Climate-mediated regeneration occurrence in Mediterranean pine forests: A modeling approach
Introduction
Natural regeneration is a key element in the evaluation of long-term sustainability of forest systems. The success of natural regeneration is dependent on different well-defined stages such as seed dispersal, predation, germination, emergence and establishment (Calama et al., 2017, Manso et al., 2014a) whereby the degree of success of one of these stages has a direct effect on the success of the next. Hence, regeneration establishment is directly dependent on the success of the regeneration emergence phase (Hunziker and Brang, 2005). Furthermore, during the change in ontogeny (seedlings to saplings), the species are influenced differently by the vegetation structure, species composition, functional attributes as well as climate conditions (Walker et al., 2007), developing different species-specific regeneration strategies (degree of shade tolerance, water use efficiency or resistance to drought). This fact, may explain the regeneration success of one species over the other and subsequent shifts in the specific composition of forest stands.
Taking into account the species-specific regeneration strategies is essential in order to determine how regeneration fellings contribute to and interact with the regeneration process (Stancioiu and O’Hara, 2006, Webb and Jarrett, 2013, Yoshida et al., 2005). In those areas where the establishment of natural regeneration is expected to be less successful due to climate change, this knowledge may be crucial. For instance, it has been observed that the role of tree canopy protection will be more important as the climatic conditions become more stressful, especially in extremely dry, hot summers (Valladares et al., 2005). It has also been well documented that some species require a progressive release of overstory cover, probably due to an increase in light requirements (Manso et al., 2014a). However, to promote favorable conditions for the occurrence and development of natural regeneration and to reduce risks, the exact moment at which these silvicultural treatments must be applied varies depending on the species, site and climate conditions (Calama et al., 2017). Therefore, the modeling of these ecological drivers, such as incoming light, temperature, etc. can be useful to improve management methods and expand our understanding of the regeneration processes of tree species.
Modeling natural regeneration has mainly been addressed using two different approaches (Kolo et al., 2017): (i) Recruitment models, which evaluate the established regeneration as a function of the environmental characteristics (Eerikäinen et al., 2007, Fortin and DeBlois, 2007); (ii) Multistage models, which consider regeneration as a multistage process consisting of underlying consecutive subprocesses –flowering & fruiting, seed dispersal, predation, germination and survival (Manso et al., 2014b, Ordóñez et al., 2006, Pukkala and Kolström, 1992). While the usefulness of these models has been widely demonstrated, current environmental concerns as well as new management challenges related to global change, species interactions, and larger management scales demand the development of new modeling tools (Costanza et al., 1990). Baker et al., 1991, Baker, 1994 improved the model formulation by simulating disturbances and regeneration of forest patches on very large landscapes (DISPATCH). These regional-level models can be used to define new strategic regional conservation and management planning since they allow the integration of new tangible information that can be used by managers to examine more complex, spatial scientific questions (Mladenoff, 2004).
Through re-measurement of permanent sample plots, regeneration occurrence can be defined as the number of new seedlings that emerge in a specific plot within a given time interval. Survival or lifetime analyses comprise many methods for modeling the probability of occurrence of a particular event after time t (Lawless, 2003). In addition, these survival models allow us to predict the probability of regeneration occurrence when the exact date of the regeneration occurrence is unknown and data records are organized in annual-intervals, minimizing uncertainties in the estimation process (Yu et al., 2013). Hence it is necessary to use an annual resolution to evaluate the climate effect on the regeneration process.
Mediterranean pine forests located on the Northern Plateau of Spain are considered a vulnerable ecosystem to climate change (Bernier and Schoene, 2009). Pinus pinea L. and Pinus pinaster Ait. are the species which are generally favored in this area due to the high economic importance of edible pine nut production in the case of P. pinea and timber and resin production in the case of P. pinaster (Gordo et al., 2012). Both species share territory, ecological conditions, historical traits and occur in either monospecific or mixed stands (Calama et al., 2017). Unfortunately, natural regeneration of both species is often unsuccessful under the current regeneration methods (Manso et al., 2013a). Under future climatic scenarios, this situation could become worse with even lower rates of natural regeneration occurrence (De Castro et al., 2005). These future climatic conditions are expected to affect the width of the germination window in the case of P. pinea (Manso et al., 2013b), and to strongly decrease seed rain in the case of P. pinaster (Ruano et al., 2015). In addition, warmer and drier conditions are expected to negatively influence seedling survival in the first summer after emergence.
This study aims to develop a probabilistic model based on survival analysis techniques in order to predict P. pinea and P. pinaster natural regeneration occurrence in the Northern Plateau of Spain. To this end, a model was designed and fitted at the regional scale, taking into account the time since regeneration felling as well as different climate and stand level conditions. Our main hypotheses were that (i) dryer conditions in the Mediterranean area, characterized by the increase in temperatures as well as the decrease in precipitation, hamper the occurrence of P. pinea and P. pinaster natural regeneration (ii) in this regard, different climate scenarios could lead to differences in the probability of success in the natural regeneration process (iii) species-specific requirements, like shade tolerance, resource use efficiency or resistance to drought, as well as stand-related variables contribute to increasing, or decreasing, the occurrence of natural regeneration.
Section snippets
Study area
The study area was located in the Northern Plateau of Spain, in both monospecific and mixed stands of stone pine (P. pinea) and maritime pine (P. pinaster) under management in Valladolid province. This region is located in a flat sedimentary area defined by the river Duero Basin, with typical soils presenting a large percentage of sand and low amounts of nutrients. In this region, the two studied species cover approximately 68.000 ha. Mixed forests of both species account for 20% of the area,
Building approach
The sequential steps to construct the model for the occurrence of natural regeneration are shown in Table 2. The number of adult trees per species, the level of grass cover and the mixture effect entered in the proportional part of the model, thus modulating the baseline regeneration occurrence. The baseline h0 of the hazard model was finally defined by the mean spring and autumn temperatures as well as the maximum summer temperatures. We detected problems of convergence when entering the
Discussion
The results of this study corroborate our main hypothesis that climatic factors control regeneration occurrence and also point to the contribution of stand structure and composition to the success of this process. Taking into account these climatic and stand-structural drivers, we also identified those future climate scenarios where natural regeneration of the studied species is expected to be an issue.
Differences in regeneration patterns resulting from differing adaptation of species to
Acknowledgements
We are grateful to the Forest Service of the Junta de Castilla y León for conducting the field experiment. The author thanks Inforiego (www.inforiego.org) of the Instituto Tecnológico Agrario de Castilla y León for the climatic data. We also thank Adam Collins for revising the English. M. Vergarechea acknowledges the FPI scholarship program from the Spanish Ministry of Education. Finally, authors would like to express their gratitude to two anonymous reviewers whose comments and suggestions
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