Diversification of Pinus halepensis forests by sowing Quercus ilex and Quercus pubescens acorns: testing the effects of different vegetation and soil treatments

Abstract

The goal of this study was to develop management strategies favouring establishment and survival of holm oak (Quercus ilex L.) and downy oak (Quercus pubescens Willd.)-two species co-occurring in Southern France-in mature Allepo pine forests (Pinus halepensis Mill subsp. halepensis). An experimental design was assessed in a partially cut mature stand of Aleppo pine in which five soil and vegetation treatments-chopping, chopping followed by scarification in one or two directions, prescribed burning, control-and two slash treatments (presence/absence) were applied. A total of 1,600 sowing points, each composed of 3 Q. ilex or Q. pubescens acorns, were installed in the different treatments in November of two consecutive years at 6 and 18 months, after the end of treatments. Survival was monitored 3 and 2 years after sowing, soil surface at the sowing points was characterized at different dates, and predawn leaf water potentials were measured during the dry season. High mortality occurred after the first summer, but survival after 1 year was 2.3-5.2 higher in Q. ilex than in Q. pubescens, confirming that Q. ilex was better adapted to the drier parts of the Mediterranean area. Survival was significantly influenced by the treatments, but there was a variable response between the two sowing years under most of the treatments. Only intense fire proved the most beneficial treatment for seedling survival in both years. The micro-local soil cover conditions induced by the treatments played a major role in explaining oak survival. In particular, grass cover (mainly Brachypodium retusum) proved to be largely unfavourable to seedling survival and growth, and this detrimental effect was also confirmed by lower predawn leaf water potential values with increasing grass cover. Acorn introduction designed to diversify mature Aleppo pine forest after soil and vegetation treatments therefore has to be considered for treatments that most efficiently impair the pre-existing competing grass cover such as prescribed high-intensity fire treatment.

Valuing acorn dispersal and resprouting capacity ecological functions to ensure Mediterranean forest resilience after fire

“…Management actions aiming at improving the growth conditions of Quercus species developing in the understory (e.g. creating a diverse range of irradiance (Gómez-Aparicio et al 2009;González-Moreno et al 2011)) or still promoting enrichment plantations of conifer stands with resprouting species (Prévosto et al 2011) could be envisaged. Overall, as stated by Puettmann et al (2008), new silvicultural tools are needed to reinforce the maintenance of the heterogeneity in ecosystem structure, composition and function (SCBD 2001).…”

Short-term dynamics of Quercus ilex advance regeneration in a Pinus nigra plantation after the creation of small canopy gaps

“…The scarcity of recent acorn seedlings indicates the dominant role that the advance regeneration played in forest dynamics during the first years after the opening. Recruitment of new acorn seedling was also observed, but the survival of young Q. ilex seedlings in full sunlight is usually low (Marañón et al, 2004;Prévosto et al, 2011b) and therefore it is difficult to predict if they will increase the density of Q. ilex meaningfully.…”

The state of forest vegetation management in Europe in the 21st century

“…density level varied between the three understorey species, with 16 plants m -2 of Calluna and Molinia appearing to permit acceptable tree growth compared to a threshold of only 10 plants m -2 for Pteridium inducing already a significant loss (38%) in pine diameter (Gaudio et al 2010). Also in this Special Issue, Prévosto et al (2010) reports that in southern France grass cover is the main factor limiting seedling survival of seeded Holm oak (Quercus ilex) and downy oak (Q. pubescens). Again in this Special Issue, Dodet et al (2010) report that on fertile sites, targeting herbaceous vegetation provides the biggest improvement in crop tree regeneration and diversity, but on less fertile sites only the most intensive vegetation control proved beneficial.…”