Predicting the impact of invasive trees from different measures of abundance.

Biological invasions produce negative impacts worldwide, causing massive economic costs and ecological impacts. Knowing the relationship between invasive species abundance and the magnitude of their impacts (abundance-impact curves) is critical to designing prevention and management strategies that effectively tackle these impacts. However, different measures of abundance may produce different abundance-impact curves. Woody plants are among the most transformative invaders, especially in grassland ecosystems because of the introduction of hitherto absent life forms. In this study, our first goal was to assess the impact of a woody invader, Pinus contorta (hereafter pine), on native grassland productivity and livestock grazing in Patagonia (Argentina), building abundance-impact curves. Our second goal, was to compare different measure of pine abundance (density, basal area and canopy cover) as predictors of pine's impact on grassland productivity. Our third goal, was to compare abundance-impact curves among the mentioned measures of pine abundance and among different measures of impact: total grassland productivity, palatable productivity and sheep stocking rate (the number of sheep that the grassland can sustainably support). Pine canopy cover, closely followed by basal area, was the measure of abundance that best explained the impact on grassland productivity, but the shape of abundance impact curves differed between measures of abundance. While increases in pine density and basal area always reduced grassland productivity, pine canopy cover below 30% slightly increased grassland productivity and higher values caused an exponential decline. This increase in grassland productivity with low levels of pine canopy cover could be explained by the amelioration of stressful abiotic conditions for grassland species. Different measures of impact, namely total productivity, palatable productivity and sheep stocking rate, drew very similar results. Our abundance-impact curves are key to guide the management of invasive pines because a proper assessment of how many invasive individuals (per surface unit) are unacceptable, according to environmental or economic impact thresholds, is fundamental to define when to start management actions.

Identifying Priorities, Targets, and Actions for the Long-term Social and Ecological Management of Invasive Non-Native Species.

Formulating effective management plans for addressing the impacts of invasive non-native species (INNS) requires the definition of clear priorities and tangible targets, and the recognition of the plurality of societal values assigned to these species. These tasks require a multi-disciplinary approach and the involvement of stakeholders. Here, we describe procedures to integrate multiple sources of information to formulate management priorities, targets, and high-level actions for the management of INNS. We follow five good-practice criteria: justified, evidence-informed, actionable, quantifiable, and flexible. We used expert knowledge methods to compile 17 lists of ecological, social, and economic impacts of lodgepole pines (Pinus contorta) and American mink (Neovison vison) in Chile and Argentina, the privet (Ligustrum lucidum) in Argentina, the yellow-jacket wasp (Vespula germanica) in Chile, and grasses (Urochloa brizantha and Urochloa decumbens) in Brazil. INNS plants caused a greater number of impacts than INNS animals, although more socio-economic impacts were listed for INNS animals than for plants. These impacts were ranked according to their magnitude and level of confidence on the information used for the ranking to prioritise impacts and assign them one of four high-level actions-do nothing, monitor, research, and immediate active management. We showed that it is possible to formulate management priorities, targets, and high-level actions for a variety of INNS and with variable levels of available information. This is vital in a world where the problems caused by INNS continue to increase, and there is a parallel growth in the implementation of management plans to deal with them.

A Global Review of Ligustrum Lucidum (OLEACEAE) Invasion.

Ligustrum lucidum is a highly invasive East Asian tree that successfully colonizes several subtropical and temperate areas around the world. Its invasion capacity results from a widespread human use mostly in urban and periurban settings, very abundant fruit and seed production, small bird-dispersed fruits, high germination rates, resprouting capacity, fast growth rates, low herbivory levels and tolerance to a wide range of light, temperature and soil. All these traits contribute to its ability to rapidly increase in abundance, alter biodiversity, landscape ecology and limit its management. This paper reviews the current knowledge on L. lucidum with particular focus on its uses, distribution, invasiveness, ecological and economic impacts and control measures. Most relevant aspect of the review highlight the negative ecological impacts of L. lucidum, its potential to continue expanding its range of distribution and the need of further studies on the eco-physiology of the species, economic impact and social perception of its invasion and early warning systems.

Effects of Landscape Structure on Medicinal Plant Richness in Home Gardens: Evidence for the Environmental Scarcity Compensation Hypothesis.

Our research involves of how Paraguayan migrants who are living in Misiones, Argentina, manage medicinal plants in home gardens, and how this practice can be related to the landscape. We examine the relationship between the richness of home garden medicinal plants and landscape variables (e.g., distance to the forest) by applying PLS analysis, which combines principal component analysis with linear regression. We surveyed 60 home gardens localized in a rural area, and we characterized the surrounding landscape with geospatial tools. Paraguayans' home gardens are extremely diverse sites (total of 136 medicinal species), where both native (82) and introduced species (50) are managed. People who live close to the native forest or mixed use areas (e.g., farms, secondary vegetation) tend to possess less native plants in their gardens because they are available nearby. While gardeners, who live in proximity to tree crops (e.g., pine plantations), have reduced access to wild medicinal resources; therefore, their effort is concentrated on maintaining native plants. These results reflect a relationship between accessibility to medicinal plants in the landscape and the management practices in the home gardens, a neglected driver in explaining the richness and composition of the medicinal plants in home gardens so far. Thus, we contributed evidence in support of the environmental scarcity compensation hypothesis. Finally, our study supports the idea that home gardens appear to function as a springboard for plant domestication.

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.

Hydraulic architecture and photoinhibition influence spatial distribution of the arborescent palm Euterpe edulis in subtropical forests.

Physiological characteristics of saplings can be considered one of the most basic constraints on species distribution. The shade-tolerant arborescent palm Euterpe edulis Mart. is endemic to the Atlantic Forest of Argentina, Brazil and Paraguay. At a local scale, saplings of this species growing in native forests are absent in gaps. We tested the hypothesis whether sensitivity to photoinhibition or hydraulic architecture constrains the distribution of E. edulis saplings in sun-exposed forest environments. Using shade houses and field studies, we evaluated growth, survival, hydraulic traits and the susceptibility of Photosystem II to photoinhibition in E. edulis saplings under different growth irradiances. Survival rates in exposed sites in the field were very low (a median of 7%). All saplings exhibited photoinhibition when exposed to high radiation levels, but acclimation to a high radiation environment increased the rate of recovery. Petiole hydraulic conductivity was similar across treatments regardless of whether it was expressed per petiole cross-sectional area or per leaf area. At the plant level, investment in conductive tissues relative to leaf area (Huber values) increased with increasing irradiance. Under high irradiance conditions, plants experienced leaf water potentials close to the turgor-loss point, and leaf hydraulic conductance decreased by 79% relative to its maximum value. Euterpe edulis saplings were able to adjust their photosynthetic traits to different irradiance conditions, whereas hydraulic characteristics at the leaf level did not change across irradiance treatments. Our results indicate that uncoupling between water demand and supply to leaves apparently associated with high resistances to water flow at leaf insertion points, in addition to small stems with low water storage capacity, weak stomatal control and high vulnerability of leaves to hydraulic dysfunction, are the main ecophysiological constraints that prevent the growth and survival of E. edulis saplings in gaps in the native forest where native lianas and bamboos show aggressive growth.

Removal of nutrient limitations in forest gaps enhances growth rate and resistance to cavitation in subtropical canopy tree species differing in shade tolerance.

A 4-year fertilization experiment with nitrogen (N) and phosphorus (P) was carried out in natural gaps of a subtropical forest in northeastern Argentina. Saplings of six dominant canopy species differing in shade tolerance were grown in five control and five N + P fertilized gaps. Hydraulic architectural traits such as wood density, the leaf area to sapwood area ratio (LA : SA), vulnerability to cavitation (P50) and specific and leaf-specific hydraulic conductivity were measured, as well as the relative growth rate, specific leaf area (SLA) and percentage of leaf damage by insect herbivores. Plant growth rates and resistance to drought-induced embolisms increased when nutrient limitations were removed. On average, the P50 of control plants was -1.1 MPa, while the P50 of fertilized plants was -1.6 MPa. Wood density and LA : SA decreased with N + P additions. A trade-off between vulnerability to cavitation and efficiency of water transport was not observed. The relative growth rate was positively related to the total leaf surface area per plant and negatively related to LA : SA, while P50 was positively related to SLA across species and treatments. Plants with higher growth rates and higher total leaf area in fertilized plots were able to avoid hydraulic dysfunction by becoming less vulnerable to cavitation (more negative P50). Two high-light-requiring species exhibited relatively low growth rates due to heavy herbivore damage. Contrary to expectations, shade-tolerant plants with relatively high resistance to hydraulic dysfunction and reduced herbivory damage were able to grow faster. These results suggest that during the initial phase of sapling establishment in gaps, species that were less vulnerable to cavitation and exhibited reduced herbivory damage had faster realized growth rates than less shade-tolerant species with higher potential growth rates. Finally, functional relationships between hydraulic traits and growth rate across species and treatments were maintained regardless of soil nutrient status.

Hydraulic differences along the water transport system of South American Nothofagus species: do leaves protect the stem functionality?

Hydraulic traits were studied for six Nothofagus species from South America (Argentina and Chile), and for three of these species two populations were studied. The main goal was to determine if properties of the water conductive pathway in stems and leaves are functionally coordinated and to assess if leaves are more vulnerable to cavitation than stems, consistent with the theory of hydraulic segmentation along the vascular system of trees in ecosystems subject to seasonal drought. Vulnerability to cavitation, hydraulic conductivity of stems and leaves, leaf water potential, wood density and leaf water relations were examined. Large variations in vulnerability to cavitation of stems and leaves were observed across populations and species, but leaves were consistently more vulnerable than stems. Water potential at 50% loss of maximum hydraulic efficiency (P(50)) ranged from -0.94 to -2.44 MPa in leaves and from -2.6 to -5.3 MPa in stems across species and populations. Populations in the driest sites had sapwood and leaves more vulnerable to cavitation than those grown in the wettest sites. Stronger diurnal down-regulation in leaf hydraulic conductance compared with stem hydraulic conductivity apparently has the function to slow down potential water loss in stems and protect stem hydraulics from cavitation. Species-specific differences in wood density and leaf hydraulic conductance (K(Leaf)) were observed. Both traits were functionally related: species with higher wood density had lower K(Leaf). Other stem and leaf hydraulic traits were functionally coordinated, resulting in Nothofagus species with an efficient delivery of water to the leaves. The integrity of the more expensive woody portion of the water transport pathway can thus be maintained at the expense of the replaceable portion (leaves) of the stem-leaf continuum under prolonged drought. Compensatory adjustments between hydraulic traits may help to decrease the rate of embolism formation in the trees more vulnerable to cavitation.

Water relations of Chusquea ramosissima and Merostachys claussenii in Iguazu National Park, Argentina.

Bamboos are prominent components of many tropical ecosystems, yet little is known about the physiological mechanisms utilized by these gigantic forest grasses. Here, we present data on the water transport properties of Chusquea ramosissima and Merostachys claussenii, monocarpic bamboo grasses native to the subtropical Atlantic forests of Argentina. C. ramosissima and M. claussenii differed in their growth form and exhibited contrasting strategies of water transport. Maximum xylem hydraulic conductivity of C. ramosissima culms was 2-fold higher than that of M. claussenii. C. ramosissima cavitated at relatively high water potentials (50% loss of conductivity at >or=1 MPa), whereas M. claussenii was more drought tolerant (50% loss at