Phytoclimatic Dynamics of Mediterranean Forests under Climate Change. A Case Study in a Southern European Pinus sylvestris L. Stand

Javier M. García-López, Carmen Allué

Regional Government of Castilla y León, Environmental Service, Burgos, Spain.
Regional Government of Castilla y León, Environmental Service, Burgos, Spain..

DOI: 10.4236/ajps.2013.43A084                

Abstract

Some effects of climate change on the composition and competitive capacity of southern European Pinus sylvestris L. forests in the Mediterranean basin were evaluated. The variation over the period 1910-2008 through 30-year mobile averages of a Phytoclimatic Suitability Index (PSI) of the main tree species of the forest cover are used to indicate the competitive hierarchy of Pinus sylvestris and Fagus sylvatica L. The methodology was applied at a specific location on the Spanish south-facing slopes of the Pyrenees mountain range in the Iberian Peninsula, where the increase in the average temperature was 1.4?C in the period of observation. The results indicated that the apparent equilibrium between the two species studied changed from the 1934-1963 average. Due to the loss of competitive capacity of Scots pine with respect to European beech, particularly from the years 1970-1999, the model predicted an inversion of the situation as it was up until now, so that beech had a higher PSI than pine. The phytoclimatic approach proposed here offers new methodological horizons for the study of the effects of climate change on the future of the forests.

Keywords

Biodiversity; Complexity; Suitability; Competition; Phytoclimatology; Scots Pine; European Beech

1. Introduction

Forests are particularly sensitive to climate change, because the long life-span of trees does not allow for rapid adaptation to environmental changes [1-3]. Thus, forest conservation, management and planning is becoming more challenging in the perspective of climate change, needing adequate links between monitoring of environmental indicators and management practices [4,5]. Forest ecosystems can respond to climate change by shifting distribution, by remaining in isolated pockets of suitable environment (refugia), becoming extinct or by adapting its composition or structure to changing conditions [6,7].

Particularly, the effects of climate change on the diversity of forest covers and on internal competitive relationships among the principal species constituting those covers seems likely to be one of the priority lines of research in the future [8,9]. Effective conservation strategies that offset the climate change threats to species persistence will be critical in maintaining species and genetic diversity [10,11].

Several studies on the effects of climate change on forest composition in the last few years have tended to focus on the variation in distributions of the main forest tree species in relation to previously defined climatic envelopes, e.g. [12,13]. Particularly, recent studies have dealt with several aspects of the influence of climate change on European forests as a whole, e.g. [3,12,14-17].

Species that lie at the limits of their natural range of distribution appear to be particularly sensitive to the effects of climate change [18,19]. One such case is Scots pine (Pinus sylvestris L.), the pine with the largest natural area and the most widespread in Europe and Asia. This species occurs more or less from east to west, from eastern Siberia to Galicia, and north to south, from Scandinavia to the south of the Iberian Peninsula (Sierra Nevada) where the most southerly specimens are found.

Scots pine is a naturally-occurring species in taiga forests in northern Europe and Asia, being its northern area of distribution more or less continuous on plains, whereas in the south, in the Mediterranean basin, it is becoming increasingly fragmented and confined to mountain areas [20]. Because of the considerable economic and ecological wealth of this species in Spain [21] the vulnerability of natural and artificial stands to climate change is a matter of particular interest and concern, especially considering that the mountain ecosystems of southern Europe may be among the worst affected by such change [22,23].

This paper reports the assay of a phytoclimatic indicator that serves to assess the possible influence of climate change on the physiognomic dynamics of arboreal covers of P. sylvestris L. in southern Europe. Several authors, e.g. [12], have reported the enrichment of European alpine forests with deciduous broadleaf species, particularly with European beech (Fagus sylvatica L.) in P. sylvestris stands in the Spanish Pyrenees [24]. No satisfactory account seems to have been offered so far of what part of these changes in forest composition are likely to be related to forest management practices [21] and what part may be due to climate dynamics [3].

Recently there has been an upsurge of interest in studying climate change in community rather than individual terms [25] and in terms of biotic interactions [26]. The methodology presented in this paper seeks to advance further in this area of research. The comparative variation over time of phytoclimatic suitability indices of P. sylvestris and other tree species present in forest formations serves as an indicator of the ability of these species to compete. This variation is also an indicator of the foreseeable composition and structure of the forest mass and the vulnerability of Scots pine to climate change, and is used in defining alternatives for future management of these pine stands.

2. Material and Methods

We used the phytoclimatic niche-based model CLIMPAIR [27], which simultaneously assesses non-linear, non statistical and dual measurements of proximity/potentiality of a target site with respect to climatic ranges of plant/vegetation units (i.e. species or woodland types).

The traditional environmental space is founded in classic ecological niche theory [28] and is defined by climatic variables regarding temperature and precipitation. Through a specific transfer function [29], this space is replaced by a suitability space. This set of phytoclimatic distances evaluates the climatic suitability of a site for a species.

The model considers a 12-dimensional climatic hyperspace corresponding to the n = 12 climatic variables (Table 1). Records of the occurrence of P. sylvestris and F. sylvatica forests are represented by a swarm of points in 12-dimentional climatic hyperspace. In this way, within the geographical scope of the model the respective species ranges or realised niches [30] can be explicitly defined by calculating convex hulls. Although many shapes could be used to enclose the points, the convex hull is defined as the smallest convex set enclosing them [31] and therefore reduces the amount of empty space compared with other volumes like parallelepipeds [32]. Each range thus takes the form of a convex hyperpolyhedron which can be explicitly defined by a set of vertices and linking edges.

In view of the complexity of explicitly determining the hyperpolyhedron for n > 2, it is best calculated by means of projections onto m climatic planes with no V_iV_j repetition formed by climatic variables V_i and V_j, where

(1)

Each variable appears in n − 1 V_iV_j projections (and). The number of possible

Table 1. Phytoclimatic variables used.

combinations is less than a factorial to avoid redundancies, as projections V_iV_j and V_jV_i give the same information. For the n = 12 climatic variables used m = 66 polyhedrons are made for the two tree species (Equation (1)).

A target point P whose coordinates are (V_iP; V_jP) in the plane V_iV_j is suitable for a specific woodland type if it occurs inside the polyhedron that represents the projection of the plane of the full range of the type, i.e. the entire hyperpolyhedron in the climatic 12-dimensional hyperspace. The target point’s position with respect to the projected polyhedron is related to its degree of compatibility or suitability by a transfer function.

The phytoclimatic position function used is a parabolic function (Equation (2)).

(2)

Between the two bounds of a variable Vi (Vi_max and Vi_min), the maximum of the curve is not a peak but a plateau or large central zone of maximum compatibility or fitness where the function takes the value 1 for a target point P. Then there is a sharp decline outwards to the edges, where it takes the value 0, and outside this range the values are negative. This effect can be achieved with a value = 2 [27,33]. assesses the capacity to predict species occurrences that the value Vi_p of a climatic variable V_i acquires at a site P and can be estimated as the inverse of the number of species considered in the model with which Vi_p is compatible.

assumes that the degree of suitability for a woodland type will be greater in occurrence sites situated far from the bounds of their 66 convex climatic ranges (), and the average of its m values in the m climatic planes can be used as a Phytoclimatic Suitability Index (PSI). Unlike statistical models, the concept of suitability in CLIMPAIR is not related to the frequency of species occurrence within their geographical ranges, and therefore the phytoclimatic position function cannot represent truncated, skewed or bimodal species response in the climatic space.

Thus, using CLIMPAIR we can generate a diagnostic spectrum of the following type in abbreviated annotation (PSI_Psy; PSI_Fsy), where PSI_Psy and PSI_Fsy (>=0 and <=1) are indices of relative phytoclimatic suitability of P. sylvestris (PSI_Psy) and F. sylvativa (PSI_Fsy) forests species with respect to the theoretical optimum (PSI = 1).

The methodology has been put into practice at the Spanish Canfranc weather station, located at an altitude of 1168 m in the Pyrenees range (lat. 42˚44'57''N, long. 00˚31'04''W), over a period stretching from 1910 to 2008 (Figure 1). The dominant forest species is currently P. sylvestris with some F. sylvatica. The last few decades

Figure 1. Situation of Canfranc weather station in the Pyrenees range.

have seen an advance of deciduous broadleaf species in areas where conifers have traditionally been predominant.

The phytoclimatic dynamics were assessed with 30- year mobile averages of climatic variables in order to filter the natural climatic variability [34] at the Canfranc station over the period from 1910 to 2008.

The phytoclimatic diagnostic model was applied to the set of climatic values in 30-year mobile averages to generate the diagnostic grid and the variation of PSI. We define D(t) = PSIPsy − PSIFsy as the difference between the suitability indices of P. sylvestris and F. sylvatica, and t is the time indicator of the series, which takes the value 1 for the first moving average (1910-1939) and 70 for the last one (1979-2008). A comparative statistical evaluation of the trend of the time series of PSI for the two target species was performed by fitting the values of the series to a time function by regression analysis. Several models of evolution with time (linear, logarithmic, quadratic, cubic, logistic, potential and hyperbolic) were assayed.

3. Results

Figure 2 shows the evolution of climatic values in 30- year mobile averages between 1910 and 2008 for the Canfranc station. We can see that precipitations are relatively stable but there is a clear increase in temperatures based on the mobile averages starting in the 1950s. The mean annual temperature (T) in particular increased by 1.4˚C between the first (1910-1939) and the last mobile average (1979-2008), and the average of minimum temperatures (TMMF) increased by 2.1˚C in the same period.

Conflicts of Interest

The authors declare no conflicts of interest.

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