The history of conifers introduced earlier elsewhere in the southern hemisphere suggests that recent invasions in Argentina, Brazil, Chile and Uruguay are likely to increase in number and size. In South Africa, New Zealand and Australia, early ornamental introductions and small forestry plantations did not lead to large-scale invasions, while subsequent large plantations were followed with a lag of about 20-30 years by troublesome invasions. Large-scale conifer plantation forestry in South America began about 50-80 years later than in South Africa, Australia and New Zealand, while reports of invasions in South America lagged behind those in the latter nations by a century. Impacts of invading non-native conifers outside South America are varied and include replacement of grassland and shrubland by conifer forest, alteration of fire and hydrological regimes, modification of soil nutrients, and changes in aboveground and belowground biotic communities. Several of these effects have already been detected in various parts of South America undergoing conifer invasion. The sheer amount of area planted in conifers is already very large in Chile and growing rapidly in Argentina and Brazil. This mass of reproductive trees, in turn, produces an enormous propagule pressure that may accelerate ongoing invasions and spark new ones at an increasing rate. Regulations to control conifer invasions, including measures to mitigate spread, were belatedly implemented in New Zealand and South Africa, as well as in certain Australian states, inspired by observations on invasions in those nations. Regulations in South America are weaker and piecemeal, but the existing research base on conifer invasions elsewhere could be useful in fashioning effective regulations in South America. Pressure from foreign customers in South Africa has led most companies there to seek certification through the Forestry Stewardship Council; a similar programme operates in Australia. Such an approach may be promising in South America.a ec_2058 489..504

Summary1. Management decisions regarding invasive plants often have to be made quickly and in the face of fragmentary knowledge of their population dynamics. However, recommendations are commonly made on the basis of only a restricted set of parameters. Without addressing uncertainty and variability in model parameters we risk ineffective management, resulting in wasted resources and an escalating problem if early chances to control spread are missed. 2. Using available data for Pinus nigra in ungrazed and grazed grassland and shrubland in New Zealand, we parameterized a stage-structured spread model to calculate invasion wave speed, population growth rate and their sensitivities and elasticities to population parameters. Uncertainty distributions of parameters were used with the model to generate confidence intervals (CI) about the model predictions. 3. Ungrazed grassland environments were most vulnerable to invasion and the highest elasticities and sensitivities of invasion speed were to long-distance dispersal parameters. However, there was overlap between the elasticity and sensitivity CI on juvenile survival, seedling establishment and long-distance dispersal parameters, indicating overlap in their effects on invasion speed. 4. While elasticity of invasion speed to long-distance dispersal was highest in shrubland environments, there was overlap with the CI of elasticity to juvenile survival. In shrubland invasion speed was most sensitive to the probability of establishment, especially when establishment was low. In the grazed environment elasticity and sensitivity of invasion speed to the severity of grazing were consistently highest. Management recommendations based on elasticities and sensitivities depend on the vulnerability of the habitat. 5. Synthesis and applications. Despite considerable uncertainty in demography and dispersal, robust management recommendations emerged from the model. Proportional or absolute reductions in long-distance dispersal, juvenile survival and seedling establishment parameters have the potential to reduce wave speed substantially. Plantations of wind-dispersed invasive conifers should not be sited on exposed sites vulnerable to long-distance dispersal events, and trees in these sites should be removed. Invasion speed can also be reduced by removing seedlings, establishing competitive shrubs and grazing. Incorporating uncertainty into the modelling process increases our confidence in the wide applicability of the management strategies recommended here.

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Summary1. Globally, temperate grasslands have been heavily modified by agricultural intensification. The ecological integrity of many remaining semi-natural grasslands is further threatened by the encroachment of invasive alien woody plant species, such as conifers. 2. Although the ecological impact of conifers is often inferred to be directly related to increasing conifer density at invaded sites, interpretation of these density-dependent effects on native biodiversity is almost always confounded by strong correlations between density and time since invasion. Here, we isolate the direct effects of exotic conifer density on native invertebrate assemblages from the confounding effects due to time since invasion, by establishing replicated experimental plantings of the invasive Pinus nigra (densities of 400, 800, 1600, 2500 and 3500 trees per ha, established in 1993) in New Zealand. 3. From the 29 549 invertebrates captured, the relative abundance of major classes and orders was largely unaffected by conifer invasion at densities below 800 trees per ha at age 14 years, but differed substantially in higher-density conifer stands (canopy cover >50%). At the species level, beetle species composition was highly sensitive to conifer invasion at densities as low as 400 trees per ha at age 14 years. Changes in beetle species composition were correlated with reduced soil moisture, increases in canopy cover, and trap distance from the nearest semi-natural grassland. 4. Synthesis and applications. The effects of exotic conifer invasion on grassland invertebrate assemblages were strongly dependent on conifer density. In our study, the negative effects of conifer invasion on biodiversity were comparatively low at densities below £800 trees per ha at 14 years after invasion. To conserve invertebrate diversity in semi-natural grasslands we recommend a dual focus of conservation actions targeting: (i) immediate control, or thinning, of high density conifer infestations (>800 trees per ha) to restore habitat to a state suitable for grassland species, and (ii) longerterm removal of sparse, pre-coning infestations in order to prevent further spread. We propose the use of canopy cover as a proxy for the confounded relationship between conifer density and time since invasion.