Jalal Kassout1,2,3, Jean-Frédéric Terral2,3, Mohammed Ater1,3*
1 Laboratoire Botanique Appliquée, Equipe bio-Agrodiversité, Faculté des Sciences, Université Abdelmalek Essaâdi. BP 2060, Tétouan 93 030, Morocco.
2 ISEM, Université de Montpellier, CNRS, IRD, EPHE. Equipe DBA, place Eugène Bataillon, CC65, 34095 Cedex, France.
3 Associated International Laboratory EVOLEA, INEE-CNRS (France – Morocco). *Corresponding author : mohammed.ater@gmail.com
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Abstract
The search for general principals to elucidate the response of species communities and ecosystems to environmental changes is a longstanding theoretical issue in ecology especially in the current ongoing global changes. By combining functional trait-based and phytoecological approaches, we quantifies wild olive communities responses to environmental factors through investigating two subspecies communities distributed in Morocco (Olea europaea subsp.
europaea var. sylvestris and O. e. subsp. maroccana) along a biogeographical, climatic and
vegetation gradient. These approaches were adopted in order to reveal main environmental factors affecting this communities diversity and to assess functional strategies developed by the olive to deal with environmental changes. Vegetation surveys, environmental variables and fifteen traits were undertaken and measured from 27 populations of wild olive along a North- South latitudinal gradient of changing environmental conditions. In order to link environmental factors with plant composition and functional traits, Hierarchical Cluster, statistical multivariate analyses and linear regression were performed. Our results indicate that the climatic variables are the main factors affecting the distribution of wild olive plant communities across the latitudinal gradient. Thus, by analysing leaf and plants size traits, we showed that wild olive functional strategies are mainly explained by a leaf economic spectrum reflecting resource use strategies. Therefore, these strategies were found to be mainly related to aridity and vegetation. Our results highlight the main factors explaining the diversity of wild olive vegetation communities, and functional strategies adopted by olive in response to environmental changes. Thus, our results can be used to model future responses of Mediterranean vegetation in the light of ongoing global changes.
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Introduction
Understanding the factors effecting the diversity, the structure and composition of plant communities as well as the ecological strategies deployed by plants along the environmental gradient are major concerns in ecology (Calow, 1987; MacArthur, 1972). In particular, to understand species assembly rules and the relation of their characteristics (traits) with the biotic and abiotic factors (Grime, 1974; Southwood, 1988; Lavorel et al., 1997). In the current context of global changes, these concerns are even more acute, particularly to predict species responses to future environmental changes (Diaz et al., 2007; Madani et al., 2018).
The spatial distribution of plant communities can be interpreted as the result of several ecological factors acting at different organizational levels, as well as at different spatial and temporal scales (de Bello et al., 2012; Götzenberger et al., 2012; Chave, 2013). In fact, under the effect of ecological factors (biotic and abiotic), species are selected along natural gradients called environmental filters scaled from a regional pool to a local habitat according to their functional traits (Woodward & Diament, 1991; Weiher & Keddy, 1995; Díaz et al., 1998). In fact, several studies have shown that plant success depend on its functional traits responses to environmental conditions (Garnier et al., 2004; MacGill et al., 2006; Cornwell & Ackerly, 2009). Traits values or attributes (sensu Violle et al., 2007) vary along environmental gradients and influence plant performance such as growth, reproduction and survival (Lavorel et al., 1997; Wright et al., 2005; Violle et al., 2007). These variations may imply a single trait or a set of interrelated traits (Wright et al., 2004; Díaz et al., 2016). Studies carried out at the interspecific (Wright et al., 2004, 2005; Diaz et al., 2004, 2016) and intraspecific levels (Albert et al., 2010; Messier et al., 2010, 2017) have shown the importance of functional trait variation in defining plants ecological strategies according to environmental factors. Moreover, the measurement of functional traits along environmental gradients has led to the identification of plant functional types particularly in relation to resource capture and allocation (Grime et al., 1997; Westoby et al., 2002; Wright et al., 2004; Diaz et al., 2004).
Amongst the prominent general rules relating plant performance to the environment, the leaf economic spectrum (LES, Wright et al., 2004) and the competitive-stress-ruderal theory (CSR,
Grime, 2001) are the most recognized concepts. The LES defines a single axis of trait variation, highlighting a fundamental trade-off between (i) acquisitive strategies of resources associated with rapid plant growth and high tissue turn-over, and characterised by high SLA (specific leaf area) values, and (ii) conservative strategies, associated with slow plant growth and low tissue turn-over and characterised by high LDMC (leaf dry matter content) values (Reich et al., 1992;
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Wright et al., 2004; Diaz et al., 2004; Shipley et al., 2006). Beside of the LES, plant size has an impact on the definition of functional strategies (Diaz et al., 2016). Grime (2001) defined adaptive strategies based on two main ecological drivers affecting plant performance and consequently their success under different conditions. The first driver is the effect of stress (S), defined as any environmental factors limiting plant growth such as limiting water resources (Hodgson et al., 1999). The second factor is the effect of disturbance (R), defined as any factors that cause destruction of plant biomass such as fire or predation (Grime, 1977). According to the differences in intensity of these factors, three main plants ecological strategies were defined: (i) stress tolerance strategy (S) in resource-poor habitats, with investment in tissue protection; (ii) ruderal strategy (R) with invest in rapid reproduction and dispersal under resource-rich environments associated with repeated disturbance; (iii) competitive strategy (C) with investment in rapid growth under highly productive habitats with less stress intensity and disturbance (Hodgson et al., 1999; Pierce et al., 2017).
These concepts or theories (LES, CSR) were mostly addressed at the interspecific level, which allowed advancing our understanding of species functional strategies (Grime, 2001; Wright et al., 2004; Diaz et al., 2004, 2016). Meanwhile, as traits are measured at the individual level (Violle et al., 2007), taking into account the intraspecific variation is important to understand plant-environment relationships within species and across communities (Albert et al., 2010; Fajardo et al., 2011; Moran et al., 2016; Messier et al., 2017; Kassout et al., 2019). In fact, the observed high intraspecific variation in some large-distributed species (Jung et al., 2010; Fajardo & Siefert, 2016, 2019) suggests a relationship between traits and environmental conditions (Lepš et al., 2011; Auger & Shipley, 2012). However, if some traits show high intraspecific variation in response to the environment (e.g. SLA, Vasseur et al., 2012), others show a limited variation, e.g. wood density (Siefert et al., 2015). At this point, it is important to underline that intraspecific variation in response to the environment may be resulted of two types of mechanisms that are difficult to differentiate: the phenotypic plasticity and/or the genetic variation (Frank, 2011).
The olive complex (Olea europaea L.) is composed by six subspecies with large distribution (Médail et al., 2001; Green, 2002; Besnard et al., 2009; Garcia-Verdugo et al., 2010). The
europaea subspecies has been domesticated and represented by two botanical varieties: O. europaea subsp. europaea var. europaea (the cultivated olive) and O. europaea subsp. europaea var. sylvestris (oleaster), common in Moroccan vegetations. In Morocco, a second
171 associated with a Macaronesian flora. Regarding to their evolutionary history and biogeographical context, these two subspecies correspond to two different lineages. Even if they are sympatric subspecies, they are isolated by ploidy: maroccana is hexaploid while
sylvestris is diploid. The distribution area of oleaster (sylvestris) is mainly determined by two
limiting climatic factors, the cold established by altitude, and the aridity represented by latitude (Ionesco & Sauvage, 1969). Within its distribution area, from north to south, oleaster belongs to different plants communities. This paper aims to (1) characterize the plant communities where oleaster is nowadays present; (2) identify the environmental factors explaining the distribution of these communities along the latitudinal gradient; (3) characterize and quantify variation(s) in functional traits according to environmental gradients; (4) test the hypothesis that the two subspecies (europaea var. sylvestris and maroccana) exhibit different adaptive strategies.