The relationship of floods, drying, flow and light to primary production and producer biomass in a prairie stream

Factors related to autochthonous production were investigated at several sites along a prairie stream at Konza Prairie Research Natural Area. Primary production, algal biomass, litter input, and ability of floods to move native substrate were measured. Additional experiments were conducted to establish the influence of light and water velocity on primary production rates and recovery of biomass following dry periods. The study period encompassed two extreme (> 50 year calculated return time) floods, thus we were able to analyze the effects of scour on periphyton biomass and productivity. Biomass of sedimentary algae was reduced greatly by flooding and did not reach preflood amounts during the 2 months following the first flood. Rates of primary production associated with sediments recovered to levels above preflood rates within 2 weeks. Biomass of epilithic periphyton was not affected as severely as that of sedimentary algae. Little relationship was observed between water velocity and photosythetic rates. Production reached maximum rates at 25% of full sun light. Epilithic chlorophyll levels recovered within eight days following a dry period, and chl a was an order of magnitude greater on rocks than sediments 51 days after re-wetting. Estimated annual rates of primary production were 2.6 times greater in the prairie than in the forest reaches of the stream. The ratio of annual autochthonous:allochthonous carbon input was 4.81 for prairie and 0.32 for the forest. Periphyton production in prairie streams is resilient with regard to flooding and drought and represents a primary carbon source for the system.     

Canopy interception of nitrogen in bulk precipitation by annually burned and unburned tallgrass prairie

Summary Nitrogen content of bulk precipitation and throughfall (canopy leachates) was measured on annually burned and unburned tallgrass prairie during a 20 month period. Throughfall amounts averaged 58% of precipitation on unburned prairie while throughfall on annually burned sites averaged 76% of precipitation inputs. Stemflow was measured in late summer and autumn. Volumes were correlated with stem density; maximum stemflow volumes measured in this study averaged about 50% of throughfall volumes.Bulk precipitation averaged 530, 456, and 420 g/l of nitrate, ammonium and organic nitrogen, respectively. Throughfall on burned sites averaged 345, 344 and 980 g/l of nitrate, ammonium and organic nitrogen, and throughfall on unburned sites averaged 258, 196 and 1701 g/l of nitrate, ammonium and organic nitrogen. Microbes on standing dead vegetation and litter of the unburned sites were estimated to remove more inorganic nitrogen from bulk precipitation than did foliage on burned sites. Only a portion of the inorganic nitrogen in bulk precipitation is immediately available for plant use, and this availability is influenced by the amount of detritus present on the prairie.     

Growth dynamics of oak seedlings (Quercus macrocarpa Michx. and Quercus muhlenbergii Engelm.) from gallery forests: implications for forest expansion into grasslands

Seedling growth dynamics of Quercus macrocarpa Michx. and Quercus muhlenbergii Engelm. were compared over a 3-month period under optimal growth conditions. These two species are the dominant trees at the western limit of the eastern deciduous forest, and are typically confined to gallery forests along stream beds in tallgrass prairie. Since tallgrass prairie is characterized by a highly variable climate and is prone to periodic drought, we hypothesized that these oaks would have rapid root growth and produce deep taproots as seedlings, enabling them to avoid drought stress and persist in this region. These traits may also facilitate forest expansion into the more xeric tallgrass prairie if fires are suppressed. Taproots of Q. macrocarpa and Q. muhlenbergii grew to approximately 140 cm and 100 cm in length, respectively, after 104 days. In both species, 65% or more of seedling biomass was allocated below ground, and root/total biomass was significantly greater in Q. muhlenbergii at 0-20 and 21-40 days after germination. The seedling taproot elongation rates reported here are much greater than rates reported in other eastern deciduous forest trees. Long-term precipitation data and soil moisture patterns from tallgrass prairie, when combined with rapid taproot elongation rates, suggest that soil moisture may not limit oak establishment or growth in tallgrass prairie in most years, although water uptake by roots was not measured in this study. Other factors, such as fire, herbivory, and seed predation and dispersal may be equally important in constraining the distribution of these species to gallery forests.     

Vegetation effects on soil resource heterogeneity in prairie and forest

A current, widespread example of vegetation change is the invasion of grassland by woody plants. This is associated with an increase in soil heterogeneity, and it has been argued that woody plants both cause and benefit from high heterogeneity. We know of no experimental demonstrations of differences between grasses and woody plants in their effects on heterogeneity. Here we compare heterogeneity between mixed-grass prairie and aspen forest, and we report the results of a soil transplant experiment that tested for differences between these vegetation types in their effects on soil resource heterogeneity. We measured the heterogeneity of resources and plant mass along 10 transects in both prairie and aspen forest in spring and summer. Light and available nitrogen (N; sum of ammonium and nitrate) were significantly more variable in forest than prairie, as were root and understory shoot mass. The variability of soil moisture and topography did not differ between prairie and forest. In our experiment, N and water in cores of prairie soil moved to forest attained the relatively high variability of forest soils. Further, forest soils moved to prairie attained the relatively low variability of prairie soils. In summary, both the biomass heterogeneity measurements and the soil transplant experiment suggested that plant uptake contributed to greater heterogeneity in forests.     

Greater seed production in elevated CO2 is not accompanied by reduced seed quality in Pinus taeda L.

For herbaceous species, elevated CO₂ often increases seed production but usually leads to decreased seed quality. However, the effects of increased atmospheric CO₂ on tree fecundity remain uncertain, despite the importance of reproduction to the composition of future forests. We determined how seed quantity and quality differed for pine trees grown for 12 years in ambient and elevated (ambient+200 μL L^?1) CO₂, at the Duke Forest free‐air CO₂ enrichment (FACE) site. We also compared annual reproductive effort with yearly measurements of aboveground net primary productivity (ANPP), precipitation (P), potential evapotranspiration (PET) and water availability [precipitation minus potential evapotranspiration (P?PET)] to investigate factors that may drive interannual variation in seed production. The number of mature, viable seeds doubled per unit basal area in high‐CO₂ plots from 1997 to 2008 (P<0.001), but there was no CO₂ effect on mean seed mass, viability, or nutrient content. Interannual variation in seed production was positively related to ANPP, with a similar percentage of ANPP diverted to reproduction across years. Seed production was negatively related to PET (P<0.005) and positively correlated with water availability (P<0.05), but showed no relationship with precipitation (P=0.88). This study adds to the few findings that, unlike herbaceous crops, woody plants may benefit from future atmospheric CO₂ by producing larger numbers of seeds without suffering degraded seed quality. Differential reproductive responses between functional groups and species could facilitate woody invasions or lead to changes in forest community composition as CO₂ rises.     

Plant responsiveness to variation in precipitation and nitrogen is consistent across the compositional diversity of a California annual grassland

Question: How does responsiveness to water and Nitrogen (N) availability vary across the compositional and functional diversity that exists in a mesic California annual grassland plant community? Location: Northern California annual grassland. Methods: A mesocosm system was used to simulate average annual precipitation totals and dry and wet year extremes observed in northern California mesic grasslands. The effects of precipitation and N availability on biomass and fecundity were measured on three different vegetation types, a mixed grass forb community, and a forb and a grass monoculture. The treatment effects on plant community composition were examined in the mixed species community. Results: While growth and seed production of the three vegetation types was inherently different, their responses to variation in precipitation and N were statistically similar. Plant density, shoot biomass, and seed production tended to increase with greater water availability in all vegetation types, with the exception of a consistent growth reduction in high precipitation (1245 mm) plots in the first year of the study. Shoot biomass responded positively to N addition, an effect that increased with greater water availability. Nitrogen addition had little effect on plant density or seed production. In the mixed grass‐forb community, biomass responsiveness to water and N treatments were consistently driven by the shoot growth of Avena barbata, the dominant grass species. Conclusions: Vegetation responses to changes in precipitation and N availability were consistent across a range of composition and structural diversity in this study. Plant growth and seed production were sensitive to both increased and decreased precipitation totals, and the magnitude of these responses to N availability varied depending on soil moisture conditions. Our results suggest the impacts of changing precipitation regimes and N deposition on annual productivity of California grasslands may be predictable under different climate scenarios across a range of plant communities.     

The role of water depth and soil temperature in determining initial composition of prairie wetland coenoclines

In this study, we examined the effects of water depth and temperature on seedling recruitment from a prairie wetland seed bank. We collected seed-bank samples from natural and restored prairie pothole wetlands in northwestern Iowa and combined them into a single sample. We examined seedling recruitment from this seed-bank sample in an experimental study using a factorial design of 4 temperature treatments (5° night and 15° day to 20° night and 30° day) and 3 water-depth treatments (0, 2, and 7 cm).Principal Components Analysis showed that both water depth and temperature had significant effects on the composition of the seedling community as measured by changes in relative stem density and biomass. Water depth had its strongest effects on stem density while temperature had its strongest effects on biomass.For the 22 most common species, stem density varied with water depth for 95% of the species and with temperature for 50% of the species. Most species with water depth responses had lower stem counts as water depth increased, and for the majority of species with temperature responses stem density increased with temperature.Total, annual, and perennial species richness was negatively correlated with water depth. Total and annual species richness was positively correlated to temperature, while perennial species richness was unresponsive to temperature. In addition, species found at low elevations as adults emerged at higher rates in the deep water treatments while species that occurred at higher elevations as adults had their highest emergence rates in the low water treatments.Our results suggest that differences in environmental conditions along coenoclines can affect the initial distribution of species emerging from the soil seed bank. Water depth sorted seedlings according to their adult water-depth tolerances, and temperature determined the proportion of annuals in the seedling community.     

Carbon allocation in forest ecosystems

Carbon allocation plays a critical role in forest ecosystem carbon cycling. We reviewed existing literature and compiled annual carbon budgets for forest ecosystems to test a series of hypotheses addressing the patterns, plasticity, and limits of three components of allocation: biomass, the amount of material present; flux, the flow of carbon to a component per unit time; and partitioning, the fraction of gross primary productivity (GPP) used by a component. Can annual carbon flux and partitioning be inferred from biomass? Our survey revealed that biomass was poorly related to carbon flux and to partitioning of photosynthetically derived carbon, and should not be used to infer either. Are component fluxes correlated? Carbon fluxes to foliage, wood, and belowground production and respiration all increased linearly with increasing GPP (a rising tide lifts all boats). Autotrophic respiration was strongly linked to production for foliage, wood and roots, and aboveground net primary productivity and total belowground carbon flux (TBCF) were positively correlated across a broad productivity gradient. How does carbon partitioning respond to variability in resources and environment? Within sites, partitioning to aboveground wood production and TBCF responded to changes in stand age and resource availability, but not to competition (tree density). Increasing resource supply and stand age, with one exception, resulted in increased partitioning to aboveground wood production and decreased partitioning to TBCF. Partitioning to foliage production was much less sensitive to changes in resources and environment. Overall, changes in partitioning within a site in response to resource supply and age were small (<15% of GPP), but much greater than those inferred from global relationships. Across all sites, foliage production plus respiration, and total autotrophic respiration appear to use relatively constant fractions of GPP – partitioning to both was conservative across a broad range of GPP – but values did vary across sites. Partitioning to aboveground wood production and to TBCF were the most variable – conditions that favored high GPP increased partitioning to aboveground wood production and decreased partitioning to TBCF. Do priorities exist for the products of photosynthesis? The available data do not support the concept of priorities for the products of photosynthesis, because increasing GPP increased all fluxes. All facets of carbon allocation are important to understanding carbon cycling in forest ecosystems. Terrestrial ecosystem models require information on partitioning, yet we found few studies that measured all components of the carbon budget to allow estimation of partitioning coefficients. Future studies that measure complete annual carbon budgets contribute the most to understanding carbon allocation.     

华北落叶松人工林蒸散及产流对叶面积指数变化的响应

定量评价林地蒸散和产流等水文过程对冠层叶面积指数(LAI)的响应,对于深入认识森林植被的生态水文过程及其发生机制,实现半干旱区林水综合管理和区域可持续发展是非常必要的。应用集总式生态水文模型BROOK90,模拟分析了不同降水年型(丰水年、平水年、枯水年)下,位于半干旱区的宁夏六盘山叠叠沟小流域内华北落叶松(Larix principis-rupprechtii)人工林的水文过程对冠层LAI变化的响应关系。结果发现:林分总蒸散量、冠层截留量、蒸腾量与LAI都呈显著的正相关关系(R~20.99,P0.01),而土壤蒸发量、产流量则与LAI均呈显著的负相关关系(R~20.99,P0.01);在不同的降水年型下,各水文过程变量与LAI的关系都可以很好地用指数函数来表达,且都存在着一个LAI阈值。当LAI低于阈值时,各水文过程变量随LAI的变化幅度较大;但高于阈值时,各变量的变化十分缓慢并趋于稳定。在不同降水年型下,各变量LAI阈值之间存在着一定的差异。一般地,丰水年各变量的LAI阈值要大于枯水年,尤其是冠层截留和土壤蒸发。在丰水年,各水文过程变量随LAI增加而变化的速率要比在平水年、枯水年更快,说明在水分充足年份中各变量的波动更多取决于LAI变化,而在水分亏缺的年份中则可能更多地受到水分条件的限制。模拟结果表明,通过减少冠层LAI(如间伐)导致的林分的降低蒸散耗水和增加产流的作用是有限的,这是由于林分蒸散降低的幅度要比LAI降低的幅度小。例如,在平水年,当LAI从4.2变为2.0(减少幅度52.4%)时,林分年蒸散仅从357.2 mm减少至333.9 mm(减少幅度6.5%)。     

Spatio-temporal variation in Lepidopteran larval assemblages associated with oak, Quercus crispula: the importance of leaf quality

Abstract.  1. Lepidoptera larval abundance and diversity in the canopies of oak ( Quercus crispula ) trees and saplings were surveyed in a cool-temperate, deciduous broadleaf forest in northern Japan.
2. In general, newly developed leaves were soft, rich in water and nitrogen, and low in tannin, whereas they became tough, poor in water and nitrogen, and high in tannin as the season proceeded. Leaf quality also varied among forest strata, such variations resulting in seasonal and among-strata differences in the structure of the Lepidoptera larval assemblage.
3. The greater Lepidoptera larval abundance and species richness may related to the higher leaf quality on spring foliage compared with summer foliage. On the other hand, diversity (Shannon's H' ) and evenness (Pielou's J' ) were greater on summer foliage than on spring foliage. Strengthened defences of the host plants against herbivory may cause these differences by filtering the larvae of Lepidoptera species and by constraining the super-dominance of a few species on summer foliage.
4. Canonical Correspondence Analysis (CCA) ordination also revealed a stratified structure of the Lepidoptera larval assemblage in the forest. In both spring and summer, the assemblage composition was more similar between sunlit and shaded canopies than between canopies and saplings. Such assemblage stratification was highly correlated with toughness and tannin content (in spring and summer) or water content (in summer).
5. This study emphasised the importance of spatio-temporal variations in leaf quality, even within the same host plant species, for promoting herbivore diversity in forests.     

Controls of nitrogen limitation in tallgrass prairie

Summary The relationship between fire frequency and N limitation to foliage production in tallgrass prairie was studied with a series of fire and N addition experiments. Results indicated that fire history affected the magnitude of the vegetation response to fire and to N additions. Sites not burned for over 15 years averaged only a 9% increase in foliage biomass in response to N enrichment. In contrast, foliage production increased an average of 68% in response to N additions on annually burned sites, while infrequently burned sites, burned in the year of the study, averaged a 45% increase. These findings are consistent with reports indicating that reduced plant growth on unburned prairie is due to shading and lower soil temperatures, while foliage production on frequently burned areas is constrained by N availability. Infrequent burning of unfertilized prairie therefore results in a maximum production response in the year of burning relative to either annually burned or long-term unburned sites.Foliage biomass of tallgrass prairie is dominated by C₄ grasses; however, forb species exhibited stronger production responses to nitrogen additions than did the grasses. After four years of annual N additions, forb biomass exceeded that of grass biomass on unburned plots, and grasses exhibited a negative response to fertilizer, probably due to competition from the forbs. The dominant C₄ grasses may out-compete forbs under frequent fire conditions not only because they are better adapted to direct effects of burning, but because they can grow better under low available N regimes created by frequent fire.     

千岛湖片段化生境中木本植物种子雨基本特征及其影响因子

为探究片段化生境中木本植物种子雨的基本特征,该研究根据2015—2020年(研究期间)在千岛湖样岛上的植物群落长期监测样地内每月收集的种子雨数据,采用Kruskal-Wallis检验对木本植物的种子雨密度进行年际差异分析,对不同传播方式物种的种子雨密度进行月份间差异性分析,并利用线性混合效应模型,探究岛屿空间特征(岛屿面积、距最近岛屿的距离、距大陆的距离)以及气候因子(0 ℃以上积温、降水量)对木本植物以及不同传播方式物种的种子雨密度的影响。结果表明:(1)2015—2020年6年间,在29个样岛用240个收集器共收集到877 178粒木本植物的成熟种子,属于26科40属52种。(2)动物传播是木本植物主要的种子传播方式,不同传播方式物种的种子雨时间动态存在较大差异。(3)木本植物的种子雨年密度与岛屿面积和年积温呈显著正相关,与年降水量呈显著负相关。(4)自主传播物种的种子雨月密度与距最近岛屿的距离呈显著正相关,而动物传播物种的种子雨月密度则与距大陆的距离呈显著正相关,风力传播物种的种子雨月密度与月积温呈极显著正相关。综上表明,生境片段化通过岛屿空间特征影响了木本植物种子雨的时间动态。     

Interannual variability in primary production in tallgrass prairie: climate,soil moisture,topographic position,and fire as determinants of aboveground biomass

From 1975 to 1993, aboveground net primary production (NPP) at the Konza Prairie Research Natural Area in NE Kansas varied from 179 g/m to 756 g/m. Across a variety of sites, NPP was significantly related to precipitation (r = 0.37), but much variability was unexplained. Thus, we evaluated the relationship between NPP with meteorological variables and soil moisture measurements in tallgrass prairie sites that varied in fire frequency and topographic position. Annually burned lowland sites had significantly higher NPP than either annually burned upland or unbumed sites. NPP in burned sites was more strongly related to meteorological variables and soil moisture when compared to unbumed sites. The lack of significant correlation between soil moisture with NPP on unbumed sites suggests that factors other than water availability limit production in these sites. When NPP data were analyzed separately by life forms, interannual variability in forb NPP was not correlated with any meteorological variables, but was negatively correlated with grass NPP (r = -0.49). The inability of a single factor, such as precipitation to explain a large portion of the interannual variability in NPP is consistent with the concept that patterns of NPP in tallgrass prairie are a product of spatial and temporal variability in light, water, and nutrients, driven by a combination of topography, fire history, and climate.     

Spatial variability and controls over biomass stocks,carbon fluxes,and resource-use efficiencies across forest ecosystems

Key message

Stand age, water availability, and the length of the warm period are the most influencing controls of forest structure, functioning, and efficiency.

Abstract

We aimed to discern the distribution and controls of plant biomass, carbon fluxes, and resource-use efficiencies of forest ecosystems ranging from boreal to tropical forests. We analysed a global forest database containing estimates of stand biomass and carbon fluxes (400 and 111 sites, respectively) from which we calculated resource-use efficiencies (biomass production, carbon sequestration, light, and water-use efficiencies). We used the WorldClim climatic database and remote-sensing data derived from the Moderate Resolution Imaging Spectroradiometer to analyse climatic controls of ecosystem functioning. The influences of forest type, stand age, management, and nitrogen deposition were also explored. Tropical forests exhibited the largest gross carbon fluxes (photosynthesis and ecosystem respiration), but rather low net ecosystem production, which peaks in temperate forests. Stand age, water availability, and length of the warm period were the main factors controlling forest structure (biomass) and functionality (carbon fluxes and efficiencies). The interaction between temperature and precipitation was the main climatic driver of gross primary production and ecosystem respiration. The mean resource-use efficiency varied little among biomes. The spatial variability of biomass stocks and their distribution among ecosystem compartments were strongly correlated with the variability in carbon fluxes, and both were strongly controlled by climate (water availability, temperature) and stand characteristics (age, type of leaf). Gross primary production and ecosystem respiration were strongly correlated with mean annual temperature and precipitation only when precipitation and temperature were not limiting factors. Finally, our results suggest a global convergence in mean resource-use efficiencies.     

延河流域植物群落功能性状对环境梯度的响应

研究群落水平上的植物功能性状特征及其随环境梯度的变化规律,对认识不同环境梯度下植物群落的形成及其对环境的适应机制具有重要意义。以延河流域不同环境梯度下的稳定的自然植物群落为对象,测量了植物群落组成物种的叶厚度、比叶面积、叶组织密度、比根长、根组织密度、单位质量叶氮含量、单位质量根氮含量、种子质量、种子体积等9个性状,然后以物种重要值为基础加权平均得到各个性状在群落水平上的平均值(即群落性状值);以现有的环境因子栅格图为基础,利用ArcGIS提出各群落对应的环境因子值,同时测定各个群落的土壤水分,分析群落各性状值与环境因子的关系,并建立关系模型。结果表明:在群落水平上,9个植物功能性状分别与13个环境因子存在不同程度的相关性,同时这9个植物功能性状对8个环境因子梯度(土壤水分、年4-10月平均气温、年7-9月总降雨量、降雨季节变化、年平均降雨量、年平均蒸发量、坡度、坡向)的响应特征较好,不同植物功能性状间具有较好相关性。群落水平上植物功能性状及其组合随环境梯度的规律性变化,反映了延河流域植被群落构建过程中环境对功能性状的筛选效应。该研究结果对该区的植被恢复重建的物种选择及植被布局规划具有重要的实践意义。     

Patterns in leaf traits of leguminous and non-leguminous dominant trees along a rainfall gradient in Ghana

Aims Both dominance distribution of species and the composition of the dominant species determine the distribution of traits within community. Leaf carbon (C) and nitrogen (N) isotopic composition are important leaf traits, and such traits of dominant species are associated with ecosystem C, water and N cycling. Very little is known how dominant species with distinct traits (e.g. N-fixing leguminous and non-leguminous trees) mediate resource utilization of the ecosystems in stressful environment.Methods Leaves of 81 dominant leguminous and non-leguminous trees were collected in forest (moist semi-deciduous and dry semi-deciduous ecosystems) and savanna (costal savanna, Guinean savanna and west Sudanian savanna ecosystems) areas and the transitional zone (between the forest and the savanna) along the transect from the south to the north of Ghana. We measured leaf traits, i.e. leaf δ 13 C, leaf δ 15 N, leaf water content, leaf mass per area (LMA) and C and N concentration. Correlation analyses were used to examine trait–trait relationships, and relationships of leaf traits with temperature and precipitation. We used analysis of covariance to test the differences in slopes of the linear regressions between legumes and non-legumes.Important findings Leaf δ 13 C, δ 15 N, leaf water content and LMA did not differ between leguminous and non-leguminous trees. Leaf N concentration and C:N ratio differed between the two groups. Moreover, leaf traits varied significantly among the six ecosystems. δ 13 C values were negatively correlated with annual precipitation and positively correlated with mean annual temperature. In contrast, leaf δ 15 N of non-leguminous trees were positively correlated with annual precipitation and negatively correlated with mean annual temperature. For leguminous trees, such correlations were not significant. We also found significant coordination between leaf traits. However, the slopes of the linear relationships were significantly different between leguminous and non-leguminous trees. Our results indicate that shifts in dominant trees with distinct water-use efficiency were corresponded to the rainfall gradient. Moreover, leguminous trees, those characterized with relative high water-use efficiency in the low rainfall ecosystems, were also corresponded to the relative high N use efficiency. The high proportion of leguminous trees in the savannas is crucial to mitigate nutrient stress.     

鼎湖山顶级森林生态系统水文要素时空规律

运用连续7a(1993～1999)的水文观测资料,对南亚热带顶级生态系统鼎湖山季风常绿阔叶林集水区水文要素时空规律进行分析,得到如下一些主要结论：(1)鼎湖山多年平均降水量为1910mm,湿季降水量占年降水量80％,干季仅占20％。6月份的降水量最大,1月份最小。(2)季风常绿阔叶林冠层截留率为31．8％,湿季的截留量占全年截留量的66．7％,截留量最大值和最小值所在的月份分别为7和1月份。各月的截留率差异很大,截留量大的月份,截留率较低;截留量小的月份,截留率较高。(3)季风常绿阔叶林集水区多年平均总径流量953．0mm,总径流系数49．9％,其中地表径流量为252．3mm,地表径流系数13．2％;地表径流与降水量之间存在二次抛物线型回归关系,与降水强度的关系不大,这说明季风常绿阔叶林的产流形式是是蓄满产流。(4)季风常绿阔叶林多年平均蒸散948．2mm,占同期降水量的49．7％;蒸散力1031．4mm,年蒸散系数为0．92,蒸散月变化规律较降水量的月变化规律有所滞后。(5)系统贮水量的月变化很大,2～8月份,系统处于蓄水阶段;9月份至翌年1月份,系统处于失水阶段。蓄水和失水的最大值分别出现在湿季和干季的第一个月,即4月份和10月份。(6)集水区多年平均水量总输入2129．9mm,实际输入1910mm(降水量),其中219．9mm的水量输入是由系统贮水量变化而产生。支出的总水量2129．9mm,实际支出1901．3mm(径流和蒸散量),其中228．6mm的水量支出是由系统贮水量变化引起的。     

Regional Patterns in Carbon Cycling Across the Great Plains of North America

The large organic carbon (C) pools found in noncultivated grassland soils suggest that historically these ecosystems have had high rates of C sequestration. Changes in the soil C pool over time are a function of alterations in C input and output rates. Across the Great Plains and at individual sites through time, inputs of C (via aboveground production) are correlated with precipitation; however, regional trends in C outputs and the sensitivity of these C fluxes to annual variability in precipitation are less well known. To address the role of precipitation in controlling grassland C fluxes, and thereby soil C sequestration rates, we measured aboveground and belowground net primary production (ANPP-C and BNPP-C), soil respiration (SR-C), and litter decomposition rates for 2 years, a relatively dry year followed by a year of average precipitation, at five sites spanning a precipitation gradient in the Great Plains. ANPP-C, SR-C, and litter decomposition increased from shortgrass steppe (36, 454, and 24 g C m^–2 y^–1) to tallgrass prairie (180, 1221, and 208 g C m^–2 y^–1 for ANPP-C, SR-C, and litter decomposition, respectively). No significant regional trend in BNPP-C was found. Increasing precipitation between years increased rates of ANPP-C, BNPP-C, SR-C, and litter decomposition at most sites. However, regional patterns of the sensitivity of ANPP-C, BNPP-C, SR-C, and litter decomposition to between-year differences in precipitation varied. BNPP-C was more sensitive to between-year differences in precipitation than were the other C fluxes, and shortgrass steppe was more responsive than were mixed grass and tallgrass prairie.     

Patterns of Annual Seed Production by Northern Hemisphere Trees: A Global Perspective

We tested whether annual seed production (masting or mast fruiting) in Northern Hemisphere trees is an evolved strategy or a consequence of resource tracking by comparing masting patterns with those of annual rainfall and mean summer temperatures, two environmental variables likely to correlate with available resources. There were generally significant negative autocorrelations between the seed crop in year x and year x+1 (year x+2 in species of Quercus requiring 2 yr to mature acorns), as expected if resources are depleted in mast years in part by switching resources from growth to reproduction. Spatial autocorrelation in annual seed production generally declined with distance but was statistically significant over large geographic areas. Variability in annual seed production was relatively high and inversely correlated with latitude and generally not bimodally distributed. Patterns of spatial autocorrelation in annual rainfall and summer temperatures are generally similar to those exhibited by annual seed production, and relative variability in annual rainfall is also inversely correlated with latitude. However, these environmental variables exhibit distinctly different patterns of temporal autocorrelation, are much less variable, and are more normally distributed than annual seed production. Combined with the inverse relationship between growth and reproduction previously documented, these results support the hypothesis that variability in annual seed production is an evolved strategy and that annual seed production is more or less normally distributed rather than an all-or-none phenomenon.     

Masting in ponderosa pine: comparisons of pollen and seed over space and time

Many plant species exhibit variable and synchronized reproduction, or masting, but less is known of the spatial scale of synchrony, effects of climate, or differences between patterns of pollen and seed production. We monitored pollen and seed cone production for seven Pinus ponderosa populations (607 trees) separated by up to 28?km and 1,350?m in elevation in Boulder County, Colorado, USA for periods of 4?C31?years for a mean per site of 8.7?years for pollen and 12.1 for seed cone production. We also analyzed climate data and a published dataset on 21?years of seed production for an eighth population (Manitou) 100?km away. Individual trees showed high inter-annual variation in reproduction. Synchrony was high within populations, but quickly became asynchronous among populations with a combination of increasing distance and elevational difference. Inter-annual variation in temperature and precipitation had differing influences on seed production for Boulder County and Manitou. We speculate that geographically variable effects of climate on reproduction arise from environmental heterogeneity and population genetic differentiation, which in turn result in localized synchrony. Although individual pines produce pollen and seed, only one-third of the covariation within trees was shared. As compared to seed cones, pollen had lower inter-annual variation at the level of the individual tree and was more synchronous. However, pollen and seed production were similar with respect to inter-annual variation at the population level, spatial scales of synchrony and associations with climate. Our results show that strong masting can occur at a localized scale, and that reproductive patterns can differ between pollen and seed cone production in a hermaphroditic plant.