Rainfall Sites Research Articles

Bio‐mitigation of carbon following afforestation of abandoned salinized farmland

AbstractAs the global demand for food continues to increase, the displacement of food production by using agricultural land for carbon mitigation, via either carbon sequestration, bioenergy or biofuel is a concern. An alternative approach is to target abandoned salinized farmland for mitigation purposes. Australia, for example, has 17 million ha of farmland that is already or could become saline. At a representative, salinized, low rainfall (350 mm yr−1) site at Wickepin, Western Australia, we demonstrate that afforestation can mitigate carbon emissions through either providing a feedstock for bioenergy or second generation biofuel production and produce salt‐tolerant fodder for livestock. A range of factors markedly affect this mitigation. These include hydrological conditions such as salinity, site factors such as slope position and soil properties and a range of silvicultural factors such as species, planting density and age of the planting. High density (2000 stems ha−1) plantings of Eucalyptus occidentalis Endl. produced a mean total biomass of 4.6 t ha−1 yr−1 (8.5 t CO2‐e ha−1 yr−1) averaged over 8 years. Atriplex nummularia Lindl. produced a mean total biomass of 3.8 t ha−1 yr−1 (6.9 t CO2‐e ha−1 yr−1) averaged over 4 years and approximately 1.9 t ha−1 yr−1 of edible dry matter annually to 8 years of age. With differences in salt tolerance between E. occidentalis and A. nummularia, we propose an integrated approach to treating salinized sites that takes salinity gradients into account, replicates natural wetland ecosystems and produces both fodder and biomass. Continued mitigation is expected as the stands mature, assuming that growth is not affected by the accumulation of salt in the soil profile. Such carbon mitigation could potentially be applied to salinized farmland globally, and this could thus represent a major contribution to global carbon mitigation without competing with food production.

Characterising wide spatial variation in population size structure of a keystone African savanna tree

Elephant impact on trees is of conservation concern in African savannas, but it’s effect on population size structure of heavily utilised species has received little attention. Although studies have shown that the population structure and density of Sclerocarya birrea subsp. caffra (marula), a heavily utilised savanna tree species, varies widely across its range, the causes of this variation are poorly understood. This study aimed to describe and compare these divergent structures and develop an understanding of the relationship between population structure and site characteristics, specifically disturbance. Four distinct groups of population structure were identified: (1) adult dominated populations consisting of only adults and no juveniles or seedlings. The size class distributions (SCDs) of this group were bell-shaped, indicating an unstable population which may have come about through circumstances that no longer occur in these areas, for example human settlement or a massive episodic recruitment event. High impala utilisation of seedlings or rodent seed predation are proposed to be the main cause of the lack of recruitment; (2) rotated sigmoid (or negative J shaped) curves, representing stable population structures, when protected from elephants, as well as at high rainfall sites with elephants, indicating that without elephants and in suitable habitats with elephants, marulas are able to attain a relatively stable size structure and instability is not inherent; (3) juvenile dominated populations with almost no adults. Adults are relegated to a few refugia, where elephants are unable to reach them; (4) juvenile dominated populations consisting of mostly juveniles and few adults. These populations, all exposed to fire and elephants, have “missing size classes” varying between 5 and 40 cm in stem diameter and 2–8 m in height. This unstable structure was most likely caused by fire suppression of juveniles combined with elephant utilisation of individuals in the “missing size classes”. Consistent application of this pressure is postulated to result in the progression from stable to “missing size class” structure, to juvenile or adult dominated populations, and finally to local extirpation, with increasing elephant densities and concomitant utilisation, interacting with fire, rainfall and soil type.

Domatia morphology and mite occupancy of Psychotria horizontalis (Rubiaceae) across the Isthmus of Panama

Leaf domatia are small plant structures in vein axials on the undersides of leaves that are often inhabited by mites of several species. The mites are presumed to benefit the plant because they are predatory or fungivorous. The domatia are thought to provide the mites shelter from predators and changes in relative humidity, and in exchange, the mites protect the plant from small herbivores and fungal spores. Differences in relative humidity can affect food availability, changing the interaction between plants and mites. We examined domatium morphology of the shrub Psychotria horizontalis (Rubiaceae) and its associated mite diversity at three sites along the rainfall gradient of the Isthmus of Panama, during the dry and wet seasons. The dry forest had a domatium morphology consistent with providing greater desiccation protection, with trichomes and a smaller domatium opening relative to domatium size (size/opening ratio). Additionally, this size/opening ratio was significantly higher in the dry season than in the wet season at all three sites. Mite diversity was highest at the intermediate rainfall site with a large degree of overlap with the other sites, whereas the dry site and wet site shared few mite species. More fungivorous mites were present in the moist forests and more facultative feeders on fungal spores and small mites in the dry forest. The average mite size at each site matched the average domatium size at each site. The dry forest had small mites in small domatia, whereas the moist forests had larger mites in larger domatia. While these data are primarily observational, the site and seasonal differences in domatium morphology and mite diversity are consistent with two main hypotheses: (1) that protection from changes in humidity would be particularly important when humidity was low, such as in the dry forest and during the dry season (2) more fungivorous mites would be found in domatia of the moist forests. The data presented here further highlight the close adaptive relationship between leaf domatia on plants and the mites that inhabit them.

Seed germination cues and the importance of the soil seed bank across an environmental gradient in the Serengeti

Soil seed banks are an important source of new individuals for many plant populations and contribute to future genetic variability. In general, the size and persistence of soil seed banks is predicted to be greater where growth occurs in unpredictable pulses, where opportunities for disturbance‐related recruitment are frequent and where the probability of recruitment failure is high. In savanna ecosystems, characterized by disturbance from fire and unpredictable water availability, soil seed banks should be relatively important sources of recruitment. However, the few studies conducted in savannas are inconclusive about the importance of soil seed banks and, more specifically, how seed banks should change across environmental gradients. We determined the number of viable seeds in the soil seed bank across savanna‐grasslands in the Serengeti, an ecosystem characterized by frequent fire and seasonal drought. Soils were exposed to a combination of smoke and heat, cues which may be required to break seed dormancy in such ecosystems. Our a priori expectation was to observe large seed banks in regions characterized by seasonal drought and comparatively smaller seed banks in regions of higher moisture availability and high fire frequencies. In contrast to our hypothesis, seed germination increased strongly with precipitation and fire frequency. In addition, there was a significant interaction effect between fire and rainfall: low rainfall sites with frequent fire had greater seed germination than low rainfall sites with low fire frequency. Moreover, in laboratory experiments, heat had a negative, smoke a positive effect on final seed germination numbers. Together, these findings suggest that fire may be a key factor in driving herbaceous seed bank dynamics in tropical savannas.

Soil water depletion and replenishment during first- and early second-rotation Eucalyptus globulus plantations with deep soil profiles

Eucalyptus globulus plantations are thought to use stored soil water when planted on ex-agricultural sites, and we hypothesized that this is likely to affect productivity of 2nd and later rotation plantations because the next rotations have access to less stored soil water. We used a combination of experiments and modeling to understand the impact of E. globulus plantations on soil water stores over the first rotation and early second rotation. The experiments were conducted at 3 contrasting sites in south-western Australia, and modeling was used to extrapolate the results to other climatic zones. Soil water dynamics were assessed to 8m depth under a range of management options, including spacing and nitrogen addition in the first rotation, and coppice or seedling re-establishment in the 2nd rotation. We found that soil water stores declined over the course of the first rotation at all sites, with some (incomplete) annual replenishment evident at the higher rainfall sites, but less replenishment at depth, especially in the lower rainfall sites. Only the wettest of the 3 sites fully replenished with soil water after harvest of the 1st rotation. Plots with higher stocking rates had higher soil water depletion early in the rotation, although by the end of the first rotation, most treatments had similar soil water deficits of around 800mm at all of the sites. Of the sites that were responsive to N fertilizer, there was a strong differential in the degree of soil water deficit between N treatments, with N fertilized trees using more of the soil water store each year, but also producing more wood. A process-based plantation growth model, CABALA, was found to be adequate for predicting soil water dynamics under the range of management options that we explored, and we applied it to understanding the potential replenishment of soil under 2nd rotation plantations in a range of climatic zones within the E. globulus estate in south-western Australia. This modeling showed that most sites with soil depths of more than 4m (i.e., most of the estate) are unlikely to be fully replenished in the 2nd rotation, and that this is likely to have a significant impact on the capacity of sites to achieve similar productivity levels in the second rotation as the first unless the sites are given an opportunity for soil water replenishment between rotations. The results from this study suggest that plantation managers will need to understand soil water dynamics at any given site to be able to predict productivity in 2nd and later rotations, and may need to explore novel management options like fallowing between rotations to allow for soil water replenishment.

The effect of situational variability in climate and soil, choice of animal type and N fertilisation level on nitrogen leaching from pastoral farming systems around Lake Taupo, New Zealand

Agricultural systems with grazing animals are increasingly under scrutiny for their contribution to quality degradation of waterways and water bodies. Soil type, climate, animal type and nitrogen (N) fertilisation are contributors to the variation in N that is leached through the soil profile into ground and surface water. It is difficult to explore the effect of these factors using experimentation only and modelling is proposed as an alternative. An agro-ecosystem model, EcoMod, was used to quantify the pastoral ecosystem responses to situational variability in climate and soil, choice of animal type and N fertilisation level within the Lake Taupo region of New Zealand. Factorial combinations of soil type (Oruanui and Waipahihi), climate (low, moderate and high rainfall), animal type (sheep, beef and dairy) and N fertilisation level (0 or 60 kg N/ha/yr) were simulated. High rainfall climates also had colder temperatures, grew less pasture and carried fewer animals overall which lead to less dung and urinary N returned. Therefore, even though a higher proportion of N returned ultimately leached at the higher rainfall sites, the total N leached did not differ greatly between sites. Weather variation between years had a marked influence on N leaching within a site, due to the timing and magnitude of rainfall events. In this region, for these two highly permeable soil types, N applied as fertiliser had a high propensity to leach, either after being taken up by plants, grazed and returned to the soil as dung and urine, or due to direct flow through the soil profile. Soil type had a considerable effect on N leaching risk, the timing of N leaching and mean pasture production. Nitrogen leaching was greatest from beef cattle, followed by dairy and sheep with the level of leaching related to urine deposition patterns for each animal type and due to the amount of N returned to the soil as excreta. Simulation results indicate that sheep farming systems with limited fertiliser N inputs will reduce N leaching from farms in the Lake Taupo catchment.

Ecotypes of wild rooibos ( Aspalathus linearis (Burm. F) Dahlg., Fabaceae) are ecologically distinct

Aspalathus linearis (Burm. F) Dahlg., Fabaceae is cultivated by small- and large-scale commercial farmers of the Cederberg and Bokkeveld Plateau in the Cape Floristic Region of South Africa, for the production of an herbal beverage called ‘rooibos’ or ‘rooibos tea’. Small-scale farmers also harvest A. linearis from the wild and market the tea as an organic and fair-trade certified product. However, little is known about the apparent ecotypes of wild A. linearis. We hypothesized that 1) rooibos ecotypes are ecologically distinct (occurring in different plant communities defined by environmental variables); 2) rooibos ecotypes are functionally distinct due to variance in water use efficiency; and 3) rooibos ecotypes are associated with threatened vegetation types/species, making populations of wild rooibos ecotypes worthy of conservation. Ecotypes of wild rooibos were identified based on plant habit and local knowledge. Plant communities were classified using Twinspan, environmental factors were tested as predictors of ecotype distribution and water use efficiency of ecotypes growing across a rainfall gradient was determined from foliar natural abundance of 13C. Wild rooibos was not generally associated with endangered vegetation types but was associated with plant species having endangered status. Wild rooibos occurred in four plant communities and comprised five wild rooibos ecotypes: shrub, tree, upright, salignus and prostrate types. Although some ecotypes clearly co-occurred, evidence is provided for habitat preference between the ecotypes: Prostrate and upright ecotypes occurred at higher elevations (> 400–600 m). Shrub ecotypes occurred at lower rainfall sites (< 200 mm p.a.) and the salignus ecotype occurred at higher rainfall sites (> 500 mm p.a.). Foliar 13C indicated greater water use efficiencies by ecotypes in relatively drier areas. The extent to which this is a plastic or inherent response requires further investigation. Considering that wild rooibos ecotypes differ both ecologically and possibly also functionally and genetically, it is concluded that populations of wild rooibos ecotypes should be considered as distinct and worthy of conservation. This distinctness should be considered when farmers apply for both harvesting and ploughing rights on land with wild rooibos ecotypes.

Are the eucalypt and non-eucalypt components of Australian tropical savannas independent?

Eucalypts (Eucalyptus and Corymbia spp.) dominate (>60%) the tree biomass of Australia's tropical savannas but account for only a fraction (28%) of the tree diversity. Because of their considerable biomass and adaptation to environmental stressors, such as fire, the eucalypts may drive tree dynamics in these savannas, possibly to the exclusion of non-eucalypts. We evaluated whether the eucalypt and non-eucalypt components in tropical savannas are dependent so that changes in one component are matched by opposite trends in the other. Using tree inventory data from 127 savanna sites across the rainfall and fire frequency gradients, we found that eucalypt and non-eucalypt basal area and species richness had a negative relationship. This relationship was maintained across the rainfall gradient, with rainfall having a positive effect on the basal area and species richness of both components, but with a greater effect in non-eucalypts. Fire frequency negatively affected basal area, but not species richness, although basal area and species richness of eucalypts and non-eucalypts did not differ in their response to fire. Rainfall appears to set the upper bounds to woody biomass in these mesic savannas, while fire maintains woody biomass below carrying capacity and facilitates coexistence of the components. The magnitude of the component responses, particularly for non-eucalypts, is determined by rainfall, but their dependence is likely due to their differential response to both rainfall and fire, but not to competition for resources. Thus, while eucalypts dominate biomass overall, at high rainfall sites non-eucalypt basal area and diversity are highest, especially where fire frequency is low.

Tropical forest soil microbial communities couple iron and carbon biogeochemistry

We report that iron-reducing bacteria are primary mediators of anaerobic carbon oxidation in upland tropical soils spanning a rainfall gradient (3500-5000 mm/yr) in northeast Puerto Rico. The abundant rainfall and high net primary productivity of these tropical forests provide optimal soil habitat for iron-reducing and iron-oxidizing bacteria. Spatially and temporally dynamic redox conditions make iron-transforming microbial communities central to the belowground carbon cycle in these wet tropical forests. The exceedingly high abundance of iron-reducing bacteria (up to 1.2 x 10(9) cells per gram soil) indicated that they possess extensive metabolic capacity to catalyze the reduction of iron minerals. In soils from the higher rainfall sites, measured rates of ferric iron reduction could account for up to 44% of organic carbon oxidation. Iron reducers appeared to compete with methanogens when labile carbon availability was limited. We found large numbers of bacteria that oxidize reduced iron at sites with high rates of iron reduction and large numbers of iron reducers. The coexistence of large populations of iron-reducing and iron-oxidizing bacteria is evidence for rapid iron cycling between its reduced and oxidized states and suggests that mutualistic interactions among these bacteria ultimately fuel organic carbon oxidation and inhibit CH4 production in these upland tropical forests.

Patterns of genotype-by-environment interaction in diameter at breast height at age 3 for eucalypt hybrid clones grown for reafforestation of lands affected by salinity

The commercial viability of plantations established for the recovery of saline lands may be supported by the deployment of improved genetic material matched to these particularly challenging environments. Patterns of genotype-by-environment interaction were investigated in a highly unbalanced data set of diameter at breast height at approximately 3 years for a total of 841 genotypes from ten Eucalyptus camaldulensis × Eucalyptus globulus and Eucalyptus camaldulensis × Eucalyptus grandis hybrid families assessed across 21 trials grown on a range of saline and non-saline low rainfall sites from southeast Queensland through central NSW, Victoria, Tasmania and southeast South Australia to southwest Western Australia using factor-analytic mixed-model methods. There was significant heterogeneity among trials in estimates of family variance, genotype-within-family variance, the ratio of family variance to total genetic variance and individual broad-sense heritability. Cluster analyses indicated that family effects were highly correlated across a main group of 19 trials and that most trials fell into two major groups for genotype-within-family effects, with an average correlation among trials within these groups of 0.55. There was, however, no obvious geographical or other explanation for the patterns, suggesting that genotypes should be deployed on the basis of broad-scale adaptation.

Assessment of suspended particulate pollution in the Bhadra River catchment, Southern India: an environmental magnetic approach

Open-cast mining generates sediment in river systems at globally significant scales. One of the challenges in attributing measured sediment loads to upstream mining activities is establishing the source of sediments that are a mixture of natural and mining-based materials. The environmental magnetic data (mass-specific magnetic susceptibility, anhysteretic remanent magnetisation, isothermal remanent magnetisation and inter-parametric ratios) on 57 samples of suspended sediment from the Bhadra River in the Sahyadri (the Western Ghat) of India have been used in this study. Samples were collected upstream, adjacent to and downstream of Kudremukh, a mountainous and high rainfall site where the largest mechanised open-cast mine in south Asia was located. Graphical and multivariate analyses and modelling of the data show that on average ~29% of the river suspended load downstream of the mine is derived from mining and allied activities at Kudremukh although the mine occupies less than 5% of the catchment. The contribution of primary ore is the maximum (18%), followed by transitional hard weathered ore (7%) and weathered ore (4%). The model has done a fairly good job of unmixing; the sum of errors is 71,000 for four samples. Modelling of samples with small mass seems to produce large errors. This investigation demonstrates the utility of environmental magnetic data, which can be obtained in a simple and rapid manner, and the unmixing of such data in identifying the contribution of mining activities to the total suspended sediment load.

Analysis of the standardized precipitation index for the Paraíba state, Brazil

A seca é um dos complexos fenômenos naturais que afeta várias partes do mundo e provoca impactos sociais, econômicos e ambientais. O estudo de secas no Estado da Paraíba é justificável, visto que cerca de 80% de sua área está inserida no semiárido nordestino. Este estudo teve como objetivo subdividir o Estado da Paraíba em três sub-regiões pluviometricamente homogêneas, utilizando a Análise de Agrupamento e, em seguida, utilizar o Índice Padronizado de Precipitação (IPP) nas escalas de tempo de 12 (IPP-12) e 24 (IPP-24) meses para monitorar as secas severas e extremas nessas sub-regiões. O período chuvoso de cada um dos 49 postos pluviométricos foi obtido pela padronização da variável precipitação. Os resultados mostraram a ocorrência de secas severas para as três sub-regiões. O IPP-12 meses revelou a ocorrência de secas com maior duração no período de 1990 a 1994 nas cidades de Araruna e Coremas. O IPP-12 meses identificou, também, secas extremas com maior duração na cidade de Monteiro, sub-região do Cariri paraibano, possivelmente devido à orografia local. Já o IPP-24 meses identificou seca severa, com maior duração em Coremas no período de 1991 a 1995. Nesses períodos, essas secas foram intensificadas, possivelmente, devido à ação do fenômeno El Niño. Em suma, o IPP-12 meses e IPP-24 meses mostraram eficazes para o monitoramento de secas no Estado.

Productivity of waterlogged and salt-affected land in a Mediterranean climate using bed-furrow systems

Waterlogging and dryland salinity in Western Australia (WA) as well as in many other parts of the world have long been recognized as major constraints to productive pastures and crops. In the late 90s, bed-furrow systems were introduced to waterlogged non-saline land in high rainfall areas of Australia. That resulted in significant yield increases of broad-acre crops through a reduction of the waterlogging. It was expected that yield gains could also be realised on waterlogged and saline land, of which more than 1 million ha is present in WA. This paper examines the productivity of crops and pasture of waterlogged and salt-affected land after the implementation of bed-furrow systems. Three sites were selected in the south western Wheatbelt of WA, with different rainfall regimes and with soil salinity ranging from none to extreme. The treatments were: pasture, cropping and raised beds (RB), no-till beds (NTB) and the Control. The distribution of salinity was mapped using an electromagnetic instrument (EM38). Pasture biomass and distribution were obtained from validated multispectral images. Grain yield and distribution were obtained from calibrated yield maps. In 4 of the 12 site-years, grain yields from the bed-furrow systems were statistically significantly compared to the Control. These were mostly associated with the site in the high rainfall region. The pasture biomass declined under the impact of summer and winter grazing and salinity. Bed-furrow systems did, in general, not generate higher grain yields for a given soil salinity compared to the Control. Relative yield - salinity relationships revealed a higher salt tolerance of the various crops compared to criteria published previously. It was also found that the interaction between waterlogging and salinity was not applicable since the timing of both issues did not coincide. Despite the grain yield gains at the high rainfall site using bed-furrow systems, cropping the low-lying salt-affected and potentially waterlogged land carries high risks. Provided these are accepted, growers with access to such land may be able to extend their cropping areas considerably. It is expected that bed-furrow systems are also beneficial in the waterlogged and salt-affected regions where topographical conditions and a precipitation surplus have created such conditions, like in some of the prairie states of North America.

Persistence and productivity of Medicago sativa subspecies sativa, caerulea, falcata and varia accessions at three intermittently dry sites in south-eastern Australia

The persistence and productivity of a diverse range of Medicago sativa germplasm including representatives of subspecies sativa, caerulea, falcata and varia were examined at 3 field sites in south-eastern Australia over 4 years. Sites were located at Tamworth, Barmedman and Hamilton, forming a 1200 km north–south transect with rainfall distribution varying from predominantly summer dominant in the north to winter dominant at the most southerly site. Several entries of subspecies varia and caerulea had herbage yields and persistence equivalent to that of M. sativa subspecies sativa cultivar Sceptre, a highly winter-active type that was used as a standard. The cultivar Cancreep, a cross of M. falcata and M. sativa, had a total yield over 3 years equivalent to 84–91% of Sceptre at the 2 sites where it was sown. Individual lines of subspecies varia demonstrated good persistence under grazing and were ranked 2nd and 6th out of 35 accessions for frequency in year 4 at Barmedman, the driest site, and 5th, 7th and 9th out of 33 accessions at Tamworth, the more summer-dominant rainfall site. Entries of subspecies falcata were among the least productive and persistent. The study indicated that germplasm from subspecies caerulea and varia offered hitherto unexploited potential for selection as persistent and drought-tolerant perennial legume alternatives to M. sativa for extensive low management grazing systems of south-eastern Australia.

Genetic Parameters and Genotype by Environment Interactions for Green and Basic Density and Stiffness of Pinus radiata D. Don Estimated Using Acoustics

Abstract Genetic parameters and genotype by environment interactions for wood properties of 13-year-old Pinus radiata were determined by sampling two progeny trials on contrasting sites in the southern slopes of New South Wales, Australia. Heritability of green and basic density were determined together with dynamic modulus of elasticity (MOE) measured using TreeTap. The phenotypic variance in MOE (CV 17-18%) was almost three times that of basic density. MOE and basic density were highly heritable at both sites (h2 0.57 &amp; 0.79 for MOE &amp; 0.59 and 0.85 for density). There was a moderately strong genetic correlation between MOE and basic density (0.64 across-sites). No genotype x environment interaction was detected in MOE or basic density. Basic density and MOE were lower on the warmer, lower rainfall site.

Managing productivity and drought risk in Eucalyptus globulus plantations in south-western Australia

More than 2.5 million ha of Eucalyptus globulus are now planted across the globe including approximately 500 000 ha in southern Australia. In this region average annual rainfall has declined since 1960 and this trend is predicted to continue in the coming decades. E. globulus is a premium species for paper manufacture and grows well under moderate seasonal water stress. The traits that underpin this rapid early growth also make the species vulnerable to prolonged water stress. We established nitrogen rate and nitrogen-by-stocking experiments in five 2-year-old E. globulus plantations along a climatic gradient in south-western Australia. We measured volume growth, predawn leaf water potential and leaf area index over 7 years or until the plantations were 9 years old. These data were used to explore the relationship between growth and water stress, to understand the mechanistic basis for the relationship and to identify best-bet management strategies for E. globulus plantations in southern Australia. Nearly all of the variation in volume growth rate between sites could be explained by a combination of climate wetness index and soil depth. There was a significant growth response to nitrogen at two low rainfall and one high rainfall site. There was no growth response to nitrogen on sites where total soil nitrogen in the top 0.1 m of soil was more than 1.9 mg g −1 and a very rapid increase in relative growth response below this threshold. The observed growth response to nitrogen was associated with an increase in water stress and on at least one site increased mortality. Matching the supply of nutrients to demand will maximise the growth at any site but this may increase the risk of drought death at water-limited sites. This will be exacerbated if forecasted changes in the climate of southern Australia are realised. Thinning to 600 stems ha −1 significantly reduced the level of water stress experienced by E. globulus in Western Australia without significantly affecting end of rotation stand volume compared to unthinned stands. These results indicate that for a range of sites in south-western Australia a final stocking density of 600 stems ha −1 coupled with application of fertiliser to maximise growth will minimise risk without sacrificing any of the site potential.

A nested multisite daily rainfall stochastic generation model

Summary This paper describes a nested multisite daily rainfall generation model which preserves the statistics at daily, monthly and annual levels of aggregation. A multisite two-part daily model is nested in multisite monthly, then annual models. A multivariate set of fourth order Markov chains is used to model the daily occurrence of rainfall; the daily spatial correlation in the occurrence process is handled by using suitably correlated uniformly distributed variates via a Normal Scores Transform (NST) obtained from a set of matched multinormal pseudo-random variates, following Wilks [Wilks, D.S., 1998. Multisite generalisation of a daily stochastic precipitation generation model. Journal of Hydrology 210, 178–191]; we call it a hidden covariance model. A spatially correlated two parameter gamma distribution is used to obtain the rainfall depths; these values are also correlated via a specially matched hidden multinormal process. For nesting, the generated daily rainfall sequences at all the sites are aggregated to monthly rainfall values and these values are modified by a set of lag-1 autoregressive multisite monthly rainfall models. The modified monthly rainfall values are aggregated to annual rainfall and these are then modified by a lag-1 autoregressive multisite annual model. This nesting process ensures that the daily, monthly and annual means and covariances are preserved. The model was applied to a region with 30 rainfall sites, one of the five sets reported by Srikanthan [Srikanthan, R., 2005. Stochastic Generation of Daily Rainfall Data at a Number of Sites. Technical Report 05/7, CRC for Catchment Hydrology. Monash University, 66p]. A comparison of the historical and generated statistics shows that the model preserves all the important characteristics of rainfall at the daily, monthly and annual time scales, including the spatial structure. There are some outstanding features that need to be improved: depths of rainfall on isolated wet days and sequences of wet days do not depend on the duration of the wet spell as well as they should; the remedy is not trivial, but will be addressed in a follow-up study. The skewness of monthly rainfall was not preserved well which appears to be a deficiency, but this is not considered important, because nearly all other validation statistics are well captured by the model and comparative cumulative frequency distribution plots of the daily depths show satisfactory matches.

Soil water, soil nitrogen and productivity of lucerne–wheat sequences on deep silt loams in a summer dominant rainfall environment

The management of water and nutrients in farming systems that incorporate alternating phases of lucerne pasture with annual grain crops posses additional challenges to rotations with annual crops only. Strategies for balancing water and nutrient resources within and across rotations will be governed by local soil, climatic, skill and economic constraints. On the Loess Plateau in China, farmers have been encouraged to grow lucerne ( Medicago sativa) to reduce soil erosion and improve soil fertility on cropping lands but with little supporting information on how to incorporate lucerne within subsistence cropping systems. Lucerne–wheat rotation experiments were established at two locations in Gansu Province and examined the yield of lucerne and wheat along with changes in soil water and nitrogen (N). Lucerne proved to be well adapted to the high water holding capacity soils and summer dominant rainfall environment of the region with annual production of around 12 t ha −1 at the higher rainfall site (Qingyang). An old (30 years), sparse stand of lucerne growing in the drier location (Dingxi) was much less productive, being dependent on incident rainfall. Apparent water use efficiency (WUE) of lucerne over individual harvest periods ranged from 4 to 56 kg ha −1 mm −1 at Qingyang. Lucerne was able to dry the soil to the crop lower limit (CLL) to depths of 3 m and there was clear evidence that lucerne roots were extracting water below this depth. Wheat following lucerne is subject to low plant available soil water at sowing, unless substantial rainfall occurs, but climate variability in this region makes this difficult to predict. Rain which falls during short fallow periods after lucerne termination provides opportunity for N fertiliser responses, which may be greater after large rainfall events that lead to N leaching. In drier environments such as Dingxi, deep drainage and leaching appear unlikely under rotations which incorporate lucerne, and here evaporative water loss from the soil surface presents a more significant management challenge. The overall variability in seasonal rainfall at both sites, even within the short period of this study, indicates that an adaptive management strategy may be required, rather than fixed rotations. Systems modelling may shed further light on the most useful strategies to manage crop rotations within this variability.

Changes in soil mineral nitrogen, nitrogen leached, and surface pH under annual and perennial pasture species

Soil mineral nitrogen (N) profiles during the growing season and changes in total soil N and available N after 3–4 years were examined under 9 different pasture swards containing annual legumes, lucerne (Medicago sativa L.), or one of 4 perennial grasses at 2 sites representative of the low and medium rainfall belt of south-eastern Australia. The effect of the presence of phalaris (Phalaris aquatica L.) or lucerne on the spatial variation in surface pH was also measured. The 9 pastures were subterranean clover (Trifolium subterraneum L.), subterranean clover with annual weeds, yellow serradella (Ornithopus compressus L.), lucerne, phalaris, cocksfoot (Dactylis glomerata L.), lovegrass (Eragrostis curvula (Schrader) Nees), wallaby grass (Austrodanthonia richardsonii (Cashm.) H.P. Linder), and a mixture of lucerne, phalaris, and cocksfoot. All the perennial treatments were sown with subterranean clover. Available mineral N values in the surface 0.10 m of soil following summer rainfall were substantially higher in pure subterranean clover or serradella (Ornithopus compressus L.) swards (24–50 μg N/g) than those containing a mixture of subterranean clover and perennials (9–20 μg N/g). Apparent leaching of soil nitrate down the profile during winter was greatest in annual pasture treatments and least in swards containing perennials. Soil pH(CaCl2) at the 0–0.10 m depth varied with proximity to perennial plants and was significantly higher (+0.2–1.1 pH units) near the base of perennial plants than in gaps between the perennials or in annual-only swards. Available mineral N to 1.0 m before cropping at the end of the pasture phase was highest following subterranean clover (175–344 kg N/ha) and serradella (202–316 kg N/ha) at both sites. Available N was lowest (91–143 kg N/ha) following perennial grass–clover swards at the drier site where the annual legume content was lower, but perennial grass–clover swards produced larger soil N values (147–219 kg N/ha) at the higher rainfall site. Removal of the pasture in August–September compared with November in the year before cropping increased available N at the time of sowing by an average of 44% (51 kg N/ha) at the drier site and 43% (74 kg N/ha) at the wetter site. Incorporating perennial pasture species in swards was found to be advantageous in reducing nitrate leaching and preventing a decline in surface soil pH; however, available soil N to following crops could be lower if the annual legume content of perennial grass-based pastures declined due to competition from the perennial species.

Variations in growth, survival and carbon isotope composition ( 13C) among Pinus pinaster populations of different geographic origins

To evaluate differences in growth and adaptability of maritime pine (Pinus pinaster Ait.), we studied growth, polycyclism, needle tissue carbon isotope composition (delta(13)C) as an estimate of water-use efficiency (WUE) and survival of seven populations at 10 years of age growing in a performance trial at a provenance test site in Escaroupim, Portugal. Six populations were from relatively high rainfall sites in Portugal and southwestern France (Atlantic group), and one population was from a more arid Mediterranean site in Spain. There were significant differences between some populations in total height, diameter at breast height, delta(13)C of bulk needle tissue, polycyclism and survival. A population from central Portugal (Leiria, on the Atlantic coast) was the tallest and had the lowest delta(13)C. Overall, the variation in delta(13)C was better explained by the mean minimum temperatures of the coldest month than by annual precipitation at the place of origin. Analyses of the relationships between delta(13)C and growth or survival revealed a distinct pattern for the Mediterranean population, with low delta(13)C (and WUE) associated with the lowest growth potential and reduced survival. There were significant negative correlations between delta(13)C and height or survival in the Atlantic group. Variation in polycyclism was correlated with annual precipitation at the place of origin. Some Atlantic populations maintained a high growth potential while experiencing moderate water stress. A detailed knowledge of the relationships between growth, survival and delta(13)C in contrasting environments will enhance our ability to select populations for forestry or conservation.