Showing papers by "Thomas T. Veblen published in 2009"

Widespread increase of tree mortality rates in the western United States

Abstract: Persistent changes in tree mortality rates can alter forest structure, composition, and ecosystem services such as carbon sequestration. Our analyses of longitudinal data from unmanaged old forests in the western United States showed that background (noncatastrophic) mortality rates have increased rapidly in recent decades, with doubling periods ranging from 17 to 29 years among regions. Increases were also pervasive across elevations, tree sizes, dominant genera, and past fire histories. Forest density and basal area declined slightly, which suggests that increasing mortality was not caused by endogenous increases in competition. Because mortality increased in small trees, the overall increase in mortality rates cannot be attributed solely to aging of large trees. Regional warming and consequent increases in water deficits are likely contributors to the increases in tree mortality rates.

Increased early growth rates decrease longevities of conifers in subalpine forests

Abstract: For trees, fast growth rates and large size seem to be a fitness benefit because of increased competitiveness, attainment of reproductive size earlier, reduction of generation times, and increased short-term survival chances. However, fast growth rates and large size entail reduced investment in defenses, lower wood density and mechanical strength, increased hydraulic resistance as well as problems with down-regulation of growth during periods of stress, all of which may decrease tree longevity. In this study, we investigated the relationship between longevity and growth rates of trees and quantified effects of spatial environmental variation (elevation, slope steepness, aspect, soil depth) on tree longevity. Radial growth rates and longevities were determined from tree-ring samples of 161 dead trees from three conifer species in subalpine forests of the Colorado Rocky Mountains (Abies lasiocarpa, Picea engelmannii) and the Swiss Alps (Picea abies). For all three species, we found an apparent tradeoff between growth rate to the age of 50 years and longevity (i.e. fast early growth is associated with decreased longevity). This association was particularly pronounced for larger P. engelmannii and P. abies, which attained canopy size, however, there were also significant effects for smaller P. engelmannii and P. abies. For the more shade-tolerant A. lasiocarpa, tree size did not have any effect. Among the abiotic variables tested only northerly aspect significantly favored longevity of A. lasiocarpa and P. engelmannii. Trees growing on south-facing aspects probably experience greater water deficits leading to premature tree death, and/or shorter life spans may reflect shorter fire intervals on these more xeric aspects. Empirical evidence from other studies has shown that global warming affects growth rates of trees over large spatial and temporal scales. For moist-cool subalpine forests, we hypothesize that the higher growth rates associated with global warming may in turn result in reduced tree longevity and more rapid turnover rates.

Bark beetle outbreaks in western North America: Causes and consequences

Abstract: Since 1990, native bark beetles have killed billions of trees across millions of acres of forest from Alaska to northern Mexico. Although bark beetle infestations are a regular force of natural change in forested ecosystems, several of the current outbreaks, which are occurring simultaneously across western North America, are the largest and most severe in recorded history.

Assessing dendroecological methods to reconstruct defoliator outbreaks on Nothofagus pumilio in northwestern Patagonia, Argentina

Abstract: We examined the use of dendroecological techniques for detecting past defoliations caused by Ormiscodes amphimone Fabricius (Saturniidae) in Nothofagus pumilio (Poepp. et Endl.) Krasser forests in ...

Pilgerodendron uviferum: The Southernmost Tree-Ring Fire Recorder Species

Abstract: This paper reports on the capability of the long-lived conifer Pilgerodendron uviferum in southwestern South America to record fire scars, making it the southernmost tree-ring fire recording species on Earth. We present detailed descriptions of the fire-scar morphology of this species and compare tree-ring fire-scar dates to independent sources of fire dates. The fire scars found on Pilgerodendron uviferum are morphologically similar to those found on conifers in western North America. All fire scars were crossdated on 80 out of 96 sampled cross-sections collected at 8 dispersed sites. Groups of micro-rings were the most common anatomical feature that impeded crossdating of fire-scar dates. A total of 597 fire-scar tips were successfully crossdated on these 80 cross-sections. The average number of fire scars per cross-section was 7.4, and the full collection of crossdated fire scars spanned from 1570 to 2004 AD. The crossdated fire-scar dates were highly synchronous within stands, and years of a...