Effects of Periodic Drought
Although there is an increase in evapotranspiration from west to east, weather that results in precipitation in the San Bernardino Mountains is generally a regional phenomenon. We have used the longest record of precipitation for the range, collected over the last 120 years at Big Bear Dam (San Bernardino Water Management District), to identify the level of drought stress experienced by ponderosa pine from year to year. Moderate drought stress is defined physiologically as reduced cell turgor that generally results in reduced stomatal conductance (reduced water loss from the leaf), and lower cellular water potential, which allows the tissue to hold onto the water that is in the leaf more tenaciously (Levitt 1980). In 1994, a year of 80 percent of the average precipitation (preceded by an above-average precipitation year), ponderosa pine experienced moderate drought stress from mid-July through the end of the growing season (Grulke 1999). Severe drought stress is also accompanied by reduced cell turgor, reduced stomatal conductance, and reduced cell-water potential. The water potential is lowered sufficiently that cell solutes are concentrated enough to disrupt enzymatic function, and cell turgor is reduced enough and for a long enough duration that cell elongation growth is limited. Needles produced in years of severe drought stress are shorter. In 1996, a year of 60 percent of the average precipitation (preceded by an above-average precipitation year), ponderosa pine experienced severe drought stress from the end of June through the end of the growing season (Grulke 1999). Over the period of the long-term precipitation record, roughly 15 percent of the years had low enough total annual precipitation to result in moderate drought stress; 30 percent of the years had low enough total annual precipitation to result in severe drought stress. Using this rough index of the level of physiological stress, ponderosa pine experienced drought stress 45 percent of the years since 1883 when precipitation records were initiated (see figure on right).
Where O3 exposure and nitrogen deposition reduce root biomass, trees are predisposed to drought stress. In general, low to moderate O3 exposures (<60 ppb hourly O3, averaged over 24 hours for the 6-month growing season) reduce water loss from trees. O3 reduces photosynthetic rates, less CO2 is required, and stomatal apertures are reduced to conserve water. However, under concentrations that are moderately high or higher, O3 exposure modifies stomatal behavior in ways that increase drought stress.
For example, sugar maple was exposed to O3 concentrations of 70 ppb during daylight hours (Tjoelker and others 1995). Early in the growing season and experiment, neither the net photosynthetic rate nor stomatal conductance was affected by the treatment. By midseason, there was a significant decrease in water-use efficiency—at the same level of carbon gain, seedlings growing in chronic O3 exposure had twice the level of water use as had control seedlings grown in charcoal-filtered air. By late season, both net photosynthesis and stomatal conductance were suppressed in plants grown in chronic O3 exposure. In a field study of sensitive and tolerant genotypes of Jeffrey pine exposed to the same ambient O3 levels (~68 ppb O3 averaged over 24 hours, for the 6-month growing season in Sequoia National Park), sensitive genotypes had lower water loss under moist, favorable conditions and higher water loss under dry, unfavorable conditions (Patterson and Rundel, 1989). Under favorable conditions, Jeffrey pine had less water loss, but because the stomatal apertures were smaller, there was also less photosynthetic carbon gain. Under unfavorable conditions (most of the day in the Sierra Nevada), sensitive Jeffrey pine had higher water loss, which would result in greater desiccation.
Although physiologists often report plant response under steady state (stable) conditions, the light environment in the forest is often dynamic. Understanding stomatal responses under rapidly changing environmental conditions with concurrent O3 exposure can perhaps better explain why trees exposed to moderately high and higher concentrations of O3 lose more water. In typical forest environments, foliage on a primary branch on the southern aspect of an open-grown tree receives flecky light two-thirds of the time (Grulke 2000). For example, the cutleaf coneflower (Rudbeckia laciniata var. digitata) is one of the most sensitive native plants to ambient O3 concentrations in Great Smoky Mountains National Park. It persists in forest gaps and on forest-meadow margins, both with flecky light environments. Tolerant plants of cutleaf coneflower had normal responses to experimentally manipulated change in light from low to high and back down to low levels. However, O3-sensitive plants had either no stomatal response or a muted stomatal response to changes in light level. The level of water loss from leaves with no or muted response to changing light level was high—they did not conserve water when light was low, and this failure to conserve water would contribute to desiccation. When humidity was lowered slowly, O3-sensitive plants closed their stomata at much lower relative humidities than did O3-tolerant plants, and this also contributed to greater desiccation (Grulke and others 2007). In a similar experiment with California black oak saplings (Quercus kelloggii) exposed to anthropogenic high O3 in a natural stand, stomatal closure in response to abruptly reduced light level was slower in plants without additional N amendment, and N amendment partially mitigated the desiccating effects of high O3 exposure (Grulke and others 2005).
Moderate to high O3 exposure can also cause stomata to remain partially open at night. In experimental O3 exposures, this was first observed in Norway spruce (Picea abies), (Weiser and Havranek 1993) and in birch (Betula pendula), (Matyssek and others 1995). Nighttime water loss rates were 25 percent as great as full daytime rates for Norway spruce, and 50 percent as great for birch. This was corroborated in ponderosa pine across the San Bernardino Mountain pollution gradient, with both higher O3 and NO2 and HNO3 exposure. In the San Bernardino Mountains, nighttime water losses were 10 percent as great as full daytime rates (Grulke and others 2005). Because these studies were largely phenomenological, a new gas exchange system was designed and built to directly test known O3 concentrations on single leaves. Chronic, moderate O3 exposure (70 ppb O3 for 8 hours per day for 1 month) significantly increased nighttime foliar water loss in California black oak and blue oak (Quercus douglasii) (Grulke and Paoletti 2005). Nighttime water losses were attributable directly to O3 exposure and were 30 percent and 20 percent, respectively, as great as daytime rates in these species. Moderately high (or higher) O3 exposure increases foliar water loss and increases tree susceptibility to drought stress.
Encyclopedia ID: p3352
