Water Balance of Mediterranean Quercus ilex L. and Pinus halepensis Mill. Forests in Semiarid Climates: A Review in A Climate Change Context

Development of a multi-objective decision support system for eco-hydrological forest management that quantifies and optimizes ecosystem services related to Carbon, Water, Fire-risk and Eco-resilience (CAFE)

Sustainable forest management (FOMA) requires explicit knowledge of the ecosystem services (ES) provided by forests and how they can be improved by different FOMA alternatives, especially when threatened by climate change and the productivity/profitability is low. Decision Support Systems (DSS) have evolved as powerful tools that facilitate decision-making in multi-objective FOMA. However, their use is often limited, as they are mono-objective, are based on empirical relationships, consider limited ES or do not define which silvicultural actions can be deployed, where and when. Thus, decision-makers and forest planners cannot adjust them to local conditions or specific criteria. CAFE (Carbon, Aqua, Fire & Eco-resilience) is a multi-objective DSS for FOMA that quantifies and optimizes different ES that stem from forest management. Its main contribution is coupling eco-hydrological process-based models and multi-objective optimization with genetic evolutionary algorithms. The output of the DSS, shown in a user-friendly interface, is a selection of the best-performing solutions (Pareto Optimal Front) of FOMA that optimize the user-selected ES. This tool allows designing and planning silvicultural operations such as thinning or planting required for meeting multiple objectives in FOMA by answering four fundamental questions: How much (thinning intensity or plantation density), where (spatial allocation), when (year of next intervention) and how (target forest strata in which the stand is vertically divided) thinning. This paper presents the design and components of CAFE, and a practical demonstration in contrasted forests. CAFE might contribute to addressing current challenges of meeting global environmental policy goals by stating baselines and additionality in FOMA.

Recycling the rain: Exploring forest management practices in a western Mediterranean watershed to recouple the atmospheric and terrestrial water cycle

Current knowledge of the role that forests play in recycling rainwater has led to the proposal of forest management practices to mitigate desertification in the western Mediterranean basin. Building upon this hypothesis, we present the Mijares watershed in eastern Spain as a detailed case study. We then perform a scientific literature review in the context of the western Mediterranean basin to identify the physical and biophysical uncertainties associated with forest management practices that aim to strengthen the atmospheric and terrestrial components of the water cycle. The review identified eleven interconnected uncertainties that result from complex vegetation, climate, soil, and water relationships. We discuss the need to deepen our understanding of the atmospheric dynamics of the Mijares by accounting for the role of forests in recycling rainfall and by adopting an adaptive management approach to apply the lessons learned in the territory. Based on these considerations, we propose forest management practices to reinforce the water cycle of the watershed. Our work contributes to understanding the uncertainties arising from forest management practices aimed at mitigating desertification, thereby supporting decision-making. These insights apply to other western Mediterranean coastal watersheds.

The Impact of Clear-Cutting on the Volume and Chemistry of Water at a Sandy Nutrient-Poor Pine Site

Clear-cutting removes a stand cover suddenly, thus impacting soil-surface conditions by the expected increase in the humus breakdown rate, due to the addition of more precipitation and solar radiation. The impacts of removing a forest were studied at a site with deep sandy soils dominated by Scots pine, where three treatments, including a stand left on site, whole-tree harvesting treatment, and stem-only harvesting treatment, were established. Clear-cut precipitation and sub-canopy throughfall were measured. The non-replicated treatments were equipped with lysimeters at depths of −10 cm and −70 cm and in addition, three exploratory wells were drilled to sample groundwater in each of the treatments. The volume and chemistry of the water samples were periodically analyzed. The mean throughfall was 84% of open-area precipitation and the pH of the water penetrating the canopy was significantly lower overall, although the subcanopy samples were only slightly more acidic. Significant differences among the treatments were shown in throughfall (total organic carbon, TOC), shallow subsurface water (pH, TOC, NO3−), subsurface water at the 70-cm depth (NO3−) and also groundwater (pH, TOC, NO3−), where some of the differences had been observed before the stand parts were clear-cut. Limits for drinking water were exceeded in groundwater pH, as the values from both clear-cut sites fell below pH 6.5 following clear-cutting. TOC greater than 5 mg·L−1 was found in the Q1-2020 samples, whereas groundwater nitrate remained low, even though the three wells differed significantly from each other. Due to the non-replicated design of the experiment, generalization of the outcomes is a matter of further research.

Post-drought conditions and hydraulic dysfunction determine tree resilience and mortality across Mediterranean Aleppo pine (Pinus halepensis) populations after an extreme drought event

Drought-related tree mortality is a global phenomenon that currently affects a wide range of forests. Key functional variables on plant hydraulics, carbon economy, growth and allocation have been identified and play a role in tree drought responses. However, tree mortality thresholds based on such variables are difficult to identify, especially under field conditions. We studied several Aleppo pine populations differently affected by an extreme drought event in 2014, with mortality rates ranging from no mortality to 90% in the most severely affected population. We hypothesized that mortality is linked with high levels of xylem embolism, i.e., hydraulic dysfunction, which would also lead to lower tree resistance to drought in subsequent years. Despite not finding any differences among populations in the vulnerability curves to xylem embolism, there were large differences in the hydraulic safety margin (HSM) and the hydraulic dysfunction level. High mortality rates were associated with a negative HSM when xylem embolism reached values over 60%. We also found forest weakening and post-drought mortality related to a low hydraulic water transport capacity, reduced plant growth, low carbohydrate contents and high pest infestation rates. Our results highlight the importance of drought severity and the hydraulic dysfunction level on pine mortality, as well as post-drought conditions during recovery processes.

The Large-Scale Effect of Forest Cover on Long-Term Streamflow Variations in Mediterranean Catchments of Central Chile

Forest ecosystems play an important role in hydrological processes as surface and subsurface runoff, as well as the storage of water at the catchment scale. Therefore, it is important to have a greater understanding of the effects of forests in the long-term water balance of Mediterranean catchments. In this sense, this study evaluates the effect of native forests, forest plantations, and the combination of both, on long-term streamflow variations in central Chile, an unusual area of Mediterranean climate characterized by a well-marked annual cycle with dry summers and wet winters. Thus, the temporal pattern of monthly streamflow was evaluated for mean flow (Qmean), maximum flow (Qmax), and minimum flow (Qmin) in 42 large-scale (>200 km2) Mediterranean catchments. Each series of monthly streamflow data was QA/QC, and then evaluated using the Mann–Kendall’s non-parametric statistical test to detect temporal variations between 1994 and 2015. In addition to the previous analysis, the monthly series were grouped into wet seasons (April–September) and dry seasons (October–April), to determine if there were any significant differences within the annual hydrological cycle. The areas covered with native and forest plantations and their relative changes were evaluated for each catchment through streamflow variations and forest cover indicators. Results revealed that streamflow variations are positive and significant when more forest cover exists. The intra-catchment relationships assessed for both species revealed the significant role of native forests and mixed masses as key ecosystems for the long-term conservation of summer streamflow in Mediterranean catchments of central Chile. These findings encourage an urgent need to create highland afforestation programs on degraded areas of central Chile, to maximize water storage in a region that is quickly drying out due to unsustainable water and land use management practices and the effects of global warming.

Species Mixing Proportion and Aridity Influence in the Height–Diameter Relationship for Different Species Mixtures in Mediterranean Forests

Estimating tree height is essential for modelling and managing both pure and mixed forest stands. Although height–diameter (H–D) relationships have been traditionally fitted for pure stands, attention must be paid when analyzing this relationship behavior in stands composed of more than one species. The present context of global change makes also necessary to analyze how this relationship is influenced by climate conditions. This study tends to cope these gaps, by fitting new H–D models for 13 different Mediterranean species in mixed forest stands under different mixing proportions along an aridity gradient in Spain. Using Spanish National Forest Inventory data, a total of 14 height–diameter equations were initially fitted in order to select the best base models for each pair species-mixture. Then, the best models were expanded including species proportion by area (mi) and the De Martonne Aridity Index (M). A general trend was found for coniferous species, with taller trees for the same diameter size in pure than in mixed stands, being this trend inverse for broadleaved species. Regarding aridity influence on H–D relationships, humid conditions seem to beneficiate tree height for almost all the analyzed species and species mixtures. These results may have a relevant importance for Mediterranean coppice stands, suggesting that introducing conifers in broadleaves forests could enhance height for coppice species. However, this practice only should be carried out in places with a low probability of drought. Models presented in our study can be used to predict height both in different pure and mixed forests at different spatio-temporal scales to take better sustainable management decisions under future climate change scenarios.

Changes in the transcript and protein profiles of Quercus ilex seedlings in response to drought stress

Quercus ilex is the dominant tree species in natural forest ecosystems across the Mediterranean Basin and in the agrosilvopastoral system dehesa, which has a high ecological and economical significance. As in other forestry species, survival in Q. ilex is threatened by long periods of drought. This paper reports the transcriptome and proteome profiles of 6-month-old seedlings subjected to severe drought conditions. Drought was imposed by water withholding in seedlings grown in perlite for 28 days. Seedling leaves were collected when leaf fluorescence had decreased by 20% and 45% relative to well-watered seedlings. The transcriptome and proteome were analyzed by using Illumina and shotgun platforms. The quality and confidence of the mRNA and protein identifications and quantifications were assessed, obtaining 25,169 transcripts and 3312 proteins. Variable transcripts and proteins were analyzed by Venn diagram, Pearson's correlation, GO enrichment, KEGG pathways, multivariate analysis and interaction networks. Despite the poor correlation between mRNA and protein, both platforms gave a complementary view of the changes in the abundance of several gene products under drought conditions and indicated that gene expression regulation and translation to phenotype is quite complex and gene-specific. As a general tendency, while transcripts and proteins of the metabolism were down-accumulated, those of stress related were up-accumulated. Out of the variable dataset, four gene products (viz., FtSH6, CLPB1, CLPB3, and HSP22) were up-accumulated at both omics levels at the two surveyed times, being the first work where they are described in drought response in forest species. These chaperones and proteases could be considered as potential drought tolerance markers to be used in the selection of elite, resilient genotypes, and in breeding programs.

20-Years of hindsight into hydrological dynamics of a mountain forest catchment in the Central Spanish Pyrenees

Mediterranean mountain forests play a significant role in hydrological regulation. In this study, hydrological dynamics was examined at different temporal scales in a small mountain forest catchment in the Central Spanish Pyrenees (San Salvador), based on a 20-year dataset (1999-2019). Mean annual runoff coefficient is 0.21, and ranged from 0.02 to 0.58. The catchment has a bi-modal hydrological behavior with two hydrological periods: a dry-period between July and December, and a wet-period between January and June. During the study period, only 108 floods were recorded, suggesting a low responsiveness of the catchment, with a high variable response. Spearman correlation analysis and stepwise multivariate regression suggest that the hydrological response in the San Salvador catchment is mainly depending on water table, with antecedent moisture conditions and rainfall depth as secondary factors. Seasonal differences were also observed: during dry season, the response was mainly related to rainfall depth and rainfall intensity; in contrast in wet season, the response was mainly related to antecedent conditions (previous rainfall and base flow). Thus, the already challenging water resources management in the Mediterranean basin is magnified by the key function of forests as natural modulators of water cycle. Consequently, the study of natural forested catchments is needed and long-datasets have to be analysed to understand the role of natural Mediterranean forest in the hydrological dynamics and its evolution and adaptation in a context of Global Change.

Assessment of Inter-Sectoral Virtual Water Reallocation and Linkages in the Northern Tianshan Mountains, China

Quantitative analysis of the reallocation and linkages of virtual water in the economic sector was important for the integrated water resources management in inland arid regions. Taking the northern Tianshan Mountains (NTM) as an example, we applied the environmental input-output model to design the accounting framework for the reallocation of blue and green virtual water (VW) in the economic sector and analyzed the correlation effect of VW reallocation among various sectors by backward and forward linkages in economic analysis. The results showed that the direct blue and green water consumption of primary industry respectively accounted for 99.2% and 100% of the total water consumption in NTM. Planting sector had the largest amount of VW outflow among all sectors. Animal husbandry, forestry and construction had a large pulling effect on VW outflow of planting sector, while planting sector and animal husbandry were the main sectors for VW export of blue and green water. We suggest that the government can increase the import of blue-green VW for agricultural raw materials through VW trade and develop industries such as service and electricity that have less pulling effect on the primary industry VW, so as to improve the economic added value of VW in the primary industry and reduce the loss of VW in primary industry production and trade flows in future water management.