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Aiken C.M.,University of Chile | Aiken C.M.,Center for Ecosystem Research in Patagonia
Dynamics of Atmospheres and Oceans | Year: 2012

A series of four field campaigns undertaken between November 2007 and August 2008 in the Baker Channel fjord complex in southern Chile provide a unique record of the seasonal evolution of its vertical and axial structure. The observations document the warming of subsurface waters during the summer and autumn of 2008 creating a subsurface temperature maximum that persists at the channel head until the following spring. An analysis of the observed horizontal and vertical structure is used to infer that the subsurface heating owes to the seasonal intrusion of relatively warm water from Penas Gulf. A series of numerical simulations provide support for the hypothesis that seasonal density fluctuations in the Penas Gulf are responsible for modulating the exchange of intermediary waters and maintaining anomalously warm water at the channel head from autumn until the following spring. The exchange mechanism involves adjustment of the Baker Channel density field to the summer buoyancy increase in the Penas Gulf, which creates an inflow of relatively warm water that fills the channel below the level of the brackish seaward flowing surface layer. The predominantly seasonal renewal of intermediary waters in Baker Channel contrasts with the more usually synoptic nature of above sill exchange in fjords. © 2012 Elsevier B.V.


Fajardo A.,Center for Ecosystem Research in Patagonia | Mcintire E.J.B.,Laval University | Mcintire E.J.B.,Natural Resources Canada
Journal of Ecology | Year: 2012

Altitudinal tree line ecotones (ATE) are among the most sensitive plant formations facing global warming as the altitudinal decrease in temperature is considered the driver controlling the upper elevation limit of tree lines world-wide. In this study, we attempted to answer the following questions: (i) how have the conditions during the last 2-3 centuries affected ATE tree growth (physiology) and recruitment (demography)? and (ii) how strong is synchrony between these two processes at the ATEs? 2. We used spatial sampling grids at different ATEs in two ecosystems on two subcontinents: Nothofagus pumilio in the Andes of Chilean Patagonia (46°SL) and Pinus albicaulis in the Rockies of Western Montana, USA (46°NL). Basal increment cores were extracted from trees to estimate the growth and recruitment date. An annual detrended basal area increment was estimated for each tree and was modelled against elevation and time. Tree growth improved over multiple centuries at all tree lines. Recently (c. 50years), however, improvements are disappearing or reversing. The uppermost tree line trees showed moderate declines in Montana and incipient declines in Patagonia. The declines are most dramatic slightly below current tree line (c. 200m). Tree recruitment patterns showed that tree lines have been moving uphill in both regions until at least 40-70years ago. These movements occurred primarily through abrupt pulses upward with infilling occurring concurrently (Patagonia) or at some time thereafter (Montana). Synchrony between growth and recruitment occurred in the 18th and 19th centuries in both regions. This synchrony was negative in Patagonia and positive in Montana, with varying lag periods. During the 20th century, these patterns of synchrony were lost at all sites. This loss of synchrony suggests that we could be entering a global period in which temperature is no longer the dominant driver of key features of tree lines. Synthesis. Our study shows that at two structurally different tree lines, recent and initial declines in growth and losses of long-term synchrony are occurring in the latter part of the 20th century. These findings are opposite to simplistic expectations of global warming effects on tree line dynamics and call for a model reformulation that uncouples drivers of growth and recruitment. © 2012 The Authors. Journal of Ecology © 2012 British Ecological Society.


Fajardo A.,Center for Ecosystem Research in Patagonia | McIntire E.J.B.,Laval University
American Journal of Botany | Year: 2010

Premise of the Study: Negative density-dependent processes have been thought to be the primary cause of shifting spatial patterns of tree populations through time. The existence of adult tree clusters might challenge this classical prediction. Here, we document the prevalence of merged stems (clustering of mature trees leading to stem fusion) in second-growth forests of Nothofagus pumilio and hypothesize that it is nonrandom but predictable in space. Methods: We stem-mapped nine sites in second-growth edge and interior forests of fire origin and in mature forests of N. pumilio (>3500 trees) in central Patagonia, Chile. The spatial structure of stand-level and individual-level features was estimated with spatial analyses (pair-correlation function and nearest-neighbor distances). Key Results: Multistemmed trees were merged clusters of separate individuals. Merged trees were predominantly found at the edge of the second-growth forests. We found strong clustering (≤5 m) at forest edge sites and none at interior sites. Nearest-neighbor distance distributions were unimodal for unmerged trees and monotonically decreasing for merged trees; interstem distances were much smaller at the edge sites than at the interior sites. Conclusions: The occurrence of merged trees at the forest edge, and the resulting high spatial aggregation of stems, is consistent with the hypothesis that establishment was probably aggregated. The spatial pattern found at the forest edge changes the standard spatial pattern sequence through time in temperate forests, altering traditional forest-stand-dynamics models. © 2010 Botanical Society of America.


McIntire E.J.B.,Laval University | Fajardo A.,Center for Ecosystem Research in Patagonia
American Naturalist | Year: 2011

Facilitation (positive interactions) has emerged as a dominant ecological mechanism in many ecosystems. Its importance has recently been expanded to include intraspecific interactions, creating the potential for higher-level natural selection within species. Using multiple lines of evidence, we show that conspecific facilitation within the southern beech tree, Nothofagus pumilio, appears to overcome competition in two life phases. In a seedling experiment addressing stress and planting-density effects, we found that mortality was lowest (~0%) where there was no stress and was indistinguishable across densities. Furthermore, in mature forests (45 years old), genetically variable, merged individuals had lower mortality (- 50%) than un-merged individuals in locations without identifiable stress. Thus, a full understanding of the occurrence of facilitation may require a more general model of resource improvements than the commonly cited stress gradient hypothesis. Additionally, the merged trees showed a density-dependent mortality pattern at the level of the group. These data demonstrate a potential mechanism (facilitation) driving natural selection at this higher level, via stem merging. These merged "superorganisms" would confirm theoretical predictions whereby facilitation acts as an ecological mechanism driving group selection. © 2011 by The University of Chicago.


Fajardo A.,Center for Ecosystem Research in Patagonia | McIntire E.J.B.,Laval University
Journal of Ecology | Year: 2011

Competition among conspecifics of the same cohort has been traditionally thought to be a main process driving population dynamics. In this classical view, however, the role of facilitation in stressful conditions has rarely been considered. Here, using a transplant experiment across a forest-prairie gradient, we test whether the stress gradient hypothesis (SGH) extends to individuals thought to be strongly competing. We transplanted 2-year-old seedlings of Nothofagus pumilio at two different densities (clusters of 10 and isolated) and at different distances from the forest edge (from 30m inside the forest up to 50m outside the forest in the prairie). We further stem-mapped all seedlings belonging to the clusters and computed a competition index (CI). After 3years of growing, survival and increment growth in diameter and height were measured and analysed using mixed-effects models. We conducted a nearest-neighbour analysis using seedlings' CI and growth and computed model fit using the area under the curve (AUC) method. Seedlings planted in dense clusters had significantly higher survival than solitary seedlings at the stressful end of the gradient. This trend was reversed at the opposite end of the gradient, supporting the SGH at the intraspecific level. Pursuing this at the level of the individual, we found that higher CIs (more neighbours) in seedlings predicted higher probabilities of their survival (facilitation) in stressful conditions. Seedlings diameter and height increment growth were not affected by planting density and only diameter varied along the stress gradient; seedlings had higher diameter increments in growth outside the forest. Finally, when compared with conceptual models, our results mostly support predictions of a higher facilitation at intermediate position along the gradient. Synthesis. We showed that facilitation overrides competition among tree seedlings even at locations under moderate stress; the facilitation process occurs in resource-mediated interactions (niche overlapping). These results represent an important shift in our way to understand the density-dependent mortality process, and calls for a model reformulation including positive interactions even when competition is expected to be strongest (conspecifics of the same cohort). © 2010 The Authors. Journal of Ecology © 2010 British Ecological Society.


Hamame M.,Center for Ecosystem Research in Patagonia | Antezana T.,University of Concepción
Deep-Sea Research Part II: Topical Studies in Oceanography | Year: 2010

E. vallentini is a circumpolar vertical migrator in the open ocean that enters the Chilean fjords through the sills to become the most common and abundant euphausiid. The diel migratory and feeding patterns of this species are studied in this distinct, enclosed and bathymetrically variable environment. The main goal was to determine the ability of E. vallentini to alter its vertical diel range in order to avoid shallow depths, or to maintain its depth migratory range and take advantage of benthic environment during daytime. Multiple opening closing nets sampled the water column in several basins along the Strait of Magellan. Fluorometrically determined stomach pigments along with on board experiments of evacuation rates allowed estimation of ingestion and consumption rates. Samples of stomach content were microscopically examined to compare day and night diet. E. vallentini migrated between the surface at night and 100-200 m in daytime. Surface night dwellers had higher chlorophyll stomach content than deep daytime dwellers. Diet composition changed from dinoflagellates and tintinnids at night, to a polychaeths-dominated diet in daytime. Stomach content, ingestion rates and consumption rates of phytoplankton were higher at night than in daytime, and accounted for an impact of 0.17% upon phytoplankton biomass. Changes of diet of E. vallentini imply unique adaptations to take advantage of both pelagic and demersal environments, which could explain its high abundance and high frequency of encounter within the southern Chilean fjords. © 2009 Elsevier Ltd. All rights reserved.


Gundale M.J.,Swedish University of Agricultural Sciences | Fajardo A.,Center for Ecosystem Research in Patagonia | Lucas R.W.,Swedish University of Agricultural Sciences | Nilsson M.-C.,Swedish University of Agricultural Sciences
Ecography | Year: 2011

Many studies at the regional scale have found either negative or hump-shaped relationships between productivity and diversity, and some theories propose that these occur because soil resource heterogeneity is either lower or less important in more productive environments. However, there have been few explicit tests of these theories in natural ecosystems. We evaluated the relationship between soil resource heterogeneity and plant richness within a well characterized system of 30 islands in northern Sweden across which soil fertility and productivity declines, and species richness increases, as a consequence of ecosystem retrogression. On each island we created a spatially explicit grid consisting of 49 sampling points in a 9.5 m quadrat, which we used to quantify spatial heterogeneity of five soil variables (NH4 +-N, amino N, PO4 --P, microbial biomass, and decomposition), and plant community composition. Using a hierarchical Bayesian approach, we estimated mean semivariograms of each variable for each island size class to compare three components of spatial heterogeneity: total variability, spatial grain, and patchiness. This analysis showed that variability within islands was usually lowest on small islands, where species richness was highest and productivity lowest; however, NH4 +-N and amino N had greater patchiness and spatial grain on small islands. We did not detect any significant across-island correlations between whole-plot plant species richness and either whole-plot standard deviation or coefficient of variation of any soil variable. Using partial Mantel tests, we found that mean correlation coefficients between within-plot plant community composition and the soil variables were never significant for any island size class, and did not differ between island size classes. Our findings do not provide any evidence that soil resource heterogeneity controls the productivity-diversity relationship in this system, and suggests other mechanisms are primarily responsible. © 2011 The Authors. Ecography © 2011 Nordic Society Oikos.


Piper F.I.,Center for Ecosystem Research in Patagonia | Fajardo A.,Center for Ecosystem Research in Patagonia
Annals of Botany | Year: 2011

Background and AimsTrees universally decrease their growth with age. Most explanations for this trend so far support the hypothesis that carbon (C) gain becomes limited with age; though very few studies have directly assessed the relative reductions of C gain and C demand with tree age. It has also been suggested that drought enhances the effect of C gain limitation in trees. Here tests were carried out to determine whether C gain limitation is causing the growth decay with tree age, and whether drought accentuates its effect.MethodsThe balance between C gain and C demand across tree age and height ranges was estimated. For this, the concentration of non-structural carbohydrates (NSCs) in stems and roots of trees of different ages and heights was measured in the deciduous temperate species Nothofagus pumilio. An ontogenetic decrease in NSCs indicates support for C limitation. Furthermore, the importance of drought in altering the C balance with ontogeny was assessed by sampling the same species in Mediterranean and humid climate locations in the southern Andes of Chile. Wood density (WD) and stable carbon isotope ratios (δ 13C) were also determined to examine drought constraints on C gain.Key ResultsAt both locations, it was effectively found that tree growth ultimately decreased with tree age and height. It was found, however, that NSC concentrations did not decrease with tree age or height when WD was considered, suggesting that C limitation is not the ultimate mechanism causing the age/height-related declining tree growth. δ 13C decreased with tree age/height at the Mediterranean site only; drought effect increased with tree age/height, but this pattern was not mirrored by the levels of NSCs.ConclusionsThe results indicate that concentrations of C storage in N. pumilio trees do not decrease with tree age or height, and that reduced C assimilation due to summer drought does not alter this pattern. © The Author 2011.


Fajardo A.,Center for Ecosystem Research in Patagonia | Piper F.I.,Center for Ecosystem Research in Patagonia
New Phytologist | Year: 2011

The focus of the trait-based approach to study community ecology has mostly been on trait comparisons at the interspecific level. Here we quantified intraspecific variation and covariation of leaf mass per area (LMA) and wood density (WD) in monospecific forests of the widespread tree species Nothofagus pumilio to determine its magnitude and whether it is related to environmental conditions and ontogeny. We also discuss probable mechanisms controlling the trait variation found. We collected leaf and stem woody tissues from 30-50 trees of different ages (ontogeny) from each of four populations at differing elevations (i.e. temperatures) and placed at each of three locations differing in soil moisture. The total variation in LMA (coefficient of variation (CV) = 21.14%) was twice that of WD (CV = 10.52%). The total variation in traits was never less than 23% when compared with interspecific studies. Differences in elevation (temperature) for the most part explained variation in LMA, while differences in soil moisture and ontogeny explained the variation in WD. Traits covaried similarly in the altitudinal gradient only. Functional traits of N. pumilio exhibited nonnegligible variation; LMA varied for the most part with temperature, while WD mostly varied with moisture and ontogeny. We demonstrate that environmental variation can cause important trait variation without species turnover. © The Authors (2010). Journal compilation © New Phytologist Trust (2010).


Fajardo A.,Center for Ecosystem Research in Patagonia | Piper F.I.,Center for Ecosystem Research in Patagonia | Piper F.I.,Institute Ecologia y Biodiversidad IEB | Cavieres L.A.,University of Concepción | Cavieres L.A.,Institute Ecologia y Biodiversidad IEB
Global Ecology and Biogeography | Year: 2011

Aim Two alternative hypotheses attempt to explain the upper elevation limit of tree lines world-wide, the carbon-limitation hypothesis (CLH) and the growth-limitation hypothesis (GLH); the altitudinal decrease of temperature is considered the driver constraining either carbon gain or growth. Using a widely distributed tree line species (Nothofagus pumilio) we tested whether tree line altitude is explained by the CLH or the GLH, distinguishing local from global effects. We elaborated expectations based on most probable trends of carbon charging with altitude according to both hypotheses, considering the alternative effects of drought. Location Two climatically contrasting tree line ecotones in the southern Andes of Chile: Mediterranean (36°54' S) and Patagonia (46°04' S). Methods At both locations, 35-50 trees of different ages were selected at each of four altitudes (including tree line), and stem and root sapwood tissues were collected to determine non-structural carbohydrate (NSC) concentrations. NSC accumulates whenever growth is more limited than photosynthesis. An altitudinal increase in NSCs means support for the GLH, while the opposite trend supports the CLH. We also determined stable carbon isotope ratios (δ13C) to examine drought constraints on carbon gain. Results NSC concentrations were positively correlated with altitude for stem tissue at the Mediterranean and root sapwood tissue at the Patagonia site. No depletion of NSC was found at either site in either tissue type. For both tissues, mean NSC concentrations were higher for the Patagonia site than for the Mediterranean site. Mean root sapwood NSC concentration values were five times higher than those of the corresponding stem sapwood at all altitudes. Values for δ13C were positively correlated with altitude in the Mediterranean site only. Main conclusions We found support for the GLH at the site without drought effects (Patagonia) and no support for the CLH at either site. It is suggested that drought moderated the effects of low temperature by masking the expected trend of the GLH at the Mediterranean site. © 2010 Blackwell Publishing Ltd.

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