Can suitability indices predict plant growth in the invaded range? The case of Acacias species.

Introduction: Forestry in many parts of the world depends on exotic species, making this industry a source of invasions in some countries. Among others, plantations of the genus Pinus, Eucalyptus, Acacia, Populus, and Pseudotsuga underpin the forestry industry and are a vital component of many countries economies. Among woody plants, the cosmopolitan genus Acacia includes some of the most commonly planted trees worldwide. In order to prevent, manage and control invasive plant species, one of the most used tools is species distribution models. The output of these models can also be used to obtain information about population characteristics, such as spatial abundance patterns or species performance. Although ecological theory suggests a direct link between fitness and suitability, this link is often absent. The reasons behind the lack of this relationship are multiple. Chile is one of the countries where Acacia species, in particular, A. dealbata and A. melanoxylon, have become invaders. Methods: Here, we used climatic and edaphic variables to predict thepotentially suitable habitats for A. dealbata and A. melanoxylon in continental Chile and evaluate if the suitability indices obtained from these models are associated with the observed performance of the trees along the country. Results: Our models show that variable importance showed significant similarities between the variables that characterize each species' niche. However, despite the high accuracy of our models, we did not observe an association between suitability and tree growth. Discussion: This disconnection between suitability and performance can result from multiple causes, from structural limitations, like the lack of biotic interactions in the models, to methodological issues, like the usefulness of the performance metric used. Whatever the scenario, our results suggest that plans to control invasive species should be cautious in assuming this relationship in their design and consider other indicators such as species establishment success.

Seed and seedling interactions in three tree species from Mediterranean forests as a knowledge base for ecological restoration.

The ecosystems of the Mediterranean regions are severely threatened by human activity, and although we have made progress in physical restoration measures, little is known about the interactions between the plants of these biomes. The objective of this study is to contribute to document interactions between seeds and seedlings of three woody species native to Chile (P. chilensis, Q. saponaria and A. caven), which could be used for restoration actions (e.g., after forest fires). In a first experiment, we evaluated the germination response, the initial elongation and the interactions between the seedlings that germinate exposed to the chemical compounds of the other species. In a second experiment, we compared the survival and growth of seedlings in monospecific versus bispecific mixtures, using a substrate similar to that which is present after a wildfire. Seed extracts of teguments promoted germination of P. chilensis, but cotyledons and whole seed extracts inhibited germination of only one species, Q. saponaria, with very high intensity. The effects of the extracts on initial seedling elongation were more variable, including five inhibitions and one facilitation. Negative effects on germination and elongation included two cases of autotoxicity. The survival of seedlings grown in mixtures showed only two differences between monospecific and bispecific mixtures, both positive, constituting a reciprocal effect between two species. Only in one case was there a significant difference in seedling growth, which was root growth inhibition. Indeed, these results reveal an interaction between species, the type and intensity of which varies according to the condition of the seed or seedling. The negative effect found in seedling root growth reflects an allelopathic interaction that conditions a vital aspect for the establishment of these species, so this information is an opportunity to improve the establishment conditions in future reforestation projects, by avoiding particular species or promoting their proportion in plantation mixtures, either by planting or direct seeding.

Size matters: point pattern analysis biases the estimation of spatial properties of stomata distribution.

Stomata distribution is an example of biological patterning. Formal methods used to study stomata patterning are generally based on point-pattern analysis, which assumes that stomata are points and ignores the constraints imposed by size on the placement of neighbors. The inclusion of size in the analysis requires the use of a null model based on finite-size object geometry. In this study, we compare the results obtained by analyzing samples from several species using point and disc null models. The results show that depending on the null model used, there was a 20% reduction in the number of samples classified as uniform; these results suggest that stomata patterning is not as general as currently reported. Some samples changed drastically from being classified as uniform to being classified as clustered. In samples of Arabidopsis thaliana, only the disc model identified clustering at high densities of stomata. This reinforces the importance of selecting an appropriate null model to avoid incorrect inferences about underlying biological mechanisms. Based on the results gathered here, we encourage researchers to abandon point-pattern analysis when studying stomata patterning; more realistic conclusions can be drawn from finite-size object analysis.

Detecting leaf pulvinar movements on NDVI time series of desert trees: a new approach for water stress detection.

Heliotropic leaf movement or leaf 'solar tracking' occurs for a wide variety of plants, including many desert species and some crops. This has an important effect on the canopy spectral reflectance as measured from satellites. For this reason, monitoring systems based on spectral vegetation indices, such as the normalized difference vegetation index (NDVI), should account for heliotropic movements when evaluating the health condition of such species. In the hyper-arid Atacama Desert, Northern Chile, we studied seasonal and diurnal variations of MODIS and Landsat NDVI time series of plantation stands of the endemic species Prosopis tamarugo Phil., subject to different levels of groundwater depletion. As solar irradiation increased during the day and also during the summer, the paraheliotropic leaves of Tamarugo moved to an erectophile position (parallel to the sun rays) making the NDVI signal to drop. This way, Tamarugo stands with no water stress showed a positive NDVI difference between morning and midday (ΔNDVI mo-mi) and between winter and summer (ΔNDVI W-S). In this paper, we showed that the ΔNDVI mo-mi of Tamarugo stands can be detected using MODIS Terra and Aqua images, and the ΔNDVI W-S using Landsat or MODIS Terra images. Because pulvinar movement is triggered by changes in cell turgor, the effects of water stress caused by groundwater depletion can be assessed and monitored using ΔNDVI mo-mi and ΔNDVI W-S. For an 11-year time series without rainfall events, Landsat ΔNDVI W-S of Tamarugo stands showed a positive linear relationship with cumulative groundwater depletion. We conclude that both ΔNDVI mo-mi and ΔNDVI W-S have potential to detect early water stress of paraheliotropic vegetation.