Modeling changes in soil organic matter in Amazon forest to pasture conversion with the Century model

Land use and land cover changes in the Brazilian Amazon region have major implications for regional and even global carbon cycling. We analyzed the effects of the predominant land use change, conversion of tropical forest to pasture, on total soil C and N, using the Century ecosystem model and data collected from the Nova Vida ranch, Western Brazilian Amazon. We estimated equilibrium organic matter levels, plant productivity and residue carbon inputs under native forest conditions, then simulated deforestation following the slash and burn procedure. Soil organic matter dynamics were simulated for pastures established in 1989, 1987, 1983, 1979, 1972, 1951, and 1911. Using input data from the Nova Vida ranch, the Century model predicted that forest clearance and conversion to pasture would cause an initial decline in soil C and N stocks, followed by a slow rise to levels exceeding those under native forest. Simulated soil total C and N levels (2500 g C m^?2 and 245 g N m^?2 in the 0–20 cm layer) prior to conversion to pasture were close to those measured in the native forest. Simulated above‐ and below‐ground biomass for the forest and pasture were comparable with literature values from this region. The model predicted the long‐term changes in soil C and N under pasture inferred from the pasture chronosequence, but there was considerable variation in soil C stocks for pastures <20 years in age. Differences in soil texture between pastures were relatively small and could not account for much of the variability between different pastures of similar ages, in either the measured or simulated data. It is likely that much of the variability in C stocks between pastures of similar ages is related to initial C stocks immediately following deforestation and that this was the largest source of variability in the chronosequence. Internal C cycling processes in Century were evaluated using measurements of microbial biomass and soil δ¹³C. The relative magnitude and long‐term trend in microbial biomass simulated by the model were consistent with measurements. The close fit of simulated to measured values of δ¹³C over time suggests that the relative loss of forest‐derived C and its replacement by pasture‐derived C was accurately predicted by the model. After 80 years, almost 90% of the organic matter in the top 20 cm was pasture derived. While our analysis represents a single ‘case study’ of pasture conversion, our results suggest that modeling studies in these pasture systems can help to evaluate the magnitude of impacts on C and N cycling, and determine the effect of management strategies on pasture sustainability.     

Can forest management have season-dependent effects on bird diversity?

Forest management modified the original structure of most European forests, and in the most extreme cases, genuinely natural and semi-natural forests were turned into plantations through clear felling and replanting, often using non-native species. We compared the bird community structure of native oak woods of northern Italy with that of their anthropogenic counter-parts: black locust and sweet chestnut woods. The three stand types were compared in terms of vegetation structure, bird species richness, diversity and abundance of foraging guilds. We analysed both the overwintering and the breeding community, to assess whether management had specific seasonal effects on bird diversity. Forestry-imposed disturbances affected bird diversity more consistently in winter than in breeding time: bird species richness and diversity were significantly greater in oak and chestnut stands, which were the preferred habitat for bark foragers and foliage gleaners. In the breeding period, bird diversity of black locust woodlands increased, and inter-stand differences were not significant. At this time of year, understorey gleaners were more abundant in black locust stands (where shrubs were denser). In winter, species richness, diversity and the abundance of several guilds were positively correlated with stand age, whereas in the breeding period canopy gleaners preferred younger woodlots. Despite the lack of inter-stand differences in breeding bird diversity, young-managed woods benefited generalist birds that need no particular conservation efforts. Conversely, priority species for forest conservation such as specialised bark foragers positively selected native and mature stands throughout the year. We suggest that detailed year-round studies on diversity and community composition could sharpen the precision with which it is possible to prescribe conservation measures in forested areas.     

Soils and vegetation of Angai forest: ecological insights from a participatory survey in South Eastern Tanzania

Soils and vegetation of a woodland in South Eastern Tanzania were studied in support of a project fostering community participation in the management of a proposed forest reserve. In nine sites within an area of 500 km², soils were characterized, frequency of occurrence of woody species was assessed and diameter at breast height of timber species recorded. Villagers identified 133 species of which only 56% were scientifically identified. However, as ordination axes derived from the complete data set were highly correlated with those derived from the restricted data set, the latter could be used. On the undulating plains, deep, sandy soils (Hypoluvic Arenosols, Profondic and Arenic Luvisols) occurred low in organic carbon, exchangeable bases and extractable aluminium content. Most common canopy species were Brachystegia spiciformis/B. boehmii and Pterocarpus angolensis. On the dissected plains on shallow, clayey soils (Leptic Cambisols) with higher organic carbon, exchangeable bases and extractable aluminium content, Acacia nigrescens and Markhamia spp. were the most common tree species. On deeper, fertile clayey soils (Profondic Lixisols and Chromic Phaeozems) vegetation consisted of species found in the two vegetation groups. Pterocarpus angolensis and Pericopsis angolensis were the only valuable timber species commonly found, most frequently on the undulating plains. As soils of this area are unfavourable for agriculture, it seems sensible to set this area aside as a forest reserve.     

Scale and the detection of land-use effects on morphology, vegetation and macroinvertebrate communities of grassland streams

1. Land‐use studies are challenging because of the difficulty of finding catchments that can be used as replicates and because land‐use effects may be obscured by sources of variance acting over spatial scales smaller than the catchment. To determine the extent to which land‐use effects on stream ecosystems are scale dependent, we designed a whole‐catchment study of six matched pairs (pasture versus native tussock) of second‐order stream catchments, taking replicate samples from replicate bedforms (pools and riffles) in each stream. 2. Pasture streams had a smaller representation of endemic riparian plant species, particularly tussock grasses, higher bank erosion, a somewhat deeper layer of fine sediment, lower water velocities in riffles, less moss cover and higher macroinvertebrate biodiversity. At the bedform scale, suspendable inorganic sediment (SIS) was higher in pools than riffles and in pasture streams there was a negative relationship between SIS and the percentage of the bed free of overhanging vegetation. Differences between stream reaches (including any interactions between land use and stream pair) were significant for SIS, substrate depth and characteristics of riparian vegetation. There were also significant differences between replicate bedforms in the same stream reaches in percentage exotic species in overhanging vegetation, percentage moss cover, QMCI (Quantitative Macroinvertebrate Community Index – a macroinvertebrate‐based stream health index) and macroinvertebrate density. 3. Significant differences among stream reaches and among replicate bedform units within the same reach, as well as interactions between these spatial units and land‐use effects, are neither trivial nor ‘noise’ but represent real differences among spatial units that typically are unaccounted for in stream studies. Our multi‐scale study design, accompanied by an investigation of the explanatory power of different factors operating at different scales, provides an improved understanding of variability in nature.     

Estimating seasonal and annual carbon inputs,and root decomposition rates in a temperate pasture following field 14C pulse-labelling

Using a ¹⁴C pulse-labelling technique, we studied the seasonal changes in assimilation and partitioning of photoassimilated C in the plant–root–soil components of a temperate pasture. Pasture and soil samples were taken after 4-h, and 35-day chase periods, to examine these seasonal ¹⁴C fluxes. Total C and ¹⁴C were determined in the shoot, root and soil system. The amounts of C translocated annually to roots and soil were also estimated from the seasonal ¹⁴C distribution and pasture growth. The in situ field decomposition of newly formed roots during different seasons, also using ¹⁴C-labelling, was studied for one year in undisturbed rhizosphere soil. The ¹⁴C-labelled roots were sampled five times and decomposition rates were calculated assuming first-order decomposition.Annual pasture production at the site was 16 020 kg DM ha^–1, and pasture growth varied with season being highest (75–79 kg ha^–1 d^–1) in spring and lowest (18–20 kg ha^–1 d^–1) in winter. The above- and below-ground partitioning of ¹⁴C also varied with the season. The respiratory ¹⁴C–CO₂ losses, calculated as the difference between the total amounts of ¹⁴C recovered in the soil-plant system at 4 h and 35 days, were high (66–70%) during the summer, autumn and winter season, and low (37–39%) during the spring and late-spring season. Pasture plants partitioned more C below-ground during spring compared with summer, autumn and winter seasons. Overall, at this high fertility dairy pasture site, 18 220 kg C/ha was respired, 6490 kg remained above-ground in the shoot, and 6820 kg was translocated to roots and 1320 kg to soil. Root decomposition rate constant (k) differed widely with the season and were the highest for the autumn roots. The half-life was highest (111 days) for autumn roots and lowest (64 days) for spring roots. About one-third of the root label measured in the spring season disappeared in the first 5 weeks after the initial 35 Day of allocation period. The late spring, summer, late summer and winter roots had intermediate half-lives (88–94 days). These results indicate that seasonal changes in root growth and decomposition should be accounted for to give a better quantification of root turnover.     

Seasonal patterns in water use and leaf turnover of different plant functional types in a species-rich woodland,south-western Australia

Woodlands in south-western Australia are evergreen and transpire throughout the year despite the long, hot and dry summers of the Mediterranean climate. Results from a case study in a species-rich Banksia woodland are used to discuss the ecological and physiological properties that appear to be essential features of this and similar communities. Tree, shrub and perennial herbaceous species with long-lived leaves dominate the community, whereas winter-green herbaceous species with short-lived leaves constitute a minor group. The total leaf area index is therefore reasonably constant in all seasons. Leaf area index is low and canopies are open, causing good coupling between the vegetation and the atmosphere, and making stomatal control an effective regulator of transpiration. Mean maximum (winter) stomatal conductances were high at approximately 300 mmol m^–2 s^–1. Deep-rootedness allows the dominant species to access soil moisture throughout the unsaturated zone, and down to the capillary fringe of the saturated zone. Shrubs and herbs with shallow roots experience greater drought stress during summer. Rates of community evapotranspiration are limited by leaf area index in the wet season, and further reduced by stomatal closure in the dry season. Deep-rooted plants appear to decrease their stomatal conductance before the development of severe drought stress. Such conservative behaviour, possibly related to plant hydraulic constraints, is a contributing factor to the limited seasonality in community water use.     

Smoke, Mulch, and Seed Broadcasting Effects on Woodland Restoration in Western Australia

Smoke, canopy‐derived mulch, and broadcast seeds were used to maximize the establishment of Banksia woodland species in sand quarries in Western Australia. Smoke, particularly aerosol smoke, had a positive effect on total seedling recruitment. Pre‐mined (woodland) sites showed a 42‐fold increase in total germinants and a 3‐fold increase in the number of species with aerosol smoke application. Post‐mined (restored) sites showed only a 3.6‐fold increase in total germinants and a 1.4‐fold increase in the number of species. Two water‐based smoke chemicals, DC10 (pH 4.5) and SC63 (pH 2.5), increased seedling recruitment at both sites but were not as effective in stimulating recruitment as aerosol smoke. Neither of the chemicals were effective in significantly increasing species richness. Application of aerosol smoke directly to seeds as a pretreatment before broadcasting had no effect on seedling recruitment. Broadcasting of seeds onto restoration sites significantly increased seedling abundance and richness. Application of a single layer of mulch from the canopy vegetation after seed broadcasting gave optimum seedling recruitment. Two layers of mulch significantly reduced recruitment, as did applying mulch before seed broadcasting. For broad‐scale restoration, the application of smoke on newly restored sites would be more effectively achieved using smoke water sprayed over the soil surface. Species that do not recruit from replaced topsoil could be effectively recovered from broadcast seed rather than from mulch.     

Growth and carbon stock change in eucalypt woodlands in northeast Australia: ecological and greenhouse sink implications

Data from 57 permanent monitoring sites are used to document the growth in woody vegetation and estimate the carbon sink in 27 M ha of eucalypt woodlands (savannas), contained within c. 60 M ha of grazed woodlands in Queensland (northeast Australia). The study sites are shown to be representative of the environment and structure of the eucalypt woodlands in the defined study area. Mean basal area increment for all live woody plants in 30 long‐term sites, with an average initial basal area of 11.86 ± 1.38 (SE) m² ha^?1, was 1.06 m² ha^?1 over a mean 14 years timeframe. The majority of the measurement period, commencing between 1982 and 1988, was characterized by below‐average rainfall. The increase in live tree basal area was due primarily to growth of existing trees (3.12 m² ha^?1) rather than establishment of new plants (0.25 m² ha^?1) and was partly offset by death (2.31 m² ha^?1). A simple but robust relationship between stand basal area and stand biomass of all woody species was developed for the eucalypt dominant woodlands. Analysis of above‐ground carbon stocks in live and standing dead woody plants gave a mean net above‐ground annual carbon increment for all 57 sites of 0.53 t C ha^?1 y^?1, similar to values estimated elsewhere in world savannas. Published root : shoot ratios were used to infer C flux in woody root systems on these sites. This results in an estimated sink in above‐ and below‐ground biomass of 18 Mt C y^?1 over the eucalypt woodlands studied, and potentially up to 35 Mt C y^?1 if extended to all grazed woodlands in Queensland. It is suggested that introduction of livestock grazing and altered fire regimes have triggered the change in tree‐grass dominance in these woodlands. Thus, change in carbon stocks in the grazed woodlands of Queensland is identified as an important component of human‐induced greenhouse gas flux in Australia, equivalent in magnitude to c. 25% of the most recently published (1999) total estimated national net emissions. The latter inventory takes into account emissions from land clearing, but does not include the sink identified in the present study. This sequestration also represents a small but significant contribution to the global terrestrial carbon sink.     

The small-scale pattern of Cynodon dactylon in Mediterranean pastures

The abundance of Cynodon dactylon was recorded in 50 plots presenting different geomorphological conditions and along a transect of 16 500 2×2 cm contiguous quadrats within a small basin 330 m in length, in the granite pediment of the Sierra de Guadarrama (Central Spain). Soil analyses were undertaken on samples from the 50 plots and the soil information matrix obtained was analysed using Principal Components Analysis and Discriminant Analysis. Results showed that in the Mediterranean pastures Cynodon dactylon was restricted to deep, well developed soils with relatively high values of cations, conductivity and organic matter, and could be used as an indicator of such soil conditions in these grasslands. The spatial pattern of the species was analysed using New Local Variances which revealed the existence of a small-scale pattern, ranging from 8 to 18 cm, present at all levels of the analysis. The size of small-scale patterns was positively correlated with species abundance and was interpreted as a relation between plant vigour and favourable soil conditions.I would like to thank Professor González Bernáldez for his interest and encouragement.     

New method for rapid assessment of the functional composition of herbaceous plant communities

Abstract We propose a rapid sampling method to assess the functional composition of herbaceous plant communities without prior knowledge of the floristic composition. To determine the community‐level value of traits (‘aggregated trait values’) for a plant community, a standardized population‐centred method exists, but requires substantial manpower and reliable botanical knowledge. We tested an alternative method, the trait transect, using four subalpine pastures in the Beaufortain region (Northern French Alps) selected along a fertility gradient. We applied both methods to measure five commonly used ‘soft traits’ known to be responsive to soil nutrient availability: plant vegetative and reproductive height, specific leaf area, leaf dry matter and nitrogen contents. We tested whether the variation of these traits along the gradient detected with the population‐centred method was also detected with the trait transect. Both methods detected expected trends in the traits in response to the fertility gradient. The trait transect method was as efficient as the population‐centred method and is recommended as an appropriate tool for monitoring ecosystem changes in response to environmental conditions and management, especially in species‐rich communities.