Regeneration aspects of three Mexican Pinus species : field and greenhouse studies Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/jm214r923

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  • In a field study, seedlings of Arizona pine (Pinus arizonica Engeim.), Apache pine (P. engelmannii Cam), and Durango pine (P. durangensis Mart.) were planted in an one open area and in canopy gaps of four forested sites of a pine-oak forest in the Sierra Madre Occidental in the state of Durango, Mexico. Competing vegetation in half of the study plots was treated with glyphosate before planting to test the effects of competition on survival, growth, and water relations. After two years in the open area, significant herbicide effects were detected only for the survival of P. engelmannii and P. durangensis. Stem height and diameters were significantly different between treated and untreated plots only for P. engelmannii. Midday xylem water potential in P. durangensis was significantly different between treated and untreated plots, and morning and midday stomatal conductance rates of P. engelmannii were significantly different between treated and untreated plots 21 months after planting. In the canopy gaps. significant herbicide effects were detected only for the survival of P. durangensis. There was little understory vegetation in canopy gaps, therefore, treated and untreated plots did not differ in seedling growth or water relations among the species. In a greenhouse study, seedlings of P. arizonica, P. engelmannii, P. durangensis, and Pacific ponderosa pine (Pinus ponderosa var. ponderosa Laws.) were subjected to well-watered and water-stressed treatments over three replicated drying cycles in the dormant season. The volumetric water content ranged between 67 and 36% in the well-watered treatment, and between 8.0 and 2.0% in the water-stressed treatment. Statistically, no water stress effect was found on seedling growth except for P. durangensis, and biologically the effect was marginal. Xylem water potential and stomatal conductance were measured in needle fascicles at the end of two drying cycles. The water stress caused significant and highly significant treatment and diurnal differences for water potential and stomatal conductance on most of the species in those two cycles. A predawn water potential threshold associated with less than 50% of the maximum midday stomata! conductance was found for each species. The threshold for the three Mexican pine species ranged from -0.6 to -0.8 MPa while P. ponderosa var. ponderosa had a higher threshold at -1.1 MPa. Through tree planting, the uncertainties of the natural regeneration process are avoided. Successful regeneration of forested sites in the Sierra Madre Occidental will require reduction in overstory stocking, control of competing vegetation where necessary and prevention of livestock grazing.
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