Trajectory analysis in community ecology

De Cáceres M., Coll L., Legendre P., Allen R.B., Wiser S.K., Fortin M.-J., Condit R., Hubbell S. (2019) Trajectory analysis in community ecology. Ecological Monographs. : 0-0.
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Doi: 10.1002/ecm.1350

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Ecologists have long been interested in how communities change over time. Addressing questions about community dynamics requires ways of representing and comparing the variety of dynamics observed across space. Until now, most analytical frameworks have been based on the comparison of synchronous observations across sites and between repeated surveys. An alternative perspective considers community dynamics as trajectories in a chosen space of community resemblance and utilizes trajectories as objects to be analyzed and compared using their geometry. While methods that take this second perspective exist, for example to test for particular trajectory shapes, there is a need for formal analytical frameworks that fully develop the potential of this approach. By adapting concepts and procedures used for the analysis of spatial trajectories, we present a framework for describing and comparing community trajectories. A key element of our contribution is the means to assess the geometric resemblance between trajectories, which allows users to describe, quantify, and analyze variation in community dynamics. We illustrate the behavior of our framework using simulated data and two spatiotemporal community data sets differing in the community properties of interest (species composition vs. size distribution of individuals). We conclude by evaluating the advantages and limitations of our community trajectory analysis framework, highlighting its broad domain of application and anticipating potential extensions. © 2019 by the Ecological Society of America

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Relative size to resprouters determines post-fire recruitment of non-serotinous pines

Sánchez-Pinillos M., Ameztegui A., Kitzberger T., Coll L. (2018) Relative size to resprouters determines post-fire recruitment of non-serotinous pines. Forest Ecology and Management. 429: 300-307.
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Doi: 10.1016/j.foreco.2018.07.009

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The persistence of non-serotinous pines in Mediterranean forests can be threatened by climate-mediated changes in fire regimes that may favor the dominance of resprouters or other fire-adapted species. Recovery of non-serotinous pines after large wildfires is often determined by their ability to grow under the canopy of promptly established resprouters. Mechanisms of facilitation or competition between resprouters and pines will thus have a profound effect on forest dynamics. We examined here the effect of neighboring oak resprouts on Pinus nigra Arn. ssp. salzmannii saplings 18 years after a wildfire. We determined the net outcome of interactions between oaks and pines and how they vary with the life stage and size of the interacting plants or the environmental conditions. We did not find any net facilitative effects of oaks on pine sapling growth. The sensitivity of pines to neighbors varied markedly with pine size, and to a lesser extent, with water availability during the growing season. Our findings suggest a self-reinforcing hierarchical process by which early-dispersed seedlings growing in low-competitive microsites can grow faster, mitigating neighboring competition in the later stage of canopy closure. These results entail a potentially critical role of management practices to promote post-fire recovery of non-serotinous pines under expected changing conditions of disturbance regimes. © 2018 Elsevier B.V.

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Managing stand density to enhance the adaptability of Scots pine stands to climate change: A modelling approach

Ameztegui A., Cabon A., De Cáceres M., Coll L. (2017) Managing stand density to enhance the adaptability of Scots pine stands to climate change: A modelling approach. Ecological Modelling. 356: 141-150.
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Doi: 10.1016/j.ecolmodel.2017.04.006

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In the Mediterranean region most climatic forecasts predict longer and more intense drought periods that can affect tree growth and mortality over broad geographic regions. One of the silvicultural treatments that has gained currency to lessen the impacts of climatic change is the reduction of stand density by thinning. However, we lack information on how the response of forest stands to different thinning treatments will be affected by climate change, and on the post-thinning temporal dynamics of water balance, specifically blue and green water. We adopted a modelling approach to explore the long-term effects of different thinning intensities on forest dynamics and water balance under climate change scenarios, coupling an individual-based model of forest dynamics (SORTIE-ND) with a mechanistic model of soil moisture dynamics and plant drought stress. We used as a case study three Scots pine plots across a gradient of climatic conditions, and we assessed the effect of site, three climatic scenarios and eight thinning intensities on tree growth, stand productivity, tree drought stress and blue water. The best thinning intensity in terms of stand productivity was obtained when between 20 and 40% of the basal area was removed, whereas the final stand stock rapidly decreased at higher thinning intensities. Moreover, the decrease in final basal area occurred at lower thinning intensities the drier the site conditions. Moderate and heavy thinnings (>30%) doubled basal area increment (BAI) of the following years in all the plots, although the effect vanished after 30–40 years, independently of the site and climate scenario. As expected, thinning was simulated to have an overall positive effect on the blue water yield and tree water status, which increased and also tended to last longer for higher thinning intensities. However, the magnitude of this effect on tree water status was most dependent on the site and climatic scenario, as drier conditions generally raised stronger and longer lasting reductions in drought stress for a given thinning intensity. Furthermore, our results highlight the existence of a site- and climate-dependent trade-off between the gain in stand productivity and the improvement in tree water status obtained by thinning, particularly for moderate or heavy thinning intensities. Our simulations suggest that thinning is a useful management tool to mitigate climate change but strongly argue against the application of general recipes across sites and appeals for carefully taking into consideration local climatic trajectories for management planning. © 2017 Elsevier B.V.

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Land-use legacies rather than climate change are driving the recent upward shift of the mountain tree line in the Pyrenees

Ameztegui A., Coll L., Brotons L., Ninot J.M. (2016) Land-use legacies rather than climate change are driving the recent upward shift of the mountain tree line in the Pyrenees. Global Ecology and Biogeography. 25: 263-273.
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Doi: 10.1111/geb.12407

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Aim: To assess the effects of climate change, past land uses and physiography on the current position of the tree line in the Catalan Pyrenees and its dynamics between 1956 and 2006. Location: More than 1000 linear kilometres of sub-alpine tree line in the Catalan Pyrenees (north-east Spain) Methods: Using aerial photographs and supervised classification, we reclassified the images into a binary raster with 'tree' and 'non-tree' values, and determined canopy cover in 1956 and 2006. We then determined the change in position of the tree line between 1956 and 2006 based on changes in forest cover. We used the distance from the position of the tree line in 1956 to the theoretical potential tree line - determined from interpretation of aerial photographs, identifying the highest old remnants of forest for homogeneous areas of the landscape in terms of bioclimatic conditions, bedrock, landform and exposure - as a surrogate of intensity of past land uses. Results: Our analyses showed that the Pyrenean tree line has moved upwards on average almost 40m (mean advance±SE: 35.3±0.5m, P

Llegeix més

Recruitment patterns of four tree species along elevation gradients in Mediterranean mountains: Not only climate matters

Benavides R., Escudero A., Coll L., Ferrandis P., Ogaya R., Gouriveau F., Peñuelas J., Valladares F. (2016) Recruitment patterns of four tree species along elevation gradients in Mediterranean mountains: Not only climate matters. Forest Ecology and Management. 360: 287-296.
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Doi: 10.1016/j.foreco.2015.10.043

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Evidence of tree regeneration failure of some species in the Iberian Peninsula forests warns us about the impact that the global change may exert on the preservation of Mediterranean forests, such as we know them. Predictions agree about an exacerbation of the summer drought there, acknowledged as the main limiting factor for the recruits' survival. On the other hand, many studies have also proved the relevant role that local heterogeneity has over the spatial distribution of forest species recruitment by providing safe sites. Therefore, to unravel how climate interacts with local factors over juveniles' performance seems crucial for the design of successful management strategies that allow facing the global warming. Here, we surveyed the natural recruitment of four dominant tree species in seven mountainous regions in the Iberian Peninsula, along entire elevational ranges as surrogates of their climatic ranges. Two of them have alpine and temperate distributions with populations at their rear edge in the Spanish mountains: Fagus sylvatica and Pinus uncinata; and the other two have a genuine Mediterranean distribution: Quercus ilex and Pinus nigra. Our main goal was to analyze for each species the effect of climate, local factors (i.e. light availability, stand structure and ground cover) and the interactions among them to identify the main drivers leading the regeneration process, assessed in terms of presence, abundance and mean annual growth of juveniles. The results showed different environmental factors determining the recruitment patterns of each species. Nevertheless, they highlighted the pervasive role exerted by both climate and fine scale factors, particularly the co-occurring vegetation on recruits' abundance, and the light availability on their growth. Moreover, we found some interactions among annual mean temperature and local factors, suggesting that climate and local heterogeneity act hierarchically, i.e. the local conditions may mitigate or exacerbate the impact of climate on juveniles. These results advocate for further research to increase our knowledge on the complex net of interactions among factors involved in recruitment at different scales, which in turn should be taken into account and incorporated in forthcoming management strategies. © 2015 Elsevier B.V.

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Tree light capture and spatial variability of understory light increase with species mixing and tree size heterogeneity

Ligot G., Ameztegui A., Courbaud B., Coll L., Kneeshaw D. (2016) Tree light capture and spatial variability of understory light increase with species mixing and tree size heterogeneity. Canadian Journal of Forest Research. 46: 968-977.
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Doi: 10.1139/cjfr-2016-0061

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Mixed and multi-layered forest ecosystems are sometimes more productive than monospecific and single-layered ones. It has been suggested that trees of different species and sizes occupy complementary positions in space, which would act as a mechanism to increase canopy light interception and wood production. However, greater canopy light interception reduces the average amount and variability of transmitted radiation, offering fewer opportunities for all species to regenerate and to maintain forest heterogeneity in the long run. We investigated whether increasing overstory heterogeneity indeed results in greater canopy light interception and lower variability in transmittance. We modeled the three-dimensional structure of forest stands with three typical forest structures, 10 mixtures of four tree species, and three different basal areas. We used the forest light interception model SAMSARALIGHT and performed three-way analyses of covariance to analyze the effects of the three varied components of forest heterogeneity. We found no evidence that increasing structural heterogeneity increases canopy light interception. However, the light interception by mixed canopies was greater than the weighted average of light interception by the corresponding pure canopies. Variability in transmittance increased in some cases with compositional heterogeneity and, to a lesser extent, with tree size inequalities. The advantage of heterogeneous forests is in opportunities for natural regeneration, as well as in opportunities to enhance canopy light interception. © 2016 NRC Research Press.

Llegeix més

Unraveling the relative importance of factors driving post-fire regeneration trajectories in non-serotinous Pinus nigra forests

Martín-Alcón S., Coll L. (2016) Unraveling the relative importance of factors driving post-fire regeneration trajectories in non-serotinous Pinus nigra forests. Forest Ecology and Management. 361: 13-22.
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Doi: 10.1016/j.foreco.2015.11.006

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In the Mediterranean, non-serotinous pinewoods are suffering an increasing occurrence of high-severity crown fires that usually drive vegetation shifts to fire-adapted communities and a decrease in pine-dominated area. Here we used a case-study approach on a large area dominated by Pinus nigra Arn. ssp. salzmannii burned in 1998 to gain further understanding of the relative importance of different factors related to local topography (elevation, aspect, slope, curvature), pre-fire vegetation (land-use history, canopy cover) and fire behavior (burn severity, presence of unburned patches) as drivers of post-fire regeneration dynamics. The results find that pine shows locally resilient responses driven mainly by factors related to fire effects (presence of unburned patches) and the characteristics of the pre-fire vegetation (i.e. stable forest areas). When fire-induced changes from pine dominance to other types of vegetation occurred, landscape 15. years post-fire was dominated by woody vegetation, with some rare grassland communities emerging under very specific conditions (mountain ridges, hilltops and rocky sites). Conversion from forest to shrubland occurred mainly in the most xeric sites (south-facing areas, in some cases with steep slopes) and areas dominated by young pine stands prior to the fire. We found manageable factors such as the pre-fire structure and composition of the vegetation strongly determine the occurrence of post-fire regeneration trajectories dominated by tree species regeneration. This knowledge can be used to define preventive management strategies oriented to direct regeneration dynamics in anticipation of fire occurrence. At landscape level, managing forest fuels to favor the occurrence of unburned patches and modify their spatial distribution along the burned landscape will favor a more resilient pine response. At stand level, adjusting silvicultural interventions to favor the natural establishment of late-successional tree species will favor post-fire oak regeneration. © 2015 Elsevier B.V..

Llegeix més

Diversifying sub-Mediterranean pinewoods with oak species in a context of assisted migration: Responses to local climate and light environment

Martín-Alcón S., Coll L., Ameztegui A. (2016) Diversifying sub-Mediterranean pinewoods with oak species in a context of assisted migration: Responses to local climate and light environment. Applied Vegetation Science. : 0-0.
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Doi: 10.1111/avsc.12216

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Questions: How do thermal migration distance and extreme cold events affect seedling emergence and survival in assisted migration schemes in the sub-Mediterranean context? What role does plant provenance play? Can biotic interactions such as nurse effect of the overstorey and shrub layer buffer the negative responses to plant translocation? Are any of these effects species-specific? Location: Three pinewoods in the Catalan Pre-Pyrenees, northeast Iberian Peninsula. Methods: We used a replicated field trial to test the early years establishment of two contrasted provenances of four Quercus species (Q. coccifera, Q. ilex, Q. faginea and Q. pubescens) that were sown and planted along gradients of elevation and understorey microsite conditions in sub-Mediterranean pinewoods. Seedling responses to translocation were evaluated through seedling emergence, seedling survival and re-sprouting after dieback events according to seedling provenance, thermal migration distance, extreme cold events and microenvironment. Results: The study reports high success of both the planting (with an overall 76.3% of initial 3-yr survival) and sowing (with an overall 50% of seedling emergence) experiments. The results show that: (1) the thermal migration distance and the occurrence of extreme cold events have strong effects on the responses of the translocated species (particularly the evergreen oaks); (2) the forest overstorey plays an important role in attenuating the negative effects of thermal migration distance on seedling survival; and (3) these responses are species-specific. The evergreen Quercus species showed more evidence of high ecotypic differentiation in terms of cold tolerance, enabling local provenances to respond better to translocation. In contrast, marcescent species, showed high phenotypic plasticity that led to a better overall establishment success. Conclusion: The implementation of assisted migration is a feasible option to increase the diversity and resilience of the sub-Mediterranean pinewoods. Assisted migration programmes should manage risks by thoroughly considering thermal migration distances and the occurrence of extreme cold events when selecting species and seed sources, since Mediterranean tree species show different strategies regarding adaptation to cold. Programme managers should also consider the advantage of planting/sowing under relatively closed canopy to buffer some of the negative responses associated with translocation. © 2016 International Association for Vegetation Science.

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Herbivory and seedling establishment in Pyrenean forests: Influence of micro- and meso-habitat factors on browsing pressure

Ameztegui A., Coll L. (2015) Herbivory and seedling establishment in Pyrenean forests: Influence of micro- and meso-habitat factors on browsing pressure. Forest Ecology and Management. 342: 103-111.
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Doi: 10.1016/j.foreco.2015.01.021

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Browsing damage is among the most determinant factors that limit the establishment of tree seedlings in forests. In some areas, this process leads to massive mortalities that can reduce or even completely prevent the regeneration of some tree species. Mediterranean mountain forests have undergone during the last decades important changes in land-uses that have significantly altered the type and abundance of herbivore populations. In this study we assessed the impact of current grazing conditions in forest regeneration using a set of experimental plantations established in the Eastern Pyrenees in areas visited by domestic livestock (cattle and horses) and wild ungulates (mainly roe deer and chamois). We analyzed during 4. years the role of seedling species and size, mesohabitat (elevation and type of forest cover) and microhabitat (herbaceous cover, distance to shrub, and light availability) on the browsing-induced mortality of more than 500 seedlings of Pinus sylvestris, Pinus uncinata, Betula pendula and Abies alba, the most common tree species in the study area. Browsing-induced mortality for the three conifer species was much lower (40%) and depended on both microhabitat - mainly on the distance to protective elements such as shrubs; and mesohabitat, with an interaction between the elevational belt (site) and the type of forest cover (gaps vs. understory). In the subalpine belt, browsing on A. alba and P. uncinata was higher during summer at plots located in the forest understory whereas, during winter, it was higher at plots located in gaps. The study shows that both mesohabitat and microhabitat can exert an effect on the patterns of plant damage by herbivores, providing useful information to adapt forest management in areas particularly exposed to them. © 2015 Elsevier B.V.

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Modelling the effect of climate-induced changes in recruitment and juvenile growth on mixed-forest dynamics: The case of montane-subalpine Pyrenean ecotones

Ameztegui A., Coll L., Messier C. (2015) Modelling the effect of climate-induced changes in recruitment and juvenile growth on mixed-forest dynamics: The case of montane-subalpine Pyrenean ecotones. Ecological Modelling. 313: 84-93.
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Doi: 10.1016/j.ecolmodel.2015.06.029

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Most predictive models forecast significant upward displacement of forest species due to increases in temperatures, but not all the species respond in the same way to changes in climate. In temperate or mountain systems, biotic competitive interactions drive species distributions, and responses to climate change will ultimately depend upon productive and demographic processes such as growth, recruitment and mortality. We parameterized and used an individual-based, spatially explicit model of forest dynamics (SORTIE-ND) to investigate the role of species-specific differences in juvenile performance induced by climate change (juvenile growth and recruitment ability) in the dynamics of mixed forests located in the montane-subalpine ecotone of the Pyrenees. We assessed this role for two types of forests composed of three species with differing light requirements and sensitivity to climate change: (1) a mixed forest with two shade-intolerant pines (Pinus uncinata and Pinus sylvestris) and (2) a mixed forest composed by a shade-intolerant pine and a shade-tolerant fir (Abies alba). Our results show that for species with similar light requirements (i.e., both pines), small differences in sapling growth response to climate change can lead to significant differences in future species composition (an increase in P. sylvestris growth of 10% leads to an increase in its abundance from 42% to 50.3%). Conversely, in pine-fir forests, shade-tolerance results more decisive than climate-induced changes in growth in driving the future forest composition. © 2015 Elsevier B.V.

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