Invasive Willow Research Articles

Jump‐starting podocarp forest establishment in a wetland dominated by invasive willow

Kahikatea (Dacrycarpus dacrydioides) is a tall, wetland‐tolerant native podocarp tree that has potential for restoration of New Zealand wetlands currently dominated by invasive Grey willow (Salix cinerea). We investigated methods to jump‐start establishment of Kahikatea saplings in the Ramsar‐listed Whangamarino Wetland. Treatments involved planting saplings after full herbicide clearance of the willow canopy (FULL), partial herbicide clearance of the canopy (PARTIAL), and manual cutting of light wells (MANUAL). Saplings were planted as individuals or clusters of seven. Treatment plots were established in 2015 and monitored until 2020. Despite Kahikatea being considered a “light‐demanding” species, saplings grew best in the PARTIAL treatment which had the lowest ambient light levels (x̄=32% of non‐intercepted sunlight). PARTIAL treatment saplings were estimated to require 24–36 years to reach the height of the surrounding willow canopy if they maintained similar growth rates. Preconditioning saplings by soaking roots before planting provided a growth advantage in the PARTIAL treatment plots, whereas nonconditioned plants grew better in the higher‐light MANUAL and FULL plots. No benefit was found for cluster planting. Restoration toward a native Kahikatea‐dominated forest will be contingent on saplings reaching willow canopy height before the faster‐growing willows reestablish, or periodic management of willows to maintain light levels above the compensation point for continued growth. Ultimately, the utility of this approach depends on what on‐going facilitation is required and whether seedlings subsequently produced by these emergent podocarp “islands” are able to jump‐start a long‐term change in the successional trajectory of the surrounding willow‐dominated wetland.

Honeydew production by the Giant Willow Aphid (Tuberolachnus salignus, Hemiptera: Aphididae) and its effect on foraging yellowjackets (Hymenoptera: Vespidae).

Understanding the factors that promote the success of invasive species is important for managing biological invasions. Interactions between invasives and other species (e.g., competitors, pathogens, or predators), could favor or limit their success. In recent decades, yellowjacket wasps, including Vespula germanica and V. vulgaris, have successfully established in Patagonia. Additionally, the invasive willow Salix fragilis has invaded areas next to watercourses, which in turn are typically colonized by the giant willow aphid (GWA, Tuberolagnus salignus), an additional species characterized as a successful invader in many regions worldwide. Aphid exudate (honeydew) has been reported to be used as a carbohydrate source by social wasps. The aim of our study was to gain a better understanding of the infestation pattern of the GWA in northwestern Patagonia, its effect on exudate availability and its relationship with yellowjacket foraging patterns. The study was conducted under the working hypothesis that the increase in the size of GWA colonies and resulting honeydew production, will fuel an increase in local Vespula spp. We found that the aphid honeydew is produced in relatively high amounts in the region (estimated at 1517 ± 139 Kg/ha/season), with strong indications that it is used by yellowjackets because of the significantly higher abundance levels of yellowjackets foraging on honeydew. The interaction of three invasive species, willows, GWA and yellowjackets, needs to receive special attention to develop future environmentally-sound mitigation tools of these nuisance pest.

Are invasive species promoting yellowjacket invasion in Patagonia?

AbstractIn recent decades, yellowjacket wasps, including Vespula vulgaris, have successfully been established in Patagonia. Additionally, the invasive willow Salix fragilis has also invaded areas next to watercourses, which in turn are typically colonized by the giant willow aphid (GWA, Tuberolagnus salignus), another invasive species. The honeydew exuded by the aphids has been reported to be used as a carbohydrate source by this social wasp. Our aim was to determine whether willows and GWA (invasive complex) on the stream shores promotes yellowjacket population growth. We worked under the hypothesis that GWA provides honeydew (i.e. carbohydrates) to yellowjackets, favouring nest growth and individual fitness. Our results indicate that habitats close (<200 m) to stream margins, where S. fragilis and GWA occur, contain not only a greater abundance of workers but also a higher nest density of V. vulgaris yellowjackets. Even though the nests were similar in size irrespective of the distance to the stream, weight, size and nutritional status of individuals suggest that foraging/nesting in proximity of the invasive complex promotes larger and better nurtured wasps, including reproductive castes, that in the long run could drive population growth and spread of the species in Patagonia. In this context, our study suggests that management of V. vulgaris populations should adopt strategies that encompass a broad approach, especially adopting new strategies designed to contemplate relevant multitrophic interactions between yellowjackets and invasive willows and aphids.

Revision of the West Palaearctic Euura bergmanni and oligospila groups (Hymenoptera, Tenthredinidae)

Eight Western Palaearctic Euura species are here assigned to the bergmanni group (bergmanni, brevivalvis, dispar, glutinosae, leptocephalus, respondens, sylvestris, and viridis) and two species to the oligospila group (frenalis and oligospila). Euura pallens (Konow, 1903) (bergmanni group) is removed from the list of West Palaearctic taxa. Euura pyramidalis (Hellén, 1948) is treated as incertae sedis within the bergmanni group. Definitions of the bergmanni and oligospila groups are primarily based on genetic sequence data (mitochondrial COI and nuclear NaK and POL2). We report likely occurrence of heteroplasmy and amplification of NUMTs among some of the treated species, complicating the use of DNA barcoding in species discrimination. Based on morphological and genetic evidence, we establish that the correct name for the invasive willow sawfly in the southern hemisphere (South America, southern Africa, Australia, New Zealand), known there only in the female sex, is Euura respondens (Förster, 1854). The species is probably native to the Palaearctic (or even Holarctic) where males are common: possibly as common as females (examined from Europe and Central Asia). The name Euura oligospila (Förster, 1854) has been incorrectly used for the species in the southern hemisphere. The examination of type material and reliable association of males and females based on genetics revealed that females of E. oligospila are morphologically extremely similar to E. respondens (and to some other E. bergmanni group species), but male penis valves and genetics enable reliable separation of these species. Morphological separation of females of E. oligospila and E. respondens is possible, but challenging. Identification keys for males and females of the bergmanni and oligospila groups are provided. The following 15 new synonymies are proposed: Nematus validicornis Förster, 1854, syn. nov. with Euura bergmanni (Dahlbom, 1835); Pteronidea woollatti Lindqvist, 1971, syn. nov. and Nematus turgaiensis Safjanov, 1977, syn. nov. with Euura brevivalvis (Thomson, 1871); Pteronidea pseudodispar Lindqvist, 1969, syn. nov. with Euura dispar (Zaddach, 1876); Nematus (Pteronidea) fastosus var. ponojense Hellén, 1948, syn. nov. and N. (P.) fastosus var. punctiscuta Hellén, 1948, syn. nov. with Euura frenalis (Thomson, 1888); Nematus declaratus Muche, 1974, syn. nov. and N. desantisi D.R. Smith, 1983, syn. nov. with Euura respondens (Förster, 1854); Pteronidea straminea Lindqvist, 1958, syn. nov., P. angustiserra Lindqvist, 1969, syn. nov., and P. disparoides Lindqvist, 1969, syn. nov. with Euura sylvestris (Cameron, 1884); Pteronidea breviseta Lindqvist, 1946, syn. nov., P. breviseta Lindqvist, 1949, syn. nov., P. abscondita Lindqvist, 1949, syn. nov., and P. lauroi Lindqvist, 1960, syn. nov. with Euura viridis (Stephens, 1835). Lectotypes are designated for 18 nominal taxa: Amauronematus longicornis Konow, 1897; A. spurcus Konow, 1904; Nematus bergmanni Dahlbom, 1835; N. brevivalvis Thomson, 1871; N. curtispina Thomson, 1871; N. (Pteronidea) fastosus var. ponojense Hellén, 1948; N. (P.) fastosus var. punctiscuta Hellén, 1948; N. glutinosae Cameron, 1882; N. microcercus Thomson, 1871; N. polyspilus Förster, 1854; N. prasinus Hartig, 1837; N. respondens Förster, 1854; N. salicivorus Cameron, 1882; N. validicornis Förster, 1854; N. virescens Hartig, 1837; Pteronidea curtispina var. luctuosa Enslin, 1916; Pteronus fastosus Konow, 1904; and P. pallens Konow, 1903.

Invasive willows drive instream community structure

Summary Exotic invasive species are known to influence taxonomic communities and ecological processes. Few studies have examined the impact on taxonomic communities across trophic levels because it is time consuming and requires a wide range of identification skills. Here, we examined the impacts of invasive willow trees (Salix spp.) on microbial and macroinvertebrate composition using a combination of high resolution ecogenomic and classical approaches. Aquatic communities were compared and contrasted between stream reaches with and without willow infestation in three temperate Australian streams. Willows significantly altered in‐stream fungal, algal and macroinvertebrate communities. Macroinvertebrate diversity was similar between riparian types. Willow‐infested sites were characterised by lower macroinvertebrate abundance and Australian biotic index scores, indicating the presence of taxa generally more tolerant to pollution. At invaded sites, grazers were replaced by generalist collector–gatherer taxa and the proportions of macroinvertebrate functional‐feeding groups (FFG) were altered such that the primary trophic pathway was changed from an autotrophic algal‐based pathway to a heterotrophic, detrital‐based pathway. Riparian invasion by willows may change the primary source of energy from autotrophy to heterotrophy in small streams, with knock‐on effects on community structure across trophic levels.

Modelling spatial and temporal changes with GIS and Spatial and Dynamic Bayesian Networks

State-and-transition models (STMs) have been successfully combined with Dynamic Bayesian Networks (DBNs) to model temporal changes in managed ecosystems. Such models are useful for exploring when and how to intervene to achieve the desired management outcomes. However, knowing where to intervene is often equally critical. We describe an approach to extend state-and-transition dynamic Bayesian networks (ST-DBNs) — incorporating spatial context via GIS data and explicitly modelling spatial processes using spatial Bayesian networks (SBNs). Our approach uses object-oriented (OO) concepts and exploits the fact that ecological systems are hierarchically structured. This allows key phenomena and ecological processes to be represented by hierarchies of components that include similar, repetitive structures. We demonstrate the generality and power of our approach using two models — one developed for adaptive management of eucalypt woodland restoration in south-eastern Australia, and another developed to manage the encroachment of invasive willows into marsh ecosystems in east-central Florida.

Knowledge of the reproductive ecology of the invasive Salix cinerea, in its invaded range, assists in more targeted management strategies

Management for invasive species is expensive, costing millions of dollars every year and is sometimes unsuccessful. Current control efforts for the highly invasive willow species, Salix cinerea, are extensive, costly, and not always effective because of rapid post-removal reinfestation. An improved knowledge of the reproductive ecology of this species will help design control strategies that minimise future expansion and make current control efforts more effective. To explicitly examine the reproductive ecology of this species in its invaded range in south-eastern Australia, field observations of population structure and sex ratios, pollination experiments and germination trials were undertaken across the Ovens catchment in Victoria. Pollination-exclusion experiments strongly suggest that Salix cinerea in Australia employs two pollen-dispersal mechanisms, with insects being the dominant vector and wind playing a small but significant role (ambophilous). Seed had high initial germination rates, above 95%, but seed is very short-lived with rapid linear decline in germinability until 8 weeks, after which there was no germination at all. An improved understanding of the limited seed bank of this species and the potential it has for long distance pollination has highlighted that preventing reinfestation after control efforts should be focussed on limiting seed production and dispersal. A greater understanding of a species reproductive ecology, particularly of weed species, in their new environment can assist land managers to more effectively target their efforts for long-term eradication strategies.

Age structure, growth performance and composition of native and invasive Salicaceae in Patagonia

In northern Patagonia, riparian ecosystems are highly modified due to alien plant invasions. The establishment of non-native willows and poplars has unknown effects for Salix humboldtiana, the only native floodplain tree species in this region. To clarify establishment processes and to assess whether interspecific competition among the Salicaceae taxa may occur and therefore whether the native species can coexist in the long term, we analysed age structure and growth performances of the dominant four taxa within mixed adult forest stands along the Rio Negro, Argentina. Low mean ages of <15 years for all four taxa within 20 stands could be detected and suggest frequent and severe disturbances resulting in the removal of existing vegetation and subsequent creation of sites for establishment. Trees of the same stand showed the same age structure indicating joint establishment events with all taxa involved. A significant better growth performance (basal area, crown diameter) could be proved for the invasive willows. Considering the upper and middle river separately in a total of 88 studied plots, S. humboldtiana was more frequent at the middle river stretch, while invasive willows showed an opposing pattern suggesting a downstream directed invasion process. The results showed that competition pressure in mature mixed stands could affect S. humboldtiana. However, these stands are usually removed by river dynamics creating new sediment bars and islands. These processes enable S. humboldtiana seeds to germinate frequently which may compensate for its inferior growth performance and potential impacts by the invasive willows and poplars.

Are Vegetative Reproduction Capacities the Cause of Widespread Invasion of Eurasian Salicaceae in Patagonian River Landscapes?

In recent decades, invasive willows and poplars (Salicaceae) have built dense floodplain forests along most of the rivers in Patagonia, Argentina. These invasion processes may affect Salix humboldtiana as the only native floodplain tree species in this region. It is assumed, that the property to reproduce vegetatively can play an important role in the establishment of invasive species in their new range. Thus, in order to contribute to a better understanding of willow and poplar invasions in riparian systems and to assess the potential impacts on S. humboldtiana the vegetative reproduction capacities of native and invasive Salicaceae were analysed. In a greenhouse experiment, we studied cutting survival and growth performance of the three most dominant invasive Salicaceae of the Patagonian Río Negro region (two Salix hybrids and Populus spec.), as well as S. humboldtiana, taking into account three different moisture and two different soil conditions. In a subsequent experiment, the shoot and root biomass of cuttings from the former experiment were removed and the bare cuttings were replanted to test their ability to re-sprout. The two invasive willow hybrids performed much better than S. humboldtiana and Populus spec. under all treatment combinations and tended to re-sprout more successfully after repeated biomass loss. Taking into account the ecology of vegetative and generative recruits of floodplain willows, the results indicate that the more vigorous vegetative reproduction capacity can be a crucial property for the success of invasive willow hybrids in Patagonia being a potential threat for S. humboldtiana.

Novel microsatellite DNA markers indicate strict parthenogenesis and few genotypes in the invasive willow sawflyNematus oligospilus

Invasive organisms can have major impacts on the environment. Some invasive organisms are parthenogenetic in their invasive range and, therefore, exist as a number of asexual lineages (=clones). Determining the reproductive mode of invasive species has important implications for understanding the evolutionary genetics of such species, more especially, for management-relevant traits. The willow sawfly Nematus oligospilus Förster (Hymenoptera: Tenthredinidae) has been introduced unintentionally into several countries in the Southern Hemisphere where it has subsequently become invasive. To assess the population expansion, reproductive mode and host-plant relationships of this insect, microsatellite markers were developed and applied to natural populations sampled from the native and expanded range, along with sequencing of the cytochrome-oxidase I mitochondrial DNA (mtDNA) region. Other tenthredinids across a spectrum of taxonomic similarity to N. oligospilus and having a range of life strategies were also tested. Strict parthenogenesis was apparent within invasive N. oligospilus populations throughout the Southern Hemisphere, which comprised only a small number of genotypes. Sequences of mtDNA were identical for all individuals tested in the invasive range. The microsatellite markers were used successfully in several sawfly species, especially Nematus spp. and other genera of the Nematini tribe, with the degree of success inversely related to genetic divergence as estimated from COI sequences. The confirmation of parthenogenetic reproduction in N. oligospilus and the fact that it has a very limited pool of genotypes have important implications for understanding and managing this species and its biology, including in terms of phenotypic diversity, host relationships, implications for spread and future adaptive change. It would appear to be an excellent model study system for understanding evolution of invasive parthenogens that diverge without sexual reproduction and genetic recombination.

Impacts of an invasive willow (Salix × rubens) on riparian bird assemblages in south-eastern Australia

We explored how a woody plant invader affected riparian bird assemblages. We surveyed 15 200-m-long transects in riparian zones in a much-changed landscape of eastern Victoria, Australia. Abundance, species-richness, foraging-guild richness and composition of birds were compared in transects in three habitat types: (i) riparian zones dominated by the invasive willow Salix × rubens; (ii) riparian zones lined with native woody species; and (iii) riparian zones cleared of almost all woody vegetation. We also measured abundance and richness of arthropods and habitat structure to explore further the effects of food resources and habitat on the avifauna. We observed 67 bird species from 14 foraging guilds. Native riparian transects had more birds, bird species and foraging guilds than willow-invaded or cleared transects. Habitat complexity increased from cleared to willow-invaded to native riparian transects, as did abundance of native and woodland-dependent birds. Native shrub and trees species had more foliage and branch-associated arthropods than did willows, consistent with a greater abundance and variety of foraging guilds of birds dependent on this resource. Willow spread into cleared areas is unlikely to facilitate greatly native bird abundance and diversity even though habitat complexity is increased. Willow invasion into the native riparian zone, by decreasing food resources and altering habitat, is likely to reduce native bird biodiversity and further disrupt connectivity of the riparian zone.

A method for mapping the distribution of willow at a catchment scale using bi-seasonal SPOT5 imagery

Summary This study showed that seasonal imagery acquired at specific stages of phenology can be used to improve the mapping accuracy of invasive willow at a catchment scale. SPOT5 XI (10 m) satellite imagery was acquired for early autumn and winter to represent the phenological stages of leaf cover and leaf fall respectively. Four classification regimes were evaluated using single- and bi-seasonal composite imagery to determine the most accurate method. Significant spectral noise was found in willow populations, especially in the winter image, due to the effects of undergrowth exposure, shadowing, topography and boundary-mixed pixels. Two noise reduction techniques were applied to the bi-seasonal composite image to improve the classification results. The noise-reduced bi-seasonal composite image was classified using the spectral angle mapper (SAM) algorithm before importation into a geographical information system. Aerial photography was used to reduce the errors of commission associated with misclassification of pastures. The class accuracy achieved for willow using the method described in this study was 77.5% (Kappa =0.87). The high cost of eradicating willow means that managers must establish priorities for control; this technique can provide a powerful tool for prioritizing control programmes and for monitoring results at a catchment scale.

Strategies for the biological control of invasive willows (Salix spp.) in Australia

Abstract Willows (Salix spp.) are Weeds of National Significance in Australia where a large number of taxa are naturalised in temperate regions and can cause serious environmental degradation of riparian and wetland habitats. Several species are of economic or ornamental value and conflicts of interest could arise with planning their suppression. Biological control of six willow species (S. alba L., S. cinerea L., S. fragilis L., S. nigra Marshall, S. viminalis L., S. x rubens Schrank) is under consideration in Australia using three approaches based on targeting different morphological structures. A rich phytophagous biota associated with Salix in their regions of origin indicate a range of organisms are available for development as biological control agents. Implementation strategies will depend on the host species targeted for biological control, their reproductive strategies, the level of control required to reduce environmental impacts and the level of conflict of interest associated with a biological control program.