Christof Bigler

Drought as an Inciting Mortality Factor in Scots Pine Stands of the Valais, Switzerland

During the 20th century, high mortality rates of Scots pine (Pinus silvestris L.) have been observed over large areas in the Rhône valley (Valais, Switzerland) and in other dry valleys of the European Alps. In this study, we evaluated drought as a possible inciting factor of Scots pine decline in the Valais. Averaged tree-ring widths, standardized tree-ring series, and estimated annual mortality risks were related to a drought index. Correlations between drought indices and standardized tree-ring series from 11 sites showed a moderate association. Several drought years and drought periods could be detected since 1864 that coincided with decreased growth. Although single, extreme drought years had generally a short-term, reversible effect on tree growth, multi-year drought initiated prolonged growth decreases that increased a tree’s long-term risk of death. Tree death occurred generally several years or even decades after the drought. In conclusion, drought has a limiting effect on tree growth and acts as a bottleneck event in triggering Scots pine decline in the Valais.

Multiple disturbance interactions and drought influence fire severity in rocky mountain subalpine forests

Disturbances such as fire, insect outbreaks, and blowdown are important in shaping subalpine forests in the Rocky Mountains, but quantitative studies of their interactions are rare. We investigated the combined effects of past disturbances, current vegetation, and topography on spatial variability of the severity of a fire that burned approximately 4500 ha of subalpine forest during the extreme drought of 2002 in northwestern Colorado. Ordinal logistic regression was used to spatially model fire severity in relation to late 1800s fires, a 1940s spruce beetle outbreak, forest cover type, stand structure, and topography. The late 1800s fires reduced the probability of burning in 2002, and the 1940s beetle outbreak slightly increased the probability of fire, particularly at high severity. Aspen (Populus tremuloides) and lodgepole pine (Pinus contorta) stands, which established after the late 1800s fires, were less likely to burn, whereas Engelmann spruce (Picea engelmannii)–subalpine fir (Abies lasiocarpa) s...

PREDICTING THE TIME OF TREE DEATH USING DENDROCHRONOLOGICAL DATA

Complex interactions of various environmental factors result in high vari- ability of tree mortality in space and time. Tree mortality functions that are implemented in forest succession models have been suggested to play a key role in assessing forest response to climate change. However, these functions are based on theoretical considerations and are likely to be poor predictors of the timing of tree death, since they do not adequately reflect our understanding of tree mortality processes. In addition, these theoretical mortality functions and most empirical mortality functions have not been tested sufficiently with respect to the accuracy of predicting the time of tree death. We introduce a new approach to modeling tree mortality based on different growth patterns of entire tree-ring series. Dendrochronological data from Picea abies (Norway spruce) in the Swiss Alps were used to calibrate mortality models using logistic regression. The autocorrelation of the data was taken into account by a jackknife variance estimator. Model performance was assessed by two criteria for classification accuracy and three criteria for prediction error. The six models with the highest overall performance correctly classified 71-78% of all dead trees and 73-75% of all living trees, and they predicted 44- 56% of all dead trees to die within 0-15 years prior to the actual year of death. For these six models, a maximum of 1.7% of all dead trees and 5% of all living trees were predicted to die .60 years prior to the last measured year. Models including the relative growth rate and a short-term growth trend as explanatory variables were most reliable with respect to inference and prediction. The generality of the mortality models was successfully tested by applying them to two independent P. abies data sets from climatologically and geologically different areas. We conclude that the methods presented improve our understanding of how tree growth and mortality are related, which results in more accurate mortality models that can ultimately be used to increase the reliability of predictions from models of forest dynamics.

Linking Increasing Drought Stress to Scots Pine Mortality and Bark Beetle Infestations

In the dry Swiss Rhone Valley, Scots pine forests have experienced increased mortality in recent years. It has commonly been assumed that drought events and bark beetles fostered the decline, however, whether bark beetle outbreaks increased in recent years and whether they can be linked to drought stress or increasing temperature has never been studied. In our study, we correlated time series of drought indices from long-term climate stations, 11-year mortality trends from a long-term research plot, and mortality probabilities modeled from tree rings (as an indicator of tree vitality) with documented occurrences of various bark beetle species and a buprestid beetle, using regional Forest Service reports from 1902 to 2003 and advisory cases of the Swiss Forest Protection Service (SFPS) from 1984 to 2005. We compared the historical findings with measured beetle emergence from a 4-year tree felling and breeding chamber experiment. The documented beetle-related pine mortality cases increased dramatically in the 1990s, both in the forest reports and the advisory cases. The incidents of beetle-related pine mortality correlated positively with spring and summer temperature, and with the tree-ring-based mortality index, but not with the drought index. The number of advisory cases, on the other hand, correlated slightly with summer drought index and temperature, but very highly with tree–ring—based mortality index. The tree-ring-based mortality index and observed tree mortality increased in years following drought. This was confirmed by the beetle emergences from felled trees. Following dry summers, more than twice as many trees were colonized by beetles than following wet summers. We conclude that increased temperatures in the Swiss Rhone Valley have likely weakened Scots pines and favored phloeophagous beetle population growth. Beetles contributed to the increased pine mortality following summer drought. Among the factors not addressed in this study, changed forest use may have also contributed to increased beetle populations and Scots pine mortality, whereas air pollution seems to be of lesser importance.

Predicting tree death for "Fagus sylvatica" and "Abies alba" using permanent plot data

Question: How well can mortality probabilities of deciduous trees (Fagus sylvatica) and conifers (Abies alba) be predicted using permanent plot data that describe growth patterns, tree species, tree size and site conditions? Location: Fagus forests in the montane belt of the Jura folds (Switzerland). Method: Permanent plot data were used to develop and validate logistic regression models predicting survival probabilities of individual trees. Backward model selection led to a reduced model containing the growth-related variable ʻrelative basal area increment ̓ (growth-dependent mortality) and variables not directly reflecting growth such as species, size and site (growth-independent mortality). Results: The growth-mortality relationship was the same for both species (growth-dependent mortality). However, species, site and tree size also influenced mortality probabilities (growthindependent mortality). The predicted survival probabilities of the final model were well calibrated, and the model showed an excellent discriminatory power (area under the receiver operating characteristic curve = 0.896). Conclusion: Mortality probabilities of Fagus sylvatica and Abies alba can be predicted with high discriminatory power using a well calibrated logistic regression model. Extending this case study to a larger number of tree species and sites could provide speciesand site-specific tree mortality models that allow for more realistic projections of forest succession.

Poor methodology for predicting large-scale tree die-off

In a recent issue of PNAS, Adams et al. (1) project a 5-fold increase in the frequency of tree die-off in pinon (Pinus edulis) under drought in the southwestern United States due to elevated temperature alone. Their study is based on 10 excavated individuals grown in containers and exposed to complete drought under either ambient or elevated temperature (+4.3 °C, 5 replicates). Trees experiencing higher temperatures died 7 weeks earlier than control trees. The authors explain this by a trend to increased respiration under warmer conditions resulting in earlier carbon starvation. In addition to the recent letter by Sala (2) pointing out that there is no direct evidence for carbon starvation as a cause of tree death to date, we are concerned with (i) the methods used to arrive at Adams et al.s (1) interpretation and (ii) the way tree die-off is extrapolated to large spatio-temporal scales from their small sample size.

Do small-grain processes matter for landscape scale questions? Sensitivity of a forest landscape model to the formulation of tree growth rate

Process-based forest landscape models are valuable tools for testing basic ecological theory and for projecting how forest landscapes may respond to climate change and other environmental shifts. However, the ability of these models to accurately predict environmentally-induced shifts in species distributions as well as changes in forest composition and structure is often contingent on the phenomenological representation of individual-level processes accurately scaling-up to landscape-level community dynamics. We use a spatially explicit landscape forest model (LandClim) to examine how three alternative formulations of individual tree growth (logistic, Gompertz, and von Bertalanffy) influence model results. Interactions between growth models and landscape characteristics (landscape heterogeneity and disturbance intensity) were tested to determine in what type of landscape simulation results were most sensitive to growth model structure. We found that simulation results were robust to growth function formulation when the results were assessed at a large spatial extent (landscape) and when coarse response variables, such as total forest biomass, were examined. However, results diverged when more detailed response variables, such as species composition within elevation bands, were considered. These differences were particularly prevalent in regions that included environmental transition zones where forest composition is strongly driven by growth-dependent competition. We found that neither landscape heterogeneity nor the intensity of landscape disturbances accentuated simulation sensitivity to growth model formulation. Our results indicate that at the landscape extent, simulation results are robust, but the reliability of model results at a finer resolution depends critically on accurate tree growth functions.

Contrasting responses of Central Asian rock glaciers to global warming

While the responses of Tien Shan glaciers – and glaciers elsewhere – to climatic changes are becoming increasingly well understood, this is less the case for permafrost in general and for rock glaciers in particular. We use a novel approach to describe the climate sensitivity of rock glaciers and to reconstruct periods of high and low rock glacier activity in the Tien Shan since 1895. Using more than 1500 growth anomalies from 280 trees growing on rock glacier bodies, repeat aerial photography from Soviet archives and high-resolution satellite imagery, we present here the worlds longest record of rock glacier movements. We also demonstrate that the rock glaciers exhibit synchronous periods of activity at decadal timescales. Despite the complex energy-balance processes on rock glaciers, periods of enhanced activity coincide with warm summers, and the annual mass balance of Tuyuksu glacier fluctuates asynchronously with rock glacier activity. At multi-decadal timescales, however, the investigated rock glaciers exhibit site-specific trends reflecting different stages of inactivation, seemingly in response to the strong increase in air temperature since the 1970s.

Site-specific water-use strategies of mountain pine and larch to cope with recent climate change

We aim to achieve a mechanistic understanding of the eco-physiological processes in Larix decidua and Pinus mugo var. uncinata growing on north- and south-facing aspects in the Swiss National Park in order to distinguish the short- and long-term effects of a changing climate. To strengthen the interpretation of the δ(18)O signal in tree rings and its coherence with the main factors and processes driving evaporative δ(18)O needle water enrichment, we analyzed the δ(18)O in needle, xylem and soil water over the growing season in 2013 and applied the mechanistic Craig-Gordon model (1965) for the short-term responses. We found that δ(18)O needle water strongly reflected the variability of relative humidity mainly for larch, while only δ(18)O in pine xylem water showed a strong link to δ(18)O in precipitation. Larger differences in offsets between modeled and measured δ(18)O needle water for both species from the south-facing aspects were detected, which could be explained by the high transpiration rates. Different soil water and needle water responses for the two species indicate different water-use strategies, further modulated by the site conditions. To reveal the long-term physiological response of the studied trees to recent and past climate changes, we analyzed δ(13)C and δ(18)O in wood chronologies from 1900 to 2013. Summer temperatures as well as summer and annual amount of precipitations are important factors for growth of both studied species from both aspects. However, mountain pine trees reduced sensitivity to temperature changes, while precipitation changes come to play an important role for the period from 1980 to 2013. Intrinsic water-use efficiency (WUEi) calculated for larch trees since the 1990s reached a saturation point at elevated CO2 Divergent trends between pine WUEi and δ(18)O are most likely indicative of a decline of mountain pine trees and are also reflected in decoupling mechanisms in the isotope signals between needles and tree-rings.

Quantifying the effects of drought on abrupt growth decreases of major tree species in Switzerland

Abstract Drought entails important effects on tree physiology, which may result in short‐ to long‐term radial growth decreases. While the majority of studies have focused on annual drought‐related variability of growth, relatively little is known about sustained growth decreases following drought years. We apply a statistical framework to identify climatic factors that induce abrupt growth decreases and may eventually result in tree mortality. We used tree‐ring data from almost 500 standing dead trees and 200 living trees in eight sites of the Swiss network of strict forest reserves, including four of the most important Central European tree species (Abies alba, Picea abies, Fagus sylvatica and Quercus spp.). First, to assess short‐term growth responses to drought under various climate and site conditions, we calculated correlations and linear mixed‐effects models between ring‐width indices (RWIs) and drought based on the Standardized Precipitation Evapotranspiration Index (SPEI). Second, to quantify drought effects on abrupt growth decreases, we applied distributed lag nonlinear models (DLNMs), which account for both delayed effects and the nonlinear relationship between the SPEI and the occurrence of abrupt growth decreases. Positive correlations between RWIs and the SPEI indicated short‐term growth responses of all species, particularly at arid sites. Results of the DLNMs revealed species‐specific growth responses to drought. For Quercus spp., abrupt growth decreases were more likely to occur several years following severe drought, whereas for P. abies, A. alba, and F. sylvatica abrupt growth decreases started frequently immediately in the drought year. We conclude that the statistical framework allows for quantifying the effects of drought intensity on the probability of abrupt growth decreases, which ultimately contributes to an improved understanding of climate impacts on forest community dynamics.