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Drought-induced positive feedback in xylophagous insects: Easier invasion of Scots pine leading to greater investment in immunity of emerging individuals

Publication year: 2012
Source: Forest Ecology and Management, Volume 270, 15 April 2012, Pages 147-152
Indrikis Krams, Janīna Daukšte, Inese Kivleniece, Guntis Brūmelis, Raimonds Cibuļskis, ...
We studied the infestation rate of Scots pinePinus sylvestrisby xylophagous insects in relation to distance from forest lakes in eastern Latvia, northern Europe. In summers of 2008 and 2009, we felled 72 pines of approximately 65 years age. Sections of the logs were incubated in insect emergence traps. The trees located near lakes were significantly less infested by xylophagous insects than those sampled at greater distances from the lakes. We also tested the ability ofTomicus piniperda, the most abundant species of xylophagous insects in our samples, to resist the entomopathogenic fungusBeauveria bassiana. The results show that beetles captured near lakes were more susceptible to the fungal infection than individuals sampled away from forest lakes, indicating that the beetles far from lakes afforded to invest more in immunity during the larval phase. During the warmest days in summer the mean maximum ambient temperature was 2.32 °C lower near lakes than away from lakes. Since increased temperatures not only trigger drought stress, but may potentially also cause temperature-sensitive mortality of conifers, a warmer microclimate may lower resistance of pine trees to attacks of xylophagous beetles. In weakened trees, insects can invade more easily resulting in increased investment in immunity in bark- and wood-boring insects. This is the first demonstration of drought-induced positive feedback in xylophagous insects infesting Scots pine. Since bark beetles are common pests of conifers, our results may be important in forest pest management.

Highlights

► Higher temperatures may result in drought stress of pines. ► Carbon starvation can lead to decreased resistance against forest pests. ► Maximum ambient temperature is lower near forest lakes than away from lakes. ► Immunity of bark beetles emerging from pines away from lakes is weak. ► Warmer microclimate lowers resistance of pines to bark beetle attacks.

The role of forest harvesting and subsequent vegetative regrowth in determining patterns of amphibian habitat use

Publication year: 2012
Source: Forest Ecology and Management, Volume 270, 15 April 2012, Pages 163-174
Viorel D. Popescu, David A. Patrick, Malcolm L. Hunter, Aram J.K. Calhoun
Conservation of forest-dependent amphibians is dependent on finding a balance between timber management and species’ habitat requirements. To examine the effect of short-term vegetative regrowth post-harvesting on amphibian habitat use, we studied the response of eight species (four forest specialists and four habitat generalists) to four forestry treatments (partial harvest, clearcut with coarse woody debris [CWD] removed, clearcut with CWD retained, and uncut control) over a 6-year period, using replicated experimental treatments in Maine, USA. Forest amphibians showed a strong negative response to clearcutting through the duration of the study, regardless of the presence of CWD, but only during the post-breeding season (i.e., summer). The spring breeding migrations of wood frogs and spotted salamanders to experimental pools were not affected by the forestry treatments. The use of partial cut treatments by forest amphibians differed between animals emerging from experimental pools (i.e., juvenile wood frogs and spotted salamanders), and animals originating from outside the experimental arrays (i.e., adults of all forest species, juvenile wood frogs and spotted salamanders). Animals emerging from our experimental pools showed no difference in the use of control and partial cut treatments, while all the other animals preferred control plots. In addition, we found a modest increase in the use of clearcuts over the 6 years following harvesting by juvenile wood frogs from experimental pools (from an 8-fold difference between forest and clearcut treatments in the first year post-clearcutting to a 3-fold difference during years 3–5). However, this increase was not significantly associated with vegetation regrowth. Forest specialists declined in abundance in all treatments beginning 2–3 years post-disturbance. Despite high yearly fluctuations in abundance, there was a shift in relative abundance towards habitat generalist species, most notably green frog juveniles. Most habitat generalist species were not affected by clearcutting or vegetative regrowth; however, we observed a lower use of clearcut treatments by green frogs starting 3 years post-harvesting, perhaps due to an increase in habitat resistance to movements associated with vegetative regrowth. These general patterns of habitat use were overridden at the local scale by site-specific variation in the use of forestry treatments, most evident in emigrating juvenile wood frogs. From a management standpoint, implementing broad silvicultural prescriptions could be a viable strategy in extensively forested landscapes, but local variation in habitat use has to be acknowledged when managers focus on a limited area.

Highlights

► We examine the effects of forestry on amphibian habitat use for 6 years post-logging. ► Forest species showed a negative response to canopy removal. ► Habitat use by generalist species was not affected by forest practices. ► Variability in habitat use was the norm at local scale, overriding general patterns. ► Six years of vegetative regrowth did not mitigate the effects of clearcutting.

Exploratory analysis for complex-life-cycle amphibians: Revealing complex forest-reproductive effort relationships using redundancy analysis

Publication year: 2012
Source: Forest Ecology and Management, Volume 270, 15 April 2012, Pages 175-182
Joanna Hawley Howard, Robert F. Baldwin, Bryan L. Brown
The relationships among complex life-cycle (CLC) amphibians and their habitats involve interaction of biotic and abiotic variables across multiple spatial and temporal scales. Most studies of species–environment relationships utilize classic distributions and regression methods which may oversimplify results, as they often involve initial reduction of data,a priorimodel specification, and parsimonious model selection. We re-analyzed a dataset collected in a study of environment-reproductive effort relationships for two amphibian species. This data was originally analyzed using negative binomial regression and AIC model selection; we applied a multivariate analysis, a variant of Redundancy Analysis (RDA), to the same dataset to examine if additional information was revealed. The multivariate analysis identified the same primary drivers as did model selection: in the context of relatively intact forests, hydroperiod was the most important factor contributing to reproductive effort. However, the RDA elucidated what may be new primary and secondary drivers related to forest structure and composition. Reproductive effort was strongly associated with tree species richness, canopy cover, and presence of individual tree species; the strength of these relationships varied between amphibian species and across spatial scales. Spotted salamanders (Ambystoma maculatum) were strongly influenced by microclimate conditions and wood frogs (Lithobates sylvaticus) by pool canopy cover and presence of wetland tree species.A priori, parsimonious model selection assists managers by delivering biologically defensible, simple explanations. However the process reduces hypothesis-generating potential and omits variables that may become important as environments change.

Highlights

► CLC amphibians require multiple habitats; drivers of their breeding effort are complex. ► These reproductive drivers may be poorly reflected in predictive models. ► Using a novel exploratory method, we found additional complexity related to breeding effort. ► We suggest considering secondary drivers revealed by multivariate techniques along with parsimonious models. ► This may be a valuable approach to identify wetland and forest variables to monitor as environmental conditions change.

In-situ measurement of twig dieback and regrowth in matureAcer saccharumtrees

Publication year: 2012
Source: Forest Ecology and Management, Volume 270, 15 April 2012, Pages 183-188
Shaik Md. Yousuf Hossain, John P. Caspersen
Crown shyness is thought to influence both the productivity and dynamics of forests, but few studies have examined the underlying causes of this common phenomenon. The few studies that exist suggest that crown shyness is caused by the reciprocal abrasion of neighboring tree crowns, resulting in the death of peripheral buds and/or the breakage of peripheral twigs (referred to here as twig dieback). However, twig dieback has not been directly observed due to the difficulty of accessing the crowns of mature canopy trees. In this study, we used a mobile canopy lift to obtain in-situ measurements of twig dieback in mature sugar maple (Acer saccharumMarsh.) trees. We measured: (1) percent dieback (% year) – the proportion of twigs that broke off or died; (2) dieback (cm year) – the length of the broken or dead portion; and (3) regrowth (cm year) – the length of the new twigs that sprouted from axillary buds. One third of the twigs suffered dieback over the course of 1 year, resulting in 1.41 cm of dieback, on average. Percent dieback was significantly higher in tree crowns located within 3 m of another crown, indicating that dieback is the result of the reciprocal abrasion of neighboring crowns. Percent dieback also increased with tree size, suggesting that tree sway increases as trees grow larger, resulting in more frequent and more intense abrasion. These trends were exacerbated by similar trends in regrowth, which was slower both in large trees and in trees located with 3 m of another crown. Our results suggest dieback may represent a substantial drain on both tree growth and stand productivity – a drain that increases as trees grow larger and stands mature.

Highlights

► One-third of twigs suffered dieback over the course of 1 year. ► Percent dieback increased significantly with stem diameter and slenderness. ► Regrowth decreased significantly with stem diameter. ► Percent dieback was significantly higher in crowns located within 3 m of one another. ► Regrowth was significantly slower in crowns located within 3 m of one another.

Determinants of tree mortality in mixed old-growthNothofagusforest

Publication year: 2012
Source: Forest Ecology and Management, Volume 270, 15 April 2012, Pages 189-199
Jennifer M. Hurst, Glenn H. Stewart, George L.W. Perry, Susan K. Wiser, David A. Norton
Rates and spatial patterns of tree mortality were examined using long-term data from old-growth, mixed-species forests of the Maruia Valley, South Island, New Zealand. The aim of the study was to investigate patterns of tree mortality in two common, co-occurring species,Nothofagus fusca(Hook. f.) Oerst. andNothofagusmenziesii(Hook. f.) Oerst. The dynamics of three old-growth stands were followed over a 23-year period, using plots sized 0.8–1.0 ha. In total the fates of 1138 individualN. fuscaand 1611N. menziesiiwere recorded, which had annual mortality rates of 0.016 and 0.0089 per year, respectively. Differing spatial and size-related patterns of mortality were found between species. For both species, individual-based logistic models showed that slower growing trees were more likely to die than faster growing trees.N. fuscatrees growing in previously disturbed stands were also more likely to die than those in undisturbed stands. Spatial point process analysis showed that deadN. fuscatrees were spatially aggregated, and were segregated from living trees, a pattern that was consistent across both small and large trees. DeadN. menziesiiwere spatially aggregated, but were not segregated from living trees. Aggregated mortality ofN. fuscatrees should favor regeneration of this light-demanding species in large canopy gaps, perpetuating its aggregated distribution, whereas the lower mortality of shade-tolerantN. menziesiiallows this species to persist. Our results demonstrate that performance differences in coexisting tree species can be manifested spatially. Between species, different mortality patterns may have implications for sustainable forest management.

Highlights

► We investigate patterns of tree mortality for two co-occurringNothofagusspecies. ► Between species, we found different spatial and size-related patterns of mortality. ► Slower growing trees were likely to die, as wereNothofagus fuscagrowing in previously disturbed stands. ► We demonstrate that performance differences between species can be manifested spatially.