Nassella trichotoma in modified tussock grasslands in New Zealand: a case study in landscape‐scale invasive plant population monitoring

An extensive metapopulation of Nassella trichotoma (nassella tussock) distributed over 878 properties, mostly pastoral farms, in the Hurunui District of North Canterbury, New Zealand, has been the subject of a regional management plan since 1941, that aimed at ‘eradication’ and more recently ‘progressive control’. Here, we describe the implementation, analysis and interpretation of a monitoring programme designed to quantify the expected decline in the weed's population. For this purpose, an annual stratified random survey was initiated in 1998. Properties were allocated to strata primarily according to their N. trichotoma plant population sizes, and sample size per stratum was determined using an optimal allocation procedure. Annually, the N. trichotoma plants on random transects within ‘homogeneous areas’ on each of about 50 randomly selected properties were counted, and the metapopulation density estimated. After adjustment for ‘ease of detection’ during monitoring, regression analysis revealed that the metapopulation of N. trichotoma has been stable from 1998 until 2014. The adaptive management response could be either to change the management to achieve population decline or change the objective to one of maintaining the stability in the metapopulation. This case study has many species‐specific features, but nevertheless, serves as an illustration of a general methodology for monitoring.     

Ecology,distribution and control of the invasive weed Nassella trichotoma (Nees) Hack. ex Arechav.: A global review of current and future challenges

Nassella trichotoma (serrated tussock) is a highly invasive perennial C₃ weed from South America. It grows in most soil conditions, can resist fire and frost, and is unpalatable to grazing animals. Each plant can produce up to 140,000 seeds annually, and together, these characteristics make it a damaging landscape weed. It has diminished the agricultural carrying capacity of pastures in south-eastern Australia, New Zealand and South Africa, and emerging populations have now been identified in Europe and the United States, and bioclimatic models suggest its distribution could significantly expand within these regions in the near future. Research into control methods for this weed has been explored, and these include herbicides applied alone and in combination, the establishment of plant competition, the introduction of seed mitigation fencing, grazing management and exclusion zones, specific biological management and alteration of soil composition. Currently, the most effective and widely used control method is the residual herbicide flupropanate (2,2,3,3-tetrafluoropropanoic acid). This review will investigate the ecology, distribution, current control techniques and past research on this species, and make recommendations for future research and management.     

Use of mathematical modeling and its inverse analysis for precise assessment of pesticide dissipation in a paddy environment

The extrapolability of the lysimeter test as a dissipation simulator in an actual paddy field was evaluated using mathematical models and their inverse analyses for predicting pesticide fate and transport processes in paddy test systems. As a source of experimental data, a four-year comparative experiment in lysimeters and paddy fields was conducted using various paddy pesticides. First, the dissipations for various active ingredients in granule pesticides under submerged applications were statistically compared using simple kinetic modeling. Second, the dissipation pathways, unobserved experimental components, and effect of the experimental setting were evaluated using a higher tier mathematical model with a novel inverse analysis protocol. Finally, owing to experimental constraints, the unobtainable parameters were extracted from the laboratory container test before being transferred to compare the outdoor experimental data under different formulation types.     

Comparison of two remote sensing models for estimating evapotranspiration: algorithm evaluation and application in seasonally arid ecosystems in South Africa

Remote sensing tools are becoming increasingly important for providing spatial information on water use by different ecosystems. Despite significant advances in remote sensing based evapotranspiration (ET) models in recent years, important information gaps still exist on the accuracy of the models particularly in arid and semi-arid environments. In this study, we evaluated the Penman-Monteith based MOD16 and the modified Priestley-Taylor (PT-JPL) models at the daily time step against three measured ET datasets. We used data from two summer and one winter rainfall sites in South Africa. One site was dominated by native broad leaf and the other by fine leafed deciduous savanna tree species and C₄ grasses. The third site was in the winter rainfall Cape region and had shrubby fynbos vegetation. Actual ET was measured using open-path eddy covariance systems at the summer rainfall sites while a surface energy balance system utilizing the large aperture boundary layer scintillometer was used in the Cape. Model performance varied between sites and between years with the worst estimates (R²<0.50 and RMSE>0.80 mm/d) observed during years with prolonged mid-summer dry spells in the summer rainfall areas. Sensitivity tests on MOD16 showed that the leaf area index, surface conductance and radiation budget parameters had the largest effect on simulated ET. MOD16 ET predictions were improved by: (1) reformulating the emissivity expressions in the net radiation equation; (2) incorporating representative surface conductance values; and (3) including a soil moisture stress function in the transpiration sub-model. Implementing these changes increased the accuracy of MOD16 daily ET predictions at all sites. However, similar adjustments to the PT-JPL model yielded minimal improvements. We conclude that the MOD16 ET model has the potential to accurately predict water use in arid environments provided soil water stress and accurate biome-specific parameters are incorporated.     

Parthenium hysterophorus in Nepal: a review of its weed status and possibilities for management

The rapid expansion of the alien invasive Parthenium hysterophorus is a new agricultural and environmental problem for Nepal. Although the weed was first recorded in 1967, the most significant expansion has occurred in the last 20 years. The weed is thought to have entered Nepal from India and currently is found in the Tarai, Siwalik and hill regions of Nepal. A CLIMEX modelling projection has shown these regions to be climatically suitable for the growth of P. hysterophorus, both under the present and a future projected increased temperature. From the initially invaded roadside vegetation, the weed has now spread into cropping land and forests. Vehicle movement and transportation of agriculture products are the main means by which its seed is spreading. The weed has reduced plant species richness and changed species composition in grasslands, while fodder collectors have developed contact dermatitis from the plant. To date, no sustainable management strategy has been developed for this weed. However, the fortuitous arrival of the biological control agents Zygogramma bicolorata and Puccinia abrupta var. partheniicola has had some impact upon the weed's growth and abundance. A CLIMEX modelling projection identifies many suitable locations for Z. bicolorata outside of its present range and within areas where the weed is likely to spread, both under the present and under a future projected climate of increased temperature. Mapping of the currently invaded areas, management through community involvement and further releases of new and redistribution of current biological control agents are required to manage P. hysterophorus in Nepal.