Short term effects of radiata pine and selected pasture species on soil organic phosphorus mineralisation

Silvopastoral systems comprise part of the continued expansion of conifer plantings on grassland in New Zealand. Greater understanding of the short term dynamics of soil organic P in such systems will further our knowledge about soil carbon and phosphorus relationships which will enable improved nutrient management in the field. A glasshouse experiment was carried out to examine the short-term effects (36 weeks) of combinations of radiata pine (Pinus radiata), lucerne (Medicago sativa L.) and perennial ryegrass (Lolium perenne L.) grown in the same soil type with a range of carbon (C) and phosphorus (P) levels on plant P uptake and the specific mineralisation rate (SMR). The SMR is defined as net mineralisation rate (i.e. gross mineralisation less microbial and geochemical uptake) and calculated from organic P decline as a percentage of organic P in the original soil before planting. This included an investigation of the effect of tree ectomycorrhizal (EM) hyphae on soil organic P. Plant P uptake was positively correlated with water soluble organic carbon (WSOC) and SMR, which in turn was closely related to soil C levels. The soils with high WSOC and C levels (which also contained high levels of labile inorganic and organic P) enabled high P uptake. Although P uptake was the greatest under radiata pine, the trees tended to deplete inorganic P to a lesser extent than the forages. When tree and forage species were combined, P uptake by forages was similar to when the forages were grown alone. The various soil and plant treatments significantly affected SMR. The two low C soils, showed the greatest organic P mineralisation while a high C soil, which contained significant levels of bicarbonate extracted inorganic P at planting and was under a long established undisturbed pasture, showed the least mineralisation. Trees grown alone showed the greatest SMR, EM hyphae and trees with lucerne were slightly lower than trees alone, while the forages showed the lowest SMR. The findings of this study showed that changes in organic P are strongly influenced by interactions between plant species (radiata pine, lucerne, ryegrass) and soil properties as determined by land use and management.     

Observations of below-ground characteristics of young redwood trees (Sequoia sempervirens) from two sites in New Zealand – implications for erosion control

Aims

Radiata pine (Pinus radiata D. Don) plantations are widely used to control erosion in New Zealand. However, other species with similar growth but longer rotation lengths and ability to coppice may offer future alternatives to radiata pine. Comparing performance of alternative species to radiata thus becomes important if policy is to be developed to promote them.

Methods

The below-ground characteristics (roots) of young redwood (Sequoia sempervirens (D. Don) Endl.) trees from two established plantations in New Zealand were examined and compared with those of radiata pine, and selected poplar and New Zealand native species.

Results

Roots with diameters less than 10 mm comprised over 99 % of total root length in 3-yr-old trees and 98 % of total root length in 4-yr-old trees. For roots greater than 2 mm in diameter, total root length of young redwood trees was greater than that of young radiata pine, poplar and the best performing New Zealand native plant. Total root length at a given root collar diameter for young (1–4 year old) redwood trees was significantly greater than for radiata pine trees. Roots of redwoods were finer and more numerous than for radiata but the below-ground biomass for a given root collar diameter showed no statistical difference between the two species.

Conclusions

Redwood, because of its comparable growth rate and the production of many fine lateral roots, has the potential to become a keystone erosion-control species in New Zealand, especially on steep lands where there is an increased risk of post-harvest landsliding associated with moderate to severe rainstorm events.     

Effects of ensiled forage legumes on performance of store finishing lambs

Male Beulah speckled face lambs (initial live weight (LW) 28.8 ± 0.31 kg) were allocated to three dietary treatments to evaluate the performance of store lambs of a hill breed when offered ensiled lucerne (Medicago sativa), red clover (Trifolium pratense) or ryegrass. Second-cut silage bales (wilted and inoculated) were prepared from 3-year old lucerne and red clover stands and a 1-year old ryegrass sward. All the lambs were group-housed and offered ad libitum ryegrass silage during a 3-week co-variate period. This was followed by a week of dietary changeover period, after which the lambs were housed individually and offered their treatment diet ad libitum. All the lambs received a flat rate supplement of pelleted molassed sugarbeet (250 g fresh weight/day). Individual intakes were determined daily, and weekly measurements of LW and body condition score (CS) were made. Additional measurements were taken by scanning the lambs for depth of Longissimus dorsi (LD) muscle and subcutaneous fat. Over an experimental period of 7 weeks, the lambs offered red clover silage had a higher voluntary silage dry matter (DM) intake, total DM intake and metabolisable energy (ME) intake (P < 0.001) than lambs offered either lucerne or ryegrass silage. This resulted in a faster (P < 0.001) growth rate and increase (P < 0.001) in CS, with no difference between lucerne and ryegrass silages. The feed conversion efficiency (FCE) was 8.0 ± 0.61 kg feed/kg gain for lambs fed red clover silage, compared with 16.6 ± 2.82 and 10.6 ± 1.94 kg feed/kg gain for lucerne and ryegrass silage, respectively. The CP intake was higher (P < 0.001) for lambs fed the lucerne and red clover silages than for the ryegrass silage treatment. The concentration of plasma total protein (TP) was higher (P < 0.05) for lambs offered ryegrass silage versus lucerne and red clover silage. Urea concentrations were highest for lambs fed lucerne silage and lowest for those fed ryegrass silage (P < 0.05). The glucose concentration was higher (P < 0.05) for lambs offered red clover silage, whereas non-esterified fatty acids (NEFA) concentration was higher (P < 0.05) for lambs offered lucerne silage. Substituting ryegrass silage with red clover silage has the potential to improve the performance of finishing store lambs.     

Use of white clover as an alternative to nitrogen fertiliser for dairy pastures in nitrate vulnerable zones in the UK: productivity, environmental impact and economic considerations

Perennial ryegrass and perennial ryegrass/white clover permanent dairy pastures are compared with respect to productivity, environmental impact and financial costs in nitrate vulnerable zones (NVZ) in the UK. With appropriate management, and utilisation of recommended perennial ryegrass and white clover cultivars, white clover is likely to stabilise at around 20% of total dry matter production in a mixed pasture. Plant dry matter production and milk production from a perennial ryegrass/white clover pasture are likely to be similar to that from a perennial ryegrass pasture receiving 200 kg N ha⁻¹ annum⁻¹ and around 70% of that obtained with perennial ryegrass supplied with 350–400 kg N ha⁻¹ annum⁻¹. Nitrate, phosphorus and methane losses from the system and decreases in biodiversity relative to a grazed indigenous sward are likely to be similar for a perennial ryegrass/white clover pasture and a perennial ryegrass pasture receiving 200 kg N ha⁻¹ annum⁻¹: nitrate leachate from both systems is likely to comply with European legislation. Greenhouse gas emissions resulting from nitrogen (N) fertiliser production would be avoided with the perennial ryegrass/white clover pasture. Within NVZ stocking rate restrictions, white clover can provide the N required by a pasture at a lower financial cost than that incurred by the application of N fertiliser.     

Root turnover in pasture species: chicory,lucerne, perennial ryegrass and white clover

For pastures, root turnover can have an important influence on nutrient and carbon cycling, and plant performance. Turnover was calculated from mini‐rhizotron observations for chicory (Cichorium intybus), lucerne (Medicago sativa), perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) grown in the Manawatu, New Zealand. The species were combined factorially with four earthworm species treatments and a no‐earthworm control. Split plots compared the effects of not cutting and cutting the shoots at intervals. Observations were made c. 18 days apart for 2.5 years. This article concentrates on differences between plant species in root turnover in the whole soil profile to 40 cm depth. At this scale, earthworm effects were generally small and short lived. For ryegrass and white clover, root length and mass were linearly related (R² = 0.82–0.99). For chicory and lucerne, the relationships were poorer (R² = 0.38–0.77), so for those species length turnover may be a poor indicator of mass turnover. Standing root length, total growth and death generally decreased in the sequence ryegrass > lucerne > chicory = white clover. In length terms, scaled turnover (growth divided by average standing root length) generally followed the sequence lucerne > white clover > perennial ryegrass = chicory. Across species the scaled turnover rate averaged 3.4 per year or 0.9% per day. Cutting shoots reduced standing root length, growth and death, but increased scaled turnover. These results indicate fast and prolonged root turnover. For ryegrass and white clover, at least there is need to reappraise how to measure and model shoot : root ratios, dry matter production and carbon cycling.     

Changes in rain forest tree diversity, dominance and rarity across a seasonality gradient in the Western Ghats, India

Aim We assessed the effects of latitude, altitude and climate on the alpha diversity of rain forest trees in the Western Ghats (WG) of India. We tested whether stem densities, dominance, the prevalence of rarity, and the proportion of understorey trees are significantly correlated with alpha diversity. Location The WG is a chain of mountains c. 1600 km in length, running parallel to the western coast of the Indian peninsula from above 8° N to almost 21° N latitude. Wet forests occur as a narrow strip in regions with heavy rainfall. Methods To assess tree diversity we used data from 40 small plots, < 1 ha in area, where all trees ≥ 3.18 cm d.b.h. had been inventoried. These plots were distributed across 7 latitudinal degrees and at elevations between 200 and 1550 m. Fisher's alpha was used as a measure of diversity. For each plot, the proportion of trees belonging to the understorey, the proportion of trees belonging to the most abundant species in the plot, as a measure of dominance, and the proportionate representation of singletons, as a measure of rarity, were estimated, and correlated with Fisher's alpha, elevation, rainfall and seasonality. Results Annual rainfall and seasonality increased towards the north, but were not significantly correlated. Tree diversity increased significantly with decreasing seasonality. Tree diversity was not significantly correlated with stem density or with the proportion of understorey tree species, but was significantly correlated with tree dominance and rarity. Dominance increased and rarity significantly decreased with increasing seasonality. Main conclusions This study demonstrates that seasonality influences rain forest tree diversity in the WG of India. The relationship between alpha diversity, dominance and rarity lends correlative support for the Janzen–Connell pest pressure hypothesis.     

Effect of plant species on the larvae of gastrointestinal nematodes which parasitise sheep

Faeces containing Trichostrongylus colubriformis and/or Ostertagia circumcincta eggs were used to provide four contaminations in each of 2 years on plots of browntop, Yorkshire fog, ryegrass, tall fescue, lucerne, chicory, cocksfoot, white clover, and prairie grass and in the second year a mixed sward of ryegrass/white clover. Third stage larvae were recovered from faeces and from four strata of herbage, 0–2.5, 2.5–5, 5–7.5 and >7.5 cm above the soil surface at 2, 4, 6, 8, 11, and 14 weeks after faeces were deposited on the swards. Herbage species had a significant (P &lt; 0.0001) effect on the number of larvae recovered. Greatest numbers of larvae, as indicated by ranking analysis, were recovered from Yorkshire fog, ryegrass, and cocksfoot and lowest numbers from white clover and lucerne. The difference between herbages in numbers of larvae recovered was due to the ‘‘development success’’, the ability of larvae to develop to the infective stage and migrate on to herbage, rather than ‘‘survival’’, the rate of population decline once on the herbage. Faecal degradation was most rapid from white clover and browntop, intermediate from tall fescue, lucerne, prairie grass, cocksfoot, and ryegrass, and slowest from Yorkshire fog swards. The numbers of larvae recovered from herbages were related (r² = 0.59, P &lt; 0.05) with the faecal mass remaining. A greater proportion of the total larvae recovered from the herbage was recovered from the bottom stratum of Yorkshire fog and prairie grass than from white clover, with the other herbages intermediate, indicating that larvae had greater difficulty migrating up Yorkshire fog and prairie grass than the other herbage species. In most herbage species, despite more larvae being recovered from the lowest stratum, larval density (L₃/kg herbage DM) was highest in the top stratum. This study has demonstrated that herbage species can have a significant impact on the population dynamics and vertical migration of T. colubriformis and O. circumcincta larvae.     

Development of heartwood in response to water stress for radiata pine in Southern New South Wales, Australia

Heartwood development and other functional changes in stem conductance in response to water stress in radiata pine were investigated using two contrasting climatic areas (high-altitude sub-alpine vs. warm–dry inland) of the Hume region of New South Wales, Australia. The study included mature (34.5–36.5 years old) and young stands (10–11 years old) measured under normal climate and during an extreme drought. The effect of water stress on heartwood development was examined using sapwood percentage, sapwood saturation, development of dry sapwood and evidence of cavitation in sapwood. Trees at the warm–dry site developed heartwood at faster rates than on the high-altitude site. At breast height, the mature stands of the warm–dry site had 8–14 % less sapwood. Extensive cavitation towards the sapwood/heartwood boundary occurred in some of the mature and young stands on the warm–dry site. We postulated that in water-limiting environments, cavitation of the inner sapwood precedes heartwood formation and is an adaptation mechanism that regulates stem conductance capacity and thus water use in the tree. The drought of 2006 led to decreases in moisture associated with cavitation not previously reported for radiata pine and demonstrated the drought hardiness of the species. In the warm–dry site, breast-height sapwood saturation dropped to 58 and 82 % for suppressed and average-sized trees in a mature unthinned stand; and 75–78 % for two young stands. These saturation levels, however, only imply average values as some cells cavitated whilst others were fully saturated. Cavitation occurred in a localized fashion affecting small to large groups of cells.     

Diurnal foraging ant–tree co-occurrence networks are similar between canopy and understorey in a Neotropical rain forest

Discussion of the vertical stratification of organisms in tropical forests has traditionally focused on species distribution. Most studies have shown that, due to differences in abiotic conditions and resource distribution, species can be distributed along the vertical gradient according to their ecophysiological needs. However, the network structure between distinct vertical strata remains little-explored. To fill this gap in knowledge, we used baits to sample ants in the canopy and understorey trees of a Mexican tropical rain forest to record the ant–tree co-occurrences. We examined the ant–tree co-occurrences in the canopy and understorey using complementary network metrics (i.e., specialization, interaction diversity, modularity, and nestedness). In addition, we evaluated co-occurrence patterns between ant species on trees, using C-score analysis. In general, we found no differences in the network structure, although the interaction diversity was greater in the understorey than in the canopy networks. We also observed that co-occurrence networks of each vertical stratum featured four ant species in the central core of highly co-occurring species, with three species unique to each stratum. Moreover, we found a similar trend toward ant species segregation in the both strata. These findings reveal a similar pattern of ant–ant co-occurrences in both vertical strata, probably due to the presence of arboreal-nesting ants in the understorey. Overall, we showed that despite the marked differences in species composition and environmental conditions between understorey and canopy strata, ant–tree co-occurrences in these habitats could be governed by similar mechanisms, related to dominance and resource monopolization by ants.     

Factors influencing brushtail possum (

Damage by introduced brushtail possums (Trichosurus vulpecula) to Pinus radiata trees was assessed in 41 compartments of a commercial forestry plantation on the Coromandel Peninsula, New Zealand. All the trees assessed were less than 3 years old. Possum damage in the compartments was low (median prevalence 3.3%) but highly variable (range 0?30%). Eight of 37 measured habitat factors differed significantly (P < 0.05) between the sites with damaged and undamaged trees. The best predictor of mean damage was stand age, but this explained only 21% of the variation in damage among compartments. Including both stand age and New Zealand bracken (Pteridium esculentum) cover improved the model significantly and explained 36% of variation in damage. Damage was apparently unrelated to compartment size, distance from the compartment boundary, and possum den-site availability. Surprisingly, the relationship between browse damage and a trap-catch index of possum abundance was weakly negative (r_S = −0.53, P = 0.05). The dense understorey associated with young pine stands tends to increase possum damage to associated P. radiata trees, but the possums in such stands may be less mobile at ground level and thus less easily trapped. Assessment of stand age and understorey characteristics, together with visual inspection for early signs of damage, is likely to be more cost-effective than possum surveys for identifying forest compartments at risk from possum browse.     

Dry matter yield,root traits,and nodule occupancy of lucerne and Caucasian clover when grown in acidic soil with high aluminium concentrations

Aims and methods

Lucerne and Caucasian clover dry matter were measured in response to recommended lime and capital P inputs for six years in an acidic soil in the New Zealand high country. The initial three years of the field experiment indicated successful establishment and persistence of both legumes. Lucerne dry matter (DM) yield was up to 4 t/ha/yr in this period and higher than Caucasian clover yields. However, a lack of persistence of lucerne was apparent from this point forward compared with Caucasian clover which produced 7.7 t DM/ha in Year 6. An experiment using tubes of soil was used to investigate whether differences in root traits, nodulation and nodule occupancy were responsible for the differences observed in field persistence over time.

Results

These showed that when rhizobia inoculant was added, the fine root length of Caucasian clover was unaffected (R² = 0.14) by aluminium (Al) content of the soil. In contrast, fine root growth of lucerne was suppressed (R² = 0.79) by the soil Al content. Nodulation of Caucasian clover was unaffected by soil pH or Al when the rhizobia inoculant was provided which suggests the viability of the commercial genotype ICC148 in this soil with a pH of 5.5 and Al ca. 7 mg/kg soil. For lucerne, the maximum nodulation score of 7.3 occurred with 2 t/ha of lime added (soil pH ca.6, Al ca. 0.3 mg/kg) plus inoculant.

Conclusions

This suggests an Al toxic threshold of <1.0 mg Al/kg soil for effective lucerne nodulation. From the lucerne nodules, eight naturalized strains of Ensifer meliloti were identified. In contrast, only one R. leguminosarum strain was detected in the Caucasian clover nodules. The competition between those rhizobia genotypes may negatively affect the efficiency of biological nitrogen fixation in lucerne. Therefore, the lack of genetic diversity of R. leguminosarum bv. trifolii in New Zealand soils might be an advantage especially if the commercial strain is acid soil tolerant.

     

Stable isotopic observation of water use sources of Pinus sylvestris var. mongolica in Horqin Sandy Land, China

Since the late 1950s, Mongolian pine (Pinus sylvestris var. mongolica) has been widely planted for vegetation restoration in arid and semi-arid areas in North China. We used stable isotope signals from precipitation, soil water, and xylem water of Mongolian pine trees, which were planted in early 1980s on sand dunes in the east-southern margin of Horqin Sandy Land, to identify water uptake sources of this tree. (1) Stable ¹⁸O isotope composition of the xylem water exhibited little seasonality, suggesting that the trees use a relatively stable water source; (2) the water source of the pine trees primarily came from a soil depth of 20–60 cm (sampling depth up to 60 cm in this study) and the trees might use groundwater when soil moisture became extremely low; and (3) there was not much difference in water sources used by the pine trees grown at the top of the fixed dune and in the inter dune lowland, although these two sites had 3–8 m elevation difference. This study suggests that it is critical for Mongolian pine trees to access relatively reliable and stable water sources to grow in sandy land habitats, and timely recharging of rainwater to the trees’ rooting depth is requisite for avoiding and/or reducing their degradation caused by water shortage.     

A distinct strain of lucerne Australian latent virus in white clover in New Zealand

A strain of lucerne Australian latent virus (LALV) was widespread in Trifolium repens (white clover) in Northland, New Zealand, occurring in 20–100% of plants tested on 23 of 26 farms. Infected plants showed chlorotic line patterns or were symptomless. In agar gel double diffusion serological tests, white clover isolates of LALV differed from lucerne isolates by two or three twofold dilution steps of antisera. In mechanical inoculation tests, white clover isolates did not infect lucerne and lucerne isolates did not systemically infect white clover. White clover and lucerne isolates also differed in their reactions in some other hosts. Six LALV isolates from white clover were indistinguishable in double diffusion serological tests and by enzyme-linked immunosorbent assay, although the isolates could be differentiated by some reactions in test plants. Purified LALV particles of lucerne and white clover isolates contained two nucleoprotein components and two RNA species of molecular weight 2.82 × 10⁶ (8150 nucleotides) and 2.46 × 10⁶ (7100 nucleotides) (estimated under denaturing conditions).     

Floristic composition and structure of vegetation under isolated trees in neotropical pastures

Abstract. Large isolated trees are a common feature of the agricultural landscape in humid tropical regions originally covered by rain forest. These isolated trees are primarily used as a source of shade for cattle and people. 13 pastures (totalling ca. 80 ha) currently used as cattle pasture were studied. In them, we registered 265 isolated trees belonging to 57 species. 50 trees of the most frequent species (Ficus spp. n = 30 and Nectandra ambigens n = 20) were selected to examine the influence of isolated trees on floristic composition and vegetation structure in the pastures. At each tree, three 4–m² quadrats were sampled: under the canopy, directly under the canopy perimeter, and beyond the canopy in the open pasture. Under-canopy vegetation was structurally and floristically different from the other two sampling sites. Mean species richness per quadrat was significantly higher under the canopy (17.8 ± 4.3 SD) than at the canopy perimeter (11.2 ± 3.4) and in the open pasture (10.6 ± 3.6) sites. Stem density was higher at under-canopy sites, where greater proportions of endozoochorous and rain-forest species were found. Isolated trees function as nursery plants for rain-forest species by facilitating the establishment of zoochorous species whose seeds are deposited under the tree canopies by frugivorous birds or bats. Our results imply that isolated trees may play a major role in seed dispersal and establishment of native species, which is of consequence for the preservation of rain-forest species in these fragmented landscapes.     

El Niño droughts and their effects on tree species composition and diversity in tropical rain forests

In this study I investigated the effects of the extreme, 1997/98 El Niño related drought on tree mortality and understorey light conditions of logged and unlogged tropical rain forest in the Indonesian province of East Kalimantan (Borneo). My objectives were to test (1) whether drought had a significant effect on tree mortality and understorey light conditions, (2) whether this effect was greater in logged than in undisturbed forest, (3) if the expected change in tree mortality and light conditions had an effect on Macaranga pioneer seedling and sapling densities, and (4) which (a)biotic factors influenced tree mortality during the drought. The 1997/1998 drought led to an additional tree mortality of 11.2, 18.1, and 22.7% in undisturbed, old logged and recently logged forest, respectively. Mortality was highest in logged forests, due to extremely high mortality of pioneer Macaranga trees (65.4%). Canopy openness was significantly higher during the drought than during the non-drought year (6.0, 8.6 and 10.4 vs 3.7, 3.8 and 3.7 in undisturbed, old logged and recently logged forest, respectively) and was positively correlated with the number of dead standing trees. The increase in light in the understorey was accompanied by a 30 to 300-fold increase in pioneer Macaranga seedling densities. Factors affecting tree mortality during drought were (1) tree species successional status, (2) tree size, and (3) tree location with respect to soil moisture. Tree density and basal area per surface unit had no influence on tree mortality during drought. The results of this study show that extreme droughts, such as those associated with El Niño events, can affect the tree species composition and diversity of tropical forests in two ways: (1) by disproportionate mortality of certain tree species groups and tree size classes, and (2) by changing the light environment in the forest understorey, thereby affecting the recruitment and growth conditions of small and immature trees.     

Fine Root Biomass and Its Relationship to Evapotranspiration in Woody and Grassy Vegetation Covers for Ecological Restoration of Waste Storage and Mining Landscapes

Production and distribution of fine roots (≤2.0 mm diameter) are central to belowground ecological processes. This is especially true where vegetation serves as a pump to prevent saturation of soil and possible drainage of excess water into or from potentially toxic waste material stored underground or in mounds aboveground. In this study undertaken near Sydney in Australia, we determined fine root biomass and evapotranspiration (ET) on a waste disposal site restored with either a 15-year-old grass sward or plantations of mixed woody species that were either 5 years old (plantation-5) with a vigorous groundcover of pasture legumes and grasses, or 3 years old (plantation-3) with sparse groundcover. These sites were compared with nearby remnant woodland; all four were located within 0.5-km radius at the same site. Ranking of fine root biomass was in the order woodland (12.3 Mg ha⁻¹) > plantation-5 (8.3 Mg ha⁻¹) > grass (4.9 Mg ha⁻¹) > plantation-3 (1.2 Mg ha⁻¹) and was not correlated with nutrient contents in soil or plants, but reflected the form and age of the vegetation covers. Trends in root length density (RLD) and root area index (RAI) followed those in root biomass, but the differences in RAI were larger than those in biomass amongst the vegetation covers. Annual ET in the dry year of 2009 was similar in the three woody vegetation covers (652–683 mm) and was at least 15% larger than for the grass (555 mm), which experienced restrained growth in winter and periodic mowing. This resulted in drainage from the grass cover while there was no drainage from any of the woody vegetation covers. In plantation-5, root biomass, RAI and RLD were reduced in the rain shadow side of the tree rows. Similarly, the amount and depth of rooting in the groundcover were reduced close to the trees compared to midway between rows. Differences in the root variables were larger than those in ET, which suggested that more roots were produced than were needed for water uptake and/or presence of considerable amounts of necromass. We conclude that vegetation covers, such as plantation-5 consisting of widely spaced trees and a heavy groundcover containing winter-active pasture legumes, will promote year-round water-use with a reduced risk of deep rooting that could breach buried wastes. This function could be sustained through progressive thinning of trees to account for not more than 25% of the whole canopy cover; this will minimize competition for limited soil-water and thereby constrain deep rooting as vegetation ages and attains climax.     

Sap flow of the southern conifer, <Emphasis Type="Italic">Agathis australis</Emphasis> during wet and dry summers

Key message

Analysis of sap flux density during drought suggests that the large sapwood and rooting volumes of larger trees provide a buffer against drying soil.

Abstract

The southern conifer Agathis australis is amongst the largest and longest-lived trees in the world. We measured sap flux densities (F _d) in kauri trees with a DBH range of 20–176 cm to explore differences in responses of trees of different sizes to seasonal conditions and summer drought. F _d was consistently higher in larger trees than smaller trees. Peak F _d was 20 and 8 g m^?2 s^?1 for trees of diameters of 176 and 20 cm, respectively, during the wet summer. Multiple regression analysis revealed photosynthetically active radiation (PAR) and vapour pressure deficit (D) were the main drivers of F _d. During drought, larger trees were more responsive to D whilst smaller trees were more responsive to soil drying. Our largest tree had a sapwood area of 3,600 cm². Preliminary analysis suggests stem water storage provides a buffer against drying soil in larger trees. Furthermore, F _d of smaller trees had higher R ² values for soil moisture at 30 and 60 cm depth than soil moisture at 10 cm depth (R ² = 0.68–0.97 and 0.55–0.67, respectively) suggesting that deeper soil moisture is more important for these trees. Larger trees did not show a relationship between F _d and soil moisture, suggesting they were accessing soil water deeper than 60 cm. These results suggest that larger trees may be better prepared for increasing frequency and intensity of summer droughts due to deeper roots and/or larger stem water storage capacity.

     

混播草地不同种群再生性的研究

在不同刈割频率和时间尺度下 ,对混播草地多年生黑麦草 (Lolium perenne)分蘖数和叶片生长、白三叶 (Trifoliumrepens)分枝数和匍匐茎生长及不同种群年产量和组分进行了连续 3年的监测研究 .结果表明 ,刈割能刺激黑麦草叶片、白三叶匍匐茎生长和分枝数发生 ,保持混播草地黑麦草和白三叶的适宜比和稳定共存 ,提高草地年生产力 ,但不同刈割频率和刈割时间对其影响差异不显著 (P >0 .0 5 ) .黑麦草叶片生长对 6月刈割效果比 8月明显 ,而白三叶匍匐茎生长则与之相反 ,黑麦草产量主要取决于叶片生长 ,白三叶产量主要取决于匍匐茎分枝数 .刈割的黑麦草、白三叶产量组分比分别为 5 0 %、15 % ,比试验前约低 10 %、5 % ,而CK为 39%、6 % .     

Grass species and their fungal symbionts affect subsequent forage growth

Persistence of forage grasses is enhanced through the deliberate and selective use of symbiotic fungal endophytes that confer benefits, particularly pest resistance. However, they have also been implicated in reduced plant community diversity as a result of directly or indirectly enhancing competitive ability. A relatively underexplored mechanism by which endophytes might influence pasture plant composition is by altering the biotic or abiotic soil conditions. To examine the soil conditioning effects of forage grass species and their fungal symbionts we tested the responses of three pasture plants, perennial ryegrass, prairie grass, and white clover in nine different soils that had been conditioned by monocultures of endophyte-containing (E+), or endophyte-free (E?), perennial ryegrass, tall fescue, or meadow fescue. Conditioning grass species had little effect on the responses of perennial ryegrass and prairie grass regardless of E+ or E? treatments. In contrast, conditioning species had a strong effect on the response of white clover, resulting in reduced biomass when grown in perennial ryegrass conditioned soils. The presence of endophyte also had significant growth consequences for white clover, but was either positive or negative depending on the conditioning grass species. In comparison to their respective E? treatments, E+ tall and meadow fescue conditioned soils resulted in reduced biomass of white clover, whereas E+ perennial ryegrass conditioned soils resulted in increased biomass of white clover. Among the conditioning strains (AR1, AR37, NEA2, WE) of E+ perennial ryegrass, white clover showed significantly different responses, but all responses were positive in comparison to the E? treatment. By examining the effects of several grass species and endophyte strains, we were able to determine the relative importance of grass species vs. fungal symbiont on soil conditioning. Overall, the conditioning effect of grass species was stronger than the effects associated with endophyte, particularly with regard to the response of white clover. We conclude that both grass species and their fungal endophytes can influence pasture plant community composition through plant–soil feedback.     

Studies on the host-finding ability of the aphid parasitoid,Trioxys complanatus (Hym.: Braconidae) in lucerne and clover

The spotted alfalfa aphid (SAA),Therioaphis trifolii (Monell)f. maculata, was first recorded in Australia in 1977. It is a major pest of lucerne and other pasture medics but not of clover (Trifolium spp.). The parasitoidTrioxys complanatus Quilis was introduced into Australia in late 1977 as part of a biological control program for SAA and made a substantial contribution to the management of the pest. Since 1989, aphids which are individually indistinguishable from SAA have been causing substantial yield loss to clover pastures in western and south-eastern Australia. This aphid, now known in Australia as the spotted clover aphid (SCA), is genetically distinct from SAA and has a much wider host range. Populations of SCA in clover are rarely parasitised byT. complanatus. A series of experiments was undertaken to determine whether the greater susceptibility of SAA in lucerne than of SCA in clover to parasitisation byT. complanatus is due to the attraction of the parasitoid preferentially to lucerne or to the differential attractiveness of the aphids themselves. SAA and SCA were offered to the parasitoids in arenas of increasing complexity from single trifoliate leaves of lucerne and clover on agar through single potted plants in small cages to groups of potted plants in a large cage. Results showed that the parasitoids exhibited a preference for lucerne though, in the cage situation, they did find and parasitise SCA on clover. They did not differentiate between SAA and SCA on lucerne.