The weight of the past: land-use legacies and recolonization of pine plantations by oak trees.

Most of the world's plantations were established on previously disturbed sites with an intensive land-use history. Our general hypothesis was that native forest regeneration within forest plantations depends largely on in situ biological legacies as a source of propagules. To test this hypothesis, we analyzed native oak regeneration in 168 pine plantation plots in southern Spain in relation to land use in 1956, oak patch proximity, and pine tree density. Historical land-use patterns were determined from aerial photography from 1956, and these were compared with inventory data from 2004-2005 and additional orthophoto images. Our results indicate that oak forest regeneration in pine plantations depends largely on land-use legacies, although nearby, well-conserved areas can provide propagules for colonization from outside the plantation, and pine tree density also affected oak recruit density. More intense land uses in the past meant fewer biological legacies and, therefore, lower likelihood of regenerating native forest. That is, oak recruit density was lower when land use in 1956 was croplands (0.004 +/- 0.002 recruits/m2 [mean +/- SE]) or pasture (0.081 +/- 0.054 recruits/m2) instead of shrubland (0.098 +/- 0.031 recruits/m2) or oak formations (0.314 +/- 0.080 recruits/m2). Our study shows that land use in the past was more important than propagule source distance or pine tree density in explaining levels of native forest regeneration in plantations. Thus, strategies for restoring native oak forests in pine plantations may benefit from considering land-use legacies as well as distance to propagule sources and pine density.

Are pine plantations valid tools for restoring Mediterranean forests? An assessment along abiotic and biotic gradients.

The ecological impacts of forest plantations are a focus of intense debate, from studies that consider plantations as "biological deserts" to studies showing positive effects on plant diversity and dynamics. This lack of consensus might be influenced by the scarcity of studies that examine how the ecological characteristics of plantations vary along abiotic and biotic gradients. Here we conducted a large-scale assessment of plant regeneration and diversity in plantations of southern Spain. Tree seedling and sapling density, plant species richness, and Shannon's (H') diversity index were analyzed in 442 pine plantation plots covering a wide gradient of climatic conditions, stand density, and distance to natural forests that act as seed sources. Pronounced variation in regeneration and diversity was found in plantation understories along the gradients explored. Low- to mid-altitude plantations showed a diverse and abundant seedling bank dominated by Quercus ilex, whereas high-altitude plantations showed a virtually monospecific seeding bank of Pinus sylvestris. Regeneration was null in plantations with stand densities exceeding 1500 pines/ha. Moderate plantation densities (500-1000 pines/ha) promoted recruitment in comparison to low or null canopy cover, suggesting the existence of facilitative interactions. Quercus ilex recruitment diminished exponentially with distance to the nearest Q. ilex forest. Richness and H' index values showed a hump-shaped distribution along the altitudinal and radiation gradients and decreased monotonically along the stand density gradient. From a management perspective, different strategies will be necessary depending on where a plantation lies along the gradients explored. Active management will be required in high-density plantations with arrested succession and low diversity. Thinning could redirect plantations toward more natural densities where facilitation predominates. Passive management might be recommended for low- to moderate-density plantations with active successional dynamics (e.g., toward oak or pine-oak forests at low to mid altitudes). Enrichment planting will be required to overcome seed limitation, especially in plantations far from natural forests. We conclude that plantations should be perceived as dynamic systems where successional trajectories and diversity levels are determined by abiotic constraints, complex balances of competitive and facilitative interactions, the spatial configuration of native seed sources, and species life-history traits.