Scientific articles

Scientific articles supported within the Agritech National Research Center  funded by the European Union Next-GenerationEU (National Recovery and Resilience Plan (NRP) - Mission 4 Component 2, Investment 1. 4 - D.D. 1032 17 /06 /2022CN000022).




https://doi.org/10.1016/j.agrformet.2022.109238

Drought legacies in mixed Mediterranean forests: Analysing the effects of structural overshoot, functional traits and site factors

Previous favorable climate conditions stimulate tree growth making some forests more vulnerable to hotter droughts. This so-called structural overshoot may contribute to forest dieback, but there is little evidence on its relative importance depending on site conditions and tree species because of limited field data. Here, we analyzed remote sensing (NDVI) and tree-ring width data to evaluate the impacts of the 2017 drought on canopy cover and growth in mixed Mediterranean forests (Fraxinus ornus, Quercus pubescens, Acer monspessulanum, Pinus pinaster) located in southern Italy. Legacy effects were assessed by calculating differences between observed and predicted basal area increment (BAI). Overall, the growth response of the study stands to the 2017 drought was contingent on site conditions and species characteristics. Most sites presented BAI and canopy cover reductions during the drought. Growth decline was followed by a quick recovery and positive legacy effects, particularly in the case of F. ornus. However, we found negative drought legacies in some species (e.g., Q. pubescens, A. monspessulanum) and sites. In those sites showing negative legacies, high growth rates prior to drought in response to previous wet winter-spring conditions may have predisposed trees to drought damage. Vice versa, the positive drought legacy found in some F. ornus site was linked to post-drought growth release due to Q. pubescens dieback and mortality. Therefore, we found evidences of structural drought overshoot, but it was restricted to specific sites and species. Our findings highlight the importance of considering site settings such as stand composition, pre-drought conditions and different tree species when studying structural overshoot. Droughts contribute to modify the composition and dynamics in mixed forests. 

Assessing Forest Vulnerability to Climate Change Combining Remote Sensing and Tree-Ring Data: Issues, Needs and AvenuesForests around the world are facing climate change. Increased drought stress and severe heat waves in recent decades have negatively impacted on forest health, making them more vulnerable and prone to dieback and mortality phenomena. Although the term vulnerability is used to indicate an increased susceptibility of forests to climate change with a worsening of their vigour status that can compromise their ability to respond to further climate extreme events, there are still uncertainties on how to evaluate it. Indeed, evaluation of forest vulnerability is complex both because of some critical issues in the estimation methods used and because of the multiple factors influencing the response of forests to ongoing climate change. A way to assess the vulnerability to environmental stresses is by combining remote sensing and dendroecological data. However, these two approaches entail multiple uncertainties, including growth/photosynthetic relationships, carbon allocation dynamics, biases of tree-ring data and noisy remote sensing data, which require further clarification for proper monitoring of pre- and post-drought forest trajectories. Our review aims to create an overview of the current literature and knowledge to understand the critical issues, needs and possible solutions that forest vulnerability research is addressing. We focus on Mediterranean forests located in a climate warming hotspot and showing a high vulnerability to increased aridification.

Radial growth, wood anatomical traits and remote sensing indexes reflect different impacts of drought on Mediterranean forests.

Drought reduces canopy cover, productivity and tree growth in forests. However, there is still little knowledge on how drought affects coupling between canopy greenness assessed by remote sensing and hydraulic conductivity detected by wood anatomy. This combination could improve the understanding of forest response to climate change. Thus, we investigated the impacts of a hot drought, which occurred in summer 2017, on radial growth, earlywood hydraulic diameter (Dh), a proxy of conductivity, and several remote-sensing indices in mixed Mediterranean hardwood forests (Quercus pubescens Willd. –Fraxinus ornus L.). In general, growth showed a higher coherence among trees and a higher responsiveness to climate. Growth decreased during the drought year, particularly for Q. pubescens, which showed high defoliation and dieback intensity. Both species showed a decline of Dh in 2018 after the drought and subsequent warm winter conditions. We found positive relationships between Dh and remote-sensing data for Q. pubescens in some of these vulnerable sites, where (i) growth was constrained by dry spring-summer conditions and (ii) Dh and growth covaried. These findings indicate a high variability among sites and tree species in their responses to drought considering earlywood anatomy, growth canopy cover and water content. However, some common patterns emerge such as links between potential hydraulic conductivity (Dh), tree cover and Dh-growth covariation in the most impacted sites. Further, F. ornus seem to perform better in terms of growth under drought conditions, showing less mortality and dieback than Q. pubescens. Future studies could explore how water transport and changes in canopy cover respond to dry and warm conditions and if that covariation indicates vulnerability to drought.

Species specific vulnerability to increased drought in temperate and Mediterranean floodplain forests


Floodplain forests are sensitive to climate warming and increased drought, as showed by recent oak (Quercus robur) dieback and mortality episodes. However, a comprehensive comparison of coexisting tree species under different climate settings or biomes are lacking. Herein, we compared growth rates, growth responses to climate and drought severity, and modeled climate mediated growth of oak and three coexisting tree species (ash, Fraxinus angustifolia; alder, Alnus glutinosa; elm, Ulmus minor). Two floodplain forests subjected to cooler (temperate climate, Ticino) and warmer (Mediterranean climate, Bosco Pantano) conditions in northern and southern Italy, respectively, were analyzed. Ash seemed to be the most sensitive to drought, particularly at the Mediterranean site where oak and elm growth were also negatively affected by water shortages. Alder appeared to be the least sensitive species in terms of growth variability to drought under both temperate and Mediterranean climate conditions. Furthermore, the growth model revealed the influence of soil moisture in spring and summer on the constrained growth of ash and oak and illustrated how oak growth could be severely reduced during drastic hotter droughts. Alder seemed to be the most drought-resistant species under both environmental conditions. These results could represent the first attempts in documenting the ecological consequences of drought in terms of projected climate trends in less investigated Mediterranean floodplain forests. Furthermore, these results highlight how climate and tree-ring data combined with growth models could be useful tools to detect early warning signals of growth decline and impending dieback in floodplain forests in response to dry spells.