Nucleated regeneration of semiarid sclerophyllous forests close to remnant vegetation
Highlights
► Regeneration of Mediterranean evergreen forests shows nucleated patterns. ► Regeneration increases with the proximity to remnant forest patches. ► Regeneration is more dependent on remnant vegetation in drier sites. ► Remnant vegetation has a facilitative role on passive restoration.
Introduction
The regeneration of mediterranean-type ecosystems has proved to be difficult and irregular in all continents, including southeast Australia (Westoby et al., 1989), California (Laycock, 1991), central Chile (Fuentes et al., 1984, Fuentes et al., 1986), the Mediterranean Basin (Kéfi et al., 2007, Acácio et al., 2009) and southwest South Africa (Vlok and Yeaton, 2000). Understanding the conditions under which natural regeneration may be possible is highly relevant for restoration planning since active restoration attempts can be notoriously unsuccessful and costly (Rey-Benayas et al., 2008). Natural regeneration may represent an opportunity for the conservation of these global hotspots of biodiversity, characterized by high levels of floristic richness and endemism (Myers et al., 2000, Olson and Dinerstein, 2002), and suppliers of provisioning, regulating and cultural services for human populations (Millennium Ecosystem Assessment, 2005).
Regeneration of the original mediterranean-type forests and evergreen shrublands is mostly slowed down by limited seedling establishment (Fuentes et al., 1984, Retana et al., 1999, Holmgren et al., 2000), and the persistence of pioneer vegetation stages that maintain the ecosystem in a condition of arrested succession (Vicente and Ales, 2006, Acácio et al., 2009, Van de Wouw et al., 2011). Experimental evidence indicates that tree seedling recruitment is generally limited by multiple ecological factors affecting seed predation and dispersion, as well as germination, growth and survival of seedlings (Fuentes et al., 1986, Retana et al., 1999, Acácio et al., 2007), which interact with land use and management practices to further difficult vegetation change (Acácio et al., 2010).
One of the most important lessons learnt from the restoration of mediterranean-type ecosystems has been the recognition of shade availability as a crucial condition for enhancing seedling establishment. In a comprehensive assessment of the experiences in Spain, Gómez-Aparicio et al. (2004) concluded that shrubs act as nurse plants enhancing the recruitment of tree seedlings in reforestation programs. Under the nurse canopy, the microclimate is cooler and often also moister, ameliorating plant thermal and water stress (Holmgren et al., 1997). This positive effect of the nurse canopy is often essential for seedlings to survive the summer drought that characterizes all mediterranean-type ecosystems (Fuentes et al., 1984, Holmgren et al., 2000, Gómez-Aparicio et al., 2004). In addition, nurse plants can increase soil nutrients (Holmgren et al., 1997, Padilla and Pugnaire, 2006) or protection from herbivores (Baraza et al., 2006). Nurse plants can also enhance seed availability by attracting seed dispersers, such as birds or mammals (Debussche et al., 1982, Pausas et al., 2006), amplifying the effects of environmental stress amelioration.
Since the net effect of the interaction between plants is an unavoidable interplay of positive and negative effects, plant interactions can switch from facilitation to competition if stress amelioration under the nurse shade is insufficient to compensate for the limitation in light availability (Holmgren et al., 1997). This can happen under very stressful conditions when resources, as soil water, are too limiting (Maestre et al., 2009), or when the capacity of nurse plants to change environmental conditions is insufficient to ameliorate stress in a significant way (Holmgren and Scheffer, 2010). For instance, in some very dry Mediterranean systems, positive interactions become neutral or negative if competition for soil water overrides the positive effects of the canopy on reducing thermal stress (Maestre and Cortina, 2004). These empirical results have inspired current theoretical models predicting either a linear increase in facilitation as environmental stress becomes more stressful (Bertness and Callaway, 1994) or a maximum effect of facilitation at intermediate conditions of abiotic stress (Maestre et al., 2009, Holmgren and Scheffer, 2010). These experimental findings and theoretical models suggest therefore that natural regeneration of mediterranean-type vegetation should be particularly dependent on nurse plants under dry sites, but indicate also potential limits to the role of positive interactions under very stressful conditions. At a landscape level, regeneration should be then characterized by a patchy spatial pattern of vegetation growth expanding from remnant vegetation patches, especially at drier sites.
In this paper, we assess if the long-term regeneration of sclerophyllous forests in central Chile shows nucleation patterns indicative of a positive effect of vegetation remnants at the landscape level, and compare the regeneration patterns between sites with distinctive climate conditions. Most of the original vegetation of this region has been destroyed by agricultural and urban growth (Schulz et al., 2010), or replaced by early successional plant communities (Fuentes et al., 1989, Van de Wouw et al., 2011) in a long historical process of landscape fragmentation and degradation (Armesto et al., 2010). Field experiments in central Chile have demonstrated that tree seedling survival can be extremely rare, particularly in drier sites, as result of desiccation and severe herbivory (Fuentes et al., 1984, Fuentes et al., 1986, Holmgren et al., 2000, Gutiérrez et al., 2007, Becerra et al., 2011), as well as intense and frequent fires (Fuentes and Muñoz, 1995, Segura et al., 1998). These field experimental results have been essential to understand the role of shrub patches on the regeneration of vegetation, but have been conducted at small scales and monitored for a couple of years only (but see Holmgren et al., 2000). On the other hand, remote-sensed studies have identified the drivers correlated with large scale patterns of vegetation change during the last 40 years (1975–2008; Schulz et al., 2011, Van de Wouw et al., 2011), but did not assess the role of vegetation remnants on the regeneration of sclerophyllous forests at a landscape scale. We studied the spatial patterns of vegetation change in a period of 52 years using remotely-sensed images to test the predictions that (1) regeneration of sclerophyllous vegetation expands from patches of remnant vegetation; and (2) regeneration is weaker under drier conditions (north-facing slopes, inland sites and further away from ravines).
Section snippets
Study area
The study sites were located in the Valparaiso and Metropolitan administrative regions of Chile (Fig. 1; Appendix A). The climate is Mediterranean with rainfalls concentrated in winter and dry summers. Microclimatic conditions differ strongly between coastal and inland sites as result of the influence of the cold Pacific Ocean. In the Coastal range temperatures are moderate, precipitation is higher than inland, and morning fogs are frequent (Cereceda, 1989), which most likely reduce plant heat
Spatial patterns of woody cover increase
The multi-temporal image analysis showed an increase of woody vegetation cover in all study sites and periods analyzed. However, vegetation regeneration was lower between 1955 and 1983 than between 1983 and 2007 in at least three sites (Cachagua, San Carlos de Apoquindo, Cerro Blanco; Fig. 2).
We found a positive spatial autocorrelation in the first lag distance (250 m), indicating that the regeneration of woody vegetation in areas adjacent to each cell was more similar and aggregated than
Aggregated spatial patterns of forest regeneration
Our statistical models indicate that patches where sclerophyllous forest regenerated are spatially autocorrelated, meaning that vegetation growth occurred in an aggregated way. The processes that could generate this spatial pattern are diverse. Two main types of spatial autocorrelation might be distinguished depending on whether endogenous or exogenous processes generate the spatial structure of the observations (Legendre and Legendre, 1998). Spatial pattern can be generated by factors that are
Conclusions
Our results highlight the importance of forest remnants for the regeneration of semiarid schlerophyllous vegetation. The aggregated pattern of vegetation recovery close to forest remnants, especially in drier Andean sites, indicate that they facilitate the regeneration of forests. Restoration efforts could aim at generating nucleated patterns of plant recruitment around forest remnants in order to increase the success of reforestation programs.
Acknowledgements
We thank Juan Armesto for facilitating this work, Cristian Delpiano for field assistance and Rodolfo Gajardo and Patricio Pliscoff for suggestions. Two anonymous reviewers provided constructive suggestions that improved the manuscript. This research was partly funded by the EU-REFORLAN (EU INCO-CT-2006-032132) and Chilean CONICYT – (RUE 33) projects.
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