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The introduced [C.sub.4] bunchgrass, Schizachyrium condensatum, is abundant in unburned, seasonally dry woodlands on the island of Hawaii, where it promotes the spread of fire. After fire, it is partially replaced by Melinis minutiflora, another invasive [C.sub.4] grass. Seed bank surveys in unburned woodland showed that Melinis seed is present in locations without adult plants. Using a combination of germination tests and seedling outplant experiments, we tested the hypothesis that Melinis was unable to invade the unburned woodland because of nutrient and/or light limitation. We found that Melinis germination and seedling growth are depressed by the low light levels common under Schizachyrium in unburned woodland. Outplanted Melinis seedlings grew rapidly to flowering and persisted for several years in unburned woodland without nutrient additions, but only if Schizachyrium individuals were removed. Nutrients alone did not facilitate Melinis establishment. Competition between Melinis and Schizachyrium naturally occurs when individuals of both species emerge from the seed bank simultaneously, or when seedlings of one species emerge in sites already dominated by individuals of the other species. When both species are grown from seed, we found that Melinis consistently outcompetes Schizachyrium, regardless of light or nutrient treatments. When seeds of Melinis were added to pots with well-established Schizachyrium (and vice versa), Melinis eventually invaded and overgrew adult Schizachyrium under high, but not low, nutrients. By contrast, Schizachyrium could not invade established Melinis pots regardless of nutrient level. A field experiment demonstrated that Schizachyrium individuals are suppressed by Melinis in burned sites through competition for both light and nutrients. Overall, Melinis is a dominant competitor over Schizachyrium once it becomes established, whether in a pot or in the field. We believe that the dominance of Schizachyrium, rather than Melinis, in the unburned woodland is the result of asymmetric competition due to the prior establishment of Schizachyrium in these sites. If Schizachyrium were not present, the unburned woodland could support dense stands of Melinis. Fire disrupts the priority effect of Schizachyrium and allows the dominant competitor (Melinis) to enter the system where it eventually replaces Schizachyrium through resource competition. Key words: biological invasion; [C.sub.4] grasses; competition; exotic grasses; fire; Hawaiian woodlands; invasion; Melinis minutiflora; nitrogen; resource limitation; Schizachyrium condensatum; seasonally dry woodland.

Despite a large body of research documenting invasive plant impacts, few studies have followed individual invaded sites over decades to observe how they change, and none have contrasted how compositional impacts from invasion compare to ecosystem-process impacts over a multi-decadal time-scale., Using direct measurements of plant density and composition and of ecosystems processes, we evaluate how ecosystem structure, above-ground net primary production ( ANPP), and above-ground and soil nutrient pools compare over 25 years since fire and C4 grass invasions disrupted seasonally dry Hawaiian woodlands. We compare structure and function between primary woodland that has never burned and is largely native species-dominated, with sites that had been the same woodland type but burned in alien-grass-fuelled fires in the 1970s and 1980s. The sites have not experienced fires since 1987., We report here that woody plant composition and structure continue to be dramatically changed by the initial invasions and fires that occurred 25 years ago and invaders continue to dominate in burned sites. This is reflected in continued low plant carbon pools in burned compared to unburned sites. Yet ANPP and N storage, which were dramatically lower in the initial decade after invasive-grass fuelled fires, have increased and are now indistinguishable from values measured in intact woodlands. Soil carbon pools were resilient to both invasion and fire initially and over time., Above-ground net primary production has recovered because of invasion of burned sites by a non-native N-fixing tree rather than because of recovery of native species. This invasive N-fixing tree is unlikely to return C storage of the invaded sites to those of unburned woodland because of its tissue and growth characteristics and its interactions with invasive grasses. It does not facilitate native species but rather promotes a persistent invasive grass/N-fixer savanna., Synthesis. We conclude that fire, an unusual disturbance in this system, has perpetuated the dominance of these sites by invasive species and that despite the dramatic recovery of above-ground net primary production and N pools, the ecosystem continues to be in a distinctly different state than the pre-fire, pre- Melinis community. Thus, despite the absence of further disturbance (fire), there is no evidence that succession towards the original ecosystem is occurring. The fact that N pools and above-ground net primary production recover because of a new invader ( Morella faya), highlights the unpredictability of ecosystem trajectories in the face of altered regional species pools. [ABSTRACT FROM AUTHOR]

Most ecosystems have multiple-plant invaders rather than single-plant invaders, yet ecological studies and management actions focus largely on single invader species. There is a need for general principles regarding invader interactions across varying environmental conditions, so that secondary invasions can be anticipated and managers can allocate resources toward pretreatment or postremoval actions. By reviewing removal experiments conducted in three Hawaiian ecosystems (a dry tropical forest, a seasonally dry mesic forest, and a lowland wet forest), we evaluate the roles environmental harshness, priority effects, productivity potential, and species interactions have in influencing secondary invasions, defined here as invasions that are influenced either positively (facilitation) or negatively (inhibition/priority effects) by existing invaders. We generate a conceptual model with a surprise index to describe whether long-term plant invader composition and dominance is predictable or stochastic after a system perturbation such as a removal experiment. Under extremely low resource availability, the surprise index is low, whereas under intermediate-level resource environments, invader dominance is more stochastic and the surprise index is high. At high resource levels, the surprise index is intermediate: Invaders are likely abundant in the environment but their response to a perturbation is more predictable than at intermediate resource levels. We suggest further testing across environmental gradients to determine key variables that dictate the predictability of postremoval invader composition. [ABSTRACT FROM AUTHOR]

Aims: Biotic homogenization results in novel communities containing multiple exotic species. Many attributes that contribute to the dominance of invaders and increase invasibility of communities have been identified, but rarely have factors governing the dominance of co-occurring invaders been considered. Here we assess the presence and performance of five exotic and one native C4 grass species across local (microsite) and more broad-scale variation in fire history, soil age, altitude and precipitation. We ask whether species show preferences for the same conditions, whether all are constrained by poor soil development, and how their performance changes when alone versus co-occurring. In addition to providing information useful to managers who want to control some of these species, such information can help to predict how distributions might change with climate change. Methods: We measured percent cover and plant height of each grass species in 64 plots in each of 15 sites, located in young volcanic substrates across environmental gradients in Hawai'i Volcanoes National Park, Hawai'i, USA. Sites varied in fire history and soil age, across altitudinal and precipitation gradients. We estimated microsite heterogeneity by measuring rock cover and soil depth, where each species was present and in plots as a whole. We analyzed effects of possible controlling variables using chi-square tests and Generalized Additive Mixed Models. Important Findings: Two species (Schizachyrium condensatum and Hyparrhenia rufa) were absent in shallow soil, restricted to more mesic sites and showed increasing performance with soil depth. By contrast, two other species (Andropogon virginicus and Melinis repens) tended to occur in shallow soil with high rock cover across a wide range of sites. One additional species (Melinis minutiflora) was weakly affected by soil depth and was the dominant at higher elevation (more mesic) and on burned sites, especially in old soil. This species was largely absent from lower elevation sites, where the more widespread and opportunistic congener, M. repens, and a drought tolerant native grass, Heteropogon contortus, were more abundant. The latter was confined to this low hot zone. Introduced C4 grasses in this region of Hawai'i are distributed according to an interaction between soil requirements, climate (correlated with elevation), competitive ability and fire response. No one strategy can explain the distributions, which will likely persist unless climate changes toward warmer and drier conditions or new fires occur. The latter would promote dominance of M. minutiflora in more mesic sites, while the former would promote either native (H. contortus) or other exotic grasses (M. repens, A. virginicus). [ABSTRACT FROM AUTHOR]

Many plant species introduced to an area outside their natural range become naturalized and may disrupt native communities, leading to a loss of biodiversity. Invasions are often facilitated by mutualisms involving animal mediated pollination and seed dispersal. A framework for understanding the significance of mutualisms in plant invasion ecology is provided.

Ecosystem eutrophication often increases domination by non-natives and causes displacement of native taxa. However, variation in environmental conditions may affect the outcome of interactions between native and non-native taxa in environments where nutrient supply is elevated. We examined the interactive effects of eutrophication, climate variability and climate average conditions on the success of native and non-native plant species using experimental nutrient manipulations replicated at 32 grassland sites on four continents. We hypothesized that effects of nutrient addition would be greatest where climate was stable and benign, owing to reduced niche partitioning. We found that the abundance of non-native species increased with nutrient addition independent of climate; however, nutrient addition increased non-native species richness and decreased native species richness, with these effects dampened in warmer or wetter sites. Eutrophication also altered the time scale in which grassland invasion responded to climate, decreasing the importance of long-term climate and increasing that of annual climate. Thus, climatic conditions mediate the responses of native and non-native flora to nutrient enrichment. Our results suggest that the negative effect of nutrient addition on native abundance is decoupled from its effect on richness, and reduces the time scale of the links between climate and compositional change. [ABSTRACT FROM AUTHOR]