Persistence of Ulex Europaeus in Managed Landscapes
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plants Review A World of Gorse: Persistence of Ulex europaeus in Managed Landscapes Nicholas Broadfield 1,2 and Melinda T. McHenry 1,* 1 Geography and Spatial Sciences, The University of Tasmania, Sandy Bay, Tasmania 7005, Australia; Nicholas.broadfi[email protected] 2 Sun Pharmaceuticals Australia, 14 Henry Street, Latrobe, Tasmania 7307, Australia * Correspondence: [email protected] Received: 19 October 2019; Accepted: 14 November 2019; Published: 19 November 2019 Abstract: Gorse (Ulex europeus L.) is a woody legume and invasive woody weed that has been introduced to temperate pastoral landscapes worldwide. Despite the apparent cosmopolitan distribution of gorse across much of the temperate agroecological landscapes of the world, research and practice pertaining to the management of gorse has been largely constrained to single-treatments, regions, or timeframes. Gorse eradication has been widely attempted, with limited success. Using the PRISMA (preferred reporting items for systematic reviews and meta-analysis) method and a quasi-metanalytical approach, we reviewed the seminal ~299 papers pertaining to gorse management. We identified (i) the ecological characteristics of the species that predispose gorse to behaving invasively, and (ii) the success of management actions (from a plant ecological life history perspective) in reducing weed vigour and impact. A broad ecological niche, high reproductive output, propagule persistence, and low vulnerability to pests allow for rapid landscape exploitation by gorse throughout much the world. Additionally, there are differences in flowering duration and season in the northern and southern hemisphere that make gorse particularly pernicious in the latter, as gorse flowers twice per year. The implications of these life history stages and resistance to environmental sieves after establishment are that activity and efficacy of control is more likely to be favourable in juvenile stages. Common approaches to gorse control, including herbicides, biological controls, and fire have not been ubiquitously successful, and may in fact target the very site resources—sward cover, soil stability, hydrological balance—that, when degraded, facilitate gorse invasion. Ongoing seedling regeneration presents difficulties if eradication is a goal, but facilitated competition may reduce costs via natural suppression. Mechanical methods of gorse removal, though highly successful, induce chronic soil erosion and land degradation and should hence be used sparingly. Keywords: woody weeds; fire; grazing; disturbance; cost-benefit; mycoherbicides; phytophagous 1. Introduction Common gorse (Ulex europaeus L.) is a thorny evergreen shrub that forms invasive thickets in pasture outside of its center of origin in Western Europe and Northern Africa [1]. First introduced for its potential as an ornamental, for fencing in agricultural landscapes, and for fodder, gorse is listed by the International Union for Conservation of Nature as one of the top 100 invasive plants on earth [2,3]. Due in part to accidental introductions due to soil translocation [4,5], gorse is now distributed from equatorial to temperate regions, but more successful in regions with limited exposure to seasonally extreme temperatures and annual rainfall between 500 and 1500 mm (Figure1). Plants 2019, 8, 523; doi:10.3390/plants8110523 www.mdpi.com/journal/plants Plants 2019, 8, 523 2 of 21 Plants 2019, 8, x FOR PEER REVIEW 2 of 21 FigureFigure 1. 1.Reported Reported globalglobal distributiondistribution ofof gorse.gorse. Green—natural distribution, distribution, red—introduced red—introduced populations,populations, black—unknown black—unknown (presumed(presumed introduced). Created Created in in ArcMap ArcMap 10.5 10.5 from from national national and and internationalinternational databases databases (after (after [ 6[6]).]). Biological,Biological, chemical, chemical, and mechanical mechanical intervention interventionss have have been been diverse diverse and numerous, and numerous, with none with noneachieving achieving ubiquity ubiquity in controlling in controlling the thenegative negative effects effects of gorse, of gorse, nor nor its itslarge large population population size. size. Additionally,Additionally, treatments treatments for for gorsegorse oftenoften occuroccur in singularity, or or target-specific target-specific life life history history stages, stages, withoutwithout opportunity opportunity to to examine examine longer-term longer-term outcomes,outcomes, nor nor contextualise contextualise these these within within environmental environmental or ecological constraints. or ecological constraints. Thus, in the present paper, we reviewed the outcomes of management actions used to control Thus, in the present paper, we reviewed the outcomes of management actions used to control gorse, throughout time and across the world, with particular emphasis on the interactions between gorse, throughout time and across the world, with particular emphasis on the interactions between species ecology, treatment efficacy, and treatment impacts upon environmental stability. We used species ecology, treatment efficacy, and treatment impacts upon environmental stability. We used peer-reviewed literature to first synthesise the life history and ecology of gorse (Objective One), peer-reviewedbefore reviewing literature management to first actions synthesise and their the lifequantified history successes and ecology and failures of gorse (Objective (Objective Two). One), beforeWe conclude reviewing this management article by suggesting actions targeted and their ma quantifiednagement successesof gorse that and reflects failures the (Objectiveecology of the Two). Wespecies conclude and thisthe environments article by suggesting that it is targetedmost successful management in invading of gorse (Objective that reflects Three). the ecology of the species and the environments that it is most successful in invading (Objective Three). 2. Findings 2. Findings 2.1. Life History and Ecological Success 2.1. Life History and Ecological Success The life history traits that facilitate the competitive success of gorse are persistent soil seedbanks andThe copious life history seed traitsoutput, that few facilitate constraints the competitiveon recruitment, success the ofability gorse to are modify persistent the surrounding soil seedbanks andenvironment copious seed to favour output, spread, few constraintsand a short gap on recruitment,between juvenile the phase ability and to fecundity modify the (Figure surrounding 2). environmentGorse tocolonises favour spread,bare ground and a short and gapmore between intact juvenilevegetation, phase such and fecundityas grassy (Figure woodland.2). ApproximatelyGorse colonises 6000–18,000 bare ground fertile and seeds more are intact produced vegetation, annually such [7–9] as grassy fromwoodland. mature individuals Approximately that 6000–18,000develop approximately fertile seeds 1000 are producedflowers per annually branch. Gorse [7–9] fromflowers mature cannot individualsself-pollinate, that ensuring develop approximatelyoutcrossing and 1000 genetic flowers variation per branch. [10]. Gorse Gorse flowers flowers in both cannot winter self-pollinate, and spring, avoiding ensuring seed outcrossing feeders andduring genetic the variation spring that [10 can]. Gorseprevent flowers up to 90% in both of seed winter production and spring, [11,12]. avoiding In the absence seed feeders of seed during feeding the springpredators, that can seed prevent set is almost up to 90% 100%. of seedSeed productionpod matura [tion11,12 takes]. In 8 the weeks. absence Mature of seed seeds feeding are dispersed predators, seedthroughout set is almost the 100%.landscape Seed initially pod maturation by the explosiv takes 8e weeks.dehiscing Mature of pods seeds (each are containing dispersed throughoutup to 12 theseeds), landscape scattering initially them by up the to explosive five meters dehiscing from the ofsource pods plant (each [13]. containing up to 12 seeds), scattering them upSoil to seedbank five meters viability from thevaries source between plant regions [13]. and soil depths, with estimates of 20 years of seed longevitySoil seedbank in the top viability five cm varies of soil, between and significantly regions and longer soil depths,lifespans with for estimatesseeds deeper of 20in yearsthe profile of seed longevitydue to reduced in the top temperature five cm of fluctuat soil, andions significantly [14]. Due to seedbank longer lifespans lifespan forand seeds an annual deeper seed in fall the from profile mature stands of 500–2000 seeds per m2, soil seedbank size can exceed 10,000 seeds per m2 [15–17]. due to reduced temperature fluctuations [14]. Due to seedbank lifespan and an annual seed fall from mature stands of 500–2000 seeds per m2, soil seedbank size can exceed 10,000 seeds per m2 [15–17]. MDPI | [SCHOOL] PlantsPlants2019 2019, 8,, 5238, x FOR PEER REVIEW 3 of3 21 of 21 Figure 2. Life history traits of common gorse, including factors influencing the success of the species in Figure 2. Life history traits of common gorse, including factors influencing the success of the species a range of global environments. in a range of global environments. Gorse seeds germinate under a wide temperature range (0 to 26 ◦C in vitro—commensurate Gorse seeds germinate under a wide temperature range (0 to 26 °C in vitro—commensurate with with observed live field emergence data [18]). Yet, mature, well-dried seeds sometimes have poor observed live field emergence