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961.
Cyanobacteria are important for global nitrogen cycle and often form complex associations referred to as cyanobacterial mats
or periphyton that are common in tropical, limestone-based wetlands. The objective of this study was to monitor the nitrogen
fixation rate using the acetylene reduction assay of these cyanobacterial mats in a tropical, unfertilized, and protected
wetland. To account for temporal and spatial variation of nitrogenase activity, we were interested in seasons in a hydrological
cycle (dry, rains, and end of rains), sites with different vascular vegetation, and rates of nitrogenase activity in a 24-h
cycle. The annual average of nitrogenase activity was 22 nmol C2H4 cm−2 h−1, with a range of <6 to 35 nmol C2H4 cm−2 h−1, and the annual nitrogen fixation rate of our study site (9.0 g N m−2 year−1) is higher than similar estimates from other freshwater wetlands. There was a clear temporal pattern in nitrogenase activity
with a maximum rate occurring during the rainy season (August) and a maximum nitrogenase activity occurring between 0600 and
1200 hours. We found spatial differences in nitrogenase activity among the four sites that could be attributed to variations
in species composition within the periphyton. 相似文献
962.
Anthropogenic N-deposition represents a significant input of N into semi-arid chaparral and coastal sage scrub (CSS) shrublands
of southern California. High levels of atmospheric N deposition have the potential to increase soil C and N mineralization,
and we hypothesize that semi-arid shrubland soil exposed to long-term (decades) high N deposition will have significantly
higher C and N mineralization potentials. This hypothesis was tested in a laboratory incubation where the inorganic N (NH4+NO3) and CO2 production of soils maintained at a constant temperature of 25°C and a soil moisture of 0.25 g H2O/g (65% water-filled pore space) were sampled sequentially over a 50-week period. The temporal trend in cumulative C and
N mineralization was well described by a first- and zero-order model, respectively. Long-term atmospheric N deposition significantly
increased potential N mineralization but not C mineralization, and both the rate and total N mineralization were significantly
positively correlated with the surface (0–10 cm) soil δ
15N natural abundance and negatively correlated with the surface soil C:N ratio. While the incubation techniques used here do
not provide realistic estimates of in situ C or N mineralization, these assays indicate that atmospheric N deposition has
significantly altered ecosystem N storage and cycling. 相似文献
963.
Ilyas Siddique Caroline Gutjahr Gamini Seneviratne Broder Breckling Sudheera W. Ranwala Ian J. Alexander 《Biology and Fertility of Soils》2007,44(1):163-170
Topsoil properties were determined in forest gardens established about 20 years ago on eroded grassland soils (abandoned tea
lands) in the wet zone of the Sri Lankan highlands. They were compared with adjacent, eroded grasslands (abandoned tea lands)
on strongly weathered soils vs soils at earlier stages of pedogenic development in a two-way analysis of variance. Soil pH
in forest gardens was, on average, 6.1, nearly one unit higher than in the adjacent grasslands. In the garden soils, the cation
exchange capacity (CEC measured at pH 4.8) was nearly double, exchangeable calcium concentrations five times and exchangeable
magnesium three times as high as in the grasslands soils. Total soil N content was found to be nearly 40% higher in the gardens.
Topsoil gravel contents in the gardens were less than half as high as in the grasslands. The increases in exchangeable bases
and N in gardens, relative to grasslands, were attributed to increased nutrient retention and acquisition. Higher retention
was partly due to the higher CECpH4.8, and probably to reduced erosion and increased, continuous fine root density in the garden topsoils. Higher field CEC in
gardens was likely to result from generally higher C contents and from the reversal of acidification, presumably caused by
base accumulation and decomposition processes. Our results suggest that forest garden establishment on degraded grasslands
can lead to accumulation of mobile nutrients in the topsoil, probably due to increased nutrient retention, subsoil uptake
and litter input exceeding nutrient uptake by the standing biomass. 相似文献
964.
Alison M. Miller Julianne M. O’Reilly-Wapstra Brad M. Potts Clare McArthur 《New Forests》2011,42(3):301-316
Mammalian herbivores regularly browse plantation seedlings. In many areas, seedlings require some form of protection if they
are to survive and grow into a productive plantation. Two general approaches for reducing browsing damage to seedlings are
to apply chemical repellents and tree guards. Both methods have existed for a long time, and new variations are constantly
being developed. Seedling stocking guards, a type of tree guard, are being used operationally in Tasmania, Australia, but
there is limited data quantifying their effectiveness and concerns with negative effects on tree performance. Conversely,
although proven effective, repellents are not being used, but are potentially cheaper and less problematic. We therefore aimed
to determine which is more effective under operational conditions, whether this effectiveness can be improved or extended,
and if either treatment has any effects on seedling form or survival. We planted Eucalyptus nitens seedlings with combinations of repellent and stocking guards in six operational plantations to examine and compare their
effectiveness. Seedlings were monitored for 12 months to examine treatment longevity. We found that both stocking guards and
repellent significantly reduced and delayed browsing severity, with their effects being additive. No negative effects on growth
were evident after 12 months, but adverse effects on seedling form warrant further investigation. Both of these methods can
be easily and relatively cheaply applied in the nursery before planting, making them appealing options to reduce browsing.
However, the ideal method for a given site will depend upon local browsing intensity. 相似文献
965.
966.
Leucaena leucocephala and Acacia farnesiana are tree species used for several agricultural purposes in the Mediterranean region. The seeds of these species exhibit dormancy,
causing delayed germination. Two experiments were conducted to investigate the effect of pre-sowing treatments (scarification,
hot water, or soaking) on seed germination of L. leucocephala and A. farnesiana. In one experiment, seeds were exposed to three pre-sowing treatments: control, sandpaper scarification, or soaking in 70°C
water for 4, 8, 12, 16, 20, or 24 min. In another experiment, seeds were soaked in 70°C water for 20 min, and then soaked
in water at room temperature for an additional 24, 48, or 72 h or blade scarified. In general, soaking the seeds of the two
species in hot water was more effective in breaking seed dormancy than scarification. Sandpaper scarification was not effective
for either species. Blade scarification increased A. farnesiana seed germination to 56%, indicating that seed dormancy was mainly a consequence of hardseededness.
L. leucocephala seeds collected from Jordan University of Science and Technology (JUST) site and soaked in 70°C water for 20 min and then
soaked for 24, 48, or 72 h had germination rates above 97%. Our results suggest that blade scarification of A. farnesiana seeds and soaking of L. leucocephala seeds in 70°C water for 20 min are effective treatments to break seed dormancy and enhance seed germination of these vital
species. 相似文献
967.
We examined the contribution of rubberwood to the timber export markets of Malaysia and Thailand. In Malaysia, rubberwood
has grown from 26% of total exported wood products in 1998 to 35% in 2007. A high proportion of furniture products (80%) is
rubberwood, whereas the contribution of rubberwood to other wooden products is less than 20%. Only 10% of sawn timber and
logs is rubberwood. In Thailand, rubberwood contributes to around 60% of total exported wood products, arising from a high
share of not only furniture products (70%) but also other wood products (around 50%) and sawn timber and logs, which have
increased in share from 40% in 1998 to 79% in 2007. We conclude that the high proportion of rubberwood products in the wood
export markets of these two countries is a result of: (1) scarcity of raw wood materials because of strict controls on the
logging of natural forests; and (2) governmental support to rubberwood production, including financial support to rubber planters
and technical assistance to downstream timber processors. 相似文献
968.
Akira Shigematsu Nobuya Mizoue Kumiko Ide Kakada Khun Muthavy Pheng Shigejiro Yoshida Katsuhisa Kohroki Noriko Sato 《New Forests》2011,42(2):149-162
We estimated the production of rubberwood from 1996 to 2011 in Cambodia. We employed data from interview surveys with three
rubberwood processing companies and statistics from estate rubber plantations and wood retailing companies. The result was
that the end use of rubberwood in Cambodia was the sawntimber products, being exported to Vietnam and China. The yield rate
from rubberwood stumpage into sawn timber was between 11.6 and 14.9%, closely matching the rates of Malaysia and Thailand.
The average production of rubberwood roundwood from 1996 to 2011 was 254,697 m3/year, of which sawn timber as well as residuals in secondary processing was 84,559 and 170,138 m3/year, respectively. Although the timber production of Cambodia has declined sharply due to the strict restriction of natural
forest logging, rubberwood could explain more than 60% of the domestic industrial roundwood production from 2000. We suggest
the necessity of monitoring the expansion of rubber plantations in new rubber producing countries and linking the rubberwood
production with the regional wood resource management. 相似文献
969.
A micropropagation method for Jaal (Salvadora persica)—a tree of arid horticulture and forestry has been developed. Nodal segments of fresh shoot sprouts originated from axillary
buds obtained from a plant around 35–40 years old lopped plant were used as explants for establishment of in vitro cultures.
Surface-sterilized explants produced optimum number of shoots through activation of axillary buds on Murashige and Skoog’s
(MS) medium containing 8.88 μM BA (6-benzyladenine) + additives (25 mgl−1 each of adenine sulphate, arginine, citric acid, 50 mgl−1 ascorbic acid). The shoot multiplication was influenced by the successive transfer of the mother explants for 4–5 passages.
The maximum number (23.1 ± 0.73 shoots per explant) of shoots were regenerated on MS supplemented with 1.11 μM BA + 1.16 μM
Kn (Kinetin) + 0.54 μM NAA (α-naphthalene acetic acid). About 90% shoots pulse-treated with a combination of 2460.27 μM Indole-3-butyric
acid (IBA) + 494.56 μM NOA (2-naphthoxy acetic acid) were rooted ex vitro on soilrite within 15–18 days. Over 80% cloned plantlets
were hardened successfully in a green house and transferred to polybag/pots. 相似文献
970.
Studies on reproductive biology are difficult but useful in species like Aegle marmelos (Linn.) Correa which is of considerable socio-economic importance and possess morphogenetic variation and qualities of wider
adaptability to different soils. Cytology, phenology, pollination, breeding system and natural regeneration of wild and cultivated
trees from India revealed the existence of diploid (2n = 18) and tetraploid trees (2n = 36) in Pachmarhi hills and only diploid
trees in Punjab plains and Shiwalik hills. The diploid and tetraploid trees showed normal meiosis and high pollen fertility.
Phenological events which included leaf fall, leaf emergence, floral bud break, flowering and fruiting are nearly the same
in wild and cultivated trees. Natural pollen transfer in the species was highly efficient. Levels of fruit set following open
pollination was quite high and is reduced considerably following hand pollination probably due to some injury caused to stigma
during experiments. Inspite of synchronous nature of anther dehiscence and stigmatic receptivity, selfing in a flower was
avoided due to herkogamy. Some selfing, however, occurred through geitonogamous mode as bagging of panicles yielded 12.21 ± 0.99
to 14.12 ± 0.91% fruit set. High pollen ovule ratio (9,250–10,600) indicated toward the obligate outbreeding nature. The species
suited to insect and wind mode of pollination. High amount of air borne pollen grains deposited on glycerine smeared glass
microscopic slides suggested towards the wind mode of pollination. Though flowers are dull coloured a variety of insects visited
the flowers due to sweet fragrance and stamen/pollen as food reward. Among insect pollinators, honey bee (Apis dorsata) was the major and legitimate pollinator while the rest were either minor pollinators or mere visitors. Inspite of high fruit/seed
set, natural regeneration through seeds was poor as fallen fruits were destroyed by fungal pathogens and white ants. The species
also propagated vegetatively through coppices and root suckers. It lacks agamospermy as bagging of emasculated flowers yielded
no fruit. It is inferred that ‘bael’ which lacks agamospermy reproduced successfully through gamospermy (xenogamy and geitonogamy)
and vegetative mode (coppices and root suckers). We also concluded that tetraploid trees growing in the Pachmarhi hills with
large sized fruits possessed better potentialities in horticulture if planted through root suckers or coppices. 相似文献