Individual Tree Mortality Research Articles

Unlocking the forest inventory data: relating individual tree performance to unmeasured environmental factors

Geographically extensive forest inventories, such as the USDA Forest Service's Forest Inventory and Analysis (FIA) program, contain millions of individual tree growth and mortality records that could be used to develop broad-scale models of forest dynamics. A limitation of inventory data, however, is that individual-level measurements of light (L) and other environmental factors are typically absent. Thus, inventory data alone cannot be used to parameterize mechanistic models of forest dynamics in which individual performance depends on light, water, nutrients, etc. To overcome this limitation, we developed methods to estimate species-specific parameters (thetaG) relating sapling growth (G) to L using data sets in which G, but not L, is observed for each sapling. Our approach involves: (1) using calibration data that we collected in both eastern and western North America to quantify the probability that saplings receive different amounts of light, conditional on covariates x that can be obtained from inventory data (e.g., sapling crown class and neighborhood crowding); and (2) combining these probability distributions with observed G and x to estimate thetaG using Bayesian computational methods. Here, we present a test case using a data set in which G, L, and x were observed for saplings of nine species. This test data set allowed us to compare estimates of thetaG obtained from the standard approach (where G and L are observed for each sapling) to our method (where G and x, but not L, are observed). For all species, estimates of thetaG obtained from analyses with and without observed L were similar. This suggests that our approach should be useful for estimating light-dependent growth functions from inventory data that lack direct measurements of L. Our approach could be extended to estimate parameters relating sapling mortality to L from inventory data, as well as to deal with uncertainty in other resources (e.g., water or nutrients) or environmental factors (e.g., temperature).

Tree Mortality following Prescribed Fire and a Storm Surge Event in Slash Pine (Pinus elliottiivar.densa) Forests in the Florida Keys, USA

In fire-dependent forests, managers are interested in predicting the consequences of prescribed burning on postfire tree mortality. We examined the effects of prescribed fire on tree mortality in Florida Keys pine forests, using a factorial design with understory type, season, and year of burn as factors. We also used logistic regression to model the effects of burn season, fire severity, and tree dimensions on individual tree mortality. Despite limited statistical power due to problems in carrying out the full suite of planned experimental burns, associations with tree and fire variables were observed. Post-fire pine tree mortality was negatively correlated with tree size and positively correlated with char height and percent crown scorch. Unlike post-fire mortality, tree mortality associated with storm surge from Hurricane Wilma was greater in the large size classes. Due to their influence on population structure and fuel dynamics, the size-selective mortality patterns following fire and storm surge have practical importance for using fire as a management tool in Florida Keys pinelands in the future, particularly when the threats to their continued existence from tropical storms and sea level rise are expected to increase.

Diachronic analysis of individual-tree mortality in a Norway spruce stand in the eastern Italian Alps

• Understanding tree mortality processes across time requires long term studies. Spatiotemporal patterns of mortality in a 200 years-old mono-layered Norway spruce stand were evaluated to determine what factors affected individual-tree mortality. • We performed an analysis on two surveys (1993 and 2005) in a 1-ha permanent plot in the Paneveggio forest (Eastern Italian Alps). Tree diameter and age distribution between surveys were compared. We examined spatial patterns of living and dead trees before 1993, in 1993 and in 2005 using univariate and bivariate Ripley’s K(d) function, and a kernel estimator of local crowding. A logistic model was used to assess the effects of diameter, age, recent growth and competitive pressure on tree mortality. • Spatial pattern analysis indicated mortality was associated to tree neighbourhood (neighbour effect at 2–5 m). An increment of regularization of tree spatial pattern occurred due to density-dependent mortality. Logistic regression showed tree diameter and recent growth were determinant on mortality risk during the monitoring period. • Even if the stand is relatively aged, mortality dynamics are those typical of stem exclusion stage. Mortality was related to competitive dynamics, and small suppressed trees with slow growth rate had higher probability to die.

Modeling Tree Mortality in Low- to Medium-Density Uneven-Aged Hardwood Stands Under a Selection System Using Generalized Estimating Equations

Abstract Tree mortality was modeled through time in uneven-aged northern hardwood stands managed under a selection system using long-term remeasurement data. Two models were fitted and compared: a traditional logistic regression model that predicted the probability of individual tree mortality over discrete time periods and a logistic regression model estimated using generalized estimating equations (GEEs) to account for autocorrelation in the longitudinal data. Model evaluation was based on the mean prediction error, mean absolute prediction error, variance of prediction errors, and mean square error calculated using an independent validation data set. The GEE model produced smaller evaluation statistics, especially for the smaller diameter classes. The predicted probability of mortality from the two models was compared with the observed mortality across 5-cm diameter classes for two time periods. Our results indicate that the GEE model was better able to capture the change in the probability of mortality over time, especially for the smaller diameter classes, than the more traditional logistic model.

Analysis of forest naturalness and tree mortality patterns in Estonia

New methods for evaluating structural properties of stands and individual tree mortality within forests are needed to enhance biodiversity assessment in forest inventories. One approach is to assess the degree of naturalness in a forest. We assessed forest naturalness by examining patterns and causes of mortality and deadwood amount and spatial distribution as indicators of naturalness, or degree of anthropogenic disturbance. This study is based on 5-year interval measurement using 294 permanent samples plots from a forest growth network in Estonia. The average annual mortality was 1.3% from stem number counting 29% of Scots pine, 27% of silver and downy birch and 20% of Norway spruce. Most common reasons for the individual tree death were growth-dependent reasons (45%), fungi (23%) and wind damage (16%). Modelling showed that relative diameter of a tree in a stand is significantly related to mortality probability. Modelling the reasons of tree death showed that with increasing relative diameter there was a greater probability that mortality was caused by wind or damage from game (mostly moose ( Alces alces L.)), insect or fungi and a lower probability that mortality was due to competition between trees. Use of structural variable such as deadwood mingling, which was based on the neighbouring trees, improved the assessment of forest naturalness and helped to distinguish recent disturbances. A comparison of deadwood mingling and nature value scores in managed and semi-natural forests showed that dead trees were more dispersed and the naturalness score was higher in semi-natural forest stands. The nature score was significantly correlated with the diversity of mortality causes indicating that mortality causes are more diverse in semi-natural stands. Mean values and distribution of the deadwood mingling index in managed and semi-natural forests were not significantly different. In middle-aged semi-natural forests, mortality is spatially more random than in managed forests, thus there is no evidence of gap formation yet.

Modeling non-catastrophic individual-tree mortality for Pinus radiata plantations in northwestern Spain

An individual-tree mortality model for radiata pine ( Pinus radiata D. Don) plantations in northwestern Spain was developed. The model relies on data from three inventories of a network of 130 permanent plots. The equation that predicts the probability of complete survival is a logistic function, which depends on tree size and competition (diameter, mean-squared diameter, stand basal area, dominant height and BALMOD competition index). Several methods of implementing the mortality model were tested (deterministic versus stochastic threshold selection and the use of expansion factors). The best results were obtained when using a fixed cut-off value to decide which trees would survive to the next growing period. When a cut-off value of 0.55 was applied, the model correctly classified 99% of the live trees and 25% of the dead trees. A cut-off of 0.71 provided an estimated mortality rate that was closest to the observed rate. With this cut-off value, the model correctly classified 95% of the live trees and 47% of the dead trees. This cut-off value is recommended when using the model to predict radiata pine individual-tree mortality in the study area.

T: A forest simulator for bioeconomic analyses based on models for individual trees

The aim of this paper is to describe a bioeconomic forest simulator based on models for individual trees. At present, the forest simulator can be applied only to stand-level analyses, but in future it is planned to be a part of a software system that may be used for decision support for large forest areas. The focus is on a flexible platform facilitating future development, and the biological submodels, i.e. distance-independent individual-tree growth and mortality models and area-based regeneration and recruitment models, are based on permanent sample plots from the Norwegian National Forest Inventory. The economic submodels estimate timber prices and harvesting costs from individual trees. For each management unit (i.e. stand), the simulator produces treatment schedules with all feasible combinations of user-defined treatment and regeneration options (e.g. precommercial thinning, thinning, different kinds of regeneration cutting with different kinds of regeneration options and selective cutting). A net present value is calculated for all treatment schedules. From the treatment schedules various aspects of forest dynamics and economics can be analysed and visualized. A case study is presented to demonstrate some of the functionality and behaviour of the simulator.

Lightning, fire and longleaf pine: Using natural disturbance to guide management

The importance of lightning as an ignition source for the fire adapted longleaf pine ( Pinus palustris) ecosystem is widely recognized. Lightning also impacts this system on a smaller scale by causing individual tree mortality. The objective of this study was to determine mortality due to lightning and other agents in longleaf stands on the Ocala National Forest in central Florida and to quantify lightning ignited fire. Mortality from lightning was also tracked in longleaf stands on the Savannah River Site in South Carolina. Lightning killed more trees than any other agent with a mean mortality of nearly 1 tree/3 ha/yr in Florida and 1 tree/8 ha/yr in South Carolina. The probability of a tree being struck by lightning increased as a function of tree height at both sites, i.e. lightning preferentially removed the largest trees from the stand. In addition lightning strikes were clumped within stands, sometimes killed multiple trees with a single strike, and often hit trees on the edge of existing gaps. The combination of these processes means gaps suitable for regeneration within longleaf stands are created quite rapidly. This information provides guidelines for the development of selection harvest systems based on this natural disturbance. Although lightning activity was greatest during the summer months in Florida and most fires occurred in June, the probability of a strike causing a fire was highest in February to May.

Modélisation de la qualité des stations et de la croissance individuelle des arbres dans des peuplements purs et mélangés de Pinus brutia dans le nord-est de la Grèce

•Simulation tools, based on individual tree growth and mortality models can produce the most detailed predictions of forest stand development under different management schedules. These models allow the manager to predict the development of any type of stand (even- and uneven-aged, and pure and mixed stands). • Different model approaches and predictors are required for pure even-aged or mixed uneven-aged forest stands. This study developed and compared two sets of models which enable tree-level simulation of the development of pure and mixed stands of Pinus brutia in north-east Greece. The first set of models for even-aged forestry consists of site index models, diameter growth models, tree height models, and mortality models. The second set, which is for uneven-aged forestry, uses a past growth index instead of a site index. • The simulations and overall fitting statistics suggest that the two types of models provide realistic and accurate predictions of forest stand development and allow one to simulate the development of complex Pinus brutia stand structures in Dadia National Park forests. • The advantages of the two approaches are discussed and it is suggested that the growth index is an effective predictor of site quality and the set of models which used such variable as predictor performed in a similar way as the models using site index, which require more information and a given stand structure (even-aged).

Predicting individual tree mortality in northern hardwood stands under uneven-aged management in southern Québec, Canada

This study proposes a generalized linear mixed model to predict individual tree mortality in northern hardwood stands under uneven-aged management. The model is based on a complementary log-log (CLL) link function, and was calibrated using permanent-plot data. Tree vigor, stem product, diameter at breast height and stand basal area were tested as explanatory variables. A plot and an interval random effect were specified to account for spatial correlations. When compared with the traditional logit link function, the CLL facilitates the inclusion of the time factor. In this case study, there was an important variability of mortality predictions between the plots and the intervals for a given plot. The interval random effect is thought to be associated with catastrophic mortality. Since both tree vigor and stem product proved to be significant mortality predictors, we recommend that these variables be evaluated to increase the accuracy of mortality models. mortality / generalized linear mixed model / random effects / hardwoods / single-tree selection cutting

Tree Mortality in Mixed Pine–Hardwood Stands Defoliated by the European Gypsy Moth (Lymantria dispar L.)

Abstract Defoliation by the European gypsy moth (Lymantria dispar L.) and subsequent tree mortality have been well documented in the northeastern United States. In this study we evaluate tree mortality after initial defoliation in mixed pine–hardwood stands in the southeastern United States as the range of European gypsy moth expands. In 1993, 46 mature pine–oak and pine–sweetgum stands were selected on the coastal plain in Maryland and Virginia along the advancing gypsy moth front. Initial stand conditions, defoliation, and tree mortality after a single defoliation outbreak were monitored for 4 years. Susceptible oaks and sweetgum were defoliated at the greatest intensities. After defoliation, mortality in attacked stands was 4.4 m2/ha higher in pine–oak stands and 1.7 m2/ha higher in pine–sweetgum stands, representing 13 and 3% of the total basal area, respectively. Mortality of pines was observed mainly among suppressed and intermediate trees. Logistic regressions were derived to correlate individual tree mortality to stand and tree factors associated with gypsy moth attack. On the basis of these models, susceptible understory species with degraded crowns in pine–oak stands were at the greatest risk of mortality. Dominant overstory pines in pine–hardwood stands appear to be at low risk.

Fine‐Scale Analysis of Mount Graham Red Squirrel Habitat Following Disturbance

ABSTRACT Habitat destruction and degradation are major factors in reducing abundance, placing populations and species in jeopardy. Monitoring changes to habitat and identifying locations of habitat for a species, after disturbance, can assist mitigation of the effects of human‐caused or ‐amplified habitat disturbance. Like many areas in the western United States, the Pinaleño Mountains of southeastern Arizona, USA, have suffered catastrophic fire and large‐scale insect outbreaks in the last decade. The federally endangered Mt. Graham red squirrel (Tamiasciurus hudsonicus grahamensis) is only found in the Pinaleño Mountains, and to assess effects of forest disturbance on habitat we modeled their potential habitat by identifying characteristics of cover surrounding their centrally defended middens. We classified high‐spatial resolution satellite imagery into ground cover classes, and we used logistic regression to determine areas used by squirrels. We also used known midden locations in conjunction with slope, elevation, and aspect to create a predictive habitat map. Squirrels selected areas of denser forest with higher seedfall for midden sites. Among active middens, those in the densest and least damaged forests were occupied in more seasons than those in more fragmented and damaged areas. The future conservation of red squirrels and the return of healthy mature forests to the Pinaleño Mountains will rely on preservation of mixed conifer zones of the mountain and active restoration of spruce‐fir forests to return them to squirrel habitat. Our ability to evaluate the spectrum of fine‐ to coarse‐scale disturbance effects (individual tree mortality to area wide boundaries of a disturbance) with high‐resolution satellite imagery shows the utility of this technique for monitoring future disturbances to habitat of imperiled species.

Spatiotemporal patterns of mortality in declining balsam fir and spruce stands

We used spatiotemporal inventory data from declining balsam fir ( Abies balsamea (L.) Mill.) and spruce ( Picea spp.) stands in New Brunswick, Canada, to determine the extent of tree-to-tree interactions and their effect on individual-tree mortality and stand break-up. Fifty circular 11.3 m radius permanent sample plots, established from 1987 to 1989 and remeasured every 3–5 years, were stem-mapped for tree and dead wood data in 2003–2005. Data included 1332 live trees and 1160 dead trees. Tree-to-tree interactions were assessed for trees within a 5 m circle of all subject trees in 12 m × 12 m subplots. Logistic regression models of individual-tree mortality were generated for two analysis periods, corresponding to about the end of a spruce budworm ( Choristoneura fumiferana (Clem.)) outbreak (1990–1992) and about 10 years after the cessation of budworm defoliation (1999–2003). From 1990–1992, significant predictor variables included annual diameter increment, moderate–severe spruce budworm defoliation, and stem defects. The mortality model also includes a distance-dependent competition index that outperformed all other measures of competition. By 1999–2003, annual diameter growth had increased by 30% following cessation of defoliation. Predictors of individual-tree mortality of trees alive in 1999–2003 included stem defects and annual wind-related mortality of neighbouring trees. Wind damage was the primary cause of death, and increased as canopies became more open. Treefall events occurred mostly in the winter, as indicated by alignment of easterly treefall direction with winter prevailing winds. Dead treefall collisions accounted for 1.1% of tree deaths and damaged 1.3% of live trees. Mortality model results suggest that when stands reach a critical stage of decline, probability of mortality for residual trees is dramatically increased due to absence of protection from neighbouring trees and increased wind exposure, effecting rapid stand break-up.

Modeling crown volume of lodgepole pine based upon the uniform stress theory

A new model was proposed based on the uniform stress theory to predict crown volume of lodgepole pine ( Pinus contorta var. latifolia Engelm.). Uniform stress theory states that trees tend to develop a balance between the size of the stem and the bending moment applied at the stem, to maintain mechanical stability. Bending moment is produced when trees receive a drag created by friction as wind acts on the crown. Data from permanent sample plots of natural stands of lodgepole pine across Alberta, Canada were used for model construction. Mean wind speed was estimated for each plot based on the Canadian Wind Energy Atlas. Diameter at breast height, a measure of leverage from the center of the crown to breast height, and wind speed were used as predictor variables. The new models explained 69.4 and 60.6% of total variation for crown volume and crown radius, respectively, when lodgepole pine trees were sampled from various subregions. Inclusion of relative depth to zero plane displacement and stand relative density resulted in virtually no improvement of the regression. The prediction of crown size with these models could be used for modelling mortality and growth of individual trees, and estimation of crown cover of stands.

An Object-Based Classification Approach in Mapping Tree Mortality Using High Spatial Resolution Imagery

In California, a newly discovered virulent pathogen (Phytophthora ramorum) has killed thousands of trees, including tanoak (Lithocarpus densiflorus), coast live oak (Quercus agrifolia), and black oak (Quercus kelloggii). Mapping the distribution of overstory mortality associated with the pathogen is an important part of disease management. In this study, we developed an object-based approach, including an image segmentation process and a knowledge-based classifier, to detect individual tree mortality in imagery of 1 m spatial resolution. The combined segmentation and classification methods provided an easy and intuitive way to incorporate human knowledge into the classification process. The object-based approach significantly outperformed a pixel-based maximum likelihood classification method in mapping the tree mortality on high-spatial-resolution multispectral imagery.

Evaluation of a post-fire tree mortality model for western USA conifers

Accurately predicting fire-caused mortality is essential to developing prescribed fire burn plans and post-fire salvage marking guidelines. The mortality model included in the commonly used USA fire behaviour and effects models, the First Order Fire Effects Model (FOFEM), BehavePlus, and the Fire and Fuels Extension to the Forest Vegetation Simulator (FFE-FVS), has not been tested with independently collected post-fire tree mortality data. The model predicts mortality for a wide range of conifer species based on crown scorch and species-specific bark thickness. We evaluated the mortality model on 13 western USA conifers: subalpine fir, red fir, white fir, Douglas-fir, incense cedar, western larch, western hemlock, Engelmann spruce, whitebark pine, lodgepole pine, ponderosa pine, Jeffrey pine, and sugar pine. Predicted stand-level mortality was within ±20% of observed mortality for all species except incense cedar, western larch, red fir, and western hemlock. Individual tree mortality prediction was most accurate for subalpine fir, incense cedar, ponderosa pine, and Jeffrey pine. Evaluation of the model provides managers with an accuracy assessment for estimating the probability of mortality for the majority of western USA conifers when using the mortality model to make land management decisions.A

Distribution, composition, and orientation of down deadwood in riparian old-growth woodlands of Zoar Valley Canyon, western New York State, USA

During 2005, we catalogued down deadwood (DDW) in forty-one 30-m × 30-m quadrats on 10 riparian upper terraces within the minimally disturbed Zoar Valley Canyon of western New York State. Woodlands on these former floodplains represent late-successional stages of a diversity of broadleaf ecotypes, with increment core-based stand ages up to 351 years. Volume of DDW averaged 84.9 ± 9.7 (±S.E.) m 3/ha among all quadrats, and ranged up to 145.3 ± 43.2 m 3/ha on individual terraces. Abundance of downed sugar maple reflected this species’ prevalence in the live overstory, but American beech deadwood was markedly overabundant due to beech bark disease mortality. Prevalence of very-shade-tolerant (sugar maple, American beech, eastern hemlock) DDW was modest for northern hardwood old growth (46.4 ± 4.3% among all quadrats), and was not related to stand age. We speculate this may reflect the floodplain origin of these woodlands. Down deadwood volume was positively associated with stand age among terraces ( R 2 = 0.446, P = 0.035), but not at the neighborhood scale of 30-m quadrats ( R 2 = 0.027), where individual tree mortality may obscure broader patterns. Orientation of DDW was non-uniform (Kolmogorov–Smirnov goodness-of-fit, P < 0.05) on five of the ten study terraces, where statistical trends in treefall direction suggested the influence of prevailing westerly winds blowing through the east-west canyon (this DDW orientation was also opposite to stream flow). This was an unexpected result, however, as we otherwise found very limited evidence of episodic wind-throw within the study area.

Modeling Postfire Mortality of Ponderosa Pine following a Mixed-Severity Wildfire in the Black Hills: The Role of Tree Morphology and Direct Fire Effects

Abstract We examined the relationship among tree size, crown and stem damage, and 5 years of postfire mortality of ponderosa pine (Pinus ponderosa Dougl. ex. P. &amp; C. Laws.) in the Black Hills following a large, mixed-severity wildfire. We measured tree morphology and direct fire effects on 963 trees and assessed individual tree mortality annually from 2001 to 2005. We used logistic regression to model tree mortality as a function of tree morphology (dbh) and bark thickness (BARK) and direct fire effects [percentage of the live crown scorched (PSCOR) and basal char measured as the percentage of the bole charred below 30 cm (CHAR)]. Models using dbh and BARK were modeled separately due to correlation between the variables. In all models, mortality decreased with increasing dbh and BARK and increased with increasing PSCOR and CHAR. Basal char contributed to the mortality of trees less than 40 cm but became less influential as dbh and BARK increased. Overall, probability of mortality modeled as a function of dbh, PSCOR, and CHAR correctly predicted the status of 78% of trees, whereas the model predicting mortality as a function of BARK, PSCOR, and CHAR had an increase in prediction accuracy of only 1%.

An analytical model of stand dynamics as a function of tree growth, mortality and recruitment: The shade tolerance-stand structure hypothesis revisited

Light competition and interspecific differences in shade tolerance are considered key determinants of forest stand structure and dynamics. Specifically two main stand diameter distribution types as a function of shade tolerance have been proposed based on empirical observations. All-aged stands of shade tolerant species tend to have steeply descending, monotonic diameter distributions (inverse J-shaped curves). Shade intolerant species in contrast typically exhibit normal (unimodal) tree diameter distributions due to high mortality rates of smaller suppressed trees. In this study we explore the generality of this hypothesis which implies a causal relationship between light competition or shade tolerance and stand structure. For this purpose we formulate a partial differential equation system of stand dynamics as a function of individual tree growth, recruitment and mortality which allows us to explore possible individual-based mechanisms—e.g. light competition—underlying observed patterns of stand structure—e.g. unimodal or inverse J-shaped equilibrium diameter curves. We find that contrary to expectations interspecific differences in growth patterns can result alone in any of the two diameter distributions types observed in the field. In particular, slow growing species can present unimodal equilibrium curves even in the absence of light competition. Moreover, light competition and shade intolerance evaluated both at the tree growth and mortality stages did not have a significant impact on stand structure that tended to converge systematically towards an inverse J-shaped curves for most tree growth scenarios. Realistic transient stand dynamics for even aged stands of shade intolerant species (unimodal curves) were only obtained when recruitment was completely suppressed, providing further evidence on the critical role played by juvenile stages of tree development (e.g. the sapling stage) on final forest structure and composition. The results also point out the relevance of partial differential equations systems as a tool for exploring the individual-level mechanisms underpinning forest structure, particularly in relation to more complex forest simulation models that are more difficult to analyze and to interpret from a biological point of view.

Relation Between Individual Tree Mortality and Tree Characteristics in a Polluted and Non-Polluted Environmentx

Data on individual tree mortality in relatively healthy (Berezinskiy biosphere reserve) and damaged (surroundings of the mineral fertilizer plant 'Achema') even-aged Scots pine (Pinus sylvestris L.) stands are presented. Tree size and competition intensity were found to be the most significant predictors of individual tree mortality in all age groups of the relatively healthy Scots pine stands, however, an essential reduction in the closeness of relations between the tree mortality rate and these variables was determined with the aging of stands. An exponential decrease in tree mortality probability with an increase of tree size is characteristic for trees suffering different competition intensity, however, this decrease becomes much more pronounced as the competition pressure increases. The relations of different tree and stand variables with tree mortality probability have been found to become much weaker in the polluted environment. An exponential increase in tree mortality probability with an increase of crown defoliation was characteristic of damaged Scots pine stands, however, the rate of the increase was different in different age and diameter classes. The impact of crown defoliation on tree mortality rate increased with the aging of stands. At the same defoliation level, individual tree mortality probability was much higher for smaller suppressed trees, however, a relative increase in tree mortality probability along with an increase of crown defoliation was more pronounced for dominant trees. a higher mortality of damaged (defoliated) trees should be considered while assessing losses in forest productivity in a polluted environment.