A Neighborhood Analysis of the Consequences of Quercus suber Decline for Regeneration Dynamics in Mediterranean Forests

Abstract

In forests, the vulnerable seedling stage is largely influenced by the canopy, which modifies the surrounding environment. Consequently, any alteration in the characteristics of the canopy, such as those promoted by forest dieback, might impact regeneration dynamics. Our work analyzes the interaction between canopy neighbors and seedlings in Mediterranean forests affected by the decline of their dominant species (Quercus suber). Our objective was to understand how the impacts of neighbor trees and shrubs on recruitment could affect future dynamics of these declining forests. Seeds of the three dominant tree species (Quercus suber, Olea europaea and Quercus canariensis) were sown in six sites during two consecutive years. Using a spatially-explicit, neighborhood approach we developed models that explained the observed spatial variation in seedling emergence, survival, growth and photochemical efficiency as a function of the size, identity, health, abundance and distribution of adult trees and shrubs in the neighborhood. We found strong neighborhood effects for all the performance estimators, particularly seedling emergence and survival. Tree neighbors positively affected emergence, independently of species identity or health. Alternatively, seedling survival was much lower in neighborhoods dominated by defoliated and dead Q. suber trees than in neighborhoods dominated by healthy trees. For the two oak species, these negative effects were consistent over the three years of the experimental seedlings. These results indicate that ongoing changes in species’ relative abundance and canopy trees’ health might alter the successional trajectories of Mediterranean oak-forests through neighbor-specific impacts on seedlings. The recruitment failure of dominant late-successional oaks in the gaps opened after Q. suber death would indirectly favor the establishment of other coexisting woody species, such as drought-tolerant shrubs. This could lead current forests to shift into open systems with lower tree cover. Adult canopy decline would therefore represent an additional factor threatening the recruitment of Quercus forests worldwide.

Microclimate and seeding predation as drivers of tree recruitment in human-disturbed oak forests

“…Besides these effects of temperature and air humidity, it is also important to note that PPFD levels in the plantation understory were almost three orders of magnitude above of those recorded within the forest, which may be linked to the wide openness of the plantation canopy. On this issue, some recent studies have shown that oak seedlings can display elevated photochemical efficiencies beneath closed forest canopies (Ibáñez et al, 2015), while elevated solar radiation cause their photoinhibition in open areas (González-Salvatierra et al, 2013). Thus, increased insolation is also likely to reduce seedling survival in the plantation.…”

Emergence and Evolution of Anthropogenic Landscapes in the Western Mediterranean and Adjacent Atlantic Regions

“…In addition, regeneration in forests in NM and SM would have been more difficult than in more humid, deciduous forests to the north [113]. Neighborhood analysis of modern Quercus woodlands suggests that the presence of trees positively affects the emergence of seedlings [114]. Climate and deforestation may have therefore altered vegetation trajectories through neighbor-specific impacts on the recruitment of arboreal species, indirectly favoring the expansion of open vegetation.…”

Impact of tree decline on spatial patterns of seedling-mycorrhiza interactions: Implications for regeneration dynamics in Mediterranean forests

“…We found that in woodland sites an important fraction of Q. suber seedlings did not show any ECM colonization (25% of the seedlings) or had low colonization values (37% of the seedlings had <20% of root tips colonized). Therefore, in these sites a higher seedling investment in ECM associations in response to tree decline could be justified in terms of an expected gain in performance, particularly taking into account that seedlings established in defoliated and dead neighborhoods are severely impaired due to stressful microclimatic conditions (Ibáñez et al, 2015) and high soil-borne pathogen loads (Gómez-Aparicio et al, 2012). This result also agrees with the ''intermediate host-plant stress hypothesis'' (Swaty et al, 2004), which postulates that stress can promote the establishment of mycorrhizal associations, at least until a certain threshold at which hosts can no longer support the C costs of the association.…”