Study of changes in life zone distribution in north-east China by climate–vegetation classification

Life zones and their changes in distribution in north-east China were studied based on climate–vegetation relationships. The warmth index (WI) and aridity index (the ratio of evaporation [evaporation rate, ER] to precipitation) were used to represent the site condition. The typical site condition of each vegetation type was determined as the classification criterion. The boundaries of the four potential vegetation zones were estimated based on the combinations of WI and ER in relation to vegetation (i.e. cold-temperate conifer forest zone, temperate broad-leaved conifer mixed forest zone, warm-temperate deciduous forest zone, and temperate steppe zone). The distribution changes in vegetation zone caused by human activities were estimated by comparing the potential vegetation with the actual one. The percentage cover of forest has shrunk from about 70% to the present 27%. About 23% of the study area was replaced by agricultural vegetation and industrial use. Nearly half of the region could have been covered by broad-leaved conifer mixed forest which was shrunk to a small area, less than 5% of the region. The broad-leaved deciduous forest zone in the southern part could have occupied about 7% of the area, and had almost no virgin stand.     

Ecosystem water fluxes for two grasslands in elevated CO2: a modeling analysis

The need to combine data from CO₂ field experiments with climate data remains urgent, particularly because each CO₂ experiment cannot run for decades to centuries. Furthermore, predictions for a given biome need to take into account differences in productivity and leaf area index (LAI) independent of CO₂-derived changes. In this study, we use long-term weather records and field data from the Jasper Ridge CO₂ experiment in Palo Alto, California, to model the effects of CO₂ and climate variability on ecosystem water fluxes. The sandstone and serpentine grasslands at Jasper Ridge provide a range of primary productivity and LAI, with the sandstone as the more productive system. Modeled soil water availability agreed well with published observations of time-domain reflectometry in the CO₂ experiment. Simulated water fluxes based on 10-year weather data (January 1985–December 1994) showed that the sandstone grassland had a much greater proportion of water movement through plants than did the serpentine; transpiration accounted for approximately 30% of annual fluxes in the sandstone and only 10% in the serpentine. Although simulated physiological and biomass changes were similar in both grasslands, the consequences of elevated CO₂ were greater for the sandstone water budget. Elevated CO₂ increased soil drainage by 20% in the sandstone, despite an approximately one-fifth increase in plant biomass; in the serpentine, drainage increased by <10% and soil evaporation was unchanged for the same simulated biomass change. Phenological changes, simulated by a 15-day lengthening of the growing season, had minimal impacts on the water budget. Annual variation in the timing and amount of rainfall was important for water fluxes in both grasslands. Elevated CO₂ increased sandstone drainage >50 mm in seven of ten years, but the relative increase in drainage varied from 10% to 300% depending on the year. Early-season transpiration in the sandstone decreased between 26% and 41%, with elevated CO₂ resulting in a simulated water savings of 54–76 mm. Even in years when precipitation was similar (e.g., 505 and 479 mm in years 3 and 4), the effect of CO₂ varied dramatically. The response of grassland water budgets to CO₂ depends on the productivity and structure of the grassland, the amount and timing of rainfall, and CO₂-induced changes in physiology. In systems with low LAI, large physiological changes may not necessarily alter total ecosystem water budgets dramatically. Received: 11 March 1997 / Accepted: 23 September 1997     

Spatial Cluster Detection for Repeatedly Measured Outcomes while Accounting for Residential History

Spatial cluster detection has become an important methodology in quantifying the effect of hazardous exposures. Previous methods have focused on cross‐sectional outcomes that are binary or continuous. There are virtually no spatial cluster detection methods proposed for longitudinal outcomes. This paper proposes a new spatial cluster detection method for repeated outcomes using cumulative geographic residuals. A major advantage of this method is its ability to readily incorporate information on study participants relocation, which most cluster detection statistics cannot. Application of these methods will be illustrated by the Home Allergens and Asthma prospective cohort study analyzing the relationship between environmental exposures and repeated measured outcome, occurrence of wheeze in the last 6 months, while taking into account mobile locations.     

Effect of irrigation frequency and nitrogen on groundnut yield and nutrient uptake

Summary Field experiments were conducted during 1979 and 1980 summer seasons on sandy loam soils of low moisture retentive capacity to study the effect of high frequency irrigation at different levels of N on groundnut yield and nutrient uptake (NPK). Four irrigation frequencies (irrigation at 2, 4, 6 and 8 cm cumulative can evaporation, corresponding to irrigation once in 3, 5, 7 and 10 days respectively) and four levels of nitrogen (0, 20, 40 and 60 kg N/ha) were tested in a factorial randomized block design with three replications. Pod yield of groundnut was maximum (3,293 kg/ha) when irrigations were scheduled at 4 cm cumulative can evaporation (once in 5 days). Addition of N did not increase the pod yield. N and P uptake by the crop was maximum (180 kg N and 18 kg P/ha) with high frequency irrigation of scheduling irrigation at 4 cm cumulative can evaporation. Highest uptake of N (183 kg/ha) and P (19 kg/ha) was with a combination of 20 kg N/ha and high frequency irrigation (4 cm CCE). K uptake was low with low irrigation frequency, while it was highest (67 kg K/ha) at 20 kg N/ha.     

The land–atmosphere water flux in the tropics

Tropical vegetation is a major source of global land surface evapotranspiration, and can thus play a major role in global hydrological cycles and global atmospheric circulation. Accurate prediction of tropical evapotranspiration is critical to our understanding of these processes under changing climate. We examined the controls on evapotranspiration in tropical vegetation at 21 pan-tropical eddy covariance sites, conducted a comprehensive and systematic evaluation of 13 evapotranspiration models at these sites, and assessed the ability to scale up model estimates of evapotranspiration for the test region of Amazonia. Net radiation was the strongest determinant of evapotranspiration (mean evaporative fraction was 0.72) and explained 87% of the variance in monthly evapotranspiration across the sites. Vapor pressure deficit was the strongest residual predictor (14%), followed by normalized difference vegetation index (9%), precipitation (6%) and wind speed (4%). The radiation-based evapotranspiration models performed best overall for three reasons: (1) the vegetation was largely decoupled from atmospheric turbulent transfer (calculated from Ω decoupling factor), especially at the wetter sites; (2) the resistance-based models were hindered by difficulty in consistently characterizing canopy (and stomatal) resistance in the highly diverse vegetation; (3) the temperature-based models inadequately captured the variability in tropical evapotranspiration. We evaluated the potential to predict regional evapotranspiration for one test region: Amazonia. We estimated an Amazonia-wide evapotranspiration of 1370 mm yr⁻¹, but this value is dependent on assumptions about energy balance closure for the tropical eddy covariance sites; a lower value (1096 mm yr⁻¹) is considered in discussion on the use of flux data to validate and interpolate models.     

Ecohydrology and the Partitioning AET Between Transpiration and Evaporation in a Semiarid Steppe

Water availability defines and is the most frequent control on processes in arid and semiarid ecosystems. Despite widespread recognition of the importance of water in dry areas, knowledge about key processes in the water balance is surprisingly limited. How water is partitioned between evaporation and transpiration is an area about which ecosystem ecologists have almost no information. We used a daily time step soil water model and 39 years of data to describe the ecohydrology of a shortgrass steppe and investigate how manipulation of soil and vegetation variables influenced the partitioning of water loss between evaporation and transpiration. Our results emphasize the overwhelming importance of two environmental factors in influencing water balance processes in the semiarid shortgrass steppe; high and relatively constant evaporative demand of the atmosphere and a low and highly variable precipitation regime. These factors explain the temporal dominance of dry soil. Annually and during the growing season 60–80% of the days have soil water potentials less than or equal to −1.5 MPa. In the 0–15 cm layer, evaporation accounts for half of total water loss and at 15–30 cm it accounts for one third of the loss. Annual transpiration/actual evapotranspiration (T/AET) ranged from 0.4–0.75 with a mean of 0.51. The key controls on both T/AET and evaporation/actual evapotranspiration in order of their importance were aboveground biomass, seasonality of biomass, soil texture, and precipitation. High amounts of biomass and late timing of the peak resulted in the highest values of T/AET.     

Estimation of evaporation rate from soil surface using stable isotopic composition of throughfall and stream water in a tropical seasonal rain forest of Xishuangbanna, Southwest China

Xishuangbanna is located at the northern edge of the distribution of tropical forests in Southeast Asia, and it has a very high frequency of radiation fog, especially during the dry season (November-April). In this study, rainwater, throughfall, intercepted fog water (fog drip) by forest canopy, and stream water were collected in January 2002 and December 2003 for stable isotopic analysis at a tropical seasonal rain forest site in Xishuangbanna, Southwest China. The objective of the study is to estimate the evaporation rate from the forest floor. The stable hydrogen (δD) and oxygen isotope composition (δ¹⁸O) of rainwater, throughfall, fog drip, and stream water was determined using an isotope ratio mass spectrometer. Stream water during the non-storm runoff period was considered to reflect the effect of evaporation from the forest floor. The evaporation rates from the forest floor were estimated using isotope composition values in stream water and the total throughfall was estimated using the Rayleigh distillation equation under equilibrium conditions. The results indicated that the annual weighted means of δD and δ¹⁸O in fog drip were consistently more enriched than those of rainwater and stream water, and the fog drip was thought to contain water that had evaporated and recycled. The weighted means of δD and δ¹⁸O in stream water during the non-storm runoff period were 5.69% and 0.39% more enriched than those of the total throughfall (rain throughfall + fog drip) in 2002, while in 2003 these were 5.05% and 0.28%, respectively. Evapotranspiration in the humid year 2002 and the dry year 2003, computed from the water balance, was 1186 mm and 987 mm, respectively, which was quite low when compared with the values reported in some humid tropical forests. Consequently, the evaporation rate from the forest floor was 5.1% of the evapotranspiration in 2002, and 3.1% in 2003. The lower evaporation rate was thought to be mainly a result of the influence of the high frequency of heavy radiation fog on the rain forest during the night in the dry season (November-April).