The objectives were to quantify aboveground, belowground and dead wood carbon pools near Mayoko in the Chaillu massif of Republic of Congo and explore relationships between carbon storage and plant ...diversity of all growth forms. A total of 190 plots (25 m by 25 m) were sampled (5072 stems, 211 species) and data analysed using recommended central‐African forest allometric equations. Mean stem diameter at breast height was 33.6 cm, mean basal area 47.7 m2 ha−1 and mean density of individuals 407 ha−1. Mean aboveground carbon (AGC) ranged from 13.93–412.66 Mg C ha−1, belowground carbon from 2.86–96.97 Mg C ha−1 and dead wood from 0.00–7.59 Mg C ha−1. The maximum AGC value recorded in a plot was 916 Mg C ha−1. The analysis performed using phytosociological association as basis rather than broad vegetation type is unique. AGC values for undisturbed terra firme forest sites featured among the highest recorded for African tropical forests. Considering only tree diversity, a weak, yet significant, relationship existed between AGC and species richness, Shannon‐Wiener index of diversity and Fisher's alpha. However, if diversity of all plant growth forms is considered, no relationship between carbon and plant diversity existed.
Résumé
Ces études visaient à quantifier les réservoirs de carbone aérien, souterrain et de bois mort près de Mayoko, dans le massif du Chaillu, en République du Congo, et à explorer les relations entre le stockage du carbone et la diversité végétale de toutes les formes de croissance. Un total de 190 parcelles (25 m par 25 m) a été échantillonné (5072 tiges, 211 espèces) et les données ont été analysées en utilisant les équations allométriques recommandées pour les forêts d'Afrique centrale. Le diamètre moyen des tiges à hauteur de poitrine était de 33,6 cm, la surface terrière moyenne était de 47,7 m² ha−1 et la densité moyenne des individus était de 407 ha−1. Le carbone aérien moyen (AGC) variait de 13,93 à 412,66 Mg C ha−1, le carbone souterrain variait de 2,86 à 96,97 Mg C ha−1 et le bois mort oscillait entre 0,00 et 7,59 Mg C ha−1. La valeur maximale de l'AGC enregistrée dans une parcelle était de 916 Mg C ha−1. L'analyse effectuée en ayant recours à l'association phytosociologique comme base plutôt qu'à un large type de végétation est unique. Les valeurs de l'AGC pour les sites forestiers en terra firme (terre ferme) non perturbés figuraient parmi les plus élevées enregistrées pour les forêts tropicales africaines. Si l'on considère uniquement la diversité des arbres, il existe une relation faible, mais significative, entre l'AGC et la richesse des espèces, l'indice de diversité de Shannon‐Wiener et l'alpha de Fisher. Toutefois, si l'on considère la diversité de toutes les formes de croissance végétale, il n'existe aucune relation entre le carbone et la diversité végétale.
Environmental circumstances shaping soil microbial communities have been studied extensively. However, due to disparate study designs, it has been difficult to resolve whether a globally consistent ...set of predictors exists, or context‐dependency prevails. Here, we used a network of 18 grassland sites (11 of those containing regional plant productivity gradients) to examine (i) if similar abiotic or biotic factors predict both large‐scale (across sites) and regional‐scale (within sites) patterns in bacterial and fungal community composition, and (ii) if microbial community composition differs consistently at two levels of regional plant productivity (low vs. high). Our results revealed that bacteria were associated with particular soil properties (such as base saturation) and both bacteria and fungi were associated with plant community composition across sites and within the majority of sites. Moreover, a discernible microbial community signal emerged, clearly distinguishing high and low‐productivity soils across different grasslands independent of their location in the world. Hence, regional productivity differences may be typified by characteristic soil microbial communities across the grassland biome. These results could encourage future research aiming to predict the general effects of global changes on soil microbial community composition in grasslands and to discriminate fertile from infertile systems using generally applicable microbial indicators.
Aim
Plant species continue to be moved outside of their native range by human activities. Here, we aim to determine whether, once introduced, plants assimilate into native communities or whether they ...aggregate, thus forming mosaics of native‐ and alien‐rich communities. Alien species might aggregate in their non‐native range owing to shared habitat preferences, such as their tendency to establish in high‐biomass, species‐poor areas.
Location
Twenty‐two herbaceous grasslands in 14 countries, mainly in the temperate zone.
Time period
2012–2016.
Major taxa studied
Plants.
Methods
We used a globally coordinated survey. Within this survey, we found 46 plant species, predominantly from Eurasia, for which we had co‐occurrence data in their native and non‐native ranges. We tested for differences in co‐occurrence patterns of 46 species between their native (home) and non‐native (away) range. We also tested whether species had similar habitat preferences, by testing for differences in total biomass and species richness of the patches that species occupy in their native and non‐native ranges.
Results
We found the same species to show different patterns of association depending on whether they were in their native or non‐native range. Alien species were negatively associated with native species; instead, they aggregated with other alien species in species‐poor, high‐biomass communities in their non‐native range compared with their native range.
Main conclusions
The strong differences between the native (home) and non‐native (away) range in species co‐occurrence patterns are evidence that the way in which species associate with resident communities in their non‐native range is not species dependent, but is instead a property of being away from their native range. These results thus highlight that species might undergo important ecological changes when introduced away from their native range. Overall, we show origin‐dependent associations that result in novel communities, in which alien‐rich patches exist within a mosaic of native‐dominated communities.
QUESTIONS: Vegetation change in arid regions with a coefficient of rainfall exceeding 33% usually displays non‐equilibrium dynamics, where abiotic factors override internal biotic controls. ...Irreversible changes have nonetheless also been described for arid regions. What are the contributions of internal/equilibrium vs external/non‐equilibrium factors to vegetation dynamics and can degradation due to overstocking be reversed after removal of livestock? LOCATION: Goegap Nature Reserve, Namaqualand, South Africa. METHODS: The descending point method was conducted annually from 1974 at two transects. Vegetation change was assessed in terms of vegetation cover, species composition, life‐form composition, range condition, species richness and diversity. Principal coordinates analysis was used to illustrate the trajectories in floristic data, and the effects of stocking density and rainfall were examined with redundancy analysis. RESULTS: Vegetation cover, species richness and Shannon‐Wiener index of diversity showed an increase and range condition improved with time. These positive changes could be related to the removal of high numbers of livestock and low wildlife numbers in the first years of survey. A gradual decline in the rate of increase in some of these parameters could be related to high grazing pressure during the later monitored years. There was a notable increase in non‐succulent chamaephytes, but the initial increase in succulent chamaephytes was not sustained. The directional change evident in perennial species composition, supports the equilibrium concept, whereby the negative changes induced by heavy grazing were partially reversed. Within the directional change, four quasi‐stable states could be distinguished, which could be reconciled with the state‐and‐transition model. The annual component showed no directional change, but displayed event‐driven, non‐equilibrium dynamics by fluctuating in reaction to the timing and quantity of rainfall. CONCLUSIONS: The vegetation change displayed elements of both equilibrium and non‐equilibrium dynamics, and demonstrated that the effects of heavy grazing in the Succulent Karoo were reversible. Overall, the recovery process proceeded slowly and was primarily detected in the perennial component of the vegetation. The increase in wildlife numbers in the later studied years and decline in perennial vegetation cover stress the need for active management of animal numbers to avoid vegetation degradation.
Because vegetation change in arid ecosystems is slow, long-term data are essential to gain an understanding of how the vegetation responds to short-term, inter-annual variation in rainfall; long-term ...cyclic rainfall patterns; and grazing pressure. The point intercept method was conducted annually over a period of more than 20 years at five transects in the plains habitat on the Goegap Nature Reserve in the Northern Cape, South Africa. Vegetation change was assessed in terms of vegetation cover, species composition, species abundance, growth form composition, range condition and plant diversity. Non-metric multidimensional scaling was used to illustrate the trajectories in floristic data and to determine the strength of the correlations with rainfall and grazing variables. The effects of the high grazing pressure on the plains were apparent in the overall decrease in total plant cover and reductions in the cover of grazing-sensitive species over the monitored period. Diversity parameters, of especially the annual component, were strongly related to rainfall. A directional change, which supports the equilibrium concept, was evident in changes in perennial species composition over time. The annual component, however, showed no directional change, but displayed eventdriven, non-equilibrium dynamics by fluctuating in reaction to the timing and quantity of rainfall.
Vachellia erioloba is a keystone tree species in the southern Kalahari. This long-term study over nearly four decades tracks two populations in different landscapes (the interior sandy duneveld ...versus the clayey Nossob riverbed) of a large conservation area and offers valuable data on this species under natural soil moisture conditions and with limited anthropogenic influences. In 1978, 18 trees were permanently marked in a 1 ha plot in the interior duneveld of the Kalahari Gemsbok National Park (Dankbaar site). In the Nossob riverbed all trees in a 1 ha plot were surveyed in 1979 (Grootkolk site). At both sites, tree height and stem circumference were subsequently measured at irregular intervals until 2016 in order to investigate growth rates and population structure. Of the 18 marked trees at Dankbaar, six died and three showed coppice regrowth following substantial dieback after a fire. A mean height increase of 60 mm/year was recorded and the mean height of the remaining uncoppiced trees was 6.8 m in 2016. Stem diameter growth rate per year varied widely between trees and between years with a mean value of 2.5 mm/year over the 38-year period. Growth rate calculated for three 10-year intervals varied. Using the mean growth rate derived in the current study and stem size of the dead trees, the mean age of the trees when they died was estimated. At the Grootkolk site, the position of the centroid in relation to the midpoint of the diameter class range suggests that this population is gradually becoming a mature to old population with limited recruitment. This was supported by the size class distribution curves. However, no differences between slopes or intercepts of the stem diameter size class distributions were found.Conservation implications: This study was conducted in a large conservation area, that is, a natural ecosystem excluding most of the anthropogenic threats that are present outside of the park. The study illustrated that in the duneveld the population studied was self-sustaining, with recruitment occurring and large individuals presumably dying of old age. Although fire caused a few individuals to coppice, no fire-related deaths were reported. In the Nossob riverbed, surveys started in a stand of predominantly young trees and the size class distribution at that stage already showed a lack of recruitment. This stand is ageing and will likely disappear at this site; however, new young stands are appearing at other sites in the Nossob riverbed. Under the current conditions with negligible anthropogenic influences, it therefore appears that some V. erioloba populations in the park are increasing in size while others are decreasing, but that overall the species will persist. The impact of global climate change on this species is, however, unknown.
There is a dearth of knowledge on the effects of annual burning of fire-breaks on species composition, plant diversity and soil properties. Whittaker's plant diversity technique was used to gather ...data on species composition and diversity in four grassland communities on the Loskop Dam Nature Reserve (LDNR). The study demonstrated that fire-beaks did not have a negative effect on plant diversity and an increase was even noted in various diversity parameters in the grassland on abandoned cropland. Fire-breaks were changing the species composition, as three of the four communities illustrated a clear separation in species composition between fire-break and unburnt plots. There was not a strong association between specific species and the fire-break or unburnt plots, except for Themeda triandra and Tristachya leucothrix, known for their association with and without fire, respectively. The change in species composition was not negatively affecting range condition. Carbon and nitrogen concentrations were slightly lower in the fire-break than unburnt soils, but the reduced concentrations were unlikely to cause severe soil degradation in fire-break zones. From a management perspective, fire-breaks appear to be a sustainable management tool as they are not adversely affecting plant diversity or range condition in the grassland association on LDNR.
Sand Forest in the Maputaland region of KwaZulu-Natal in South Africa is deemed the most valuable, but also probably the most complex vegetation type of this part of the Maputaland–Pondoland–Albany ...hotspot of biodiversity. However, Sand Forest is under threat from the current human population growth in that region as well as from uncontrolled increases in wild herbivore numbers in conservation areas. The present study compares the impacts of herbivores and humans on the state of woody resources between two sites under differing utilisation regimes. Sand Forest was found to be a complex association of tree assemblages defined by different canopy and subcanopy properties. Although marked differences in the abundance of selected species were noted at the two sites, the Sand Forest remained dominated by fine-grained species under both utilisation regimes. The fine-grained nature of Sand Forest implies that regeneration depends on the creation of small canopy gaps either by natural processes, humans or elephants, while the creation of large gaps could transform it into woodland. Management of conservation areas where Sand Forest occurs should therefore concentrate efforts on regulating animal populations to levels that provide gap properties that favour forest regeneration.
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