semi‐arid African savannah

The objective of this study was to investigate the influence of mopane canopy cover on litter decomposition in a semi‐arid African savannah. We used a randomized block design with five blocks of 100 × 100 m demarcated in a 10‐ha pocket of open mopane woodland. Litterbags were placed beneath large (8.3 m crown diameter) and small mopane trees (2.7 m crown diameter) and in the intercanopy area. Decomposition was fastest in the intercanopy area exposed to solar radiation (k = 0.35 year^?1), intermediate beneath small trees (k = 0.28 year^?1) and slowest beneath large trees (k = 0.23 year^?1). Soil temperatures beneath small and large trees were 3–5 and 6–9°C lower than in the intercanopy area, respectively. Bacterial and fungal counts were significantly higher (P < 0.05) beneath large than small trees and in the intercanopy area. The amount of N and P released did not vary significantly among sampling sites. Soil moisture in the dry season was similar among sampling sites but rainy‐season soil moisture was significantly greater (P < 0.05) beneath large than small trees and in the intecanopy area. Mopane canopy cover retarded litter decomposition suggesting that photodegradation could be an important factor controlling carbon turnover in semi‐arid African savannahs.     

Experimental changes in disturbance type do not induce short‐term shifts in plant community structure in three semi‐arid grasslands of the Victorian Riverine Plain managed for nature conservation

Summary Since the mid 1990s, there has been a significant increase in the area of semi‐arid grasslands included in the National Reserve Systems in the Victorian Riverine Plain. This expansion has not been matched by an improved understanding of the alternate disturbance regimes that might produce better outcomes for native ecosystem conservation. Over the past 150 years, stock grazing has completely replaced fire in these grasslands. As a result, the impact of fire on native (and exotic) plant biodiversity is little understood. This study compared the current grazing regime (i.e. ‘status quo’) with burning and the removal of grazing (‘deferred’ management) across three grasslands in the Victorian Riverine Plain to determine the effects of short‐term exposure to alternate disturbances on community structure. Our results showed little change in species density, composition or abundance under the three disturbance treatments. A long exposure to stock grazing may have reduced the abundance of species likely to respond positively to burning. The cover of the biological soil crust responded positively to fire; such changes are known to significantly influence establishment and the functional composition of communities. As such, further investigation of the functional attributes of these communities may broaden our understanding of short‐term responses to alternate disturbance events. To better understand the utility of fire as a management tool, a long‐term commitment to expanding the implementation of this regime from its current extent will greatly increase the understanding of alternate disturbances in this landscape.     

Effect of cropping systems on cereal stemborers in the cool‐wet and semi‐arid ecozones of the Amhara region of Ethiopia

1 Field experiments were conducted on maize and sorghum at three locations in the Amhara state of Ethiopia to determine the effects of mixed cropping on stemborer infestation, borer natural enemies and grain yields. In the cool‐wet ecozone of western Amhara, sole maize was compared with maize intercropped with faba bean, mustard, potatoes and cowpea. In the semi‐arid ecozone of eastern Amhara, the trial was conducted on both maize and sorghum with the companion crops haricot bean, sesame, cowpea and sweet potatoes. 2 The results showed that the predominant borer species in western and eastern Amhara were, respectively, Busseola fusca and Chilo partellus. In Addis Zemen, western Amhara, maize intercropped with mustard and potatoes had significantly lower pest numbers and percent tunnelling than other intercrops and the maize monocrop during the vegetative stage. In eastern Amhara, the cropping system did not significantly affect pest densities but damage to stem, ear or heads tended to be greatest when cereals were intercropped with sweet potatoes. 3 Parasitism of C. partellus by the braconid Cotesia flavipes was greater on maize than sorghum, and on maize it was greater with sweet potatoes than in other intercrops or sole maize. Cocoon mass number per plant did not vary significantly between treatments. 4 There were significant differences between treatments in yields of both sorghum and maize (per plant and per unit area) with the lowest yields observed when they were intercropped with a tuber crop. 5 The results suggest that simultaneous planting of the crop species selected has little advantage over monocropped maize.     

Context‐dependent interactions between adult shrubs and seedlings in a semi‐arid shrubland

Abstract. Question: In semi‐arid systems, rainfall gradients can cause plant‐plant interactions to shift from negative to positive or vice versa. However, the importance of a second major abiotic factor, soil nutrients, has rarely been considered. We consider different combinations of both factors and ask: do net adult‐seedling interactions become less competitive and more facilitative with increasing overall abiotic harshness? Location: Succulent Karoo, Western Cape, South Africa. Methods: We examined the interactions between seedlings and adult shrubs at two sites. Sites differ in rainfall, and each contain two habitats: Nutrient‐rich mounds associated with underground termitaria and a relatively nutrient‐poor matrix. We carried out a spatial pattern analysis of community‐wide seedling‐adult associations. We then conducted field and greenhouse experiments to test the effects of soil and the presence of neighbouring shrubs on the growth and survival of six seedling species. Results: At the higher rainfall site, both competitive and facilitative effects of adults on seedlings were found but did not differ by habitat, despite the more benign conditions in the mound habitat. At the lower rainfall site, adult shrubs generally had neutral effects on seedlings in the matrix habitat. In the nutrient‐rich mound habitat, however, adult shrubs had strong and consistently competitive effects on seedlings. Conclusion: Seedling‐adult interactions could not be predicted by a simple overall gradient of abiotic harshness, demonstrating the need for more complex, mechanistic models to predict plant‐plant interactions. We suggest that rainfall and soil nutrients affect seedling‐adult relations through their interactive effects on the life‐history attributes of the species involved.     

Range‐wide genetic structure of a cooperative mouse in a semi‐arid zone: Evidence for panmixia

Landscape topography and the mobility of individuals will have fundamental impacts on a species’ population structure, for example by enhancing or reducing gene flow and therefore influencing the effective size and genetic diversity of the population. However, social organization will also influence population genetic structure. For example, species that live and breed in cooperative groups may experience high levels of inbreeding and strong genetic drift. The western pebble‐mound mouse (Pseudomys chapmani), which occupies a highly heterogeneous, semi‐arid landscape in Australia, is an enigmatic social mammal that has the intriguing behaviour of working cooperatively in groups to build permanent pebble mounds above a subterranean burrow system. Here, we used both nuclear (microsatellite) and mitochondrial (mtDNA) markers to analyse the range‐wide population structure of western pebble‐mound mice sourced from multiple social groups. We observed high levels of genetic diversity at the broad scale, very weak genetic differentiation at a finer scale and low levels of inbreeding. Our genetic analyses suggest that the western pebble‐mound mouse population is both panmictic and highly viable. We conclude that high genetic connectivity across the complex landscape is a consequence of the species’ ability to permeate their environment, which may be enhanced by “boom‐bust” population dynamics driven by the semi‐arid climate. More broadly, our results highlight the importance of sampling strategies to infer social structure and demonstrate that sociality is an important component of population genetic structure.     

Germination sensitivities to water potential among co‐existing C3 and C4 grasses of cool semi‐arid prairie grasslands

An untested theory states that C₄ grass seeds could germinate under lower water potentials (Ψ) than C₃ grass seeds. We used hydrotime modelling to study seed water relations of C₄ and C₃ Canadian prairie grasses to address Ψ divergent sensitivities and germination strategies along a risk‐spreading continuum of responses to limited water. C₄ grasses were Bouteloua gracilis, Calamovilfa longifolia and Schizachyrium scoparium; C₃ grasses were Bromus carinatus, Elymus trachycaulus, Festuca hallii and Koeleria macrantha. Hydrotime parameters were obtained after incubation of non‐dormant seeds under different Ψ PEG 6000 solutions. A t‐test between C₃ and C₄ grasses did not find statistical differences in population mean base Ψ (Ψ_b(50)). We found idiosyncratic responses of C₄ grasses along the risk‐spreading continuum. B. gracilis showed a risk‐taker strategy of a species able to quickly germinate in a dry soil due to its low Ψ_b(50) and hydrotime (θ_H). The high Ψ_b(50) of S. scoparium indicates it follows the risk‐averse strategy so it can only germinate in wet soils. C. longifolia showed an intermediate strategy: the lowest Ψ_b(50) yet the highest θ_H. K. macrantha, a C₃ grass which thrives in dry habitats, had the highest Ψ_b(50), suggesting a risk‐averse strategy for a C₃ species. Other C₃ species showed intermediate germination patterns in response to Ψ relative to C₄ species. Our results indicate that grasses display germination sensitivities to Ψ across the risk‐spreading continuum of responses. Thus seed water relations may be poor predictors to explain differential recruitment and distribution of C₃ and C₄ grasses in the Canadian prairies.     

Big‐sized trees overrule remaining trees' attributes and species richness as determinants of aboveground biomass in tropical forests

Large‐diameter, tall‐stature, and big‐crown trees are the main stand structures of forests, generally contributing a large fraction of aboveground biomass, and hence play an important role in climate change mitigation strategies. Here, we hypothesized that the effects of large‐diameter, tall‐stature, and big‐crown trees overrule the effects of species richness and remaining trees attributes on aboveground biomass in tropical forests (i.e., we term the “big‐sized trees hypothesis”). Specifically, we assessed the importance of: (a) the “top 1% big‐sized trees effect” relative to species richness; (b) the “99% remaining trees effect” relative to species richness; and (c) the “top 1% big‐sized trees effect” relative to the “99% remaining trees effect” and species richness on aboveground biomass. Using environmental factor and forest inventory datasets from 712 tropical forest plots in Hainan Island of southern China, we tested several structural equation models for disentangling the relative effects of big‐sized trees, remaining trees attributes, and species richness on aboveground biomass, while considering for the full (indirect effects only) and partial (direct and indirect effects) mediation effects of climatic and soil conditions, as well as interactions between species richness and trees attributes. We found that top 1% big‐sized trees attributes strongly increased aboveground biomass (i.e., explained 55%–70% of the accounted variation) compared to species richness (2%–18%) and 99% remaining trees attributes (6%–10%). In addition, species richness increased aboveground biomass indirectly via increasing big‐sized trees but via decreasing remaining trees. Hence, we show that the “big‐sized trees effect” overrides the effects of remaining trees attributes and species richness on aboveground biomass in tropical forests. This study also indicates that big‐sized trees may be more susceptible to atmospheric drought. We argue that the effects of big‐sized trees on species richness and aboveground biomass should be tested for better understanding of the ecological mechanisms underlying forest functioning.     

Use of environmental predictors for vegetation mapping in semi‐arid mountain rangelands and the determination of conservation hotspots

Question: Can we predict the spatial distribution of plant communities in semi‐arid rangelands based on a limited set of environmental variables? Where are priority areas for conservation located? Location: Al Jabal al Akhdar, Sultanate of Oman. Methods: A Classification Tree Analysis (CTA) was used to model the presence/absence of seven rangeland communities and agricultural areas based on seven selected environmental predictor variables. The latter were either obtained from existing digital datasets or derived from a digital elevation model and satellite images, whereas the grazing intensity was spatially modelled with the kernel density estimation technique. The resulting decision rules of a CTA were applied for predictive mapping within the study area (400 km², resolution of 5 m) by means of ENVI's decision tree classifier. Plant communities of natural rangelands were subsequently evaluated to determine priority areas for nature conservation. Results: Altitude, grazing intensity and landform revealed the highest predictive power. Most of the rangelands were predicted as Sideroxylon–Oleetum. The overall classification accuracy was 89%, whereby agricultural areas and the Ziziphus spina‐christi‐Nerium oleander community at wadi sites had no misclassification. Inaccuracies occurred mainly because of low sample numbers and errors in available maps of predictor variables. The highest rank for nature conservation was observed for the Teucrio‐Juniperetum occupying 20% of the study area. Conclusions: Vegetation mapping using CTA is a valuable tool for rangeland monitoring and identification of key representative areas for nature conservation. An extrapolation of the model used might be feasible to regions adjacent to the central Hajar Mountains.