Analysis of the performance of different implementations of a heuristic method to optimize forest harvest scheduling

Research ArticleFinding an optimal solution of forest management scheduling problems with even flow constraints while addressing spatial concerns is not an easy task. Solving these combinatorial problems exactly with mixed-integer programming (MIP) methods may be infeasible or else involve excessive computational costs. This has prompted the use of heuristics. In this paper we analyze the performance of different implementations of the Simulated Annealing (SA) heuristic algorithm for solving three typical harvest scheduling problems. Typically SA consists of searching a better solution by changing one decision choice in each iteration. In forest planning this means that one treatment schedule in a single stand is changed in each iteration (i.e. one-opt move). We present a comparison of the performance of the typical implementation of SA with the implementation where up to three decision choices are changed simultaneously in each iteration (i.e. treatment schedules are changed in more than one stand). This may allow avoiding local optimal. In addition, the impact of SA – parameters (i.e. cooling schedule and initial temperature) are tested. We compare our heuristic results with a MIP formulation. The study case is tested in a real forest with 1000 stands and a total of 213116 decision choices. The study shows that when the combinatorial problem is very large, changing simultaneously the treatment schedule in more than one stand does not improve the performance of SA. Contrarily, if we reduce the size of the problem (i.e. reduce considerably the number of alternatives per stand) the two-opt moves approach performs betterinfo:eu-repo/semantics/publishedVersio

A progressive hedging approach to solve harvest scheduling problem under climate change

Due to the long time horizon typically characterizing forest planning, uncertainty plays an important role when developing forest management plans. Especially important is the uncertainty related to recently human-induced global warming since it has a clear impact on forest capacity to contribute to biogenic and anthropogenic ecosystem services. If the forest manager ignores uncertainty, the resulting forest management plan may be sub-optimal, in the best case. This paper presents a methodology to incorporate uncertainty due to climate change into forest management planning. Specifically, this paper addresses the problem of harvest planning, i.e., defining which stands are to be cut in each planning period in order to maximize expected net revenues, considering several climate change scenarios. This study develops a solution approach for a planning problem for a eucalyptus forest with 1000 stands located in central Portugal where expected future conditions are anticipated by considering a set of climate scenarios. The model including all the constraints that link all the scenarios and spatial adjacency constraints leads to a very large problem that can only be solved by decomposing it into scenarios. For this purpose, we solve the problem using Progressive Hedging (PH) algorithm, which decomposes the problem into scenario sub-problems easier to solve. To analyze the performance of PH versus the use of the extensive form (EF), we solve several instances of the original problem using both approaches. Results show that PH outperforms the EF in both solving time and final optimality gap. In addition, the use of PH allows to solve the most di cult problems while the commercial solvers are not able to solve the EF. The approach presented allows the planner to develop more robust management plans that incorporate the uncertainty due to climate change in their plansinfo:eu-repo/semantics/publishedVersio

Accounting for climate change in a forest planning stochastic optimization model

An approach is proposed for incorporating the variations in timber growth and yield due to climate change uncertainty into the forest harvesting decision process. A range of possible climate scenarios are transformed by a forest growth and yield model into tree growth scenarios, which in turn are integrated into a multistage stochastic model that determines the timber cut in each future period so as to maximize net present value over the planning horizon. For comparison purposes, a deterministic model using a single average climate scenario is also developed. The performance of the deterministic and stochastic formulations are tested in a case study of a medium-term forest planning problem for a Eucalyptus forest in Portugal where climate change is expected to severely impact production in the coming years. Experiments conducted using 32 climate scenarios demonstrate the stochastic model's superior results in terms of present value, particularly in cases of relatively high minimum timber demand. The model should therefore be useful in supporting forest planners' decisions under climate uncertainty

Stability of mechanically exfoliated layered monochalcogenides under ambient conditions

Abstract Monochalcogenides of groups III (GaS, GaSe) and VI (GeS, GeSe, SnS, and SnSe) are materials with interesting thickness-dependent characteristics, which have been applied in many areas. However, the stability of layered monochalcogenides (LMs) is a real problem in semiconductor devices that contain these materials. Therefore, it is an important issue that needs to be explored. This article presents a comprehensive study of the degradation mechanism in mechanically exfoliated monochalcogenides in ambient conditions using Raman and photoluminescence spectroscopy supported by structural methods. A higher stability (up to three weeks) was observed for GaS. The most reactive were Se-containing monochalcogenides. Surface protrusions appeared after the ambient exposure of GeSe was detected by scanning electron microscopy. In addition, the degradation of GeS and GeSe flakes was observed in the operando experiment in transmission electron microscopy. Additionally, the amorphization of the material progressed from the flake edges. The reported results and conclusions on the degradation of LMs are useful to understand surface oxidation, air stability, and to fabricate stable devices with monochalcogenides. The results indicate that LMs are more challenging for exfoliation and optical studies than transition metal dichalcogenides such as MoS2, MoSe2, WS2, or WSe2