Tree Mortality In Forests Research Articles

Stand-scale tree mortality factors differ by site and species following drought in southwestern mixed conifer forests

Impacts of drought on tree mortality in high-elevation mixed-conifer forests of southwestern U.S. are poorly understood. A recent extended and severe drought in the region provided an opportunity to investigate the patterns and factors associated with tree mortality in this forest type. Specifically, we quantified mortality that occurred between 1995 and 2008 of four tree species, white fir (Abies concolor), limber pine (Pinus flexilis), trembling aspen (Populus tremuloides), and Douglas-fir (Pseudotsuga menziesii), in mixed-conifer forests over three sites in northern Arizona within 84, 0.02ha plots. We found: (1) varied but substantial tree mortality (4–56% by basal area) in most species between 1996 and 2006 in association with recent severe and prolonged drought; (2) tree mortality differed among sites and species with aspen and white fir having the most mortality (>30% by basal area); (3) relationships between tree mortality and most climatic factors (e.g. temperature, precipitation, Palmer Drought Severity Index) were lagged 1–4yr; (4) bark beetle attack and intraspecific tree basal area were consistently and positively related to tree mortality for most species and sites, whereas topographic and other stand characteristics were less consistently related to mortality. Results show that aspen, Douglas-fir, and white fir were more vulnerable to recent drought-associated mortality than limber pine. Associations between tree mortality and intraspecific basal area support further evaluation of treatments that reduce intraspecific competition within stands to lower risks of tree mortality in southwestern mixed conifer forests.

Causes, consequences, and the future of forest mortality due to climate change

Mortality of forest trees is growing at an accelerating rate due in part to increasing severity and frequency of droughts. The consequences of increasing forest mortality range from impacts on timber and tourism revenue, reductions in fuelwood availability and carbon storage, and feedbacks that accelerate climate changes. Unfortunately, our understanding of where, when, and how forests die is extremely limited, precluding us from forecasting future changes in forest composition and services. Here we review the state of current knowledge regarding mortality processes, and highlight the urgent scientific challenges that must be overcome if we are to adequately plan for future changes in forest survival and mortality. We suggest that forest monitoring through inventory networks and newly emerging remote sensing techniques should be a high priority for maintenance and development, because only through these observations can we determine what regions and species are most vulnerable to climate change. These datasets are also critical for evaluation of regional to global models of forest dynamics. Without accurate models, forecasts of future forest composition cannot be considered reliable. Furthermore, experimental tests of how the dominant global forest species die are needed to enable models to simulate mortality correctly. Lastly, experimental tests of forest management strategies that can alleviate stress under the novel climate regimes the globe is now experiencing are essential to allow planning for mitigation options in the forestry sector. Increasing forest mortality is now inevitable given the global energy portfolio; however, the ability of science and forestry to quantify, understand, predict, and mitigate impacts on forests remains a viable option to allow planning for future forest management to minimize impacts on the Earth’s forests.

Spatio-temporal prediction of tree mortality based on long-term sample plots, climate change scenarios and parametric frailty modeling

An approach is presented to predict the effects climate change may have on mortality of forest trees. Mortality is modeled using long-term observations from the Pan-European Programme for Intensive and Continuous Monitoring of Forest Ecosystems plots, retrospective climate data and frailty models having a parametric baseline hazard function. The linear predictor is modeled by B-spline regression techniques to allow for nonlinear cause-and-effect curves. Spatio-temporal predictions of mortality of four major tree species in the German state of Baden-Württemberg were derived in terms of unconditional hazard ratios and based on climate projection data.According to the model, marginal risk of tree death for 100year old Norway spruce trees will be doubled until 2100. Hazard rates of common beech will be halved in low elevation areas and will be reduced by 25% in higher elevations until 2100. Hazard rates of silver fir will be less affected by a changing climate and will increase by at least 25% and by a maximum of 100% in mountainous regions. Scots pines hazard rates will be halved on higher elevation sites and will increase on lower elevation sites.

The effects of α‐cellulose extraction and blue‐stain fungus on retrospective studies of carbon and oxygen isotope variation in live and dead trees

Tree-ring carbon and oxygen isotope ratios from live and recently dead trees may reveal important mechanisms of tree mortality. However, wood decay in dead trees may alter the δ(13)C and δ(18)O values of whole wood obscuring the isotopic signal associated with factors leading up to and including physiological death. We examined whole sapwood and α-cellulose from live and dead specimens of ponderosa pine (Pinus ponderosa), one-seed juniper (Juniperous monosperma), piñon pine (Pinus edulis) and white fir (Abies concolor), including those with fungal growth and beetle frass in the wood, to determine if α-cellulose extraction is necessary for the accurate interpretation of isotopic compositions in the dead trees. We found that the offset between the δ(13)C or δ(18)O values of α-cellulose and whole wood was the same for both live and dead trees across a large range of inter-annual and regional climate differences. The method of α-cellulose extraction, whether Leavitt-Danzer or Standard Brendel modified for small samples, imparts significant differences in the δ(13)C (up to 0.4‰) and δ(18) O (up to 1.2‰) of α-cellulose, as reported by other studies. There was no effect of beetle frass or blue-stain fungus (Ophiostoma) on the δ(13)C and δ(18)O of whole wood or α-cellulose. The relationships between whole wood and α-cellulose δ(13)C for ponderosa, piñon and juniper yielded slopes of ~1, while the relationship between δ(18)O of whole wood and α-cellulose was less clear. We conclude that there are few analytical or sampling obstacles to retrospective studies of isotopic patterns of tree mortality in forests of the western United States.

The contribution of competition to tree mortality in old-growth coniferous forests

Competition is a well-documented contributor to tree mortality in temperate forests, with numerous studies documenting a relationship between tree death and the competitive environment. Models frequently rely on competition as the only non-random mechanism affecting tree mortality. However, for mature forests, competition may cease to be the primary driver of mortality. We use a large, long-term dataset to study the importance of competition in determining tree mortality in old-growth forests on the western slope of the Sierra Nevada of California, U.S.A. We make use of the comparative spatial configuration of dead and live trees, changes in tree spatial pattern through time, and field assessments of contributors to an individual tree's death to quantify competitive effects. Competition was apparently a significant contributor to tree mortality in these forests. Trees that died tended to be in more competitive environments than trees that survived, and suppression frequently appeared as a factor contributing to mortality. On the other hand, based on spatial pattern analyses, only three of 14 plots demonstrated compelling evidence that competition was dominating mortality. Most of the rest of the plots fell within the expectation for random mortality, and three fit neither the random nor the competition model. These results suggest that while competition is often playing a significant role in tree mortality processes in these forests it only infrequently governs those processes. In addition, the field assessments indicated a substantial presence of biotic mortality agents in trees that died. While competition is almost certainly important, demographics in these forests cannot accurately be characterized without a better grasp of other mortality processes. In particular, we likely need a better understanding of biotic agents and their interactions with one another and with competition.

Widespread Amazon forest tree mortality from a single cross‐basin squall line event

Climate change is expected to increase the intensity of extreme precipitation events in Amazonia that in turn might produce more forest blowdowns associated with convective storms. Yet quantitative tree mortality associated with convective storms has never been reported across Amazonia, representing an important additional source of carbon to the atmosphere. Here we demonstrate that a single squall line (aligned cluster of convective storm cells) propagating across Amazonia in January, 2005, caused widespread forest tree mortality and may have contributed to the elevated mortality observed that year. Forest plot data demonstrated that the same year represented the second highest mortality rate over a 15‐year annual monitoring interval. Over the Manaus region, disturbed forest patches generated by the squall followed a power‐law distribution (scaling exponent α = 1.48) and produced a mortality of 0.3–0.5 million trees, equivalent to 30% of the observed annual deforestation reported in 2005 over the same area. Basin‐wide, potential tree mortality from this one event was estimated at 542 ± 121 million trees, equivalent to 23% of the mean annual biomass accumulation estimated for these forests. Our results highlight the vulnerability of Amazon trees to wind‐driven mortality associated with convective storms. Storm intensity is expected to increase with a warming climate, which would result in additional tree mortality and carbon release to the atmosphere, with the potential to further warm the climate system.

Impacts of tropical cyclones on U.S. forest tree mortality and carbon flux from 1851 to 2000

Tropical cyclones cause extensive tree mortality and damage to forested ecosystems. A number of patterns in tropical cyclone frequency and intensity have been identified. There exist, however, few studies on the dynamic impacts of historical tropical cyclones at a continental scale. Here, we synthesized field measurements, satellite image analyses, and empirical models to evaluate forest and carbon cycle impacts for historical tropical cyclones from 1851 to 2000 over the continental U.S. Results demonstrated an average of 97 million trees affected each year over the entire United States, with a 53-Tg annual biomass loss, and an average carbon release of 25 Tg y(-1). Over the period 1980-1990, released CO(2) potentially offset the carbon sink in forest trees by 9-18% over the entire United States. U.S. forests also experienced twice the impact before 1900 than after 1900 because of more active tropical cyclones and a larger extent of forested areas. Forest impacts were primarily located in Gulf Coast areas, particularly southern Texas and Louisiana and south Florida, while significant impacts also occurred in eastern North Carolina. Results serve as an important baseline for evaluating how potential future changes in hurricane frequency and intensity will impact forest tree mortality and carbon balance.

Pourquoi et où les arbres adultes meurent dans une jeune forêt tempérée? Le rôle du voisinage

• The density and identity of tree neighbourhood is a key factor to explain tree mortality in forests, especially during the stem exclusion phase. • To understand this process, we built a logistic model for mortality in a spatially explicit context, including tree and neighbourhood predictors. Additionally, we used this model to build mortality risk frequency distributions. Finally, we tested this model against a random mortality model to predict the spatial pattern of the forest. • Annual mortality rate was high for pedunculate oak (Quercus robur, 6.99%), moderate for birch (Betula celtiberica, 2.19%) and Pyrenean oak (Q. pyrenaica, 1.58%) and low for beech (Fagus sylvatica, 0.26%). Mortality risk models for pedunculate oak and birch included stem diameter, tree height, canopy position and neighbourhood. Mortality was affected by the specific nature of the neighbourhood showing a clear competitive hierarchy: beech > pedunculate oak > birch. Models based on random mortality and logistic regression model were able to predict the spatial pattern of survivors although logistic regression predictions were more accurate. • Our study highlights how simple models such as the random mortality one may obscure much more complex spatial interactions.

Tree mortality due to anEl Niñoflood along the lower Tana River, Kenya

This study investigated forest tree mortality as a result of the extremely strong 1997-1998 El Nino flood in Tana River, Kenya, directly tested how mortality varied in relation to tree species and diameter at breast height (DBH), and indirectly tested how mortality varied in rela- tion to floodwater depth. The study forest was under 2 m of water for approximately 71 days, from December 1997 until February 1998. Twenty-five liana, subcanopy tree, and canopy tree species were selected based on their importance in the diet of the Tana River mangabey (Cercocebus galeritus Peters), a critically endangered pri- mate endemic to the Tana River. Reproductive-sized indi- viduals of these species were enumerated in 16.25 ha. I also enumerated trees killed by the El Nino flood, recorded their location in the forest, and measured their DBH. Ninety-two trees in fifteen species were killed by the flood. There were significant differences in mortality by species and by 0.25 ha quadrat. There were negative correlations between number of dead and DBH, number of dead and distance from the river, and per cent of quadrat killed and distance from the river. There appear to have been few long-term consequences of the El Nino flood in the forest or for the mangabeys.

Spatial Clusters and Variability Analysis of Tree Mortality

Spatial patterns of tree mortality are important factors in analysis of forest health and sustainable forest management. We analyzed spatial cluster characteristics of forest tree mortality among hardwoods, softwoods, and all species groups between 1989 and 1997 in Georgia. We used for this purpose the 1989 and 1997 USDA Forest Service Forest Inventory and Analysis (FIA) permanent sample plot data, spatial scan statistic and semivariogram analysis methods, and geographic information system (GIS) techniques. In Georgia, forest tree mortality for softwoods and all species groups formed clusters in North Georgia in 1989. These clusters became more widespread, but the aggregation intensity of tree mortality was decreased compared with that of the 1989 clusters. Another finding is that the locations for mortality clusters of softwoods were almost the same as those of all species combined both in 1989 and in 1997. In 1989 the forest tree mortality of softwoods and all species groups agglomerated with small clusters forming in north Georgia. In 1997, these clusters became more widespread, but the magnitudes of tree mortality within them decreased compared with those of 1989 clusters. Semivariogram analysis suggests that hardwoods, softwoods, and all species combined followed a similar trend for spatial variation in that they fit a spherical model of tree mortality. However, the strengths of spatial dependence were significantly different among the hardwoods, softwoods, and all species in 1989 and 1997. The higher the agglomeration of tree mortality, the stronger the spatial dependence. Spatial analysis of tree damage indicates that agents of tree damage were also agglomerated in the state. Eight kinds of tree damage, possibly combined with other kinds of damage, directly determined the spatial patterns of forest tree mortality. Out of these eight, only vegetation competition, insects, and weather caused the formation of the north Georgia tree mortality clusters.

Tropical forest tree mortality, recruitment and turnover rates: calculation, interpretation and comparison when census intervals vary

Summary Mathematical proofs show that rate estimates, for example of mortality and recruitment, will decrease with increasing census interval when obtained from censuses of non‐homogeneous populations. This census interval effect could be confounding or perhaps even driving conclusions from comparative studies involving such rate estimates. We quantify this artefact for tropical forest trees, develop correction methods and re‐assess some previously published conclusions about forest dynamics. Mortality rates of > 50 species at each of seven sites in Africa, Latin America, Asia and Australia were used as subpopulations to simulate stand‐level mortality rates in a heterogeneous population when census intervals varied: all sites showed decreasing stand mortality rates with increasing census interval length. Stand‐level mortality rates from 14 multicensus long‐term forest plots from Africa, Latin America, Asia and Australia also showed that, on average, mortality rates decreased with increasing census interval length. Mortality, recruitment or turnover rates with differing census interval lengths can be compared using the mean rate of decline from the 14 long‐term plots to standardize estimates to a common census length using λcorr = λ × t0.08, where λ is the rate and t is time between censuses in years. This simple general correction should reduce the bias associated with census interval variation, where it is unavoidable. Re‐analysis of published results shows that the pan‐tropical increase in stem turnover rates over the late 20th century cannot be attributed to combining data with differing census intervals. In addition, after correction, Old World tropical forests do not have significantly lower turnover rates than New World sites, as previously reported. Our pan‐tropical best estimate adjusted stem turnover rate is 1.81 ± 0.16% a−1 (mean ± 95% CI, n = 65). As differing census intervals affect comparisons of mortality, recruitment and turnover rates, and can lead to erroneous conclusions, standardized field methods, the calculation of local correction factors at sites where adequate data are available, or the use of our general standardizing formula to take account of sample intervals, are to be recommended.

Using the Non-Parametric Classifier CART to Model Forest Tree Mortality

Abstract A binary classification tree (CART) was used to predict forest tree mortality for two conifer species. CART models were fitted using binary recursive splitting of the data set into increasingly homogeneous subsets. Models were compared in terms of improvement of prediction accuracy, representativeness in average size of the predicted mortality trees, and interpretability of the results. For shade intolerant ponderosa pine, crown ratio, diameter increment prediction, or variables indicating the relative position of a tree in a stand were used for splitting. For shade tolerant white fir, height increment prediction and stand density were selected for splitting. The prediction accuracies for mortality trees of the best CART models were between 28-36% for ponderosa pine and between 11-17% for white fir. CART was also compared with logistic regression using a stochastic and a deterministic assignment of mortality. Efficiencies similar to those achieved with CART were reached with deterministic logistic models using thresholds probabilities. However, CART and the logistic model tended to utilize different predictor variables, especially for white fir. CART uncovered additional factors for white fir important for predicting mortality not identified by the logistic regression. For. Sci. 44(4):507-516.

A Brief Assessment of Forest Health in Northeastern United States and Southeastern Canada

Air pollution is rightfully considered as a very real and important threat to our forests. However, determining the possible role of air pollutants in causing changes in forest health and productivity has been difficult. Ever-present diseases, insects, and abiotic stressors (and their continual interactions) must also be monitored to effectively determine what, if any, additional threat is being posed by air pollutants to forests. Recent forest health surveys in northeastern North America have failed to provide evidence of cause-effect for air pollutants as the most likely incitant of certain forest species declines, yet, the public has been led to believe that acid rain and air pollution have caused widespread mortality of forest trees. Recent surveys have proven that our forests are generally healthy and productive.

Simulation of forest tree mortality

Simulation of forest tree mortality