Milena Holmgren

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...

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.

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...

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...

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

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.

... 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. ...

... by mere fishing is like pushing the ball uphill in the front stability landscape: if you stop ... 8. DYNAMICINTERACTION OF SOCIETIES AND ECOSYSTEMS 209 Experiments show that fish have little effect on ... in eating zoo-plankton only during the first months of their life, causing a ...

Africa is home to some of the most vulnerable natural ecosystems and species on the planet. Around 7000 protected areas seek to safeguard the continent's rich biodiversity, but many of them face increasing management challenges. Human disturbances permeating into the parks directly and indirectly affect the ecological functioning and integrity of protected areas. With the envisaged expansion of the protected area network and further expected population and economic growth in the region, the competition between nature conservation and resources demands is likely to increase. The regular monitoring of land cover in and around protected areas can support the early detection of conservation conflicts. In this paper, we evaluate the use of the annual time series of MODIS Land Cover (LC) type product between 2003 and 2009 to monitor land cover changes at continental scale. We use the mean classification confidence and change frequency as indicators to assess the temporal consistency o...

Background/Question/Methods Predicting ecosystem resilience to climate and disturbance events is one of the most urgent and challenging problems in ecology today. We used the MODIS remotely-sensed estimations of tree cover (%) at 1 km2 to explore the patterns of tree cover distribution in tropical regions of South America, Africa, and Australia, and their relationship with climate. We used the tree-cover frequency distributions to infer ecological resilience and to map the potential vulnerability of tropical ecosystems in the three continents. Results/Conclusions We found striking tri-modal distributions corresponding to treeless (< 5 %), savanna (20 %), and forest (> 60 %) states which are strongly associated to mean annual precipitation. The transitional conditions between treeless-savanna (5-10 % tree cover) and between savanna-forest (55-60 % tree cover) occur at very low frequencies, which we interpret as an indication of transient unstable conditions. We found that not o...

Although the boreal region is warming twice as fast as the global average, the way in which the vast boreal forests and tundras may respond is poorly understood. Using satellite data, we reveal marked alternative modes in the frequency distributions of boreal tree cover. At the northern end and at the dry continental southern extremes, treeless tundra and steppe, respectively, are the only possible states. However, over a broad intermediate temperature range, these treeless states coexist with boreal forest (∼75% tree cover) and with two more open woodland states (∼20% and ∼45% tree cover). Intermediate tree covers (e.g., ∼10%, ∼30%, and ∼60% tree cover) between these distinct states are relatively rare, suggesting that they may represent unstable states where the system dwells only transiently. Mechanisms for such instabilities remain to be unraveled, but our results have important implications for the anticipated response of these ecosystems to climatic change. The data reveal tha...

The Chilean matorral is characterized by multispecific shrub clumps in dry areas but has a continuous canopy in wetter sites. It has been hypothesized that this difference is due to easier recolonization of open patches by shrub seedlings under more mesic conditions. Within the mesic range of the matorral we designed a field experiment to compare shrub seedling emergence, growth,

The hypothesis that high and low intensity human-made fires can produce ecologically different effects in the Chilean matorral is examined. We compared the abundance of naturally established seedlings under shrubs burned by low and high intensity fires on five north and five south facing slopes. On south facing slopes, we found 54 shrubs burned by low intensity fires and only 4 shrubs

ABSTRACT Climatic warming is substantially intensifying the global water cycle and is projected to increase rainfall variability. Using satellite data, we show that higher climatic variability is associated with reduced tree cover in the wet tropics globally. In contrast, interannual variability in rainfall can have neutral or even positive effects on tree cover in the dry tropics. In South America, tree cover in dry lands is higher in areas with high year-to-year variability in rainfall. This is consistent with evidence from case studies suggesting that in these areas rare wet episodes are essential for opening windows of opportunity where massive tree recruitment can overwhelm disturbance effects, allowing the establishment of extensive woodlands. In Australia, wet extremes have similar effects, but the net effect of rainfall variability is overwhelmed by negative effects of extreme dry years. In Africa, effects of rainfall variability are neutral for dry lands. It is most likely that differences in herbivore communities and fire regimes contribute to regulating tree expansion during wet extremes. Our results illustrate that increasing climatic variability may affect ecosystem services in contrasting, and sometimes surprising, ways. Expansion of dry tropical tree cover during extreme wet events may decrease grassland productivity but enhance carbon sequestration, soil nutrient retention and biodiversity.