The occurrence of C4 plants and their morphological functional types in the vegetation of Xinjiang, China

Floristic composition, morphological functional types and habitat distributions for C₄ species were studied in Xinjiang, North-western China. 89 species, in 9 families and 41 genera, were identified with C₄ photosynthesis. 48 % of these C₄ species were found in Monocotyledoneae, e.g. Cyperaceae (5 species), Gramineae (38 species), the other 52 % was in Dicotyledoneae, e.g. Chenopodiaceae (29 species), Amaranthaceae (7 species), and Polygonaceae (5 species). Compared with those in semi-arid grasslands in North China and tropical forests in South China, more plant families were found with the occurrence of C₄ plants in this arid region. Relatively higher annual species (63 %), shrubs (18 %), and Chenopodiaceae C₄ species (33 %) compositions were the primary characteristics for the C₄ species occurring in Xinjiang, and this was remarkably related with its arid environment. More Chenopodiaceae C₄ species occurring in the region suggested that this type of C₄ species may have higher capacity to fit the air and soil droughts. There was a strong relationship between C₄ occurrence and habitat distributions, more than half of the total 89 C₄ species were found in disturbed and cultivated lands and early stages of vegetation successions, indicating C₄ occurrence was not only related with climate changes, but also with land uses and vegetation dynamics.     

C4 Plants in the Vegetation of Tibet,China: Their Natural Occurrence and Altitude Distribution Pattern

Floristic composition, life forms for C₄ species, and the pattern of altitude distribution were studied on Tibetan Plateau. 79 species, in 7 families and 46 genera, were identified with C₄ photosynthesis. 95 % of these C₄ species belong to Gramineae (51 species), Cyperaceae (14 species), and Chenopodiaceae (10 species), indicating that C₄ plants mainly occur in very few families (7 of 204) on the Tibetan Plateau. High altitude distribution for all the Chenopodiaceae C₄ species (> 3 000 m above sea level) suggests that plants of this kind have large tolerance to cold, dryness, and strong ultraviolet radiation. Most Gramineae and Cyperaceae C₄ species occurrences are consistent with extensive distribution of steppes and meadows in the vast flat of the central Plateau (1 000–3 000 m a.s.l.). Relatively high amount of hemicryptophyte form plants (44 %) in the region indicates that the vegetation, especially grassland, meadows, and steppe, are in good condition. There is a strong relationship between numbers of C₄ species and altitude in the Tibetan Plateau. Occurrence of C₄ species is significantly less in both high and low altitude plateaux in Tibet. Altitude distribution pattern for C₄ species in the region is not only consistent with the altitude and climate, but also with the vegetation types in altitude gradient.     

C4 Species and their Response to Large-Scale Longitudinal Climate Variables Along the Northeast China Transect (NECT)

Natural occurrence of C₄ species, life forms, and their longitudinal distribution patterns along the Northeast China Transect (NECT) were studied. Six vegetation regions experiencing similar irradiation regimes, but differing in longitude, precipitation, and altitude were selected along the NECT from 108 to 131 °E, around altitude of 43.5 °N. Seventy C₄ species were identified in 41 genera and 13 families. 84 % of the total C₄ species were found in four families: Gramineae (38 species), Chenopodiaceae (11 species), Cyperaceae (5 species), and Amaranthaceae (5 species). C₄ grasses make up 54 % of the total identified C₄ species along the NECT and form the leading C₄ family in meadow, steppe, and desert along the NECT. C₄ Chenopodiaceae species make up about 16 % of the C₄ species and become less important, particularly in the meadow and the eastern end of the NECT. 57 % of the total C₄ species are therophytes and 37 % are hemicryptophytes, which is consistent with floristic composition and land utilization. In general, the number of C₄ species decreased significantly from the west to the east or from dry to moist areas along the NECT, and was remarkably correlated with annual precipitation (r ²= 0.677) and aridity (r ²= 0.912), except for salinized meadow region. The proportion of C₄ species from all the six vegetation regions was considerably correlated with these two climatic parameters (r ²= 0.626 or 0.706, respectively). These findings suggest that the natural occurrence of C₄ species varies significantly along the large-scale longitudinal gradient of the NECT. The notable relationship of C₄ species number and proportion in the flora with variations in annual precipitation and aridity suggest that these two climatic parameters are the main factors controlling the longitudinal distribution patterns of C₄ species along the NECT.     

Photosynthetic Pathways,Life Forms,and Reproductive Types for Forage Species Along the Desertification Gradient on Hunshandake Desert,North China

Floristic compositions, life forms, reproductive types for forage species, and their responses to desertification in Hunshandake desert were studied. 164 species, in 30 families and 94 genera, were identified with C₃ (137 species), C₄ (25 species), and CAM (2 species) photosynthesis. Of the 25 C₄ species, 76 % were grasses and Chenopodiaceae species (hereafter chenopods). This suggests that the C₄ species mainly occurred in a few families in the desert region. The reduction of C₃ species and the increase of C₄ species with desertification indicated that C₄ species might have higher tolerance to environmental stresses (e.g. dry and poor soil). Relatively more hemicrytophyte and therophyte forms in the desert are related to the local temperate climate and vegetation dynamics. Relatively greater proportions of C₄/C₃ and clonal species/sexual species at mobile dune showed that the C₄ species and clonal species could make greater contribution to sand land restoration in the Hunshandake desert.     

Photosynthetic and morphological functional types for native species from mixed prairie in Southern Saskatchewan, Canada

Photosynthetic pathways (e.g. C₃, C₄) and morphological functional types (e.g. trees, shrubs, high perennial grasses, perennial forbs) were identified for the native species from the Saskatchewan mixed prairie, using the data from references published between 1950 and 2003. Of the total 219 identified species in 145 genera and 45 families, 208 species in 137 genera and 44 families were found with C₃ photosynthesis, and most of these species are dominants (e.g. Agropyron dasystachyum Hook. and Stipa spartea var. curtiseta Hitchc.). 11 species in 10 genera and 3 families were identified with C₄ photosynthesis (e.g. Atriplex argentea Nutt., Andropogon scoparius Michx., Boutelou gracilis Lag., Calamovilfa longifolia Hook.). The amount of total identified C₄ species in the region is much less than that from the South Dakota mixed prairie (27 species). Gramineae is the leading family with C₄ photosynthesis (8 species), Chenopodiaceae ranks the second (2 species). Relatively less forb types [50 % perennial forbs (PEF) and 12 % annual forbs (ANF)] and more graminoid types (25 %) composition suggested that the rangelands in the region are relatively stable. Lacking of the knowledge on the optimal traits for PFTs classification in the region, further studies (e.g. C₃ and C₄ plant identification and optimal trait selection) are needed to explore the relationships between PFTs and vegetation variations, as well as land-use and climate changes.     

C4 plants in the vegetation of Mongolia: their natural occurrence and geographical distribution in relation to climate

The natural geographical occurrence, carbon assimilation, and structural and biochemical diversity of species with C₄ photosynthesis in the vegetation of Mongolia was studied. The Mongolian flora was screened for C₄ plants by using ¹³C/¹²C isotope fractionation, determining the early products of ¹⁴CO₂ fixation, microscopy of leaf mesophyll cell anatomy, and from reported literature data. Eighty C₄ species were found among eight families: Amaranthaceae, Chenopodiaceae, Euphorbiaceae, Molluginaceae, Poaceae, Polygonaceae, Portulacaceae and Zygophyllaceae. Most of the C4 species were in three families: Chenopodiceae (41 species), Poaceae (25 species) and Polygonaceae, genus Calligonum (6 species). Some new C₄ species in Chenopodiaceae, Poaceae and Polygonaceae were detected. C₄ Chenopodiaceae species make up 45% of the total chenopods and are very important ecologically in saline areas and in cold arid deserts. C₄ grasses make up about 10% of the total Poaceae species and these species naturally concentrate in steppe zones. Naturalized grasses with Kranz anatomy,of genera such as Setaria, Echinochloa, Eragrostis, Panicum and Chloris, were found in almost all the botanical-geographical regions of Mongolia, where they commonly occur in annually disturbed areas and desert oases. We analyzed the relationships between the occurrence of C₄ plants in 16 natural botanical-geographical regions of Mongolia and their major climatic influences. The proportion of C₄ species increases with decreasing geographical latitude and along the north-to-south temperature gradient; however grasses and chenopods differ in their responses to climate. The abundance of Chenopodiaceae species was closely correlated with aridity, but the distribution of the C₄ grasses was more dependent on temperature. Also, we found a unique distribution of different C₄ Chenopodiaceae structural and biochemical subtypes along the aridity gradient. NADP-malic enzyme (NADP-ME) tree-like species with a salsoloid type of Kranz anatomy, such as Haloxylon ammodendron and Iljinia regelii, plus shrubby Salsola and Anabasis species, were the plants most resistant to ecological stress and conditions in highly arid Gobian deserts with less than 100 mm of annual precipitation. Most of the annual C₄ chenopod species were halophytes, succulent, and occurred in saline and arid environments in steppe and desert regions. The relative abundance of C₃ succulent chenopod species also increased along the aridity gradient. Native C₄ grasses were mainly annual and perennial species from the Cynodonteae tribe with NAD-ME and PEP-carboxykinase (PEP-CK) photosynthetic types. They occurred across much of Mongolia, but were most common in steppe zones where they are often dominant in grazing ecosystems. Received: 17 March 1999 / Accepted: 1 November 1999     

Natural Occurrence and Backwater Infection of C4 Plants in the Vegetation of the Yangtze Hydropower Three Gorges Project Region

Natural occurrence of C₄ species, life form, altitude pattern, and infection by the Three Gorges Project (TGP) were studied in the TGP region. 76 species (about 2.5 % of the total 2 685 vascular plant species in the region), in 6 families and 42 genera, were identified with C₄ photosynthesis. 91 % of these C₄ species belong to Monocotyledoneae, e.g. Cyperaceae (14 species), Gramineae (54 species), and Commelinaceae (1 species). Of these C₄ species, Gramineae was the leading C₄ family: 54 C₄ grass species (71 % of the total C₄ species), about 36 % of the total grasses, were identified in the TGP region. 98 % C₄ species was found in therophyte (55 %) and hemicryptophyte (43 %). This is consistent with high grass and sedge compositions in the region. Most habitats of more than a half of these C₄ species (65 %) will be submerged permanently, but no species will be endangered or extinct, because 95 % C₄ species can be found from 500 to 800 m above sea level. The abundance of some C₄ species will be dropped due to the reduction of distribution scope. It will take a long-term to explore the effects of the TGP on plants, vegetation, and environment.     

C<Subscript>4</Subscript> plants in the deserts of China: occurrence of C<Subscript>4</Subscript> photosynthesis and its morphological functional types

C₄ photosynthetic pathway and morphological functional types were determined for 104 species in 45 genera and 10 families from the deserts of China. 67 C₄ species (64.4 %) were found in Dicotyledoneae (e.g. Chenopodiaceae, Polygonaceae, and Amaranthaceae), the other 37 species were in Monocotyledoneae (e.g. Gramineae, Cyperaceae, and Commelinaceae). 36.5 % of the Chenopodiaceae species (predominantly members of the genera Anabasis, Atriplex, Kochia, Salsola, and Suaeda) identified in the desert regions were found with C₄ photosynthesis, which was about 48 % of the total C₄ species. Many C₄ species (58.7 %) were annuals (e.g. Amaranthus, Atriplex, Digitaria, Eragrostis, Kochia, and Salsola) and experienced long-term droughts, high temperature, and high irradiance. Relatively more shrub C₄ species (28 species of 104) were found in Chenopodiaceae (e.g. Anabasis, Camphorosma, Haloxylon, and Salsola) and Polygonaceae (e.g. Calligonum) in the desert regions. Most of shrub C₄ species with small leaf area were no more than 1 m in height and distributed in sandy soils. Composition of relatively more annual species, shrubs, and Chenopodiaceae C₄ species was the primary characteristic for the C₄ species occurrence in deserts, and this was remarkably related with the arid environmental conditions.     

Photosynthetic pathway and morphological functional types in the vegetation from North-Beijing agro-pastoral ecotone, China

Photosynthetic pathways (C₃, C₄, and CAM) and morphological functional types were identified for the species from vegetation in agro-pastoral ecotone, North Beijing. 792 vascular plant species (nearly half of the total species in the ecotone), in 66 families and 317 genera, were identified with C₃, C₄, and CAM photosynthesis (Table 1). 710 species (90 % of the identified species in Table 1) in 268 genera and 64 families were found with C₃ photosynthesis, 68 species (9 % of the total identified species) in 40 genera and 7 families with C₄ photosynthesis, and 14 species in 4 genera and 1 family with CAM photosynthesis. Gramineae is the leading family with C₄ photosynthesis (43 species), Cyperaceae ranks the second (16 species) followed by Chenopodiaceae (5 species). The significant increase of C₄ proportion (C₄/total species) with land deterioration suggested the plants of this type are remarkably responsive to land use in the ecotone. 792 species were classified into nine morphological functional types and the changes of most of these types (e.g. perennial forbs (PEF), annual grasses (ANG), and annual forbs (ANF)) were consistent with habitats and vegetation dynamics in the agro-pastoral ecotone. Hence the photosynthetic pathways, combined with the morphological functional types, are efficient indications for studying the linkage between species and ecosystems in the ecotone.     

Plant functional types and their ecological responses to salinization in saline grasslands, Northeastern China

Photosynthetic pathways (C₃, C₄, and CAM) and morphological functional types (e.g. shrubs, high perennial grasses, short perennial graminaceous plants, annual grasses, annual forbs, perennial forbs, halophytes, and hydrophytes) were identified for the species from salinity grasslands in Northeastern China, using the data from both stable carbon isotope ratios (¹³C) and from the references published between 1993 and 2002. 150 species, in 99 genera and 37 families, were found with C₃ photosynthesis, and most of these species are dominants [e.g. Leymus chinensis (Trin.) Tzvel., Calamagrostis epigeios (L.), Suaeda corniculata (C.A. Mey.) Bunge]. 40 species in 25 genera and 8 families were identified with C₄ photosynthesis [e.g. Chloris virgata Sw., Aeluropus littoralis (Gouan) Parlat] and 1 species with CAM photosynthesis. Gramineae is the leading family with C₄ photosynthesis (27 species), Chenopodiaceae ranks the second (5 species). The significant increase of C₄ proportions with intense salinity suggested this type plant is remarkable response to the grassland salinization in the region. 191 species were classified into eight morphological functional types and the changes of most of these types (e.g. PEF, HAL, and HPG) were consistent with habitats and vegetation dynamics in the saline grassland. My findings suggest that the photosynthetic pathways, combined with morphological functional types, are efficient means for studying the linkage between species and ecosystems in this type of saline grassland in Northeastern China.This revised version was published online in March 2005 with corrections to the page numbers.     

Distribution of C3 and C4 grasses at different altitudes in a temperate arid region of Argentina

Summary The distribution of native C₃ and C₄ grasses in a temperate arid region of Mendoza, Argentina, was studied in six areas at different altitudes. C₄ species predominate at low elevations in both relative species abundance and plant cover. At high elevations C₃ species are dominant in cover and composition. At medium altitudes (1100–1600 m) grass species composition is balanced but plant cover of C₃ species is greater. Of 31 genera in the whole area, 19 were C₄. Only the genera Stipa (C₃) and Aristida (C₄) were present in all the six areas surveyed. The pattern of grass distribution shows high correlation with evapotranspiration and temperature parameters, but low correlation with rainfall. The relation between grass distribution and different climatic parameters is discussed.     

Photosynthetic Pathways and Life Form Types for Native Plant Species from Hulunbeier Rangelands,Inner Mongolia,North China

Photosynthetic pathway Types (C₃, C₄, and CAM) and life forms of native species from Hulunbeier rangelands, north China were studied. Of the total 258 species, 216 species in 132 genera and 42 families had C₃ photosynthetic pathway, including dominant herbs, e.g. Stipa baicalensis Roshev. and Leymus chinensis (Trin.) Tzvel., Filifolium sibiricum Kitam. and Arudinella hirta (Thunb.) Koidz. 38 species in 28 genera and 10 families were found with C₄ photosynthesis, and 4 species in 2 genera and 1 family had CAM photosynthetic pathway. The occurrence of C₄ species was common in Gramineae and Chenopodiaceae, and the two families were leading ones within C₄ plants. More than 52 % of the total 258 species were in H form, 21 % in Th form, 19 % in G form; the other life form Types, e.g. Ch, M, N, and HH, formed less than 3 %. 68 % of C₄ species were in Th form and 24 % in H form, indicating that these Types were the dominant life forms for C₄ species in the rangeland region. The occurrence of C₄ species was closely related with plant habitats, disturbed lands had the highest C₄ abundance (55 % of the total C₄ species), followed by grasslands and sandy soil, and forests had the lowest C₄ abundance (8 %). Hence the occurrence of C₄ species could be efficient indicator for rangeland dynamics in Hulunbeier rangelands.     

Photosynthetic pathway variation among C4 grasses along a precipitation gradient in Argentina

Aim Based on the biochemical and physiological attributes of C₄ grasses, and on the close association between decarboxylation pathways and the taxa in which they evolved, the hypotheses tested were: (1) that C₄ grasses would become progressively more abundant as precipitation decreased, with grasses of the NADP‐me subtype more abundant in wetter sites and those of the NAD‐me subtype more common in arid regions; and (2) that the distribution of grass subfamilies would also be correlated with annual precipitation. Location The study was conducted along a precipitation gradient in central Argentina, from the eastern Pampas (>1000 mm year^?1) to the western deserts and semi‐deserts near the Andes (<100 mm year^?1). Methods Percentage of species and relative cover of C₃ and C₄ grasses (including C₄ subtypes) in local floras from 15 lowland sites of central Argentina were obtained from our own unpublished data and from recently published floristic surveys. Pearson correlation coefficients were obtained between grass distribution parameters and the available climatic data. Results The percentage of C₄ grasses increased towards the arid extreme and showed a strong negative correlation with annual rainfall (r = ?0.74, P < 0.01). Within the C₄ subtypes, the NADP‐me species showed a higher proportional representation at the wetter extreme, whereas the representation of NAD‐me species increased towards the more arid extreme. The relationship of PEP‐ck species with climatic parameters in central Argentina was less evident. The distributions of the Panicoideae and Chloridoideae subfamilies along the precipitation gradient were diametrically opposed, with the Panicoideae positively (r = 0.86, P < 0.001) and the Chloridoideae negatively (r = ?0.87, P < 0.001) correlated with annual precipitation. Main conclusions Our data are consistent with the broad observation that C₄ grasses tend to dominate in areas where the wet season falls in the warmer summer months. In agreement with previously reported results for Africa, Asia, Australia and North America, we describe here for the first time a significant relationship between annual precipitation and the prevalence of the NADP‐me and NAD‐me photosynthetic pathways along climatic gradients for the Neotropics. We also report for the first time that correlations between C₄ species and annual rainfall are stronger when the relative cover of grass species is considered. The association of grass subfamilies Panicoideae and Chloridoideae with rainfall is as strong as that recorded for the NADP‐me and NAD‐me variants, respectively, suggesting that characteristics other than decarboxylation type may be responsible for the geographic patterns described in this study.     

The C4 Photosynthetic Pathway and Life Forms in Grassland Species from North China

C₄ photosynthetic pathway and life form were determined for 159 species in 71 genera and 13 families in the grassland of North China. 45 % of the C₄ species were found in Graminae, 19 % in each of Cyperaceae and Chenopodiaceae. More than 51 % of these C₄ species were in therophyta and 36 % hemicryptophyta, while fewer species were in nanophanerophyta (9 %) or geophyta (5 %). The numbers of C₄ species and their life forms were closely related with grassland deterioration and succession in North China. This indicated that the C₄ species had greater capacity to tolerate environmental stress (e.g. drought and salinity) caused by animal grazing and cultivation.     

Vegetation variation within and among palustrine wetlands along an altitudinal gradient in KwaZulu-Natal,South Africa

The variation in graminoid species composition and diversity and the distribution of photosynthetic pathways among 66 wetlands in KwaZulu-Natal, South Africa, and within six of these wetlands was described and related to measured physical parameters, using multivariate and univariate techniques. Altitude, which ranged from 550 m to 2120 m, accounted for most variation among wetlands, with an almost complete turnover of species along this gradient. Landform setting was less important in explaining overall species composition, but relationships of individual species were revealed (e.g. Eleocharis dregeana showed an affinity for depressions). Within a wetland there was an almost complete turnover of species along a gradient of wetness, as described using soil morphological criteria. Most species were consistently associated with the same wetness zones across different wetland sites, e.g., Phragmites australis with the wettest zone, Pycreus macranthus with the intermediate zone, and Eragrostis plana with the least wet zone. The occurrence and abundance of different photosynthetic pathway types depended on altitude and degree of wetness. At high altitudes, C₃ sedges, notably Carex acutiformis, dominated the wettest zone and C₃ and C₄ grasses and sedges dominated the intermediate and least wet zones. At mid altitudes, C₃ and C₄ sedges and C₃ grasses dominated the wettest zone, C₃ and C₄ grasses and sedges dominated the intermediate zone and C₄ grasses dominated the least wet zone. Low altitude sites showed a similar distribution of photosynthetic pathways as mid-altitude sites, but C₃ species were less abundant. Species richness was positively associated with the log of wetland size and, at the level of an individual wetland, species richness and evenness were found to be consistently greater in the intermediate and least wet zone compared with the wettest zone. The management implications of the results are discussed in the light of continuing anthropogenic loss of wetlands in the study area and global climate change.     

C4 photosynthesis evolved in warm climates but promoted migration to cooler ones

C₄ photosynthesis is considered an adaptation to warm climates, where its functional benefits are greatest and C₄ plants achieve their highest diversity and dominance. However, whether inherent physiological barriers impede the persistence of C₄ species in cool environments remains debated. Here, we use large grass phylogenetic and geographical distribution data sets to test whether (1) temperature influences the rate of C₄ origins, (2) photosynthetic types affect the rate of migration among climatic zones, and (3) C₄ evolution changes the breadth of the temperature niche. Our analyses show that C₄ photosynthesis in grasses originated in tropical climates, and that C₃ grasses were more likely to colonise cold climates. However, migration rates among tropical and temperate climates were higher in C₄ grasses. Therefore, while the origins of C₄ photosynthesis were concentrated in tropical climates, its physiological benefits across a broad temperature range expanded the niche into warmer climates and enabled diversification into cooler environments.     

Plant functional types and their ecological responses to salinization in saline grasslands,Northeastern China

Photosynthetic pathways (C₃, C₄, and CAM) and morphological functional types (e.g. shrubs, high perennial grasses, short perennial graminaceous plants, annual grasses, annual forbs, perennial forbs, halophytes, and hydrophytes) were identified for the species from salinity grasslands in Northeastern China, using the data from both stable carbon isotope ratios (δ¹³C) and from the references published between 1993 and 2002. 150 species, in 99 genera and 37 families, were found with C₃ photosynthesis, and most of these species are dominants [e.g. Leymus chinensis (Trin.) Tzvel., Calamagrostis epigeios (L.), Suaeda corniculata (C.A. Mey.) Bunge]. 40 species in 25 genera and 8 families were identified with C₄ photosynthesis [e.g. Chloris virgata Sw., Aeluropus littoralis (Gouan) Parlat] and 1 species with CAM photosynthesis. Gramineae is the leading family with C₄ photosynthesis (27 species), Chenopodiaceae ranks the second (5 species). The significant increase of C₄ proportions with intense salinity suggested this type plant is remarkable response to the grassland salinization in the region. 191 species were classified into eight morphological functional types and the changes of most of these types (e.g. PEF, HAL, and HPG) were consistent with habitats and vegetation dynamics in the saline grassland. My findings suggest that the photosynthetic pathways, combined with morphological functional types, are efficient means for studying the linkage between species and ecosystems in this type of saline grassland in Northeastern China.     

Changes in plant form and function across altitudinal and wetness gradients in the wetlands of the Maloti-Drakensberg,South Africa

A survey of 93 wetlands in six catchments across the Maloti-Drakensberg is used to assess the distribution of plant functional types across altitudinal and wetness gradients. Altitudes range from 1,000 to 3,200 m a.s.l. Within each catchment, the wetlands were selected to cover the complete range in altitude and wetland types. In each of the selected wetlands, vegetation was sampled in 3 by 3 m quadrats covering the entire range of wetness represented in the wetland, from temporarily wet to permanently inundated soils. Plant species were allocated to one of 11 different functional types (examples are C₃ grasses, C₄ sedges, rosette plants, and shrubs), and the proportion of the vegetation in each sample occupied by each functional type was calculated from the species’ abundances. Canonical Correspondence Analysis shows that “wetness” clearly has the highest impact on the distribution of functional types, followed by altitude. The most important plant functional types in wetlands are grasses and sedges, however, at higher altitudes, forbs (especially rosette plants) and bulbous plants become a more prominent feature in the wetlands. The total amount of graminoids gradually decreases with altitude. The general trend is that sedges tend to increase with increasing wetness and C₃ plants (grasses and sedges) increase with increasing altitude, but these effects are not independent. The distributions of C₄ sedges and C₄ grasses along an altitudinal gradient are quite different, and C₄ grasses grow abundantly at much higher altitudes than C₄ sedges. C₄ sedges are very scarce at the altitudes represented in the Maloti-Drakensberg area, whereas C₃ grasses occur in the permanently wet parts of the wetlands, especially at higher altitudes (normally mostly occupied by sedges). Shrubs are rare in wetlands and tend to be an indication of disturbance. This study complements previous studies on the distribution of grasses and sedges at the lower altitudes within KwaZulu-Natal, which found that at altitudes below 1,000 m a.s.l. C₄ sedges were much more prominent, while forbs and rosette plants were largely absent. This confirms that C₄ as an adaptation to hotter and warmer climates is sometimes a less favorable metabolism in wet high altitude areas. At high altitudes, rosette plants and bulbous plants become more competitive in wetlands, probably because grasses and sedges present at these altitudes generally grow smaller than they do in low altitude wetlands.     

Geographical distribution and chorology of grasses in the Arabian Peninsula

Arid regions of Saudi Arabia occupy most of the area of the Arabian Peninsula. These areas are at the meeting position of plants from Mediterranean, Irano-Turanian, Saharo-Arabian, and Sudanian phytogegraphical regions. Geomorphology of the area reveals a wide diversity of landforms including coastal lines, desert plains, and high mountains. Grasses are well represented in the flora of Saudi Arabia and form an appropriate group for studying the relation of grass distribution, chorology, and photosynthetic pathways. In this paper, geographical distribution of C₃ and C₄ grasses was studied in an area extending between latitude 17°N and latitude 31°N. Two regions were recognized in the study area, namely; a (relatively) cold region north of latitude 24°N with ample winter rainfall, and a hot region south of latitude 24°N with scarce summer rainfall. Work involved field observations and collection of grass species in the study area. Work also depended on published carbon discrimination values of grasses and biochemical analysis of C₄ species subtypes. Climatic conditions in the study area vary considerably, and the distribution of grass species was found to follow patterns that reveal adaptive advantages of different photosynthetic pathways. Grass species in the cold northern region with ample winter rainfall are generally C₃ grasses belonging mainly to Mediterranean/Irano-Turanean chorotypes. C₃ grass species found in the southern hot region were recorded at high altitudes of southern mountains characterized by low temperatures. Grass species recorded at low altitudes in the south hot region with scarce summer rainfall were mainly C₄ grasses belonging to Tropical and Saharo-Arabian-Sudanean chorotypes. Pronounced spatial variations of temperature profoundly control the geographical distribution of C₃ and C₄ grasses. Low temperatures in the northern cold region and at high altitudes of the southern hot region limit the occurrence of C₄ grasses and shift the ecological balance in favor of C₃ grasses. Results are discussed in terms of heat sensitivity of the CO₂ carboxylating enzyme of C₃ grasses and high temperature optima for CO₂ assimilation of C₄ grasses. Results are also discussed in comparison with geographical distribution of grasses in other parts of the world.