Ecology of Acacia species in Chihuahuan Desert rangeland.

摘要

The main objective of this study was to determine over 2 growing seasons the growth behavior of viscid acacia (Acacia neovernicosa Isely) and whitethorn acacia (Acacia constricta Benth) at different locations in the northern Chihuahuan Desert in New Mexico.; Two sites for viscid acacia and 2 sites for whitethorn acacia were selected. At each site, 5 small and 5 large plants of each acacia species were randomly selected. On each plant, 8 twigs were selected and marked. Measurements were made on these twigs every 15 days from May to October 1993 and 1994. Mean canopy volume of plants was determined 2 times each year. Associated plant species with these acacias were determined once every year in summer. Chemical composition of plant leaves and pods for these species was determined 2 times each year. Seed germination, root and shoot growth rates and weights were measured under greenhouse conditions during 1994.; Analysis of the data indicated that both species had small and near regular increases in twig diameter. In general large plants had a greater increase in twig diameter than small plants. Neither species showed any detectable seasonal increase in twig length and number of nodes and both followed a sympodial growth pattern. Twigs of viscid acacia produced about 2 times as many leaves as those of whitethorn acacia. Both species were deciduous and generally, twigs of large plants of both species produced more flowers and pods than twigs of small plants. Entire twigs, leaves, flowers and pods of viscid acacia were sticky by secretions of glands, while on whitethorn acacia, this stickiness was not common. Production of secondary branches on twigs of whitethorn acacia was higher than that of viscid acacia. However, the number of these secondary branches per twig was less than 1 and the new secondary branches exhibited non-sympodial growth during their first growing season and sympodial growth during the second growing season. Generally large plants of both species at each site had greater increase in volume than small plants during both years. During 1994, because rainfall was below average, twig characteristics and plant volume measured were smaller than those measured for 1993. Moreover, low rainfall in 1994 changed the composition and densities of other species associated with these acacias.; Under greenhouse conditions, scarified seeds of viscid and whitethorn acacia had about 41% and 43% germination, respectively, in petri dishes. Pot tests revealed germination of viscid and whitethorn acacia of 36% and 53%, respectively. Seedlings of whitethorn acacia had higher root-shoot elongation and weights than those of viscid acacia in root boxes with sandy loam soil from the Jornada Plain. Leaves and pods of viscid acacia had about 15-20% crude protein while those of whitethorn acacia had 20-25% crude protein. Leaves and pods of both species had high concentrations of condensed tannins and total phenolics.