Drought-induced stomatal closure probably cannot explain divergent white spruce growth in the Brooks Range, Alaska, USA.

Increment cores from the boreal forest have long been used to reconstruct past climates. However, in recent years, numerous studies have revealed a deterioration of the correlation between temperature and tree growth that is commonly referred to as divergence. In the Brooks Range of northern Alaska, USA, studies of white spruce (Picea glauca) revealed that trees in the west generally showed positive growth trends, while trees in the central and eastern Brooks Range showed mixed and negative trends during late 20th century warming. The growing season climate of the eastern Brooks Range is thought to be drier than the west. On this basis, divergent tree growth in the eastern Brooks Range has been attributed to drought stress. To investigate the hypothesis that drought-induced stomatal closure can explain divergence in the Brooks Range, we synthesized all of the Brooks Range white spruce data available in the International Tree Ring Data Bank (ITRDB) and collected increment cores from our primary sites in each of four watersheds along a west-to-east gradient near the Arctic treeline. For cores from our sites, we measured ring widths and calculated carbon isotope discrimination (δ13C), intrinsic water-use efficiency (iWUE), and needle intercellular CO2 concentration (C(i)) from δ13C in tree-ring alpha-cellulose. We hypothesized that trees exhibiting divergence would show a corresponding decline in δ13C, a decline in C(i), and a strong increase in iWUE. Consistent with the ITRDB data, trees at our western and central sites generally showed an increase in the strength of the temperature-growth correlation during late 20th century warming, while trees at our eastern site showed strong divergence. Divergent tree growth was not, however, associated with declining δ13C. Meanwhile, estimates of C(i) showed a strong increase at all of our study sites, indicating that more substrate was available for photosynthesis in the early 21st than in the early 20th century. Our results, which are corroborated by measurements of xylem sap flux density, needle gas exchange, and measurements of growth and δ13C along moisture gradients within each watershed, suggest that drought-induced stomatal closure is probably not the cause of 20th century divergence in the Brooks Range.

Evidence of soil nutrient availability as the proximate constraint on growth of treeline trees in northwest Alaska.

The position of the Arctic treeline, which is a key regulator of surface energy exchange and carbon cycling, is widely thought to be controlled by temperature. Here, we present evidence that soil nutrient availability, rather than temperature, may be the proximate control on growth of treeline trees at our study site in northwest Alaska. We examined constraints on growth and allocation of white spruce in three contrasting habitats. The habitats had similar aboveground climates, but soil temperature declined from the riverside terrace to the forest to the treeline. We identified six lines of evidence that conflict with the hypothesis of direct temperature control and/or point to the importance of soil nutrient availability. First, the magnitude of aboveground growth declined from the terrace to the forest to the treeline, along gradients of diminishing soil nitrogen (N) availability and needle N concentration. Second, peak rates of branch extension, main stem radial and fine-root growth were generally not coincident with seasonal air and soil temperature maxima. At the treeline, in particular, rates of aboveground and fine-root growth declined well before air and soil temperatures reached their seasonal peaks. Third, in contrast with the hypothesis of temperature-limited growth, growing season average net photosynthesis was positively related to the sum of normalized branch extension, main stem radial and fine-root growth across trees and sites. Fourth, needle nonstructural carbohydrate concentration was significantly higher on the terrace, where growth was greatest. Fifth, annual branch extension growth was positively related to snow depth, consistent with the hypothesis that deeper snow promotes microbial activity and greater soil nutrient availability. Finally, the tree ring record revealed a large growth increase during late 20th-century climate warming on the terrace, where soil N availability is relatively high. Meanwhile, trees in the forest and at the treeline showed progressively smaller growth increases. Our results suggest temperature effects on tree growth at our study sites may be mediated by soil nutrient availability, making responses to climate change more complex and our ability to interpret the tree ring record more challenging than previously thought.

Variation in carbohydrate source-sink relations of forest and treeline white spruce in southern, interior and northern Alaska.

Two opposing hypotheses have been presented to explain reduced tree growth at the treeline, compared with growth in lower elevation or lower latitude forests: the carbon source and sink limitation hypotheses. The former states that treeline trees have an unfavorable carbon balance and cannot support growth of the magnitude observed at lower elevations or latitudes, while the latter argues that treeline trees have an adequate carbon supply, but that cold temperatures directly limit growth. In this study, we examined the relative importance of source and sink limitation in forest and treeline white spruce (Picea glauca) in three mountain ranges from southern to northern Alaska. We related seasonal changes in needle nonstructural carbohydrate (NSC) content with branch extension growth, an approach we argue is more powerful than using needle NSC concentration. Branch extension growth in the southernmost Chugach Mountains was much greater than in the White Mountains and the Brooks Range. Trees in the Chugach Mountains showed a greater seasonal decline in needle NSC content than trees in the other mountain ranges, and the seasonal change in NSC was correlated with site-level branch growth across mountain ranges. There was no evidence of a consistent difference in branch growth between the forest and treeline sites, which differ in elevation by approximately 100 m. Our results point to a continuum between source and sink limitation of growth, with high-elevation trees in northern and interior Alaska showing greater evidence of sink limitation, and those in southern Alaska showing greater potential for source limitation.

Vascular plant colonisation of Surtsey Island (1965-1990) - a dataset.

BACKGROUND: The process of ecosystem development over time that takes place on a new substrate devoid of biological activity (such as, for example, lava) is called primary succession. Research on primary succession is not easy, as it is limited to rare occasions when a piece of land totally lacking in any pre-existing life occurs. The emergence of volcanic islands is such an occasion; it is a unique event that allows a natural experiment in the study of colonisation processes and primary succession. Surtsey (located in the Vestmannaeyar archipelago off the southern coast of Iceland) is an iconic example of a place where primary succession has been studied for decades and where human disturbance has been minimised due to significant geographic isolation and early protection efforts. Here, we present a georeferenced dataset of vacular plant occurrences collected during the field studies carried out on Surtsey Island during the first three decades of its existence. NEW INFORMATION: To date, no dataset containing plant distribution data documenting the process of early stages of colonisation of Surtsey has been published. What is more, to our knowledge, there is no other dataset that can be compared with our Surtsey data that is readily available for researchers working on plant colonisation dynamics and primary succession processes. Here, we present a complete, geo-referenced dataset of all plant occurrences (10,094 in total) collected on Surtsey between 1965 and 1990.

Natural causes of the tundra-taiga boundary.

The tundra-taiga interface is characterized by a change in tree cover or density, tree size and shape, tree growth, and reproduction. Generally, trees get denser, taller, and less damaged as one moves from the tundra into the taiga proper. The environmental covariates and possible mechanisms resulting in these patterns are addressed in the paper. Low seed rain density, lack of safe sites caused by microclimatic variation, low surface substrate moisture, and low soil nutrient availability may limit the density of the tree species. Tree growth may be limited by a short growing season and further diminished, by shoot and root damage reducing carbon and nutrient stores as well as by reducing carbon and nutrient uptake capacities. Positive and negative feedbacks of tree density on tree growth exist at treeline. Increased tree density leads to increased air temperature and decreased wind damage, but also to lower soil temperature, reduced nutrient availability, and greater nutrient competition.

The dynamics of the tundra-taiga boundary: an overview and suggested coordinated and integrated approach to research.

The tundra-taiga boundary stretches for more than 13,400 km around the Northern Hemisphere and is probably the Earth's greatest vegetation transition. The trees that define the boundary have been sensitive to climate changes in the past and models of future vegetation distribution suggest a rapid and dramatic invasion of the tundra by the taiga. Such changes would generate both positive and negative feedbacks to the climate system and the balance could result in a net warming effect. However, the boundary is becoming increasingly affected by human activities that remove trees and degrade forest-tundra into tundra-like areas. Because of the vastness and remoteness of the tundra-taiga boundary, and of methodological problems such as problematic definitions and lack of standardized methods to record the location and characteristics of the ecotone, a project group has been established under the auspices of the International Arctic Science Committee (IASC). This paper summarizes the initial output of the group and focuses on our uncertainties in understanding the current processes at the tundra-taiga boundary and the conflicts between model predictions of changes in the location of the boundary and contrasting recently observed changes due to human activities. Finally, we present recommendations for a coordinated international approach to the problem and invite the international community to join us in reducing the uncertainties about the dynamics of the ecotone and their consequences.