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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. Source

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. Source

Hamame M.,Center for Ecosystem Research in Patagonia | Antezana T.,University of Concepcion
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. Source

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. Source

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. Source

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