Stand dynamics of old-growth hemlock forests in central Bhutan are shaped by natural disturbances

Understanding how past disturbances have influenced the development of forests is critical for deciphering their current structure and composition and forecasting future changes.In this study,dendrochronological methods were applied to uncover the disturbance history of old-growth hemlock-dominated forests in central Bhutan.Analysis of tree-ring samples from two old-growth hemlock stands,located in two different topographic settings,identified the importance of gap-phase dynamics in facilitating recruitment and growth releases and producing complex,multi-aged structure s over time.One site showed evidence of a near stand-replacing disturbance in the late 1700s,while the other showed no evide nce of high-severity disturbance at any time over the last 400 years.At both sites low-to medium-severity disturbances,some of which appear to be associated with cyclones originating in the Bay of Bengal,dominated the disturbance regime.The hemlock stands exhibited a significant positive association between cyclone occurrence and growth release events and between recruitment pulses and growth release events.From 1800 to 1970 there was an increase in recruitment of angiosperm tree species at most sites and a corresponding decline in conifer recruitment.Over the past 50 years there has been little new recruitment;this may be due to light limitation in the understory from shade-tolerant angiosperms and bamboo in the lower strata of these stands.Significant variations in disturbance dynamics and recruitment were observed across the study sites,suggesting that other factors,such as topography and climate,may be influencing long-term stand development patterns.This study highlights the complex interplay between historical disturbance regimes and tree recruitment in shaping the age and size structures of old-growth hemlock forests in central Bhutan.It also provides new insights into the dynamics of these forests that can be used to support effective forest conservation and management in the future.

Evaluating and quantifying the effect of various spruce budworm intervention strategies on forest carbon dynamics in Atlantic Canada

Spruce budworm (SBW) outbreaks are one of the most devastating natural disturbances in spruce-balsam fir forests of eastern North America. Both early intervention strategy (EIS) and foliage protection strategy (FP) are being tested to limit forest losses, but the quantitative impact on forest carbon (C) dynamics is still unclear. In this study, we designed 19 separate scenarios of no intervention or varying success of EIS, FP, and their combination on SBW caused defoliation and mortality. We then used the TRIPLEX-Insect model to quantify their effects on forest C dynamics in the forests of the four provinces of Atlantic Canada. A scenario applying FP to 10%of the area with the greatest potential C losses of living biomass, protecting foliage in 10%of the forests is more realistic than higher proportion of FP given the high cost and large areas involved, resulted in reducing average cumulative net ecosystem productivity (NEP) from 2020 to 2039 by 56%–127%compared to a no outbreak scenario.Our results showed that FP would have to be applied everywhere to reduce tree mortality and increase NEP more than 8 years of successful EIS applied. However, if EIS can be successfully implemented for 12 years, it will maintain more forest C than FP applied everywhere during a moderate outbreak. We also found that the combination of EIS followed by FP in 10%of the areas disturbed by the SBW could maintain average cumulative NEP at similar levels to no defoliation in every province of Atlantic Canada. Black/red spruce forests younger than 60years old underwent the smallest changes in C dynamics whether using EIS, FP, or both. This highlights the importance of forest species, forest age, and their interactions on the effectiveness of a treatment during SBW outbreak. Overall, 31%–76%of the study area in Atlantic Canada could convert from a C sink to a source by 2039,if no protective measures are used under the worst-case scenarios, thus contributing to future climate warming.

Effects of forest canopy density and epixylic vegetation on nutrient concentrations in decaying logs of a subalpine fir forest

Background:Deadwood and the associated epixylic vegetation influence nutrient cycles in forest ecosystems.Open canopies strongly regulate deadwood decomposition and disrupt epixylic vegetation on logs.However,it is unclear how the forest canopy density and epixylic vegetation growth affect the nutrient concentrations in deadwood.Methods:We measured the concentrations of nitrogen(N),phosphorus(P),potassium(K),calcium(Ca),sodium(Na),magnesium(Mg),and manganese(Mn)in experimentally exposed decaying logs placed in gaps,at the edge of gaps,and under the closed canopy during a four-year decomposition experiment in a Subalpine Faxon fir forest(Abies fargesii var.faxoniana)on the eastern Qinghai-Tibetan Plateau,China.To assess the effect of the epixylic vegetation,we experimentally removed it from half of the logs used in the study.Results:Under open canopy conditions in the gap and at the edge,the concentrations for most of the nutrients in the bark and the highly decayed wood were lower than under the closed canopy.The effect of the epixylic treatment on nutrient concentrations for all but K and Na in barks varied with the decay classes.Significantly lower concentrations of N,P,Ca,and Mn following the removal of epixylic vegetation were observed in the wood of decay class IV.Epixylic vegetation significantly increased most nutrient concentrations for decaying barks and wood under open canopy conditions.In contrast,epixylic vegetation had no or minimal effects under the closed canopy.Conclusions:Forest canopy density and epixylic vegetation significantly alter the nutrient concentrations in decaying logs.Open canopies likely accelerate the rate of nutrient cycling between the epixylic vegetation and decaying logs in subalpine forests.

Conceptual models of forest dynamics in environmental education and management:keep it as simple as possible,but no simpler

Background：Conceptual models of forest dynamics are powerful cognitive tools,which are indispensable for communicating ecological ideas and knowledge,and in developing strategic approaches and setting targets for forest conservation,restoration and sustainable management.Forest development through time is conventionally described as a directional,or ＂linear＂,and predictable sequence of stages from ＂bare ground＂ to old forest representing the ＂climax-state＂.However,this simple view is incompatible with the current knowledge and understanding of intrinsic variability of forest dynamics.Hypothesis：Overly simple conceptual models of forest dynamics easily become transformed into biased mental models of how forests naturally develop and what kind of structures they display.To be able to communicate the essential features and diversity of forest dynamics,comprehensive conceptual models are needed.For this end,Kuuluvainen（2009） suggested a relatively simple conceptual model of forest dynamics,which separates three major modes of forest dynamics,and incorporates state changes and transitions between the forest dynamics modes depending on changes in disturbance regime.Conclusions：Conceptual models of forest dynamics should be comprehensive enough to incorporate both longterm directional change and short-term cyclic forest dynamics,as well as transitions from one dynamics mode to another depending on changes in the driving disturbance regime type.Models that capture such essential features of forest dynamics are indispensable for educational purposes,in setting reference conditions and in developing methods in forest conservation,restoration and ecosystem management.

Variable retention harvesting in Great Lakes mixed-pine forests: emulating a natural model in managed ecosystems

Variable retention harvesting(VRH)systems have gained wide use in many different forest types across the globe,but largely have been implemented in forests characterized by severe,infrequent disturbance regimes.There has been less attention given to developing VRH approaches in forests that are characterized as having a mixed-severity disturbance regime that often results in only partial mortality of canopy trees in spatially heterogeneous patterns.One example of such a forest type is red pine(Pinus resinosa Ait.)-dominated ecosystem of the western Great Lakes region of North America.The purpose of this review is to provide a conceptual foundation for developing VRH approaches in red pine ecosystems that are based on a mixed-severity disturbance regime.Our contention is that red pine forests managed following a natural model are more resilient to disturbances and external threats such as climate change.For the red pine ecosystem,VRH application should reflect the often severe,but partial canopy removal from natural disturbance that is characteristic of this ecosystem and that results in more than trivial numbers of surviving overstory trees across a range of spatial configurations in regenerating stands.Retained live trees should span a range of diameters,but favor the larger end of the diameter distribution,as this reflects the likely pattern of survival after natural disturbance and is often a key structural element lacking from managed areas.VRH should be applied in ways that vary the spatial pattern of legacy trees in and among stands,but largely in ways that reflect the pattern of spatially patchy canopy structure,with large openings surrounded by a less disturbed matrix,as occurs with a natural disturbance regime.Legacy trees and deadwood structures should reflect the composition of the pre-disturbance forest,including species in addition to dominant red pine.Finally,retained structures should be viewed as dynamic entities that grow,die,and decay and that need to be documented and accounted for over time.While more organizations are incorporating some form of VRH into policy and practice for red pine-dominated ecosystems,this application is not always based on a comprehensive understanding of the actual natural model of development,which reflects a mixed-severity disturbance regime.Our goal is to review the ecological evidence for this disturbance regime and interpret the structural and compositional outcomes of the disturbance model,so as to advance VRH approaches that better emulate the actual disturbance and development model for this regionally important ecosystem.

Gap Dynamics and Tree Species Diversity in a Tropical Montane Rain Forest of Hainan Island,China

Based on investigation of 53 gaps and 25 quadrats (15m×15m each) of non-gap closed stand in an old-growth tropical montane rain forest of Hainan Island, China, canopy disturbance regimes and gap regeneration were studied. Gaps were elliptical in horizontal form, the ratio of long axis /short axis was about 1.4. Percentage of expanded gaps (EG) and canopy gaps (CG) area in the landscape were 53.5% and 25.2% respectively. EG ranged from 31.4 m2 to 488.2m2 and CG/rom 14.9m2 to 354.2m2, their average sizes ...