Most of the precipitation recorded during 2004–2005 fell during the autumn of 2004, with almost no precipitation from December 2004 until June 2005 (Fig. ). Freezing below −10 °C occurred on several consecutive days during February and March. Many of these cold, dry days had clear skies with many hours of intense irradiance (>1900 µmol PAR m−2 s−1), giving daily irradiances <30 mol PAR m−2 d−1 after March 2005 (Fig. ). Air temperature was higher and varied more at the sun than shade site; minimum temperatures in the sun were >2·5 °C lower and maximum temperatures were up to 4·5 °C higher than those in the shade (Fig. ). Leaf temperatures were very close to that of the air in the shade, while they were up to 4 °C higher than that of the air in the sun during the central hours of clear spring and summer days (data not shown). Air humidity was higher in the shade than in the sun throughout the year, leading to VPDs significantly lower than those in the sun (Table ). Daily irradiance was, on average, only one-third in the shade of that in the sun (Table ). Away from plants, soil water content (θ) at 20 cm depth was greater and more affected by precipitation in the sun than in the shade (Fig. ). However, a different and more complex seasonal pattern was found for the θ below the sampled plants. It was higher in the shade than in the sun from August 2004 until March 2005, but by the end of the dry spring and during the 2005 summer the pattern was reversed and soils were slightly but significantly more moist below plants in the sun (Fig. ). Depending on the species, this seasonal pattern of θ paralleled that of the seasonal changes in Ψ of the target plants (Table , Fig. ). θ of Q. ilex was less negative in the sun than in the shade in both summers but, in contrast, differences for A. uva-ursi were mostly insignificant (Fig. ). Ψ was significantly lower at the end of the summer 2005 than in 2004, reaching values as low as –6·5 MPa in A. uva-ursi.
Fig. 3. (A) Soil water content under the study plants. (B) Water potential (Ψ; pre-dawn measurements plus midday values in August 2004) of Quercus ilex and Arctostaphylos uva-ursi in sun and shade habitats. Each symbol is the mean of a minimum of six (more ...)
Photochemical efficiency of PSII (Fv/Fm) differed significantly among species, habitats and times of the year, with complex interactions between these three factors (Table , Fig. ). Pre-dawn Fv/Fm tended to be lower in sun than in shade plants, and were particularly so for A. uva-ursi, at the end of both summers (Fig. A). In contrast, Q. ilex exhibited similar values in the shade and in the sun for most of the year, with the exception of March 2005, where the combination of freezing temperatures and high irradiance resulted in mean Fv/Fm values as low as 0·4 at the sun site (Fig. A). Values of pre-dawn Fv/Fm at the end of August 2005 were significantly lower than those of August 2004 for all combinations of species and habitats. Although midday Fv/Fm (measured after 30 min in darkness) was lower than pre-dawn Fv/Fm, the seasonal and species trends were similar to those observed pre-dawn, with very low values in the sun for A. uva-ursi in summer and for Q. ilex in early spring (Fig. B). However, midday Fv/Fm was also reduced in the shade at some times of the year, with mean values of 0·6 during the summer in A. uva-ursi and during the early spring in Q. ilex. As was the case for pre-dawn Fv/Fm, midday Fv/Fm was significantly lower in August 2005 than in August 2004.
Fig. 4. Pre-dawn (A) and midday (B) photochemical efficiency (Fv/Fm) of Quercus ilex and Arctostaphylos uva-ursi in sun and shade habitats. Dashed lines indicate the threshold of 0·8 as a reference for optimum Fv/Fm. Each symbol is the mean of a minimum (more ...)
Maximum rates of light-saturated net photosynthesis (Psat; Fig. A, B) and stomatal conductances (gs; Fig. C, D) were low for both species during all seasons and in both habitats. However, when considering data from both species for May, July and August 2005 (i.e. the dates where full sets of gas exchange measurements were obtained for both species), significant interactions between habitat and date were found, and between species and date, with season being the main source of variation for these two gas exchange variables (Fig. ; Table ). While maximum rates of Psat were significantly higher in the sun than in the shade in both species during the spring (i.e. May), there were no differences between species and habitats in summer (July–August; Fig. ). Sun and shade plants also exhibited similar rates of Psat in mid-winter (i.e. January 2005); for Q. ilex, average rates of Psat (in μmol CO2 m−2 s−1 ± s.e.) were 3·56 ± 0·32 (shade) and 3·33 ± 0·50 (sun). In contrast, rates of Q. ilex Psat in early spring (i.e. March 2005) were higher in the shade (1·77 ± 0·13) than in the sun (0·25 ± 0·24). At saturating irradiance, gs decreased significantly with the onset of drought during the summer in both species (Fig. ). gs was significantly higher in the shade than in the sun in Q. ilex during the winter (data not shown) and early spring (Fig. ), while gs of A. uva-ursi was always very low in the shade (Fig. ).
Fig. 5. (A, B) Maximum net photosynthetic rate (Psat) and (C, D) stomatal conductance (gs) under ambient (Am) and saturating (Sa) light at ambient temperature and CO2 concentrations of Quercus ilex (A, C) and Arctostaphylos uva-ursi (B, D) in sun and shade habitats. (more ...)
To gain an insight into actual rates of carbon assimilation in the shaded environment, Pnet of shade plants was also measured at the prevailing ambient irradiance of the understorey (i.e. typically 100–450 µmol photons m−2 s−1). For Q. ilex, Pnet at ambient irradiance (Pamb) was available for all months. However, for A. uva-ursi, Pamb rates were only available for May, July and August 2005; Fig. shows values for both species from May to August 2005. For other months, Pamb values (in μmol CO2 m−2 s−1 ± s.e.) of Q. ilex shaded plants were: August 2004 = 1·07 ± 0·18; January 2005 = 3·56 ± 0·32; March 2005 = 1·01 ± 0·39. Comparison of these rates with those in Fig. shows that in late winter (i.e. March) and spring (May), Pamb was lower than Psat in Q. ilex; however, following the onset of summer, no differences were found between Pamb and Psat. Thus, while low irradiances at the shade site limited Pnet in winter/spring, factors other than irradiance limited Pnet of shaded plants in the summer. For A. uva-ursi, little difference was found between Pamb and Psat of shaded plants through May–August, with Pamb declining with the onset of summer (Fig. ).
The very low gas exchange rates of both species in the two habitats over most of the year led to low WUEs (Fig. ). However, significant differences were found among species and habitats, with A. uva-ursi exhibiting greater WUE than Q. ilex in the sun, particularly during the spring. Intrinsic WUE was greater in the shade than in the sun in Q. ilex, while both intrinsic and instantaneous WUEs were in general greater in the sun than in the shade in A. uva-ursi (Fig. ).
Fig. 6. Instantaneous and intrinsic water use efficiency [WUE, mean Pnet/mean transpiration under ambient light (A, B); and mean maximum net photosynthetic rate/mean maximum conductance (Psat/gs) under saturating light (C, D), respectively] of Quercus ilex (A, (more ...)
Carbon isotopic composition (δ13C) of the leaves collected at the end of the period of the study exhibited a significant species × habitat interaction (Table ): δ13C was more negative in the sun than in the shade in Q. ilex, whereas the reverse was true in A. uva-ursi (Fig. ). No differences were found between the two species in the shade, while the highest differences were found between sun individuals of the two species.
Fig. 7. Carbon isotope composition (δ13C) of leaves of Quercus ilex and Arctostaphylos uva-ursi in sun and shade habitats collected at the end of the summer 2005; letters at the base of each bar indicate statistically homogenous groups (ANOVA, Fisher (more ...)