Abstract
Conifers dominate temperate and boreal forests around the world and many species are economically important for structural timber production. They are also adapted to extreme climatatic conditions, such as drought and cold temperatures. Numerous conifer species occur within the Pacific Northwest United States (PNW) including firs, hemlocks, cedars, larches, and pines. The diversity of hydrologic and geographic variables across the PNW creates distinctive localized climates with differing limiting factors on conifer growth, such as temperature, moisture, and nutrient availability. Dendrochronology, or tree-ring dating, is a multidisciplinary methodology that utilizes annual tree ring widths to investigate environmental conditions influencing trees or stands throughout their life. A substantial amount of information is known about tree-growth responses to climate and stand dynamics. However, investigations into interacting silvicultural and climatological influences, as well as spatial variability of growth relationships, may inform future management and dendrochronological techniques. The first three chapters of this dissertation utilize dendrochronology to investigate multiple drivers of species-specific tree growth.
The first chapter investigates the impacts of density reductions, via different thinning intensities, on tree growth in moist mixed coniferous forests in northern Idaho, USA. Species that respond rapidly to available sunlight and/or nutrients, like western larch and western redcedar, show the greatest growth increases following thinning. The less consistent responses to thinning by western hemlock and grand fir were likely due to their autecological characteristics and inherent lack of responses to greater growing space. Findings from Chapter 1 align with past thinning experiments and found that thinning is an effective tool for increasing growth in most species. However, if the objectives are to favor injury-prone and less competitive species like western hemlock and grand fir, precautions must be taken during and after treatment to limit tree damage that could produce undesired responses.
Chapter 2 presents the temporal variability of growth-drought relationships for the same species from Chapter 1, and how that relationship is influenced by thinning. The four species in this study, western larch, grand fir, western redcedar, and western hemlock, show a wide range of responses to drought depending on timing of drought, length of drought, intensity of drought, and forest stand density. Findings indicate that length and season of drought, species-specific drought tolerance, and stage of stand development, influence growth-drought responses. Drought sensitivity often involves trade-offs among other limiting factors like direct competition for resources. Moreover, trees growing in moist forests may not be as highly susceptible to droughts as those in dry forests. Therefore, it is suggested that stands be managed as complex adaptive systems by prioritizing species, age, and structural diversity. Results from Chapter 1 and Chapter 2 demonstrate that strip clearcutting in moist forest results in diverse conifer species composition and structure that can be further managed to create complexity through mid-rotation thinning.
Chapter 3 focused at a larger spatial scale (e.g., greater PNW) to examine the effects of geographical factors on flow-growth relationships of four different conifer species. Streamflow correlated negatively with subalpine fir and mountain hemlock growth, species commonly found at cool, moist, high elevation sites, indicating that they are likely more sensitive to severe environmental variation like those experienced with climate change. Drier-site species, Douglas-fir and ponderosa pine were mostly positively correlated with flow, though a few had significant negative correlations, indicating that they are species with high adaptive capacity. Results help simplify planning for field collections and strengthen methodologies for future streamflow reconstructions by supplying knowledge about which streams, species, elevations, and directions will yield the most robust models in the spatially diverse terrain of the PNW.
Chapter 4 is a collaborative synthesis of climate change research in the Columbia River Basin (CRB). Results show that spatial distribution and thematic content of research varies across an international border, with greater concentrations of research in the United States than Canada. A general scarcity of social science research and limited interaction between social and biophysical content reinforces the need for increased collaboration between disparate disciplines. Future research focus areas should include research related to climate change adaptation and mitigation, increased integration between social and biophysical sciences, and collaborations that bridge the international border for a more unified basin-wide focus. Focusing on these new directions for research will increase the potential for science and management communities to co-produce actionable science and effective responses to climate change.
With the utilization of dendrochronological techniques, many of the interacting drivers of species-specific tree growth in the PNW were discovered. Shade tolerance, disturbance dynamics, and hydroclimate all influence conifer growth in the region. The relationships between streamflow and growth are heightened for trees growing in extreme climates, and these relationships are driven by geographical features. Overall, this dissertation provides insight into dendrochronological techniques as well as silvicultural management in moist mixed conifer forests; it also lends support that forest management can assist tree growth and alter growth-drought relationships depending on species. Finally, the dissertation offers additional evidence to the decades-long theory that trees growing at the edges of their ranges show higher sensitivity to limiting factors.