The below list represents only planned manuscripts. Some of these
manuscripts have not been received by the Editorial Office yet. Papers
submitted to MDPI journals are subject to peer-review.
Title: Tree-Water Relationships at the Southernmost European Fir (Abies pinsapo Boiss.) Drought Avoidance Matter
Author: Juan Carlos Linares Calderón
Affiliation: Universidad Pablo de Olavide, Sevilla, Spain
Abstract: Drought adaptations on forest trees have been widely studied. However, the current scenario of global warming poses considerable uncertainty regarding the magnitude and direction of these recent environmental changes. Drought sensitivity as well as differences in adaptive capacity for water balance adjustment may determine the persistence or local extinction of a species in drought prone sites. Thus, detailed ecophysiological knowledge is need to understand likely population dynamics in response to global change. For two consecutive years, we studied water relationships in natural populations of Abies pinsapo, a relict fir species which recently showed extensive forest decline. This study took place along an altitudinal gradient with microclimatic differences that affect water availability. Stomatal conductance, xylem water potential, needles osmotic potential, and net photosynthesis rate were measured. The ecophysiological variables were related to soil water content, air temperature and relative humidity (RH), atmospheric water potential, and vapour pressure deficit. A. pinsapo showed a drought avoidance strategy that is reflected on a quick stomata closure even at relatively high soil water content. Its ecophysiological features are similar to those of most temperate tree species. However, the seasonal pattern for tree-water relationships was typically Mediterranean, with maximum values of stomatal conductance in spring and stomatal closure in summer; decreasing xylem water potential at dawn (parallel to increasing soil drought); relatively high photosynthesis rates in winter, and summer minimums. A. pinsapo does not show any osmotic adjustment capacity and it does not seem to substantially modify its intrinsic water use efficiency as a response to drought. The effects of water stress are stronger at the warmer lower elevation. Our results allow to expect a gradual drought-induced decline of these relict populations in favour of better adapted tree species, basically Quercus and Pinus under a warmer scenario.
Type of Paper: Article
Title: Comparative water use and drought responses of Pinus sylvestris and Quercus ilex: ecophysiological mechanisms underlying a vegetation shift
Authors: David Aguadé1,2, Rafael Poyatos1, Jordi Martínez-Vilalta1,2
Affiliations:1 CREAF, Cerdanyola del Vallès 08193, Spain
2 Universitat Autònoma Barcelona, Cerdanyola del Vallès 08193, Spain
Abstract:The ecological impact of drought-related tree die-off episodes will be largest where and when they result in vegetation shifts. In this study we compare the water-use strategies and drought responses of two dominant tree species (Pinus sylvestris and Quercus ilex) growing together in a dry-prone Mediterranean ecosystem where the former species is being replaced by the latter in a process that has been documented since the early 1990s. Relative to Pinus sylvestris, Quercus ilex is more anisohydric and its stomatal conductance and water-use are less sensitive to changes in vapour pressure deficit or soil water availability. Seasonal fluctuations in non-structural carbohydrates, particularly starch, associated to drought were more pronounced in Pinus sylvestris. Small differences were observed when pure Quercus ilex stands were compared to mixed (Pinus sylvestris and Quercus ilex) stands, suggesting that Quercus ilex will be able to retain dominance in the canopy after Pinus sylvestris die-off.
Title: Continued Changes in Tree Populations in Northern Arizona Mixed - conifer and Ponderosa Pine Forest in the Decade Following a Climate - Related Mortality Pulse
Author: Joseph Ganey
Abstract: Recent studies documented climate-mediated tree mortality in forests and woodlands throughout the southwestern U. S. Mortality occurred rapidly during and immediately following an extreme climate year in 2002 and was non - random with respect to tree species. We sampled trees in northern Arizona mixed - conifer and ponderosa pine (Pinus ponderosa) forests in 2004 and 2014, to see if these changes continued in subsequent years. Tree density did not differ significantly between 2004 and 2014 in either forest type, and basal area increased significantly in ponderosa pine forest. Spatial variation in extent of change in tree density and basal area among sample plots was high. Percent change in basal area was negatively associated with 2004 basal area in mixed - conifer forest and positively associated with elevation in ponderosa pine forest. Basal area increased in some species whereas other species, especially quaking aspen (Populus tremuloides) and white fir (Abies concolor), showed large proportional decreases. Our results suggest a degree of short-term resilience in these forest types. Study duration was short relative to forest ecology and climate cycles, however, and considerable uncertainty remains regarding long - term, climate - related trends in these forest types.
Title: Are diverse tropical tree plantations more resistant and resilient to drought than monocultures?
Author: Norbert Kunert1) and Alida Mercado Cárdenas2)
Affiliation: 1) Department for Biogeochemical Processes, Max Planck Institute for Biogeochemistry, Jena, Germany
2) Institute of Ecology, Friedrich Schiller University, Jena, Germany
Abstract: Tropical tree plantations usually consist of a single exotic fast growing species, but recent research describes positive mixture effects on ecosystem functions of mixed tropical tree plantations. Converting mono-specific stands into mixed stands may improve stand stability and might reduce increasing abiotic and biotic disturbances due to climate change. However, only little is known about the extent to which tropical tree species or tropical tree communities can resist increasing disturbances in the short term, e.g. water limitations due to increasing dry season intensity or length, or about their resilience after such disturbances and their capacity to adapt to changing conditions in the long term. Studies relating drought resistance and resilience to community diversity are completely missing. In this review, we present the current knowledge of drought resistance of mixed species plantations and summarise evidence from a tree biodiversity experiment in Panama. Further, we highlight the urgent need of a multifactorial manipulative throughfall reduction experiment in tropical environments. The outcome of such studies would greatly assist the forestry sector in tropical regions to maintain highly productive and ecologically sound forest plantations in a changing climate.
Keywords: drought stress, neotropics, native tree species, biodiversity
Type of Paper: Article
Title: Species Selection and Its Dependency on the Ecological Niche
Authors: Tanja Sanders, Thomas Riedel, Wolfgang Beck
Affiliations: Thünen-Institut für Waldökosysteme, Thünen Institute of Forest Ecosystems, Eberswalde, Germany
Abstract: Bioclimatic envelop (BE) models are a convenient tool for scientist and politicians alike: they provide an easy to understand message about the future distribution of species. Hung up on the readily available parameters temperature and precipitation large areas can be classified depending on their suitability for certain species BE can be defined as the climatic component of the fundamental ecological niche, or the ‘climatic niche’ (Pearson & Dawson 2003). Lacking, however, are a minimum of two factors 1) climate is not the only determent for species distribution and 2) species undergo adaptive processes (for general critics cf.: Pearson & Dawson 2003, Dormann 2007, Huntley et al. 2010, Araújo & Townsend Peterson 2012). Four major limitations of BE models may be identified regarding the model assumptions: (1) species distribution ranges are determined only be climate variables, (2) there is an equilibrium between the realized species range and the its potential range determined by the climate, (3) species lack the ability to adapt to changing environmental conditions and (4) species interactions (e.g. competitive interactions) remain constant in future. A dual approach enables us to address two of these challenges: (1) calculation of species distribution based on the mean available water capacity between 1961-90 based on the national forest inventory data (BWI) in Germany and two future scenarios; and (2) quantification of adaptive capacity for Fagus sylvatica and Picea abies at selected sites using dendrochronological methods. Using available water capacity as a proxy to determine future species ditribution certain areas become apparent which would be classed as to dry or wet under the current point of adaption, respectivly. However, using dendroecological methods we can quantify the potential in adaptation and acclimatisation to add as potential new ranges for certain species.
Title: Rhizobium strains influence biomass gain and tissue nitrogen content in nitrogen-fixing Leucaena leucocephala seedlings under drought.
Authors: Gabriela Pereyra, Henrik Hartmann, Beate Michalzik, Waldemar Ziegler, Susan Trumboree
Abstract: Nitrogen fixing plants donate carbon (C) rich compounds to symbiotic diazotrophic bacteria, typically living in nodular structures on plant roots, and receive nitrogen (N) in exchange. Nitrogen-fixing tree species like Leucaena leucocephala L. are important for cattle pasture and reforestation in tropical and subtropical regions. However, anticipated more frequent heat and drought waves may have several negative effects on the usefulness of leguminous trees, including their ability to fix nitrogen. We investigated how different rhizobial strains influenced the symbiotic ratio in Leucaena, defined as the amount of N taken up and present in plant tissues compared to the amount of C accumulated in nodule biomass. We inoculated seedlings with two different species of symbiont bacteria, Mesorhizobium loti or Rhizobium tropici, and measured plant and nodule biomass as well as tissue C and N concentrations during seedling development and during a drought pulse. The number of nodules and nodule biomass of both strains was correlated with total plant biomass and with tissue N content during early development. Symbiotic N fixation and developmental stage had variable effects on the 15N abundance in different plant organs. Also, a strong 15N enrichment was observed in nodules of both strains compared to other plant tissues. In symbiosis with R. tropici, plants continued to allocate C resources to rhizobia during drought in the form of non-structural carbohydrates, with positive effects on total plant biomass and nitrogen content in leaves and roots. This was not the case for plants in symbiosis with M. loti, where nodule growth continued without N gain for the plant. In addition, asparagine concentration, a key amino acid involved in regulatory mechanisms of nodule activity, was reduced by 98% after 15 days of drought in M. loti seedlings. Our results suggest that not only the choice of tree species but also the choice of an efficient symbiont is important for legume trees used in reforestation and agroforestry systems, especially in drier environments. Therefore we recommend the use of R. tropici in association with Leucaena trees for forest restoration or agroforestry.
Keywords: symbiotic ratio; nodulation; nitrogen fixation; Rhizobium tropici; Mesorhizobium loti; Leucaena.