Introducing resprouters to enhance Mediterranean forest resilience: importance of functional traits to select species according to a gradient of pine density

Abstract

International audienceResprouter species are important for Mediterranean ecosystem resilience, but they are scarce in landscapes dominated by pioneer pines. Sound knowledge of resprouter seedling functional responses and establishment success across different pine habitats would help guide forest restoration efforts. Four broadleaved resprouter species, that is two shrubs - Arbutus unedo and Pistacia terebinthus - and two trees - Fraxinus ornus and Sorbus domestica - were planted under an experimentally created gradient of pine cover from totally open conditions to dense pine cover. Seedling survival and growth were monitored for 5years. Phenological, physiological and morphological traits were measured to gain insight into the mechanisms of pine-seedling interaction. Interaction outcomes varied according to species identity, pine cover treatment and time. Light-to-moderate pine cover induced greater height growth and little or no effects on diameter and survival of the two trees. Competition was always detected for shrubs, whereas competition and facilitation were noted for trees. Within pine stands, negative interactions increased with tree cover, making dense stands the most limiting. Interactions were only detectable after 2-3years, but increased in intensity with time. Functional responses to increasing pine cover indicated a seedling strategy of carbon gain optimization in the shade (increased specific leaf area and leaf area, decreased leaf dry matter content), particularly in tree species seedlings. Increasing pine cover induced higher water stress, but lower photochemical stress. The deciduous species had a longer vegetation season under pine canopies, whereas the evergreen shrub Arbutus unedo showed a lower polycyclism rate and flower production.Synthesis and applications. Moderate pine cover facilitates the establishment of deciduous trees at the expense of shrubs. This pattern can be attributed to a higher shade tolerance of the trees than the shrubs combined with a low tolerance of trees to photoinhibition. We therefore recommend using evergreen shrubs for the restoration of open land and high-specific-leaf-area deciduous trees for pine stand diversification. However, in dense forests, thinning is needed to increase light availability prior to seedling introduction. Moderate pine cover facilitates the establishment of deciduous trees at the expense of shrubs. This pattern can be attributed to a higher shade tolerance of the trees than the shrubs combined with a low tolerance of trees to photoinhibition. We therefore recommend using evergreen shrubs for the restoration of open land and high-specific-leaf-area deciduous trees for pine stand diversification. However, in dense forests, thinning is needed to increase light availability prior to seedling introduction

Fire recurrence and emergency post-fire management influence seedling recruitment and growth by altering plant interactions in fire-prone ecosystems

“…Similarly, in subsoiled stands following two large fires, the highest positive effect of resprouter shrubs (P. tridentatum and E. australis) was on seedling growth in the second growing season after fire and total height, while their highest negative effect was on root biomass, and, to a minor degree, on aerial biomass. In all managed stands, greater annual seedling growth and height under shrubs than in open ground resulted in lower aerial and root biomass, thus, indicating elongation in response to shade, and net aboveground plant competition for light (Calvo et al, 2008 and references therein), as well as belowground competition for water and nutrients at the initial stages of post-fire seedling establishment (Gavinet et al, 2016;Maestre and Cortina, 2004;Prévosto et al, 2012). Better or worse performance of maritime pine seedlings under canopy cover than in open ground at early stages of development has been associated to differences in local climate (Fernandes et al, 2017;Rodríguez-García et al, 2010;Ruano et al, 2009), stand environmental characteristics (Rodríguez-García et al, 2010, and population-specific life-history strategies (Rodríguez-García and Bravo, 2013;Vizcaíno et al, 2014).…”

“…(i) the partial cutting of dead trees as an alternative to salvage logging to retain some snags that favour seedling growth avoiding belowground competition (Castro et al, 2011), where a sufficient number of seedlings is naturally recruited after one large fire (i.e., no action is the best option since planting or seeding are unneces Taboada et al Forest Ecology and Management xxx (2017) Moreira et al, 2012) and local inhabitants demand compensatory burned timber selling; (ii) direct seeding to mimic massive seed rain immediately after fire, leaving unprofitable immature burned trees in place to act as potential nurses buffering microclimate conditions (Castro et al, 2011;Marzano et al, 2013) and enhancing soil fertility (Marañón-Jiménez and , instead of subsoiling and planting of seedlings (relatively expensive compared to direct seeding; Moreira et al, 2012), where recruitment is undermined by frequent large fires and slopes are smooth (i.e., active restoration with seeds originating from nearby stands with the same serotiny level; de las Gil et al, 2009); in this case, increasing the amount of fine woody debris prior to seeding could aid the protection of seeds from predation and adverse climate (Madrigal et al, 2005;Rodríguez et al, 2008;Rodríguez-García et al, 2010); and (iii) either nurse-based planting of seedlings in manually-dug holes close to standing immature burned trees (i.e., active restoration with seedlings grown from seeds collected in nearby stands; de las or alternately allowing natural succession to resprouter-dominated shrublands highly-adapted to recurrent fires (i.e., forest conversion to an alternative successional stage; Gavinet et al, 2016;Moreira et al, 2012), rather than seeding, where recruitment is lessened by repeated fires and slopes are steep and stony.…”

“…Benefactor plants improve recruitment and performance of beneficiary ones, through numerous processes like nutrient accumulation, microclimate amelioration, and herbivory protection (Gómez-Aparicio et al, 2005Maestre et al, 2003). Facilitation generally prevails under high abiotic stress or disturbance (Bertness and Callaway, 1994); but the net outcome of facilitative/competitive interactions acting simultaneously is the result of multiple factors (reviewed by Maestre et al, 2009), including the identity and life-strategies of the interacting species (Gavinet et al, 2016;Gómez-Aparicio, 2009;Gómez-Aparicio et al, 2008;Prévosto et al, 2012;Schöb et al, 2013) and the nature and degree of the environmental stress (Maestre et al, 2003;Pugnaire and Luque, 2001).…”

“…We expected that (i) under the stressful conditions after a single large stand-replacing fire the regrowth of potential nurse plants (obligate seeder shrubs in particular) will protect pine seedlings against the harsh environment, resulting in net facilitation and improved seedling performance (Castro et al, 2004;Fernandes et al, 2017;Rodríguez-García et al, 2011;Ruano et al, 2009); (ii) post-fire salvage logging after a large fire will eliminate potential nurse objects (burned tree trunks, branches, logs, and snags), increase soil compaction, and damage the soil seed bank, delaying facilitative interactions (basically, between obligate seeder shrubs and pine seedlings), and lessening seedling performance (Castro et al, 2011;; (iii) the occurrence of two subsequent large high-severity fires will favour highly-adapted resprouter shrubs over obligate seeder shrubs (Calvo et al, 2012;Keeley, 1986;Pausas and Vallejo, 1999), shifting plant-plant interactions from facilitative to competitive ones, and reducing seedling performance (Gavinet et al, 2016); (iv) mechanical site preparation prior to planting of seedlings after recurrent fires will decrease soil resistance, improve water and air transport, and promote the development of a deep root system of the planted trees, reducing competition and enhancing seedling performance (Espelta et al, 2003; but see Löf et al, 2012).…”

Restoring forests: regeneration and ecosystem function for the future

“…clearcutting and plantation forestry). Other practices, such as forest enrichment by underplanting, contribute to enhance structural complexity of managed forests yet their effectiveness is dependent upon choice of thinning intensity and underplanted species attributes (Gavinet et al 2016;Lesko and Jacobs 2018;Lu et al 2018). Because several disciplines are involved in the discussion, any attempts to formulate a universal definition of restoration will continue to generate discussion (Mansourian 2018; Stanturf et al 2014a, b).…”