**4. Discussion**

364 International Perspectives on Global Environmental Change

Alhama Genil Monachil Dílar Dúrcal Chico Soportújar Poqueira Trevélez

**B C**

**A A**

Alhama Genil Monachil Dílar Dúrcal Chico Soportújar Poqueira Trevélez

Alhama Genil Monachil Dílar Dúrcal Chico Soportújar Poqueira Trevélez

**A B C**

**<sup>C</sup> A C A B**

DMIN

**MIN**

MIN

**A**

**C**

**B C B C**

**A B A**

Northwest South

**d) MIN**

Northwest South

**f) DMIN**

**A B C D**

Northwest South

**D**

**B D A B D A B C**

**B C**

**B C D**

**C**

**B C D**

**A A B**

**b) EVI\_sCV**

**A B A A**

0.0 0.1 0.2 0.3 0.4 0.5 0.6

CV

CV

**<sup>D</sup> C D**

Alhama Genil Monachil Dílar Dúrcal Chico Soportújar Poqueira Trevélez

**A A**

**A**

Alhama Genil Monachil Dílar Dúrcal Chico Soportújar Poqueira Trevélez

**AC**

Alhama Genil Monachil Dílar Dúrcal Chico Soportújar Poqueira Trevélez

12 Jul 26 Jun 10 Jun

25 May 9 May 23 Apr 7 Apr 22 Mar 6 Mar 18 Feb 2 Feb 17 Jan 1 Jan 19 Dec 3 Dec 17 Nov 1 Nov

28 Jul

**MAX**

MAX

**MMAX**

DMAX

**A**

**B A B E** **C**

**A B E**

**a) EVI\_mean**

Northwest South

**c) MAX**

**A**

**A B**

**A**

**A**

**<sup>B</sup> A B**

Northwest South

**A B C**

Northwest South

**e) DMAX**

**BD A B C D**

**A B C D**

 Median 25%-75% Non-Outlier Range Outliers Extremes z

Fig. 5. Functional characterization of the oak woods of Sierra Nevada (Spain) based on the EVI attributes for the 2001-2009 period. Letters show significant differences in *post hoc* comparisons. a) EVI annual mean, an estimator of annual primary production; b) EVI seasonal Coefficient of Variation, a descriptor of seasonality; c) Maximum and d) Minimum EVI annual values, indicators of the maximum and minimum photosynthetic activity; Dates when the e) Maximum and f) Minimum EVI values are reached, indicators of phenology.

**D**

**<sup>C</sup> AB A B D**

**A C D** 

**Media EVI**

EVI\_mean

#### **4.1 Baseline conditions and trends in the ecosystem functioning of the Pyrenean oak woods of Sierra Nevada National Park**

Our approach, based on a time series of satellite-derived images of the EVI, provided a description of how different attributes of ecosystem functioning change across the remaining locations of Pyrenean oak woodlands in Sierra Nevada. This reference description provides the baseline conditions of ecosystem functioning that can be used to assess the effects of environmental changes on ecosystems processes. The Pyrenean oak woodlands of Sierra Nevada showed a unimodal EVI seasonal dynamics with a unique and well-defined growing season centered in summer and winter minima, as observed in previous works (Alcaraz-Segura et al., 2009a). Differences among locations mainly occurred during the winter non-growing season and at the beginning of the growing season (spring) and were mainly related to the location in the north or south slopes of Sierra Nevada. The lower EVI\_mean values in the northern oak woods (Figure 5a) are closely linked to the presence of lower winter MIN values than in the southern woods (Figure 5d) and with the more abrupt EVI decrease during the autumn. In contrast, southern woods maintained relatively high EVI values throughout their longer growing season (Figure 4). The greater annual vegetation greenness of southern woods is probably due to the greater incidence of solar radiation that favors longer growing seasons, milder temperatures during the winter, and an extra water supply from humid air masses coming from the Mediterranean sea that compensate the very high evapotranspiration rates during the summer, in comparison to the colder and more continental locations of the northern slope (Costa Tenorio et al., 2005). Contrary, summer maximum EVI values (MAX) would not cause significant differences in annual vegetation greenness between the northern and southern locations. In consequence, the northern slope shows much greater seasonality (EVI\_sCV) than the southern slope since MAX values are similar in both orientations, though the northern woods showed lower MIN values than the southern ones (Figure 5d). From the analysis of the shape of the EVI seasonal curves and according to previous studies (Alcaraz-Segura et al., 2009a), the main limiting factors for vegetation greenness in the oak woodlands of Sierra Nevada are low winter temperatures and lower solar irradiation in the northern slope, which favors a longer presence of snow (Figure 5d). An important point to consider is that the greater vegetation greenness of the southern woodlands during the non-growing season is not related to the activity of the oak trees (because they are winter semi-deciduous), but to the shrubs and herbaceous vegetation occupying the undergrowth vegetation and the patches without trees (Figure 8). In the same way, since the snow melt happens faster and earlier in the southern woods, undergrowth vegetation is also responsible for the earlier and more pronounced rise in vegetation greenness during the start of the growing season than in the northern woods (Figure 3).

Our study also showed that though the oak woodlands of Sierra Nevada have not experienced significant changes of the EVI\_mean during the 2001-2009 period, they have suffered seasonal functional changes that mainly affected the beginning of the growing season. In contrast to this relative stability of annual mean vegetation greenness (EVI\_mean) since 2001, previous evaluations showed a significant increase in vegetation greenness throughout the eighties and nineties in Sierra Nevada (see Alcaraz-Segura et al., 2008b for the 1981-2003 period, and Alcaraz-Segura et al., 2009b for the 1982-2006 period). Such evaluations used the GIMMS-AVHRR (Global Inventory Modelling and Mapping Studies - Advanced Very High Resolution Radiometer) NDVI dataset. Though there is some debate on the existence of a long-term bias in the GIMMS dataset towards NDVI increases in some

Satellite-Based Monitoring of Ecosystem Functioning in Protected Areas:

Recent Trends in the Oak Forests (*Quercus pyrenaica* Willd.) of Sierra Nevada (Spain) 367

approach for the Park management, the monitoring program should include the identification of the key ecological processes that can be related to this functional description and that are central for the maintenance of the ecological integrity. For instance, the differences in the strength of the EVI trends among different oak forest patches could be associated to the two modes of climatic variability that affect Sierra Nevada. The observed weaker start of the growing season during study period could be related to the increase of positive phases of the North Atlantic Oscillation (NAO Index), which are the main control of winter precipitation and temperature, particularly in the north-western slope (Liras, 2011). In addition, we also observed EVI increases during the summer (July-August) in the southern slope (Figure 4), which could be related to the increase of active phases of the Western Mediterranean Oscillation (WeMO), increasing late summer precipitation during the study period (Liras, 2011; Cabello et al., Accepted). In this sense, the obtained results in the EVI trends for the different woods could be used to prioritize management actions in relation to climate change adaptation in the most threatened sites. Nevertheless, this should be only one of the guiding hypotheses for adaptive management, since other processes such as insect damage and forest succession could also be taking place in the park (Sierra Nevada

National Park managers, *personal communication,* Stöver et al., 1996; CMJA, 2008).

**5. Conclusions** 

without affecting the annual means.

A monitoring system based on the tools and analysis shown here could embrace several monitoring objectives, as it simultaneously informs managers about the changes in productivity, phenology, and seasonality of the ecosystems. For example, changes in the EVI attributes could be directly related to changes in the amount, seasonality, and phenology of ecosystem carbon gains. In addition, linking the EVI dynamics of the Pyrenean oak woodlands to the ecology of species of conservation concern could be used to evaluate and monitor the conservation status of the habitat of such species. This could be the case of the blue tit (*Parus caeruleus*), whose reproductive success is related to the ecosystem status of *Quercus pyrenaica* forests, especially at the beginning of female reproductive period (April-May), which is associated with the start of the growing season (Arriero et al., 2006). Such association implies that delays in the start of the growing season or forest degradation would negatively affect the reproduction success of this bird. Moreover, the information derived from this monitoring approach could help guiding land-use planning to avoid overexploitation of Sierra Nevada oak woodlands. For instance, livestock pressure should be limited in those periods of the year that are experiencing strong negative EVI trends.

Our approach shows how satellite based monitoring systems can be very useful to assess the effects of environmental changes on protected areas and to orientate adaptive management actions. Overall, this study provides a reference characterization against which to assess changes in ecosystem functioning of the oak woods of Sierra Nevada, and identifies functional changes that occurred during the 2001-2009 period. Such information helps to fill the lack of knowledge about these woodlands, as demanded by the Spanish Ministry of Environment (García & Mejías, 2009). In practical terms, it allows the incorporation of ecosystem functional aspects of ecosystems to nature conservation and to the maintenance of ecosystem services, in particular those related to carbon sequestration in this protected area. Our results imply that conservation and management policies cannot be only based on static situations, since ecosystems are changing. In addition, annual summaries are not enough as monitoring indicators, since functional changes may occur at key seasonal stages

regions of the world including the Canadian Boreal forest (Alcaraz-Segura et al., 2010a) and South America (Baldi et al., 2008), the NDVI increases observed in Sierra Nevada with GIMMS during the 1980's and 1990's agreed with other independent datasets. Alcaraz-Segura et al. (2010b) showed that the positive NDVI trends that Sierra Nevada displayed in previous studies with the GIMMS dataset were observed for the 1981-1999 period using other independent datasets such as PAL (Pathfinder AVHRR Land), FASIR (Fourier-Adjustment, Solar zenith angle corrected, Interpolated Reconstructed), and LTDR (Land Long-Term Data Record) datasets. Positive NDVI trends were also observed in Sierra Nevada during the 1989-2002 period using the MEDOKADS (Mediterranean Extended Daily One-km AVHRR Data Set) archive (Martínez & Gilabert, 2009).

The EVI decrease observed at the beginning of the growing season during the 2000-2009 period in Sierra Nevada oak woodlands (Figures 3 and 4), is also in contrast with the NDVI seasonal increase in autumn, winter, and spring that was reported for the 1982-2006 period using GIMMS images of the entire Park (see Figure 2 in: Alcaraz-Segura et al., 2008a). Such contrasting trends lead to think that the increase of spring vegetation greenness that occurred throughout de eighties and nineties (Alcaraz-Segura et al., 2008a) ended around the year 2000 when the spring started to return to lower greenness values. Yet, the trends towards greater vegetation greenness in autumn and winter reached during the eighties and nineties (Alcaraz-Segura et al., 2008a) was maintained after the year 2000, since we did not find significant EVI trends in these seasons. The strong EVI decreases at the beginning of the growing season and the presence of some EVI summer increases during the senescence period lead to think that the growing season of southern oak woods (Figure 4) might be starting later but strengthening towards the summer (with the exception of Poqueira; Figure 4c).

An important outcome of our work is that significant functional changes, i.e. a significant decrease of vegetation greenness at the beginning of the growing season, took place in Sierra Nevada oak woodlands without implying significant trends in the annual averages. Despite the EVI annual mean, an estimator of annual primary production, is extensively used as an integrative descriptor of ecosystem functioning and status, our work highlights the importance of studying variables beyond the annual summaries (like seasonality and phenology) as significant trends in particular months of the year may not significantly affect the EVI annual mean but may have broad ecological consequences in critical periods such as the start of the growing season.

#### **4.2 Application to forest monitoring and management**

Since satellite images are regularly captured over large regions and under common protocols, the spectral vegetation indices represent an adequate approach to implement ecosystems monitoring programs in protected areas and to promote adaptive management actions (Alcaraz-Segura et al., 2008a; Alcaraz-Segura et al., 2008b; Cabello et al., 2008). Our work provides interesting information for the prioritization and the orientation of management actions for the Pyrenean oak forests of Sierra Nevada National Park. First, we provided a regional functional reference characterization of all oak woodlands of the Park for the 2001-2009 period. Our monitoring approach uses EVI-derived descriptors of ecosystem functioning that may allow managers to detect the spatial and temporal anomalies (Oyonarte et al., 2010), and to guide specific management actions in particular areas. The spatial and temporal deviations from the baseline conditions detected could be alerting of inconspicuous "within-state" changes in the forests as a result of cumulative impacts (Vogelmann et al., 2009). However, to improve the ecological significance of this approach for the Park management, the monitoring program should include the identification of the key ecological processes that can be related to this functional description and that are central for the maintenance of the ecological integrity. For instance, the differences in the strength of the EVI trends among different oak forest patches could be associated to the two modes of climatic variability that affect Sierra Nevada. The observed weaker start of the growing season during study period could be related to the increase of positive phases of the North Atlantic Oscillation (NAO Index), which are the main control of winter precipitation and temperature, particularly in the north-western slope (Liras, 2011). In addition, we also observed EVI increases during the summer (July-August) in the southern slope (Figure 4), which could be related to the increase of active phases of the Western Mediterranean Oscillation (WeMO), increasing late summer precipitation during the study period (Liras, 2011; Cabello et al., Accepted). In this sense, the obtained results in the EVI trends for the different woods could be used to prioritize management actions in relation to climate change adaptation in the most threatened sites. Nevertheless, this should be only one of the guiding hypotheses for adaptive management, since other processes such as insect damage and forest succession could also be taking place in the park (Sierra Nevada National Park managers, *personal communication,* Stöver et al., 1996; CMJA, 2008).

A monitoring system based on the tools and analysis shown here could embrace several monitoring objectives, as it simultaneously informs managers about the changes in productivity, phenology, and seasonality of the ecosystems. For example, changes in the EVI attributes could be directly related to changes in the amount, seasonality, and phenology of ecosystem carbon gains. In addition, linking the EVI dynamics of the Pyrenean oak woodlands to the ecology of species of conservation concern could be used to evaluate and monitor the conservation status of the habitat of such species. This could be the case of the blue tit (*Parus caeruleus*), whose reproductive success is related to the ecosystem status of *Quercus pyrenaica* forests, especially at the beginning of female reproductive period (April-May), which is associated with the start of the growing season (Arriero et al., 2006). Such association implies that delays in the start of the growing season or forest degradation would negatively affect the reproduction success of this bird. Moreover, the information derived from this monitoring approach could help guiding land-use planning to avoid overexploitation of Sierra Nevada oak woodlands. For instance, livestock pressure should be limited in those periods of the year that are experiencing strong negative EVI trends.
