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ABSTRACT: In this field study we analysed the regional and local scale effects of disturbance and climate on altitudinal treelines dominated by Nothofagus pumilio in northern Patagonia. We compared two regions west and east of the Andes at 40° S, slopes with warm vs cool aspects and undisturbed vs locally disturbed treelines. This spatial framework allowed us to test (1) for differences among treelines affected by different types of local disturbance and (2) the traditional hypothesis that low temperature limits treeline. Contingency tables and ANOVA showed that local disturbance occurred more frequently than expected on slopes with cool aspects, steep slope angles and concave slope configuration. Disturbed treelines were locally lowered with longer ecotones and lower krummholz growth rates and vegetation cover than undisturbed treelines. Three-way ANOVA showed the significant influences of study area (regional climate) and aspect (local climate) on treeline elevation, krummholz growth rates and density, tree density and vegetation cover, while accounting for local disturbance. Treeline elevations were higher east of the Andes reflecting the more continental climate in Argentina than in Chile, plus regional impacts of volcanic eruptions. Tree density and vegetation cover were greater west of the Andes reflecting greater precipitation in Chile. Within study areas, local climate had different influences on treeline elevations and krummholz growth rates west and east of the Andes. We predict that increased tree growth and upslope advance of treeline in response to global warming is more likely in Chile than in Argentina near 40° S, unless precipitation increases east of the Andes. To test these predictions, we recommend research be stratified to account for the influences of local disturbance, which may confound climatic impacts. In northern Patagonia, suitable control (undisturbed) study sites will most likely be found at upper slope positions with low slope angles, simple microtopography and straight topographic configuration.
Journal of Vegetation Science 04/2009; 14(5):733 - 742. · 2.77 Impact Factor
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Phillip J van Mantgem,
Nathan L Stephenson,
John C Byrne, Lori D Daniels,
Jerry F Franklin,
Peter Z Fulé,
Mark E Harmon,
Andrew J Larson,
Jeremy M Smith,
Alan H Taylor,
Thomas T Veblen
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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.
Science 02/2009; 323(5913):521-4. · 31.20 Impact Factor
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ABSTRACT: Dans cette étude, nous avons utilisé des coupes minces pour améliorer la précision et l'exactitude de la détermination de l'âge de semis de Nothofagus pumilio. Nous avons comparé deux méthodes pour déterminer le nombre de cernes sur 85 coupes transversales prélevées au collet des semis : (1) le décompte de cernes de rondelles de bois observées à la lumière réfléchie et (2) le décompte de cernes de coupes minces observées à la lumière transmise. Les échantillons comptaient de 14 à 53 cernes. La comparaison des décomptes de cernes entre les deux méthodes a révélé des différences de un à 12 ans pour 85 % des semis. Deux sources d'erreur ont été identifiées. Premièrement, sur 70 des 85 échantillons, jusqu'à 12 cernes incomplets pouvaient expliquer les différences dans les décomptes de cernes de deux rayons distincts sur la même rondelle de bois. Deuxièmement, les courts rayons, le grand nombre de cernes et la nature poreuse et diffuse du bois ont résulté en de fréquentes erreurs lors de la détection visuelle des cernes sur les rondelles de bois. De 1 à 3 faux cernes ont été détecté chez 15 semis. Cinquante-sept échantillons présentaient des cernes étroits et comprimés qui ont causé des sous-estimations de une à 12 ans dans le décompte de cernes des galettes par rapport aux coupes minces. La grande variabilité dans la présence de cernes étroits a limité nos tentatives d'interdater visuellement les patrons de largeurs de cernes entre les semis. Cependant, les décomptes obtenus à partir des coupes minces nous ont permis d'identifier les faux cernes et les cernes incomplets améliorant ainsi la précision et l'exactitude des nos estimations de l'âge des semis. Pour les études sur des semis à croissance lente qui nécessitent des estimés d'âge très précis, nous recommandons la préparation et la datation de coupes minces tel que décrit dans cet article.
Ecoscience 01/2009; · 1.28 Impact Factor
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ABSTRACT: Tree radial growth and seedling establishment of Nothofagus pumilio at al-pine treeline near 40 S latitude in Chile and Argentina show time-and site-dependent relationships to interannual-and decadal-scale climate variation. Six treelines were sampled at two spatial scales corresponding to regional and local climates. A shift in climate from cool–wet to warm–dry conditions facilitated comparison of climate–vegetation relationships during two distinct periods: 1957–1976 and 1977–1996. For each treeline, tree radial growth and seedling establishment were correlated against monthly and seasonal temperature, pre-cipitation, moisture availability, and two indices of El Niñ o–Southern Oscillation (ENSO): southern oscillation index (SOI) and sea surface temperature (SST). Four key aspects of climatic influences on N. pumilio radial growth and seedling establishment were as follows. (1) The relationship between krummholz radial growth and temperature variation was non-linear. (2) Moisture availability was the dominant climatic factor influencing seedling es-tablishment, although temperature–precipitation interactions resulted in variability among study areas. (3) Climate conditions that facilitated Nothofagus pumilio seedling establish-ment were distinct, and often opposite, from those that enhanced radial growth. (4) The relationships of radial growth and seedling demography with climate and ENSO differed among study areas and have been unstable over the past 40 years. Observed spatial and temporal instabilities in vegetation–climate relationships demonstrate the complexity of treeline dynamics in northern Patagonia under a changing climate. We conclude that a directional increase in temperature, as predicted by current global climate scenarios, will not necessarily result in an upslope expansion of the N. pumilio forests growing at altitudinal treeline in northern Patagonia.
Ecology. 01/1284; 85:1284-1296.
