Ground Area Basis Research Articles

Exploration capacity versus specific enzymatic activity of ectomycorrhizas in response to primary productivity and soil phosphorus availability in Bornean tropical rainforests.

Ectomycorrhizal (ECM) fungi are functionally important in biogeochemical cycles in tropical ecosystems. Extracellular enzymatic activity of ECM on a ground-area basis is the product of two attributes; exploration capacity (ECM surface-area) and specific enzymatic activity. Here, we elucidated which attribute better explained the ECM enzymatic activity in response to different levels of soil phosphorus (P) and Nitrogen (N) availability in five Bornean tropical rainforests. We determined the surface area of ECM root tips as well as the enzymatic activities per ECM surface area for carbon (C), N and P degrading enzymes in each site. We evaluated the relationship of ECM enzyme activities with the resource availabilities of C (Above-ground net primary production; ANPP), N, and P of ECM by a generalized linear mixed model. The ECM enzymatic activities on a ground-area basis were more significantly determined by specific enzymatic activity than by the exploration capacity. Specific enzymatic activities were generally negatively affected by C (ANPP) and soil P availability. ECM fungi enhance the specific enzyme activity rather than the exploration capacity to maintain the capacity of nutrient acquisition. The less dependence of ECM fungi on the exploration capacity in these forests may be related to the limitation of C supply from host trees. We highlighted the adaptive mechanisms of ECM fungi on nutrient acquisition in tropical ecosystems through the response of enzymatic activity to nutrient availability across the elements.

Lolium perenne engineered for elevated leaf lipids exhibits greater energy density in field canopies under defoliation

Co-expression of diacylglycerol O -acyltransferase (DGAT1) and a lipid droplet-stabilizing protein called Cys-oleosin (collectively ‘HME’ technology) increases foliar lipid concentrations and plant biomass, which could elevate energy yields from perennial ryegrass (PR, Lolium perenne L.)-based pastures. Quantifying the feasible benefits of HME PR necessitates that herbage growth and energy density are quantified on a ground area basis under regular defoliation. We examined the translation, from spaced pots indoors to field canopies (swards), of the fatty acid (FA), gross energy (GE), and biomass enhancing traits associated with HME technology expression in PR. Under controlled indoor conditions in spaced pots, hemizygous HME seedlings with 32% higher leaf FA content (1.2%DW) than null controls exhibited a faster rate of leaf extension. These effects translated into a stable 0.2–0.5 kJ/gDW increase in herbage GE concentration, and a 6–10% herbage growth rate advantage when HME ‘miniswards’ were grown under moderate light competition (LAI 7–9) for five defoliation and regrowth cycles (harvests). However, subsequent growth in a dense canopy (LAI>10) eliminated this HME herbage growth advantage. In two field trials in the Midwest United States, hemizygous HME swards exhibited 25–34% (0.8–1%DW) higher end-of-season herbage FA content than null control swards, and 0.2–0.5 kJ/gDW higher end-of-season herbage GE. Herbage growth rates were generally similar for HME and null control swards across both field seasons. This thorough analysis of trait translation shows that the benefits of enhanced FA and GE translate to HME PR canopies grown in the field. The intrinsic growth advantage of HME PR may be diminished in field canopies such that yield benefits are realized only under limited conditions.

Isoprene and monoterpene emissions from alder, aspen and spruce short-rotation forest plantations in the United Kingdom

Abstract. An expansion of bioenergy has been proposed to help reduce fossil-fuel greenhouse gas emissions, and short-rotation forestry (SRF) can contribute to this expansion. However, SRF plantations could also be sources of biogenic volatile organic compound (BVOC) emissions, which can impact atmospheric air quality. In this study, emissions of isoprene and 11 monoterpenes from the branches and forest floor of hybrid aspen, Italian alder and Sitka spruce stands in an SRF field trial in central Scotland were measured during two years (2018–2019) and used to derive emission potentials for different seasons. Sitka spruce was included as a comparison as it is the most extensive plantation species in the UK. Winter and spring emissions of isoprene and monoterpenes were small compared to those in summer. Sitka spruce had a standardised mean emission rate of 15 µgCg-1h-1 for isoprene in the dry and warm summer of 2018 – more than double the emissions in 2019. However, standardised mean isoprene emissions from hybrid aspen were similar across both years, approximately 23 µgCg-1h-1, and standardised mean isoprene emissions from Italian alder were very low. Mean standardised total monoterpene emissions for these species followed a similar pattern of higher standardised emissions in the warmer year: Sitka spruce emitting 4.5 and 2.3 µgCg-1h-1 for 2018 and 2019, aspen emitting 0.3 and 0.09 µgCg-1h-1, and Italian alder emitting 1.5 and 0.2 µgCg-1h-1, respectively. In contrast to these foliage emissions, the forest floor was only a small source of monoterpenes, typically 1 or 2 orders of magnitude lower than foliage emissions on a unit of ground area basis. Estimates of total annual emissions from each plantation type per hectare were derived using the MEGAN 2.1 model. The modelled total BVOC (isoprene and monoterpenes) emissions of SRF hybrid aspen plantations were approximately half those of Sitka spruce for plantations of the same age. Italian alder SRF emissions were 20 times smaller than from Sitka spruce. The expansion of bioenergy plantations to 0.7 Mha has been suggested for the UK to help achieve net-zero greenhouse gas emissions by 2050. The model estimates show that, with such an expansion, total UK BVOC emissions would increase between <1 % and 35 %, depending on the tree species planted. Whereas increases might be small on a national scale, regional increases might have a larger impact on local air quality.

Soil faunal effect on plant litter decomposition in mineral soil examined by two in-situ approaches: Sequential density-size fractionation and micromorphology

Potentially significant effects of soil fauna on plant litter decomposition are difficult to quantify by the commonly used litter bag approach due to physical constraints of meshed bag and inevitable loss of litter fragments through the mesh. We thus tested the applicability of two bag-free approaches, physical fractionation and micromorphological analysis, to examine litter decomposition in an Andosol under a temperate humid climate. In-situ decomposition of two winter cover crop litter, C-rich wild oat (WO) and N-rich hairy vetch (HV), was compared with a conventionally farmed (CF) field which received no litter as a control. The surface soils (Ap holizons) samples taken at 0, 2, and 8 weeks after the litter incorporation were separated by density to recover a low-density fraction (LF, <1.6 g cm−3) followed by its size fractionation via wet sieving (4.0, 2.0, 1.0, 0.5, and 0.1 mm mesh sizes) based on the idea that litter fragmentation occurs in parallel with decomposition and that the size of the faunal excrements depends on that of the organism. While the LF total mass did not show clear change over the eight weeks in all treatments, its size distribution exhibited a significant decline in >4 mm size fraction and concurrent increases in 0.5–2 mm fractions primarily as macrofauna excrements in the WO field. While the loss due to its fragmentation below 0.1 mm is possible, the identified excrements (1–2 mm size fractions) produced over eight-weeks accounted for 4.6 ± 0.2% and 5.5 ± 0.4%, of the C and N present in the WO litter on a ground area basis, and 0.67 ± 0.01% and 0.43 ± 0.01% of the surface soil C and N, respectively. Micromorphology of thin soil sections confirmed the abundance of larvae (Coleoptera) excrements that were mainly comprised of WO stems. In contrast, the faunal excrements found in HV soil were largely from mesofauna (mites). The significant difference found between the litter decomposition patterns was partly attributable to the faunal activity. Specifically, the decomposition of the tough tissues of WO was promoted by macrofauna, while that of HV legume litter was associated with mesofauna without any indications of macrofaunal influence. The physical fractionation based approach we developed here allowed to estimate the production rate of macrofaunal excrements under the field condition for the first time, which can help to elucidate the impact of soil fauna on biogeochemical cycling.

Distinctive Roles of Two Aggregate Binding Agents in Allophanic Andisols: Young Carbon and Poorly-Crystalline Metal Phases with Old Carbon

Interaction of organic matter (OM) with soil mineral components plays a critical role in biophysical organization (aggregate structure) as well as in biogeochemical cycling of major elements. Of the mineral components, poorly-crystalline phases rich in iron (Fe) and aluminum (Al) are highly reactive and thus contribute to both OM stabilization and aggregation. However, the functional relationship among the reactive metal phases, C stability, and aggregation remains elusive. We hypothesized that relatively young C acts as a binding agent to form the aggregates of weak physical stability, whereas the reactive metal phases and older C bound to them contribute to stronger aggregation. Using four surface horizons of Andisols having a gradient of soil C concentration due to decadal OM management, we conducted sequential density fractionation to isolate six fractions (from &lt;1.6 to &gt;2.5 g cm−3) with mechanical shaking, followed by selective dissolution and radiocarbon analysis. After 28 years of no-till with litter compost addition, not only C and N but inorganic materials including the reactive metal phases (pyrophosphate-, oxalate-, and dithionite-extractable metals) showed clear shifts in their concentrations towards lower-density fractions (especially &lt;2.0 g cm−3) on a ground-area basis. This result was explained by the binding of compost-derived OM with soil particles. Major portions of the reactive metal phases in bulk samples were distributed in mid-density fractions (2.0–2.5 g cm−3) largely as sonication-resistant aggregates. Theoretical density calculations, together with depletion in radiocarbon (Δ14C: −82 to −170‰) and lower C:N ratio, implied that the sorptive capacity of the reactive metal phases in these fractions were roughly saturated with pre-existing OM. However, the influx of the compost-derived, modern C into the mid-density fractions detected by the paired-plot comparison suggests decadal C sink in association with the reactive metal phase. Our results supported the concept of aggregate hierarchy and further provided the following new insights. At the high hierarchy level where shaking-resistant aggregates form, soil organo-mineral particles appeared to be under a dynamic equilibrium and the changes in OM input regime controlled (dis)aggregation behavior due to the binding effect of relatively young C. At a lower hierarchy level, the reactive metal phases were bound to N-rich, 14C-depleted OM and together functioned as persistent binding agent. Our study suggests that the recognition of binding agents and aggregate hierarchy level would help to untangle the complex organo-mineral interactions and to better understand soil C stability.

The effects of subsurface irrigation at different soil water potential thresholds on the growth and transpiration of Populus tomentosa in the North China Plain

SummaryIn order to find the optimal subsurface drip irrigation (SDI) scheduling for mature triploid Populus tomentosa plantations in the North China Plain, a field experiment was conducted in 2010 and 2011 to investigate the effects of SDI at different soil water potential (SWP) thresholds on the growth and transpiration of a P. tomentosa plantation when it was six and seven years old. The experiment included three SWP treatments, which initiated irrigation when the SWP at 20 cm depth and 10 cm distance from a drip emitter reached −25 (T25), −50 (T50) and −75 (T75) kPa, respectively. A control non-irrigation treatment (CK) was also included. Long-term SWP, soil water content (SWC), transpiration, tree growth, meteorological factors and groundwater level were monitored. Results showed that SDI influenced the SWC only in 0–80 cm of soil. From April to July, on average, the cumulative stand-level transpiration on a ground area basis (E) and growth of basal area at breast height accounted for 81% and 93% of their corresponding whole-season values, while the cumulative reference crop potential evapotranspiration (ET0) was 43% higher than the rainfall. In contrast, from August to October, the growth rate of P. tomentosa was very slow, while the cumulative rainfall was 36% higher than ET0 and the average groundwater level was relatively high (125 cm). Relative to CK, the E under the SWP treatments was significantly (P < 0.05) increased by 20–73%. Decreasing the SWP irrigation threshold from −25 to −50 kPa significantly reduced E by 31% (P < 0.05), but decreasing the threshold from −50 to −75 kPa did not further reduce P. tomentosa E. Relative to CK, T25, T50 and T75 increased the annual above-ground dry biomass (ADB) increment by 54% (P < 0.05), 34% (P < 0.05) and 24% (P > 0.05) in 2010, and by 28% (P > 0.05), 29% (P > 0.05) and 32% (P < 0.05) in 2011, respectively. However, no significant difference in ADB increment was detected among the SWP treatments. Based on these results, it can be concluded that when planting P. tomentosa at sites with similar characteristics to ours in the North China Plain: (1) SDI could be promoted in the cultivation of P. tomentosa to improve tree growth; (2) a range of −50 to −75 kPa at a depth of 20 cm and 10 cm distant from a drip emitter is recommended as the irrigation threshold for scheduling SDI in P. tomentosa plantations and (3) irrigation should be applied between April and July, while drainage should be implemented between August and October.

永定河沿河沙地杨树人工林蒸腾耗水特征及其环境响应

Poplar plantations are the most prevalent and the fastest expanding plantations in Northern China. However,the high water use of poplars necessitates careful consideration of their suitability for water-stressed regions. It is essential to find out the relationships between the water transpired by poplar plantation and environmental factors during different growing periods,which is very important in poplar plantation sustainable management. Many researchers have done a lot of job on this subject,but few referred to a mature fast growing poplar stand. In the current study the water use and response to environmental factors was evaluated by simultaneous measurements of sap flow using the TDP( Thermal Dissipation Probe)method,micrometeorological conditions and soil moisture in a 13-year old poplar( Populus euramericana cv. 74 / 76 )plantation located on the floodplain of Yongding River in Daxing district,the southern suburb of Beijing,China from 2010 to 2011. Our objectives were to illustrate the water used for transpiration by this 13-year old fast growing poplar plantationand explore how the tree / stand transpiration responses to the different environmental conditions. The results showed that the daily pattern of sap flow was a bell curve. The daily water consumption of individual trees ranged from 6.4 to 59.7 kg / d,varying by season and sapwood which was calculated through the trunk diameter at breast height. On a ground area basis,the growing season transpiration was 114 and 175 mm in 2010 and 2011,respectively. It accounted for 30% —37% of precipitation over the same periods. Compared to earlier reports for poplars in literature,the observed water use was relatively low,likely attributable to growing age and no available irrigation during the study period. A diurnal hysteresis was observed between sap flow and vapor pressure deficit( VPD). Similarly,this situation was also true between sap flow and net radiation( Rn). The hysteresis was greater with high than low soil moisture,suggesting that transpiration did response to the change of soil moisture though indirectly. On daily scale,significant correlations were observed between daily stand transpiration and Rn,VPD and soil moisture. Besides,the relationship between transpiration and soil moisture was related to the depth of soil. The soil moisture under the surface 30—90cm affected transpiration most effectively. When the soil moisture changed,the relationship between stand transpiration and Rn or VPD varied. The growth of VPD stimulated the stand transpiration when it was not greater than 1 k Pa. The over great value of VPD would slow down the growth of stand transpiration. Compared to the data of earlier research at this site,we found that the transpiration of the plantation declined with the stand age. On a monthly and annual scale,transpiration was correlated with rainfall,suggesting limited soil water storage. Simultaneously,we found that during dry months when transpiration could not be satisfied by rainfall,soil water storage offered the water to be transpired. However,the source of soil water storage was also come from rainfall thus the transpiration between 2010 and 2011 were quite different because of great rainfall difference in these two years.

Phreatophytes under stress: transpiration and stomatal conductance of saltcedar (Tamarix spp.) in a high-salinity environment

We sought to understand the environmental constraints on an arid-zone riparian phreatophtye, saltcedar (Tamarix ramosissima and related species and hybrids), growing over a brackish aquifer along the Colorado River in the western U.S. Depth to groundwater, meteorological factors, salinity and soil hydraulic properties were compared at stress and non-stressed sites that differed in salinity of the aquifer, soil properties and water use characteristics, to identify the factors depressing water use at the stress site. Saltcedar leaf-level transpiration (EL), LAI, and stomatal conductance (GS) were measured over a growing season (June–September) with Granier and stem heat balance sensors and were compared to those for saltcedar at the non-stress site determined in a previous study. Transpiration on a ground-area basis (EG) was calculated as EL × LAI. Environmental factors were regressed against hourly and daily EL and GS at each site to determine the main factors controlling water use at each site. At the stress site, mean EG over the summer was only 30 % of potential evapotranspiration (ETo). GS and EG peaked between 8 and 9 am then decreased over the daylight hours. Daytime GS was negatively correlated with vapor pressure deficit (VPD) (P < 0.05). By contrast, EG at the non-stress site tracked the daily radiation curve, was positively correlated with VPD and was nearly equal to ETo on a daily basis. Depth to groundwater increased over the growing season at both sites and resulted in decreasing EG but could not explain the difference between sites. Both sites had high soil moisture levels throughout the vadose zone with high calculated unsaturated conductivity. However, salinity in the aquifer and vadose zone was three times higher at the stress site than at the non-stress site and could explain differences in plant EG and GS. Salts accumulated in the vadose zone at both sites so usable water was confined to the saturated capillary fringe above the aquifer. Existence of a saline aquifer imposes several types of constraints on phreatophyte EG, which need to be considered in models of plant water uptake. The heterogeneous nature of saltcedar EG over river terraces introduces potential errors into estimates of ET by wide-area methods.

Scaling foliar respiration to the stand level throughout the growing season in a Quercus rubra forest

Stand-level, canopy foliar carbon loss (R(can)) was modeled for a virtual Quercus rubra L. monoculture at two sites differing in soil water availability in a northeastern deciduous forest (USA) throughout the 2003 growing season. Previously reported foliar respiratory temperature responses of Q. rubra were used to parameterize a full distributed physiology model that estimates R(can) by integrating the effects of season, site and canopy position, and represents the best estimation of R(can). Model sensitivity to five simplified parameterization scenarios was tested, and a reasonable procedure of simplification was established. Neglecting effects of season, site or canopy position on respiration causes considerable relative error in R(can) estimation. By contrast, assuming a constant E(0) (a temperature response variable of the respiration model), or a constant night temperature (mean nighttime temperature) caused only a small relative error (< 10%) compared with the full model. From June 8 to October 28, 2003, modeled R(can) of the virtual Q. rubra monoculture was, on average, 45.3 mmol CO(2) m(-2) night(-1) on a ground-area basis (or 334 mmol CO(2) kg(-1) night(-1) on a biomass basis) and 101 mmol CO(2) m(-2) night(-1) (or 361 mmol CO(2) kg(-1) night(-1)) at the drier site and the more mesic site, respectively. To model R(can) of Q. rubra (or other Quercus species with similar respiratory properties), variations in the base respiration rate across season, site and canopy position need to be fully accounted for, but E(0) may be assumed constant. Modeling R(can) at the mean nighttime temperature would not strongly affect estimated canopy carbon loss.

Influence of irrigation and fertilization on transpiration and hydraulic properties of Populus deltoides

Long-term hydraulic acclimation to resource availability was explored in 3-year-old Populus deltoides Bartr. ex Marsh. clones by examining transpiration, leaf-specific hydraulic conductance (G(L)), canopy stomatal conductance (G(S)) and leaf to sapwood area ratio (A(L):A(S)) in response to irrigation (13 and 551 mm year(-1) in addition to ambient precipitation) and fertilization (0 and 120 kg N ha(-1) year(-1)). Sap flow was measured continuously over one growing season with thermal dissipation probes. Fertilization had a greater effect on growth and hydraulic properties than irrigation, and fertilization effects were independent of irrigation treatment. Transpiration on a ground area basis (E) ranged between 0.3 and 1.8 mm day(-1), and increased 66% and 90% in response to irrigation and fertilization, respectively. Increases in G(L), G(S) at a reference vapor pressure deficit of 1 kPa, and transpiration per unit leaf area in response to increases in resource availability were associated with reductions in A(L):A(S) and consequently a minimal change in the water potential gradient from soil to leaf. Irrigation and fertilization increased leaf area index similarly, from an average 1.16 in control stands to 1.45, but sapwood area was increased from 4.0 to 6.3 m(2) ha(-1) by irrigation and from 3.7 to 6.7 m(2) ha(-1) by fertilization. The balance between leaf area and sapwood area was important in understanding long-term hydraulic acclimation to resource availability and mechanisms controlling maximum productivity in Populus deltoides.

Acclimation of Wax Begonia to Light Intensity: Changes in Photosynthesis, Respiration, and Chlorophyll Concentration

Physiological acclimation of plants to light has been studied mostly at the leaf level; however whole-plant responses are more relevant in relation to crop growth. To examine the physiological changes associated with different daily light integrals (DLI) during growth of wax begonia (Begonia semperflorens-cultorum Hort.), we grew plants under DLI of 5.3, 9.5, 14.4, and 19.4 mol·m-2·d-1 in a whole-plant gas exchange system. Photosynthesis-light response curves of groups of 12 plants were determined after 25 d of growth. Physiological parameters were estimated per m2 ground area and per m2 leaf area. On a ground area basis, significant increases in dark respiration (Rd), quantum yield (α), the light compensation point (LCP), and maximum gross photosynthesis (Pg,max) were seen with increasing DLI. Variations in physiological parameters among different treatments were small when corrected for differences in leaf area. On a leaf area basis, α, LCP, and the light saturation point (LSP) did not change significantly, whereas significant increases in Rd and Pg,max were seen with increasing DLI. There was a small decrease in leaf chlorophyll concentration (6.3%, measured in SPAD units) with increasing DLI. This study indicates that wax begonias acclimate to low DLI by increasing their leaf chlorophyll concentration, presumably to more efficiently capture the available light, and to high DLI by increasing Pg,max to efficiently utilize the available light, thereby maximizing carbon gain under both situations.

Estimating yields of tropical maize genotypes from non-destructive, on-farm plant morphological measurements

Maize is the main grain crop grown in the highlands of sub-Saharan Africa, on a broad range of soil fertility and management conditions. Important yield variability has been reported at different scales, reflecting the intensity and spatial distribution of growth-limiting and growth-reducing factors. Maize yield estimation represents a valuable tool to assess within-farm variability in soil fertility through crop performance. The objective of this study was to develop mathematical relationships between plant morphological attributes and grain yield of tropical maize genotypes, based on plant allometric characteristics. These models were used to estimate maize yields and the estimates were validated against independent data collected from experimental and farmers’ fields in western Kenya. Three commercial hybrids and three local varieties were considered. Multiple linear regression models including plant height and either ear length or ear diameter as explanatory variables, and simple linear regressions including only plant height, were the most accurate to estimate both total aboveground biomass and grain dry matter yields per plant ( r 2 : 0.76–0.91). Average values for the harvest index ranged between 0.34 and 0.42, varying with the total aboveground biomass produced per plant. Yield estimations on ground area basis for farmers’ fields were acceptably accurate. Plant height measurements can be easily taken at any moment after maize flowering and used in simple models to estimate maize yield. This approach proved also a valuable tool to discuss yield variability with farmers.

Coupling whole-tree transpiration and canopy photosynthesis in coniferous and broad-leaved tree species

We used sap flow as a measure of whole-tree function to examine how coniferous and broad-leaved species in mixed temperate forests differ in canopy-level transpiration and photosynthetic rates. We used heat dissipation probes to measure whole-tree sap flow in three species throughout one full year and then combined these measurements with micrometeorological monitoring and leaf-level gas exchange to determine whole-tree carbon gain. Both broad-leaved species (red oak, Quercus rubra L.; red maple, Acer rubrum L.) had two- to four-fold greater annual fluxes of water and carbon on a ground area basis than did the conifer (eastern hemlock, Tsuga canadensis (L.) Carrière), with red oak trees additionally showing 60–80% higher fluxes than red maple. Despite fixing one-third of its carbon when broad-leaved species were leafless, hemlock was not able to compensate for its low photosynthetic rates during the growing season. Productivity measures derived from annual growth rings and eddy covariance confirmed that whole-tree sap flow provided a valuable estimate of both the magnitude of current forest fluxes and differences in individual species' fluxes. Our results indicate that the predicted loss of hemlock from mixed temperate forests could potentially increase whole-forest water loss and carbon gain by two- to four-fold, provided sufficient nitrogen and water remain available to support such a change.

Deuterium enriched irrigation indicates different forms of rain use in shrub/grass species of the Colorado Plateau.

We contrasted the seasonal use of simulated large rain events (24mm) by three native species of the arid Colorado Plateau: the perennial grass Hilaria jamesii and two shrubs Artemesia filifolia and Coleogyne ramosissima. Deuterium-enriched water was used to distinguish shallow "pulse" water from water in deeper soil layers that were unaffected by the water input. We also measured the leaf gas exchange rates of watered and unwatered control plants for 5days after the rain event. H. jamesii had twice the pulse water proportion in its xylem than the two shrubs in spring (approx. 70% vs 35%). In summer, the pulse water proportions of all species were around 70%. The increase in the relative pulse water uptake of the two shrubs was caused primarily by a reduction in the rate of water uptake from deeper sources, consistent with the decrease in the availability of stored winter water. Rain increased the rates of gas exchange in C. ramosissima in both seasons, in H. jamesii only in summer and had no significant effect on A. filifolia. In H. jamesii, summer rain also increased water use efficiency. This suggests three principle mechanisms for rainwater use: (1) immediate increase in gas exchange via stomatal opening (C. ramisissima), (2) immediate increase in water use efficiency through restoration of the photosynthetic apparatus (H. jamesii) and (3) conservation of deeper soil water, potentially extending photosynthetic activity into later drought periods (A. filifolia). On a ground-area basis, A. filifolia was by far the largest consumer of spring and summer rain, due to its greater ground cover, while rain use by H. jamesii was negligible. We hypothesize that a population's fraction of the total community Leaf Area Index, more than species identity, determines which species takes up most of the spring and summer precipitation and we discuss this idea in the context of Walter and Stadelmann's (1974, In: Brown JW Jr (ed) Desert biology. Academic Press, New York, pp 213-310) water partitioning hypothesis.

Plant Growth Response of Two Runner Peanut Cultivars to Reduced Seeding Rate1

Abstract The ability of peanut plants to adjust to wide plant-to-plant spacing and the use of more precise mechanical planters have not been thoroughly exploited to reduce seeding rates. The magnitude of plant growth adjustment of two runner peanut cultivars to reduced seeding rate was studied in 1992 and 1993. A precision vacuum planter was used to plant the cultivars GK-7 and Southern Runner (SR) in single rows per bed at 8, 12, and 22 seed/m2. Total plant dry matter weight (TDM), leaf area (LA), and main stem height, nodes, and branches were measured six times during each season. Vegetative growth adjustment to reduced seeding rate was detected as early as 52 d after planting (DAP) in 1992 and 42 DAP in 1993. Plants fully adjusted to reduced seeding rate when they began forming pods at 70 DAP in 1992 and 75 DAP in 1993. By this stage, individual plants in the 8 seed/m2 treatment accumulated 2 to 2.7 times as much total dry matter (TDM) as plants in the 22 seed/m2, and all seeding treatments within each cultivar accumulated equivalent amounts of TDM on a unit ground area basis. The 8 seed/m2 treatment produced significantly greater TDM/m2 and leaf area index (LAI) than the 22 seed/m2 treatment by the end of both seasons (132 DAP in 1992 and 152 DAP in 1993). The partitioning of dry weight to pods and leaves was also greatest for the 8 seed/m2 seeding treatment. By the end of the season, the 8 seed/ m2 seeding treatment produced as much as 30% more pod dry weight (PDW) per m2 than the 22 seed/m2 treatment. Plants in the 8 seed/m2 seeding were significantly shorter but produced more main stem nodes and branches than the 22 seed/m2 seeding. Under the conditions of this study, plants of these runner cultivars showed the capacity to grow larger and compensate fully for reduced seeding rate before any measurable pod production began. Pod production, therefore, was not reduced and in some situations increased by reducing the seeding rate to as low as 8 seed/m2.

Yield improvements through modification of planting density and harvest frequency in short rotation coppice Salix spp.—1. Yield response in two morphologically diverse varieties

An experiment comparing the dry matter yield of intensively managed short rotation coppice (SRC) under factorial combinations of two plant varieties ( S. viminalis cv. Jorunn and S. x dasyclados), five planting densities (10,000– 111,000 plants ha −1 ) and two harvesting frequencies is reported. Data are presented from the first biennial and triennial harvest cycles at two sites (East Anglia and Warwickshire, UK) planted in spring 1996 and flailed in autumn 1996. Higher annual yields were attainable by more intensive packing of plants combined with more frequent harvesting. With S. viminalis cv. Jorunn, yield increased by 34% between the lowest and highest planting densities. Biennial harvesting increased yield compared with triennial harvests. S. viminalis, on average, yielded 2.7 t ha −1 yr −1 more than S. x dasyclados, with peak yields of 11.4 t ha −1 yr −1 . Significant differences in yield between sites were noted, with an average of 1 t ha −1 yr −1 benefit on a mineral soil compared with a peaty loam. There were no statistically significant interactions in the data. Biennial plots reached peak heights of 254– 297 cm ( S. viminalis) and 239– 273 cm ( S. x dasyclados) in 1998. Peak heights of triennial plots (achieved in 1999) were 401– 515 cm ( S. viminalis) and 316– 420 cm ( S. x dasyclados). There was a gradual increase in height through the three years and a non-significant trend towards decreased height with increasing planting density at the ends of the growing seasons. Both varieties demonstrated phenotypic plasticity, with individual plant weight and stem number decreasing as a function of increasing density. A range of parameters including stem width reduction, stem mortality and plant mortality were seen to vary. The population of primary stems was found to increase up to the time of canopy closure, on a per plant basis and per unit ground area basis. After this time, competition for light resulted in self-thinning of stems through until final harvest for S. dasyclados but not S. viminalis. Stem numbers per plant, recorded after leaf fall, showed different responses between both harvesting frequency and variety. With S. viminalis, stem numbers remained constant after cut back for up to 2 yr , but increased rapidly as a result of a harvest after 2 yr . With S. x dasyclados, a similar regrowth of stems was observed after a 2-yr harvest. Prior to the harvest, stem numbers per plant decreased steadily for 2 of the 3 yr depending on harvest cycle. After 3 yr stem populations with S. x dasyclados were also decreasing. The degree to which non-destructive measurements could be used to (a) determine annual increment and (b) be used to predict eventual yield was examined. Cylindrical volume was the only growth measurement that enabled a reasonable fit between crop morphology and final yield to be made. The relationship was found to be a good fit and the fit improved with a longer harvest interval. Regression equations were not significantly different between sample sites but were significantly different between both variety ( P<0.001) and harvest interval ( P<0.05). In summary, we deduce that modern varieties of S. viminalis such as Jorunn, are more suited to higher planting density and intensive harvesting, due to more erect growth reducing intra-specific competition at high planting densities.

Control of transpiration in a 220-year-old Abies amabilis forest

We measured sap flow at the branch and tree levels, and calculated tree transpiration at the stand level, in a 220-year-old Abies amabilis (Dougl.) Forbes forest. Temporal and spatial patterns of branch sap flow rate per unit leaf area reflected differences in canopy position and diurnal variation in radiation exposure. Average leaf area normalized branch conductance of upper canopy branches ranged from 0.50 to 1.01 mm s −1. Maximum leaf area normalized tree sap flow rates were similar to those previously measured in a younger A. amabilis forest (about 80 g m −2 leaf area h −1), but dominant trees in the old growth stand transpired approximately three times more per day (up to 281 kg H 2O per day) than dominant trees in the younger forest. This difference was attributed primarily to leaf area: dominant trees in the old growth stand had approximately three times more leaf area than those in the younger Abies stand. Crown conductance on a ground area basis varied by an order of magnitude between the smallest and largest trees measured (maximum 4.84 mm s −1 and mean 1.44 mm s −1 versus maximum 37.54 mm s −1 and average 12.16 mm s −1, respectively). There was considerable spatial and temporal variation in stomatal versus boundary layer control of transpiration as expressed by the Ω decoupling coefficient. Ω for the largest measured tree ranged from less than 0.1 to greater than 0.6, and remained above 0.3 for more than 9% of the daylight hours monitored. The degree of decoupling decreased with tree size, with Ω of the smallest tree sampled never exceeding 0.3. Daily stand transpiration ranged from less than 0.4 mm to greater than 3.3 mm depending on radiation and vapor pressure deficit.

Effects of nutrition and soil water availability on water use in a Norway spruce stand.

We investigated effects of nutrition and soil water availability on sap flux density, transpiration per unit leaf area (EL), and canopy stomatal conductance (GS) of Norway spruce (Picea abies L. (Karst.)) in northern Sweden during the 1996 growing season. Our objectives were to determine (1) if artificially imposed drought (65% rain diversion) reduces soil water sufficiently to cause physiological limitations to whole-tree and plot-scale water transport, and (2) whether increased capacity for water transport resulting from fertilization-induced increases in leaf (> 3-fold) and sapwood areas (> 2.3-fold) deplete soil water sufficiently to cause a negative feedback on GS and EL. We monitored soil water content (theta) and soil water potential (PsiS) in control (C), drought (D), fertilized (F) and irrigated + fertilized (IL) treatment plots, along with site meteorological conditions. Ten trees per plot were monitored for sap flow. Although there were significant treatment differences in mean daily EL (C > D > F; P < 0.01) and GS (C > D > F; P < 0.05), variation in absolute magnitudes was small. Therefore, transpiration differences on a unit ground area basis (EC) were nearly proportional to leaf area differences. Precipitation was well distributed throughout the study period and so PsiS remained high, except during short dry periods in Plot F when it declined rapidly. Thus, although soil water was not limiting to GS, EL or EC when precipitation was uniformly distributed throughout the growing season, we cannot conclude that water availability would not limit GS in fertilized stands if the seasonal distribution of precipitation were altered.

Measuring and modeling conductances of black spruce at three organizational scales: shoot, branch and canopy.

To investigate the extent to which the energy balance of a globally important ecosystem is controlled by biological and environmental processes, measurements of water vapor flux were made on individual black spruce (Picea mariana [Mill.] B.S.P.) shoots, branches, and a whole canopy at the BOREAS Southern Study Area Old Black Spruce (SSA OBS) site. These measurements were used to estimate stomatal, branch boundary layer and canopy boundary layer conductances to water vapor. On a projected needle area basis, stomatal conductances varied between 14 and 92 mmol m(-2) s(-1), and total branch conductance varied seasonally between zero and about 35 mmol m(-2) s(-1). On a ground area basis, total canopy conductance varied between 24 and 105 mmol m(-2) s(-1). Total canopy conductance was partitioned into aerodynamic and physiological components by using shoot-scale measurements scaled by leaf area index. Good agreement was found with an independent estimate of aerodynamic conductance measured when the canopy was wet. Compared with most coniferous forests, the canopy was relatively uncoupled from the atmosphere, and at the ecosystem scale, the control of water vapor flux was approximately equally divided between physiological and abiotic conductances. Two widely used steady-state models of stomatal conductance were parameterized from the shoot and branch measurements. Parameters varied considerably throughout the growing season. A time-constant term was added to these static models to construct dynamic models of stomatal conductance under naturally varying environmental conditions. The dynamic versions of the models outperformed the static versions in explaining stomatal response to rapidly changing environmental conditions. The length of the time-constant term, derived using the dynamic models, suggested that stomata were slow to respond to changing environmental conditions, and that the speed of the response was strongly temperature-dependent.

Seasonal soil‐surface carbon fluxes from the root systems of young Pinus radiata trees growing at ambient and elevated CO2 concentration

SummaryElevated atmospheric CO2 concentration may result in increased below‐ground carbon allocation by trees, thereby altering soil carbon cycling. Seasonal estimates of soil surface carbon flux were made to determine whether carbon losses from Pinus radiata trees growing at elevated CO2 concentration were higher than those at ambient CO2 concentration, and whether this was related to increased fine root growth.Monthly soil surface carbon flux density (f) measurements were made on plots with trees growing at ambient (350) and elevated (650 μmol mol−1) CO2 concentration in large open‐top chambers. Prior to planting the soil carbon concentration (0.1%) and f (0.28 μmol m−2 s−1 at 15 °C) were low. A function describing the radial pattern of f with distance from tree stems was used to estimate the annual carbon flux from tree plots. Seasonal estimates of fine root production were made from minirhizotrons and the radial distribution of roots compared with radial measurements of f. A one‐dimensional gas diffusion model was used to estimate f from soil CO2 concentrations at four depths.For the second year of growth, the annual carbon flux from the plots was 1671 g y−1 and 1895 g y−1 at ambient and elevated CO2 concentrations, respectively, although this was not a significant difference. Higher f at elevated CO2 concentration was largely explained by increased fine root biomass. Fine root biomass and stem production were both positively related to f. Both root length density and f declined exponentially with distance from the stem, and had similar length scales. Diurnal changes in f were largely explained by changes in soil temperature at a depth of 0.05 m.Ignoring the change of f with increasing distance from tree stems when scaling to a unit ground area basis from measurements with individual trees could result in under‐ or overestimates of soil‐surface carbon fluxes, especially in young stands when fine roots are unevenly distributed.