5,760 research outputs found
Forest Soil Carbon and Nitrogen Cycles under Biomass Harvest: Stability, Transient Response, and Feedback
Biomass harvest generates an imbalance in forest carbon (C) and nitrogen (N) cycles and the nonlinear biogeochemical responses may have long-term consequences for soil fertility and sustainable management. We analyze these dynamics and characterize the impact of biomass harvest and N fertilization on soil biogeochemistry and ecosystem yield with an ecosystem model of intermediate complexity that couples plant and soil C and N cycles. Two harvest schemes are modeled: continuous harvest at low intensity and periodic clear-cut harvest. Continuously-harvested systems sustain N harvest at steady-state under net mineralization conditions, which depends on the C:N ratio and respiration rate of decomposers. Further, linear stability analysis reveals steady-state harvest regimes are associated with stable foci, indicating oscillations in C and N pools that decay with time after harvest. Modeled ecosystems under periodic clear-cut harvest operate in a limit-cycle with net mineralization on average. However, when N limitation is strong, soil C–N cycling switches between net immobilization and net mineralization through time. The model predicts an optimal rotation length associated with a maximum sustainable yield (MSY) and minimum external N losses. Through non-linear plant–soil feedbacks triggered by harvest, strong N limitation promotes short periods of immobilization and mineral N retention, which alter the relation between MSY and N losses. Rotational systems use N more efficiently than continuous systems with equivalent biomass yield as immobilization protects mineral N from leaching losses. These results highlight dynamic soil C–N cycle responses to harvest strategy that influence a range of functional characteristics, including N retention, leaching, and biomass yield
Critical random forests
Let denote a random forest on a set of vertices, chosen
uniformly from all forests with edges. Let denote the forest
obtained by conditioning the Erdos-Renyi graph to be acyclic. We
describe scaling limits for the largest components of and , in
the critical window or . Aldous
described a scaling limit for the largest components of within the
critical window in terms of the excursion lengths of a reflected Brownian
motion with time-dependent drift. Our scaling limit for critical random forests
is of a similar nature, but now based on a reflected diffusion whose drift
depends on space as well as on time
A Stochastic Model for the Species Abundance Problem in an Ecological Community
We propose a model based on coupled multiplicative stochastic processes to
understand the dynamics of competing species in an ecosystem. This process can
be conveniently described by a Fokker-Planck equation. We provide an analytical
expression for the marginalized stationary distribution. Our solution is found
in excellent agreement with numerical simulations and compares rather well with
observational data from tropical forests.Comment: 4 pages, 3 figures, submitted to PR
A L\'evy area by Fourier normal ordering for multidimensional fractional Brownian motion with small Hurst index
The main tool for stochastic calculus with respect to a multidimensional
process with small H\"older regularity index is rough path theory. Once
has been lifted to a rough path, a stochastic calculus -- as well as solutions
to stochastic differential equations driven by -- follow by standard
arguments. Although such a lift has been proved to exist by abstract arguments
\cite{LyoVic07}, a first general, explicit construction has been proposed in
\cite{Unt09,Unt09bis} under the name of Fourier normal ordering. The purpose of
this short note is to convey the main ideas of the Fourier normal ordering
method in the particular case of the iterated integrals of lowest order of
fractional Brownian motion with arbitrary Hurst index.Comment: 20 page
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