Carlos Gracia

Ecosystem service supply and vulnerability to global change in Europe

Global change will alter the supply of ecosystem services that are vital for human well-being. To investigate ecosystem service supply during the 21st century, we used a range of ecosystem models and scenarios of climate and land-use change to conduct a Europe-wide assessment. Large changes in climate and land use typically resulted in large changes in ecosystem service supply. Some of these trends may be positive (for example, increases in forest area and productivity) or offer opportunities (for example, “surplus land” for agricultural extensification and bioenergy production). However, many changes increase vulnerability as a result of a decreasing supply of ecosystem services (for example, declining soil fertility, declining water availability, increasing risk of forest fires), especially in the Mediterranean and mountain regions.

Extreme climatic events shape arid and semiarid ecosystems

6 Climatic changes associated with the El Nino Southern Oscillation (ENSO) can have a dramatic impact on ter- restrial ecosystems worldwide, but especially on arid and semiarid systems, where productivity is strongly lim- ited by precipitation. Nearly two decades of research, including both short-term experiments and long-term studies conducted on three continents, reveal that the initial, extraordinary increases in primary productivity percolate up through entire food webs, attenuating the relative importance of top-down control by predators, providing key resources that are stored to fuel future production, and altering disturbance regimes for months or years after ENSO conditions have passed. Moreover, the ecological changes associated with ENSO events have important implications for agroecosystems, ecosystem restoration, wildlife conservation, and the spread of disease. Here we present the main ideas and results of a recent symposium on the effects of ENSO in dry ecosystems, which was convened as part of the First Alexander von Humboldt International Conference on the El Nino Phenomenon and its Global Impact (Guayaquil, Ecuador, 16-20 May 2005).

Likely effects of climate change on growth of Quercus ilex, Pinus halepensis, Pinus pinaster, Pinus sylvestris and Fagus sylvatica forests in the Mediterranean region

Mediterranean forest growth is constrained by drought and high temperatures during summer. Effects of climate change on these forests depend on how changes in water availability and temperature will take place. A process-based forest growth model, growth of trees is limited by water in the Mediterranean (GOTILWAþ), was applied in the Mediterranean region on Quercus ilex, Pinus halepensis, P. pinaster, P. sylvestris and Fagus sylvatica forests. The effects of climate change on growth were analysed, as well as the effect of thinning cycle length, combined with the assumption of different soil depths. Thinning cycle lengths was included because it can affect the response of stands to climatic conditions, and soil depth because this is positively related to soil water-holding capacity and consequently may change the effects of drought. The simulation period covered 140 years (1961–2100). Model results show that leaf area index (LAI) may increase, favoured by the increase of atmospheric CO2, particularly at sites where rainfall is relatively high and climatic conditions not too warm. The predicted increase in temperature significantly influenced mean leaf life span (MLLS). MLLS of F. sylvatica would increase with climate change, implying a longer growing period. Conversely, MLLS of evergreen species would be reduced, accelerating leaf turnover. In general, our results showed a higher production promoted by projected climate change in response to the increasing atmospheric CO2 concentration and rainfall in the region. Temperature increase would have different consequences for production. In F. sylvatica, the longer growing period would promote higher production, particularly when water is not limiting. On the other hand, Q. ilex and Pinus species would expend more carbon in maintaining and producing leaves to replace those lost in increased turnover rate. As expected, access of roots to deeper soil results in an increased final wood yield (FWY) due to an improved water balance that promotes higher transpiration, photosynthesis and growth. In general, the shorter the harvest cycle, the larger the FWY, because of less tree mortality between harvesting events. According to our results, temperature and rainfall may constrain growth during certain periods but if rainfall increases in the future, a positive effect on growth is likely. # 2002 Elsevier Science B.V. All rights reserved.

Annual and seasonal changes in fine root biomass of a Quercus ilex L. forest

The biomass, production and mortality of fine roots (roots with diameter <2.5 mm) were studied in a typical Mediterranean holm oak (Quercus ilex L.) forest in NE Spain using the minirhizotron methodology. A total of 1212 roots were monitored between June of 1994 and March of 1997. Mean annual fine root biomass in the holm oak forest of Prades was 71±8 g m−2 yr−1. Mean annual production for the period analysed was 260+11 g m−2 yr−1. Mortality was similar to production, with a mean value of 253±3 g m−2 yr−1. Seasonal fine root biomass presented a cyclic behaviour, with higher values in autumn and winter and lower in spring and summer. Production was highest in winter, and mortality in spring. In summer, production and mortality values were the lowest for the year. Production values in autumn and spring were very similar. The vertical distribution of fine root biomass decreased with increasing depth except for the top 10–20 cm, where values were lower than immediately below. Production and mortality values were similar between 10 and 50 cm depth. In the 0–10 cm and the 50–60 cm depth intervals, both production and mortality were lower.

GOTILWA: An Integrated Model of Water Dynamics and Forest Growth

GOTILWA is a simulation model of forest growth. Its name, GOTILWA, is an acronym for Growth Of Trees Is Limited by WAter. The name itself defines the main characteristic of the model. Water is, very often, the limiting factor for plant growth (Pinol et al. 1991; Sala 1992; Chap. 13) and thus it constitutes a key factor in the model (Tello et al. 1994). In a standard simulation, daily climatic data are analyzed. From the interaction between daily rainfall and the forest structure, the amount of intercepted water by the canopy layer, throughfall and stemflow are estimated. This effective rainfall increases the water stored in the soil which is used by the trees. The proportion of sapwood to heartwood, the leaf area of each tree and, consequently, the leaf area index (LAI) of the forest are all highly dependent on water availability in the model.

Canopy structure within a Quercus ilex forested watershed: variations due to location, phenological development, and water availability

Spatial and temporal changes in canopy structure were studied in 1988 and 1989 in a Mediterranean Quercus ilex forest in north-eastern Spain. Due to differences in precipitation patterns the 1989 growing season was drier than the 1988 growing season. Sampling was conducted in parallel at two sites which represent endpoints along a slope gradient within a watershed (ridge top at 975 m, and valley bottom at 700 m). At both sites, similar inter-annual changes in canopy structure were observed in response to differences in water availability. Samples harvested in the upper 50 cm of the canopy during 1989 exhibited a decrease in both average leaf size and the ratio of young to old leaf and stem biomass relative to samples obtained in 1988. At the whole canopy level, a decrease in leaf production efficiency and an increase in the stem to leaf biomass ratio was observed in 1989. Temporal changes in canopy leaf area index (LAI) were not statistically significant. Average LAI values of Q. ilex at the two sites were not significantly different despite differences in tree stature and density (4.6 m2 m−2 at the ridge top, and 5.3 m2 m−2 at the valley bottom). Vertical distribution of leaves and stems within the canopy was very similar at the two locations, with more than 60% of the total LAI in the uppermost metre of the canopy. The possible significance of such an LAI distribution on the canopy carbon budget is discussed.

STRUCTURE AND DYNAMICS OF THE ROOT SYSTEM

The belowground component of terrestrial ecosystems is much less understood than any of the aboveground components, yet important ecosystem processes such as nutrient recycling, water storage, and long-term carbon accumulation occur largely in this compartment. For instance, belowground structures accounted for up to 83% of the total biomass in 13 Mediterranean woody communities (Hilbert and Canadell 1995), and belowground primary production was 60–80% of the total net primary production in a variety of woody systems (Coleman 1976; Agren et al. 1980; Fogel 1985). Yet both root biomass and production are infrequently studied and technical difficulties make the measurements often inaccurate. Furthermore, plant root distribution and maximum rooting depths play important roles in overall ecosystem function, but it was not until recently that ecosystem-level and global comprehensive studies have been undertaken (Canadell et al. 1996; Jackson et al. 1996).

Nutrient content in Quercus ilex canopies: Seasonal and spatial variation within a catchment

Spatial and temporal changes in canopy nutrient content were studied in 1988 and 1989 in a Mediterranean Quercus ilex ssp. ilex L. forest in north-eastern Spain. Sampling was conducted in parallel at two sites which represent endpoints along a slope gradient within a small catchment (ridge top at 975 m and valley bottom at 700 m). Deeper soils resulted in significantly higher N and P concentrations, and N content on a leaf area basis at the valley bottom site. In contrast, K concentration in leaves was significantly higher at the ridge top site, where soil K concentration was also higher. At both sites, N and P content on a leaf area basis was highest at the top of the canopy, where leaf area is highest. N resorption efficiency decreased from top to bottom of the canopy. Results suggested a minor role of shaded leaves as nutrient storage sites. Lower P resorption efficiency was found at the ridge top site. Seasonal changes of P and N concentration on a leaf area basis suggest P replenishment, and to a lesser degree N, during periods of lower growth activity due to low temperatures, but coinciding with higher water availability (autumn-early spring period). Thus, N and P resorption from the remaining foliage in the canopy took place, and to a larger degree at the valley bottom site, coinciding with a slightly higher leaf area index and productivity at this site.

Functional Responses to Thinning

Holm oak (Quercus ilex L.) has adaptive traits to survive or easily regenerate after disturbances. Its high resprouting capacity (Chap. 5) can explain the structure of most holm oak forests with a very high tree density, which determines a strong competition for the resources and a very low growth rate (Chap. 3). Although self-thinning is the natural mechanism of reduction in tree density as the size of trees increase, the low growth rate leads these forests to an almost permanent state of stagnation in which most of the gross primary production (GPP) is invested in respiration (Chap. 12) leading to a very low net primary production (NPP). This low NPP and the high tree density produce a very slow diameter growth, which often is much less than 1 mm year-1 (Gracia et al. 1996; Chap. 3).

Seasonality of monoterpene emission potentials in Quercus ilex and Pinus pinea: Implications for regional VOC emissions modeling

[1] VOC emissions from terrestrial ecosystems provide one of the principal controls over oxidative photochemistry in the lower atmosphere and the resulting air pollution. Such atmospheric processes have strong seasonal cycles. Although similar seasonal cycles in VOC emissions from terrestrial ecosystems have been reported, regional emissions inventories generally omit the effect of seasonality on emissions. We compiled measurement data on seasonal variations in monoterpene emissions potentials for two evergreen species (Quercus ilex and Pinus pinea) and used these data to construct two contrasting seasonal response functions for the inclusion in monoterpene emission models. We included these responses in the Niinemets et al. model and compared simulation results to those of the MEGAN model, both with and without its predicted seasonality. The effect of seasonality on regional monoterpene emissions inventories for European Mediterranean forests dominated by these species was tested for both models, using the GOTILWA+ biosphere model platform. The consideration of seasonality in the Niinemets et al. model reduced total estimated annual monoterpene emissions by up to 65% in some regions, with largest reductions at lower latitudes. The MEGAN model demonstrated a much weaker seasonal response than that in the Niinemets et al. model, and did not capture the between species seasonality differences found in this study. Results suggest that previous regional model inventories based on one fixed emission factor likely overestimate regional emissions, and species-specific expressions of seasonality may be necessary. The consideration of seasonality both largely reduces monoterpene emissions estimates, and changes their expected seasonal distribution.