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Context-dependent response of the trait-related flowering pattern in sub-Mediterranean grasslands
No full textThe timing of flowering is a key component of community assembly, being critical to competition for resources or avoidance of stress. In grasslands, the functional differentiation of
species inside the community drives the flowering pattern, so that each phenological phase
relates to a specific set of functional traits. Although the temporal dimension is an important
factor in determining species assemblages, how plant traits respond to the heterogeneity of
resource availability has been identified mostly through observations of spatial variations.
To understand the role of temporal variation of resources in the species assemblage processes, we performed a trait-based phenological study in sub-Mediterranean grasslands (central
Apennines). We counted flowering shoots of species of three plant communities along a
productivity gradient in 1 m × 1 m plots, during the growing season. We calculated the mean
proportions of flowering shoots of species sharing traits related to resource acquisition and
stress tolerance and analysed their relations with soil temperature, soil humidity, and canopy
height, comparing their temporal patterns using indicator species analysis, redundancy analysis, variation partitioning, and generalized linear modeling.
Our results highlighted that the temporal gradient, which synthesizes the variation trend of
the environmental conditions and vegetation structure of the grassland communities during
the growing season, is a major factor in determining the trait-based flowering pattern. In the
early flowering phases, there is a predominance of species with strategies of slow resource
acquisition and storage but rapid growth rate, while in the periods with optimal environmental conditions, the blooming species are the ones with strategies of slow growth rate and more
efficient resource acquisition, conservation, and use. No common patterns emerged among
plant communities for the late flowering species from mid-summer to autumn.
We also observed that the shift in soil temperature, soil relative humidity and canopy
height among communities reflected a modification over time of the flowering expression
of some traits, such as leaf anatomy and persistence, type of storage organ, and vertical
space occupation. Harsh conditions filter species with resource-retaining strategies, drought
resistance, and avoidance ability, whereas in productive conditions, competition for light
promotes the coexistence of species with dissimilar resource acquisition strategies on a fine
spatial scale. This seems to substantiate the hypothesis that the functional response to the
seasonal variation in environmental conditions largely retraces the modifications, at the community level, of the functional composition across spatial resource gradients.
Our results show that the amplitude of the environmental fluctuations influences the type
and number of strategies positively filtered by the system. Environmental fluctuations in time
influenced the trait-related flowering pattern more in highly productive conditions (having the highest differences in soil relative humidity and canopy height between spring and summer)
than in less productive ones. In fact, in productive conditions, the phenological responses to
environmental fluctuations are mostly related to traits that limit competition with dominant
species by spatial niche segregation (vegetative propagation, vertical space occupation, and
plant height) and to species that fit their life cycle to the variation of environmental conditions, through different life and leaf span. Conversely, such traits showed weaker trends in the
driest community, where the flowering pattern was less dependent on temporal fluctuations
of environmental conditions. These findings are consistent with the fluctuation niche theory,
based on the different growth/phenological response of species to the variation of resource
availability over time
Micro-topographyc gradients affect recovery processes after grazing cessation
No full textThough the interplay of grazing intensity and the availability of resources is a key driver in grassland composition, very few studies focused on trait changes after abandonment along productivity gradients. Through a comparative approach, we aimed to assess the context-dependent effects of long-term grazing cessation on functional composition and diversity in sub-Mediterranean grasslands. We hypothesized that variability of topography, soil and vegetation structure on a fine scale drives the trait-based dynamics after long-term abandonment, also influencing the patterns of functional diversity. On a calcareous mountain ridge of central Italy, we collected data on species cover and traits, site characteristics, soil depth and vegetation structure in 0.5 m × 0.5 m plots located in extensively grazed pastures and in grasslands abandoned since the early 1970s. We analysed patterns of species and traits in relation to environmental variables and management type, and trends in functional diversity (FD, Rao’s quadratic entropy) along a productivity gradient. We found that grazing cessation reduced the overall FD and that the direction of species and trait response after long-term grazing cessation were affected, on a fine scale, by the soil depth / productivity gradient. In dryer conditions, species and functional responses were less affected by abandonment, and were devoted to resistance to both stress and disturbance. In abandoned pastures we detected a significant decrease in FD with increasing productivity, leading to a shift from functional strategies devoted to grazing avoidance and tolerance to those devoted to competition for light and resource acquisition. This trend was related to the filtering effect of coarse tall grasses, which spread in highly productive conditions. In grazed grasslands, we detected an overall increasing trend of FD with increasing productivity, confirming the key role of extensive grazing in maintaining high levels of FD
An optimization–diversification approach to portfolio selection
No full textThe classical approaches to optimal portfolio selection call for finding a feasible portfolio that optimizes a risk measure, or a gain measure, or a combination thereof by means of a utility function or of a performance measure. However, the optimization approach tends to amplify the estimation errors on the parameters required by the model, such as expected returns and covariances. For this reason, the Risk Parity model, a novel risk diversification approach to portfolio selection, has been recently theoretically developed and used in practice, mainly for the case of the volatility risk measure. Here we first provide new theoretical results for the Risk Parity approach for general risk measures. Then we propose a novel framework for portfolio selection that combines the diversification and the optimization approaches through the global solution of a hard nonlinear mixed integer or pseudo Boolean problem. For the latter problem we propose an efficient and accurate Multi-Greedy heuristic that extends the classical single-threaded greedy approach to a multiple-threaded setting. Finally, we provide empirical results on real-world data showing that the diversified optimal portfolios are only slightly suboptimal in-sample with respect to optimal portfolios, and generally show improved out-of-sample performance with respect to their purely diversified or purely optimized counterparts
Portfolio selection problems in practice: a comparison between linear and quadratic optimization models
No full textSeveral portfolio selection models take into account practical limitations on the number of assets to include and on their weights in the portfolio. We present here a study of the Limited Asset Markowitz (LAM), of the Limited Asset Mean Absolute Deviation (LAMAD) and of the Limited Asset Conditional Value-at-Risk (LACVaR) models, where the assets are limited with the introduction of quantity and cardinality constraints. We propose a completely new approach for solving the LAM model, based on reformulation as a Standard Quadratic Program and on some recent theoretical results. With this approach we obtain optimal solutions both for some well-known financial data sets used by several other authors, and for some unsolved large size portfolio problems. We also test our method on five new data sets involving real-world capital market indices from major stock markets. Our computational experience shows that, rather unexpectedly, it is easier to solve the quadratic LAM model with our algorithm, than to solve the linear LACVaR and LAMAD models with CPLEX, one of the best commercial codes for mixed integer linear programming (MILP) problems. Finally, on the new data sets we have also compared, using out-of-sample analysis, the performance of the portfolios obtained by the Limited Asset models with the performance provided by the unconstrained models and with that of the official capital market indices
Recovery of a grassland community invaded by Brachypodium rupestre after multi-annual mowing tratment
No full textA long-term experiment of mowing activities demonstrated the effectiveness of disturbance in recovering the invaded grassland communities (by Brachypodium rupestre) by the species richness increase and amelioration of the other indicators
Intra-specific multi-trait approach reveals scarce ability in the variation of resource exploitation strategies for a dominant tall-grass under intense disturbance
Full text linkMowing is a suitable practice to restore the grassland coenological composition after invasion of tall-grasses. However, how it affects the tall-grass intraspecific variation is still unknown. We investigated if mowing decreases the competitive ability of the tall-grass Brachypodium rupestre, and if this reduction is due to its poor ability to adapt to the new conditions by reducing resource investment in leaf and flower production, resource storage in belowground organs, increasing seed output and reducing seed mass. In 2017, we measured vegetative and reproductive traits of B. rupestre in two fenced areas in central Apennines (Italy). Half of each area had been mown twice a year since 2010 and half remained unmown. To investigate the effect of mowing on B. rupestre's functional strategies, we used linear mixed-effects modelling and correlation analysis. Mowing significantly reduced the trait values related to competitive ability/resource acquisition (vegetative height and leaf traits), resource storage (mature hypogeogenous rhizome), and sexual reproduction (mean seed mass, number of flowers, reproductive height), but increased seed output. We did not find significant variation of dry matter content in epigeogenous rhizomes and in the shoot number. B. rupestre is not prone to endure a prolonged disturbance, not being able to reallocate resources from acquisitive and retentive to reproductive structures. However, it is able to guarantee the presence of individuals to a certain degree by changing the reproductive strategies, such as decreasing the seed mass and increasing the seed output
Maximum risk diversification for portfolio selection
No full textWe consider a new measure of diversification for a portfolio of risky assets, and we address the problem of finding portfolios with maximum diversification, possibly with the addition of return constraints. The diversification measure is based on a convexity gap between the risk of a convex combination of assets and the convex combination of their risks. We first provide some theoretical results on this diversification measure, and we establish connections with the Herfindahl index for risk. Then we formulate the portfolio diversification model for several risk measures. Finally, we provide some preliminary computational results
Changes in species and functional composition in the herb layer of sub-Mediterranean Ostrya carpinifolia abandoned coppices
Full text linkIn recent decades, the traditional management of woods has ceased in several parts of the Apennine ridge, with the result that some woods have not undergone forestry for 40–70 years. The research aim was to assess the variation of species and functional composition in the herb layer of Ostrya carpinifolia woods (central Italy), after cessation of the usual management (coppice-with-standards). Using a space-for-time substitution, we compared stands at the end of the usual rotation cycle (20–25 years) with stands not subjected to silvicultural treatments for about 40–45 years and collected environmental and structural data. The main drivers of the herb layer modification, assessed using Ellenberg indicator values analysis and redundancy analysis (RDA), were primarily related to time since the last coppicing and wood structure. Results of indicator species analyses (ISAs) and Wilcoxon rank sum tests indicated that in abandoned coppices, the regenerative processes proceed through the spread of late-successional species, while the light filtering through canopy fosters species usually considered of fringe habitats. The functional strategies revealed by ISA and RDA underlined still stressful conditions in stands at the end of the usual coppicing rotation cycle, which might be thought as a legacy of the post-logging condition, and processes of recovery/maturation of the forest systems in abandoned woods leading to a better spatial and temporal niche partitioning. The preferential distribution of species usually growing in the Apennine beech woods supports the hypothesis that the studied O. carpinifolia woods are secondary forest ecosystems originating from the degradation of mixed beech woods
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