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Analysis of the patterns of distribution of photosynthetic pathways and representativity of the family Poaceae on Martin García Island, Río de La Plata, Buenos Aires, Argentina
Flora (2002) 197, 351–360 http://www.urbanfischer.de/journals/flora
Analysis of the patterns of distribution of photosynthetic pathways and representativity of the family Poaceae on Martin García Island, Río de La Plata, Buenos Aires, Argentina Mariana Cecilia Bianchin Belmonte* & Zulma E. Rúgolo de Agrasar Instituto de Botánica Darwinion, Labardén 200, (1642) San Isidro, Buenos Aires, República Argentina * e-mail corresponding author: [email protected] Received: Oct 17, 2001, in revised form: Feb 26, 2002 · Accepted: May 25, 2002
Summary Martin García Island lies on an unique geographical location in South America where species from three different phytogeographical provinces meet. Although its surface area is very small (168 hectares), it has a wide variety of vegetation types ranging from gallery forests to xerophytic woods. The floristic inventory of this island was updated for the family Poaceae by collecting and identifying material with an outcome of 118 taxa, 32 of which are cited for the first time for the area. The plants collected were classified according to their tribe, subfamily, genus, species and photosynthetic pathway. The distribution of the photosynthetic types C3/C4 in the various ecological units of Martin García Island was discussed on the basis of sun/shade and humidity parameters. The results show that C3 taxa prevail in humid and shady habitats and C4 taxa in dry and sunny areas. Key words: South America, Poaceae, C3/C4
Introduction The flora of a certain area can characterize in detail its ecological situation and relevant knowledge forms the basis for an analysis of the vegetation dynamics that is necessary also for its management and conservation. Grasses (Poaceae) constitute in many vegetation units a considerable amount of all plants and dominate the landscape aspect in most semiarid to semihumid regions of the world. Grasslands are widespread also in Argentina, by nature and even more under human influence, and a high diversity can be found within the members of this family (Parodi 1969; Cabrera 1979, 1976; Hueck & Seibert 1981). A closer taxonomic information allows further research studies regarding the phylogenetic links of the encountered plant taxa and their particular anatomical structures. An important contribution in this perspective is the study of the photosynthetic pathways in relation to their distribution in different environments. In the grass family as a whole (and also in few other Angiosperm families) the genera differ in the ways of carbon dioxide fixation. Depending on whether the initial product is the 0367-2530/02/197/05-351 $ 15.00/0
three carbon molecule phosphoglyceric acid as in the Calvin Benson Cycle they belong to the C3 type or to the C4 plants. In the latter case the primary CO2 fixation results into the four-carbon molecule oxaloacetic acid which is subsequently transformed to malic and/or aspartic acid, following the Hatch-Slack cycle (Werger & Ellis 1981). The various photosynthetic pathways seem to have their advantages under different environmental conditions (Werger & Ellis 1981). The relation between the distribution of C3 and C4 plants and climatic conditions (temperature, humidity, altitude and rainfall) have been studied by various authors: Stowe & Teeri (1978) in North America, Chazdon (1978) in Costa Rica, Meinzer (1978) in Central America, Klink & Joly (1986) in Brasil, Vogel et al. (1978) and Werger & Ellis (1981) in South Africa, Rundel (1980) in Hawaii, and Hattersley (1983) in Australia. Generally, these studies have been carried out at a broader geographical level. However, Klink & Joly (1986) state that the effect of local variables (mainly sunlight/shade) upon the distribution of C3 and C4 grasses should also be considered, since they provide a new FLORA (2002) 197
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N
References Gallery Forests Xerophytic Woods Periurban Woods Riverside Woods Sand Dunes Transitional Areas Riverside Thicket & Grassland Wetlands I Urban Area
Wetlands II Urban Area
River Plate Pier
River Plate
0m
200 m 300 m
Fig. 1. Martin García Island, Rio de la Plata – Ecological Units.
approach to the study of the distribution of C3 and C4 plants. Our aim is to study the distribution of C3 and C4 grasses in a very small area with a wide variety of habitats at a local level.
Material and methods Area description Martin García Island lies in the Northwestern part of Río de La Plata, 40 km away from Buenos Aires and 3.5 km away from Uruguay, at 34°11 09 S and 58°15 09 W. It is bounded by the Infierno Channel to the East, the East Channel to the North and Buenos Aires Channel to the South. The island is mainly an outcrop of crystalline basement corresponding to the Brazilian Shield (precambrian formation of igneous rocks) virtually covered by pleistocene and holocene deposits (Dalla Salda 1981; Ravizza 1984; González & Ravizza 1987). It has a surface area of 168 ha, and its plateau lies about 27 m above sea level, with beaches of precambrian outcropping rocks to the South and clayish sedimentary beaches to the North, East and West. 352
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The climate is mild and humid, with an average annual mean temperature of 17.6°C, an annual precipitation of 980 mm and a high level of humidity reaching up to 81% in June and July. Prevailing winds blow from the North, Northeast, East and Southeast. Frost is very unusual and short, probably due to the influence of the warm waters of the Uruguay River. The island lies on an unique geographical location where species coming from three different phytogeographical provinces meet: Paranense (gallery rainforests that penetrate the area through the rivers Uruguay and Paraná), Pampeana (plants from the “Pampas” steppe grassland) and Espinal (xerophytic forest species of the inland areas of the country) (Cabrera 1976; Soriano 1992). Being a reservation the population of Martin García island is limited to the people required to look after the power plant, to cover basic needs (grocery, bakery, school) and services for tourism (restaurant, inn, launches) adding up to a number of 181. There are launches coming and going three times a week and a special launch with supplies once a week. The island also has a runway that crosses its land from North to South. Due to an exceptional biogeographical diversity, the island presents special features that have been studied by Lahitte &
Hurrell (1994) defining the following ecological units (Fig. 1): • Gallery forests: locally known as “monte blanco”. They grow on the shore, presenting more than one stratum of trees (Myrtaceae, Lauraceae, Euphorbiaceae and Myrsinaceae), shrub, shade herb and moss strata and a great variety of epiphytes, climbing plants and vines. • Riverside woods: mesomorphic woods that grow in the South of the island. They are more open and have less vines. • Peri-urban woods: mesomorphic woods that grow in the areas surrounding the plateau and the urban settlement, mainly the ravines. Both native and exotic species grow in these woods that have vines and climbing plants (though less in number than the gallery forests). • Xerophytic woods: woods with a tree stratum of xerophytic trees of low hight, scrub vegetation and a stratum of sand herbs. • Sand dunes: scattered on a sandy soil we find a thin shrub stratum and a diverse grass stratum. There are also temporary ponds lying at different spots. • Riverside thicket & grassland: grasses and broad-leaved herbs grow on rocky outcrops on the southern shore of the island with patches of thicket scattered about. • Wetlands I “Pajonales”: inundated areas colonized by various reed-like species of the families Alismataceae, Cyperaceae and Poaceae. • Wetlands II “Juncales”: inundated areas colonised by Schoenoplectus californicus of the Cyperaceae family.The wetlands units stretch to the north linking Martin García Island with the Uruguayian Timoteo Dominguez Islet. An artificial boundary was set to separate both pieces of land. • Transitional areas: vegetation types with special features due to the fact that they originate from a mixture of the surrounding units. • Urban area: this area of permanent settlements of inhabitants lies mainly on the plateau. It has prairies surrounding it where exotic and native grass species are found.
Methods Several floristic surveys were carried out at different times of the year (so as to encompass all seasons). Plants were collected and the unit where they were found was noted together with any further observation worth mentioning (e.g. a grass found in the shade in an open area like the dunes). Canopy cover of mesophanerophytes and nanophanerophytes were estimated in the different ecological units. The latter were redefined on the basis of the parameters considered (sunlight exposure and humidity). Thus riverside woods and periurban woods were considered one unit (mesomorphic woods) since they have a similar grass composition and similar features regarding coverage and humidity. The northern transitional area and the ponds were included as new ecological units since they show special features. The northern transitional area occupies the land bounding the runway to the North abutting on gallery forests to the East and West (shown as a part of the xerophytic woods in the map). Though it is a transitional area with a high degree of alteration
due to human activity (depicted in the number of exotic species), it is a forest with an uniform pattern of sunlight exposure (70% coverage) and humidity (high towards the north since it lies by the coast), with some dry and more irradiated patches with xerophytic vegetation to the South, on the border of the runway. The ponds were also considered as a separate unit because of the humidity difference they have with the areas they are found in, that is, the dunes. The wetlands II (“Juncales”) were not included in the present study because they do not present grass vegetation. Floristic inventories were drawn up following the classical methods for plant determination. The material collected was then processed, classified and deposited in the Vascular Division sector of the Museo de La Plata and the Instituto de Botánica Darwinion in Buenos Aires, Argentina. The taxa were also classified according to their affiliation to the various tribes and subfamilies of the Poaceae (Nicora & Rúgolo de Agrasar 1987) to find the best represented tribes. The current study was complemented with a thorough revision of the material preserved in the following herbariums: BA, BAA, BAB, BAF, LP, SI (Holmgren et al., 1990). Herbarium data and the pertinent literature were reviewed for further information on the species distribution on the island (Hauman 1925; Parodi 1925a, b; Martínez Crovetto 1942; Cabrera 1953, 1970; Parodi 1944, 1953, 1956, 1958, 1969; Nicora 1968, Rosengurtt & Izaguirre de Artucio 1968; Burkart 1969; Caro & Sánchez 1969; Cabrera & Zardini 1978; Zuloaga 1979, 1981, 1989; Zuloaga et al. 1994, 1998; Torres 1993; Bianchin 1995, Rúgolo de Agrasar et al.1996). The species herbarized were classified according to their photosynthetic pathway based on literature informations (Sánchez 1968; Ellis 1977; Dengler et al. 1985; Pendergast 1986, 1989; Pendergast & Hattersley 1987; Klink & Joly 1989; Brown 1997) and own data (Rúgolo de Agrasar et al. 1991). The distribution on the island of taxa with the different photosynthetic types was compared with sunlight exposure and humidity parameters of the respective habitats (standard meteorological data). Other climatic data such as temperature and rainfall on the various sites were not considered since they do not much vary from site to site.
Results The grass flora of Martin García Island comprises 118 taxa of which 8 genera, 34 species and 1 variety are cited for the island for the first time (Table 1). The inventory shows 57 taxa with the C3 photosynthetic pathway and 61 of the C4 type. C3 species are distributed among 24 genera belonging to 9 tribes (Arundineae, Aveneae, Bambuseae, Bromeae, Meliceae, Oryzeae, Poeae, Paniceae, Stipeae, Triticeae) and 4 subfamilies (Arundinoideae, Bambusoideae, Panicoideae, Pooideae). C4 species are found in 25 genera belonging to 5 tribes (Andropogoneae, Aristideae, Cynodonteae, Eragrostideae, Paniceae) and 3 subfamilies (Arundinoideae, Chloridoideae, Panicoideae). The tribe Paniceae encompasses the largest number of genera (15) and species (45) and FLORA (2002) 197
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is the only one including both photosynthetic types (31 C4 / 14 C3). The patterns of distribution of the C3 or C4 types (tab. 1) within the different ecological units are shown in Fig. 2. The species Aristida murina (C4), Melica hyalina (C3), Piptochaetium bicolor var. minor (C3) and Setaria verticillata (C4) are not included in the graph and table since the material found at the herbaria belonged to old collections which do not mention the ecological unit where they were collected. The most obvious contrast in occurrence of the two
photosynthetic types was found in the Wetland I with eight C4 and only three C3 species. Gallery forests also show a marked preponderance of one photosynthetic type, but shifted into the other direction: 17 taxa belong to the C3 type (85%) and three are C4 taxa (15%). Woods follow a similar pattern with eight C 3 species (73%) and only three C4 taxa (27%). Xerophytic woods and dunes show a higher number of C4 types, with eleven C4 species (65%) and six C3 species (35%) in the first case and 15 species having the C4 syndrome (62%) and nine species of the C3 photo-
Table 1. Distribution of Poaceae taxa on Martin García Island, Rio de la Plata (º Taxons cited for the first time; * exotic species). Gallery forests C3 Briza rufa (J. Presl) Steud. Briza subaristata Lam. ºBromus brachyanthera Döll var. brachyanthera (Arechav.) J. A. Cámara Bromus brachyanthera Döll var. uruguayensis (Arechav.) J. A. Cámara Bromus hordeaceus L.* ºElymus breviaristatus (Hitchc.) Á. Löve subsp. scabrifolius (Döll) Á. Löve ºIchnanthus pallens (Sw.) Munro ex Benth. Ichnanthus tenuis (J. Presl) Hitchc. & Chase Melica sarmentosa Nees ºNassella megapotamica (Spreng. ex
Trin.) Barkworth Oplismenus hirtellus (L.) P. Beauv. subsp. setarius (Lam.) Mez ex Ekman ºPanicum cordovense E. Fourn. Panicum laxum Sw. ºPanicum missionum Ekman Panicum sabulorum Lam. var. polycladum (E. Ekman) R. A. Palacios Pseudechinolaena polystachya (H. B. K.) O. Stapf Steinchisma decipiens (Nees ex Trin.) W. V. Br. C4 Ischaemum minus J. Presl Panicum prionitis Nees Setaria parviflora (Poir.) Kerguélen
Mesomorphic woods C3 Chaetotropis elongata (H. B. K.) Björkman var. elongata Lolium multiflorum Lam. Melica sarmentosa Nees Oplismenus hirtellus (L.) P. Beauv. subsp. setarius (Lam.) Mez ex Ekman ºPanicum missionum Ekman Panicum sabulorum Lam. var. sabulorum
Pseudechinolaena polystachya (H. B. K.) O. Stapf Steinchisma decipiens (Nees ex Trin.) W. V. Br. C4 Eleusine tristachya (Lam.) Lam. Eragrostis cilianensis (All.) Vignolo Lutati ex Janch.* ºPanicum bergii Arechav.
Xerophytic woods C3 Cortaderia selloana (Schult. & Schult. f.) Asch. & Graebn. Deyeuxia viridiflavescens (Poir.) Kunth var. montevidensis (Nees) Cabrera & Rúgolo Panicum laxum Sw. Poa bonariensis (Lam.) Kunth Steinchisma hians (Elliott) Nash Stipa papposa Nees
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C4 ºAxonopus fissifolius (Raddi) Kuhlm ºBothriochloa laguroides (DC.) Herter Eragrostis bahiensis Schrad. ex Schult. Eragrostis lugens Nees ºEustachys retusa (Lag.) Kunth Muhlenbergia schreberi Gmelin ºPaspalum nicorae Parodi Schizachyrium condensatum (H. B. K.) Nees ºSetaria fiebrigii Hermann Setaria parviflora (Poir.) Kerguélen Sporobolus indicus (L.) R. Br.
Table 1. (continued) Dunes C3 Briza subaristata Lam. Cortaderia selloana (Schult. & Schult. f.) Asch. & Graebn. Lolium multiflorum Lam. Piptochaetium montevidense (Spreng.) Parodi ºPiptochaetium panicoides (Lam.) E. Desv. Poa lanuginosa Poir. ºStipa neesiana Trin. & Rupr. var. longiaristata Arechav. Vulpia australis (Nees ex Steud.) Blom Vulpia myuros (L.) Gmel. f. megalura (Nutt.) Stace & Cotton
C4 Cenchrus pauciflorus Benth. ºCynodon affinis Caro & E.A. Sánchez Cynodon hirsutissimus (Litard. & Maire) Caro & E. A. Sánchez * ºDigitaria ciliaris (Retz.) Koeler ºDigitaria swalleniana Henrard Eragrostis bahiensis Schrad. ex Schult. Eragrostis lugens Nees Eragrostis neesii Trin. Eustachys bahiensis (Steud.) Herter var. bahiensis ºEustachys distichophylla (Lag.) Nees ºEustachys retusa (Lag.) Kunth ºPanicum bergii Arechav. Panicum racemosum (P. Beauv.) Spreng Setaria parviflora (Poir.) Kerguélen Sporobolus indicus (L.) R. Br.
Ponds C3 Leersia hexandra Sw. Luziola peruviana Gmelin Oplismenopsis najada (Hack. & Arechav.) Parodi Steinchisma decipiens (Nees ex Trin.) W. V. Br.
C4 Echinochloa helodes (Hack.) Parodi Echinochloa polystachya (H. B. K.) Hitchc. var. spectabilis (Nees) Mart. Crov. Paspalum repens Berg. Paspalum urvillei Steud.
Riverside grassland & thicket C3 Briza rufa (J. Presl) Steud. Chaetotropis elongata (H. B. K.) Björkman var. elongata ºChaetotropis elongata (H. B. K.) Björkman var. longearistata Nicora Leersia hexandra Sw. Luziola peruviana Gmelin Nassella hyalina (Nees) Barkworth Panicum sabulorum Lam. var. polycladum (E. Ekman) R. A. Palacios Steinchisma decipiens (Nees ex Trin.) W. V. Br.
C4 Axonopus compressus (Sw.) P. Beauv. Digitaria aequiglumis (Hack. & Arechav.) Parodi Echinochloa polystachya (H.B.K.) Hitchc. var. spectabilis (Nees) Mart. Crov. Ischaemum minus J. Presl Panicum gouinii E. Fourn. Paspalum minus E. Fourn. Paspalum notatum Flüggé var. latiflorum Döll Pennisetum latifolium Spreng. Stenotaphrum secundatum (Walter) Kuntze
Wetlands I C3 Panicum grumosum Nees Panicum pernambucense (Spreng.) Mez ex Pilg. Steinchisma decipiens (Nees ex Trin.) W. V. Br. C4 Echinochloa helodes (Hack.) Parodi ºEragrostis hypnoides (Lam.) Britton,
Stern & Pogg Hemarthria altissima (Poir.) Stapf & C. E. Hubb. Panicum elephantipes Nees ex Trin. ºPaspalum haumanii Parodi Paspalum repens Berg. Paspalum urvillei Steud. Paspalum vaginatum Sw. FLORA (2002) 197
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Table 1. (continued) North Transitional Area C3 Bromus catharticus Vahl Deyeuxia viridiflavescens (Poir.) Kunth var. montevidensis (Nees) Cabrera & Rúgolo Nassella hyalina (Nees) Barkworth Panicum pernambucense (Spreng.) Mez ex Pilg. Piptochaetium montevidense (Spreng.) Parodi Poa annua L.*
Rostraria cristata (L.) Tavelev ºStipa neesiana Trin. & Rupr. var. longiaristata Arechav. Stipa papposa Nees C4 ºBothriochloa laguroides (DC.) Herter Eriochloa montevidensis Griseb. Paspalum notatum Flüggé var. latiflorum Döll Paspalum urvillei Steud. Setaria rosengurtii Nicora
Transitional Areas C3 Briza minor L.* Briza subaristata Lam. Deyeuxia viridiflavescens (Poir.) Kunth var. montevidensis (Nees) Cabrera & Rúgolo Hordeum murinum L. subsp. leporinum (Link) Arcang. * Glyceria multiflora Steud Lolium multiflorum Lam. Melica argyrea Hack. Melica brasiliana Ard. ºNassella formicarium (Delile) Barkworth Panicum laxum Sw. Phalaris angusta Nees ex Trin. ºPolypogon monspeliensis (L.) Desf. * ºPolypogon viridis (Gouan) Breistr. * Stipa brachychaeta Godr. Stipa neesiana Trin. & Rupr. var. neesiana Setaria parviflora (Poir.) Kerguélen Setaria vaginata Spreng. ºSorghum halepense (L.) Pers. var. halepense*
Vulpia myuros (L.) Gmel. f. megalura (Nutt.) Stace & Cotton C4 ºAndropogon ternatus (Spreng.) Nees Axonopus compressus (Sw.) P. Beauv. ºBothriochloa laguroides (DC.) Herter Bouteloua megapotamica (Spreng.) Kuntze Chloris canterae Arechav. var. grandiflora (Roseng. & Izag.) Anderson Chloris ciliata Sw. Eleusine tristachya (Lam.) Lam. Eragrostis lugens Nees ºEriochloa punctata (L.) Desv. ex Hamilton Eustachys bahiensis (Steud.) Herter var. bahiensis ºEustachys distichophylla (Lag.) Nees ºPanicum bergii Arechav. Paspalum dilatatum Poir. Paspalum lividum Trin. ex Schltdl. Sporobolus indicus (L.) R. Br. Sporobolus pyramidatus (Lam.) Hitchc. Stenotaphrum secundatum (Walter) Kuntze
Urban Area C3 ºArundo donax L.* Briza minor L.* Briza subaristata Lam Bromus catharticus Vahl Bromus hordeaceus L.* Chaetotropis elongata (H. B. K.) Björkman var. elongata Deyeuxia viridiflavescens (Poir.) Kunth var. montevidensis (Nees) Cabrera & Rúgolo ºHordeum euclaston Steud. Lolium multiflorum Lam. ºPhyllostachys bambusoides Siebold & Zuccarini* ºPiptochaetium stipoides (Trin. & Rupr.) Hack. Poa annua L.* ºPolypogon monspeliensis (L.) Desf. * Rostraria cristata (L.) Tavelev Stipa brachychaeta Godr. Stipa papposa Nees
Vulpia bromoides (L.) S. F. Gray * C4 ºBothriochloa laguroides (DC.) Herter Bouteloua megapotamica (Spreng.) Kuntze Cynodon dactylon (L.) Pers. * Cynodon hirsutissimus (Litard. & Maire) Caro & E. A. Sánchez * Echinochloa polystachya (H. B. K.) Hitchc. var. spectabilis (Nees) Mart. Crov. Eragrostis lugens Nees Eriochloa montevidensis Griseb. ºEustachys retusa (Lag.) Kunth Muhlenbergia schreberi Gmelin Paspalum dilatatum Poir. ºSetaria fiebrigii Hermann Setaria parviflora (Poir.) Kerguélen Setaria rosengurtii Nicora ºSorghum halepense (L.) Pers. var. muticum (Hack.) Hayek * Stenotaphrum secundatum (Walter) Kuntze
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Fig. 2. Distribution of C3 / C4 photosynthetic types of grasses in the vegetation units of Martin García Island – Abbreviations: GF: gallery forests, MW: mesomorphic woods, XW: xerophytic woods, SD: sand dunes, PO: ponds, RGT: riverside grassland & thickets, WL : wetland I (“Pajonales“), NTA: northern transitional area, TA: transitional areas, UA: urban area.
synthetic pathway (38%) in the dunes. Similar numbers are found in the north transitional area, but this time with a C3 dominance (64%). The other transitional areas show a higher number of C4 taxa (20) than C3 species (16), and the urban area has a higher number of C3 (17) as compared to C4 taxa (15), but there are no marked differences. Likewise the riverside thicket & grassland and the ponds show almost equal numbers of species belonging to the two photosynthetic types: 9 C4 / 8 C3 and 4 C4 / 4 C3, respectively.
Discussion Although the Poaceae are typically found in the prairies, they are well represented also in gallery forest communities, woodlands and swamps. In the Rio de la Plata gallery forests we find more broad-leaved, shady hydrophyte taxa among which we may mention in particular Pseudoechinolaena polystachya, Oplismenus hirtellus subsp. setarius, Panicum decipiens, Ichnanthus pallens and I. tenuis. In the lower but more open lands we observe a higher number of hydrophytes and tender
grasses with small-linear leaves. The same applies to the xerophytic communities with Eragrostis bahiensis and E. lugens, Eustachys retusa, Cenchrus pauciflorus, Stipa papposa and Deyeuxia viridiflavescens var. montevidensis as the prevailing taxa in the xerophytic forests, and Poa lanuginosa, Eragrostis bahiensis, E. lugens and E. neesi, Panicum racemosum, Cenchrus pauciflorus, Digitaria swalleniana, Eustachys retusa and Sporobolus indicus typically found in the dunes where irradiation is high and water availability low, all taxa characterized by more linear and harder leaf blades. As we move closer to the central plateau (urban area) exotic species can be found in higher numbers, probably due to human activity (Table 1). The relations between climatic conditions and photosynthetic types have been usually studied at a broad geographical level. Klink & Joly (1986) state, however, that studying these patterns at this level of approach is not enough since there are local variables such as sun and shade that greatly can influence vegetation composition. Martin García Island is very suitable for a small scale analysis since it presents different ecological units with local variations in sunlight exposure and humidity in a very small surface area. FLORA (2002) 197
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The parameters taken into account in this study were sun / shade exposure and humidity of the various vegetation units. There are areas which are totally exposed to sunlight (dunes, wetlands), units with varying degrees of shade (woods), and very shaded areas (gallery forests). Likewise there are very humid areas (gallery forests), less humid sites (mesomorphic woods) and dry ones (dunes and xerophytic woods). Other variables such as altitude, temperature and rainfall do not influence differently the vegetation cover because they do not differ within the small area of the whole island. The observed patterns of grass species distribution on Martin García Island are consistent with previous studies at much larger geographical scales that link C3 plants to humid and shaded areas and C4 ones to dryer zones with a high sunlight exposure. Gallery forests, humid habitats with 95% coverage by plants, show a distinct C3 dominance. Woods follow this pattern with a slightly less extreme difference between the two photosynthetically separated types of grasses. This vegetation type has more open areas and trails and is consequently less humid (Fig. 2, Table 1). In the xerophytic woods we observe a C4 dominance. This is consistent with the expectations as far as the humidity parameter is concerned (arid zones with a sandy soil and xerophytic vegetation). On the first glimpse, there might be an inconsistency with respect to the sunlight exposure in woodlands. However, these are open woods with scattered trees and an average coverage of 65%, leaving many open spots where grasses grow, more or less fully exposed to the sun. The high number of C4 species in the dunes is easily explained by the good adaptability of these species to a high sunlight exposure and a dry/arid habitat. (Fig. 2, Table 1). In the north transitional area, which has a canopy coverage of 70% and is quite humid, we find a high number of C3 species. If these figures and those of the gallery forests are compared the difference between the amount of C3 and of C4 grasses is less drastic. This may be accounted for by a more open wood there with sunlight and drier patches towards the South (where the runway strip is found). The other transitional areas show no differences in photosynthetic type numbers. This may be due to the fact that these areas do not have a specific vegetation pattern and its sunlight/shade and humidity conditions may vary from one unit to the other (Fig. 2, Table 1). The analysis of the relation between the variables “sun exposure” and “humidity” and the corresponding relations of grasses with different photosynthetic pathways allows some interesting conclusions. As stated already by Rundel (1980), the relative dominance of these two photosynthetic systems along environmental gradients results from a complex interaction of several climatic factors. In areas where both parameters favour 358
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one photosynthetic type (e.g. high ambient humidity combined with shade, like in gallery forests), we observe a marked distribution difference. In areas such as the riverside thicket & grassland and the ponds with a considerable sun exposure (favourable for the development of C4) and high humidity (favourable for the development of C3) the numbers of grass taxa following the one or the other photosynthetic type are similar, in the mentioned examples 9 C4 / 8 C3 and 4 C4 / 4 C3 species, respectively (Fig. 2, Table 1). The wetland I “pajonal” (100% sunlight exposure and highly humid) does not follow this pattern, presenting a strong C4 dominance (8 C4 / 3 C3) (Fig. 2, Table1). This contradiction to the expectation of a balanced C3/C4 relationship may be solved by the fact that the riverside grasslands and the ponds have nevertheless some shaded patches. These result from thin thicket intermingled in the typical pajonal vegetation. Few shrubs are growing also in the dunes surrounding the ponds. The conclusion would be too far reaching, however, that sun exposure generally has a stronger influence than humidity on the distribution pattern of photosynthetic types. Sage (1999) states that although C4 photosynthesis may enable survival in sites too harsh for C3 vegetation (e.g. very xeric locations), abiotic stress such as drought is not a prerequisite for C4 dominance. In virtually any warm terrestrial habitat, including wetlands, C4 taxa can be superior due to their higher temperature optima for growth, provided that ecological disturbance minimizes competition from C3 wood vegetation. Since the overall results are consistent with comparable supra-regional research work, the current study shows that a focus on sunlight exposure and humidity variations at a local level proves to be a useful methodology to be applied also in the analysis of the distribution of Poaceae photosynthetic types in smaller areas. From the knowledge about sun exposure and aridity of a patchy vegetation a raw estimate of the grass diversity should be possible. On the other hand, already a floristic survey of grass-dominated vegetation types, supplemented by informations about the genus- or speciesspecific photosynthetic pathways can render bioclimatic informations for the particular habitats.
Acknowledgements The authors are grateful to Dr. Silvia Burkart for her revision of the manuscript and her insightful suggestions. The assistance of F. O. Zuloaga, M. Molina, M. Arriaga and J. F. Pensiero in the classification of the genera Panicum, Eustachys, Eriochloa and Setaria is gratefully acknowledged; thanks are expressed also to the directors and curators of the herbariums mentioned. Finally, the authors thank Elisa Nicora, for her constant support, the identification of some species and her valuable company in one of the field trips.
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Analysis of the patterns of distribution of photosynthetic pathways and representativity of the family Poaceae on Martin García Island, Río de La Plata, Buenos Aires, Argentina Mariana Cecilia Bianchin Belmonte* & Zulma E. Rúgolo de Agrasar Instituto de Botánica Darwinion, Labardén 200, (1642) San Isidro, Buenos Aires, República Argentina * e-mail corresponding author: [email protected] Received: Oct 17, 2001, in revised form: Feb 26, 2002 · Accepted: May 25, 2002
Summary Martin García Island lies on an unique geographical location in South America where species from three different phytogeographical provinces meet. Although its surface area is very small (168 hectares), it has a wide variety of vegetation types ranging from gallery forests to xerophytic woods. The floristic inventory of this island was updated for the family Poaceae by collecting and identifying material with an outcome of 118 taxa, 32 of which are cited for the first time for the area. The plants collected were classified according to their tribe, subfamily, genus, species and photosynthetic pathway. The distribution of the photosynthetic types C3/C4 in the various ecological units of Martin García Island was discussed on the basis of sun/shade and humidity parameters. The results show that C3 taxa prevail in humid and shady habitats and C4 taxa in dry and sunny areas. Key words: South America, Poaceae, C3/C4
Introduction The flora of a certain area can characterize in detail its ecological situation and relevant knowledge forms the basis for an analysis of the vegetation dynamics that is necessary also for its management and conservation. Grasses (Poaceae) constitute in many vegetation units a considerable amount of all plants and dominate the landscape aspect in most semiarid to semihumid regions of the world. Grasslands are widespread also in Argentina, by nature and even more under human influence, and a high diversity can be found within the members of this family (Parodi 1969; Cabrera 1979, 1976; Hueck & Seibert 1981). A closer taxonomic information allows further research studies regarding the phylogenetic links of the encountered plant taxa and their particular anatomical structures. An important contribution in this perspective is the study of the photosynthetic pathways in relation to their distribution in different environments. In the grass family as a whole (and also in few other Angiosperm families) the genera differ in the ways of carbon dioxide fixation. Depending on whether the initial product is the 0367-2530/02/197/05-351 $ 15.00/0
three carbon molecule phosphoglyceric acid as in the Calvin Benson Cycle they belong to the C3 type or to the C4 plants. In the latter case the primary CO2 fixation results into the four-carbon molecule oxaloacetic acid which is subsequently transformed to malic and/or aspartic acid, following the Hatch-Slack cycle (Werger & Ellis 1981). The various photosynthetic pathways seem to have their advantages under different environmental conditions (Werger & Ellis 1981). The relation between the distribution of C3 and C4 plants and climatic conditions (temperature, humidity, altitude and rainfall) have been studied by various authors: Stowe & Teeri (1978) in North America, Chazdon (1978) in Costa Rica, Meinzer (1978) in Central America, Klink & Joly (1986) in Brasil, Vogel et al. (1978) and Werger & Ellis (1981) in South Africa, Rundel (1980) in Hawaii, and Hattersley (1983) in Australia. Generally, these studies have been carried out at a broader geographical level. However, Klink & Joly (1986) state that the effect of local variables (mainly sunlight/shade) upon the distribution of C3 and C4 grasses should also be considered, since they provide a new FLORA (2002) 197
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N
References Gallery Forests Xerophytic Woods Periurban Woods Riverside Woods Sand Dunes Transitional Areas Riverside Thicket & Grassland Wetlands I Urban Area
Wetlands II Urban Area
River Plate Pier
River Plate
0m
200 m 300 m
Fig. 1. Martin García Island, Rio de la Plata – Ecological Units.
approach to the study of the distribution of C3 and C4 plants. Our aim is to study the distribution of C3 and C4 grasses in a very small area with a wide variety of habitats at a local level.
Material and methods Area description Martin García Island lies in the Northwestern part of Río de La Plata, 40 km away from Buenos Aires and 3.5 km away from Uruguay, at 34°11 09 S and 58°15 09 W. It is bounded by the Infierno Channel to the East, the East Channel to the North and Buenos Aires Channel to the South. The island is mainly an outcrop of crystalline basement corresponding to the Brazilian Shield (precambrian formation of igneous rocks) virtually covered by pleistocene and holocene deposits (Dalla Salda 1981; Ravizza 1984; González & Ravizza 1987). It has a surface area of 168 ha, and its plateau lies about 27 m above sea level, with beaches of precambrian outcropping rocks to the South and clayish sedimentary beaches to the North, East and West. 352
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The climate is mild and humid, with an average annual mean temperature of 17.6°C, an annual precipitation of 980 mm and a high level of humidity reaching up to 81% in June and July. Prevailing winds blow from the North, Northeast, East and Southeast. Frost is very unusual and short, probably due to the influence of the warm waters of the Uruguay River. The island lies on an unique geographical location where species coming from three different phytogeographical provinces meet: Paranense (gallery rainforests that penetrate the area through the rivers Uruguay and Paraná), Pampeana (plants from the “Pampas” steppe grassland) and Espinal (xerophytic forest species of the inland areas of the country) (Cabrera 1976; Soriano 1992). Being a reservation the population of Martin García island is limited to the people required to look after the power plant, to cover basic needs (grocery, bakery, school) and services for tourism (restaurant, inn, launches) adding up to a number of 181. There are launches coming and going three times a week and a special launch with supplies once a week. The island also has a runway that crosses its land from North to South. Due to an exceptional biogeographical diversity, the island presents special features that have been studied by Lahitte &
Hurrell (1994) defining the following ecological units (Fig. 1): • Gallery forests: locally known as “monte blanco”. They grow on the shore, presenting more than one stratum of trees (Myrtaceae, Lauraceae, Euphorbiaceae and Myrsinaceae), shrub, shade herb and moss strata and a great variety of epiphytes, climbing plants and vines. • Riverside woods: mesomorphic woods that grow in the South of the island. They are more open and have less vines. • Peri-urban woods: mesomorphic woods that grow in the areas surrounding the plateau and the urban settlement, mainly the ravines. Both native and exotic species grow in these woods that have vines and climbing plants (though less in number than the gallery forests). • Xerophytic woods: woods with a tree stratum of xerophytic trees of low hight, scrub vegetation and a stratum of sand herbs. • Sand dunes: scattered on a sandy soil we find a thin shrub stratum and a diverse grass stratum. There are also temporary ponds lying at different spots. • Riverside thicket & grassland: grasses and broad-leaved herbs grow on rocky outcrops on the southern shore of the island with patches of thicket scattered about. • Wetlands I “Pajonales”: inundated areas colonized by various reed-like species of the families Alismataceae, Cyperaceae and Poaceae. • Wetlands II “Juncales”: inundated areas colonised by Schoenoplectus californicus of the Cyperaceae family.The wetlands units stretch to the north linking Martin García Island with the Uruguayian Timoteo Dominguez Islet. An artificial boundary was set to separate both pieces of land. • Transitional areas: vegetation types with special features due to the fact that they originate from a mixture of the surrounding units. • Urban area: this area of permanent settlements of inhabitants lies mainly on the plateau. It has prairies surrounding it where exotic and native grass species are found.
Methods Several floristic surveys were carried out at different times of the year (so as to encompass all seasons). Plants were collected and the unit where they were found was noted together with any further observation worth mentioning (e.g. a grass found in the shade in an open area like the dunes). Canopy cover of mesophanerophytes and nanophanerophytes were estimated in the different ecological units. The latter were redefined on the basis of the parameters considered (sunlight exposure and humidity). Thus riverside woods and periurban woods were considered one unit (mesomorphic woods) since they have a similar grass composition and similar features regarding coverage and humidity. The northern transitional area and the ponds were included as new ecological units since they show special features. The northern transitional area occupies the land bounding the runway to the North abutting on gallery forests to the East and West (shown as a part of the xerophytic woods in the map). Though it is a transitional area with a high degree of alteration
due to human activity (depicted in the number of exotic species), it is a forest with an uniform pattern of sunlight exposure (70% coverage) and humidity (high towards the north since it lies by the coast), with some dry and more irradiated patches with xerophytic vegetation to the South, on the border of the runway. The ponds were also considered as a separate unit because of the humidity difference they have with the areas they are found in, that is, the dunes. The wetlands II (“Juncales”) were not included in the present study because they do not present grass vegetation. Floristic inventories were drawn up following the classical methods for plant determination. The material collected was then processed, classified and deposited in the Vascular Division sector of the Museo de La Plata and the Instituto de Botánica Darwinion in Buenos Aires, Argentina. The taxa were also classified according to their affiliation to the various tribes and subfamilies of the Poaceae (Nicora & Rúgolo de Agrasar 1987) to find the best represented tribes. The current study was complemented with a thorough revision of the material preserved in the following herbariums: BA, BAA, BAB, BAF, LP, SI (Holmgren et al., 1990). Herbarium data and the pertinent literature were reviewed for further information on the species distribution on the island (Hauman 1925; Parodi 1925a, b; Martínez Crovetto 1942; Cabrera 1953, 1970; Parodi 1944, 1953, 1956, 1958, 1969; Nicora 1968, Rosengurtt & Izaguirre de Artucio 1968; Burkart 1969; Caro & Sánchez 1969; Cabrera & Zardini 1978; Zuloaga 1979, 1981, 1989; Zuloaga et al. 1994, 1998; Torres 1993; Bianchin 1995, Rúgolo de Agrasar et al.1996). The species herbarized were classified according to their photosynthetic pathway based on literature informations (Sánchez 1968; Ellis 1977; Dengler et al. 1985; Pendergast 1986, 1989; Pendergast & Hattersley 1987; Klink & Joly 1989; Brown 1997) and own data (Rúgolo de Agrasar et al. 1991). The distribution on the island of taxa with the different photosynthetic types was compared with sunlight exposure and humidity parameters of the respective habitats (standard meteorological data). Other climatic data such as temperature and rainfall on the various sites were not considered since they do not much vary from site to site.
Results The grass flora of Martin García Island comprises 118 taxa of which 8 genera, 34 species and 1 variety are cited for the island for the first time (Table 1). The inventory shows 57 taxa with the C3 photosynthetic pathway and 61 of the C4 type. C3 species are distributed among 24 genera belonging to 9 tribes (Arundineae, Aveneae, Bambuseae, Bromeae, Meliceae, Oryzeae, Poeae, Paniceae, Stipeae, Triticeae) and 4 subfamilies (Arundinoideae, Bambusoideae, Panicoideae, Pooideae). C4 species are found in 25 genera belonging to 5 tribes (Andropogoneae, Aristideae, Cynodonteae, Eragrostideae, Paniceae) and 3 subfamilies (Arundinoideae, Chloridoideae, Panicoideae). The tribe Paniceae encompasses the largest number of genera (15) and species (45) and FLORA (2002) 197
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is the only one including both photosynthetic types (31 C4 / 14 C3). The patterns of distribution of the C3 or C4 types (tab. 1) within the different ecological units are shown in Fig. 2. The species Aristida murina (C4), Melica hyalina (C3), Piptochaetium bicolor var. minor (C3) and Setaria verticillata (C4) are not included in the graph and table since the material found at the herbaria belonged to old collections which do not mention the ecological unit where they were collected. The most obvious contrast in occurrence of the two
photosynthetic types was found in the Wetland I with eight C4 and only three C3 species. Gallery forests also show a marked preponderance of one photosynthetic type, but shifted into the other direction: 17 taxa belong to the C3 type (85%) and three are C4 taxa (15%). Woods follow a similar pattern with eight C 3 species (73%) and only three C4 taxa (27%). Xerophytic woods and dunes show a higher number of C4 types, with eleven C4 species (65%) and six C3 species (35%) in the first case and 15 species having the C4 syndrome (62%) and nine species of the C3 photo-
Table 1. Distribution of Poaceae taxa on Martin García Island, Rio de la Plata (º Taxons cited for the first time; * exotic species). Gallery forests C3 Briza rufa (J. Presl) Steud. Briza subaristata Lam. ºBromus brachyanthera Döll var. brachyanthera (Arechav.) J. A. Cámara Bromus brachyanthera Döll var. uruguayensis (Arechav.) J. A. Cámara Bromus hordeaceus L.* ºElymus breviaristatus (Hitchc.) Á. Löve subsp. scabrifolius (Döll) Á. Löve ºIchnanthus pallens (Sw.) Munro ex Benth. Ichnanthus tenuis (J. Presl) Hitchc. & Chase Melica sarmentosa Nees ºNassella megapotamica (Spreng. ex
Trin.) Barkworth Oplismenus hirtellus (L.) P. Beauv. subsp. setarius (Lam.) Mez ex Ekman ºPanicum cordovense E. Fourn. Panicum laxum Sw. ºPanicum missionum Ekman Panicum sabulorum Lam. var. polycladum (E. Ekman) R. A. Palacios Pseudechinolaena polystachya (H. B. K.) O. Stapf Steinchisma decipiens (Nees ex Trin.) W. V. Br. C4 Ischaemum minus J. Presl Panicum prionitis Nees Setaria parviflora (Poir.) Kerguélen
Mesomorphic woods C3 Chaetotropis elongata (H. B. K.) Björkman var. elongata Lolium multiflorum Lam. Melica sarmentosa Nees Oplismenus hirtellus (L.) P. Beauv. subsp. setarius (Lam.) Mez ex Ekman ºPanicum missionum Ekman Panicum sabulorum Lam. var. sabulorum
Pseudechinolaena polystachya (H. B. K.) O. Stapf Steinchisma decipiens (Nees ex Trin.) W. V. Br. C4 Eleusine tristachya (Lam.) Lam. Eragrostis cilianensis (All.) Vignolo Lutati ex Janch.* ºPanicum bergii Arechav.
Xerophytic woods C3 Cortaderia selloana (Schult. & Schult. f.) Asch. & Graebn. Deyeuxia viridiflavescens (Poir.) Kunth var. montevidensis (Nees) Cabrera & Rúgolo Panicum laxum Sw. Poa bonariensis (Lam.) Kunth Steinchisma hians (Elliott) Nash Stipa papposa Nees
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C4 ºAxonopus fissifolius (Raddi) Kuhlm ºBothriochloa laguroides (DC.) Herter Eragrostis bahiensis Schrad. ex Schult. Eragrostis lugens Nees ºEustachys retusa (Lag.) Kunth Muhlenbergia schreberi Gmelin ºPaspalum nicorae Parodi Schizachyrium condensatum (H. B. K.) Nees ºSetaria fiebrigii Hermann Setaria parviflora (Poir.) Kerguélen Sporobolus indicus (L.) R. Br.
Table 1. (continued) Dunes C3 Briza subaristata Lam. Cortaderia selloana (Schult. & Schult. f.) Asch. & Graebn. Lolium multiflorum Lam. Piptochaetium montevidense (Spreng.) Parodi ºPiptochaetium panicoides (Lam.) E. Desv. Poa lanuginosa Poir. ºStipa neesiana Trin. & Rupr. var. longiaristata Arechav. Vulpia australis (Nees ex Steud.) Blom Vulpia myuros (L.) Gmel. f. megalura (Nutt.) Stace & Cotton
C4 Cenchrus pauciflorus Benth. ºCynodon affinis Caro & E.A. Sánchez Cynodon hirsutissimus (Litard. & Maire) Caro & E. A. Sánchez * ºDigitaria ciliaris (Retz.) Koeler ºDigitaria swalleniana Henrard Eragrostis bahiensis Schrad. ex Schult. Eragrostis lugens Nees Eragrostis neesii Trin. Eustachys bahiensis (Steud.) Herter var. bahiensis ºEustachys distichophylla (Lag.) Nees ºEustachys retusa (Lag.) Kunth ºPanicum bergii Arechav. Panicum racemosum (P. Beauv.) Spreng Setaria parviflora (Poir.) Kerguélen Sporobolus indicus (L.) R. Br.
Ponds C3 Leersia hexandra Sw. Luziola peruviana Gmelin Oplismenopsis najada (Hack. & Arechav.) Parodi Steinchisma decipiens (Nees ex Trin.) W. V. Br.
C4 Echinochloa helodes (Hack.) Parodi Echinochloa polystachya (H. B. K.) Hitchc. var. spectabilis (Nees) Mart. Crov. Paspalum repens Berg. Paspalum urvillei Steud.
Riverside grassland & thicket C3 Briza rufa (J. Presl) Steud. Chaetotropis elongata (H. B. K.) Björkman var. elongata ºChaetotropis elongata (H. B. K.) Björkman var. longearistata Nicora Leersia hexandra Sw. Luziola peruviana Gmelin Nassella hyalina (Nees) Barkworth Panicum sabulorum Lam. var. polycladum (E. Ekman) R. A. Palacios Steinchisma decipiens (Nees ex Trin.) W. V. Br.
C4 Axonopus compressus (Sw.) P. Beauv. Digitaria aequiglumis (Hack. & Arechav.) Parodi Echinochloa polystachya (H.B.K.) Hitchc. var. spectabilis (Nees) Mart. Crov. Ischaemum minus J. Presl Panicum gouinii E. Fourn. Paspalum minus E. Fourn. Paspalum notatum Flüggé var. latiflorum Döll Pennisetum latifolium Spreng. Stenotaphrum secundatum (Walter) Kuntze
Wetlands I C3 Panicum grumosum Nees Panicum pernambucense (Spreng.) Mez ex Pilg. Steinchisma decipiens (Nees ex Trin.) W. V. Br. C4 Echinochloa helodes (Hack.) Parodi ºEragrostis hypnoides (Lam.) Britton,
Stern & Pogg Hemarthria altissima (Poir.) Stapf & C. E. Hubb. Panicum elephantipes Nees ex Trin. ºPaspalum haumanii Parodi Paspalum repens Berg. Paspalum urvillei Steud. Paspalum vaginatum Sw. FLORA (2002) 197
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Table 1. (continued) North Transitional Area C3 Bromus catharticus Vahl Deyeuxia viridiflavescens (Poir.) Kunth var. montevidensis (Nees) Cabrera & Rúgolo Nassella hyalina (Nees) Barkworth Panicum pernambucense (Spreng.) Mez ex Pilg. Piptochaetium montevidense (Spreng.) Parodi Poa annua L.*
Rostraria cristata (L.) Tavelev ºStipa neesiana Trin. & Rupr. var. longiaristata Arechav. Stipa papposa Nees C4 ºBothriochloa laguroides (DC.) Herter Eriochloa montevidensis Griseb. Paspalum notatum Flüggé var. latiflorum Döll Paspalum urvillei Steud. Setaria rosengurtii Nicora
Transitional Areas C3 Briza minor L.* Briza subaristata Lam. Deyeuxia viridiflavescens (Poir.) Kunth var. montevidensis (Nees) Cabrera & Rúgolo Hordeum murinum L. subsp. leporinum (Link) Arcang. * Glyceria multiflora Steud Lolium multiflorum Lam. Melica argyrea Hack. Melica brasiliana Ard. ºNassella formicarium (Delile) Barkworth Panicum laxum Sw. Phalaris angusta Nees ex Trin. ºPolypogon monspeliensis (L.) Desf. * ºPolypogon viridis (Gouan) Breistr. * Stipa brachychaeta Godr. Stipa neesiana Trin. & Rupr. var. neesiana Setaria parviflora (Poir.) Kerguélen Setaria vaginata Spreng. ºSorghum halepense (L.) Pers. var. halepense*
Vulpia myuros (L.) Gmel. f. megalura (Nutt.) Stace & Cotton C4 ºAndropogon ternatus (Spreng.) Nees Axonopus compressus (Sw.) P. Beauv. ºBothriochloa laguroides (DC.) Herter Bouteloua megapotamica (Spreng.) Kuntze Chloris canterae Arechav. var. grandiflora (Roseng. & Izag.) Anderson Chloris ciliata Sw. Eleusine tristachya (Lam.) Lam. Eragrostis lugens Nees ºEriochloa punctata (L.) Desv. ex Hamilton Eustachys bahiensis (Steud.) Herter var. bahiensis ºEustachys distichophylla (Lag.) Nees ºPanicum bergii Arechav. Paspalum dilatatum Poir. Paspalum lividum Trin. ex Schltdl. Sporobolus indicus (L.) R. Br. Sporobolus pyramidatus (Lam.) Hitchc. Stenotaphrum secundatum (Walter) Kuntze
Urban Area C3 ºArundo donax L.* Briza minor L.* Briza subaristata Lam Bromus catharticus Vahl Bromus hordeaceus L.* Chaetotropis elongata (H. B. K.) Björkman var. elongata Deyeuxia viridiflavescens (Poir.) Kunth var. montevidensis (Nees) Cabrera & Rúgolo ºHordeum euclaston Steud. Lolium multiflorum Lam. ºPhyllostachys bambusoides Siebold & Zuccarini* ºPiptochaetium stipoides (Trin. & Rupr.) Hack. Poa annua L.* ºPolypogon monspeliensis (L.) Desf. * Rostraria cristata (L.) Tavelev Stipa brachychaeta Godr. Stipa papposa Nees
Vulpia bromoides (L.) S. F. Gray * C4 ºBothriochloa laguroides (DC.) Herter Bouteloua megapotamica (Spreng.) Kuntze Cynodon dactylon (L.) Pers. * Cynodon hirsutissimus (Litard. & Maire) Caro & E. A. Sánchez * Echinochloa polystachya (H. B. K.) Hitchc. var. spectabilis (Nees) Mart. Crov. Eragrostis lugens Nees Eriochloa montevidensis Griseb. ºEustachys retusa (Lag.) Kunth Muhlenbergia schreberi Gmelin Paspalum dilatatum Poir. ºSetaria fiebrigii Hermann Setaria parviflora (Poir.) Kerguélen Setaria rosengurtii Nicora ºSorghum halepense (L.) Pers. var. muticum (Hack.) Hayek * Stenotaphrum secundatum (Walter) Kuntze
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Fig. 2. Distribution of C3 / C4 photosynthetic types of grasses in the vegetation units of Martin García Island – Abbreviations: GF: gallery forests, MW: mesomorphic woods, XW: xerophytic woods, SD: sand dunes, PO: ponds, RGT: riverside grassland & thickets, WL : wetland I (“Pajonales“), NTA: northern transitional area, TA: transitional areas, UA: urban area.
synthetic pathway (38%) in the dunes. Similar numbers are found in the north transitional area, but this time with a C3 dominance (64%). The other transitional areas show a higher number of C4 taxa (20) than C3 species (16), and the urban area has a higher number of C3 (17) as compared to C4 taxa (15), but there are no marked differences. Likewise the riverside thicket & grassland and the ponds show almost equal numbers of species belonging to the two photosynthetic types: 9 C4 / 8 C3 and 4 C4 / 4 C3, respectively.
Discussion Although the Poaceae are typically found in the prairies, they are well represented also in gallery forest communities, woodlands and swamps. In the Rio de la Plata gallery forests we find more broad-leaved, shady hydrophyte taxa among which we may mention in particular Pseudoechinolaena polystachya, Oplismenus hirtellus subsp. setarius, Panicum decipiens, Ichnanthus pallens and I. tenuis. In the lower but more open lands we observe a higher number of hydrophytes and tender
grasses with small-linear leaves. The same applies to the xerophytic communities with Eragrostis bahiensis and E. lugens, Eustachys retusa, Cenchrus pauciflorus, Stipa papposa and Deyeuxia viridiflavescens var. montevidensis as the prevailing taxa in the xerophytic forests, and Poa lanuginosa, Eragrostis bahiensis, E. lugens and E. neesi, Panicum racemosum, Cenchrus pauciflorus, Digitaria swalleniana, Eustachys retusa and Sporobolus indicus typically found in the dunes where irradiation is high and water availability low, all taxa characterized by more linear and harder leaf blades. As we move closer to the central plateau (urban area) exotic species can be found in higher numbers, probably due to human activity (Table 1). The relations between climatic conditions and photosynthetic types have been usually studied at a broad geographical level. Klink & Joly (1986) state, however, that studying these patterns at this level of approach is not enough since there are local variables such as sun and shade that greatly can influence vegetation composition. Martin García Island is very suitable for a small scale analysis since it presents different ecological units with local variations in sunlight exposure and humidity in a very small surface area. FLORA (2002) 197
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The parameters taken into account in this study were sun / shade exposure and humidity of the various vegetation units. There are areas which are totally exposed to sunlight (dunes, wetlands), units with varying degrees of shade (woods), and very shaded areas (gallery forests). Likewise there are very humid areas (gallery forests), less humid sites (mesomorphic woods) and dry ones (dunes and xerophytic woods). Other variables such as altitude, temperature and rainfall do not influence differently the vegetation cover because they do not differ within the small area of the whole island. The observed patterns of grass species distribution on Martin García Island are consistent with previous studies at much larger geographical scales that link C3 plants to humid and shaded areas and C4 ones to dryer zones with a high sunlight exposure. Gallery forests, humid habitats with 95% coverage by plants, show a distinct C3 dominance. Woods follow this pattern with a slightly less extreme difference between the two photosynthetically separated types of grasses. This vegetation type has more open areas and trails and is consequently less humid (Fig. 2, Table 1). In the xerophytic woods we observe a C4 dominance. This is consistent with the expectations as far as the humidity parameter is concerned (arid zones with a sandy soil and xerophytic vegetation). On the first glimpse, there might be an inconsistency with respect to the sunlight exposure in woodlands. However, these are open woods with scattered trees and an average coverage of 65%, leaving many open spots where grasses grow, more or less fully exposed to the sun. The high number of C4 species in the dunes is easily explained by the good adaptability of these species to a high sunlight exposure and a dry/arid habitat. (Fig. 2, Table 1). In the north transitional area, which has a canopy coverage of 70% and is quite humid, we find a high number of C3 species. If these figures and those of the gallery forests are compared the difference between the amount of C3 and of C4 grasses is less drastic. This may be accounted for by a more open wood there with sunlight and drier patches towards the South (where the runway strip is found). The other transitional areas show no differences in photosynthetic type numbers. This may be due to the fact that these areas do not have a specific vegetation pattern and its sunlight/shade and humidity conditions may vary from one unit to the other (Fig. 2, Table 1). The analysis of the relation between the variables “sun exposure” and “humidity” and the corresponding relations of grasses with different photosynthetic pathways allows some interesting conclusions. As stated already by Rundel (1980), the relative dominance of these two photosynthetic systems along environmental gradients results from a complex interaction of several climatic factors. In areas where both parameters favour 358
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one photosynthetic type (e.g. high ambient humidity combined with shade, like in gallery forests), we observe a marked distribution difference. In areas such as the riverside thicket & grassland and the ponds with a considerable sun exposure (favourable for the development of C4) and high humidity (favourable for the development of C3) the numbers of grass taxa following the one or the other photosynthetic type are similar, in the mentioned examples 9 C4 / 8 C3 and 4 C4 / 4 C3 species, respectively (Fig. 2, Table 1). The wetland I “pajonal” (100% sunlight exposure and highly humid) does not follow this pattern, presenting a strong C4 dominance (8 C4 / 3 C3) (Fig. 2, Table1). This contradiction to the expectation of a balanced C3/C4 relationship may be solved by the fact that the riverside grasslands and the ponds have nevertheless some shaded patches. These result from thin thicket intermingled in the typical pajonal vegetation. Few shrubs are growing also in the dunes surrounding the ponds. The conclusion would be too far reaching, however, that sun exposure generally has a stronger influence than humidity on the distribution pattern of photosynthetic types. Sage (1999) states that although C4 photosynthesis may enable survival in sites too harsh for C3 vegetation (e.g. very xeric locations), abiotic stress such as drought is not a prerequisite for C4 dominance. In virtually any warm terrestrial habitat, including wetlands, C4 taxa can be superior due to their higher temperature optima for growth, provided that ecological disturbance minimizes competition from C3 wood vegetation. Since the overall results are consistent with comparable supra-regional research work, the current study shows that a focus on sunlight exposure and humidity variations at a local level proves to be a useful methodology to be applied also in the analysis of the distribution of Poaceae photosynthetic types in smaller areas. From the knowledge about sun exposure and aridity of a patchy vegetation a raw estimate of the grass diversity should be possible. On the other hand, already a floristic survey of grass-dominated vegetation types, supplemented by informations about the genus- or speciesspecific photosynthetic pathways can render bioclimatic informations for the particular habitats.
Acknowledgements The authors are grateful to Dr. Silvia Burkart for her revision of the manuscript and her insightful suggestions. The assistance of F. O. Zuloaga, M. Molina, M. Arriaga and J. F. Pensiero in the classification of the genera Panicum, Eustachys, Eriochloa and Setaria is gratefully acknowledged; thanks are expressed also to the directors and curators of the herbariums mentioned. Finally, the authors thank Elisa Nicora, for her constant support, the identification of some species and her valuable company in one of the field trips.
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