Effects of mineral nutrition on susceptibility to cavitation were examined in four hybrid poplar clones. Two drought-sensitive and two drought-resistant hybrid clones of black cottonwood (Populus trichocarpa Torr. & Gray) and eastern cottonwood (P. deltoides Bartr.) were grown at three concentrations of nitrogen (N) applied factorially with two concentrations of phosphorus (P) in a greenhouse, and subjected to varying degrees of drought stress before measurement of cavitation and of anatomical features that might affect cavitation. Mean vessel pit pore diameters were 0.132 micro m at low P, and 0.074 micro m at high P, but no other significant effects of mineral nutrition on vessel dimensions were observed. Vessel diameter and specific conductivity were greater in the drought-resistant clones than in the drought-susceptible clones. Drought-resistant clones did not reach such low water potentials as drought-sensitive clones during the cavitation induction experiments, suggesting better stomatal and cuticular control of water loss. Scanning electron microscope observations showed less damage to pit membranes, also suggesting greater membrane strength in drought-resistant clones than in drought-sensitive clones. High concentrations of N increased cavitation, whereas high concentrations of P decreased cavitation as measured by both hydraulic flow apparatus and dye perfusion techniques. For one test, cavitation was 48% at high N and low P, but only 28% at high N and high P. We consider that N fertilization may make poplars more susceptible to cavitation on dry sites, but P fertilization may reduce this effect.

No abstract

Effects of N and K nutrition on drought and cavitation resistance were examined in six greenhouse-grown poplar clones: Populus trichocarpa (Torr. & Gray) and its hybrids with P. deltoides Bartr. and P. euramericana (Dole) Guinier, before and after preconditioning to water stress. Both tendency to cavitate and water-use efficiency (WUE) increased when N supply was increased, whereas K supply had little impact on cavitation. Mean xylem vessel diameters increased from 36.6 &mgr;m at low-N supply to 45.2 &mgr;m at high-N supply. Drought-hardy clones, which were relatively resistant to cavitation, had the smallest mean vessel diameters. Vulnerability to cavitation had a weakly positive relationship with vessel diameter, and a negative correlation with transpiration. Drought hardening offered no protection against cavitation in a subsequent drought. Under drought conditions, increasing N supply increased leaf loss and decreased water potentials, whereas increasing K supply decreased leaf loss. Drought-resistant clones exhibited similar WUE to drought-susceptible clones, but had smaller, more numerous stomata and greater leaf retention under drought conditions.

A growth chamber experiment was conducted with Pinuscontorta Dougl. ex Loud. var. latifolia Engelm. seedlings grown in soil compacted at 0.1, 2.0, 4.0, 6.0, and 8.0 MPa pressure. Three moisture regimes were applied factorially to compaction levels by watering from above or by maintaining 2- or 10-cm water tables at the base of the 40-cm soil columns. All treatments were grown at either 22:14 °C or 26:18 °C (light:dark) for 13 weeks. Soil compaction increased bulk density, penetrometer resistance, and soil CO2 and ethylene. The presence of water tables resulted in elevated soil gravimetric water content, which rose with increased compaction and resulted in reduced penetrometer resistance and soil O2. Increased compaction was associated with decreased needle lengths, root dry weights, and net photosynthesis and increased rates of shoot respiration. Compaction had a small effect on height growth, with the tallest seedlings occurring at the greatest compaction rate. Shoot concentrations of mineral nutrients also decreased as soil compaction increased. Within the water table treatments, increased gravimetric soil water content was generally paralleled by a rise in the negative effects of compaction on growth, root/shoot dry weight ratios, and shoot mineral nutrient concentrations.

Conlin, T. S. S. and van den Driessche, R. 2000. Response of soil CO 2 and O 2 concentrations to forest soil compaction at the Long-term Soil Productivity sites, Central British Columbia. Can. J. Soil Sci. 80: 625-632. The concentrations of soil CO 2 and O 2 at three Long-term Soil Productivity sites located in the Sub-Boreal Spruce biogeoclimatic zone of British Columbia, Canada, were monitored over several growing seasons. These sites were treated with three levels of soil compaction and three levels of organic material removal following forest harvest. Both compaction and depth had a significant effect upon soil CO 2 concentrations and average values increased in response to deeper sampling and compaction. Removal of the forest floor and other organic materials had no significant effect upon soil CO 2 and O 2 concentrations. Mean soil CO 2 concentrations varied from site to site and data collected over three growing seasons also showed that mean CO 2 values in response to compaction fluctuated significantly from year to year. The high levels of soil CO 2 observed in this study (up to 40 000 µL L -1 ), especially in response to the compaction treatments, may have had a substantial effect upon whole plant carbon allocation as well as soil nutrition available to trees growing on the treated plots. Although mean soil O 2 also decreased in response to compaction and sampling depth, these lower concentrations did not approach the levels required to influence plant growth.Key words: Soil, compaction, carbon dioxide, oxygen, long-term soil productivity Conlin, T. S. S. et van den Driessche, R. 2000. Réponse à la compaction des concentrations de CO 2 et de O 2 des sols forestiers aux Emplacements d'observation de longue durée de la productivité du sol dans le centre de la Colombie-Britannique. Can. J. Soil Sci. 80: 625-632. Nous avons surveillé durant plusieurs saisons de croissance l'évolution des concentrations de CO 2 et de O 2 dans le sol à trois placeaux d'observation de longue durée de la productivité du sol dans la zone biogéoclimatique sud-boréale à épinette (épicéa) en Colombie-Britannique (Canada). Ces placeaux étaient exposés à trois intensités de compaction et à trois niveaux d'enlèvement de la couche organique à la suite de la récolte de la forêt. La compaction et la profondeur d'échantillonnage exerçait toutes deux un effet significatif sur les concentrations de CO 2 du sol, les valeurs moyennes augmentant en fonction du degré de compaction et de la profondeur de l'échantillon. L'enlèvement de la couche organique et d'autres matières organiques n'avaient pas d'effets significatifs sur les concentrations de CO 2 et de O 2 du sol. Les concentrations moyennes de CO 2 variaient d'un emplacement à l'autre et en outre leur réaction à la compaction fluctuait significativement d'une année à l'autre. Les concentrations élevées de CO 2 du sol observées (jusqu'à 40 000 µL L -1 ), surtout en réponse à l'intensité de la compaction, pourraient avoir un effet substantiel sur la disponibilité du carbone et des éléments ...