Response of understory vegetation to variable tree mortality following a mountain pine beetle epidemic in lodgepole pine stands in northern Utah

We analyzed the long-term dynamics of aboveground biomass ofLeymus chinense steppe in relation to interannual variation of precipitation and temperature during 1980–1989 at levels of community, growth form and species in the Xilin river basin, Inner Mongolia Autonomous Region, China. Annual aboveground net primary production (ANPP) varied from 154.00 g m^-2 yr^-1 in 1980 to 318.59 g m^-2 yr^-1 in 1988, with a mean of 248.63 g m^-2 yr^-1 and the coefficient of variation of 25%. ANPP was not significantly correlated to annual precipitation and total precipitation during April–September atp0.05 level, but precipitation in May and August accounted for 69% of interannual variation of ANPP. The means of rain use efficiency and water use efficiency ofL. chinense steppe were 8.1 kg DM ha^-1 mm^-1 yr^-1 and 0.89 mg DM g^-1 H₂O respectively. Aboveground biomass of various growth forms and species had different response patterns to interannual variation of precipitation and temperature. Monthly and seasonal distribution of precipitation and temperature were the key controls of aboveground biomass of species.     

Increasing precipitation event size increases aboveground net primary productivity in a semi-arid grassland

Water availability is the primary constraint to aboveground net primary productivity (ANPP) in many terrestrial biomes, and it is an ecosystem driver that will be strongly altered by future climate change. Global circulation models predict a shift in precipitation patterns to growing season rainfall events that are larger in size but fewer in number. This “repackaging” of rainfall into large events with long intervening dry intervals could be particularly important in semi-arid grasslands because it is in marked contrast to the frequent but small events that have historically defined this ecosystem. We investigated the effect of more extreme rainfall patterns on ANPP via the use of rainout shelters and paired this experimental manipulation with an investigation of long-term data for ANPP and precipitation. Experimental plots (n = 15) received the long-term (30-year) mean growing season precipitation quantity; however, this amount was distributed as 12, six, or four events applied manually according to seasonal patterns for May–September. The long-term mean (1940–2005) number of rain events in this shortgrass steppe was 14 events, with a minimum of nine events in years of average precipitation. Thus, our experimental treatments pushed this system beyond its recent historical range of variability. Plots receiving fewer, but larger rain events had the highest rates of ANPP (184 ± 38 g m⁻²), compared to plots receiving more frequent rainfall (105 ± 24 g m⁻²). ANPP in all experimental plots was greater than long-term mean ANPP for this system (97 g m⁻²), which may be explained in part by the more even distribution of applied rain events. Soil moisture data indicated that larger events led to greater soil water content and likely permitted moisture penetration to deeper in the soil profile. These results indicate that semi-arid grasslands are capable of responding immediately and substantially to forecast shifts to more extreme precipitation patterns.     

Evidence for the promotion of aboveground grassland production by native large herbivores in Yellowstone National Park

We examined the effect of native large herbivores on aboveground primary production of nonforested habitat in Yellowstone National Park, Wyoming. Productivity of vegetation grazed by elk (Cervus elaphus) and bison (Bison bison) was compared with that of ungrazed (permanently fenced) vegetation at four sites. Two methods were used that, we believed, would provide the most accurate measurements under the different grazing regimes encountered in the study. Production of ungrazed vegetation in permanent exclosures (10×10 m or 15×15 m, 3 per site) and that of vegetation that was grazed only in the winter was taken as peak standing crop. Production of vegetation grazed during the growing season was the sum of significant increments (P<0.05) in standing crop inside temporary exclosures (1.5×1.5 m, 6 per site) moved every four weeks to account for herbivory.Aboveground productivity of grazed vegetation was .47% higher than that of ungrazed vegetation across sites (P<0.0003). This result could be explained by either a methodological or grazer effect. We believe it was the latter. Results from a computer simulation showed that sequential sampling with temporary exclosures resulted in a slight underestimation of production, suggesting that the reported differences between treatments were conservative. We suggest that stimulation of aboveground production by ungulates may be, in part, due to the migratory behavior of native ungulates that track young, high quality forage as it shifts spatially across the Yellowstone ecosystem.     

三江平原北部生态系统服务价值的时空演变

以三江平原北部地区2市5县为研究区,采用生态系统服务价值评估方法对1954—2009年间三江平原北部地区生态系统服务价值进行估算,以期全面分析土地生态系统服务价值随时空变化的特点。结果表明:(1)1954—2009年间三江平原北部地区的不同土地利用类型面积变化显著,农田变化速度最快,其次为湿地,人口数量的增加及保证粮食增产是土地利用变化的主要驱动力。(2)1954—2009年三江平原北部地区生态系统服务价值总量逐渐减少,共减少779.51亿元。长期的土地垦殖是三江平原北部地区生态系统服务价值减少的主要驱动因素。(3)就各土地利用类型生态系统服务价值而言,55年间除农田生态系统和水域生态系统生态服务价值是增加外,其余各生态系统服务价值均在减少。湿地生态系统服务价值减少值最大,其次为林地生态系统服务价值。各项生态系统服务功能除与农田相关的食物生产在增加,其余也均呈减少趋势。(4)不同地区单位面积生态系统服务价值损失量也不相同,湿地、林地面积比重下降幅度大、且农田面积比重上升幅度大的区域,其单位面积生态系统服务价值损失量也较大。     

Conservation of nitrogen increases with precipitation across a major grassland gradient in the Central Great Plains of North America

Regional analyses and biogeochemical models predict that ecosystem N pools and N cycling rates must increase from the semi-arid shortgrass steppe to the sub-humid tallgrass prairie of the Central Great Plains, yet few field data exist to evaluate these predictions. In this paper, we measured rates of net N mineralization, N in above- and belowground primary production, total soil organic matter N pools, soil inorganic N pools and capture in resin bags, decomposition rates, foliar ¹⁵N, and N use efficiency (NUE) across a precipitation gradient. We found that net N mineralization did not increase across the gradient, despite more N generally being found in plant production, suggesting higher N uptake, in the wetter areas. NUE of plants increased with precipitation, and δ¹⁵N foliar values and resin-captured N in soils decreased, all of which are consistent with the hypothesis that N cycling is tighter at the wet end of the gradient. Litter decomposition appeared to play a role in maintaining this regional N cycling trend: litter decomposed more slowly and released less N at the wet end of the gradient. These results suggest that immobilization of N within the plant–soil system increases from semi-arid shortgrass steppe to sub-humid tallgrass prairie. Despite the fact that N pools increase along a bio-climatic gradient from shortgrass steppe to mixed grass and tallgrass prairie, this element becomes relatively more limiting and is therefore more tightly conserved at the wettest end of the gradient. Similar to findings from forested systems, our results suggest that grassland N cycling becomes more open to N loss with increasing aridity.     

Pilot trophic model for subantarctic water over the Southern Plateau, New Zealand: a low biomass, high transfer efficiency system

The Southern Plateau subantarctic region, southeast of New Zealand, is an important feeding area for birds, seals and fish, and a fishing ground for commercially significant species. The Southern Plateau is a major morphometric feature, covering approximately 433,620 km² with average depth of 615 m. The region is noted for its relatively low levels of phytoplankton biomass and primary production that is iron-limited. In order to evaluate the implications of these attributes for the functioning of this ecosystem a steady-state, 19-compartment model was constructed using Ecopath with Ecosim software of Christensen et al. [www.ecopath.org]. The system is driven by primary production that is primarily governed by the supply of iron and light. The total system biomass of 6.28 g C m⁻² is very low compared with systems so far modelled with a total system throughput of 1136 g C m⁻² year⁻¹. In the model, the Southern Plateau retains 69% of the biomass in the pelagic system and 99% of total production. Although fish are caught demersally, most of their food is part of production in the pelagic system. Top predators represent about 0.3% of total biomass and account for about 0.24 g C m⁻² year⁻¹ of food consumed made up of birds 0.058 g C m⁻² year⁻¹, seals 0.041 g C m⁻² year⁻¹, and toothed 0.094 g C m⁻² year⁻¹ and baleen whales 0.051 g C m⁻² year⁻¹. This amounts to 105,803 tonnes carbon over the whole of the Southern Plateau and is about 17% of the total amount of food eaten by non-mesopelagic fish. Mean transfer efficiencies between trophic levels II and IV of 23% are at the high end of the range reported in the literature. In the model, adult fish production is almost completely accounted for by the fisheries take (32%), consumption by seals (7%), toothed whales (21%), other adult fish (13%), and squid (20%). Fish and squid catches are at the trophic levels of 4.8 and 5.0, respectively. The gross efficiency of the fishery is 0.018% (catch/primary production). Although not all data come from direct knowledge of this system, the model reflects its general characteristics, namely a low primary production system dominated by the microbial loop, low sedimentation to the seafloor, high transfer efficiencies, a long food web and supporting high-level predators.     

Topographic and climatic controls on soil environments and net primary production in a rugged temperate hardwood forest in Korea

Eight years (1994–2001) of field data and a biogeochemical process model, BIOME-BGC, were used to examine effects of local topography and inter-annual climatic variability on soil physical (i.e., soil moisture and temperature) and biogeochemical (i.e., organic matter content, soil respiration, and leaf litter production) variables in a temperate hardwood forest in Korea. The field data were collected from adjacent south-facing (S) and north-facing (N) slopes, respectively, to examine effects of local topography, and were utilized to validate predictability according to BIOME-BGC which was applied to model unmeasured hydro-ecological processes [i.e., evapotranspiration, net primary production (NPP), and net ecosystem exchange of carbon]. Our field-data analyses indicated that soil-related variables including soil temperature, water content, organic matter, soil respiration, and floor leaf litter store significantly differed between the S and N slopes, while leaf litter production did not differ as significantly as the soil-related variables. The BIOME-BGC predictions showed good agreement with the mean field data aggregated across the slopes. Our simulation results and field observations indicated that the inter-annual variations of leaf litter production and maximum leaf area index were best explained by precipitation, both at a 1-year lag, while variation in annual NPP was well correlated with precipitation without a temporal lag. Our results imply that: (1) local topography needs to be explicitly considered in ecosystem studies as a forcing function generating spatial heterogeneity in soil physical and biogeochemical variables within a rugged landscape, and (2) water limits vegetation productivity in our study forest, in spite of a relatively high annual precipitation rate (1,579 mm year^–1).     

Litterfall as a measure of primary production in Mediterranean holm-oak forest

In this paper we discuss the use of litterfall as a method to measure primary production and its between year relation to climatic fluctuation. Seven years of study in a mediterranean holm-oak forest showed a moderate inter-annual variability of litterfall (C.V. 11%), while the annual variability of the actual evapotranspiration was three times higher (C.V.30%). Furthermore, the inter- and intra-annual variability of nutrient content in the various fractions are presented in relation to water availability. Monthly and seasonal variability was higher than the annual variability for all analyzed elements.     

Clonal plants in a spatially heterogeneous environment: effects of integration on Serengeti grassland response to defoliation and urine-hits from grazing mammals

It has been hypothesized that clonal integration between ramets inenvironments with spatially variable rates of herbivory and nutrientavailabilities leads to increased growth and fitness in the genet. An increasein genet growth could potentially influence ecosystem processes such as primaryproductivity and nutrient cycling. I tested the idea that clonal integrationwould lead to greater aboveground productivity, compensatory response todefoliation, and N re-distribution in a factorial experiment in fieldplots in the Serengeti Ecosystem, Tanzania. Each plot had either all stolonssevered or left intact, had repeated defoliation (to simulate grazing bythe African buffalo) or none, and was either located next to a plot thatreceived urea (to simulate a urine-hit) or next to a plot thatremained untreated. Plots that received stolon severing treatments had32% less peak biomass than did connected control plots, and this suggeststhat plants grew better when ramets remained connected. However, compensatoryresponse by plots to repeated defoliation was inconsistent with the hypothesizedbenefits of ramet integration at the ecosystem level: productivityresponse to defoliation was similar between plots with connected vs. severedramets (i.e. no stolon severing × defoliation interaction wasfound). When averaged across other treatments, defoliated plots hadincreased productivity compared to nondefoliated plots during the growingseason. Thus, ramet connection and defoliation did increase productivity whenthey were considered alone, but productivity response to defoliation wasunaffected by ramet connections. Urea additions, which led to a 78%increase in productivity in adjacent urine-hits, had noconsistent effect on productivity but did increase leaf percent N in adjoiningstudy plots. Thus, in the Serengeti, urine hits probably have very localizedeffects on productivity during the initial growing season.