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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]

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]