926 research outputs found
Assessing the growth and climate sensitivity of secondary forests in highly deforested Amazonian landscapes
Tropical forests hold 30% of Earthâs terrestrial carbon and at least 60% of its terrestrial biodiversity, but forest loss and degradation are jeopardizing these ecosystems. Although the regrowth of secondary forests has the potential to offset some of the losses of carbon and biodiversity, it remains unclear if secondary regeneration will be affected by climate changes such as higher temperatures and more frequent extreme droughts. We used a data set of 10 repeated forest inventories spanning two decades (1999â2017) to investigate carbon and tree species recovery and how climate and landscape context influence carbon dynamics in an older secondary forest located in one of the oldest postâColumbian agricultural frontiers in the Brazilian Amazon. Carbon accumulation averaged 1.08 Mg·haâ1·yrâ1, and species richness was effectively constant over the studied period. Moreover, we provide evidence that secondary forests are vulnerable to drought stress: Carbon balance and growth rates were lower in drier periods. This contrasts with drought responses in primary forests, where changes in carbon dynamics are driven by increased stem mortality. These results highlight an important climate changeâvegetation feedback, whereby the increasing dryâseason lengths being observed across parts of Amazonia may reduce the effectiveness of secondary forests in sequestering carbon and mitigating climate change. In addition, the current rate of forest regrowth in this region was low compared with previous panâtropical and Amazonian assessmentsâour secondary forests reached just 41.1% of the average carbon and 56% of the tree diversity in the nearest primary forestsâsuggesting that these areas are unlikely to return to their original levels on politically meaningful time scales
Necessary Optimality Conditions for Higher-Order Infinite Horizon Variational Problems on Time Scales
We obtain Euler-Lagrange and transversality optimality conditions for
higher-order infinite horizon variational problems on a time scale. The new
necessary optimality conditions improve the classical results both in the
continuous and discrete settings: our results seem new and interesting even in
the particular cases when the time scale is the set of real numbers or the set
of integers.Comment: This is a preprint of a paper whose final and definite form will
appear in Journal of Optimization Theory and Applications (JOTA). Paper
submitted 17-Nov-2011; revised 24-March-2012 and 10-April-2012; accepted for
publication 15-April-201
Leguminous cover crops improve the profitability and the sustainability of rainfed olive (Olea europaea L.) orchards: from soil biology to physiology of yield determination
The olive sector has a crucial economic, social, cultural and ecological relevance in the Mediterranean region, where tillage and
herbicides application still are generalized practices. However, these techniques oppose to the recommendations of UE policy.
Thus, other methods are needed to reduce adverse environmental impacts and to improve biodiversity, soil carbon sink and
fertility, to save fossil fuels and to increase yield and the safety and nutritional value of food products. Meanwhile, since
Mediterranean basin is particularly vulnerable to climate change, including lower precipitation in summer, olive tree will
experiment some hard changes, mainly under rainfed conditions. Therefore, we propose an adequate management of cover crops
to shift tillage and herbicides, in order to minimize runoff and evaporation water losses, conserve soil moisture storage and
promote the infiltration of water in soil. The experiment was carried out during 4 years on a commercial orchard (cv.
Cobrançosa) in Northeast Portugal. The treatments laid out were: (1) ordinary tillage techniques (OT) used by local growers (two
tillage trips per year); (2) cover crop with self-reseeding annual legume species (AL); (3) natural vegetation fertilized (NVF)
with 60 kg N hm2 (as in OT); (4) natural vegetation (NV) left unfertilized. The results revealed that AL treatment is the best
option, reaching 37, 53 and 95% higher cumulative yield than NVF, OT and NV, respectively, in a closely association with
greater physiological performance during the summer, mainly evidenced by lower oxidative damage and by favourable changes
in water status and net photosynthetic rate, due to lower stomatal and mesophyll limitations. Moreover, the AL covered soil
presented considerable microbial diversity and enzymatic activities, which may contribute to promote and conserve soil quality
and health, as well the stability of ecosystems. Thus, leguminous cover crops improve the profitability and the sustainability of
rainfed olive orchards.info:eu-repo/semantics/publishedVersio
Green manure legumes affect seasonal soil and leaf CO2 exchange rates in an olive rainfed orchard
Management methods that decrease requirement for agricultural chemicals are needed
to reduce adverse environmental impacts. Moreover, they can play an important role in
atmospheric C02 emission and fixation. We propose green manure legumes to reduce
erosion, soil compaction and nitrate pollution, to improve biodiversity, soil carbon sink
and fertility, to save fossil fuels and to increase yield and the nutritional value of food
products. The research was carried out near Mirandela, Northeast Portugal, on a 15-
year commercial olive orchard (Olea europaea L. cv. Cobrançosa), grown under
rainfed conditions
Magnetic Charge in a Nonassociative Field Theory
The violation of the Jacobi identity by the presence of magnetic charge is
accomodated by using an explicitly nonassociative theory of octonionic fields.
It is found that the dynamics of this theory is simplified if the Lagrangian
contains only dyonic charges, but certain problems in the constrained
quantisation remain. The extension of these concepts to string theory may
however resolve these difficulties.Comment: 10 pages, REVTeX, no figure
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