Apical dominance ratio as an indicator of the growth conditions favouring Abies alba natural regeneration under Mediterranean environment

Abstract

As reported in several studies, the presence of Abies alba Mill (silver fir) has been declining throughout its natural range over a large part of mountainous belt in the Mediterranean area. In such a context, regeneration establishment depends mainly on the occurrence of a suitable combination of water and light availability. Thus, before planning any forest management practice, it is essential to investigate on the optimal microclimate conditions influencing the success of natural regeneration of silver fir. To this aim, changes in growth and photosynthesis together with C, N and O isotope compositions have been investigated on silver fir naturally recruited saplings, growing in mixed stands with Fagus sylvatica on Apennine mountains (southern Italy). The apical dominance ratio (ADR, ratio between apical shoot length and length of first whorl lateral twigs) has been used as an indicator for microclimate conditions in which saplings grow. Based on the range of ADR values (i.e. from 0.10 to 1.30), saplings were distributed in four classes. As expected, increases in height, root collar diameter and radial growth correspond to enhancing ADR values, gaining the optimal conditions in class IV. This latter also displayed the best performance in terms of maximum CO2 assimilation at saturating light (Amax) and water-use efficiency as assessed by carbon isotope discrimination analysis. Conversely, class I and II seem to display the highest performance in terms of CO2 respiration rate (Rd) and absolute water loss saving as assessed by the application of oxygen isotopes. We conclude that, in relatively mild Mediterranean areas, forest managers should promote silvicultural treatments favouring light conditions and migration of saplings towards class IV of ADR. This class represents the optimal microclimate for regeneration establishment of silver fir

Coordination of morphological and physiological traits in naturally recruited Abies alba Mill. saplings: insights from a structural equation modeling approach

“…Therefore, based on the insights from our model, we argue that if ADR is associated with the potential of the species in the vertical light gradient (Dobrowolska 2008;Grassi and Giannini 2005;Ripullone et al 2016), then morphology could represent the total cost in terms of resource investment to ensure both mechanical stability and a suitable space arrangement (King 1990), while physiology could represent the photosynthetic efficiency of plants (Givnish 1995). Then, the relationship between morphology and physiology may be interpreted in terms of a synergism likely to influence the fitness of understory woody plants under different conditions of light availability (Küppers 1989;Westoby and Wright 2006).…”

“…The main data came from an experiment carried out recently by Ripullone et al (2016), which surveyed a silver fir natural regeneration plot within the Pollino National Park (39°07′ N; 16°13′ E; 1350 m a.s.l. ), Southern Italy.…”

“…At the beginning of October 2012, lateral current-year shoots of the first whorl were taken from each sapling for determination of carbon (δ 13 C), oxygen (δ 18 O), and nitrogen (δ 15 N) isotope compositions. For further details, please see Ripullone et al (2016).…”

“…It is well established that the observed isotope discrimination in leaf, that reflects the balance between the leaf photosynthetic capacity and its stomatal conductance, is very sensitive to environmental conditions such as light regimes, and in turn on ADR (Yakir and Israeli 1995;Ripullone et al 2016). Therefore, based on a semi-empirical approach, Phys was defined by the LMA, leaf nitrogen content (N leaf ), δ 13 C, δ 18 O, and δ 15 N isotope compositions.…”

“…Could this be considered a useful step towards helpful modeling of silver fir natural regeneration? For our model exercise, we used the silver fir saplings cohorts investigated by Ripullone et al (2016) growing on Monte Pollino (Southern Italy), which was one of the most important refugial areas for silver fir in the glacial period and still retains high conservation value for this species (Piotti et al 2017).…”