Predicting species dominance shifts across elevation gradients in mountain forests in Greece under a warmer and drier climate
The Mediterranean Basin is expected to face warmer and drier conditions in the future, following projected increases in temperature and declines in precipitation. The aim of this study is to explore how forests dominated by Abies borisii-regis, Abies cephalonica, Fagus sylvatica, Pinus nigra and Quercus frainetto will respond under such conditions. We combined an individual-based model (GREFOS), with a novel tree ring data set in order to constrain tree diameter growth and to account for inter- and intraspecific growth variability. We used wood density data to infer tree longevity, taking into account inter- and intraspecific variability. The model was applied at three 500-m-wide elevation gradients at Taygetos in Peloponnese, at Agrafa on Southern Pindos and at Valia Kalda on Northern Pindos in Greece. Simulations adequately represented species distribution and abundance across the elevation gradients under current climate. We subsequently used the model to estimate species and functional trait shifts under warmer and drier future conditions based on the IPCC A1B scenario. In all three sites, a retreat of less drought-tolerant species and an upward shift of more drought-tolerant species were simulated. These shifts were also associated with changes in two key functional traits, in particular maximum radial growth rate and wood density. Drought-tolerant species presented an increase in their average maximal growth and decrease in their average wood density, in contrast to less drought-tolerant species.
KeywordsMediterranean mountainous forests Greece Forest gap models Modelling Drought Climate change Trait plasticity
In areas surrounding the Mediterranean Basin, forests are an important element of the established vegetation, covering around 20–30% of the total land areas in the northern part and reaching up to 50% in Greece (Archibold 1995; Scarascia-Mugnozza et al. 2000). These forests are expected to experience warmer and drier conditions in the near future due to global warming (Giorgi and Lionello 2008; Gualdi et al. 2013), as well as potential shifts in fire frequency driven by both climatic and anthropogenic forcing (Barbero et al. 1990; Pausas 2004; Moriondo et al. 2006). Observational evidence of shifts in forest structure and function over the last century has started to accumulate and are frequently attributed to climatic changes. Tree growth changes related to temperature increase (Jump et al. 2006; Linares and Tíscar 2010), decline in precipitation (Sarris et al. 2011) and/or CO2 fertilisation (Martínez-Vilalta et al. 2008) have been documented at both lowland and upland Mediterranean regions. Forest dieback has been attributed to drought (Van Mantgem et al. 2009; Allen et al. 2010), and/or to pathogens outbreaks (Desprez-Loustau et al. 2006; Chrysopolitou et al. 2013), while drought-induced changes in species composition have also been reported (Allen and Breshears 1998; Peñuelas and Boada 2003). Furthermore, some studies report an increase in fire frequency associated with the recent warming, both in the north-western (Pausas and Fernández-Muñoz 2012) and the north-eastern (Koutsias et al. 2013) part of the Mediterranean Basin. The way these shifts will progress under future global change conditions is important in terms of nature conservation and climate change adaptation (Bonan 2008).
Mediterranean plants have evolved under low water availability and thus have developed a number of morphological and physiological adaptations that enable them to withstand prolonged drought periods (Sardans and Peñuelas 2013). Recent studies suggest that in a Mediterranean climate, there is a continuum of plant drought performance, ranging from fast-growing deciduous species with a high resource use and high drought vulnerability to conservative slow-growing evergreen species with low water use and high drought tolerance (Lopez-Iglesias et al. 2014). The two ends of the spectrum reflect a drought tolerance versus a drought avoidance strategy, and functional traits such as rooting depth per leaf area, relative growth rate and net carbon assimilation rate were found to be good predictors of seedling drought survival time (Lopez-Iglesias et al. 2014). Seedlings of fast-growing species were less drought-tolerant in contrast to slow-growing species that exhibited a higher drought tolerance. A similar (weak) trade-off between growth and survival has been reported for mature trees with wood density being a good (negative) predictor of relative growth rate (Martínez-Vilalta et al. 2010). The above suggest that based on their functional configuration, Mediterranean species are responding individualistically to drought and thus their distribution could be controlled by water availability (Piedallu et al. 2013).
Simulations from both local- and global-scale vegetation models suggest that forests established under a Mediterranean climate are particularly vulnerable to climate change (Morales et al. 2007; Fyllas and Troumbis 2009; Hickler et al. 2012), although the large climatic stochasticity of Mediterranean ecosystems (Blondel and Aronson 1999) could increase the uncertainty in such modelling exercises. Under climate change conditions, some typical Mediterranean forests dominated by species such as Pinus halepensis are projected to be more resilient than others that are mainly found in the temperate zone (Keenan et al. 2011). One of the key drivers of vegetation and/or productivity shifts in these predictions is the increased soil moisture deficits, following an increase in temperature and a decrease in precipitation under climate change (Morales et al. 2007). However, other factors, such as fire frequency and CO2 fertilisation, could also interact with water limitation, leading to complex ecosystem responses (Fyllas and Troumbis 2009; Keenan et al. 2011). Disentangling the role of water limitation, fire and CO2 in forest ecosystem processes is important in order for their current dynamics and future risks to be better understood. Field studies, specifically designed to constrain the way such processes are simulated in vegetation dynamics models, could increase our understanding of forest function under current conditions and enhance our confidence in the projections of their future state.
In addition to the above, the role of inter- and intra-specific functional trait variation has been recently highlighted as an important component that needs to be incorporated into vegetation dynamics model (Scheiter et al. 2013; Fyllas et al. 2014; van Bodegom et al. 2014; Sakschewski et al. 2015). Traditionally, parameterisation of species and/or plant functional types (PFTs) is based on the use of some “average” or “appropriate” mean trait value, for characteristics that have a direct effect on the regeneration, the growth and the mortality of the simulated species. For example, specific leaf area has been used as a parameter to differentiate the turnover rate of leaf biomass between PFTs (Sitch et al. 2008), or as a parameter to describe the architecture (in terms of foliage area/biomass) of different tree species (Bugmann 2001; Fyllas et al. 2007). The selection of one “average” trait value could lead to “static” model behaviour as the population variability in the response of species/PFTs is a priori restricted, just because of the constant value given to some key functional characters (Fyllas et al. 2012, 2014). Ignoring this intraspecific variability is not in agreement with what is observed in real plant communities and comprises a key element of natural selection and evolution. In addition, depending on the way vegetation dynamics models are built, variability in some functional characters could affect more than one simulated process, through either direct or indirect routes. For example, given that most vegetation dynamics models include a “carbon starvation” mortality term, the influence of a specific growth parameter on model behaviour could be manifested directly through growth and indirectly through mortality. Ignoring functional variation in simulated plant communities could be an important bias, especially when projecting vegetation dynamics under climate change conditions, where alternative “functional configurations” could lead to viable life strategies.
Individual-based modelling is a tool widely used to simulate vegetation dynamics (Grimm et al. 2006). Forest gap dynamics models are a special group of individual-based models that follow the life of each tree in a stand and simulate key processes of interest such as regeneration, competition and mortality (Bugmann 2001). Gap dynamics models have a long history in modelling forest ecosystems structure and function with applications all over the world (Shugart 1984; Bugmann 2001; Fyllas et al. 2007; Ngugi et al. 2013). Because these models are based on empirical equations of growth and mortality, they provide reasonable approximations of stand growth, succession and disturbance patterns. Furthermore, as these models focus on individual tree performance, they provide an excellent framework to consider intra-specific trait variability and explore the potential shifts in species- or community-level trait variation under changing environmental conditions. However, to our knowledge there is no study that incorporates trait variability in the widely used forest-gap dynamics modelling framework.
Evaluate the predictive ability of the model and compare the “static” (single trait values) versus the “plastic” (varying trait values) model set-up under current climatic conditions.
Explore how the species composition and functional variation of these forests will respond to a gradually warmer and drier climate.
Materials and methods
Study sites and dominant tree species
Study site description
Pindos South (Agrafa)
Pindos North (Valia Kalda)
Elevation range (m asl)
Average annual temperature (°C)
Total annual precipitation (mm)
P. nigra, A. cephalonica
Q. frainetto, A. borisii-regis
P. nigra, F. sylvatica
A detailed description of GREFOS model is provided elsewhere (Fyllas et al. 2007). The model has been developed, parameterised and used for forest species found in the north-eastern part of the Mediterranean area (Fyllas et al. 2007, 2010; Fyllas and Troumbis 2009; Kint et al. 2014). GREFOS takes into account the discrete life history strategies (LHS) of Mediterranean tree and shrub species (Pausas 1999), by assigning a distinct recruitment density and resprouting capacity to each LHS. Regeneration in the model is based on empirical relationships between stand-level LAI and recruitment density (Fyllas et al. 2008, 2010), where a maximum threshold of LAI “ceases” the establishment of saplings due to light limitation. Individuals are competing for light through a height-based hierarchy with taller trees shading all smaller ones. A daily soil water balance model is used to calculate relative water content (θ) and subsequently the annual drought duration in order to adjust growth (Granier et al. 1999; Fyllas and Troumbis 2009). Evaporation is estimated following the Priestley and Taylor (1972) method, while the pedotransfer functions of Wösten et al. (1999), along with site-specific soil texture and depth measurements, are used to calculate soil water retention and release parameters.
As in most forest gap dynamics models, annual tree growth is estimated through the concept of optimum diameter increment, i.e. the growth that an individual of a certain species and size can reach under no-resource limitation or competition (Moore 1989; Bragg 2001; Risch et al. 2005). The “actual” diameter increment is subsequently estimated by adjusting the optimum diameter growth, as a function of the abiotic (temperature and water availability) and biotic (shading) conditions that prevailed for a given time for each tree in the stand. Here we use a novel ring width data to estimate the parameters (and their variation) of a commonly used optimum growth equation (Zeide 1993), as described in “Optimum growth curve and intraspecific plasticity” section.
In GREFOS mortality has three components. The growth-related component (“carbon starvation”) estimated as a function of a tree’s past growth, the background (“intrinsic”) mortality representing species longevity and the fire-related mortality, which is linked to species LHS. Background mortality is usually estimated in forest gap models through species longevity or maximum size. Here we use wood density as a proxy for background mortality (Martínez-Vilalta et al. 2010). Intra- and inter-specific variation in wood density is incorporated in the model based on a novel wood density data set, as described in the “Background mortality” section. Species with a higher wood density generally have lower growth and mortality rates (Reich 2014). This growth versus survival trade-off has been incorporated in the model by hardwiring a relationship between maximum diameter growth rate and wood density based on the combined analysis of the tree ring width and wood density data sets (“Growth–longevity trade-off” section).
Optimum growth curve and intraspecific plasticity
In order to parameterise the diameter growth curve, a minimum of 30 cores were taken from trees of each dominant species at each study area (Fyllas et al. 2017). All cores were collected at breast height with a 5-mm increment borer. In the laboratory, the cores were glued on channelled wood, dried at room temperature and sanded with progressively finer-grade abrasive paper until cells were clearly visible under magnification. All samples were visually cross-dated using visual recognition of tree ring patterns and lists of marker years (those with narrow rings) (Yamaguchi 1991). Tree ring widths were measured to 0.01 mm using Time Series Analysis and Presentation (TSAP) software package and LINTAB measuring table. Raw ring width series were synchronised according to their Gleichläufigkeit score, which represents the overall accordance of two series t values, which are sensitive to extreme values such as marker years and the cross-date index (CDI), which is a combination of both (Rinn 2003). Finally, the COFECHA software was used to perform a data quality control and to evaluate the cross-dating (Grissino-Mayer 2001).
Growth curve parameter estimates along with their confidence intervals for each tree species
Gm (mm year−1)
Mean wood density estimates along with their confidence intervals for each tree species
DW (g cm−3)
At each study area, the model was set up to simulate stand dynamics along an elevation (500 m long) gradient, with an step of 50 m. Soil depth was set to 2.0 m in all study sites. All simulations started from bare ground and lasted for a 1000-year-long simulation period. In this study, the fire component of the model has been disabled in order to explore for merely the effects of drought. At each study site, only the two species known to occur abundantly are allowed to establish. It should be noted that these two species are the dominant elements of vegetation accounting for more than 80% of the total basal area in our study plots. Two climate scenarios were used, namely: (1) the baseline (BL) climate representing the current climatic conditions with the climate of the twentieth century at each study area randomly repeated for the simulation period and (2) the IPCC A1B climate change (CC) scenario with an approximately 3 °C increase in temperature and 20% reduction in precipitation taken as one of the intermediate projections cases from an ensemble regional climate model projections for the Mediterranean area (Gualdi et al. 2013). The baseline climate was extracted from the E-OBS gridded climatology (Haylock et al. 2008) for the time period between 1950 and 2013. Across the elevation gradients, temperature was corrected with a lapse rate of 6.5 °C km−1. Precipitation was assumed not to change with elevation. In both cases, a spin-up period of 500 years, during which the observed 1950–2013 climate was randomly replicated, was used until vegetation reached an equilibrium with climate. For the CC scenario during the spin-up period, climate was assumed to be similar to BL conditions, followed by a transient period of 100 years during which temperature and precipitation anomalies were linearly applied until climate stabilised after simulation year 600.
The model was applied following a “static” (no intraspecific variability) and “plastic” (with intraspecific variability) set-up, and 30 iterations were performed for each elevation and climate scenario. The steady-state (simulation year 600–1000) average basal area of each species under baseline conditions was estimated and compared with local observations. Changes in the species average values of the two key traits, Gm and DW, with time and climate scenario were also explored. Potential shifts in these traits indicate the way species could adapt to warmer and drier conditions, by adjusting the two traits that are directly linked to their growth and mortality.
At Mt. Taygetos, P. nigra’s average wood density and maximum growth showed no clear trend with elevation (Fig. 6). Under climate change conditions, an overall small decrease in DW, associated with an increase in Gm for P. nigra, was simulated. For A. cephalonica, a small decrease in DW with elevation was simulated, especially under BL conditions. At Agrafa, DW increased and Gm decreased with elevation for the more drought-tolerant Q. frainetto under BL conditions. This trend was not as strong under CC conditions (Fig. 7). No clear trend in DW or Gm was simulated for A. borisii-regis although at higher elevations, the average DW value was smaller compared to lower altitudes. At Valia Kalda (Fig. 8), the elevational shifts of simulated DW and Gm were more pronounced. A lower Gm and a higher DW were achieved at higher elevation for the more drought-tolerant P. nigra, with the opposite trend found for F. sylvatica. Overall, Gm increased and DW decreased under CC conditions for the more drought-tolerant species, while the opposite was true (decreasing Gm and increasing DW) for the less drought-tolerant species across all sites.
This study presents the parameterisation of a forest gap model, with novel tree ring width and wood density measurements, aiming to better understand and to project the dynamics of typical mountainous forests in Greece. The inclusion of species-specific parameters leads to a realistic prediction of the distribution patterns and the basal area of the dominant tree species, observed in three study sites under current climate. In general, the predicted steady-state average basal area is accurately simulated for lower elevations and early successional species, but was slightly underestimated at higher elevation and late successional species. By accounting for intraspecific variability in growth and mortality through two functional traits (maximum growth rate and wood density), the model simulates a higher contribution of slow-growing species across all elevations. We suggest that vegetation dynamics models, and particularly those that are implemented at local scales, should be constrained with site-specific information that takes into account the variability of key functional traits.
The inclusion of growth and mortality plasticity is an important aspect of this work. Currently, there is great effort to include functional trait variation into vegetation dynamics models, in order to account for the potential plasticity in the ecosystems’ response to climate change (Scheiter et al. 2013; Fyllas et al. 2014; van Bodegom et al. 2014). In this study, we present a simple method to implement trait variation within existing forest gap dynamics models, based on the measured intraspecific variability in functional traits associated with tree growth and longevity. Our approach is based on the inclusion of a fundamental trade-off between growth and survival. Wood density is selected as the core trait to represent this trade-off. Lower wood density is related to higher growth and mortality rates both within and between the species of interest. On the other hand, higher wood density is associated with smaller maximum growth and mortality rates. By parameterising this theoretical trade-off with species-specific measurements of tree growth and wood density, the model adequately captures the vegetation dynamics in our three study sites. Additional axes of functional trait variation that represent ecophysiological trade-offs could be potentially integrated in this modelling framework in future studies. These could relate to species hydraulic properties and/or response to disturbance (Pausas 1999; Sánchez-Gómez et al. 2006).
In all study sites, CC simulations suggest an upward shift in the dominance patterns, with more drought-tolerant species (Pinus nigra and Quercus frainetto) increasing their contribution with elevation. The inclusion of trait plasticity does not significantly alter the CC simulations outcomes, with the only exception at Mt Taygetos, where the A. cephalonica was more persistent at higher elevations when trait plasticity was included. The overall trend of shifting dominance patterns with CC is related to the drought tolerance of the studied species. Compared to A. borisii-regis and F. sylvatica, the higher elevation dominant species at South and North Pindos, A. cephalonica is considered more drought tolerant, and thus, it maintains its contribution to the stand’s basal area under CC conditions. It should be noted that species response to drought is simulated in this version of the model by counting the number of days with water stress that feeds into a species-specific drought response function (Granier et al. 1999; Fyllas and Troumbis 2009). Hence, there is no direct effect of trait plasticity to a species and/or individual’s drought tolerance. Various studies suggest a relationship between cavitation resistance (Hacke et al. 2001) and drought tolerance (Poorter and Markesteijn 2008; Preston et al. 2006, but see Hoffmann et al. 2011) with DW, and thus, such a relationship could be implemented in future versions of this modelling framework.
In this study, simulated shifts in species dominance were also associated with changes in the two functional traits used to describe variation in growth and mortality. In all three study sites, the CC scenario resulted in higher Gm of the drought-tolerant species (P. nigra and Q. frainetto) and lower DW. Shifts in Gm and DW were not as clear for the less drought-tolerant species, with the exception of F. sylvatica at North Pindos, where a systematic decrease in Gm and increase in DW were simulated. These outputs suggest that across the studied elevation gradients, species that are able to tolerate longer dry periods could increase their growth rate in contrast to less drought-tolerant species. It should be noted that our modelling framework not only is taking into account the uncertainty of vegetation processes (Reyer et al. 2016) by including variation in growth and mortality, but also enables tree populations to shift their functional characteristics. This is particularly important in terms of adaptation to changing biotic and abiotic conditions.
Empirical evidence of species upward shifts (Peñuelas and Boada 2003; Moser et al. 2010; Pauli et al. 2012) has been documented, and modelling exercises identify the vulnerability of Mediterranean ecosystems to climate change (Morales et al. 2007; Fyllas and Troumbis 2009; Hickler et al. 2012). Simulations in this study suggest that climate change could lead to significant shifts of species distribution in mountainous Mediterranean forests. The importance of considering intraspecific variability for modelling purposes is highlighted here. Although in two of the study sites incorporating trait variability did not “enhance” the ability of the least drought-tolerant species to adapt to a changing climate, in Mt Taygetos the response of A. cephalonica to drier conditions was more gradual under the plastic model set-up. Such responses will be dependent on local environmental conditions as well as the ecophysiological ranges of species performance. Field studies that quantify these ranges as well as the way functional traits coordinate and interact to form fundamental ecological strategies could help us better parameterise models of vegetation dynamics.
This study presents the application of a forest gap dynamics model to explore the potential effects of drought on the dynamics of mountainous Mediterranean forests in Greece. Emphasis was given on incorporating intraspecific variability in growth and mortality. Simulations under climate change conditions suggest an upward shift of the more drought-tolerant species. These changes are also accompanied by intraspecific shifts in two key functional traits that express the growth and mortality patterns of the tree species. In general, populations of more drought-tolerant species increase their maximum radial growth and decreased their wood density in contrast to less drought-tolerant species. Incorporating trait variability and accounting for fundamental ecological trade-offs in vegetation dynamics models could increase the realism in projecting the fate of forest ecosystems under global change conditions.
We acknowledge the E-OBS data set from the EU-FP6 Project ENSEMBLES (http://ensembles-eu.metoffice.com) and the data providers in the ECA&D Project (http://www.ecad.eu). This work was financed by the “Mediterranean Forests in Transition/MEDIT” Grant to NF. The research project is implemented within the framework of the Action “Supporting Postdoctoral Researchers” of the Operational Program “Education and Lifelong Learning” (Action’s Beneficiary: General Secretariat for Research and Technology) and is co-financed by the European Social Fund (ESF) and the Greek State (PE10 (927)).
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