Dendroentomological and forest management implications in the Interior Douglas-fir zone of British Columbia, Canada

Douglas-fir (Pseudotsuga menziesii var. glauca Mirb. Franco) forests in the Interior of British Columbia, Canada, show periodic defoliation due to western spruce budworm (WSB) (Choristoneura occidentalis Freeman) outbreaks. Tree defoliation causes a reduction in radial growth and is therefore visible in tree rings. In this paper, we identify WSB defoliation history, and critically examine the potential for using dendrochronological analysis by comparing tree-ring estimates with insect surveys. WSB defoliation history was investigated using cores from Douglas-fir growing in the Lac du Bois region of the Kamloops Forest District. Years with an abrupt decrease in radial growth were considered as negative pointer years that potentially reflected WSB outbreaks. The comparison with ponderosa pine (Pinus ponderosa Dougl., ex P. & C. Laws.) (a non-host species) permitted differentiation between growth reductions in Douglas-fir due to climatic effects and those due to defoliation by WSB. The dendrochronological data were matched with information reporting visible damage in Forest Insect Disease Survey (FIDS) and British Columbia Ministry of Forest records. Our objective-based method using ring-width measurements from host and non-host chronologies was compared with qualitative techniques based on the software program OUTBREAK. We were able to distinguish seven distinct outbreak events in 300 years of record.     

The potential for using Larix decidua ring widths in reconstructions of larch budmoth (Zeiraphera diniana) outbreak history: dendrochronological estimates compared with insect surveys

In the Alps, larch (Larix decidua Mill.) forests show periodic discolouration due to larch budmoth (LBM) outbreaks (Zeiraphera diniana Guénée, Lepidoptera: Tortricidae). Tree defoliation causes severe reductions in radial growth, visible in tree rings. This paper aims at reconstructing LBM outbreak history, and critically examining the potential for using dendrochronological data by comparing tree-ring estimates with insect surveys. The occurrence of LBM outbreaks was investigated using 249 cores from larch growing near the timberline in three regions of the French Alps (Briançonnais, Maurienne, and Tarentaise). Years with an abrupt decrease in radial growth (-40%) were considered as negative pointer years reflecting the potential impact of LBM. The comparison with three non-host conifers (Norway spruce, stone and mountain pines) permitted us to distinguish growth reductions in larch due to climatic effects from those due to defoliation by LBM. The dendrochronological data were matched with information reporting conspicuous discolouration in old forestry reports or recovered from systematic field observations. Twenty-two outbreaks are discernible within the period 1800-1983. A peak reduction in larch growth occurred at intervals of 8.86ǃ.01 years in the following years: 1801, 1811, 1820, 1830, 1838, 1846, 1857, 1867, 1875, 1884, 1892, 1901, 1909, 1918, 1926, 1936, 1945, 1953, 1963, 1972, 1980, and 1987. Our objective method based on ring measurements and comparison with non-host trees was compared with qualitative techniques based on the visual detection of conspicuous latewood anomalies. Larch in the Briançonnais (which experiences a continental climate) are infested first, whereas the Tarentaise region exhibits a much weaker impact of LBM. Complete tree recovery was observable 3 years after outbreak peaks.     

Linking aerial survey data of forest insect defoliation and tree ring data to estimate forest level growth losses

Aerial surveys provide valuable information on the scale and severity of defoliation and mortality caused by forest insects. However, methods to estimate the losses in growth using aerial survey data are not available. This study presents a method to link aerial survey data of location and severity of insect defoliation to tree ring series of radial growth in order to quantify potential growth losses due to defoliation. The method includes estimation of potentially confounding effects of climate. As an example we used aerial surveys of hemlock looper defoliation in the Mt. Revelstoke region of British Columbia, and tree ring series of radial growth of 351 western hemlock trees located in 22 plots and exposed to various levels of periodic defoliation. An autoregressive conditional heteroskedasticity time-series model (ARCH) of standardized growth indices identified significant direct and lagged effects of climate and defoliation on radial growth between 1930 and 2005. Defoliation effects were statistically significant in all plots (P < 0.05). The variation in both timing and strength of estimated growth reductions varied considerably among plots. During years of observed defoliation the radial growth was reduced by an average of 10%. Growth reductions attributed to defoliation were most pronounced in the second and third year of a looper outbreak. Our modelling approach linked tree ring data to aerial survey data of defoliation through a logistic distance dependent threshold function and affords a quantitative estimation of regional growth losses.     

Spatial Distribution of Mountain Pine Beetle Outbreaks in Relation to Climate and Stand Characteristics： A Dendroecological Analysis

Principal components analysis, followed by K-means cluster analysis, was used to detect variations in the timing and magnitude of Pinus contorta Dough ex Loud. growth releases attributed to mountain pine beetle outbreaks in 31 stands of central British Columbia. Four major growth release patterns were identified from 1970 to 2000. Variations in the timing of growth releases among clustered stands corresponded well to aerial survey data indicating the timing of beetle outbreaks in the study area. Redundancy analysis was used to determine how variations in the timing and magnitude of growth releases attributed to beetle outbreaks changed with variations in climate or stand conditions over the study area. The first RDA axis, which accounted for 39% of the variations in growth patterns among stands, was significantly （P〈0.05） correlated with gradients in the percentage of pine in stands killed by mountain pine beetle, summer aridity, variation in summer precipitation, distance from initial infestation site, average pine age, and maximum August temperatures. The second RDA axis explained 6% of the variations and was significantly correlated with gradients in the beetle climate suitability index, extreme cold month temperatures, and site index. Comparisons of growth release patterns with aerial survey data and redundancy analyses indicated that dendrochronological techniques are useful for identifying mountain pine beetle outbreaks in central British Columbia, particularly among stands that had a density high enough to produce a growth release signal. Provided future studies account for interannual weather fluctuations, identification of growth increases due to stand thinning caused by beetle outbreaks will be useful for reconstructing the history of beetle outbreaks over much longer time periods.     

Variation in wood anatomical structure of Douglas-fir defoliated by the western spruce budworm: a case study in the coastal-transitional zone of British Columbia,Canada

Key message

An outbreak of the western spruce budworm temporarily modifies cellular wood anatomy of stem wood in natural and mature Douglas-fir stands impacting wood quality properties.

Abstract

Western spruce budworm (Choristoneura occidentalis Freeman) is a widespread and destructive defoliator of commercially important coniferous forests in western North America. In British Columbia, Canada, Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] is the most important and widely distributed host. Permanent sample plots were established at a number of locations in southern interior at the beginning of a severe western spruce budworm outbreak in the 1970s. Two of the sites were sampled in 2012 to determine whether modifications had occurred in the anatomical characteristics of stem wood formed during outbreak years. We determined that rings formed during the western spruce budworm 1976–1980 outbreak had a significantly lower proportion of latewood, reduced mean cell wall thickness and smaller radial cell diameters. While the cellular characteristics of the earlywood remained fairly constant, significant reductions in lumen area occurred in 1978 and 1979 at each site. Our study shows that western spruce budworm outbreaks not only reduce annual radial growth, but also temporarily modify cellular characteristics in latewood cells, which has implications for wood density and quality in Douglas-fir.     

El Niño-Southern Oscillation modulates insect outbreaks in humid subtropical China: Evidences from tree rings and carbon isotopes

Insect outbreaks are key disturbances triggering decline episodes in humid subtropical evergreen forests in eastern China. However, their long-term variability and linkages to climate are still unclear largely due to the shortness of records reporting forest damage. A retrospective approach would improve our understanding of long-term outbreak variability and its dependence on climate and relevant atmospheric circulation patterns such as the El Niño-Southern Oscillation (ENSO). This study provides a dendrochronological reconstruction of Dendrolimus punctatus outbreaks affecting Pinus massoniana in humid subtropical eastern China. We used tree-ring width and carbon isotope data from sites where outbreaks were or not were reported for the past fifty years, here considered as host and non-host sites, respectively. The results showed that inter-annual changes in carbon isotope discrimination are more sensitive to outbreaks than changes in tree-ring width. Based on carbon isotopes, we inferred that photosynthetic rates increased as a compensatory mechanism in response to insect defoliation. We reconstructed eleven insect outbreaks since 1915, which corresponded to ENSO positive phases and below-average sea surface temperatures in the central and eastern tropical Pacific Ocean. These conditions correspond to “La Niña” episodes which induce hot and dry climate conditions across the study region and these may facilitate the occurrence of D. punctatus outbreaks.     

Spatial Distribution of Mountain Pine Beetle Outbreaks in Relation to Climate and Stand Characteristics:A Dendroecological Analysis

Principal components analysis, followed by K-means cluster analysis, was used to detect variations in the timingand magnitude of Pinus contorfa Dougl. ex Loud. growth releases attributed to mountain pine beetle outbreaks in31 stands of central British Columbia. Four major growth release patterns were identified from 1970 to 2000.Variations in the timing of growth releases among clustered stands corresponded well to aerial survey dataindicating the timing of beetle outbreaks in the study area. Redundancy analysis was used to determine howvariations in the timing and magnitude of growth releases attributed to beetle outbreaks changed with variationsin climate or stand conditions over the study area. The first RDA axis, which accounted for 39% of the variations ingrowth patterns among stands, was significantly (P<0.05) correlated with gradients in the percentage of pine instands killed by mountain pine beetle, summer aridity, variation in summer precipitation, distance from initialinfestation site, average pine age, and maximum August temperatures. The second RDA axis explained 6% of thevariations and was significantly correlated with gradients in the beetle climate suitability index, extreme coldmonth temperatures, and site index. Comparisons of growth release patterns with aerial survey data and redun-dancy analyses indicated that dendrochronological techniques are useful for identifying mountain pine beetleoutbreaks in central British Columbia, particularly among stands that had a density high enough to produce agrowth release signal. Provided future studies account for interannual weather fluctuations, identification ofgrowth increases due to stand thinning caused by beetle outbreaks will be useful for reconstructing the history ofbeetle outbreaks over much longer time periods.     

Assessing the climatic sensitivity of Douglas-fir at its northern range margins in British Columbia, Canada

Northern hemisphere tree species growing at their northern range margins may be particularly responsive to climate change and could provide important information regarding future broad-scale responses. We analyzed and compared tree-ring data between five Douglas-fir [Pseudotsuga menziesii var. glauca (Mirb.) Franco] populations growing at the species’ northernmost distribution in British Columbia, Canada, and five populations located 150 km to the southeast. We quantified climate–growth relationships using uni- and multivariate techniques at different temporal scales. Our data suggest that (1) even at its northernmost distributions, precipitation limits long-term mature Douglas-fir radial growth more than temperatures, (2) northernmost Douglas-fir populations are distinct from populations located further within the species’ range in terms of certain key short-term growth responses, and (3) northernmost Douglas-fir growth sensitivities to climate may be increasing over time. In the future, mature Douglas-fir productivity in the northern portion of its range may be primarily limited by precipitation, and responses may be strongest at the species’ range margins.     

Predicted short-term radial-growth changes of trees based on past climate on Vancouver Island,British Columbia

Tree-ring radial expansion estimator (TREE) is an integrated radial growth model that allows users to define short-term climate change scenarios to anticipate the impact upon mature trees found growing at high elevation on Vancouver Island, British Columbia. Five individualistic models were built to represent the radial growth behaviour of mountain hemlock (Tsuga mertensiana (Bong.) Carr), yellow-cedar (Chamaecyparis nootkatensis (D. Don) Spach), western red-cedar (Thuja plicata Donn), Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), and western hemlock (Tsuga heterophylla (Raf.) Sarg.) trees. The models were developed on climate-radial growth relationships incorporating Nanaimo climate station data, and were able to explain from 55 to 68 per cent of the variance in radial growth. The models can be run with modifications to yearly precipitation and temperature variables, giving the user the ability to investigate the radial-growth impacts of a wide range of possible climate change scenarios. Results from eight such scenarios show that species growing within their ecological limits illustrate a limited change in radial growth to forecasted climate, while species growing at an ecotonal boundary are usually very sensitive to a specific climate variables (e.g., July temperature). A forecasted alteration to this key variable will then radically alter the radial-growth rate of the species.     

Spatial Distibution of Mountain Pine Beetle Outbreaks in Relation to Climate and Stand Characteristics:A Dendroecological Analysis

Principal components analysis, followed by K-means cluster analysis, was used to detect variations in the timing and magnitude of Pinus contorta Dougl. ex Loud. growth releases attributed to mountain pine beetle outbreaks in 31 stands of central British Columbia. Four major growth release patterns were identified from 1970 to 2000.Variations in the timing of growth releases among clustered stands corresponded well to aerial survey data indicating the timing of beetle outbreaks in the study area. Redundancy analysis was used to determine how variations in the timing and magnitude of growth releases attributed to beetle outbreaks changed with variations in climate or stand conditions over the study area. The first RDA axis, which accounted for 39% of the variations in growth patterns among stands, was significantly (P＜0.05) correlated with gradients in the percentage of pine in stands killed by mountain pine beetle, summer aridity, variation in summer precipitation, distance from initial infestation site, average pine age, and maximum August temperatures. The second RDA axis explained 6% of the variations and was significantly correlated with gradients in the beetle climate suitability index, extreme cold month temperatures, and site index. Comparisons of growth release patterns with aerial survey data and redundancy analyses indicated that dendrochronological techniques are useful for identifying mountain pine beetle outbreaks in central British Columbia, particularly among stands that had a density high enough to produce a growth release signal. Provided future studies account for interannual weather fluctuations, identification of growth increases due to stand thinning caused by beetle outbreaks will be useful for reconstructing the history of beetle outbreaks over much longer time periods.     

A dendroecological reconstruction of western spruce budworm outbreaks (Choristoneura occidentalis) in the Front Range,Colorado, from 1720 to 1986

Radial increment cores from Douglas-fir (Pseudotsuga menziesii) and blue spruce (Picea pungens), defoliated by western spruce budworm (Choristoneura occidentalis), were analyzed by means of dendrochronological methods and compared with cores from undefoliated ponderosa pine (Pinus ponderosa) and lodgepole pine (Pinus contorta) growing on the same sites in the Front Range, Colorado. Extensive deforestation during the gold and silver booms in the second part of the nineteenth century led to dense and almost pure stands of shadetolerant budworm host species. By using the skeleton plot method, the number of trees with clear growth reductions is obtained, thus representing an exact record of forest insect attacks. The analysis of abrupt growth reductions revealed at least nine outbreaks of western spruce budworm between 1720 and 1986, the majority occurring in the nineteenth century. The outbreaks were graphically compared with periods of attack in New Mexico and Colorado which were detected by other scientists employing tree-ring measurement techniques. No increase in the frequency of severe outbreaks during the twentieth century was observed, yet there is some evidence that the most recent outbreak might be the most severe ever recorded. Open Douglas-fir stands on higher sites were more susceptible to heavy budworm attack than dense stands on lower sites. Blue spruce was less frequently and less severely attacked than Douglas-fir. The spatial pattern of historical outbreaks generally was very patchy.     

Tree-ring analysis of larch sawfly (Pristiphora erichsonii (Hartig)) defoliation events and hydrological growth suppression in a peatland

Insect defoliation events are a major forest disturbance in the boreal forest in Canada. Reconstructing previous events are crucial to understanding natural factors that lead to insect defoliation periods, improving our ability to predict future infestations and increasing the reliability of forest management plans and pest control programs. Researchers have often been limited in their ability to draw accurate conclusions regarding the history of larch sawfly (Pristiphora erichsonii (Hartig)) infestation events in North America. It is well known that floods can affect survival of larch sawfly populations, as well as suppress radial growth of eastern larch (Larix laricina (Du Roi) K. Koch) trees. Eastern larch often inhabits peatlands where high water-table levels can lead to a decrease in tree-ring widths. Water-table level increases result in similar radial-growth patterns to when trees are defoliated by larch sawfly, making accurate diagnoses of larch sawfly events a challenge. This fact becomes more accentuated when non-host species used for standard dendroecological analyses (often black spruce (Picea mariana (Mill.) Britton) present an opposite relationship to water-table variability, enhancing chances of drawing erroneous conclusions when using program OUTBREAK.This paper introduces a novel case study to advance the understanding of the complex landscapes where these larch sawfly events are taking place. It stresses the necessity of using upland sites less influenced by the hydrological regime to accurately reconstruct larch sawfly defoliation events. We found that where investigations are taking place solely in peatlands or lowlands, and where they are heavily influenced by hydrological conditions, the use of other lines of evidence such as pale rings and long-term hydrological records are crucial to understand the dynamics in the system. In our case study, program OUTBREAK identified five defoliation events in a peatland and upland site; however, hydrological data and pale ring evidence indicated that the last event identified in the peatland site by the host/non-host analysis was confounded by hydrological growth suppression.     

Insect defoliation modulates influence of climate on the growth of tree species in the boreal mixed forests of eastern Canada

Increasing air temperatures and changing precipitation patterns due to climate change can affect tree growth in boreal forests. Periodic insect outbreaks affect the growth trajectory of trees, making it difficult to quantify the climate signal in growth dynamics at scales longer than a year. We studied climate‐driven growth trends and the influence of spruce budworm (Choristoneura fumiferana Clem.) outbreaks on these trends by analyzing the basal area increment (BAI) of 2058 trees of Abies balsamea (L.) Mill., Picea glauca (Moench) Voss, Thuja occidentalis L., Populus tremuloides Michx., and Betula papyrifera Marsh, which co‐occurs in the boreal mixedwood forests of western Quebec. We used a generalized additive mixed model (GAMM) to analyze species‐specific trends in BAI dynamics from 1967 to 1991. The model relied on tree size, cambial age, degree of spruce budworm defoliation, and seasonal climatic variables. Overall, we observed a decreasing growth rate of the spruce budworm host species, A. balsamea and P. glauca between 1967 and 1991, and an increasing growth rate for the non‐host, P. tremuloides, B. papyrifera, and T. occidentalis. Our results suggest that insect outbreaks may offset growth increases resulting from a warmer climate. The observation warrants the inclusion of the spruce budworm defoliation into models predicting future forest productivity.     

Tree ring δ<Superscript>15</Superscript>N as validation of space-for-time substitution in disturbance studies of forest nitrogen status

Forest ecosystem nitrogen (N) response to disturbance has often been examined by space-for-time substitution, but there are few objective tests of the possible variation in disturbance type and intensity across chronosequence sites. We hypothesized that tree ring δ¹⁵N, as a record of ecosystem N status, could validate chronosequence assumptions and provide isotopic evidence to corroborate N trends. To test this we measured soil N availability, soil δ¹⁵N, and foliar N attributes of overstory Douglas-fir (Pseudotsuga menziesii) and understory western hemlock (Tsuga heterophylla) across three old-growth stands and nine second-growth plantations on southeast Vancouver Island, British Columbia (Canada). Increment cores for wood δ¹⁵N were retrieved from three co-dominant Douglas-fir per plot. Bulk soil δ¹⁵N was well aligned with both foliar and recent wood δ¹⁵N, demonstrating the utility of wood δ¹⁵N in monitoring ecosystem N status. Strongly contrasting trends in tree ring δ¹⁵N were evident among second-growth stands, with most trees from plantations older than 50 years exhibiting steep declines (3–4‰) in δ¹⁵N but with no temporal trends detected for younger plantations. The discrepancy in tree ring δ¹⁵N suggests disturbance history varied considerably among second-growth sites, likely because of greater slash loads and hotter broadcast burns on older cutblocks. As a consequence, the pattern of increased soil N availability and foliar N concentration with time since disturbance derived from the chronosequence could not be validated. Tree ring δ¹⁵N may provide insights into disturbance intensity, especially fire, and correlations with foliar N concentration could inform the extent of changes in stand nutrition.     

Growth ring response in Paratecoma peroba (Record) Kuhlm. of seasonal semideciduous forest in Southeast Brazil

Seasonal climate can influence the radial growth of woody species, resulting in physical indications of regional environmental events that are identifiable through dendrochronological analysis. The objective of this work was to investigate the dendroclimatological potential of the Paratecoma peroba occurring in the last remnant of seasonal semideciduous forest in Rio de Janeiro, Brazil, and to characterize the anatomical structure of its growth rings. This important species has a large ecological role in the forest as a late succession but is classified as “Endangered" because it is extensively and illegally exploited due to the use potential of wood. Samples were collected and analyzed using dendrochronological methods and wood anatomy. P. peroba has distinct annual growth rings with diffuse to semi-porous porosity, marginal parenchyma, radial flattening, and greater thickening of the fibre wall in the latewood. It may sometimes present weakly distended rays and a higher frequency of vessels in the initial wood. The analyzed individuals are 30–77 years of age and exhibit a radial increase of 2–4 mm year^-1. Our results indicate that precipitation and temperature influence the growth of P. peroba in this forest and that temperature is the climatic factor with the most influence on the growth ring of the species. Given the importance of the species and the study area, knowing its growth rates and the factors that influence it is possible to offer better criteria for managing species and its reforestation for conservation.     

Precision of dating insect outbreaks using wood anatomy: the case of Anoplophora in Japanese maple

To control invasive Anoplophora outbreaks, it is crucial to accurately date infestation dynamics. Dating of Anoplophora outbreaks is possible as these xylobiont insects induce wounds in living trees by forming e.g., exit holes. This study investigates to what precision these wounds can be dated with dendrochronological techniques. In an experimental setting, we studied the precision of wound dating on Acer palmatum, an ornamental tree occasionally containing larvae of A. chinensis. We studied the development of wound reactions at the beginning, during and after the growing season, both in relation to leaf phenology and intra-annual tree-ring growth. We found that the precision of dating exit holes is limited due to the highly variable intra-annual tree-ring growth whereby only an accurate distinction can be made between wounds originated during, or after tree-ring formation. The resolution was improved using local growth—the local reactivation of xylem growth around the wound—as a marker for wounds that occurred at the end of the growing season. We conclude that the intra-annual precision of dating Anoplophora outbreaks in Acer palmatum in the temperate North-western European climate is limited to three distinct phases: (i) The period of dormancy and leaf emergence (ca. October until April/beginning of May), when the wounds are located at the tree-ring boundary (ii) The period of tree-ring growth in which wounds are located within the tree ring (ca. end of April/beginning of May until late August/beginning of September), (iii) end of growing season (ca. end of August/September) in which local growth occurs.     

Recent decline of high altitude coniferous growth due to thermo-hydraulic constrains: evidence from the Miyaluo Forest Reserve,Western Sichuan Plateau of China

Tree growth decline has been reported in many places around the globe under the context of increasingly warming climate, and strengthening drought intensity is detected to be the primary factor for such decline, particularly in northern forest sites, as well as arid and semi-arid areas. Yet, the forest growth decline in high altitude, high mountain sites certainly merits investigation. Here, we reported faxon fir (Abies fargesii var. faxoniana) forest growth decline (slope = -0.64) at the tree line (4150 m above sea level) in Miyaluo Forest Reserve (MFR) at the Western Sichuan Plateau, southwestern China since 2000. We investigated the cause of tree growth decline by applying dendrochronological approaches. We took tree-ring samples from fir trees at the tree line and developed tree-ring width (TRW) chronology. The tree growth – climate relationship analysis showed that maximum temperature (Tmax) was the primary factor limiting the radial growth of fir trees in the investigated area. The moving correlation analysis indicated the strengthening positive influence of Tmax, spring precipitation, and cloud cover during winter and monsoon period on radial growth since 2000s. Our results have shown that both thermal and hydraulic constrains accounted for the radial growth decline of fir trees at the tree line of MFR in the western Sichuan Plateau.     

Tests of the role of a nuclear polyhedrosis virus in the population dynamics of its host,douglas-fir tussock moth,Orgyia pseudotsugata (Lepidoptera: Lymantriidae)

Summary Outbreaks of the Douglas-fir tussock moth, Orgyia pseudotsugata (McDunnough), have recurred periodically, at 7- to 10-year intervals, since the first recorded observation in 1916 in Chase, British Columbia, Canada. Anderson and May (1981) hypothesized that microparasites are responsible for the periodic population fluctuations of some defoliating insects. We chose the association between the Douglas-fir tussock moth and a viral disease, caused by a nuclear polyhedrosis virus (NPV), to test whether their model, and variants thereof, can predict the observed population cycles. Density-dependent mortality, vertical transmission of the virus and an incubation period were added to the free-living stages model of Anderson and May (1981). Parameter values for the models were derived from published data and from an experiment.Sensitivity analyses conducted for each model showed that none of the models generated the behavior of the Douglas-fir tussock moth as observed in the field. Thus, the periodicity of the outbreaks in field populations of tussock moths cannot be explained solely by the dynamics of the viral disease as described by Anderson and May's class of models; the virus is too short-lived and the growth rate of the insect population too high. Dynamics of other system components such as predators, parasites or food of the tussock moth probably play a significant role in the insect's population dynamics.     

Summer temperature dependency of larch budmoth outbreaks revealed by Alpine tree-ring isotope chronologies

Larch budmoth (LBM, Zeiraphera diniana Gn.) outbreaks cause discernable physical alteration of cell growth in tree rings of host subalpine larch (Larix decidua Mill.) in the European Alps. However, it is not clear if these outbreaks also impact isotopic signatures in tree-ring cellulose, thereby masking climatic signals. We compared LBM outbreak events in stable carbon and oxygen isotope chronologies of larch and their corresponding tree-ring widths from two high-elevation sites (1800–2200 m a.s.l.) in the Swiss Alps for the period AD 1900–2004 against isotope data obtained from non-host spruce (Picea abies). At each site, two age classes of tree individuals (150–250 and 450–550 years old) were sampled. Inclusion of the latter age class enabled one chronology to be extended back to AD 1650, and a comparison with long-term monthly resolved temperature data. Within the constraints of this local study, we found that: (1) isotopic ratios in tree rings of larch provide a strong and consistent climatic signal of temperature; (2) at all sites the isotope signatures were not disturbed by LBM outbreaks, as shown, for example, by exceptionally high significant correlations between non-host spruce and host larch chronologies; (3) below-average July to August temperatures and LBM defoliation events have been coupled for more than three centuries. Dampening of Alps-wide LBM cyclicity since the 1980s and the coincidence of recently absent cool summers in the European Alps reinforce the assumption of a strong coherence between summer temperatures and LBM defoliation events. Our results demonstrate that stable isotopes in tree-ring cellulose of larch are an excellent climate proxy enabling the analysis of climate-driven changes of LBM cycles in the long term.