Seasonal Influences on the Carbon-Water Relations in Ponderosa Pine Forests in the Northern Boundary of the North American Monsoon System

Climate models have projected that arid and semiarid lands will experience warmer and drier conditions for the next 100 years. For the last twenty years, the Southwestern US has been experiencing one of the worst droughts over the last century, not only threatening ecological systems but also the water security of its population. Understanding the environmental processes that affect arid and semiarid forests are essential to better understand the water and carbon cycles, and tree-ring research has contributed valuable knowledge in this regard. There is a common understanding that moisture-stress has significant impacts on forested ecosystems and thereby on the global carbon and water cycles. Under persistent moisture deficit, a decline in growth, an increased proportion of wildfires, insect outbreaks, and mass-tree-mortality are often observed in arid and semi-arid forests, having large impacts on their carbon budgets and their capacity to act as a carbon sink. This study addresses the seasonal and regional climatic influences on the water-carbon relations in the ponderosa pine forests of the southwestern US (SW). This region is characterized by a complex climatology related to the North American Monsoon system (NAMS). A topic of interest in this dissertation is the role of the summer rainfall after the early-summer hyper-arid period in the region, providing a unique seasonal condition for these ecosystems to thrive. While these forests clearly rely on winter snowpack to drive much of their annual net primary productivity, the temporal and regional extent to which they supplement winter moisture with summer monsoon moisture needs to be clarified. The core of this dissertation is a study of the spatial and temporal variability of the stable carbon and oxygen isotopes in the cellulose of subsections of the tree rings (e.g., earlywood and latewood) collected from a network of thirteen sites along a latitudinal gradient extending from southern Arizona and New Mexico, through southwest Colorado, and up to northern Utah. The analysis is based on biological and physical processes and their close relationships with isotope effects to infer eco-physiological responses to climate variations over the last century. The stable carbon isotopes are used to derive intrinsic Water-Use Efficiency (iWUE) defined by the molar ratio of carbon gain to water loss. The stable oxygen isotope ratio is used to infer the variations on evaporative flux at the leaf level, which depend on stomatal conductance, atmospheric vapour pressure deficit at the leaf surface, and variations in the isotopic ratio of the source water. Both isotopic ratios are used to document variations in tree productivity and hydrologic vulnerability within the context of climate change impacts on this region. During the study, it was found that climate change in the SW has impacted the carbon and water cycles of these forests for at least the past twenty years. Additionally, seasonality influence the eco-physiology of ponderosa pine change along the latitudinal gradient, as shown by significant differences between EW and LW. These differences are explained by the large shifts in seasonal VPD, which are more evident in the southern part of our study region due to the mid-summer arrival of monsoon rains. These findings will be useful for regional natural resource managers and improves our understanding of seasonal influences on forest water–carbon relationships. This approach will also be useful to develop seasonally resolved paleoclimate and paleo-ecophysiological reconstructions to characterize the long-term influence of winter versus summer moisture on carbon-water relations in forested ecosystems

Dendrochronology and Middle Miocene Petrified Oak: Modern Counterparts and Interpretation

This study reports the first successful statistical crossdating among many ring width time series from petrified wood, thus providing a replicable continuous annual resolution window into tree growth and environmental influences during the middle Miocene. The petrified samples, of the genus Quercus, originated at the Stinking Water (SW) locality in Oregon, a Miocene-aged exposure associated with the Columbia River Basalts. 40AR/39AR dating on pillow basalt from the locality yielded a weighted Plateau Age of 13.79 ±0.09 Ma placing the death of the trees at the end of the Langhian Stage of the Middle Miocene (15.97±0.05 to 13.65±0.05 Ma), during the middle Miocene Climate Transition (MMCT). Ring width time series from 26 radii, 17 different trees, show significant intercorrelation. A Modified Coexistence Approach was applied to determine the likely climate range when the SW trees were growing. The modified approach included regression of site-mean ring width time series statistic values on estimated soil moisture for the site locations, using site-mean data from 126 modern Quercus sites from across the United States. Identification of highly significant linearities indicated strong relationships between ring width intercorrelation and soil moisture and ring width variability and soil moisture. Comparison of individual modern site-mean statistical values with values calculated for the SW locality suggests a mesic growing environment for the SW Quercus, with moderate temperatures. Geographic placement of modern Quercus sites with site-mean statistics similar to the SW values indicates a modern analogue in the eastern United States. Modern distributions of mesic species in the genera present at the SW locality suggest similarities with the central and southern Appalachian Mountains and the Ozark/Ouachita Mountains, indicating a mean annual temperature range of between ring width intercorrelation and soil moisture and ring width variability and soil moisture. Comparison of individual modern site-mean statistical values with values calculated for the SW locality suggests a mesic growing environment for the SW Quercus, with moderate temperatures. Geographic placement of modern Quercus sites with site-mean statistics similar to the SW values indicates a modern analogue in the eastern United States. Modern distributions of mesic species in the genera present at the SW locality suggest similarities with the central and southern Appalachian Mountains and the Ozark/Ouachita Mountains, indicating a mean annual temperature range of ≅10 ˚C to ≅ 15 ˚C and a mean annual precipitation range of ≅750 mm to ≅1200 mm when the SW Quercus were growing

Fire, logging and establishment patterns of second-growth forests in south-central Chile: implications for their management and restoration

Second-growth forests represent the greatest potential resource for forest management and large-scale ecological restoration in many regions. In south-central Chile, second-growth forests include those dominated by Nothofagus obliqua, N. dombeyi, Drimys winteri, and a mixture of evergreen species, especially hardwoods. This article examines the influence of fire and logging on the establishment patterns and development of second-growth forests in south-central Chile. We characterize the size structure and composition of these four types of forests with sampling plots. The identification of the type of disturbance and its date of occurrence was determined from evidence such as fire scars and even-aged pulses of tree establishment. The size, structure and species composition of these forests indicate an intermediate state of development with an average density and basal area ranging from 1294 to 5038 trees ha-1 and from 59 to 85 m² ha-1, respectively. Logging and/or devastating fires that occurred in the early decades of the 1900s promoted the relatively rapid establishment and growth of pioneer species (Nothofagus obliqua, N. dombeyi, D. winteri). In the Mixed Evergreen second-growth forests, mid-shade or shade tolerant species (e.g., Gevuina avellana, Eucryphia cordifolia, Amomyrtus luma, and A. meli) became established mostly through vegetative sprouting. Fires and logging have been pervasive factors in determining the structural and compositional uniformity of the native forests of south-central Chile. Ecological restoration at a landscape level, either by ecological processes (i.e., a reduction in fire frequency) and/or the structure and composition of second-growth forests, provide a relevant approach to accelerating the generation of attributes of old-growth forests, therefore meeting manifold societal demands for forest goods and services.Los bosques secundarios representan el mayor recurso forestal para el manejo y la restauración de gran escala en muchas regiones del mundo. En el centro-sur de Chile los bosques secundarios están dominados por Nothofagus obliqua, N. dombeyi, D. winteri, y Siempreverde mixtos. El presente artículo examina la influencia del fuego y tala en los patrones de establecimiento y desarrollo de bosques secundarios de la región centro-sur de Chile. Para la caracterización de la estructura y composición de cada bosque secundario se seleccionaron rodales representativos estableciendo 4-6 parcelas de 900 m². El tipo de disturbio y su fecha de ocurrencia fue establecido a través de evidencias tales como cicatrices de fuego y pulsos de establecimiento de árboles. La estructura de tamaños y composición indican un estado de desarrollo intermedio o de reiniciación del sotobosque presentando en promedio una densidad y área basal que varía entre 1294 y 5038 árboles ha-1 y entre 59 y 85 m² ha-1, respectivamente. Incendios de gran severidad y/o madereo ocurridos en las primeras décadas de los 1900 promovieron un rápido establecimiento de las principales especies pioneras (Nothofagus obliqua, N. dombeyi, D. winteri). En los bosques secundarios Siempreverde mixtos, el establecimiento de especies tolerantes o semi-tolerantes a la sombra (ej., Gevuina avellana, Eucryphia cordifolia), fue principalmente por rebrotes vegetativos. El fuego y madereo han sido agentes clave en determinar la uniformidad estructural y composicional de los bosques nativos del centro-sur de Chile. La restauración ecológica a escala de paisaje, tanto de procesos ecológicos (frecuencia incendios) como de la estructura y composición de los bosques secundarios, ofrece una relevante aproximación para acelerar la generación de atributos de bosques antiguos que satisfagan los múltiples bienes y servicios ecosistémicos demandados por la sociedad

The Influence of Winter Snowpack on the Use of Summer Rains in Montane Pine Forests Across the Southwest U.S.

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Recovery of Pilgerodendron uviferum (D. Don) Florin forest after antropogenic disturbance in the Valdivian Costal Range, Chile Recuperación de bosques de Pilgerodendron uviferum (D. Don) Florin luego de perturbaciones antrópicas en la Cordillera de la Costa de Valdivia, Chile

The loss and decline of one ecosystem can be triggered by continuous changes in structure made by frequent and/or severe disturbances. Therefore we quantify the structural losses and the posteriori recovery after man-made disturbances of two fragments of P. uviferum in the Reserva Nacional Valdivia (RNV; 40°S) by studying the evidence of human disturbance and forest structure. Our results indicate frequent burned trees and stumps, indicating that slash and burn are the main factor shaping the present structure of these forest fragments. In both cases 86% of the basal area was represented by dead tress, and 14% of basal area by living individuals. The results show a severe past disturbance and the recovery of those P. uviferum forests by the abundant recruitment with a very high flora richness which is characteristic of this forest ecosystem. Nevertheless we found that some human-made disturbances can alter the successional patchways of this endangered ecosystem in the Cordillera de la Costa Valdiviana.Los cambios provocados por sucesivas y/o severas alteraciones pueden causar la transformación o pérdida de un sistema ecológico. En el presente trabajo se cuantifica la pérdida estructural y su posterior recuperación después de alteraciones antrópicas en dos fragmentos de Pilgerodendron uviferum (D. Don) Florin ubicadas en la Reserva Nacional Valdivia. Se estudió la estructura y composición de los fragmentos, y las evidencias de alteraciones antrópicas. La presencia de abundantes árboles quemados y tocones indica que el fuego y la extracción de madera han sido los principales agentes que han determinado la estructura y composición actual de dichos fragmentos. Para ambos casos, el 86 % del área basal corresponde a individuos muertos y el 14 % a individuos vivos. Se cuantificó una abundante regeneración natural postalteración, con una alta riqueza de especies en el sotobosque característica de estos bosques. No obstante, actualmente ambos fragmentos presentan amenazas que podrían alterar el curso de la sucesión secundaria de este frágil ecosistema ubicado en la Cordillera de la Costa valdiviana

Ecophysiological Responses of Nothofagus obliqua Forests to Recent Climate Drying Across the Mediterranean‐Temperate Biome Transition in South‐Central Chile

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Tree growth sensitivity to climate varies across a seasonal precipitation gradient

Spatial patterns of precipitation in the southwestern United States result in a complex gradient from winter-to-summer moisture dominance that influences tree growth. In response, tree growth exhibits seasonal-to-annual variability that is evident in the growth of whole tree rings, and in sub-annual sections such as earlywood and latewood. We evaluated the influence of precipitation and temperature on the growth of Pinus ponderosa trees in 11 sites in the southwestern US. Precipitation during the year of growth and the prior year accounted for about half of the climate influence on annual growth, with the other half reflecting conditions 2–4 years prior to growth, indicating that individual trees do indeed exhibit multi-year “memory” of climate. Trees in wetter sites exhibited weaker influence of past precipitation inputs, but longer memory of climatic variability. Conversely, trees in dry sites exhibited shorter memory of long-term climatic variability, but greater sensitivity to past precipitation effects. These results are consistent with the existence of complex interactions between endogenous (phenotype) effects and exogenous (climate) effects in controlling climate memory in trees. After accounting for climate, residual variability in latewood growth was negatively correlated with earlywood growth, indicating a potential tradeoff between latewood versus earlywood growth. This study provides new insights that will assist the accurate prediction of woody biomass growth and forest carbon sequestration across a southwestern US precipitation gradient

A Seasonally Varying Tree Physiological Response to Environmental Conditions: Results From Semi‐Arid China

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Seasonal divergence between soil water availability and atmospheric moisture recorded in intra-annual tree-ring delta O-18 extremes

Intra-annual variability of tree-ring oxygen stable isotopes (delta O-18) can record seasonal climate variability and a tree's ecophysiological response to it. Variability of sub-annual tree-ring delta O-18 maxima and minima, which usually occur in different parts of the growing season, may exhibit different climatic signals and can help in understanding past seasonal moisture conditions, especially in Asian monsoon areas. We developed minimum and maximum tree-ring delta O-18 series based on sub-annual tree-ring delta O-18 measurements ofPinus massonianaat a humid site in southeastern China. We found that interannual variability in minimum tree-ring delta O-18 is primarily controlled by the July-September soil water supply and source water delta O-18, whereas the maximum latewood tree-ring delta O-18 is primarily controlled by the relative humidity (RH) in October. The maximum of variability of earlywood tree-ring delta O-18 records the RH of October of the previous year. We used minimum and maximum tree-ring delta O-18 to develop two reconstructions (1900-2014) of seasonal moisture availability. The summer soil water supply (July-September self-calibrated Palmer drought severity index) and the RH in fall show contrasting trends, which may be related to late-growing seasonal warming leading to a high vapor capacity and high atmospheric moisture. Our findings are valuable for research that aims to explore seasonal moisture changes under anthropogenic climate change and the ecological implications of such contrasting trends.Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]