Trying to Link Vegetation Units with Biomass Data: The Case Study of Italian Shrublands
, Serena Marras2, 3, Costantino Sirca2, 3, Donatella Spano2, 3 and Riccardo Valentini4
(1)
Department of Agricultural and Forestry Sciences, University of Palermo, Palermo, Italy
(2)
Department of Natural and Territorial Science, University of Sassari, Sassari, Italy
(3)
Impacts on Agriculture, Forest and Natural Ecosystem Division (IAFENT), Euro-Mediterranean Center on Climate Changes (CMCC), Sassari, Italy
(4)
Impacts on Agriculture, Forest and Natural Ecosystem Division (IAFENT), Euro-Mediterranean Center on Climate Changes (CMCC), Viterbo, Italy
Abstract
Although their carbon stock is relevant in assessing the baseline for the negotiation of future agreements with respect to carbon balance, there still are few available studies concerning the biomass and the net ecosystem exchange capacity of Mediterranean shrublands. In this chapter a preliminary overview on the biomass values concerning Italian shrubland communities and/or their dominant/characteristic woody species is provided. Many useful data on above- and belowground biomass issued from investigations carried out in other Mediterranean countries and concerning plant communities, which share the same ecological, floristic and structural traits of Italian shrublands. A preliminary finding of this research is the uneven degree of knowledge concerning the different non-forest woody communities. For example, there is still no literature on the biomass of some 2/3 of all the considered phytosociological units. Besides, both the above and the below-ground biomass of many Mediterranean shrubs show a very wide range of variation as they are strongly influenced by progressive succession processes and by the nature, the intensity and the frequency of disturbance factors. Thus, direct measuring of these values for each vegetation unit and dominant woody species should be encouraged and intensified. Monitoring activities concerning biomass increase are recommended as well: as a matter of fact, at present reference data on this topic are so limited and variable that it is not possible to confidently estimate the annual growth of shrubland communities.
14.1 Introduction
Shrublands may represent the most mature local communities and not be able to “evolve” towards proper forests. In these cases, their presence is mostly due to very strong limiting (stress) factors, such as long-lasting warm or cold periods, scarce water availability, nutrient-poor or toxic soils, intensive drainage, etc. Elsewhere, shrublands represent unsteady steps within a dynamic process, which may be progressive or retrogressive. In the first case, they issue from the recent colonization performed by woody species after land abandonment (Corona et al. 2008; La Mantia et al. 2008), and their complexity increases along with cover rate till they become mature forests. In retrogressive succession, in contrast, severe and/or frequent disturbance (i.e., grazing, burning, cutting, etc.) causes the structural simplification of pre-existing forest communities. As a consequence, shrublands and open maquis communities may have similar physiognomies even when they have rather different histories to tell. Thus, a lack of historical information may bias any trial to assess how plant biomass is likely to change in the future. On the other hand, as shrub communities often develop when forests degrade, the knowledge of their carbon stock is relevant in assessing the baseline for negotiating future agreements with respect to carbon balance.
At present the official data concerning non-forest Italian vegetation do not match perfectly: in fact, according to the most recent (2005) National Forestry Inventory (http://www.sian.it/inventarioforestale/jsp/home.jsp) shrublands cover 990,916 ha and woodlands 46,678 ha, while following the official data provided by the National Forestry Inventory carried out in 1985, Corona et al. (1997) estimate that altogether Italian shrubby, rupicolous, and riparian plant communities occupy 2,164,500 ha and have a carbon stock of 26.4 (18–35) Mt. The same authors also report 256 Mt as the average value of carbon stock in Italian forests (high forests + coppices) and tree plantations.
Beyond these discrepancies, there is no doubt about the important role played by shrublands in the global carbon stock, as indicated by the Kyoto Protocol (the “Marrakesh Agreements” explicitly refer to shrub vegetation colonizing abandoned agricultural land). On the other hand, there are few available studies concerning the biomass and the net ecosystem exchange capacity of shrublands (Navarro Cerrillo and Blanco Oyonarte 2006). This is also true for the most common woody species (hereinafter abbreviated “WS”) typical to Mediterranean (Usò et al. 1997) and Italian (Costa and La Mantia 2005; Corona et al. 2012) shrublands, and even less information is available concerning biomass assessment at the community level. This knowledge gap not only depends on the lack of field investigations focused on the dominant WS but also on the high floristic and structural variability of shrublands themselves.
Besides, most of Mediterranean WS show high size variability (Corona et al. 2012 and references within), so that the common techniques and algorithms used for the forest tree species do not always fit well with maquis ones. Even ring analysis techniques often fail for these species because high environmental variability makes interpretation of ring data difficult (Cherubini et al. 2003). Because a standardised methodology is unavailable for these species, researchers have estimated maquis biomass and the annual variation in the biomass using a variety of methods. As a result, the available data are seldom comparable (Corona et al. 2012).
During the last four decades, plant ecologists have published hundreds of papers and thousands of phytosociological relevés concerning the non-forest Italian plant communities. This huge amount of data is mostly focused on floristic similarities among coenoses, but many of the papers provide only basic information on the abiotic context (e.g., soil chemistry, bioclimate, slope, aspect, disturbance intensity and frequency, etc.), and very few report the main structural patterns (vertical stratification, cover of each vegetation layer, etc.). This makes difficult to compare the relevés published in different works, even if they concern the same plant community. In this chapter an attempt to interpret phytosociological data has been carried out in order to provide a preliminary overview on the biomass values of Italian shrubland communities and/or their dominant/characteristic WS.
14.2 Basic Assumptions on Italian Shrubland Vegetation
Several thousands of phytosociological relevés concerning the Italian non-forest woody communities published till June 2011 were considered (data not shown). In order to facilitate modelling, we focused our attention on the similarities concerning the following six parameters: (1) whole vascular flora (all taxa); (2) ‘weighted’ vascular flora (all taxa also considering coverage values); (3) dominant WS; (4) most frequent WS (i.e. whose frequence was ≥ 50 %); (5) ecological (i.e., species with similar edapho-climatic requirements); and (6) structural (i.e. cover rate, stratification, etc.) patterns.
The nomenclature of vegetation units follows Biondi and Blasi (2009). All those classes in which WS are dominant or frequent in only one or a few associations, like the pioneer vegetation of screes and ravines (class Thlaspietea rotundifolii), the chasmophilous communities of undisturbed cliffs (class Asplenietea trichomanis), the meso-xerophilous perennial grasslands (class Festuco–Brometea), the chamaephytic lithophilous communities prone to salt spray (class Crithmo–Staticetea pro parte), the rush-dominated prairies on salt-rich and humid soils (class Juncetea maritimi) were excluded. The same decision was taken for high maquis formations dominated by Quercus ilex, Quercus suber, and Pinus spp. because they should be ascribed to proper woodland communities (class Quercetea ilicis, order Quercetalia ilicis).
Many useful data on above- and belowground biomass (hereinafter Ab and Bb) issued from investigations which have been carried out in other Mediterranean countries (e.g. Spain, southern France, Greece and Israel) and concerned plant communities which shared the same ecological, floristic and structural traits of Italian shrublands.
14.3 The Study Case of Italian Shrublands
Italian shrubland plant communities are ascribed to 17 classes, 26 orders, 56 alliances (Table 14.1) and c. 730 associations and subassociations (data not shown). The most widespread communities resulted to be: (1) the Mediterranean maquis and scrubland (class Quercetea ilicis, order Quercetalia calliprini); (2) the submediterranean deciduous mantle assemblages (class Rhamno cathartici–Prunetea spinosae); (3) the acidophilous (classes Cisto ladaniferi–Lavanduletea stoechadis and Cytisetea scopario–striati) and basiphilous (Cisto cretici–Micromerietea julianae and Rosmarinetea officinalis) garrigues and heaths; (4) the coastal sub-halophilous shrublands and garrigues (Crithmo–Staticetea pro parte); (5) the psammophilous garrigues typical to the fixed dunes (Helichryso italici–Crucianelletea maritimae pro parte); (6) the alpine, sub-alpine, and apennine scrublands and heaths (classes Erico–Pinetea pro parte, Calluno–Ulicetea and Pino–Juniperetea); (7) the oromediterranean heath communities typical to the mountain tops of Central and Southern Italy, Sardinia, and Sicily (Carici–Genistetea lobelii and Rumici–Astragaletea siculi); (8) the thermo-hygrophilous open riparian communities (class Nerio–Tamaricetea) and (9) the halo-xerophilous (class Pegano–Salsoletea) and halo-xero-nitrophilous (class Sarcocornietea fruticosae) scrub communities dominated by shrubby chenopods.
Table 14.1
Overview of the dominant and/or most frequent WS within an up-to-date syntaxonomic scheme of Italian pre-forest vegetation
Phytosociological unit | Dominant WS | Other characteristic WS | Importance–biomass (see note 1) | Available data on biomass estimation (cfr. Table 14.2) | ||
|---|---|---|---|---|---|---|
Class | Order | Alliance | ||||
Crithmo-Staticetea | Crithmo-Limonietalia (incl. Senecionetalia cinerariae) | Plantagini-Thymelaeion hirsutae | Thymelaea hirsuta | Helichrysum spp., Senecio cineraria s.l. | M–M | N |
Anthyllidion barbae-jovis | Anthyllis barba-jovis | Helichrysum spp. | M–M | N | ||
Crucianellion rupestris | Crucianella rupestris | Limonium spp. | L–L | N | ||
Sarcocornietea fruticosae | Sarcocornietalia fruticosae | Sarcocornion alpinii | Sarcocornia alpinii | M–M | N | |
Sarcocornion fruticosae | Sarcocornia fruticosa | Halimione portulacoides | M–M | Y | ||
Arthrocnemion macrostachyi | Arthrocnemum macrostachyum | Halocnemum macrostachyum | M–L | Y | ||
Suaedion verae | Suaeda vera | Halimione portulacoides, Limoniastrum monopetalum | M–M | Y | ||
Limonietalia | Limoniastrion monopetali (= Limonion ferulacei) | Limonium ferulaceum | Limoniastrum monopetalum | L–L | N | |
Cisto ladaniferi-Lavanduletea stoechadis | Lavanduletalia stoechadis | Cistion ladaniferi | Lavandula stoechas | Cistus spp., Erica scoparia | M–L | Y |
Calicotomo-Genistion tyrrhenae | Calicotome spp., Genista gr. ephedroides | Cytisus villosus, Genista monspessulana | M–M | N | ||
Teucrion mari | Teucrium marum, Genista spp. | Cistus spp., Rosmarinus officinalis, Halimium halimifolium | M | N | ||
Helichryso-Crucianelletea maritimae | Helichryso-Crucianelletalia maritimae | Crucianellion maritimae | Crucianella maritima | Ephedra distachya, Helichrysum microphyllum, Thymelaea tartonraira | L–L | N |
Euphorbion pithyusae | Euphorbia pithyusa subsp. pithyusa | Helichrysum microphyllum, Astragalus thermensis | L–L | N | ||
Cisto cretici-Micromerietea julianae | Cisto cretici-Ericetalia manipuliflorae | Cisto cretici-Ericion manipuliflorae | Cistus creticus, Erica manipuliflora | Sarcopoterium spinosum | M–M | Y |
Rosmarinetea officinalis | Ononidetalia striatae | Lavandulo-Genistion cinereae | Lavandula stoechas, Genista cinerea | M–L | Y | |
Rosmarinetalia officinalis | Rosmarinion officinalis | Rosmarinus officinalis | Globularia alypum | M–M | Y | |
Aphyllanthion | Aphyllanthes monspeliensis | L–L | N | |||
Cisto eriocephali-Ericion multiflorae | Cistus eriocephalus, Erica multiflora | Sarcopoterium spinosum | M–M | Y | ||
Euphorbion ligusticae | Euphorbia ligustica | L–L | N | |||
Cytisetea scopario-striati | Cytisetalia scopario-striati | Violion messanensis | Sarothamnus scoparius | Adenocarpus spp. | M–L | N |
Cytiso villosi-Telinetalia monspessulanae | Telinion monspessulano-linifoliae | Genista monspessulana, Cytisus villosus | Ulex europaeus | M–L | N | |
Rhamno cathartici-Prunetea spinosae | Prunetalia spinosae | Cytision sessilifolii | Cytisus sessilifolius | Crataegus laevigata, Amelanchier ovalis | M–M | N |