Physiological and growth responses of switchgrass (Panicum virgatum L.) in native stands under passive air temperature manipulation

In the central Great Plains of North America, climate change predictions include increases in mean annual temperature of 1.5–5.5 °C by 2100. Ecosystem responses to increased temperatures are likely to be regulated by dominant plant species, such as the potential biofuel species Panicum virgatum (switchgrass) in the tallgrass prairie. To describe the potential physiological and whole‐plant responses of this species to future changes in air temperatures, we used louvered open‐sided chambers (louvered OSC; 1 × 1 m, adjustable height) to passively alter canopy temperature in native stands of P. virgatum growing in tallgrass prairie at varying topographic positions (upland/lowland). The altered temperature treatment decreased daily mean temperatures by 1 °C and maximum temperatures by 4 °C in May and June, lowered daytime stomatal conductance and transpiration, decreased tiller density, increased specific leaf area, and delayed flowering. Among topographic contrasts, aboveground biomass, flowering tiller density, and tiller weight were greater in lowland sites compared to upland sites, with no temperature treatment interactions. Differences in biomass production responded more to topography than the altered temperature treatment, as soil water status varied considerably between topographic positions. These results indicate that while water availability as a function of topography was a strong driver of plant biomass, many leaf‐level physiological processes were responsive to the small decreases in daily mean and maximum temperature, irrespective of landscape position. The varying responses of leaf‐level gas exchange and whole‐plant growth of P. virgatum in native stands to altered air temperature or topographic position illustrate that accurately forecasting yields for P. virgatum in mixed communities will require greater integration of physiological responses to simulated climate change (increased temperature) and resource availability over natural environmental gradients (soil moisture).     

Spatial and temporal variation in islands of fertility in the Sonoran Desert

In many arid and semi-arid ecosystems, canopy trees and shrubs have a strong positive influence on soil moisture and nutrient availability, creating islands of fertility where organic matter and nutrients are high relative to areas outside the canopy. Previous studies of canopy effects on soil processes have rarely considered how landscape context may modulate these effects. We measured the effects of velvet mesquite trees (Prosopis velutina) on soil moisture and the biogeochemistry of nitrogen at different positions along a topographic gradient from upland desert to riparian zone in the Sonoran Desert of central Arizona. We also examined how landscape position and patterns of precipitation interact to determine the influence of P. velutina on soil moisture, N availability assessed using ion exchange resins, net N mineralization and net nitrification, and microbial biomass C and N. P. velutina clearly created islands of fertility with higher soil organic matter, net N mineralization and net nitrification rates, and microbial biomass under mesquite canopies. These effects were consistent across the landscape and showed little temporal variability. Magnitude and direction of effect of mesquite on soil moisture changed with landscape position, from positive in the upland to negative in the terrace, but only when soil moisture was >4%. Resin N showed responses to mesquite that depended on precipitation and topographic position, with highest values during wet seasons and under mesquite on terraces. We suggest changes in proximity of P. velutina to groundwater lead to shifts in biogeochemical processes and species interactions with change in landscape position along a topographic gradient.     

A TEN-YEAR RECORD OF ABOVEGROUND BIOMASS IN A KANSAS TALLGRASS PRAIRIE: EFFECTS OF FIRE AND TOPOGRAPHIC POSITION

Measurements of mid-season live and dead aboveground biomass are reported for a 10-yr period (1975–84) in a northeast Kansas tallgrass prairie. Study sites included shallow, rocky upland and deep, non-rocky lowland soils in annually burned (April) and unburned watersheds. Lowland sites had significantly greater live biomass than upland sites for both burned and unburned prairie for the 10-yr period. Moreover, live biomass was greater on burned than unburned lowland sites, but was not significantly increased by fire on the upland sites. Averaged across upland and lowland sites, mid-season live biomass was 422 g m^–2 on annually burned and 364 g m^–2 on unburned sites for the 10-yr period. Each site had its lowest live biomass value during the severe drought year of 1980 (range = 185–299 g m^–2). During the study period, live biomass was most strongly correlated with seasonal pan water evaporation (r = –0.45 to –0.82), whereas dead biomass was correlated with the previous yr's precipitation (r = 0.61 and 0.90 for upland and lowland sites, respectively). When aboveground biomass was sampled throughout the 1984 season and separated into several components, biomass of the graminoids was 40% lower, whereas that of forbs and woody plants was 200–300% greater in the unburned than in the annually burned site.     

Carbon dioxide fluxes in a spatially and temporally heterogeneous temperate grassland

Landscape position, grazing, and seasonal variation in precipitation and temperature create spatial and temporal variability in soil processes, and plant biomass and composition in grasslands. However, it is unclear how this variation in plant and soil properties affects carbon dioxide (CO₂) fluxes. The aim of this study is to explore the effect of grazing, topographic position, and seasonal variation in soil moisture and temperature on plant assimilation, shoot and soil respiration, and net ecosystem CO₂ exchange (NEE). Carbon dioxide fluxes, vegetation, and environmental variables were measured once a month inside and outside long-term ungulate exclosures in hilltop (dry) to slope bottom (mesic) grassland throughout the 2004 growing season in Yellowstone National Park. There was no difference in vegetation properties and CO₂ fluxes between the grazed and the ungrazed sites. The spatial and temporal variability in CO₂ fluxes were related to differences in aboveground biomass and total shoot nitrogen content, which were both related to variability in soil moisture. All sites were CO₂ sinks (NEE>0) for all our measurments taken throughout the growing season; but CO₂ fluxes were four- to fivefold higher at sites supporting the most aboveground biomass located at slope bottoms, compared to the sites with low biomass located at hilltops or slopes. The dry sites assimilated more CO₂ per gram aboveground biomass and stored proportionally more of the gross-assimilated CO₂ in the soil, compared to wet sites. These results indicate large spatio-temporal variability of CO₂ fluxes and suggest factors that control the variability in Yellowstone National Park.     

典型草原通量塔通量贡献区地上生物量和叶面积指数的时空变异

引入循环采样方法,在生长季(7月初、7月末和8月末)测量了典型草原植被生物量和叶面积指数,并利用地统计方法分析了二者的空间分布格局.结果表明:在7月初、7月末和8月末3个时期,地上生物量递增,叶面积指数(LAI)先增大后减小,地上生物量和LAI均具有良好的空间自相关性,二者空间格局相似且具有相似的时间演变规律,由条带状逐渐变为斑块状.从7月初到8月末,地上生物量和LAI的块金值/基台值[C0/(C+C0)]显著降低,表明其空间自相关程度从中度自相关变为强相关;变程逐渐缩小,空间连续性变差;分维值逐渐降低,空间依赖性增强.地形和草地管理方式是影响草原植被生物量和叶面积指数空间分布格局的重要因素,导致水热因子空间差异,进而对牧草生长产生影响.     

Arctic shrub growth trajectories differ across soil moisture levels

The circumpolar expansion of woody deciduous shrubs in arctic tundra alters key ecosystem properties including carbon balance and hydrology. However, landscape‐scale patterns and drivers of shrub expansion remain poorly understood, inhibiting accurate incorporation of shrub effects into climate models. Here, we use dendroecology to elucidate the role of soil moisture in modifying the relationship between climate and growth for a dominant deciduous shrub, Salix pulchra, on the North Slope of Alaska, USA. We improve upon previous modeling approaches by using ecological theory to guide model selection for the relationship between climate and shrub growth. Finally, we present novel dendroecology‐based estimates of shrub biomass change under a future climate regime, made possible by recently developed shrub allometry models. We find that S. pulchra growth has responded positively to mean June temperature over the past 2.5 decades at both a dry upland tundra site and an adjacent mesic riparian site. For the upland site, including a negative second‐order term in the climate–growth model significantly improved explanatory power, matching theoretical predictions of diminishing growth returns to increasing temperature. A first‐order linear model fit best at the riparian site, indicating consistent growth increases in response to sustained warming, possibly due to lack of temperature‐induced moisture limitation in mesic habitats. These contrasting results indicate that S. pulchra in mesic habitats may respond positively to a wider range of temperature increase than S. pulchra in dry habitats. Lastly, we estimate that a 2°C increase in current mean June temperature will yield a 19% increase in aboveground S. pulchra biomass at the upland site and a 36% increase at the riparian site. Our method of biomass estimation provides an important link toward incorporating dendroecology data into coupled vegetation and climate models.     

Using Information Theory to Determine Optimum Pixel Size and Shape for Ecological Studies: Aggregating Land Surface Characteristics in Arctic Ecosystems

Quantifying vegetation structure and function is critical for modeling ecological processes, and an emerging challenge is to apply models at multiple spatial scales. Land surface heterogeneity is commonly characterized using rectangular pixels, whose length scale reflects that of remote sensing measurements or ecological models rather than the spatial scales at which vegetation structure and function varies. We investigated the ‘optimum’ pixel size and shape for averaging leaf area index (LAI) measurements in relatively large (85 m² estimates on a 600 × 600-m² grid) and small (0.04 m² measurements on a 40 × 40-m² grid) patches of sub-Arctic tundra near Abisko, Sweden. We define the optimum spatial averaging operator as that which preserves the information content (IC) of measured LAI, as quantified by the normalized Shannon entropy (E _S,n) and Kullback–Leibler divergence (D _KL), with the minimum number of pixels. Based on our criterion, networks of Voronoi polygons created from triangulated irregular networks conditioned on hydrologic and topographic indices are often superior to rectangular shapes for averaging LAI at some, frequently larger, spatial scales. In order to demonstrate the importance of information preservation when upscaling, we apply a simple, validated ecosystem carbon flux model at the landscape level before and after spatial averaging of land surface characteristics. Aggregation errors are minimal due to the approximately linear relationship between flux and LAI, but large errors of approximately 45% accrue if the normalized difference vegetation index (NDVI) is averaged without preserving IC before conversion to LAI due to the nonlinear NDVI-LAI transfer function. Author Contributions:  PS devised and undertook the analyses and wrote the paper. MW devised and implemented the measurement plan, and reviewed the analysis. LS assisted in the spatial data analysis and derivation of the terrain indices. RAB provided the macro-scale dataset. APB generated the DEM from aircraft data. JGE provided meteorological data. MvW generated the micro-scale field data with the help of Lorna Street and Sven Rasmussen. All authors contributed to the text.     

Primary Production of Lowland Natural Grasslands and Upland Sown Pastures Across a Narrow Climatic Gradient

Variation in aboveground net primary production (ANPP) is usually studied across wide environmental gradients focusing on spatial averages of zonal natural communities. We studied the spatial and temporal variation of ANPP of upland sown pastures and lowland natural grasslands across a narrow gradient of precipitation and temperature. The Flooding Pampa (Argentina) encompasses an 850–1000 mm range of mean annual precipitation and a 13.8–16.0°C range of mean annual temperature. For 15 100 × 100 km cells, we obtained mean monthly precipitation, temperature, and paddock-level ANPP of upland pastures and lowland grasslands during 8 years. Mean annual ANPP of lowland grasslands and upland sown pastures was positively related to mean annual precipitation. ANPP of upland pastures was 60–80% larger and increased more steeply with mean annual precipitation. ANPP seasonality also changed across the gradient. In lowland grasslands, as mean annual precipitation increased, ANPP monthly maximum increased, minimum decreased, and the duration of the growing season shortened. In contrast, in upland pastures, ANPP monthly maximum was constant, minimum increased, and the growing season lengthened with increasing precipitation. ANPP was more stable across years for lowland grasslands than for upland pastures. The response of annual ANPP to current-year precipitation decreased across the gradient, while the importance of the previous-year precipitation increased. In summary, we found strong spatial and temporal patterns of ANPP across a narrow environmental gradient. In addition, landscape position and species composition heavily influenced those patterns.     

Soil water dynamics and vegetation patterns in a semiarid grassland

Long-term (1985–1992) dynamics and spatial variations in soil water below the evaporative zone were evaluated for a shortgrass steppe with a low and variable precipitation regime. Each of a sandy loam, clay loam, and two sandy clay loam sites comprised a toposequence with upland, midslope and lowland positions. Soil water was monitored at 15 cm intervals providing estimates covering 22.5 to 97.5 cm depths.Soil water throughout the profile was highest in the clay loam site and lowest in the sandy loam site. However, stored soil water did not vary systematically among slope positions. Total vegetation cover was highest on the lowland in two sites, but was greatest on the midslope position in the other two. Total vegetation cover was greatest on the CL site, which was the wettest in terms of soil water. Soil water depletion was related to the depth-distribution of roots. There was an inverse relationship between aboveground production and soil water content of the 30, 45 and 60 cm layers during the growth period. Root distributions through the profile did not, however, vary with soil texture or with different soil water profiles controlled by texture. The less variable water content of deeper soil layers is a resource which potentially buffers the impact of pronounced variability in precipitation and thus contributes to vegetation stability of the shortgrass community.     

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.     

Landscape‐scale lidar analysis of aboveground biomass distribution in secondary Brazilian Atlantic Forest

Secondary forests account for more than half of tropical forests and represent a growing carbon sink, but rates of biomass accumulation vary by a factor of two or more even among plots in the same landscape. To better understand the drivers of this variability, we used airborne lidar to measure forest canopy height and estimate biomass over 4529 ha at Serra do Conduru Park in Southern Bahia, Brazil. We measured trees in 30 georeferenced field plots (0.25‐ha each) to estimate biomass using allometry. Then we estimated aboveground biomass density (ABD) across the lidar study area using a statistical model developed from our field plots. This model related the 95th percentile of the distribution of lidar return heights to ABD. We overlaid this map of ABD on a Landsat‐derived forest age map to determine rates of biomass accumulation. We found rapid initial biomass regeneration (~6 Mg/ha yr), which slowed as forests aged. We also observed high variability in both height and biomass across the landscape within forests of similar age. Nevertheless, a regression model that accounted for spatial autocorrelation and included forest age, slope, and distance to roads or open areas explained 62 and 77 percent of the landscape variation in ABD and canopy height, respectively. Thus, while there is high spatial heterogeneity in forest recovery, and the drivers of this heterogeneity warrant further investigation, we suggest that a relatively simple set of predictor variables is sufficient to explain the majority of variance in both height and ABD in this landscape.     

Hierarchical classification of switchgrass genotypes using SSR and chloroplast sequences: ecotypes,ploidies, gene pools,and cultivars

Switchgrass (Panicum virgatum L.) is an important crop for bioenergy feedstock development. Switchgrass has two main ecotypes: the lowland ecotype being exclusively tetraploid (2n = 4x = 36) and the upland ecotype being mainly tetraploid and octaploid (2n = 8x = 72). Because there is a significant difference in ploidy, morphology, growth pattern, and zone of adaptation between and within the upland and lowland ecotypes, it is important to discriminate switchgrass plants belonging to different genetic pools. We used 55 simple sequence repeats (SSR) loci and six chloroplast sequences to identify patterns of variation between and within 18 switchgrass cultivars representing seven lowland and 11 upland cultivars from different geographic regions and of varying ploidy levels. We report consistent discrimination of switchgrass cultivars into ecotype membership and demonstrate unambiguous molecular differentiation among switchgrass ploidy levels using genetic markers. Also, SSR and chloroplast markers identified genetic pools related to the geographic origin of the 18 cultivars with respect to ecotype, ploidy, and geographical, and cultivar sources. SSR loci were highly informative for cultivar fingerprinting and to classify plants of unknown origin. This classification system is the first step toward developing switchgrass complementary gene pools that can be expected to provide a significant heterotic increase in biomass yield.     

Environmental and Biotic Controls over Aboveground Biomass Throughout a Tropical Rain Forest

The environmental and biotic factors affecting spatial variation in canopy three-dimensional (3-D) structure and aboveground tree biomass (AGB) are poorly understood in tropical rain forests. We combined field measurements and airborne light detection and ranging (lidar) to quantify 3-D structure and AGB across a 5,016 ha rain forest reserve on the northeastern flank of Mauna Kea volcano, Hawaii Island. We compared AGB among native stands dominated by Metrosideros polymorpha found along a 600–1800 m elevation/climate gradient, and on three substrate-age classes of 5, 20, and 65 kyr. We also analyzed how alien tree invasion, canopy species dominance and topographic relief influence AGB levels. Canopy vertical profiles derived from lidar measurements were strong predictors (r ² = 0.78) of AGB across sites and species. Mean AGB ranged from 48 to 363 Mg ha⁻¹ in native forest stands. Increasing elevation corresponded to a 53–84% decrease in AGB levels, depending upon substrate age. Holding climate constant, changes in substrate age from 5 to 65 kyr corresponded to a 23–53% decline in biomass. Invasion by Psidium cattleianum and Ficus rubiginosa trees resulted in a 19–38% decrease in AGB, with these carbon losses mediated by substrate age. In contrast, the spread of former plantation tree species Fraxinus uhdei corresponded to a 7- to 10-fold increase in biomass. The effects of topographic relief at both local and regional scales were evident in the AGB maps, with poorly drained terrain harboring 76% lower biomass than forests on well-drained relief. Our results quantify the absolute and relative importance of environmental factors controlling spatial variation in tree biomass across a rain forest landscape, and highlight the rapid changes in carbon storage incurred following biological invasion. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Author Contributions  GPA and RFH conceived of or designed the study. GPA, RFH, TAV, DEK, and TKB performed research and analyzed data. GPA, RFH, DEK, and TKB contributed new methods or models. GPA wrote the article.     

Microtopography‐induced transient waterlogging affects switchgrass (Alamo) growth in the lower coastal plain of North Carolina,USA

Very limited information is currently available on growth responses of switchgrass (lowland cultivars) to transient waterlogging in lowland or poorly drained areas. This study investigated impacts of microtopography‐induced transient waterlogging on switchgrass (Alamo cultivar) growth, represented by leaf‐level gas exchange and biomass yield, in an established experimental field located in the Atlantic coastal plain of North Carolina, USA. Intensive leaf‐level gas exchange measurements were conducted on switchgrass at paired spots with distinct elevations in three sub‐blocks. Aboveground biomass was randomly collected across the study field to explore the potential impacts of the transient waterlogging on biomass yield. The sum of excess water (SEW) was calculated based on measured instantaneous water table depth to generalize the relationship between biomass yield and intensity of transient waterlogging. Results showed significant (P ≤ 0.0001) treatment effects on leaf‐level gas exchange, characterized by evident reduction in both CO₂ assimilation rate and stomatal conductance when water table was at or near the soil surface at low positions. Negative impacts of transient waterlogging on leaf‐level gas exchange became more evident with the increasing of elevation differences between paired subplots. Stomatal closure was found to be the main mechanism responsible for the decline of net assimilation under transient waterlogging. Aboveground biomass yields of switchgrass showed relatively high spatial variability and were positively and linearly correlated with microtopography (represented by elevation in the analysis) (P < 0.03, R² > 0.77). Further analysis showed that biomass yields were negatively correlated with SEW (P < 0.001, R² > 0.6) with an exponential relationship. Results of this study strongly demonstrated transient waterlogging could negatively affect switchgrass growth by suppressing leaf‐level gas exchange rates and ultimately reducing biomass yield. Findings from this study have critical implications for evaluating the economic viability of growing switchgrass on marginal lands that are subject to transient waterlogging stresses.     

Effects of ecotypes and morphotypes in feedstock composition of switchgrass (Panicum virgatum L.)

Switchgrass (Panicum virgatum L.) is a C4 grass with high biomass yield potential and is now a model species for the Bioenergy Feedstock Development Program. Two distinct ecotypes (e.g., upland and lowland) and a range of plant morphotypes (e.g., leafy and stemmy) have been observed in switchgrass. The objective of this study was to determine the influence of ecotype and morphotype on biomass feedstock quality. Leaf and stem tissues of leafy and stemmy morphotypes from both lowland and upland ecotypes were analyzed for key feedstock traits. The leaf : stem ratio of leafy morphotype was more than 40% higher than the stemmy morphotype in both upland and lowland ecotypes. Therefore, the stemmy morphotype has significant advantages over leafy morphotype during harvesting, storage, transportation and finally the feedstock quality. Remarkable differences in feedstock quality and mineral composition were observed in switchgrass genotypes with distinct ecotypic origins and variable plant morphotypes. Lignin, hemicelluloses and cellulose concentrations were higher in stems than in the leaves, while ash content was notably high in leaves. A higher concentration of potassium was found in the stems compared to the leaves. In contrast, calcium was higher and magnesium was generally higher in the leaves compared to stems. The upland genotypes demonstrated considerably higher lignin (14.4%) compared with lowland genotypes (12.4%), while hemicellulose was higher in lowland compared with upland. The stemmy type demonstrated slightly higher lignin compared with leafy types (P < 0.1). Differences between the ecotypes and morphotypes for key quality traits demonstrated the potential for improving feedstock composition of switchgrass through selection in breeding programs.     

Airborne LiDAR technique for estimating biomass components of maize: A case study in Zhangye City,Northwest China

Crop biomass is an important ecological indicator of growth, light use efficiency, and carbon stocks in agro-ecosystems. Light detection and ranging (LiDAR) or laser scanning has been widely used to estimate forest structural parameters and biomass. However, LiDAR is rarely used to estimate crop parameters because the short, dense canopies of crops limit the accuracy of the results. The objective of this study is to explore the potential of airborne LiDAR data in estimating biomass components of maize, namely aboveground biomass (AGB) and belowground biomass (BGB). Five biomass-related factors were measured during the entire growing season of maize. The field-measured canopy height and leaf area index (LAI) were identified as the factors that most directly affect biomass components through Pearson's correlation analysis and structural equation modeling (SEM). Field-based estimation models were proposed to estimate maize biomass components during the tasseling stage. Subsequently, the maize height and LAI over the entire study area were derived from LiDAR data and were used as input for the estimation models to map the spatial pattern of the biomass components. The results showed that the LiDAR-estimated biomass was comparable to the field-measured biomass, with root mean squared errors (RMSE) of 288.51 g/m² (AGB), and 75.81 g/m² (BGB). In conclusion, airborne LiDAR has great potential for estimating canopy height, LAI, and biomass components of maize during the peak growing season.     

The regional variation of aboveground live biomass in old-growth Amazonian forests

The biomass of tropical forests plays an important role in the global carbon cycle, both as a dynamic reservoir of carbon, and as a source of carbon dioxide to the atmosphere in areas undergoing deforestation. However, the absolute magnitude and environmental determinants of tropical forest biomass are still poorly understood. Here, we present a new synthesis and interpolation of the basal area and aboveground live biomass of old‐growth lowland tropical forests across South America, based on data from 227 forest plots, many previously unpublished. Forest biomass was analyzed in terms of two uncorrelated factors: basal area and mean wood density. Basal area is strongly affected by local landscape factors, but is relatively invariant at regional scale in moist tropical forests, and declines significantly at the dry periphery of the forest zone. Mean wood density is inversely correlated with forest dynamics, being lower in the dynamic forests of western Amazonia and high in the slow‐growing forests of eastern Amazonia. The combination of these two factors results in biomass being highest in the moderately seasonal, slow growing forests of central Amazonia and the Guyanas (up to 350 Mg dry weight ha^?1) and declining to 200–250 Mg dry weight ha^?1 at the western, southern and eastern margins. Overall, we estimate the total aboveground live biomass of intact Amazonian rainforests (area 5.76 × 10⁶ km² in 2000) to be 93±23 Pg C, taking into account lianas and small trees. Including dead biomass and belowground biomass would increase this value by approximately 10% and 21%, respectively, but the spatial variation of these additional terms still needs to be quantified.     

Large altitudinal increase in tree root/shoot ratio in tropical mountain forests of Ecuador

Tropical rain forests decrease in tree height and aboveground biomass (AGB) with increasing elevation. The causes of this phenomenon remain insufficiently understood despite a number of explanations proposed including direct or indirect effects of low temperature on carbon acquisition and carbon investment, adverse soil conditions and impaired nutrient supply. For analysing altitudinal patterns of aboveground/belowground carbon partitioning, we measured fine (<2 mm in diameter) and coarse root (2–5 mm) biomass and necromass and leaf area index (LAI), and estimated AGB from stand structural parameters in five tropical mountain rain forests at 1050, 1540, 1890, 2380 and 3060 m along an altitudinal transect in the South Ecuadorian Andes. Average tree height and AGB were reduced to less than 50% between 1050 and 3060 m, LAI decreased from 5.1 to 2.9. The leaf area reduction must have resulted in a lowered canopy carbon gain and thus may partly explain the reduced tree growth in the high-elevation stands. In contrast, both fine and coarse root biomass significantly increased with elevation across this transect. The ratio of root biomass (fine and coarse) to AGB increased more than ten-fold from 0.04 at 1050 m to 0.43 at 3060 m. Under the assumption that fine root biomass does reflect root productivity, our data indicate a marked belowground shift in C allocation with increasing elevation. Possible explanations for this allocation shift are discussed including reduced N supply due to low temperatures, water logging or adverse soil chemical conditions. We conclude that the fine root system and its activity may hold the key for understanding the impressive reduction in tree size along tropical mountain slopes in Ecuador. Analyses of fine root turnover and longevity in relation to environmental factors along altitudinal transects in tropical mountains are urgently needed.     

Variability in Leaf Area and Stemwood Increment Along a 300-year Lodgepole Pine Chronosequence

Large disturbances such as the 1988 Yellowstone fires produce considerable spatial heterogeneity in ecosystem processes across landscapes, in part by affecting vegetation structure. However, the persistence of this heterogeneity with time since disturbance, and thus the role of large disturbances in shaping the heterogeneity of ecosystem processes over large spatial and temporal scales, remains unclear. Such an inquiry requires that variability as well as mean conditions of forest structure and growth be examined if changes are to be projected for heterogeneous postdisturbance landscapes. We studied a chronosequence of unburned, mature lodgepole pine stands (stand ages ranging from 50 to 300 or more years) to examine the variability in stand density, leaf-area index (LAI), and stem growth [basal area increment (BAI), a surrogate for aboveground net primary productivity (ANPP)] with stand age, the relationships between these factors, and how these factors were related to stand and site characteristics. Variation in LAI and BAI was explained primarily by differences in stand density and age (r²=0.51 for both LAI and BAI), and both LAI and BAI were most variable in the youngest age class [coefficient of variation (CV), 38% and 41% for LAI and BAI]. The relationship between LAI or BAI and stand density was significantly weaker (r² < 0.20) at stand ages characterized by canopy closure (50–175 years), suggesting that stand structure and production are closely linked. Thus, the spatial variability of stand production, which is initially very high following large fires in this landscape, is detectable for over a century before successional changes in forest structure greatly affect the initial postdisturbance landscape pattern of stand production. Given the recent focus on spatial heterogeneity of ecosystem processes across large landscapes, projecting changes in postdisturbance patterns of stand production has very strong significance for ecosystem science.     

Root production, turnover and respiration under two grassland types along an altitudinal gradient: influence of temperature and solar radiation

 We have measured the rates of root production and death and of root respiration in situ under two grasslands along an altitudinal gradient in the northern Pennines, UK, represented by a lowland site at 171 m in an agricultural setting, and three upland sites between 480 and 845 m. One grassland was dominated by Festuca ovina and was on a brown earth soil; the other was dominated by Juncus squarrosus and Nardus stricta and occurred on a peaty gley. The natural altitudinal gradient was extended by transplantation. Although root biomass and root production (estimated using minirhizotrons) both showed pronounced seasonal peaks, there was no simple altitudinal gradient in either variable, and neither root production nor root death rate was a simple function of altitude. Increased root accumulation in summer was a function of change in the length of the growing season, not of soil temperature. Root populations in winter were similar at all sites, showing that increased production at some sites was accompanied by increased turnover, a conclusion confirmed by cohort analyses. Respiration rate, measured in the field by extracting roots and measuring respiration at field temperature in an incubator, was unrelated to temperature. The temperature sensitivity of respiration (expressed as the slope of a plot of log respiration rate against temperature) showed no simple seasonal or altitudinal pattern. Both root growth (under Festuca) and respiration rate were, however, closely related to radiation fluxes, averaged over the previous 10 days for growth and 2 days for respiration. The temperature sensitivity of respiration was a function of soil temperature at the time of measurement. These results show that root growth and the consequent input of carbon to soil in these communities is controlled by radiation flux not temperature, and that plants growing in these upland environments may acclimate strongly to low temperatures. Most carbon cycle models assume that carbon fluxes to soil are powerfully influenced by temperature, but that assumption is based largely on short-term studies and must be reassessed. Received: 11 August 1997 / Accepted: 25 October 1997