Photosynthetic response, carbon isotopic composition, survival, and growth of three stock types under water stress enhanced by vegetative competition

Selecting the proper stock type for reforestation on dry sites can be critical for the long-term survival and growth of seedlings. In this study, we use a novel approach to understand stock type selection on a site where drought was induced with vegetative competition. Three ponderosa pine (Pinus ponderosa Lawson & C. Lawson var. ponderosa C. Lawson) seedling stock types were planted in the field and subjected to three levels of competition. Winter wheat (Triticum aestivum L. em.) was sown in three densities (0, 150, and 300 plants·m-²) and was successfully used as a model competitor to create drought conditions. High rates of net photosynthesis (A) indicated that seedlings with adequate soil moisture and without vegetative competition were established within three weeks. Conversely, low A, low soil moisture, and low predawn water potential measurements indicated that seedlings planted with vegetative competition were moisture-stressed and not established. Drought conditions created by the wheat caused 100% mortality among smaller stock types, whereas the largest stock type had a 63%--75% mortality rate. Measures of stable carbon isotopes showed stratification based on water availability, with significant δ¹³C enrichment in competition treatments. Soil moisture is critical for seedlings to establish quickly after planting. Our data suggest that proper stock type selection on drought- or vegetation-prone sites can confer survival and growth benefits. (author)