Ghosts from the past: even comprehensive sampling of the native range may not be enough to unravel the introduction history of invasive species-the case of Acacia dealbata invasions in South Africa.

PREMISE OF THE STUDY: Knowledge about the introduction history (source(s), number and size of introduction events) of an invasive species is a crucial prerequisite to understand invasion success and to facilitate effective and sustainable management approaches, especially for effective biological control. We investigated the introduction history of the Australian legume tree Acacia dealbata in South Africa. Results of this study will not only provide critical information for the management of this species in South Africa, but will also broaden our overall knowledge on the invasion ecology of this globally important invasive tree. METHODS: We used nuclear microsatellite markers to compare the genetic diversity and structure between 42 native Australian and 18 invasive South African populations and to test different and competing introduction scenarios using Approximate Bayesian Computation analyses. KEY RESULTS: Australian populations were characterized by two distinct genetic clusters, while South African populations lacked any clear genetic structure and showed significantly lower levels of genetic diversity compared to native range populations. South African populations were also genetically divergent from native populations and the most likely introduction scenario indicated an unknown source population. CONCLUSIONS: Although we cannot definitely prove the cause of the observed genetic novelty/diversification in South African Acacia dealbata populations, it cannot be attributable to insufficient sampling of native populations. Our study highlights the complexity of unravelling the introduction histories of commercially important alien species.

Range-wide genetic structure and diversity of the endemic tree line species Polylepis australis (Rosaceae) in Argentina.

PREMISE OF THE STUDY: Knowledge on the range-wide distribution of genetic structure and diversity is required to facilitate the understanding of historical tree migration and for predicting responses to current climate change. With respect to post-glacial migration patterns known from the northern hemisphere, we tested the prediction that the southernmost populations of a subtropical tree line species have lower within-population genetic diversity and higher genetic differentiation than the central and northernmost populations. METHODS: We used AFLP to assess the genetic structure of 18 populations of the wind-pollinated Polylepis australis (Rosaceae) sampled over its entire distributional range in three Argentinean high mountain regions. Genetic diversity was calculated as a percentage of polymorphic bands (P) and Nei's expected heterozygosity (He); genetic differentiation was assessed using AMOVA, Φ(ST)--statistics, and Bayesian cluster analysis. KEY RESULTS: Contrary to our expectations, the northernmost Polylepis australis stands had lower within-population genetic diversity and higher genetic differentiation than the central and southernmost stands. Populations grouped into two major clusters, the first containing the southern populations and four central populations and the second containing the northern and one central population. CONCLUSIONS: Patterns of Polylepis australis genetic structure and diversity differ from historical migration scenarios observed for the northern hemisphere. The decline in genetic diversity toward the north may point to an equatorward migration following past climatic changes. Populations within the south and central part appear to be connected by effective long-distance pollination while gene flow in the northern part is probably hampered by geographic isolation.

Plant biodiversity in the face of global change.

Le Roux et al. suggest that documented increases in local plant richness in response to climate change should consider the nature of 'new' species. They find that species responsible for increases in richness in areas that have experienced significant disturbance and climate change are often invasive and/or weedy species.

Recent Anthropogenic Plant Extinctions Differ in Biodiversity Hotspots and Coldspots.

During the Anthropocene, humans are changing the Earth system in ways that will be detectable for millennia to come [1]. Biologically, these changes include habitat destruction, biotic homogenization, increased species invasions, and accelerated extinctions [2]. Contemporary extinction rates far surpass background rates [3], but they seem remarkably low in plants [4, 5]. However, biodiversity is not evenly distributed, and as a result, extinction rates may vary among regions. Some authors have contentiously argued that novel anthropic habitats and human-induced plant speciation can actually increase regional biodiversity [6, 7]. Here, we report on one of the most comprehensive datasets to date, including regional and global plant extinctions in both biodiversity hotspots (mostly from Mediterranean-type climate regions) and coldspots (mostly from Eurasian countries). Our data come from regions covering 15.3% of the Earth's surface and span over 300 years. With this dataset, we explore the trends, causes, and temporal dynamics of recent plant extinctions. We found more, and faster accrual of, absolute numbers of extinction events in biodiversity hotspots compared to coldspots. Extinction rates were also substantially higher than historical background rates, but recent declines are evident. We found higher levels of taxonomic uniqueness being lost in biodiversity coldspots compared to hotspots. Causes of plant extinctions also showed distinct temporal patterns, with agriculture, invasions, and urbanization being significant drivers in hotspots, while hydrological disturbance was an important driver in coldspots. Overall, plant extinctions over the last three centuries appear to be low, with a recent (post-1990) and steady extinction rate of 1.26 extinctions/year.

Introduction to the special issue: Tree invasions: towards a better understanding of their complex evolutionary dynamics.

Many invasive plants show evidence of trait-based evolutionary change, but these remain largely unexplored for invasive trees. The increasing number of invasive trees and their tremendous impacts worldwide, however, illustrates the urgent need to bridge this knowledge gap to apply efficient management. Consequently, an interdisciplinary workshop, held in 2015 at Stellenbosch University in Stellenbosch, South Africa, brought together international researchers to discuss our understanding of evolutionary dynamics in invasive trees. The main outcome of this workshop is this Special Issue of AoB PLANTS. The collection of papers in this issue has helped to identify and assess the evolutionary mechanisms that are likely to influence tree invasions. It also facilitated expansion of the unified framework for biological invasions to incorporate key evolutionary processes. The papers cover a wide range of evolutionary mechanisms in tree genomes (adaptation), epigenomes (phenotypic plasticity) and their second genomes (mutualists), and show how such mechanisms can impact tree invasion processes and management. The special issue provides a comprehensive overview of the factors that promote and mitigate the invasive success of tree species in many parts of the world. It also shows that incorporating evolutionary concepts is crucial for understanding the complex drivers of tree invasions and has much potential to improve management. The contributions of the special issue also highlight many priorities for further work in the face of ever-increasing tree invasions; the complexity of this research needs calls for expanded interdisciplinary research collaborations.

Non-native populations of an invasive tree outperform their native conspecifics.

Introduced plants often face new environmental conditions in their non-native ranges. To become invasive, they need to overcome several biotic and abiotic filters that may trigger adaptive changes in life-history traits, like post-germination processes. Such early life cycle traits may play a crucial role in the colonization and establishment success of invasive plants. As a previous study revealed that seeds of non-native populations of the woody Siberian elm, Ulmus pumila, germinated faster than those of native populations, we expected growth performance of seedlings to mirror this finding. Here, we conducted a common garden greenhouse experiment using different temperature and watering treatments to compare the biomass production of U. pumila seedlings derived from 7 native and 13 populations from two non-native ranges. Our results showed that under all treatments, non-native populations were characterized by higher biomass production and enhanced resource allocation to aboveground biomass compared to the native populations. The observed enhanced growth performance of non-native populations might be one of the contributing factors for the invasion success of U. pumila due to competitive advantages during the colonization of new sites.

Evolutionary dynamics of tree invasions: complementing the unified framework for biological invasions.

Evolutionary processes greatly impact the outcomes of biological invasions. An extensive body of research suggests that invasive populations often undergo phenotypic and ecological divergence from their native sources. Evolution also operates at different and distinct stages during the invasion process. Thus, it is important to incorporate evolutionary change into frameworks of biological invasions because it allows us to conceptualize how these processes may facilitate or hinder invasion success. Here, we review such processes, with an emphasis on tree invasions, and place them in the context of the unified framework for biological invasions. The processes and mechanisms described are pre-introduction evolutionary history, sampling effect, founder effect, genotype-by-environment interactions, admixture, hybridization, polyploidization, rapid evolution, epigenetics, and second-genomes. For the last, we propose that co-evolved symbionts, both beneficial and harmful, which are closely physiologically associated with invasive species, contain critical genetic traits that affect the evolutionary dynamics of biological invasions. By understanding the mechanisms underlying invasion success, researchers will be better equipped to predict, understand, and manage biological invasions.