Milena Holmgren | Wageningen University (original) (raw)

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Papers by Milena Holmgren

Research paper thumbnail of Landscape features associated with the passive recovery of Mediterranean sclerophyllous woodlands of central Chile

Research paper thumbnail of Persistent Acacia savannas replace Mediterranean sclerophyllous forests in South America

Forest Ecology and Management

Mediterranean ecosystems are global hotspots of biodiversity threaten by human disturbances. Grow... more Mediterranean ecosystems are global hotspots of biodiversity threaten by human disturbances. Growing evidence indicates that regeneration of Mediterranean forests can be halted under certain circumstances and that successional stages can become notoriously persistent. The Mediterranean sclerophyllous forest in central Chile is been largely transformed into savannas dominated by the invasive legume tree Acacia caven as result of interacting management and ecological factors. We used multi-temporal satellite imag- ery to study the transition dynamics of these major vegetation types over the last four decades (1975– 2008). Vegetation changes were related to indicators of resource availability (topography, water avail- ability, solar radiance), potential propagule availability (distance to forest remnant patches) and distur- bance regimes (grazing, fire occurrence and distance to roads and cities). During this study period, forests were mostly converted into Acacia savannas (46.1%). Aca...

Research paper thumbnail of Understanding the effects of rainfall variability on dryland vegetation

Background/Question/Methods Rainfall variability is expected to increase with climate warming. We... more Background/Question/Methods Rainfall variability is expected to increase with climate warming. We used the MODIS remotely-sensed estimations of tree cover (%) at 1 km2 to explore the patterns of tropical tree cover distribution in relation to rainfall variability. Results/Conclusions We found that rainfall variability is associated with reduced tree cover in the wet tropics globally. In contrast, high year-to-year variability is positive (South America), neutral (Africa) or negatively (Australia) related to tree cover in the dry tropics. We reflect on long-term observations to identify potential mechanisms that may explain these contrasting responses of tree cover to extreme rainfall events in drylands.

Research paper thumbnail of Shrub facilitation increases plant diversity along an arid scrubland–temperate rain forest boundary in South America

Journal of Vegetation Science

Aim: Theoretical models predict nurse plant facilitation enhances species richness by amelioratin... more Aim: Theoretical models predict nurse plant facilitation enhances species richness by ameliorating stressful environmental conditions and expanding distributional ranges of stress-intolerant species into harsh environments. We studied the role of nurse facilitation on the recruitment of perennial plants along an arid scrubland—temperate rain forest boundary to test the following predictions: (1) nurse shrub canopy increases seedling abundance and species richness along the rain forest—scrubland boundary; (2) scrubland species are less dependent on facilitative interactions than temperate rain forest species, especially at the moister, upper end of the gradient. Location: Bosque Fray Jorge National Park, north-central Chile, South America (30° 39′ S — 71° 40′ W). Methods: We examined seedling abundance and species richness of perennial plants in the open and under different types of patches that may facilitate species recruitment (living shrubs, dead shrubs, perennial grasses and tre...

Research paper thumbnail of Nucleated regeneration of semiarid sclerophyllous forests close to remnant vegetation

Forest Ecology and Management

Natural regeneration of mediterranean plant communities has proved difficult in all continents. I... more Natural regeneration of mediterranean plant communities has proved difficult in all continents. In this paper we assess whether regeneration of sclerophyllous forests shows nucleated patterns indicative of a positive effect of vegetation remnants at the landscape level and compare the regeneration patterns between sites with distinctive climate conditions. We studied the spatial patterns of vegetation change during 52 years in central Chile using remotely-sensed images to test the predictions that (1) regeneration of sclerophyllous vegetation expands from patches of remnant vegetation; and (2) regeneration is more dependent on remnant vegetation in drier sites. Our results show that the regeneration of the sclerophyllous vegetation in central Chile is a slow process that may be possible under certain conditions. We found that the fraction of regenerated vegetation increases with the proximity to remnant sclerophyllous forest in an aggregated pattern. Especially in drier sites, veget...

Research paper thumbnail of Herbivory and plant growth rate determine the success of El Niño Southern Oscillation-driven tree establishment in semiarid South America

Global Change Biology, 2006

While climatic extremes are predicted to increase with global warming, we know little about the e... more While climatic extremes are predicted to increase with global warming, we know little about the effect of climatic variability on biome distribution. Here, we show that rainy El Niño Southern Oscillation (ENSO) events can enhance tree recruitment in the arid and semiarid ecosystems of north-central Chile and northwest Peru. Tree-ring studies in natural populations revealed that rainy El Niño episodes

Research paper thumbnail of The balance of competition and facilitation in plant communities: A graphical model

It has been hypothesized that plants are increasingly shade intolerant in drier conditions. Altho... more It has been hypothesized that plants are increasingly shade intolerant in drier conditions. Although many field patterns can be understood from this theory, the conspicuous {open_quotes}nurse plant{close_quotes} phenomenon in dry areas seems to contradict the theory. We derive a graphical model to illustrate how the interplay of facilitation and competition can be understood from two ingredients: the plant responses to

Research paper thumbnail of ENSO rainfall and herbivore control enhance tree establishment in semiarid Chile: experimental evidence

Research paper thumbnail of Interactive effects of human and ecological factors determine persistence of cork oak forests in southern Portugal

Research paper thumbnail of Nurse plant effects on shrub seedling establishment along a topographical gradient of water availability in north-central Chile

Research paper thumbnail of Tapping climatic variation for dryland restoration

Research paper thumbnail of Could ENSO events change the invisibility of semiarid herbaceous communities?

Research paper thumbnail of Tree-seedling establishment in fragmented Mediterranean forests of central Chile

Research paper thumbnail of Tipping points in tropical tree cover: linking theory to data

It has recently been found that the frequency distribution of remotely sensed tree cover in the t... more It has recently been found that the frequency distribution of remotely sensed tree cover in the tropics has three distinct modes, which seem to correspond to forest, savanna, and treeless states. This pattern has been suggested to imply that these states represent alternative attractors, and that the response of these systems to climate change would be characterized by critical transitions and hysteresis. Here, we show how this inference is contingent upon mechanisms at play. We present a simple dynamical model that can generate three alternative tree cover states (forest, savanna, and a treeless state), based on known mechanisms, and use this model to simulate patterns of tree cover under different scenarios. We use these synthetic data to show that the hysteresis inferred from remotely sensed tree cover patterns will be inflated by spatial heterogeneity of environmental conditions. On the other hand, we show that the hysteresis inferred from satellite data may actually underestimate real hysteresis in response to climate change if there exists a positive feedback between regional tree cover and precipitation. Our results also indicate that such positive feedback between vegetation and climate should cause direct shifts between forest and a treeless state (rather than through an intermediate savanna state) to become more likely. Finally, we show how directionality of historical change in conditions may bias the observed relationship between tree cover and environmental conditions.

Research paper thumbnail of The mystery of missing trubs revisited: a response to McGlone et al. and Qian and Ricklefs

Trends in Ecology & Evolution, 2015

Research paper thumbnail of Why trees and shrubs but rarely trubs?

Trends in Ecology & Evolution, 2014

Research paper thumbnail of Reduced herbivory during simulated ENSO rainy events increases native herbaceous plants in semiarid Chile

Research paper thumbnail of El Niño effects on the dynamics of terrestrial ecosystems

Trends in Ecology & Evolution, 2001

Research paper thumbnail of Shrub facilitation increases plant diversity along an arid scrubland-temperate rain forest boundary in South America

Journal of Vegetation Science, 2012

... into smaller patches on the mountain plateau and is replaced by a matrix of semi-arid scrub v... more ... into smaller patches on the mountain plateau and is replaced by a matrix of semi-arid scrub vegetation on the inland-facing slopes and lowlands (Squeo et al. ... The bot-tom mesh was perforated with holes of 4 cm2 at each seed-ling planting point to facilitate seedling growth. ...

Research paper thumbnail of Rapid root extension during water pulses enhances establishment of shrub seedlings in the Atacama Desert

Journal of Vegetation Science, 2011

Research paper thumbnail of Landscape features associated with the passive recovery of Mediterranean sclerophyllous woodlands of central Chile

Research paper thumbnail of Persistent Acacia savannas replace Mediterranean sclerophyllous forests in South America

Forest Ecology and Management

Mediterranean ecosystems are global hotspots of biodiversity threaten by human disturbances. Grow... more Mediterranean ecosystems are global hotspots of biodiversity threaten by human disturbances. Growing evidence indicates that regeneration of Mediterranean forests can be halted under certain circumstances and that successional stages can become notoriously persistent. The Mediterranean sclerophyllous forest in central Chile is been largely transformed into savannas dominated by the invasive legume tree Acacia caven as result of interacting management and ecological factors. We used multi-temporal satellite imag- ery to study the transition dynamics of these major vegetation types over the last four decades (1975– 2008). Vegetation changes were related to indicators of resource availability (topography, water avail- ability, solar radiance), potential propagule availability (distance to forest remnant patches) and distur- bance regimes (grazing, fire occurrence and distance to roads and cities). During this study period, forests were mostly converted into Acacia savannas (46.1%). Aca...

Research paper thumbnail of Understanding the effects of rainfall variability on dryland vegetation

Background/Question/Methods Rainfall variability is expected to increase with climate warming. We... more Background/Question/Methods Rainfall variability is expected to increase with climate warming. We used the MODIS remotely-sensed estimations of tree cover (%) at 1 km2 to explore the patterns of tropical tree cover distribution in relation to rainfall variability. Results/Conclusions We found that rainfall variability is associated with reduced tree cover in the wet tropics globally. In contrast, high year-to-year variability is positive (South America), neutral (Africa) or negatively (Australia) related to tree cover in the dry tropics. We reflect on long-term observations to identify potential mechanisms that may explain these contrasting responses of tree cover to extreme rainfall events in drylands.

Research paper thumbnail of Shrub facilitation increases plant diversity along an arid scrubland–temperate rain forest boundary in South America

Journal of Vegetation Science

Aim: Theoretical models predict nurse plant facilitation enhances species richness by amelioratin... more Aim: Theoretical models predict nurse plant facilitation enhances species richness by ameliorating stressful environmental conditions and expanding distributional ranges of stress-intolerant species into harsh environments. We studied the role of nurse facilitation on the recruitment of perennial plants along an arid scrubland—temperate rain forest boundary to test the following predictions: (1) nurse shrub canopy increases seedling abundance and species richness along the rain forest—scrubland boundary; (2) scrubland species are less dependent on facilitative interactions than temperate rain forest species, especially at the moister, upper end of the gradient. Location: Bosque Fray Jorge National Park, north-central Chile, South America (30° 39′ S — 71° 40′ W). Methods: We examined seedling abundance and species richness of perennial plants in the open and under different types of patches that may facilitate species recruitment (living shrubs, dead shrubs, perennial grasses and tre...

Research paper thumbnail of Nucleated regeneration of semiarid sclerophyllous forests close to remnant vegetation

Forest Ecology and Management

Natural regeneration of mediterranean plant communities has proved difficult in all continents. I... more Natural regeneration of mediterranean plant communities has proved difficult in all continents. In this paper we assess whether regeneration of sclerophyllous forests shows nucleated patterns indicative of a positive effect of vegetation remnants at the landscape level and compare the regeneration patterns between sites with distinctive climate conditions. We studied the spatial patterns of vegetation change during 52 years in central Chile using remotely-sensed images to test the predictions that (1) regeneration of sclerophyllous vegetation expands from patches of remnant vegetation; and (2) regeneration is more dependent on remnant vegetation in drier sites. Our results show that the regeneration of the sclerophyllous vegetation in central Chile is a slow process that may be possible under certain conditions. We found that the fraction of regenerated vegetation increases with the proximity to remnant sclerophyllous forest in an aggregated pattern. Especially in drier sites, veget...

Research paper thumbnail of Herbivory and plant growth rate determine the success of El Niño Southern Oscillation-driven tree establishment in semiarid South America

Global Change Biology, 2006

While climatic extremes are predicted to increase with global warming, we know little about the e... more While climatic extremes are predicted to increase with global warming, we know little about the effect of climatic variability on biome distribution. Here, we show that rainy El Niño Southern Oscillation (ENSO) events can enhance tree recruitment in the arid and semiarid ecosystems of north-central Chile and northwest Peru. Tree-ring studies in natural populations revealed that rainy El Niño episodes

Research paper thumbnail of The balance of competition and facilitation in plant communities: A graphical model

It has been hypothesized that plants are increasingly shade intolerant in drier conditions. Altho... more It has been hypothesized that plants are increasingly shade intolerant in drier conditions. Although many field patterns can be understood from this theory, the conspicuous {open_quotes}nurse plant{close_quotes} phenomenon in dry areas seems to contradict the theory. We derive a graphical model to illustrate how the interplay of facilitation and competition can be understood from two ingredients: the plant responses to

Research paper thumbnail of ENSO rainfall and herbivore control enhance tree establishment in semiarid Chile: experimental evidence

Research paper thumbnail of Interactive effects of human and ecological factors determine persistence of cork oak forests in southern Portugal

Research paper thumbnail of Nurse plant effects on shrub seedling establishment along a topographical gradient of water availability in north-central Chile

Research paper thumbnail of Tapping climatic variation for dryland restoration

Research paper thumbnail of Could ENSO events change the invisibility of semiarid herbaceous communities?

Research paper thumbnail of Tree-seedling establishment in fragmented Mediterranean forests of central Chile

Research paper thumbnail of Tipping points in tropical tree cover: linking theory to data

It has recently been found that the frequency distribution of remotely sensed tree cover in the t... more It has recently been found that the frequency distribution of remotely sensed tree cover in the tropics has three distinct modes, which seem to correspond to forest, savanna, and treeless states. This pattern has been suggested to imply that these states represent alternative attractors, and that the response of these systems to climate change would be characterized by critical transitions and hysteresis. Here, we show how this inference is contingent upon mechanisms at play. We present a simple dynamical model that can generate three alternative tree cover states (forest, savanna, and a treeless state), based on known mechanisms, and use this model to simulate patterns of tree cover under different scenarios. We use these synthetic data to show that the hysteresis inferred from remotely sensed tree cover patterns will be inflated by spatial heterogeneity of environmental conditions. On the other hand, we show that the hysteresis inferred from satellite data may actually underestimate real hysteresis in response to climate change if there exists a positive feedback between regional tree cover and precipitation. Our results also indicate that such positive feedback between vegetation and climate should cause direct shifts between forest and a treeless state (rather than through an intermediate savanna state) to become more likely. Finally, we show how directionality of historical change in conditions may bias the observed relationship between tree cover and environmental conditions.

Research paper thumbnail of The mystery of missing trubs revisited: a response to McGlone et al. and Qian and Ricklefs

Trends in Ecology & Evolution, 2015

Research paper thumbnail of Why trees and shrubs but rarely trubs?

Trends in Ecology & Evolution, 2014

Research paper thumbnail of Reduced herbivory during simulated ENSO rainy events increases native herbaceous plants in semiarid Chile

Research paper thumbnail of El Niño effects on the dynamics of terrestrial ecosystems

Trends in Ecology & Evolution, 2001

Research paper thumbnail of Shrub facilitation increases plant diversity along an arid scrubland-temperate rain forest boundary in South America

Journal of Vegetation Science, 2012

... into smaller patches on the mountain plateau and is replaced by a matrix of semi-arid scrub v... more ... into smaller patches on the mountain plateau and is replaced by a matrix of semi-arid scrub vegetation on the inland-facing slopes and lowlands (Squeo et al. ... The bot-tom mesh was perforated with holes of 4 cm2 at each seed-ling planting point to facilitate seedling growth. ...

Research paper thumbnail of Rapid root extension during water pulses enhances establishment of shrub seedlings in the Atacama Desert

Journal of Vegetation Science, 2011