Exotic Herbivores as Drivers of Plant Invasion and Switch to Ecosystem Alternative States

This paper discusses the evidence of the way in which coupled herbivore and plant invasions altered the structure and functioning of ecosystems in central Chile, and compares these patterns to results from other Mediterranean ecosystems around the world. In pre-Columbian times, sclerophyllous matorral forests largely covered mediterranean Chile. Remnants of the original, continuous matorral can now be found only on relatively humid slopes of the Coastal Range. In the Intermediate Depression, a long central valley, it has been replaced by extensive savannas dominated by the exotic Acacia caven, whereas the matorral remains on dry slopes as isolated shrub clumps. Experimental work suggests that introduced cattle have played a major role in promoting the shift to Acacia savanna, whereas introduced European rabbits prevent the recolonization of clearings by matorral on the slopes. As in other regions of the world, the loss of woodlands in Chile is likely to have resulted in reduced carbon storage capacity and nutrient recycling, and in increased erosion and climatic aridity within the region. Numerous exotic herbs and grasses now dominate the open areas, and experiments indicate that the dry conditions in the absence of woodland cover in combination with loss of soil fertility and severe grazing by introduced mammals has pushed the competitive balance from native to exotic species. These changes are probably very difficult to reverse. Even if grazing is drastically reduced, drought may prevent the recovery of matorral species in much of the region. However, recent theoretical work suggests that if such grazer reduction is timed to coincide with wet El Niño episodes, recovery of the original woodlands may be triggered.     

Nurse plants vs. nurse objects: effects of woody plants and rocky cavities on the recruitment of the Pilosocereus leucocephalus columnar cactus

BACKGROUND AND AIMS: Most studies on cactus recruitment have focused on the role of woody plants as seedling facilitators. Although the spatial association of cacti with objects had been described, the mechanisms underlying this association remain unknown. The aims of this study were to identify which mechanisms facilitate the establishment of a columnar cactus under the shade and protection of objects and to compare these mechanisms with those involved in plant-plant facilitation. METHODS: Three split-split-plot field experiments were conducted to compare the effects of two microhabitats (inside rocky cavities and beneath plant canopies) on seed removal, germination, seedling survivorship and dry weight. Flat, open spaces were used as the control. For each microhabitat, the effect of seed or seedling protection and substrate limitation were explored; aboveground microclimate and some soil properties were also characterized. KEY RESULTS: The permanence of superficial seeds was greater inside rocky cavities than beneath woody plant canopies or on flat, open areas. Germination was similar in cavities and beneath plant canopies, but significantly higher than on flat, open areas. Seedling survivorship was greater beneath plant canopies than inside cavities or on flat, open spaces. CONCLUSIONS: The mechanisms of plant facilitation are different from those of object facilitation. There are seed-seedling conflicts involved in the recruitment of P. leucocephalus: nurse plants favour mainly seedling survivorship by providing a suitable microenvironment, while nurse objects mainly favour seed permanence, by protecting them from predators.     

Alleviation of Summer Drought Boosts Establishment Success of <Emphasis Type="Italic">Pinus sylvestris</Emphasis> in a Mediterranean Mountain: An Experimental Approach

We performed an irrigation experiment to study the impact of summer drought on Pinus sylvestris establishment at its southernmost distribution limit. Watering was done during the first growing season simulating mesic summer conditions, and we monitored the consequences for survival and growth during the first growing season and the delayed consequences on the second growing season. In addition, we considered the heterogeneity created by the microhabitats, where seeds are found after dispersal (bare soil, under shrubs, and under adult pines). Summer drought was the main mortality factor in all the microhabitats. Watering increased emergence and doubled seedling survival compared to non-watered control sites. Differences were even higher when the cumulative effect on emergence and survival was considered, with an overall recruitment of 22.4% in watered plots vs. 7.9% in control. Irrigation increased growth in bare soil and under shrubs, but had scant effect on growth under pines, suggesting that radiation was the limiting factor in this microhabitat. The positive effect of irrigation on growth parameters persisted during the second growing season despite water was not added the second year, showing delayed consequences of drought on seedling performance. Summer drought thus limits Pinus sylvestris establishment in these southernmost forests by reducing both recruitment and growth. This might lead to the development of a remnant dynamic in these relict populations under the current regional increase in dryness and rainfall variability associated with global warming.     

Spatial distribution and density dependent growth and flowering of Asphodelus aestivus Brot.

Distribution patterns of A. aestivus at four different sites of a degraded mediterranean-type ecosystem (asphodel desert), Thessally, Greece, have been studied. A first order spatial structure was identified at small block sizes and was related to asexual reproduction, a second order structure, at large block sizes, was related to sexual reproduction. Spatial distribution of A. aestivus was associated with soil depth while ecological neighbourhood was discussed in relation to degradation phenomena, especially erosion. Density dependent growth of aboveground biomass and flowering has also been explored. Self-thinning was not detected, whereas the percentage of plants flowering decreases linearly with density and this was attributed to competition for nutrients. A reciprocal equation describes the growth of the aboveground biomass with time. A two-sided competition was displayed by initial plant weight, while the rate of growth of the aboveground biomass was proved independent of density.     

Spatio‐temporal variation in the effect of herbaceous layer on woody seedling survival in a Chilean mediterranean ecosystem

Questions: What is the effect of herbaceous layer on seedling establishment of three woody pioneer species in open areas of central Chile under a semi‐arid mediterranean climate? How do inter‐annual and habitat conditions (slope aspect) modulate this effect? Under high stress conditions such as the drier year and habitat (north‐facing slope) do herbs reach low abundance and have neutral effects on woody seedlings? Under medium stress conditions for these woody species, such as the wetter year and south‐facing slope, does the herbaceous layer reach greater abundance and have positive effects on woody seedlings due to increasing soil water content? Location: A watershed on the outskirts of Santiago, Chile, subjected to clearing of woody vegetation through firewood extraction and human‐set fires. Methods: In spring 2007, we set up 20 plots (3 m × 2 m). Half of each plot had herbs removed manually and by application of herbicide. In both halves of each plot, one seedling (8 months old) of each of the three native woody species (Colliguaya odorifera, Schinus polygamus and Quillaja saponaria) was planted and survival monitored subsequently. The experiment was repeated in two consecutive growing seasons (2007–2008 and 2008–2009) that differed significantly in total precipitation (152 and 256.5 mm, respectively), and replicated in two sites that differed in aspect and abiotic conditions: a moister south‐ and a drier north‐facing slope. Results: In the first and drier year, the herbaceous layer had low cover and no significant effect on seedling survival of woody species. During the second year, herbs had greater cover and a significant positive effect on spring survival of C. odorifera in the north‐facing slope, which was lost after summer. During this wetter year on the south‐facing slope, herb cover had a positive effect on survival of S. polygamus (mainly during summer). Conclusions: The role of mostly ruderal herbs on woody seedling establishment depended on the species, rainfall of the current year and slope aspect, and may be explained by soil moisture patterns. This suggests that the effect of ruderal herbs on woody seedlings shifts from neutral under high stress conditions produced by drought to positive under moderate stress conditions. Our results contribute to understand interactions between ruderal herbs and woody species under contrasting abiotic conditions. Therefore, control of the herbaceous layer may not be needed in restoration programmes for this region. Moreover, herbs may benefit restoration of woody cover in mesic habitats.