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Accueil du site → Doctorat → États-Unis → 1981 → POPULATION DYNAMICS OF GRASS-FORB COMPLEXES ON A SEMIDESERT RANGE AND SOME DEMOGRAPHIC ASPECTS OF JAMES RUSHPEA (HOFFMANSEGGIA JAMESII TORR. AND GRAY)

New Mexico State University (1981)

POPULATION DYNAMICS OF GRASS-FORB COMPLEXES ON A SEMIDESERT RANGE AND SOME DEMOGRAPHIC ASPECTS OF JAMES RUSHPEA (HOFFMANSEGGIA JAMESII TORR. AND GRAY)

SULIMAN, MUSTAFA MOHAMED

Titre : POPULATION DYNAMICS OF GRASS-FORB COMPLEXES ON A SEMIDESERT RANGE AND SOME DEMOGRAPHIC ASPECTS OF JAMES RUSHPEA (HOFFMANSEGGIA JAMESII TORR. AND GRAY)

Auteur : SULIMAN, MUSTAFA MOHAMED

Université de soutenance : New Mexico State University

Grade : Doctor of Philosophy (PhD) 1981

Résumé
Dynamics of forbs and important grass species from 1915 to 1974 were investigated utilizing records from long-term charted quadrats on the Jornada Experimental Range, USDA, SEA-AR, in southern New Mexico. A short-term demographic study focusing on longevity and age-structure of the perennial forb, James rushpea (Hoffmanseggia jamesii), was also conducted using a technique for counting growth increments. A polythetic agglomerative hierarchical clustering analysis was used to cluster a total number of 42 quadrats into ecological groups. The standard Euclidean distance as a measure of similarity and the flexible strategy with Beta set to the value of -.25 were adopted in this analysis. Three clusters were obtained. These were found to coincide nicely with the characteristic features of black grama (Bouteloua eriopoda), tarbush (Flourensia cernua) and mesquite (Prosopis juliflora) sand dunes vegetation types, respectively. Quadrats that were protected from livestock grazing did not exhibit any special vegetational pattern to place them together in a separate cluster. Grama vegetation type was floristically richer and more heavily populated by forb species than tarbush and mesquite sand dunes vegetation types. Ninety-six, 67 and 51 percent of the 49 forb species used in the clustering analysis were found in grama, tarbush and mesquite sand dunes vegetation types, respectively. The grass-forb complex has undergone a considerable change over the period 1915-1974. The general pattern in grass cover (percent) was a buildup from the deteriorated conditions that prevailed prior to the establishment of the Experimental Range. During the period 1926, 1927, 1929 and 1931-1933, average basal grass cover values were 12.63, 10.54 and 8.49 percent in grama, tarbush and mesquite sand dunes vegetation types, respectively. During the extremely dry year of 1934, reduction in grass cover was 29, 42 and 19 percent in these vegetation types, respectively. A substantial recovery in grass cover occurred during the years 1937-1940. During the severe drought of 1950-1956, average grass cover was only 3.28, 3.86 and 0.93 percent in the three vegetation types, respectively. Following the drought years, grass cover increased gradually but never reached the cover percent it had prior to the drought period. An inverse relationship was found between forb density and grass cover. Annual forbs were found to capitalize heavily on years of above-average precipitation, particularly in those years that were preceded by years of below-average precipitation. The general behavior of the grass-forb complexes as influenced by the precipitation pattern indicated a short cycle of vegetative change. It begins with high density of annual forbs, relatively low density of perennial forbs and poor grass cover. The cycle may end with a reverse shift in the composition of these life forms when moisture conditions become more favorable. The dynamics of black grama, dropseed (Sporobolus spp.), tobosa (Hilaria mutica), threeawns (Scleropogon spp.) and burrograss (Schleropogon brevifolius) from 1927 to 1974 indicated a firmly established dominance sequence among them. Their dynamics also indicated the degree of disturbance that occurred since 1927 in the three vegetation types studied and the position of each site relative to the climax status in the area. Interesting results were obtained by the study of some demographic aspects of James rushpea. A maximum number of 9 growth increments was found for this species. A highly significant correlation (P < .001) was found between the number of growth increments and the cormlike base circumference (cm.). Age-structure of this species was described by two patterns : A J-shaped pattern on protected and highly disturbed sites, and an inverse J-shaped pattern on lightly grazed sites.

Mots clés  : Range plants—Arid regions. Hoffmanseggia jamesii.

Présentation (Jornada NMSU)

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Page publiée le 6 avril 2015, mise à jour le 11 décembre 2018