We tested the hypothesis that fertilized containerized Pinus canariensis seedlings increases survival when planted in semiarid sites through the improvement of their physiological status during the establishment phase by an increment in root growth. Seedlings were cultured under two diVerent regimes: traditional (in non-fertilized natural soil) and alternative (in fertilized peat). Morphological attributes and nitrogen content were measured before planting. Measurements of survival and growth in the plantation were made periodically for 2 years and physiological plant responses (leaf water potential, gas exchange and chlorophyll Xuorescence) during the third summer after planting were tested and Wnally a set of plants were excavated to measure the same parameters as before planting. Seedlings cultivated using fertilized peat achieved the highest values for all of evaluated parameters. During the third dry season, big seedlings exhibited better physiological status. Therefore, enhanced root growth can result in better water uptake during the dry period thereby increasing survival and growth in the next few years after planting. A feed-back physiological model is proposed to explain P. canariensis establishment in a semiarid environment.
Gas exchange characteristics and chlorophyll fluorescence of the Canarian endemic pine (Pinus canariensis) were measured during the day for a year in a field stand on Tenerife, Canary Islands, Spain. Diurnal tendencies of gas exchange were variable depending on ambient conditions. In general they paralleled photosynthetic photon flux density with only one peak at midday, except on summer days with high air vapour pressure deficit (VPD), when needles exhibited a severe midday depression of CO 2 assimilation rate (A), in parallel with a reduction of stomatal conductance (g s ). The internal CO 2 concentration tendencies during the day suggest that stomatal closure was the main cause of the midday depression of photosynthesis. Chlorophyll fluorescence data corroborate this assertion, with the parameter F v /F m reaching high values throughout day and year. P. canariensis living in the sub-tropic exhibited high values of A (maximal A value of 17 mol m 2 s 1 ) and high optimal needle temperature for photosynthesis (25C) which were at the upper limit of the values given for conifers and similar only to data obtained for some pine species adapted to habitats at similar latitudes. g s was reduced to half when VPD attained 40 mbar, allowing this pine to have high A/g s values during high evaporative demand conditions.
We investigated the seasonal variation in the gas exchange of current and 1-year-old needles in the upper sun and lower shade crown of adult Pinus canariensis trees. In general, current year needles displayed lower gas exchange rates than the 1-year-old needles. In both needle age classes, gas exchange was signiWcantly lower in the shade than in the sun crown. However irrespective of crown position and needle age, maximum daily net photosynthesis, transpiration, and stomatal conductance for water vapour were generally higher during the wet and cold winter as compared to the dry and hot summer. These higher gas exchange values obtained during the cold and wet season can mainly be explained by higher soil-water availability and lower evaporative demand as compared to the warm and dry seaon. In addition, we also observed a displacement in the temperature optimum of net photosynthesis towards lower temperatures during the cold and wet season as compared to the warm and dry season. The observed gas exchange characteristics indicate a conservative water saving strategy and thus allowing P. canariensis needles to maintain a positive carbon gain even at periods of high evaporative demand and low soil-water availability.
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