Root length density and carbon content of agroforestry and grass buffers under grazed pasture systems in a Hapludalf

Enhancement of root development helps to improve soil physical properties, carbon sequestration, and water quality of streams. The objective of this study was to evaluate differences in root length density (RLD) and root and soil carbon content within grass buffer (GB), agroforestry buffer (AgB), rotationally grazed pasture (RG) and continuously grazed pasture (CG) treatments. Pasture and GB areas included red clover (Trifolium pretense L.) and lespedeza (Kummerowia stipulacea Maxim.) planted into fescue (Festuca arundinacea Schreb.) while AgB included Eastern cottonwood trees (Populus deltoids Bortr. ex Marsh.) planted into fescue. One-meter deep soil cores were collected from each treatment in August 2007 and 2008 with a soil probe. Three soil cores were sampled at six replicate sampling positions. Soil cores were collected in plastic tubes inserted inside the metal soil probe. Soils were segregated by horizons, and roots were separated into three diameter classes (0–1, 1–2, >2 mm) by soil horizon. Root length was determined using a flatbed scanner assisted with computer software. Buffer treatments (167 cm/100 cm³) had 4.5 times higher RLD as compared to pasture treatments (37.3 cm/100 cm³). The AgB treatment had the highest (173.5 cm/100 cm³) RLD and CG pasture had the lowest (10.8 cm/100 cm³) value. Root carbon was about 3% higher for the buffers compared to RG treatment. Soil carbon was about 115% higher for the buffers compared to pasture treatments. Results from this study imply that establishment of agroforestry and GB on grazed pasture watersheds improve soil carbon accumulation and root parameters which enhance soil physical and chemical properties thus improving the environmental quality of the landscape.     

Water Use of Tree Lines: Importance of Leaf Area and Micrometeorology in Sub-Humid Kenya

In this research the relative importance of leaf area and microclimatic factors in determining water use of tree lines was examined in sub-humid Western Kenya. Measurements of tree water-use by a heat-balance technique, leaf area, bulk air saturation deficit, daily radiation, and soil water content were done in an experiment with tree lines within crop fields. The tree species were Eucalyptus grandis W. Hill ex Maiden, Grevillea robusta A. Cunn. and Cedrella serrata Royle, grown to produce poles on a phosphorus-fixing Oxisol/Ferralsol with (+P) or without (−P) phosphorus application. Doubling the leaf area of Cedrella and Grevillea doubled water use in a leaf area (LA) range of 1–11 m² per tree. The response of Eucalyptus water use (W) to increases in leaf area was slightly less marked, with W = LAⁿ, n<1. Transpiration rate per unit leaf area (Tr) was the other important determinant of water use, being affected by both tree species and phosphorus fertilization. A doubling of the saturation deficit (SD) halved the water use of all trees except for Cedrella +P, in which water use increased. A direct effect of soil water content on water use was only found in Grevillea -P, with a small increase (60%) as available water increased from 1.4 to 8.9% above wilting point (32%). This low direct response to soil water content is probably due to the extensive tree-root systems and the deep clayey soils supplying sufficient water to meet the evaporative demand. Indirect responses to soil water content via decreases in leaf area occurred in the dry season. The results showed that water use of tree lines was more determined by leaf area and transpiration rate per unit leaf area than by micro meteorological factors. The linear response of tree water use to leaf area, over a wide range leaf areas, is a specific characteristic of tree line configurations and distinguished them from forest stands. In tree lines light interception and canopy conductance increase with leaf area much more than a similar leaf area increase would have caused in a closed forest canopy.     

Key roles of leaves, stockplant age, and auxin concentration in vegetative propagation of two African mahoganies: Khaya anthotheca Welw. and Khaya ivorensis A. Chev

Leaf area, IBA concentration and age of stockplants were all found to be important factors for successful rooting for vegetative propagation using single-node cuttings of the two main African mahogany species: Khaya anthotheca and K. ivorensis. Cuttings with leaf area of 30–50 cm² had the best rooting percentage and cuttings with about 30 cm² had the most number of roots per cutting in K. anthotheca. Khaya ivorensis, cuttings with 10–30 cm² leaf area had the highest rooting percentage. Cuttings collected from 1-year-old stockplants recorded the highest rooting percentage and largest number of roots per cutting. Cuttings from 3-year-old stockplant of the same seeds sources had the lowest rooting suggesting aging negatively impacted rooting ability. The effect of auxin concentration, on rootability was examined with cuttings of K. anthotheca. An IBA concentration of 0.8% was the best exogenous auxin concentration for percentage rooting, number of roots per cutting and the length of the longest root per cutting.     

Long-term nitrogen addition regulates root nutrient capture and leaf nutrient resorption in Larix gmelinii in a boreal forest

Human activities accelerate global nitrogen (N) deposition, and elevated N availability may alter the stoichiometric balance of nutrients and then affect nutrient absorption by plants. The boreal forest is considered one of the world’s most N-limited ecosystems, and its response to N deposition is already a hot issue. In order to explore how long-term nitrogen addition influences nutrient uptake and distribution in Larix gmelinii in a boreal forest, four N treatment levels (0, 25, 50 and 75 kg N ha^?1 yr^?1) have been applied in a boreal forest since May 2011. Nitrogen addition significantly reduced the soil pH, significantly changed the soil N availability, increased the total N and N/P in needles and fine roots, and decreased the total P in needles and the C/N in soil. Nitrogen addition significantly reduced nitrogen resorption efficiency, and its impacts on P resorption efficiency were not significant. Nitrogen addition significantly increased the root length, surface area and diameter of 4th- and 5th-order transport fine roots. The N and N/P of needles showed seasonal variation. The needle N concentration and N/P were positively correlated with N addition, while the needle P was negatively correlated with nitrogen addition. With increase in nitrogen addition, Larix gmelinii increased its investment in its belowground parts, which may explain why Larix gmelinii tended to put more C in long-lived roots to improve its C utilization efficiency. Given the P deficiency caused by N addition, Larix gmelinii may be more likely to absorb P from the soil and adjust its C distribution to meet its P demand rather than relying on internal nutrient resorption.

     

Assessing soil conservation strategies for upland cropping in Northeast Thailand with the WaNuLCAS model

Soil conservation approaches and agroforestry systems can play an important role in controlling erosion from tropical hillside cropping systems. Experimental testing of their potential application domain and design, however, is costly and time consuming. We, therefore, tested the ability of the Water, Nutrient and Light Capture in Agroforestry Systems (WaNuLCAS) model. The specific objectives of the study were: (i) to evaluate the WaNuLCAS model for predicting water induced erosion under different soil conservation strategies, (ii) to use the model for a better understanding of various soil conservation measures in controlling erosion, and (iii) to assess the magnitude and dynamics of key processes influencing the efficiency of soil conservation measures. A 3-year-data set (2003–2005) from a field experiment from the Loei province in Northeast Thailand on the impact of soil conservation (Leucaena hedgerow, Jack bean relay cropping) under minimum tillage on runoff and soil loss was tested. Results indicated that WaNuLCAS was able to predict soil loss and runoff well at the test site; i.e. R ² = 0.80 and 0.82, respectively. In the calibration procedure a sub data set was used, where adjusting crop development parameters was an important step for improving simulated soil loss (R ² = 0.75) and runoff (R ² = 0.89). Soil conservation measures such as Leucaena hedges were effective techniques to control runoff and soil loss. Implementing a dynamic soil structure module in combination with minimum tillage reduced runoff and soil loss via an increase in available macropores and hence drainage over time which improved simulation results. Relay cropping with Jack bean played an important role in the control treatment in reducing soil loss during the third year due to its additional soil cover and positive impact on soil fertility as suggested by the model. Hence, the WaNuLCAS model is a valuable tool to study and understand processes and to explore management options for improving tropical hillside cropping threatened by soil degradation.     

Understory plant diversity and biomass in hybrid poplar riparian buffer strips in pastures

Understory plant biomass, species richness and canopy openness were measured in six-year old hybrid poplar riparian buffer strips, in the understory of two unrelated clones (MxB-915311 and DxN-3570), planted along headwater streams at three pasture sites of southern Quebec. Canopy openness was an important factor affecting understory biomass in hybrid poplar buffers, with lower understory biomass observed on sites and under the clone with lower canopy openness. Although tree size was an important factor affecting canopy openness, relationships between total stem volume and canopy openness, for each clone, also support the hypothesis of a clonal effect on canopy openness. Understory biomass and canopy openness as low as 3.6 g m⁻² and 7.6% in 1 m² microplots were measured under clone MxB-915311 at the most productive site. This reduction of understory plant growth could compromise important buffer functions for water quality protection (runoff control, sediment trapping and surface soil stabilisation), particularly were concentrated runoff flow paths enter the buffer. On the other hand, tree buffers that maintain relatively low canopy openness could be interesting to promote native and wetland plant diversity. Significant positive relationships between canopy openness and introduced species richness (R ² = 0.46, p < 0.001) and cover (R ² = 0.51, p < 0.001) were obtained, while no significant relationship was observed between canopy openness and native (wetland) species richness and cover. These results suggest that planting riparian buffer strips of fast-growing trees can rapidly lead to the exclusion of shade-intolerant introduced species, typical colonisers of disturbed habitats such as riparian areas of pastures, while having no significant effect on native (wetland) diversity. Forest canopy created by the poplars was probably an important physical barrier controlling introduced plant richness and abundance in agricultural riparian corridors. A strong linear relationship (R ² = 0.73) between mean total species richness and mean introduced species richness was also observed, supporting the hypothesis that the richest communities are the most invaded by introduced species, possibly because of higher canopy openness, as seen at the least productive site (low poplar growth). Finally, results of this study highlight the need for a better understanding of relationships between tree growth, canopy openness, understory biomass and plant diversity in narrow strips of planted trees. This would be useful in designing multifunctional riparian buffer systems in agricultural landscapes.     

Partitioning of simulated rainfall in a kaolinitic soil under improved fallow–maize rotation in Zimbabwe

Research on improved fallows has concentrated on soil fertility benefits neglecting possible benefits to soil and water conservation. The effects of improved fallows on rainfall partitioning and associated soil loss were investigated using simulated rainfall on a kaolinitic soil in Zimbabwe. Simulated rainfall at an intensity of 35 mm h⁻¹ was applied onto plots that were under planted fallows of Acacia angustissima and Sesbania sesban, natural fallow and maize (Zea mays L.) for two years. At the end of 2-years in October 2000, steady state infiltration rates could not be determined in A. angustissima and natural fallow plots, but they were 24 mm h⁻¹ in S. sesban and 5 mm h⁻¹ in continuous maize. The estimated runoff losses after 30 min of rainfall were 44% from continuous maize compared with 22% from S. sesban and none from A. angustissima and natural fallow plots. Infiltration rate decay coefficients were 36 mm and 10 mm for S. sesban and continuous maize, respectively. In October 2001 after one post-fallow crop, it was still not possible to determine the steady state infiltration rates in A. angustissima and natural fallows, but they were 8 and 5 mm h⁻¹ for, S. sesban and continuous maize systems, respectively. The runoff loss, averaged across tilled and no-tilled plots, increased to 30% in the case of S. sesban fallowed plots and 57% for continuous maize; there was still no runoff loss from the other treatments. There were significant differences (P<0.05) in infiltration rate decay coefficients among treatments. The infiltration rate decay coefficient was 25 mm for S. sesban and it remained unchanged at 10 mm for continuous maize. It is concluded that planted tree fallows increase steady state infiltration rates and reduce runoff rates, but these effects markedly decrease after the first year of maize cropping in non-coppicing tree fallows. This revised version was published online in June 2006 with corrections to the Cover Date.     

Production and quality of senesced and green litterfall in a pine–oak forest in central-northwest Mexico

Litterfall is an important ecological process in forest ecosystems, influencing the transfer of organic matter, carbon (C), nitrogen (N), phosphorous (P) and other nutrients from vegetation to the soil. We examined the production of different litterfall fractions as well as nutrient content and nutrient inputs by senesced and green leaf-litter in a semiarid forest from central Mexico. From September 2006 to August 2007, monthly litter sampling was carried out in monospecific and mixed stands of Quercus potosina and Pinus cembroides. Litterfall displayed a marked bimodal pattern with the largest annual amount (5993 ± 655 kg ha⁻¹ yr⁻¹) recorded in mixed stands, followed by Q. potosina (4869 ± 510 kg ha⁻¹ yr⁻¹), and P. cembroides (3023 ± 337 kg ha⁻¹ yr⁻¹). Leaves constituted the largest fraction of total litterfall reaching almost 60%, while small branches contributed with 20–30%. Overall, N content in leaf-litter was higher while lignin content was significantly lower for Q. potosina than for P. cembroides. Thus, greater litter quality together with higher litter production caused the largest C, N and P inputs to forest soils to occur in monospecific Q. potosina stands. Green leaf fall displayed significantly lower lignin:N and C:N ratios in Q. potosina than P. cembroides suggesting faster decomposition and nutrient return rates by the former. Although we recorded only two green leaf fall events, they accounted for 18% and 11% of the total N and P input, respectively, from leaf-litter during the study period. Apart, from the large spatiotemporal heterogeneity introduced by differences in litter quantity and quality of evergreen, deciduous and mixed stands, green litterfall appears to represent a much more important mechanism of nutrient input to semiarid forest ecosystems than previously considered.     

Vegetative propagation of Prunus africana: effects of rooting medium, auxin concentrations and leaf area

The region of West and Central Africa is endowed with high-value fruit trees and medicinal plants, which are currently traded locally as well as on regional and international markets. Unfortunately, they are all exploited from the wild and there has been little or no focussed effort to domesticate and cultivate them. Prunus africana is one of these important medicinal plant under domestication. A series of nursery experiments were conducted to assess the effects of rooting medium (sawdust, sand and a 50:50 mixture of sand and sawdust), auxin concentration (0, 50, 100, 150, and 200 μg IBA), and leaf area (0, 5, 10, 20, and 25 cm²) on rooting success of juvenile cuttings of P. africana. The percentage of cuttings rooted was significantly greater (P < 0.05) in sawdust (80%), than in sand alone (72%) or in mixture with sawdust (71%). Leaf area also significantly affected the percentage of rooting. Leafless cuttings did not root and were all dead by week 6, but in leafy cuttings rooting ability increased proportionally with leaf area up to 20 cm² (79%). Larger leaf cuttings (25 cm²) rooted at the same level as those of 20 cm². The cuttings with the largest leaves also had the greatest mean number of roots per cutting (14 roots cutting⁻¹), while those with the smallest (5 cm²) leaf area produced the fewest roots (5 roots cutting⁻¹). The application of auxin (IBA) promoted rooting (P < 0.05) up to an optimum application of 100–200 μg IBA per cutting, but 300 μg was supraoptimal. It can be concluded that P. africana is amenable to vegetative propagation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Densification, stand-replacement wildfire, and extirpation of mixed conifer forest in Cuyamaca Rancho State Park, southern California

A century of fire suppression culminated in wildfire on 28 October 2003 that stand-replaced nearly an entire 4000 ha “sky island” of mixed conifer forest (MCF) on Cuyamaca Mountain in the Peninsular Range of southern California. We studied the fire affected Cuyamaca Rancho State Park (CRSP), which represents a microcosm of the MCF covering approximately 5.5 × 10⁶ ha (14%) of California, to evaluate how fire suppression unintentionally destabilizes this ecosystem. We document significant changes in forest composition, tree density, and stem diameter class distribution over a 75-year period at CRSP by replicating ground-based measurements sampled in 1932 for the Weislander Vegetation Type Map (VTM) survey. Average conifer density more than doubled, from 271 ± 82 trees ha⁻¹ (standard error) to 716 ± 79 ha⁻¹. Repeat aerial photographs for 1928 and 1995 also show significant increase in canopy cover from 47 ± 2% to 89 ± 1%. Changes comprise mostly ingrowth of shade-tolerant Calocedrus decurrens [Torr.] Floren. in the smallest stem diameter class (10–29.9 cm dbh). The 1932 density of overstory conifer trees (>60 cm dbh) and 1928 canopy cover at CRSP were similar to modern MCF in the Sierra San Pedro Mártir (SSPM), 200 km S in Baja California, Mexico, where fire suppression had not been practiced, verifying that the historical data from the early twentieth century represent a valid “baseline” for evaluating changes in forest structure. Forest successions after modern crown fires in southern California demonstrate that MCF is replaced by oak woodlands and shrubs. Post-fire regeneration in severely burned stands at CRSP includes abundant basal sprouting of Quercus chrysolepis Liebm. and Quercus kelloggii Newb., but only few seedlings of Abies concolor [Gord. and Glend.] Lindl (average 16 ± 14 ha⁻¹), while whole stands of C. decurrens, Pinus lambertiana Dougl., and Pinus ponderosa Laws. were extirpated. Prescribed burning failed to mitigate the crown fire hazard in MCF at CRSP because the low-intensity surface fires were small relative to the overall forest area, and did not thin the dense understory of sapling and pole-size trees. We propose that larger, more intense prescribed understory burns are needed to conserve California's MCF.     

Regional influences of soil available water-holding capacity and climate,and leaf area index on simulated loblolly pine productivity

We simulated loblolly pine (Pinus taeda L.) net canopy assimilation, using BIOMASS version 13.0, for the southeastern United States (1° latitude by 1° longitude grid cells) using a 44-year historical climate record, estimates of available water-holding capacity from a natural resource conservation soils database, and two contrasting leaf area indices (LAI) (low; peak LAI of 1.5 m² m⁻² projected, and high; 3.5 m² m⁻²). Median (50th percentile) available water-holding capacity varied from 100 to 250 mm across the forest type for a normalized 1.25 m soil profile. Climate also varied considerably (growing season precipitation ranged from 200 to 1600 mm while mean growing season temperature ranged from 13° to 26°C). Net canopy assimilation ranged from 9.3 to 19.2 Mg C ha⁻¹ a⁻¹ for high LAI and the 95th percentile of available water-holding capacity simulations.We examined the influence of soil available water-holding capacity, and annual variation in temperature and precipitation, on net canopy assimilation for three cells of similar latitude. An asymptotic, hyperbolic relationship was found between the 44-year average net canopy assimilation and soil available water-holding capacity. Shallow soils had, naturally, low water-holding capacity (<100 mm) and, subsequently, low productivity. However, median available water-holding capacity (125–150 mm) was sufficient to maintain near maximum production potential in these cells.Simulations were also conduced to examine the direct affects of soil available water on photosynthesis (P_N) and stomatal conductance (g_S) on net canopy assimilation. In the absence of water limitations on P_N and g_S, net canopy assimilation increased by only 10% or less over most of the loblolly pine region (when compared to simulations for median available water-holding capacity with water influences in place). However, the production differences between high and low LAI, at the median soil available water-holding capacity, ranged from 30% to 60% across the loblolly pine range. Vapor pressure deficit was found to dramatically reduce productivity for stands of similar LAI, incident radiation, rainfall, and available water-holding capacity. Thus, these simulations suggest that, regionally, loblolly pine productivity may be more limited by low LAI than by soil available water-holding capacity (for soils of median available water-holding capacity or greater). In addition, high atmospheric forcing for water vapor will reduce net assimilation for regions of otherwise favorable available water and LAI.     

Validation of a canopy photosynthesis model for cocksfoot pastures grown under different light regimes

Daily net canopy photosynthesis (P _n) was predicted for cocksfoot (Dactylis glomerata L.) canopies grown under different light regimes by integration of a leaf photosynthesis model developed for the light-saturated photosynthetic rate (P _max), photosynthetic efficiency (α) and the degree of curvature (θ) of the leaf light–response curve. When shade was the only limiting factor, the maximum P _n (P _nmax) was predicted to decrease approximately linearly from 33.4 g CO₂ m⁻² d⁻¹ to zero as photosynthetic photon flux density (PPFD) fell from full sunlight (1800 μmol m⁻² s⁻¹ PPFD) to 10% of this in a fluctuating light regime. It was also predicted that at 50% transmissivity P _nmax was higher for a continuous light regime (10.4 g CO₂ m⁻² d⁻¹) than for a fluctuating light regime with the same intensity (8.4 g CO₂ m⁻² d⁻¹). The canopy photosynthesis model was then used to predict dry matter (DM) production for cocksfoot field grown pastures under a diverse range of temperature, herbage nitrogen content and water status conditions in fluctuating light regimes. This prediction required inclusion of leaf area index and leaf canopy angle from field measurements. The model explained about 85% of the variation in observed cocksfoot DM production for a range from 6 to 118 kg DM ha⁻¹ d⁻¹. The proposed model improves understanding of pasture growth prediction through integration of relationships between shade limitations in fluctuating light regimes and other environmental factors that affect the canopy photosynthetic rate of cocksfoot pastures in silvopastoral systems.     

Influence of coarse wood and pine saplings on nitrogen mineralization and microbial communities in young post-fire Pinus contorta

Nitrogen (N) limits productivity in many coniferous forests of the western US, but the influence of post-fire structure on N cycling rates in early successional stands is not well understood. We asked if the heterogeneity created by downed wood and regenerating pine saplings affected N mineralization and microbial community composition in 15-yr old lodgepole pine (Pinus contorta var. latifolia) stands established after the 1988 fires in Yellowstone National Park (Wyoming, USA). In three 0.25-ha plots, we measured annual in situ net N mineralization in mineral soil using resin cores (n = 100 per plot) under pine saplings, downed wood (legacy logs that survived the fire, and fire-killed trees that had fallen and were contacting or elevated above the ground), and in bare mineral soil. Annual in situ net N mineralization and net nitrification rates were both greater in bare mineral soil (8.4 ± 0.6 and 3.6 ± 0.3 mg N kg_soil⁻¹ yr⁻¹, respectively) than under pine saplings, contact logs, or elevated logs (ca. 3.9 ± 0.5 and 0.8 ± 0.1 mg N kg_soil⁻¹ yr⁻¹, respectively). Net nitrification was positively related to net N mineralization under all treatments except for elevated logs. In laboratory incubations using ¹⁵N pool dilution, NH₄⁺ consumption exceeded gross production by a factor of two in all treatments, but consumption and gross production were similar among treatments. Contrary to our initial hypothesis, microbial community composition also did not vary among treatments. Thus, two- to three-fold differences in in situ net N mineralization rates occurred despite the similarity in microbial communities and laboratory measures of gross production and consumption of NH₄⁺ among treatments. These results suggest the importance of microclimate on in situ annual soil N transformations, and differences among sites suggest that broader scale landscape conditions may also be important.     

Water relations of pine seedlings in contrasting overstory environments

Overstory conditions influence understory microclimate and resource availability, leading to gradients in evaporative demand and moisture availability that influence seedling water relations. Partial canopies may either reduce seedling moisture stress by ameliorating environmental conditions, or increase moisture stress by reducing soil moisture availability. This study used stable isotope ratios of oxygen (δ¹⁸O) and carbon (δ¹³C) and mass-based foliar nitrogen concentrations to investigate changes in transpiration (E), stomatal conductance (g_s) and intrinsic water use efficiency (iWUE) of pine seedlings across an overstory gradient from open canopy gap environments to closed canopy forest. Foliar δ¹⁸O increased sharply from basal areas of 0–10 m² ha⁻¹ in Pinus banksiana, Pinus resinosa, and Pinus strobus seedlings, followed by a more gradual increase with further increases in basal area. Foliar δ¹³C followed a similar, but less pronounced pattern in P. banksiana and P. strobus seedlings, and had no apparent relationship with overstory basal area in P. resinosa seedlings. The slope of the δ¹⁸O:δ¹³C relationship was positive for every species. Foliar nitrogen concentrations were not correlated with overstory basal area. These results suggest seedling E declined as overstory basal area increased due to reductions in g_s, while iWUE increased slightly from open gaps to partial canopy environments. Open gap environments appear to provide sufficient moisture to sustain high leaf-level gas exchange rates in the species we studied, while relatively small increases in overstory basal area apparently promote rapid declines in g_s, leading to greatly reduced seedling water loss and small increases in iWUE.     

Space-time dynamics of fine root biomass of six forests in the Maoershan forest region, northeast China

The Maoershan forestry centre is situated in the Zhangguangcai Mountain of the Changbai mountain range. The main forest types in the Maoershan region are plantation (Pinus sylvestris var. mongolica, Pinus koraiensis and Larix gmelinii) and natural secondary forests (Fraxinus mandshurica, Quercus mongolica and Populus davidiana). Fine roots have enormous surface areas, growing and turning over quickly, which plays an important role in terms of substance cycling and energy flow in the forest ecosystem. This study deals with the dynamics of live, dead, and total fine roots (≤ 5 mm) biomass in the 0–30 cm soil layer using the soil core method. Differences between the six stands in the Maoershan region showed the following results: 1) the fine root biomass in the various stands showed obvious differences. The total fine root biomass of six stands from high to low were F. mandshurica (1,030.0 g/m²) > Q. mongolica (973.4 g/m²) > Pinus koraiensis (780.9 g/m²) > L. gmelinii (718.2 g/m²) > Populus davidiana (709.1 g/m²) > Pinus sylvestris var. mongolica (470.4 g/m²); 2) except for L. gmelinii, the development of live fine root biomass agreed with the trend of total fine root biomass. The maximum biomass of live fine roots in Pinus koraiensis or L. gmelinii stand appeared in May, others in June; in the F. mandshurica stand, the minimum biomass of live fine roots occurred in September, others in July or August; 3) the proportions of dead fine root biomass varied in different stands; 4) the vertical distribution of fine roots was affected by temperature, water, and nutrients; the proportion of fine root biomass was concentrated in the 0–10 cm soil layer. The fine root biomass of six stands in the 0–10 cm soil layer was over 40% of the total fine root biomass; this proportion was 60.3% in F. mandshurica. Space-time dynamics of the various stands had different characteristics. When investigating the substance cycling and energy flows of all forest ecosystems, we should consider the characteristics of different stands in order to improve the precision of our estimates. __________ Translated from Scientia Silvae Sinicae, 2006, 42(6): 13–19 [译自: 林业科学]     

Responses of fine root mass,length, production and turnover to soil nitrogen fertilization in <Emphasis Type="Italic">Larix gmelinii</Emphasis> and <Emphasis Type="Italic">Fraxinus mandshurica</Emphasis> forests in Northeastern China

The responses of fine root mass, length, production and turnover to the increase in soil N availability are not well understood in forest ecosystems. In this study, sequential soil core and ingrowth core methods were employed to examine the responses of fine root (≤1 mm) standing biomass, root length density (RLD), specific root length (SRL), biomass production and turnover rate to soil N fertilization (10 g N m⁻² year⁻¹) in Larix gmelinii (larch) and Fraxinus mandshurica (ash) plantations. N fertilization significantly reduced fine root standing biomass from 130.7 to 103.4 g m⁻² in ash, but had no significant influence in larch (81.5 g m⁻² in the control and 81.9 g m⁻² in the fertilized plots). Similarly, N fertilization reduced mean RLD from 6,857 to 5,822 m m⁻² in ash, but did not influence RLD in larch (1,875 m m⁻² in the control and 1,858 m m⁻² in the fertilized plots). In both species, N fertilization did not alter SRL. Additionally, N fertilization did not significantly alter root production and turnover rate estimated from sequential soil cores, but did reduce root production and turnover rate estimated from the ingrowth core method. These results suggested that N fertilization had a substantial influence on fine root standing biomass, RLD, biomass production and turnover rate, but the direction and magnitude of the influence depended on species and methods.     

Modeling leaf maximum net photosynthetic rate of <Emphasis Type="Italic">Festuca pallescens</Emphasis>, the dominant perennial grass of Patagonian pine-based silvopastoral systems

The integrated relationship in a simple mechanistic model between the critical environmental factors controlling leaf photosynthesis of understory species would be a useful tool to optimize the management of the silvopastoral systems. Individual effect of leaf temperature, water stress and light environment over net maximum photosynthetic rate (Pmax) was evaluated on Festuca pallescens leaves grown in a silvopastoral system of two Pinus ponderosa canopy covers (350 and 500 trees ha⁻¹) and natural grassland. The aim was to integrate individual functions for Pmax against these environmental factors into a multiplicative model. We measured pre-dawn water potential (ψ _pd), leaf temperature and net photosynthetic rate (Pn), stomatal conductance (gs) and intercellular CO₂ concentration (Ci) as a function of photosynthetic photon flux density (PPFD). The highest Pmax under non-limiting conditions was 20.4 μmol CO₂ m⁻² s⁻¹ and was defined as standardized dimensionless Pmax _s  = 1 for comparison of environmental factors. The leaf temperature function showed an optimum range between 20.2 and 21.8°C where Pmax _s  = 1. Then, Pmax _s declined by an average 1 μmol CO₂ m⁻² s⁻¹ C⁻¹ from the optimum to 4.7 and 38.5°C. Pmax _s decreased at a rate of 9.49 μmol CO₂ m⁻² s⁻¹ MPa⁻¹ as water potential reaches −1.9 MPa and showed a lower slope as water potential decreased down to −4.3 MPa. The light environment was estimated from hemispherical photograph analysis. Pmax _s was 20% higher in leaves of open control plants than under the maximum tree canopy cover. The simple multiplicative model accounted for 0.82 of the variation in Pmax. Such a simple mechanistic model is the first step towards a more effective decision support tool.     

Effect of increased rainfall on water dynamics of larch (<Emphasis Type="Italic">Larix cajanderi</Emphasis>) forest in permafrost regions,Russia: an irrigation experiment

Sap flow measurements, from July to August 2004, were coupled with micrometeorological, soil moisture, and soil temperature measurements to analyze forest water dynamics in irrigated and undisturbed (control) larch (Larix cajanderi) forest plots in eastern Siberia. Plots were irrigated with 120 mm (20 mm day⁻¹) of water from 17 to 22 July. Sap flow measurements of ten trees at each plot were scaled up to daily stand canopy transpiration (E _c ). Canopy transpiration at the irrigation and control plots was similar before irrigation. Forest evapotranspiration (E _a ) was obtained from Ohta et al. (Agric For Meteorol 148:1941–1953, 2008) while E _a in the irrigation plot was estimated based on the E _{c_irrig}/E _{c_cont} ratio. Rainfall during July–August was 63.4 mm but, after including water from thawing soil layers, the actual water input was 109.9 and 218.5 mm in the control and irrigation plots, respectively. Despite this large difference, a corresponding difference in E _c (and E _a ) was not observed [42.6 (61.5) mm and 46.4 (71.8) mm in control and irrigation plots, respectively]. Daily canopy conductance (g _c ) increased as long as moisture was well supplied in the upper soil layers and evaporative demand was high. Soil moisture and rainfall contribution to E _a was 36.9 and 24.6 mm in the control plot and 34.5 and 37.3 mm in the irrigation plot, respectively. Water supply from soil thawing layers in the control plot and high runoff (105.6 mm) rates in the irrigation plot accounted for the similarity in water dynamics. Under increased precipitation, the forest used less soil water stored from previous growing seasons.     

Below-ground root yield and distribution in natural and replanted mangrove forests at Gazi bay, Kenya

Estimation of total biomass in woody ecosystems is important because of its relevance to nutrient turnover and the potential to store carbon. Most work on mangrove biomass, particularly in the Western Indian Ocean Region, has concentrated on the above-ground component; comparatively little is known on below-ground biomass. The current study was conducted at Gazi bay on the southern coast of Kenya. The objective was to determine the below-ground biomass of three species of mangrove, Rhizophora mucronata Lamarck, Avicennia marina (Forsk.) Vierh and Sonneratia alba J. Smith, in natural and replanted stands. The effects of distance from the tree base and of soil depth on root biomass and size distributions were also studied using coring. Live below-ground biomass (mean ± S.E.) ranged from 7.5 ± 0.4 t/ha to 35.8 ± 1.1 t/ha, 48.4 ± 0.7 t/ha to 75.5 ± 2.0 t/ha and 39.1 ± 0.7 t/ha to 43.7 ± 1.7 t/ha for R. mucronata, S. alba and A. marina, respectively, depending on the age of the stand. Including dead roots produced total biomass values of 34.9 ± 1.8–111.5 ± 5.6 t/ha, 78.9 ± 3.3–121.5 ± 7.3 t/ha and 49.4 ± 1.1–84.7 ± 5.4 t/ha for R. mucronata, S. alba and A. marina. These values imply carbon contents of live roots ranging between 3.8 ± 0.2 C t/ha and 17.9 ± 0.6 C t/ha, 24.2 ± 0.4 C t/ha and 37.7 ± 1.0 C t/ha and 19.5 ± 0.4 C t/ha and 21.9 ± 0.9 C t/ha for R. mucronata, S. alba and A. marina stands, respectively, and 17.4 ± 0.9 C t/ha and 55.7 ± 2.8 C t/ha, 39.4 ± 1.7 C t/ha and 60.7 ± 3.6 C t/ha and 24.7 ± 0.6 C t/ha and 42.4 ± 2.9 C t/ha for R. mucronata, S. alba and A. marina stands, respectively if dead roots are included. Stand densities were 4650 ± 177 stems/ha, 3800 ± 212 stems/ha and 3567 ± 398 stems/ha for R. mucronata 6-year old, 12-year-old and natural stands respectively. Mean stem diameter, and basal area were highest in the 12-year-old plantation while below-ground root biomass increased with age. Stand density for S. alba, was highest in the 12-year-old plantation (7900 ± 141 stems/ha) while the 9-year-old stand had trees with the largest diameter (7.7 ± 0.9 cm). Below-ground biomass was highest in the 12-year old (75.5 ± 2.0 t/ha) and lowest in the natural stand (48.4 ± 0.7 t/ha). Stand density for A. marina was highest in the 12-year-old plantation (4300 ± 919 stems/ha) while mean stem diameter (7.9 ± 0.7 cm) and basal area (16.2 ± 2.1 m²/ha) were highest in the natural stand. Below-ground biomass in the 12-year-old (43.7 ± 1.7 t/ha) and natural stands (39.1 ± 0.7 t/ha) was similar. Root densities decreased with soil depth and with distance from the base of trees for all species and stands. Fine roots (diameter <5 mm) constituted between 24% and 45% of the total stand live root biomass. The information generated is important in establishing the total biomass and thus the potential amount of carbon sequestered by mangroves in the study area.     

Foliar Responses of Understorey Abies lasiocarpa to Different Degrees of Release Cutting in a Betula papyrifera and Conifer Mixed Species Stand

Foliar responses of subalpine fir [Abies lasiocarpa (Hook.) Nutt.] to thinning were studied in a 35-yr-old mixed stand of paper birch (Betula papyrifera Marsh.) and conifers. The stand regenerated naturally after a wildfire with a canopy dominated by paper birch (average height 9.8 m) and an understorey dominated by subalpine fir (average height 1.6 m). The stand was thinned to four densities of birch: 0, 600 and 1200 stems ha-¹ and control (unthinned at 2300-6400 stems ha-¹) in the autumn of 1995. The understorey conifers, mainly subalpine fir, were thinned to 1200 stems ha-¹. The study used a completely randomized split-plot design. Three sample trees were systematically selected from each treatment replicate and each tree stratum (upper, intermediate and lower understorey). One-year-old and older age class needles were collected from one south-facing branch within the fifth whorl from the tree top. Thinning of paper birch significantly (p <0.001) increased leaf area and dry weight per 100 needles for intermediate and short trees except in the 0 birch treatment. Understorey subalpine fir trees in 600 stems ha¹ birch (T3) had the largest leaf area and leaf dry weight per 100 1-yr-old needles. Specific leaf area (SLA) decreased from unthinned (T1) to 0 birch (T4). Lower understorey trees had the largest SLA. One-year-old needles had significantly higher N, P and K concentrations in all the thinning treatments. These responses are consistent with the shade tolerance of subalpine fir. The results suggest that when managing a paper birch-conifers mixed-wood forest it may be of benefit to understorey conifers to leave a birch canopy as a nursing crop.