Xylem Functional Traits as Indicators of Health in Mediterranean Forests

Purpose of Review: We conducted a literature survey and meta-analysis to assess, in Mediterranean forests impacted by drought, the role of xylem functional traits as indicators of tree health, and their potential to adjust over a range of climatic conditions to support tree performance and survival. We aimed also to depict the geographic variability of xylem functional traits among Mediterranean forest trees as a tool for regional scale-oriented vulnerability assessment. Recent Findings: Associations were investigated among xylem functional traits, hydraulic safety, and whole plant drought resistance for tree species in Mediterranean-type climates. Variable data were available from a number of study cases of Mediterranean forest communities impacted by intense drought, wherein tree growth and xylem functional traits were investigated along with tree decline and dieback episodes. Variable data were available from a number of studies that analyzed xylem trait adjustments to climatic conditions at different temporal scales. Summary: We observed differing growth patterns and xylem conduit area responses in healthy and unhealthy trees and we sketched hydraulic strategies that may fit observed patterns. Overall, a clear role of xylem conduit size as stand-alone tree health indicator did not emerge. We showed that xylem traits may adjust along different temporal scales and may support the performance and health of Mediterranean tree species over a range of climatic conditions. We outlined substantial geographic variability in xylem traits across the Mediterranean region, suggesting patchy responses to increasing drought. Knowledge gaps and needed lines of research are highlighted

Tip-to-base conduit widening remains consistent across cambial age and climates in Fagus sylvatica L

: Water transport, mechanical support and storage are the vital functions provided by the xylem. These functions are carried out by different cells, exhibiting significant anatomical variation not only within species but also within individual trees. In this study, we used a comprehensive dataset to investigate the consistency of predicted hydraulic vessel diameter widening values in relation to the distance from the tree apex, represented by the relationship Dh ∝ Lβ (where Dh is the hydraulic vessel diameter, L the distance from the stem apex and β the scaling exponent). Our analysis involved 10 Fagus sylvatica L. trees sampled at two distinct sites in the Italian Apennines. Our results strongly emphasize that vessel diameter follows a predictable pattern with the distance from the stem apex and β ~ 0.20 remains consistent across cambial age and climates. This finding supports the hypothesis that trees do not alter their axial configuration represented by scaling of vessel diameter to compensate for hydraulic limitations imposed by tree height during growth. The study further indicates that within-tree variability significantly contributes to the overall variance of the vessel diameter-stem length exponent. Understanding the factors that contribute to the intraindividual variability in the widening exponent is essential, particularly in relation to interspecific responses and adaptations to drought stress

Nitrogen deposition outweighs climatic variability in driving annual growth rate of canopy beech trees: Evidence from long-term growth reconstruction across a geographic gradient

In this study, we investigated the role of climatic variability and atmospheric nitrogen deposition in driving long-term tree growth in canopy beech trees along a geographic gradient in the montane belt of the Italian peninsula, from the Alps to the southern Apennines. We sampled dominant trees at different developmental stages (from young to mature tree cohorts, with tree ages spanning from 35 to 160 years) and used stem analysis to infer historic reconstruction of tree volume and dominant height. Annual growth volume (G V ) and height (G H ) variability were related to annual variability in model simulated atmospheric nitrogen deposition and site-specific climatic variables, (i.e. mean annual temperature, total annual precipitation, mean growing period temperature, total growing period precipitation, and standard precipitation evapotranspiration index) and atmospheric CO 2 concentration, including tree cambial age among growth predictors. Generalized additive models (GAM), linear mixed-effects models (LMM), and Bayesian regression models (BRM) were independently employed to assess explanatory variables. The main results from our study were as follows: (i) tree age was the main explanatory variable for long-term growth variability; (ii) GAM, LMM, and BRM results consistently indicated climatic variables and CO 2 effects on G V and G H were weak, therefore evidence of recent climatic variability influence on beech annual growth rates was limited in the montane belt of the Italian peninsula; (iii) instead, significant positive nitrogen deposition (N dep ) effects were repeatedly observed in G V and G H ; the positive effects of N dep on canopy height growth rates, which tended to level off at N dep values greater than approximately 1.0 g m −2  y −1 , were interpreted as positive impacts on forest stand above-ground net productivity at the selected study sites

Mediterranean old-growth forests exhibit resistance to climate warming

Old-growth mountain forests represent an ideal setting for studying long-term impacts of climate change. We studied the few remnants of old-growth forests located within the Pollino massif (southern Italy) to evaluate how the growth of conspecific young and old trees responded to climate change. We investigated two conifer species (Abies alba and Pinus leucodermis) and two hardwood species (Fagus sylvatica and Quercus cerris). We sampled one stand per species along an altitudinal gradient, ranging from a drought-limited low-elevation hardwood forest to a cold-limited subalpine pine forest. We used a dendrochronological approach to characterize the long-term growth dynamics of old (age > 120 years) versus young (age < 120 years) trees. Younger trees grew faster than their older conspecifics during their juvenile stage, regardless of species. Linear mixed effect models were used to quantify recent growth trends (1950–2015) and responses to climate for old and young trees. Climate sensitivity, expressed as radial growth responses to climate during the last three decades, partially differed between species because high spring temperatures enhanced conifer growth, whereas F. sylvatica growth was negatively affected by warmer spring conditions. Furthermore, tree growth was negatively impacted by summer drought in all species. Climate sensitivity differed between young and old trees, with younger trees tending to be more sensitive in P. leucodermis and A. alba, whereas older F. sylvatica trees were more sensitive. In low-elevation Q. cerris stands, limitation of growth due to drought was not related to tree age, suggesting symmetric water competition. We found evidence for a fast-growth trend in young individuals compared with that in their older conspecifics. Notably, old trees tended to have relatively stable growth rates, showing remarkable resistance to climate warming. These responses to climate change should be recognized when forecasting the future dynamics of old-growth forests for their sustainable management. © 2021 The Author

Tree-ring growth and climate response of silver fir (Abies alba Mill.) in Basilicata (Southern Italy)

A dendroclimatic study on silver fir (Abies alba Mill.) was carried out in the Abetina of Ruoti (PZ, Basilicata - southern Italy), a nearly natural mixed Quercus cerris - Silver fir forest. Observations were made on twenty-three large size dominant silver fir trees and dendroecological analyses performed for the period 1866-2007. Results show that late spring/summer precipitations, rather than temperatures, are the most critical factor affecting silver fir growth in this area

Drought-induced oak decline in the western Mediterranean region: an overview on current evidences, mechanisms and management options to improve forest resilience

Increased forest vulnerability is being reflected as more widespread and severe drought-induced decline episodes. In particular, the Mediterranean area is revealing a high susceptibility to phenomena of loss in tree vitality across species. Within tree species, oaks (Quercus spp.) are experiencing extensive decline in many countries. However, in the wake of the so-called “oak decline phenomenon”, the attention on these species has generally been limited. In this paper, we review the current available literature on oak-decline cases reported within the Mediterranean Basin, with particular remark for those occurred in Italy and Spain. More specifically our main aims were to: (i) provide an update on the patterns and mechanisms of decline by focusing on tree-ring and wood-anatomical variables; (ii) provide some hints for improving the resistance and resilience of oak stands experiencing decline. Our review reveals that drought is reported as the main driver triggering oak decline within the Mediterranean Basin, although other causes (i.e., increasing temperature, pathogens attack or excessive stand density) could exacerbate decline. In most reported cases, drought induced a substantial reduction of growth and changes in some wood anatomical properties. Indeed, growth decline prior death is also indicated as an early-warning signal of impending death. In ring-porous oak species, declining trees were often characterized by a very low production of latewood and a decrease in lumen area of the widest earlywood vessels, suggesting a potential reduction of hydraulic conductivity. Moreover, hydraulic dysfunction is reported as the main cause of decline. Finally, regarding management actions that should be considered for improving the resilience of declining stands and preserve the species-specific stand composition, it could be useful to shorten the rotation period of coppice stands or promoting their gradual conversion towards high forests, and favoring more drought-resistant species should also be considered. In addition, regeneration prior to regeneration cuts should be improved by anticipating seed dispersal or by planting oak seedlings obtained from local germoplasm

Correction: Xylem Functional Traits as Indicators of Health in Mediterranean Forests (Current Forestry Reports, (2020), 6, 3, (220-236), 10.1007/s40725-020-00124-5)

The original version of this article unfortunately contained a mistake. The figures previously used are the old version and some symbols in the figure captions were displayed incorrectly. Final versions of figures with their captions are provided here. The original article has been corrected. (Figure presented.)

Applications of structural equation modeling (SEM) in forestry research

Forest ecosystem dynamics are driven by a complex array of simultaneous cause-and-efect relationships. In recent years, the functional traits of plants their morphological, anatomical, and physiological characteristics have been broadly used in research on forest ecology to understand how vegetation properties and function change along key ecological gradients. Presently, several comparative studies have dealt with the relationships among traits by using simple bivariate approaches, while few of them scaled up to multivariate ones, which are capable of accounting for the complexity of the relationships among variables. Indeed, many changes in tree structure and function occurring throughout ontogeny likely involve trade-ofs which are dificult to discern. Therefore, although multiple regression is sometimes used to examine interacting traits, understanding complex multivariate data requires specialized powerful techniques such as Structural Equation Modeling (SEM). The SEM approach may be particularly useful at modeling complex and more realistic functional relationships in trees. Its strength consists in the ability to resolve the uncertainties in regression models by taking into account the nature and magnitude of direct and indirect efects of multiple interacting variables. Here, we discuss the SEM model applications on forest ecological studies, and we report a case study of SEM application to investiga - te the mechanisms influencing the apical dominance ratio (ADR), recently reported as suitable indicator of the growth conditions favoring silver fir (Abies alba Mill.) natural regeneration in Mediterranean areas. The core objective of the application study was to provide a conceptual framework to understand the mechanisms linking a set of morphological and physiological traits to ADR, and to quantify the system of interrelationships explaining this linkage. Our SEM model demonstrated that ADR is influenced by the synergistic efects of several traits linked to whole-tree architecture and physiology not apparent in univariate relationships. We argue that ADR can express the short-term physiological adaptive responses of silver fir saplings under Mediterranean climatic constraints

Assessing the effectsof nitrogen deposition and climate on carbon isotope discriminationand intrinsic water-use efficiency of angiosperm and conifer trees underrising CO2 conditions

The objective of this study is to globally assess the effects of atmospheric nitrogen deposition and climate, associated with rising levels of atmospheric CO2, on the variability of carbon isotope discrimination (Δ13C) and intrinsic water-use efficiency (iWUE) of angiosperm and conifer tree species. Eighty-nine long-term isotope tree-ring chronologies, representing 23 conifer and 13 angiosperm species for 53 sites worldwide, were extracted from the literature, and used to obtain long-term time series of Δ13C and iWUE. Δ13C and iWUE were related to the increasing concentration of atmospheric CO2 over the industrial period (1850-2000) and to the variation of simulated atmospheric nitrogen deposition and climatic variables over the period 1950-2000. We applied generalized additive models and linear mixed-effects models to predict the effects of climatic variables and nitrogen deposition on Δ13C and iWUE. Results showed a declining Δ13C trend in the angiosperm and conifer species over the industrial period and a 16.1% increase of iWUE by between 1850 and 2000, with no evidence that the increased rate was reduced at higher ambient CO2 values. The temporal variation in Δ13C supported the hypothesis of an active plant mechanism that maintains a constant ratio between intercellular and ambient CO2 concentrations. We defined linear mixed-effects models that were effective to describe the variation of Δ13C and iWUE as a function of a set of environmental predictors, alternatively including annual rate (Nrate) and long-term cumulative (Ncum) nitrogen deposition. No single climatic or atmospheric variable had a clearly predominant effect, however Δ13C and iWUE showed complex dependent interactions between different covariates. A significant association of Nrate with iWUE and Δ13C was observed in conifers and in the angiosperms, and Ncum was the only independent term with a significant positive association with iWUE, while a multi-factorial control was evident in conifers