Forests play an important role in the earth system, regulating climate, maintaining biodiversity, and provisioning human communities with water, food and fuel. Interactions between climate and forest dynamics are not well constrained, and high uncertainty characterizes projections of global warming impacts on forests and associated ecosystem services. Recently observed tree mortality and forest die-off forewarn an acceleration of forest change with rising temperature and increased drought. However, the processes leading to tree death during drought are poorly understood, limiting our ability to anticipate future forest dynamics. The objective of this dissertation was to improve understanding of drought-associated tree mortality through literature synthesis and tree-ring studies on trees that survived and died during droughts in the southwestern USA. Specifically, this dissertation 1) documented global tree mortality patterns and identified emerging trends and research gaps; 2) quantified relationships between growth, climate, competition and mortality of piñon pine during droughts in New Mexico; 3) investigated tree defense anatomy as a potentially key element in piñon avoidance of death; and, 4) characterized the climate sensitivity of piñon resin ducts in order to gain insight into potential trends in tree defenses with climate variability and change. There has been an increase in studies reporting tree mortality linked to drought, heat, and the associated activity of insects and pathogens. Cases span the forested continents and occurred in water, light and temperature-limited forests. We hypothesized that increased tree mortality may be an emerging global phenomenon related to rising temperatures and drought (Appendix A). Recent radial growth was 53% higher on average in piñon that survived versus died during two episodes of drought-associated mortality, and statistical models of mortality risk based on average growth, growth variability, and abrupt growth changes correctly classified the status of ~70% of trees. Climate responses and competitive interactions partly explained growth differences between dying and surviving trees, with muted response to wet/cool conditions and enhanced sensitivity to competition from congeners linked to growth patterns associated with death. Discrimination and validation of models of mortality risk varied widely across sites and drought events, indicating shifting growth-mortality relationships and differences in mortality processes across space and time (Appendix B). Pre-formed defense anatomy is strongly associated with piñon survivorship over a range of sites and stand conditions. Models of mortality risk that account for both growth and resin duct attributes had≈10¹⁹ more support than models that contained only growth. The greatest improvement in classification was among trees from the 2000s drought, suggesting an enhanced role for tree defense allocation and/or bark beetle activity during recent warm versus historic cool drought. Accounting for defense characteristics and growth-defense allocation is likely to be important for improving representation of drought-associated mortality (Appendix C). Piñon resin duct chronologies contain climate responses that are coherent and distinct from those of radial growth. Growth responds positively and strongly to previous fall and current winter precipitation, and negatively to late spring and early summer temperature. A relatively equal positive resin duct response to winter precipitation and positive response to mid-to-late summer drought suggests that changes in climate will affect tree defense anatomy in complex ways, with the outcome determined by seasonal changes in precipitation and temperature (Appendix D).

As populations of the European gypsy moth (Lymantria dispar L.) move into the southeast, laboratory studies indicate that the potential exists for defoliation and possible tree mortality in mixed pine-hardwood stands. This dissertation describes a field study that was initiated to determine the susceptibility and vulnerability of pure and mixed stands of loblolly pine, oaks and sweetgum in pine-oak and pine-sweetgum cover types in the Atlantic Coastal Plain physiographic province. Sixteen stands were defoliated between 1992 and 1996. Stand susceptibility was significantly related to stand composition; stands with a greater proportion of susceptible species experienced greater defoliation. Oaks and sweetgum were heavily defoliated. Pines did not suffer extensive defoliation, and results of the study indicate that the probability of widespread gypsy moth defoliation in pine plantations appears to be low. A multiple linear regression model for the prediction of mean stand defoliation is also presented. Trends in tree mortality were similar to those previously described in northeastern forests. Susceptible tree species were heavily impacted, however, oaks displayed greater vulnerability than sweetgum. Suppressed and intermediate trees in the understory, and trees that were in poor or fair condition, had a greater probability of dying subsequent to defoliation. However, under- and overstory basal area mortality rates were not significantly different in the pine-oak type, and stem mortality rates were not significantly different in either type. Loblolly pine mortality was observed, but it was difficult to isolate the source, and thereby determine whether gypsy moth defoliation was a primary causal factor. In general, the potential for extensive pine mortality in mixed pine-hardwood stands does not appear to exist. Logistic regression was found to be a useful tool in the prediction of individual tree mortality and two logistic regression equations were derived and validated for use in pine-oak and pine-sweetgum cover types. / Ph. D.

Over the last decade, millions of acres of western North American forest have been reduced to areas of standing dead trees following eruptions in bark beetle populations. This thesis provides up-to-date information on streamflow response to the recent bark beetle outbreak in subalpine forests of the Colorado Rockies. Streamflow and climate measures are evaluated in eight central Colorado catchments with long-term data records and varying levels of beetle outbreak. No detectable streamflow change is observed in 7 of 8 highly impacted catchments. A significant reduction in streamflow is observed in 1 highly impacted catchment and is likely driven by tree mortality and record warm temperatures. These findings deviate from expected results and have important implications for vegetation and streamflow change under a warmer climate.

Sustainable management of mixed-species forests for timber is underpinned by research on forest stand dynamics and quantification of tree recruitment, growth and mortality rates. Different performance among species across light gradients theoretically prevents more shade-tolerant species from excluding shade-intolerant species, driving succession and allowing species coexistence. This research investigates stand dynamics and performance trade-offs for co-occurring tree species: Nothofagus fusca (red beech) and Nothofagus menziesii (silver beech), which together dominate extensive areas of New Zealand’s indigenous forest. Using permanent plot data, measurements of permanently tagged individuals are used to quantify recruitment, growth and mortality rates for each species, across size classes and life-history stages (i.e. seedlings, trees). First, seedling growth and mortality is examined in relation to microhabitats (e.g., light, substrate type) and contrasted with patterns of seedling abundance. Second, spatially explicit permanent plot data are used to examine tree growth in relation to competition, local disturbance and tree size over a 23-year period. Third, the influence of competition and disturbance on tree mortality and spatial patterns of tree mortality are examined. Fourth, a simulation model for tree population dynamics is parameterised for mixed-Nothofagus forest and used to evaluate long term consequences of disturbances (e.g. alternate harvesting regimes) on structure and composition. Small-scale disturbance favoured each species at different life stages and for different measures of performance (i.e. recruitment, growth or mortality). N. fusca seedlings and trees grew fast in high light microhabitats, such as those created by small-scale disturbances, but adult N. fusca mortality was elevated near sites of recent disturbance. By contrast, N. menziesii trees grew faster near sites of recent disturbance, which may help this species persist. Consequently, simulation results showed that small-scale disturbance frequency was a major determinant of forest composition and structure, determining whether N. fusca or N. menziesii is dominant. The simulation model could be developed further and used to inform the sustainable management of mixed-Nothofagus forests.

Drought-induced tree mortality is an emerging global phenomenon that appears related to climate change and rising temperatures in particular, and may be an early indication of vegetation change. However, vegetation response to climate change is uncertain, particularly for future novel climates. Notably, no current models of vegetation change attempt to mechanistically predict plant mortality, and in particular, mortality of trees, which exerts strong influences on ecological function. Resolving uncertainties surrounding the physiological mechanism and temperatures sensitivity of tree mortality is a current challenge in global change ecology. The objectives of this dissertation were to 1) consider tree mortality consequences for earth system processes related to carbon, water, and energy exchange that include climate regulation; 2) explore tree mortality effects on the water cycle by developing hypotheses and research needs; 3) quantify the temperature sensitivity of drought-induced tree mortality and gain insight into the physiological mechanism of mortality; 4) quantify the relationships among temperature, stored carbohydrate resources, and gas exchange to further elucidate physiological tree mortality mechanisms; and 5) quantify the sensitivity of two species of pine seedlings to progressively elevated temperatures and relate mortality to the effect of temperature on carbon metabolism. Major findings of this dissertation relate to the temperature sensitivity, physiological mechanism, and implications of tree mortality. Assessment of the potential consequences of tree mortality for earth system processes documented the contrasting influences of tree mortality on the terrestrial C cycle and land-surface energy exchange, the balance of which will determine the net effects on climate regulation (Appendix A). Following a survey of the ecohydrology literature, thresholds for tree mortality to cause watershed changes were hypothesized at ~20% loss of canopy cover, ~500 mm of annual precipitation, and whether flows are snowmelt dominated (Appendix B). Elevated temperature (~+4°C) accelerated tree mortality by 28% during experimental drought, a difference related to cumulative respiration dynamics in piñon pine (Appendix C). Stored carbohydrate resources were declined during lethal drought but were not entirely depleted prior to mortality (Appendix D). Seedlings exhibited progressive declines in time-to mortality with increased temperatures, a response related to C metabolism (Appendix E).

No description available.

In this thesis I examined the impacts of variable retention harvesting on residual tree mortality and natural regeneration of white spruce [Picea glauca (Moench (Voss)] in northern Alberta. The VR was done in four overstory canopy compositions (ranging from deciduous dominated to conifer dominated) and at six rates of canopy retention (2%, 10%, 20%, 50%, 75% and 100%). After 10 years there was 32.9 % mortality of aspen (Populus tremuloides Michx.) and 16.9 % mortality of spruce in the VR cuts. Mortality of individual trees was greater with low density of trees, in the conifer stands and for trees with short live crowns, which are large and trees near machine corridors. Natural regeneration of spruce was greatest with higher availability of seed trees (>30 ha-1) and on machine corridors, where stocking reached 74%. By contrast, stocking was 14% on retention strips, when seed tree density was 11 seed trees ha-1. / Forest Biology and Management

Submitted by Claudia Rocha (claudia.rocha@udesc.br) on 2017-12-11T12:45:44Z No. of bitstreams: 1 PGEF16MA067.pdf: 68542 bytes, checksum: 23aa33dada350b049cf9be80cf0294d9 (MD5) / Made available in DSpace on 2017-12-11T12:45:44Z (GMT). No. of bitstreams: 1 PGEF16MA067.pdf: 68542 bytes, checksum: 23aa33dada350b049cf9be80cf0294d9 (MD5) Previous issue date: 2016-06-24 / FAPESC / Uniedu / This dissertation aimed to identify the impact of reservoir filling of a hydroeletric power station on the floristic-structural compostion of the tree component in ecotonal fragment between Mixed Ombrophilous Forest and Deciduous Forest, located in the municipality of São José de Cerrito, SC. Two survey were conducted: in the first and second year after the reservoir water filling. For this, 48, 200 m2, plots were allocated through stratified systematic sampling, in three sectors: bottom (sector 1), adjacent to the reservoir, middle slope and upper slope. The trees within plots with circumference at breast height (cbh) ≥ 15,7 cm were identified and measured (cbh), and, in the second evaluation year, the dead trees were counted and recruits incorporated to survey. In each plot, the following environmental variables were obtained: edaphic, topographic and environmental impact. The data were analyzed by phytosociological estimates, demographic dynamics, multivariate analysis and modelling techniques. In the first year, the high number of dead trees (second greatest value of relative importance in community: 9.59%) stood out, with a significantly great number in the sector 1 (p < 0,05), which could be associated to the proximity to water reservoir. The dynamics demonstrated stability in richness and individuals number, whereas there were differences among sectors, with sector 1 presenting greater basal area loss (2.51%.year-1) than others. There was a weak relation between dynamics and evaluated environmental variables, indicating that the trees development is mostly associated to non-evaluated variables, such as reservoir influence. Thus, the present study indicates that the first effects of reservoir can be observed two years after the process of water filling / A presente dissertação objetivou identificar o impacto do enchimento do reservatório de uma Usina Hidreletrica (UHE) na composição florístico-estrutural do componente arbóreo de um fragmento em ecótono entre Floresta Ombrófila Mista e Floresta Estacional Decidual localizado no município de São José do Cerrito, SC. Foram realizadas duas avaliações: no primeiro e no segundo ano após o enchimento do reservatório. Para isso, foram alocadas 48 parcelas de 200 m² cada, de forma estratificada-sistemática, em três setores do fragmento: adjacente às margens do reservatório (setor 1), meio da encosta e topo da encosta. Os indivíduos arbóreos dentro das parcelas com circunferência a altura do peito (CAP) ≥ 15,7 cm foram identificados e mensurados (CAP), sendo que, no segundo ano de avaliação, os indivíduos mortos foram quantificados e os recrutas incorporados ao levantamento. Em cada parcela, foram mensuradas as variáveis ambientais: características edáficas, topográficas, de luminosidade e de impacto ambiental. Os dados foram analisados por estimativas fitossociológicas, cálculos de dinâmica, análises multivariadas e técnicas de modelagem. Destaca-se, no primeiro ano, o elevado número de indivíduos mortos (segundo maior valor de importância na comunidade: 9,59%), sendo significativamente maior no setor 1 (p < 0,05), o que pode estar associada à maior proximidade com a lâmina d’água. A dinâmica apresentou estabilidade na riqueza e em número de indivíduos, porém, entre os setores houve distinções, sendo o setor 1 caracterizado pela maior taxa de perda em área basal (2,51%.ano-1). Houve pouca relação das taxas de dinâmica com as variáveis ambientais analisadas, indicando que a maior influência encontrada sobre o desenvolvimento das árvores está associada a variáveis não utilizadas nas análises, como a influência do reservatório. Assim, o presente estudo indica que, dois anos após o enchimento do reservatório, foi possível perceber os primeiros efeitos do enchimento do lago da hidrelétrica na comunidade arbórea

Widespread, high levels of tree mortality, termed forest die-off, associated with drought and rising temperatures, are disrupting forests worldwide. Drought will likely become more frequent with climate change, but even without more frequent drought, higher temperatures can exacerbate tree water stress. The temperature sensitivity of drought-induced mortality of tree species has been evaluated experimentally for only single-step changes in temperature (ambient compared to ambient + increase) rather than as a response surface (multiple levels of temperature increase), which constrains our ability to relate changes in the driver with the biological response. Here we show that time-to-mortality during drought for seedlings of two western United States tree species, Pinus edulis (Engelm.) and Pinus ponderosa (Douglas ex C. Lawson), declined in continuous proportion with increasing temperature spanning a 7.7 degrees C increase. Although P. edulis outlived P. ponderosa at all temperatures, both species had similar relative declines in time-to-mortality as temperature increased (5.2% per degrees C for P. edulis; 5.8% per. C for P. ponderosa). When combined with the non-linear frequency distribution of drought duration-many more short droughts than long droughts-these findings point to a progressive increase in mortality events with global change due to warming alone and independent of additional changes in future drought frequency distributions. As such, dire future forest recruitment patterns are projected assuming the calculated 7-9 seedling mortality events per species by 2100 under business-as-usual warming occur, congruent with additional vulnerability predicted for adult trees from stressors like pathogens and pests. Our progressive projection for increased mortality events was driven primarily by the non-linear shape of the drought duration frequency distribution, a common climate feature of drought-affected regions. These results illustrate profound benefits for reducing emissions of carbon to the atmosphere from anthropogenic sources and slowing warming as rapidly as possible to maximize forest persistence.

Many fire-prone forests in western North America suffer from hazardous fuel conditions. Mechanical mastication is an increasingly common method of fuels treatment, but little is known regarding long-term effectiveness. A randomized block design at two sites (northern Sierras and southern Cascades) compared ladder fuels and overstory growth among treatments including mastication alone and mastication followed with prescribed fire or herbicide 10 or 11 years post-treatment. Subsequent herbicide application reduced ladder fuels in comparison to mastication alone or with prescribed fire. Prescribed fire further reduced ladder fuels at the southern Cascades site, however, in the northern Sierras postire ladder fuels were positively related to overstory absence. Mastication alone effectively released ponderosa pine at the southern Cascades site, whereas neither herbicide nor prescribed fire affected pine radial growth. This study demonstrates the feasibility of prescribed fire and herbicide for increasing treatment longevity of mastication, but also highlights potential limitations and important considerations.