Cristina Moreno-Gutiérrez

Summary: Plant–plant interactions shape the structure and composition of plant communities, but shifts in interaction outcomes might occur in the face of ongoing climate change. We assessed the influence of Pinus halepensis plantations on the long‐term ecophysiological performance of understorey vegetation, by conducting a retrospective comparison (1989–2007) of growth‐ring widths, δ¹³C and δ¹⁸O between Rhamnus lycioides shrubs from two contrasting vegetation types: P. halepensis plantations vs. open woodlands. We also measured the leaf δ¹³C, δ¹⁸O, δ¹⁵N, and nutrient concentrations of understorey R. lycioides shrubs growing at varying distances from planted trees within pine plantations. Dendroecological and stable isotope data revealed strong competitive effects of planted P. halepensis trees on R. lyciodies. Shrubs in pine plantations showed lower radial growth and higher growth ring δ¹³C and δ¹⁸O than those in open woodlands, indicating lower stomatal conductance and photosynthesis in the former. The strong competitive effects of P. halepensis on understorey R. lycioides were most evident in wet, productive years. Conversely, in very dry years, there were indications of a facilitative effect of planted P. halepensis canopies on understorey shrubs. Within pine plantations, understorey R. lycioides shrubs growing at shorter distances from planted trees were forced to rely on more superficial and ephemeral soil water sources, which reduced their stomatal conductance (higher leaf δ¹⁸O) and interfered with nutrient uptake (lower leaf N and P concentrations and more negative δ¹⁵N). The intrinsic water use efficiency of R. lycioides shrubs growing in open woodlands has increased during recent decades as a result of their ability to adjust their stomatal conductance in response to increasing temperature and atmospheric CO₂. However, this adaptive response was much weaker or absent in the severely drought‐stressed shrubs from pine plantations. Pine afforestation strongly reduces water and nutrient availability for understorey shrubs in drylands, with potential long‐term consequences for ecosystem biodiversity, structure and functioning. Competition by P. halepensis on R. lycioides clearly outweighed facilitation in the long‐term, thus compromising the ability of understorey shrubs in semi‐arid pine plantations to cope with climate change.

Summary: Variation in the stable carbon and oxygen isotope composition (δ¹³C, Δ¹⁸O) of co‐occurring plant species may reflect the functional diversity of water use strategies present in natural plant communities. We investigated the patterns of water use among 10 coexisting plant species representing diverse taxonomic groups and life forms in semiarid southeast Spain by measuring their leaf δ¹³C and Δ¹⁸O, the oxygen isotope ratio of stem water and leaf gas exchange rates. Across species, Δ¹⁸O was tightly negatively correlated with stomatal conductance (g_s), whereas δ¹³C was positively correlated with intrinsic water use efficiency (WUE_i). Broad interspecific variation in Δ¹⁸O, δ¹³C andWUE_iwas largely determined by differences ing_s, as indicated by a strong positive correlation between leaf δ¹³C and Δ¹⁸O across species The 10 co‐occurring species segregated along a continuous ecophysiological gradient defined by their leaf δ¹³C and Δ¹⁸O, thus revealing a wide spectrum of stomatal regulation intensity and contrasting water use strategies ranging from ‘profligate/opportunistic’ (highg_s, lowWUE_i) to ‘conservative’ (lowg_s, highWUE_i). Coexisting species maintained their relative isotopic rankings in 2 yr with contrasting rainfall, suggesting the existence of species‐specific ‘isotopic niches’ that reflect ecophysiological niche segregation in dryland plant communities.

We investigated whether stand structure modulates the long‐term physiological performance and growth ofPinus halepensisMill. in a semiarid Mediterranean ecosystem. Tree radial growth and carbon and oxygen stable isotope composition of latewood (δ¹³C_LWandδ¹⁸O_LW, respectively) from 1967 to 2007 were measured inP. halepensistrees from two sharply contrasting stand types: open woodlands with widely scattered trees versus dense afforested stands.In both stand types, tree radial growth,δ¹³C_LWandδ¹⁸O_LWwere strongly correlated with annual rainfall, thus indicating that tree performance in this semiarid environment is largely determined by inter‐annual changes in water availability.However, trees in dense afforested stands showed consistently higherδ¹⁸O_LWand similarδ¹³C_LWvalues compared with those in neighbouring open woodlands, indicating lower stomatal conductance and photosynthesis rates in the former, but little difference in water use efficiency between stand types. Trees in dense afforested stands were more water stressed and showed lower radial growth, overall suggesting greater vulnerability to drought and climate aridification compared with trees in open woodlands.In this semiarid ecosystem, the negative impacts of intense inter‐tree competition for water onP. halepensisperformance clearly outweigh potential benefits derived from enhanced infiltration and reduced run‐off losses in dense afforested stands.

Silvicultural thinning usually improves the water status of remaining trees in water‐limited forests. We evaluated the usefulness of a dual stable isotope approach (δ¹³C,δ¹⁸O) for comparing the physiological performance of remaining trees between forest stands subjected to two different thinning intensities (moderate versus heavy) in a 60‐year‐oldPinus halepensisMill. plantation in semiarid southeastern Spain. We measured bulk leafδ¹³C andδ¹⁸O, foliar elemental concentrations, stem water content, stem waterδ¹⁸O (δ¹⁸O_stem water), tree ring widths and leaf gas exchange rates to assess the influence of forest stand density on tree performance. Remaining trees in low‐density stands (heavily thinned) showed lower leafδ¹⁸O, and higher stomatal conductance (g_s), photosynthetic rate and radial growth than those in moderate‐density stands (moderately thinned). By contrast, leafδ¹³C, intrinsic water‐use efficiency, foliar elemental concentrations andδ¹⁸O_stem waterwere unaffected by stand density. Lower foliarδ¹⁸O in heavily thinned stands reflected higherg_sof remaining trees due to decreased inter‐tree competition for water, whereas higher photosynthetic rate was largely attributable to reduced stomatal limitation to CO₂uptake. The dual isotope approach provided insight into the early (12 months) effects of stand density manipulation on the physiological performance of remaining trees.

Stable isotopes and their potential for detecting various and complex ecosystem processes are attracting an increasing number of scientists. Progress is challenging, particularly under global change scenarios, but some established views have been challenged. The IX meeting of the Spanish Association of Terrestrial Ecology (AAET, Úbeda, 18–22 October 2009) hosted a symposium on the ecology of stable isotopes where the linear mixing model approach of partitioning sinks and sources of carbon and water fluxes within an ecosystem was challenged, and new applications of stable isotopes for the study of plant interactions were evaluated. Discussion was also centred on the need for networks that monitor ecological processes using stable isotopes and key ideas for fostering future research with isotopes.