Recruitment into the seedling bank of an undisturbed Mediterranean pinewood: Increasing forest resistance to changing climates
Introduction
In the northern Hemisphere, the low-altitude and southern limits of tree ranges are expected to move upwards and northwards as conditions become more arid due to ongoing climate change (Dyderski et al., 2018). This shift in species distribution should be the product of two processes: increased adult tree mortality and reduced self-recruitment, insufficient to compensate for adult decline. In this regard, some studies have documented recent episodes of adult mortality due to increased climatic dryness in the Mediterranean region (Candel-Pérez et al., 2012, Vilà-Cabrera et al., 2013). However, the long-term impact of climate change on tree recruitment may be difficult to assess for at least two reasons: (i) trees are long-lived, iteroparous plants that may fail many times before achieving successful regeneration, and (ii) desiccation is already the main cause of seedling mortality among Mediterranean tree species (Castro et al., 2005, Ruano et al., 2009, Matías et al., 2012, Tíscar et al., 2017). These limitations can be overcome by combining observational and experimental data as in the present study.
Despite the major importance of water availability for the successful regeneration of Mediterranean tree species, irradiance plays a general key role as well (Puerta-Piñero et al. 2007). The response of tree seedlings to the amount of light reaching the forest floor is specific. Therefore, foresters commonly rank tree species on a continuum of shade tolerance and apply regeneration methods accordingly, removing most trees from the stand in order to regenerate shade intolerant species, or maintaining a canopy of mature trees to allow the establishment of shade tolerant ones (O'Hara, 2014). This general rule can however vary under the stressful conditions of Mediterranean forests. Shade buffers against high temperatures and excessive evapotranspiration, and consequently improves water balance (Gómez-Aparicio et al., 2004). Thus, several studies have reported increased seedling survival of both shade tolerant and intolerant pine species under the canopy of taller plants in Mediterranean forests (Castro et al., 2005, Ruano et al., 2009, Tíscar and Linares, 2014, Andivia et al., 2018). The beneficial effects of shade are still present even when water is artificially added to simulate the stormy summers that sporadically occur in Mediterranean mountains (Gómez-Aparicio et al., 2008). As a result, shade by itself, or combined with increased water availability during atypically wet summers, may facilitate pine recruitment under the canopy of undisturbed stands. This generates an opportunity to consider tree regeneration in managed Mediterranean pinewoods not only in relation to silvicultural disturbances, but also in relation to the probable presence of seedling banks. Seedling banks are formed by those seedlings that develop beneath an intact canopy and the establishment of which is not consequently related to the entries of planned cutting cycles in managed forests (O'Hara, 2014). Mediterranean pinewoods currently tend to remain silviculturally undisturbed for decades, due to poor conditions of the timber market and/or the implementation of extended rotations for biodiversity conservation and carbon sequestration (Bravo et al., 2008). Consequently, whilst expanded drought due to climate change might prevent seedling establishment at the time planned by management cutting cycles, the seedling bank could be crucial for increasing forest resistance. Resistance was considered to be the ability to recruit new seedlings even though increasingly arid conditions will make this more difficult.
An increasing trend in temperatures and a decreasing trend in precipitation are already being observed in southern parts of the Iberian Peninsula (Linares and Tíscar, 2010, Ruiz-Sinoga et al., 2011). Considering this, I used both observational and experimental data to evaluate the impacts of the resulting more arid conditions on the recent recruitment dynamics of Pinus nigra subsp. salzmannii (P. nigra hereafter) in an undisturbed forest. By working with silviculturally undisturbed stands, I could quantify recruitment into the seedling bank. Forests of P. nigra occupy 405,983 ha in the eastern calcareous mountains of the Iberian Peninsula. Summer drought is considered to be the major limiting factor for the recruitment of P. nigra (Tíscar and Linares, 2011). However irradiance is equally important, as seedling establishment occurs more frequently under intermediate levels of canopy cover (Tíscar and Linares, 2014).
Models fitted to simulate forest occupancy under a climate change scenario project an important future decline of P. nigra forests (Benito-Garzón et al., 2008). Thus, some signs of range contraction could be already evident at the southernmost distribution limit of this species (Matías and Jump, 2014). In this regard, I used observational data obtained from 198 forest inventory plots to test whether the regeneration niche of P. nigra has moved upwards during recent decades by comparing current seedling and pole tree distributions along an altitudinal gradient that would reproduce projected climate changes in temperature (Rabasa et al., 2013). Specifically, I understood that a higher positive effect of altitude should be expected on the establishment of seedlings rather than on pole trees, because seedlings would have established under warmer, drier climatic conditions. Since water availability is decreasing for seedling establishment due to contemporary climate change, I also used experimental data from an irrigation experiment to analyse the influence of water availability on the establishment of P. nigra seedling banks (Matías et al., 2012). I additionally accounted for the effects of several variables acting at the microhabitat scale that could affect water infiltration and soil desiccation under closed canopies, namely micro-slope, soil compaction, depth and cover of the litter layer, and light availability. Hereafter, I will refer to all these variables including water availability as abiotic factors. Finally, I also used temperature and precipitation data to confirm climate trends towards more arid conditions in the study forest.
The following questions are specifically addressed: (1) is summer aridity progressively increasing in the study area? (2) is the range of P. nigra moving upwards in the southernmost forest of the species distribution? (3) what are the effects of abiotic factors on the recruitment of undisturbed P. nigra forests? (4) what is the potential role of seedling banks in the natural regeneration of managed P. nigra forests?
Section snippets
Study site
The study was carried out in the Cazorla, Segura and Las Villas Natural Park (southeast Spain) (37°54′N–2°53′W) in a managed forest named ‘Navahondona’. The site is part of the most extensive P. nigra forest at the southernmost limit of the species distribution area (Fig. A.1.). Climate is Mediterranean. Snowfalls and frost are common during the winter, but summers are dry and hot. The Navahondona forest covers 16456 ha along an altitudinal gradient from 780 to 2108 m above sea level (a.s.l.).
Climatic trends and recruitment of P. nigra into the forest seedling bank
The monthly mean temperature, precipitation and water balance (precipitation minus potential evapotranspiration) of the study site are shown in Table 1. The Mann-Kendall tests showed significant increases in the tendency of mean temperature since 1972. Thus, all months with the exception of September yielded significant temperature increases. A negative significant precipitation trend was found for the month of June, whereas water balance showed a significant negative trend in the summer months
Climatic trends and recruitment into the seedling bank
Results indicated, at the local scale provided by the study site, a trend towards hotter and drier summers in congruence with climate change models that predict increased aridity for the Mediterranean Region due to both progressive warming and decrease in precipitation (Giorgi and Lionello, 2008). Considering that elevated seedling mortality due to desiccation during the summer is generally acknowledged as the main limiting factor for P. nigra recruitment (Tíscar and Linares, 2011, Tíscar and
Acknowledgements
Consejería de Medio Ambiente y Ordenación del Territorio (Junta de Andalucía) provided the means to carry out the research. I am grateful to Benjamín Viñegla from the University of Jaén for lending me the SunScan plant canopy analyzer and the handheld soil moisture sensor.
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