Mediterranean forests are characterized by a remarkable set of features that make them naturally and aesthetically attractive, but also quite fragile [1
]. Mediterranean forestry is focused on a multi-functional approach, providing a wide range of goods and services for society ranging from products with high market value (fuelwood, cork, mushroom, pinecones, etc.) and non-market value ecosystem services (soil and landscape protection, water regulation, biodiversity conservation, carbon dioxide fixation, recreation, aesthetic view, etc.). The latter is more significant than their productive value, especially their significant role for carbon sequestration [2
]. One of the notable characteristics of Mediterranean forests is their rich biodiversity, reflected by high genetic variability, exemplified by the large number of tree species in comparison to Nordic forests resulting from the survival of many conifer and broadleaf species during the glacial periods. Long-term exploitation (manipulation) of trees and forestland since ancient times is another feature of Mediterranean forest which has resulted in the dispersion of species such as Pinus pinea, Castanea sativa
, and Quercus suber
all over the Mediterranean basin [1
]. Dry, hot, harsh climate along with long-lasting and frequent droughts, pests, and diseases, increasing risk of large-scale fires and severe water scarcity are the main challenges for the Mediterranean forests, which largely impact on forest health, growth, and productivity. The role of mixed forest for promoting forest productivity while coping with these challenges has been increased in the Mediterranean region in recent decades.
Mixed forest has received a great deal of attention in recent decades due to its greater provision of goods and services and its high ecological value in comparison to monoculture forest [3
]. Mixed forest is defined as a forest unit of at least 0.5 ha that composes at least two tree species at any developmental stage, shares common resources (water, light, soil nutrients) and its structure and component species are altered over the time [5
]. The main characterizations of mixed forests are described not only by better protection, preservation, maintaining and monitoring of biodiversity but also have high resistance capacity against both natural and anthropogenic disturbances such as climate change, storm, pests, disease, and air pollution and its consequences. The economic importance of mixed forest is not negligible because of its multi-use, multi-source in comparison with pure stands [6
]. High biodiversity levels are linked with high forest productivities [7
] leading to a general rule of thumb regarding the promotion of mixed versus pure forests. However, forest multifunctionality must include also other components not directly assessed by diversity or biomass yield. Despite this generally positive relationship between diversity and productivity for temperate forests, a large variation has been reported in terms of species richness highlighting the importance of species identity in the mixture [8
]. Species identity can modify the mixture outcomes; by assessing the impact of different mixtures composition, we can gain an insight into the outcomes and delineate adequate management strategies. The loss of biodiversity imposed by anthropogenic influence and climactic change has highlighted the importance of worldwide diversity over the last few decades [7
]. In that time, scientists have been taking into account the importance of biodiversity for many ecosystem functions and services, such as productivity, stability, sustainability, sinking carbon dioxide, preserving soil fertility, controlling pest outbreaks, retaining water, etc. [11
]. Among these, the importance of tree species diversity on biomass productivity has been studied based on the variety of genes, species, or functional traits of organisms in hundreds of types of ecological communities [7
]. A series of biodiversity–ecosystem functioning studies have revealed that biodiversity (including taxonomic, functional, and phylogenetic diversity) promotes the functionality of ecosystems such as primary production, decomposition, nutrient cycling, trophic interactions, etc., and consequently supports a broad range of ecosystem services such as food production, climate regulation, pest control, and pollination [13
]. However, contradictory results have been documented for the relationship between species diversity and biomass, finding negative impacts [10
] or no change [15
] in biomass yield with species diversity levels. In addition, loss of biodiversity ranks among the most pronounced changes to the environment [16
]. Reductions in diversity and species composition alter fluxes of energy and the essential services that ecosystem provides to human society, such as the production of food, pest and disease control, or water purification [17
]. Globally, biodiversity is largely and irreversibly being degraded and lost due to direct drivers, i.e., habitat disturbance, habitat fragmentation, land use change, over-exploitation and the spread of alien species, and indirect drivers, i.e., climate change, population growth, economic growth and increasing demand for food, materials, water and energy [18
]. The loss of biodiversity weakens species’ connections and impairs the ecosystems, leading to the extinction of species and local populations, which will disrupt the capacity of ecosystem to contribute to human well-being and sustain future generations.
Tree diversity plays a fundamental role in forest diversity because it is often linked with the major properties of the forest ecosystem, leading to the possible enhancement in diversity of other forest assemblies [14
] and providing the required resources and suitable habitats for other forest species [19
]. The most used representation of ecological diversity is species diversity, which is defined by the number of species and abundance of each species living within a certain area [20
]. The species coexisting in a certain area are interconnected and dependent on one another for survival. While doing so, they perform important ecosystem functions and offer different ecosystem services for human life and society: provisional services (products obtained from ecosystem: many different type of food, fresh water, etc.); regulating services (the benefits obtained from the regulation of ecosystem processes: air quality and pollination); cultural services (the non-material benefits that people obtain: spiritual enrichment, recreation and aesthetic experiences); supporting service needed to maintain other services (i.e., photosynthesis and nutrient recycling). The provision of ecosystem for such goods and services depends basically on functions performed by living plants [21
Two main mechanisms explain the reasons that biodiversity influences on productivity: selection effects and complementary effects. Different plant species in a mixture have different physiologies, morphologies and life history traits that might allow them to fully utilize limiting resources in different space and time compared to a monoculture of any species [21
]. Complementarity between species traits can lead to over- or under-yielding in mixed forests in relation to monocultures. For instance, tree species that have different root morphologies occupy different soil profiles, which potentially allows them to exploit soil resources from different soil depths. Additionally, differences in shade tolerance or in foliage persistence during the year can lead to complementarity. However, it should be noted that these complementarities occur solely when co-existing species exhibit various forms of niche differentiation that allow them to capture resources in different space or time [22
]. Another mechanism of productivity that diversity affects is selection effect (sampling effect) which describes species specific-effect on biomass: a greater productivity in more diverse communities is due to the most productive species which become dominant in the community due to competition. The likelihood of becoming a highly productive species increases as diversity increases. Thus, this leads to the increment in the total productivity of the community. Such considerations have led to the general perception of having higher productivity in an area where more plant species co-exist.
Therefore, our main objective was to explore the relationship between tree and structural diversity and forest biomass in Mediterranean mixed forests. To achieve that, two intensive sample plots were analyzed in the Spanish northern plateau and all individual trees in each 1-ha plot were recorded for their species, measured and geopositioned. With these data, different biodiversity metrics and forest biomass values were calculated. Several lineal models were fitted, evaluated, and selected to provide an insight into the diversity and biomass relationship.