[1] Measurement of fine root production and turnover rate, the reciprocal of mean life span of a root population, is crucial to the understanding of the carbon cycle of an ecosystem as fine roots account for up to 30% of global terrestrial net primary production. Our goal was to characterize fine root production, mortality, standing crop, and turnover rate in a Mediterranean climate. Using simulations, we established that our sampling interval must be less than monthly to keep the turnover rate error to less than 10%. Adhering to this interval, we measured fine root turnover rate by mark-recapture modeling methods and compared predicted with observed turnover rates. The best selected model indicated that these rates were a function of diameter, length, soil temperature, and soil water content. Turnover rate increased with decreasing diameter and length and increasing soil temperature and soil water content. We found a yearly pattern of hysteresis between fine root production, mortality, and turnover rate relative to soil temperature. This was explained by soil temperature-moisture hysteresis using our best selected model. Production and turnover rate were greater in spring to early summer when both soil temperature and soil moisture were high, resulting in a seasonal variation of belowground net primary production. We suggest that this behavior could be a result of fine roots' strategy to cope with a limited growing season of a semiarid Mediterranean climate.Citation: Kitajima, K., K. E. Anderson, and M. F. Allen (2010), Effect of soil temperature and soil water content on fine root turnover rate in a California mixed conifer ecosystem,
[1] Trees and shrubs growing in California's mountains rely on deep roots to survive the hot and dry Mediterranean climate summer. The shallow montane soil cannot hold enough water to support summer transpiration, and plants must access deeper moisture from the weathered bedrock. We used the HYDRUS-1D model to simulate the moisture flux through the soil-plant continuum in Southern California's San Jacinto Mountains. The mechanisms facilitating deep water access are poorly understood, and it is possible that either or both hydraulic lift and capillary rise contribute to the survival and activity of trees and soil microorganisms. We modified HYDRUS to incorporate hydraulic lift and drove it with meteorological and physiological data. The modeled quantity of water lifted hydraulically ranged from near zero during the wet months to~28 mm month À1 in midsummer. Likewise, modeled capillary rise was negligible during the winter and averaged~15 mm month À1 during June through November. Both mechanisms provided water to support evapotranspiration during the dry months. Isotopic measurements of xylem water for eight shrub and tree species confirmed the importance of a deep source of water. Conventional and automated minirhizotron observations showed that fine-root and rhizomorph biomass remained relatively constant year-round, while mycorrhizal hyphae biomass varied markedly, peaking in the wet season and declining by~70% in the dry season. Model results predict that hydraulic lift and capillary rise play key roles in Southern California's mountains: they support evapotranspiration and photosynthesis during the summer drought; they contribute to the year-round survival of fine roots and soil microorganisms.Citation: Kitajima, K., M. F. Allen, and M. L. Goulden (2013), Contribution of hydraulically lifted deep moisture to the water budget in a Southern California mixed forest,
SummaryUnderstanding the temporal variation of soil and root dynamics is a major step towards determining net carbon in ecosystems. We describe the installation and structure of an in situ soil observatory and sensing network consisting of an automated minirhizotron with associated soil and atmospheric sensors.Ectomycorrhizal hyphae were digitized daily during 2011 in a Mediterranean climate, highelevation coniferous forest. Hyphal length was high, but stable during winter in moist and cold soil. As soil began to warm and dry, simultaneous mortality and production indicating turnover followed precipitation events. Mortality continued through the dry season, although some hyphae persisted through the extremes. With autumn monsoons, rapid hyphal re-growth occurred following each event.Relative hyphal length is dependent upon soil temperature and moisture. Soil respiration is related to the daily change in hyphal production, but not hyphal mortality.Continuous sensor and observation systems can provide more accurate assessments of soil carbon dynamics.
Rapid responses of microbial biomass and community composition following a precipitation event have been reported for soil bacteria and fungi, but measurements characterizing ectomycorrhizal fungi remain limited. The response of ectomycorrhizal fungi after a precipitation event is crucial to understanding biogeochemical cycles and plant nutrition. Here, we examined changes in ectomycorrhizal formation, diversity, and community composition at the end of a summer drought and following precipitation events in a conifer–oak mixed forest under a semiarid, Mediterranean-type climate in CA, USA. To study the effects of different amounts of precipitation, a water addition treatment was also undertaken. Ectomycorrhizal fungal diversity and community composition changed within 6 days following precipitation, with increased simultaneous mortality and re-growth. Ectomycorrhizal diversity increased and community composition changed both in the natural rainfall (less than 10 mm) and water addition (50 mm) treatments, but larger decreases in ectomycorrhizal diversity were observed from 9 to 16 days after precipitation in the water addition treatment. The changes were primarily a shift in richness and abundance of Basidiomycota species, indicating higher drought sensitivity of Basidiomycota species compared with Ascomycota species. Our results indicate that ectomycorrhizal formation, diversity, and community composition rapidly respond to both precipitation events and to the amount of precipitation. These changes affect ecosystem functions, such as nutrient cycling, decomposition, and plant nutrient uptake, in semiarid regions.Electronic supplementary materialThe online version of this article (10.1007/s00572-018-0859-3) contains supplementary material, which is available to authorized users.
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