Search

Spruce–fir (Picea–Abies) forests of the North American Acadian Forest Region are at risk of disappearing from the northeastern United States and Canada due to climate change. Species distribution models (SDMs) have been used to predict changes in this critical transitional ecosystem in the past, but none have addressed how seasonal patterns of temperature and precipitation interact to influence tree species abundance. Inferences have also been limited by contemporary inventory data that could not fully characterize species ranges because they either, (1) only sampled species occurrence after large‐scale human disturbance and settlement, or (2) did not span critical geopolitical boundaries (e.g., the US–Canadian border) that intersect the focal species' range(s). Here, we built new SDM models to better assess the bioclimatic distribution of four spruce–fir species and to test the importance of seasonal climate interactions. We compiled an extensive database of tree occurrence and abundance from recent (~1955–2012) and historical time periods (1623–1869) to model current species distributions and to predict how these might change under future climate. We found that including historical tree data in our SDMs revealed previously unrecognized suitable habitat along the southern edge of species' contemporary ranges. Random forest models predicted occurrence with high accuracy (area under receiver operator curve >0.98), and the seasonal climate variables that emerged as most important for these cold‐adapted species all included interactions that reflected sensitivity to colder temperatures, and preferences for wet weather concentrated in the winter months. Under moderate climate warming (representative concentration pathway 6.0), the northeastern United States retained additional suitable habitat when historical data were included through 2060 for three of the four species: red spruce (Picea rubens), black spruce (Picea mariana), and balsam fir (Abies balsamea), while white spruce (Picea glauca) habitat contracted into Canada. In contrast, future predictions from models that used contemporary data alone forecast extirpation for all four species from the northeastern United States. Overall, these findings highlight that prediction of species ranges in transitional ecosystems that span geopolitical boundaries and gradients of intense land use are improved when historical data and seasonal climate interactions of both temperature and precipitation variables are incorporated. [ABSTRACT FROM AUTHOR]

Numerous models are available in northeastern North America to estimate aboveground tree biomass, yet most have focused on trees ≥12.5 cm diameter, and these models are often poor predictors of small tree biomass (<12.5 cm diameter). Additionally, models available to estimate small tree biomass often lack independent evaluation with field data. We developed a new set of additive biomass component (foliage, branch, and bole) models for five naturally regenerated hardwood species, four hybrid poplar (Populus sp.) clones and white spruce (Picea glauca (Moench.), from trees in eastern Maine. Biomass measurements were used to evaluate national and regional models for naturally regenerated species, and species specific models for planted stock. Results showed that the regional and national models provided similar fits for total biomass across all species. Moreover, the small tree biomass models currently used to predict woody biomass substantially underestimated biomass from 19.0% to 36.6% for all of the naturally regenerated hardwood species at our site. This substantial underestimation of small tree biomass may have contributed to the recent 34% decrease in estimated sapling woody biomass, and 15% decrease in aboveground biomass of all living trees in Maine, where nearly one-quarter of the 70,000 km2 forestlands are dominated by saplings. This problem may exist in other regions of the United States if substantial proportions of forestlands are dominated by small trees. Further model evaluation is warranted to assess the performance of the current models. [ABSTRACT FROM AUTHOR]

Counts of dead and living trees in open alvar indicated that the 2012 drought had a significant impact on the Burnt Lands Alvar in eastern Ontario, with mortality of woody species ranging from 10% to 100% at open alvar sites. The maximum age of dead trees suggested that the most recent drought of similar impact occurred more than 35 years ago, possibly in 1974. Some of the killed junipers were 87-90 years old. All dry periods indicated on a summer rainfall diagram may not be severe enough to kill woody vegetation, but a drought causing an average 50% kill of woody vegetation in open alvar may have occurred at least once every 30 years over the past century. Consequently the encroachment of woody species into alvar grassland appears not to be a threat to this habitat. Open alvar could have existed on the landscape as isolated occurrences since early postglacial times, if the climate fluctuated in the distant past as it has recently. With evidence for drought in a fluctuating climate to maintain the disjunct occurrences of midwestern plant and animal species, these occurrences may be considered as reliable indicators of a more continuous distribution of such species more than 10 000 years ago. [ABSTRACT FROM AUTHOR]

Intraspecific assisted migration ( ISAM) through seed transfer during artificial forest regeneration has been suggested as an adaptation strategy to enhance forest resilience and productivity under future climate. In this study, we assessed the risks and benefits of ISAM in white spruce based on long-term and multilocation, rangewide provenance test data. Our results indicate that the adaptive capacity and growth potential of white spruce varied considerably among 245 range-wide provenances sampled across North America; however, the results revealed that local populations could be outperformed by nonlocal ones. Provenances originating from south-central Ontario and southwestern Québec, Canada, close to the southern edge of the species' natural distribution, demonstrated superior growth in more northerly environments compared with local populations and performed much better than populations from western Canada and Alaska, United States. During the 19-28 years between planting and measurement, the southern provenances have not been more susceptible to freezing damage compared with local populations, indicating they have the potential to be used now for the reforestation of more northerly planting sites; based on changing temperature, these seed sources potentially could maintain or increase white spruce productivity at or above historical levels at northern sites. A universal response function ( URF), which uses climatic variables to predict provenance performance across field trials, indicated a relatively weak relationship between provenance performance and the climate at provenance origin. Consequently, the URF from this study did not provide information useful to ISAM. The ecological and economic importance of conserving white spruce genetic resources in south-central Ontario and southwestern Québec for use in ISAM is discussed. [ABSTRACT FROM AUTHOR]

Forest plantations in the northeastern United States comprise a small proportion of the total forest area. Most plantations are typically softwood dominated and managed for sawlog and pulpwood production, while high-yield hardwood plantations for bioenergy feedstocks have not been as widely investigated. The objective of this study was to compare the biomass production of planted white spruce ( Picea glauca (Moench) Voss) and hybrid poplar ( Populus spp.) plantations (four clones) in monoculture, and in mixture of the two on a typical reforestation site in Maine. Three years following planting, hybrid poplar height and ground line diameter growth rates began to diverge among clones, and by 6 years, the Populus nigra × Populus maximowiczii (NM6) clone clearly outperformed three Populus deltoides × Populus nigra clones (D51, DN10 and DN70) both in pure stands and in mixtures with white spruce. In mixture, we found the yield of white spruce to decline as the yield of hybrid poplar increased. Overall, yields of white spruce monocultures were comparable to those reported in eastern Canada, while the hybrid poplar biomass yields were substantially lower than those reported from studies on abandoned agricultural lands, likely due to the harsher soil conditions at our site. The dominance of rocky and poorly drained sites (like the one tested in this study) across Maine will likely limit the feasibility of widespread hybrid poplar plantations, and thus constrains their potential use as a bioenergy feedstock. [ABSTRACT FROM AUTHOR]

Fire, which is the dominant disturbance in the boreal forest, creates substantial heterogeneity in soil burn severity at patch and landscape scales. We present results from five field experiments in Yukon Territory, Canada, and Alaska, USA that document the effects of soil burn severity on the germination and establishment of four common boreal trees: Picea glauca, Picea mariana, Pinus contorta subsp. latifolia, and Populus tremuloides. Burn severity had strong positive effects on seed germination and net seedling establishment after 3 years. Growth of transplanted seedlings was also significantly higher on severely burned soils. Our data and a synthesis of the literature indicated a consistent, steep decline in conifer establishment on organic soils at depths greater than 2.5 cm. A meta-analysis of seedling responses found no difference in the magnitude of severity effects on germination versus net establishment. There were, however, significant differences in establishment but not germination responses among deciduous trees, spruce, and pine, suggesting that small-seeded species experience greater mortality on lightly burned, organic soils than large-seeded species. Together, our analyses indicate that variations in burn severity can influence multiple aspects of forest stand structure, by affecting the density and composition of tree seedlings that establish after fire. These effects are predicted to be most important in moderately-drained forest stands, where a high potential variability in soil burn severity is coupled with strong severity effects on tree recruitment. [ABSTRACT FROM AUTHOR]

Abstract: The northern treeline is generally limited by available warmth. However, in recent years, more and more studies have identified drought stress as an additional limiting factor for tree growth in northern boreal forests and at treelines. Three growth responses to warming have been identified: increase in growth, decrease in growth, and nonsignificant correlation of tree growth with climate. Here we investigate the effect of drought stress on radial growth of white spruce at northern treelines along a longitudinal gradient spanning the entire Brooks Range in Alaska. We systematically sampled 687 white spruce at seven treeline sites. Where possible, we sampled three site types at a given site: high-density forest, low-density forest, and floodplain forest. We investigated the relationship of site and site type to tree growth responses. In the western part of our study area, we found very high numbers of trees responding with increase in growth to recent warming; while in the eastern part, trees responding with decrease in growth to recent warming are predominant. Within a given site, more trees reacting positively to warming grow on site types characterized by low tree density. These patterns coincide with precipitation decreases from west to east and local water availability gradients, therefore pointing to drought stress as the controlling factor for the distribution of trees responding with increase or decrease in growth to recent warming. Compared to 20th century climate, we project a 25–50% basal area (BA) increase in the western region for the 21st century due to climate warming as projected by five general circulation models, 4–11% in the central region and decreases (+1 to −11%) in the eastern region. The overall net change in projected 21st century BA increase at each site seems to be controlled by the relative proportion of responder groups. If these are similar, differences in the magnitude of increase versus decrease in growth control BA projections for that site. This study highlights the importance of regional-scale investigations of biosphere–climate interactions, since our results indicate a substantial gain in aboveground biomass as a result of future warming only in the western regions; while in the eastern regions, climate warming will decrease overall wood production and therefore carbon uptake potential. [Copyright &y& Elsevier]

Northern and high-latitude alpine treelines are generally thought to be limited by available warmth. Most studies of tree-growth–climate interaction at treeline as well as climate reconstructions using dendrochronology report positive growth response of treeline trees to warmer temperatures. However, population-wide responses of treeline trees to climate remain largely unexamined. We systematically sampled 1558 white spruce at 13 treeline sites in the Brooks Range and Alaska Range. Our findings of both positive and negative growth responses to climate warming at treeline challenge the widespread assumption that arctic treeline trees grow better with warming climate. High mean temperatures in July decreased the growth of 40% of white spruce at treeline areas in Alaska, whereas warm springs enhance growth of additional 36% of trees and 24% show no significant correlation with climate. Even though these opposing growth responses are present in all sampled sites, their relative proportion varies between sites and there is no overall clear relationship between growth response and landscape position within a site. Growth increases and decreases appear in our sample above specific temperature index values (temperature thresholds), which occurred more frequently in the late 20th century. Contrary to previous findings, temperature explained more variability in radial growth after 1950. Without accounting for these opposite responses and temperature thresholds, climate reconstructions based on ring width will miscalibrate past climate, and biogeochemical and dynamic vegetation models will overestimate carbon uptake and treeline advance under future warming scenarios. [ABSTRACT FROM AUTHOR]

Presents multi-proxy tree-ring data from twenty productive stands of white spruce in the interior of Alaska. Evidence that radial tree growth has decreased with increasing temperature; Effect of temperature-induced drought; Suggestion that drought stress may have been an important factor limiting carbon intake in North American forests; Implications for the future capacity of northern latitudes to sequester carbon.

Investigates competition and microenvironmental modification in an Alaskan postfire boreal forest. Establishment of seedlings by white spruce and paper birch in patches dominated by horsetail and bluestem; Reduction of survivorship, growth and nitrogen accumulation in the presence of aboveground vegetation.

Needle hardiness of introduced yellow pine, Pinus banksiana Lamb., lodgepole pine, P. contorta Dougl. and native white spruce, Picea glauca (Moench) Voss, were assessed by the effective prefreezing temperature method. Yellow pine needles were less hardy than lodgepole pine or white spruce needles in Alaska on each date measured. Although hardiness decreased in springtime in all species, decreases in hardiness in yellow pine began before temperatures were above -20°C, apparently in response to day length, while decreases in hardiness in lodgepole pine and white spruce began only when mean temperatures were above 0°C. Hardiness was increased by decreasing the water content of yellow pine and spruce needles. However, only the latter increased its field hardiness by decreased water contents, and only to a small degree. Large decreases in phospholipid occurred during the dehardening period, indicating the presence of major membrane-associated changes. However, changes in hardiness did not closely parallel those in phospholipid; hardiness decreased before phospholipid did in spruce and after phospholipid did in lodgepole pine. In yellow pine, changes in hardiness were more closely related to changes in phospholipid content. Decreases in phospholipid appeared to be correlated with the day length in all species. [ABSTRACT FROM AUTHOR]

The Spatial Alaskan Forest Ecosystem Dynamics (SAFED) model was validated across four of the most common vegetation types found in interior Alaska. The vegetation types were an alder (Alnus spp.) - balsam poplar (Populus balsamifera L.) site (FP2), an old-growth balsam poplar and white spruce (Picea glauca (Moench) Voss) site (FP3), a mixed deciduous (primarily birch (Betula papyrifera Marsh.) and aspen (Populus tremuloides Michx.)) and white spruce site (UP2), and a mature white spruce site (UP3). The FP site types are common on the floodplain along the Tanana River and the UP site types are common in the uplands in interior Alaska. SAFED is based on nitrogen productivity for vegetation growth, litter fall quantity and quality, and microbial efficiency for forest floor decomposition. The state factors (climate, topography, and disturbance) are used to describe a broad-scale classification of the landscape to define basic limitations for the driving variables. Climate and ecosystem-level disturbances are handled as restricted stochastic processes. The model has been programed in a spatial framework as an ARC/INFO AML within the GRID package. The current version of the model has been validated as functional from an individual tree basis (1-m2 cell size) in a number of forest types found in interior Alaska. The growth, litter fall, and forest floordecomposition were compared with data from the sites. An estimate ofyearly carbon balance for the four sites was calculated. [ABSTRACT FROM AUTHOR]