Predicting weed distribution at the landscape scale : using naturalized Brassica as a model system

1. Quantifying and predicting the distribution of naturalized plant species is a major concern for weed risk assessment and understanding the potential impacts of gene-flow between crop species and feral or naturalized populations. 2. We developed a rapid field assessment to quantify the distribution of naturalized Brassica populations at the landscape scale, using the Canterbury Plains region of New Zealand as a model system. Internationally, brassicas are one of the most widely cultivated crops, and have well-documented problems of crop escapes, gene-flow and hybridization with wild relatives. Brassicas are cultivated intensively in the study system for both seed production and forage, are widely naturalized and have raised concerns for gene-flow from feral populations to cultivated varieties. 3. Generalized linear models (GLM) were constructed in order to predict the presence-absence of brassicas using survey data of naturalized populations located in 50 3 × 3 km plots. This model was validated subsequently both by ground-truthing predicted presence-absence in a second field survey, and by comparing cumulative probability distributions from a presence-only botanical survey. 4. The best model produced from the field survey predicted naturalized Brassica presence-absence robustly (r² = 0.52) and was well validated against results from the presence-only survey carried out in the same year. Results from the second field survey during a different growing season did not match predictions from the original model, suggesting that strong between-year differences influence large-scale Brassica distribution. The differences between years were driven by an overall increase in the prevalence of brassicas over the course of the study that was not associated with any predictive variable or climate. 5. Synthesis and applications. These results have important implications both for developing models of species distribution and for predicting gene-flow risk at the landscape scale. Despite constructing a model that was subsequently validated from presence-only data in one year, it did not predict Brassica distribution in a subsequent year; this suggests that between-year or stochastic factors operate strongly and are a major consideration for risk assessment. Models used to predict species distributions, in general, need to account for these between-year effects either by incorporating mechanistic processes or long-term data.9 page(s

Cross-scale management strategies for optimal control of trees invading from source plantations

Biological invasion by non-native tree species can transform landscapes, and as a consequence, has received growing attention from researchers and managers alike. This problem is driven primarily by the naturalisation and invasion of tree species escaping from cultivation or forestry plantations. Furthermore, these invasions can be strongly influenced by the land-use matrix of the surrounding region, specific management of the source populations, and environmental conditions that influence seed dispersal or habitat quality for the invader. A major unresolved challenge for managing tree invasions in landscapes is how management should be deployed to contain or slow the spread of invading populations from one or more sources (e.g. plantations). We develop a spatial simulation model to test: (1) how to best prioritise the control of invasive tree populations spatially to slow or contain the biological invader when habitat quality varies in the landscape, and (2) how to allocate control effort among different management units when trees spread from many source populations. We first show that to slow down spread effectively, management strategy is less important than management effort. We then identify the conditions affecting the relative performance of different management strategies. At the landscape scale, targeting peripheral stands consistently yielded the best results whereas at the regional scale, management strategies needed to account for both habitat quality and tree life-history. Overall, our findings demonstrate that knowledge of how habitat affects tree life-history stages can improve management to contain or slow tree invasions by improving the spatial match between management effort and efficacy

HyPix: 1D physically based hydrological model with novel adaptive time-stepping management and smoothing dynamic criterion for controlling Newton–Raphson step

International audienceThe newly developed open-source Hydrological Pixel model, HyPix, written in the fast and flexible Julia lan- guage, efficiently solves the mixed form of the Richardson–Richards’ equation (RRE). HyPix uses a cell-centred, finite-volume scheme for the spatial discretization, with an implicit Euler scheme for the temporal discretization, by using the weighted average inter-cell hydraulic conductivity. HyPix includes the following modules: (a) rainfall interception, (b) root water uptake with compensation algorithm and root growth, (c) soil evaporation, (d) ponding using a novel method for computing sorptivity, and (e) runoff. HyPix includes a wide range of top and boundary conditions (flux, pressure, free drainage). To control the Newton–Raphson iterations, HyPix incorporates a novel dynamic physical smoothing criterion, which improves not only the model performance but also its accuracy compared with using the traditional absolute convergence criterion. To control the time-step, the traditional physical time-step management based on changes in the soil water content was specifically designed to solve RRE based on soil water content. This work adapts the time-step management such that it is specifically designed to solve RRE based on soil water pressure without introducing further parameters. The novel time-step management also requires only one parameter and was found to be more efficient than the traditional time-step management. HyPix implements an option to solve the derivatives numerically, enabling the RRE to be modified and tested (e. g., the inter-cell hydraulic conductivity) by changing only a few lines of code. Numerically calculating de- rivatives was found to be as accurate as deriving the derivatives analytically, and only 10–25% slower.The well-established hydrological model HYDRUS was used to validate HyPix without the sink term. The HyPix results show good agreement to HYDRUS, validating the algorithms implemented in HyPix. Even for challenging conditions, HyPix can provide accurate and reliable results using the recommended standard op- tions. Moreover, the algorithm developed in HyPix is more efficient than the one used in HYDRUS, particularly for coarse texture soils. The recommended options were also tested by running HyPix with sink term using field data

Multistep optimization of HyPix model for flexible vertical scaling of soil hydraulic parameters

International audienceEfficient simulation of water-flow processes in the vadose zone is crucial to increase agricultural productivity within environmental limits. This requires deriving detailed soil hydraulic parameters of the soil profile that is highly challenging, particularly for heterogeneous soils. We therefore developed an alternative indirect meth- odology to calibrate the hydraulic parameters from soil water content time series measured at multiple depths by using the new physically based hydrological model HyPix.We propose a novel, efficient, multistep optimization algorithm for layered soils that derives an optimal set of hydraulic parameters for a desired number of soil layers. For each selected soil layer, HyPix derives five physical, bimodal, Kosugi hydraulic parameters that describe the soil water retention and hydraulic conductivity by using a novel algorithm that reduces the degree of sensitivity and freedom of the parameters. The optimization al- gorithm upscales the soil hydraulic parameters by gradually incorporating the soil heterogeneity. This method overcomes the problems associated with optimization of the hydraulic parameters of each layer individually, which leads to poor results because it does not represent the cohesive soil water dynamics across the unsaturated zone.We tested the method using soil water content measurements at different depths at five heterogeneous experimental sites in New Zealand. We show how the accuracy of the simulated water balance components increases with the number of soil layers. The multistep optimization upscales a detailed, layered profile of soil hydraulic parameters into a model with fewer layers. The methodology developed provides an estimate of the uncertainty of using a reduced number of soil layers. We also show that a pedological description can provide an indication of the minimum soil layers of vertical discretization required to accurately compute the soil water balance components

Global to community scale differences in the prevalence of convergent over divergent leaf trait distributions in plan asssemblages.

Aim The drivers of species assembly, by limiting the possible range of functional trait values, can lead to either convergent or divergent distributions of traits in realized assemblages. Here, to evaluate the strengths of these species assembly drivers, we partition trait variance across global, regional and community scales. We then test the hypothesis that, from global to community scales, the outcome of co-occurring trait convergence and divergence is highly variable across biomes and communities. Location Global: nine biomes ranging from subarctic highland to tropical rain forest. Methods We analysed functional trait diversity at progressively finer spatial scales using a global, balanced, hierarchically structured dataset from 9 biomes, 58 communities and 652 species. Analyses were based on two key leaf traits (foliar nitrogen content and specific leaf area) that are known to drive biogeochemical cycling. Results While 35% of the global variance in these traits was between biomes, only 15% was between communities within biomes and as much as 50% occurred within communities. Despite this relatively high within-community variance in trait values, we found that trait convergence dominated over divergence at both global and regional scales through comparisons of functional trait diversity in regional and community assemblages against random (null) models of species assembly. Main conclusions We demonstrate that the convergence of traits occurring from global to regional assemblages can be twice as strong as that from regional to community assemblages, and argue that large differences in the nature and strength of abiotic and biotic drivers of dominant species assembly can, at least partly, explain the variable outcome of simultaneous trait convergence and divergence across sites. Ultimately, these findings stress the urgent need to extend species assembly research to address those scales where trait variance is the highest, i.e. between biomes and within communities. © 2011 Blackwell Publishing Ltd

Global effects of non-native tree species on multiple ecosystem services

Non-native tree (NNT) species have been transported worldwide to create or enhance services that are fundamental for human well‐being, such as timber provision, erosion control or ornamental value; yet NNTs can also produce undesired effects, such as fire proneness or pollen allergenicity. Despite the variety of effects that NNTs have on multiple ecosystem services, a global quantitative assessment of their costs and benefits is still lacking. Such information is critical for decision-making, management and sustainable exploitation of NNTs. We present here a global assessment of NNT effects on the three main categories of ecosystem services, including regulating (RES), provisioning (PES) and cultural services (CES), and on an ecosystem disservice (EDS), i.e. pollen allergenicity. By searching the scientific literature, country forestry reports, and social media, we compiled a global data set of 1683 case studies from over 125 NNT species, covering 44 countries, all continents but Antarctica, and seven biomes. Using different meta-analysis techniques, we found that, while NNTs increase most RES (e.g. climate regulation, soil erosion control, fertility and formation), they decrease PES (e.g. NNTs contribute less than native trees to global timber provision). Also, they have different effects on CES (e.g. increase aesthetic values but decrease scientific interest), and no effect on the EDS considered. NNT effects on each ecosystem (dis)service showed a strong context dependency, varying across NNT types, biomes and socio-economic conditions. For instance, some RES are increased more by NNTs able to fix atmospheric nitrogen, and when the ecosystem is located in low-latitude biomes; some CES are increased more by NNTs in less-wealthy countries or in countries with higher gross domestic products. The effects of NNTs on several ecosystem (dis)services exhibited some synergies (e.g. among soil fertility, soil formation and climate regulation or between aesthetic values and pollen allergenicity), but also trade-offs (e.g. between fire regulation and soil erosion control). Our analyses provide a quantitative understanding of the complex synergies, trade-offs and context dependencies involved for the effects of NNTs that is essential for attaining a sustained provision of ecosystem services