Change of the Most Probable Escape Path in a Fast-Slow Insect Outbreak System Under Different Noise Amplitude Ratios

In the present paper, noise-induced escape from the domain of attraction of a stable fixed point of a fast-slow insect outbreak system is investigated. According to Dannenberg's theory(Dannenberg PH, Neu JC, 2014)[1], different noise amplitude ratios μ lead to the change of the Most Probable Escape Path(MPEP). Therefore, the research emphasis of this paper is to extend their study and discuss the changes of the MPEPs in more detail. Firstly, the case for μ=1, wherein the MPEP almost traces out the critical manifold, is considered. Via projecting the full system onto the critical manifold, a reduced system is obtained and the quasi-potential of the full system can be partly evaluated by that of this reduced system. In order to test the accuracy of the computed MPEP, a new relaxation method is then presented. Then, as μ converges to zero, an improved analytical method is given, through which a better approximation for the MPEP at the turning point is obtained. And then, in the case that the value of μ is moderate, wherein the MPEP will peel off the critical manifold, to determine the changing point of the MPEP on the critical manifold, an effective numerical algorithm is given. In brief, in this paper, a complete investigation on the structural changes of the MPEPs of a fast-slow insect outbreak system under different values of μ is given, and the results of the numerical simulations match well with the analytical ones.

The effect of insect disturbance on forest carbon flux: a multisite analysis

<p>Insect outbreaks have a substantial effect on forest carbon cycle, including turning forests from carbon sink to carbon source. However, there is still lack of evaluation of the impact of insect disturbance on forests carbon cycle across different forest types at the global scale. Hence, we conducted a multi-site analysis to compare ecosystem CO2 fluxes’ change after an insect outbreak by compiling flux data from the literature or flux database. The final database consists of 21 site-years of eddy covariance data from 17 forest sites among diversity of forest types, namely temperate forest, mangrove, larch forest, etc. Our research showed that insect outbreak had significant negative effects on GEP that GEP reduces -186 gCm-2y-1 on average, as well as NEP reducing -146 gCm-2y-1 while had no significant positive effect on Re. Additionally, similar conclusion was gained when analyzing the recovery procedure that GEP increases with time at the slope of 67.6 gCm-2y-2 and NEP increases at slightly lower rate of 49.2 gCm-2y-2. But there is no evidence that Re will increase. As insects cause more severe damage to forests, all three carbon variables share negative correlation that GEP drops at the slope of -6.7 gCm-2y-1 and NEP also decreases in relatively similar rate at 66.3 gCm-2y-1 with Re decreasing by -3.4 gCm-2y-1. Also, it was found that girdling experiments has different impact on carbon budget from normal insect outbreaks while drought could dampen the damage to GEP followed with a greater recovery rate.</p>

Disturbance history modulates how litter and herbaceous cover influence conifer regeneration after fire

Climate-driven increases in disturbance frequency and extent augment the potential for compounded disturbances. Drawing on well-studied forests that experienced successive disturbances, we asked: (1) how does post-fire cover of litter, herbaceous cover and bare ground vary between stands affected by combinations of blow-down, insect outbreak, and fire? (2) How do post-fire relationships between ground cover and conifer regeneration vary with recent disturbance history? We measured ground cover and conifer regeneration from 2003 to 2014 following stand-replacing fires in 2002. Burned stands were either blown down in 1997, affected by a 1940s Dendroctonus rufipennis (spruce beetle; SB) outbreak, or neither. Implementing mixed-effects models, we measured the relationships between pre-fire stand attributes (structural stage, canopy dominance and combination of disturbances) and post-fire ground cover and between post-fire ground cover and conifer regeneration. Fire-only stands had more litter and herbaceous cover post fire than other stands (P<0.05). Fir regeneration increased with litter in stands that only burned, but decreased with litter in stands that were first blown down. Similarly, pine and fir regeneration increased with herbaceous cover after fire-only, but did the opposite in stands affected by the SB outbreak. Pre-fire legacies can modulate the effects of ground cover on plant regeneration.