Climatic changes have induced striking altitudinal and latitudinal vegetation shifts throughout history. These shifts will almost certainly recur in the future; threatening other flora and fauna, and influencing climate feedback loops. Changes in the spatial distribution of vegetation are most conspicuous at physiognomically distinct ecotones, particularly between the subalpine forest and alpine tundra. Traditionally, ecological research has linked abiotic variables with the position of this ecotone (e.g., cold temperatures inhibit tree survival at high elevations). Thus, the prevailing assumption states that this ecotone is in equilibrium or quasi-equilibrium with the surrounding physical environment and that any dynamic shifts express direct linkages with the physical environment. This dissertation employs a landscape ecology approach to examine the abiotic and biotic ecological mechanisms most important in controlling tree establishment at this ecotone. The study site is on the western slopes of Mount Rainier, which was severely burned by a slash fire in 1930. Therefore, a crucial underlying assumption is that the ecological mechanisms controlling tree establishment are similar at disturbed and undisturbed sites. I exploited the use of 1970 CORONA satellite imagery and 2003 aerial photography to map 33 years of changes in arboreal vegetation. I created detailed maps of abiotic variables from a LIDAR-based DEM and biotic variables from classified remotely sensed data. I linked tree establishment patterns with abiotic and biotic variables in a GIS, and analyzed the correlations with standard logistic regression and logistic regression in the hierarchical partitioning framework at multiple spatial resolutions. A biotic factor (proximity to previously existing trees) was found to exert a strong influence on tree establishment patterns; equaling and in most cases exceeding the significance of the abiotic factors. The abiotic setting was more important at restricted spatial extents near the extreme upper limits of the ecotone and when analyzing coarse resolution data, but even in these cases proximity to existing trees remained significant. The strong overall influence of proximity to existing trees on patterns of tree establishment is unequivocal. If the underlying assumption of this dissertation is true, it challenges the long-held ecological assumption that vegetation in mountainous terrain is in equilibrium with and most strongly influenced by the surrounding physical environment.

This thesis offers a method that informs on the most appropriate sites for successful tree establishment and provides protection to the planted trees in CDM forestry projects. It also offers a method that can accurately and precisely estimate woody biomass in CDM forestry projects established on an agricultural landscape. The thesis has established a set of evaluation criteria that are defined and generally agreed upon by a panel of local forestry experts in Malawi. These criteria express the degree of appropriateness of particular locations to support successful tree establishment and growth, and protect the planted trees from deforestation and forest degradation. They also influence farmers’ decision-making to allocate land for either tree planting or other competing land use options, e.g. crop cultivation. The thesis uses these evaluation criteria to functionally identify priority sites on the landscape of Central Malawi for the establishment of CDM forestry projects. The priority sites are identified using trade-off analysis tools of the GIS-based MCA protocols. Besides site identification, the thesis has established a magnitude of discrepancy that can result from the use of imported allometric models in estimating woody biomass in CDM forestry projects. Imported allometric models consistently under-estimate the woody biomass of the growing trees in the study area with a mean prediction error of as high as 50%. Local allometric models are therefore developed and they are found to be both accurate and precise in estimating woody biomass. These results imply that identification of priority sites, and accurate and precise estimation of woody biomass in CDM forestry project activities are likely and can be attained. In turn, successful establishment of CDM project activities lead to enhanced productivity that will attract expanded investments in CDM forestry projects. The thesis uses Malawi as a case study. The findings of this thesis can be scaled out to other countries whose socio-ecological characteristics are similar to Malawi. These are mostly tropical countries of the Sub-Saharan Africa. In so doing, the thesis contributes to improving productivity and estimation of carbon in CDM forestry project.

The factors affecting tree establishment in boreal forests after fire will help determine the community composition of the regenerating forest. These may have large consequences on the community dynamics for years after the fire disturbance. Factors such as burn severity and soil moisture among others have been shown to play a key role in influencing several facets of establishment. However, tree establishment after megafire in boreal forest in Europe has not yet been fully understood. Here I capitalise on a megafire in Sweden in 2014 to investigate the relative impact of different abiotic factors and preconditions on tree establishment six years after the fire. This study used a systematic survey of tree saplings (height >30cm) at 625 locations inside the nature reserve set up within the burnt area. Tested factors were: The number of dead trees lying down, slope and slope aspect, elevation, soil wetness, pre-fire standing volume, distance to fire perimeter, forest stand age, stand productivity index, previous stand dominant tree species, humus thickness after fire and depth of burn. Generalized Linear Mixed Models (GLMMs) were used to estimate the effect of these factors on specific tree species abundance. Strong influences from previous wood volume, soil wetness, elevation, and dead wood lying down had an effective influence on sapling abundance but were typically species-specific. Only elevation and wood volume had a consistent effect on all species’ abundances. Habitat context was important on a landscape scale. These results support the pattern of increasing boreal deciduousness caused by high burn severity and shorter disturbance intervals, in turn, caused by hotter, drier weather, which will have implications on the composition of boreal forests of tomorrow.

Recent research has challenged the long-standing hypothesis that forests in the Upper Midwest of the United States developed during wetter periods and retreated during dry periods. We explored this debate by examining patterns of tree establishment on an oak savanna in east-central Minnesota within the context of variable moisture availability and fire suppression. We used superposed epoch analyses (SEA) to evaluate the mean moisture conditions for a 21-year window surrounding tree establishment dates. Before effective fire suppression (1809–1939), 24 of 42 trees with pith dates (62%) grew to 30-cm height during dry years (Palmer Drought Severity Index < -1), versus only 5 of 42 (12%) that established in wet years (PDSI > 1). Significantly more trees established during dry periods (negative PDSI values) than would be expected with the proportion of wet-to-dry years (x²= 10.738, df = 1, p-value = 0.001). Twenty of the complete sample of 74 trees with pith dates (27%) established during drought in the 1930s. We hypothesize that dry conditions limited plant productivity, which in turn decreased competition between grasses and tree seedlings and reduced rates of accumulation of fine fuels, enabling seedlings to grow tall enough to resist subsequent fires. We recommend SEA as a methodological approach to compare historical climate conditions with the timing of regeneration success in other regions of forest expansion.

Developing quantified establishment period estimates for newly planted trees will help set realistic goals for plant performance in the urban landscape. Nine years of tree planting records obtained from the city of Boston, MA and the town of Brookline, MA were used to derive samples of trunk caliper for hedge maple (Acer campestre), London planetree (Platanus x acerifolia), and red oak (Quercus rubra). Several site characteristics were measured to identify effects on newly planted tree growth. Breakpoint estimates of the piecewise regression models fell at 4 years and 6 years for London planetree and hedge maple respectively, showing establishment periods for urban trees are longer than the conventional 3 years. Pit area has a significant effect on caliper growth for London planetree and red oak, with larger growing spaces yielding larger trees. Observed soil bulk densities did not have a significant effect on caliper measurement for any of the species tested. Mean soil bulk density was 1.4 g/cm3, 1.45 g/cm3, and 1.49 g/cm3 for hedge maple, London planetree, and red oak respectively. Soil texture was significant for hedge maple only, with percent clay showing significant positive effects and percent sand as marginally significant. Pit type showed a significant effect on caliper measurement of London planetree with grass strips producing the largest caliper trees followed by open pits. Planting locations with tree grates produced the smallest caliper trees. Presence of perforated air vents produced significantly larger caliper trees than sites with an air vent present for red oak.