Long-Term Tree Regeneration of Fragmented Agroforestry Systems Under Varying Climatic Conditions

Abstract

Iberian dehesas and montados are agroforestry systems protected by the European Habitats Directive due to high levels of biological diversity associated to their savannah-like structure. Tree scattering in dehesas, montados and other agroforestry systems is, however, known to compromise tree regeneration, although recent work suggests that it may protect tree populations from climate warming by alleviating plant-plant competition. We analyze how climatic conditions, tree isolation and their interactions influence the outcomes of regeneration stages, from flower production to early seedling establishment, using data gathered during the long-term monitoring (2001–2018) of ca. 300 Holm oak Quercus ilex trees located in central Spain. Holm oak reproductive effort, predispersal seed losses, and early seedling recruitment were sensitive to climate change, especially to year-round drought. Effort and early seedling recruitment decreased, while abortion and predispersal seed predation increased, with higher drought intensity. Spring warming increases pollination effectiveness, but had no further effect on acorn crops. Forest clearing seemed to have little scope to ameliorate these negative effects, as shown by weak or no interactive effects between the spatial configuration of trees (cover or isolation) and climate variables (spring temperature or drought intensity). Forest opening aimed at decreasing adult tree mortality under climate change scenarios would then have little or no effects on tree recruitment. Landscape-scale rotations alternating shrub encroachment and thinning along periods adapted to changing climate are proposed as the main management option to preserve both oak forests and dehesas in the long term.

Biological integrity enhances the qualitative effectiveness of conditional mice-oak mutualisms

“…Also, it supports the view that biological integrity (presence of the full set of producers, consumers, dispersers and predators) is key for ensuring seed dispersal effectiveness in synzoochorous conditional mutualisms [5,52]. This may be particularly important in man-made habitats like dehesas, which depend on conditional mutualisms to ensure their long-term sustainability [38,55]. selective behavior only occurred when (A) ungulates were absent and (B) there was no predator scent.…”

“…Consequently, predation rates were reduced and larger acorns had a higher probability of dispersal. In Mediterranean systems, seedlings from larger acorns are more resistant to summer drought [29,60], which represents the main recruitment bottleneck for oak regeneration [38,61]. Thus, our simulation results suggest that suboptimal conditions can enhance both, the quantity and quality component of dispersal effectiveness by mice.…”

“…They are savanna-like habitats, simpler than natural forests but diverse enough to maintain all key elements influencing the oak-mice conditional mutualism. In spite of this, dehesas, as well as other man-made systems dominated by scattered trees, suffer from a chronic lack of tree regeneration that compromises its long-term sustainability [37,38]. Depending on the local intensity of management, nearby areas can have contrasting levels of shrub cover, mice densities and competition with ungulates [39,40].…”

Habitat Suitability for Small Mammals in Mediterranean Landscapes: How and Why Shrubs Matter

“…Nonetheless, the combination of long-term monitoring of small mammal communities with LiDAR-based high-resolution measurements of vegetation structure allowed us to parametrise clear-cut responses of small mammals to human-driven vegetation changes. Bearing in mind the keystone role of small mammals in the long-term sustainability of managed Mediterranean forest [72], this investigation can provide useful criteria for the management of Mediterranean forests under the current global change conditions [17,74].…”

Ungulate presence and predation risks reduce acorn predation by mice in dehesas

“…These unexpected results may respond to some particularities of our study system. On one hand, dehesas are characterized by a high acorn production [ 53 , 54 ], and hence, the effects of competition for seeds may have been attenuated [ 13 ]. On the other, ungulates not only compete with rodents for acorns, but also are important modulators of vegetation structure in dehesas.…”