Águeda M. González-Rodríguez

Laurel forests in Tenerife, Canary Islands

Abstract. The efficiency of the conductive system in about 40-year-old Laurus azorica trees growing in a laurel forest was evaluated by comparing main stems and leaves (petioles) on the basis of theoretical sap flow values (1) calculated from vessel anatomy (taking vessels as ideal capillaries), (2) derived from measured dye velocity and (3) data taken from direct sap flow measurements. It was found that actual sap flow rate per wood area increases in stems from the pith towards the cambium. The outermost part of the stem is the most important part of the tree for conducting water. Maximum actually measured transpiration (sap flow rate) for the stand was practically identical to the theoretical rate calculated based on petiole anatomy, but it was about 45 times lower than that calculated based on stem anatomy. This illustrates the safety features of stem wood, which due to its high vessel density, is capable of transporting all the water required even when only a small area of its vessels is working. In the petioles, xylem is more efficiently used, but almost all vessels must work in order to supply water to leaves and any disturbance may cause leaf loss.

Laurel forests in Tenerife, Canary Islands. II. Leaf distribution patterns in individual trees

Abstract Branching pattern, leaf distribution (area, dry weight, number) and allometric relations on individual trees from a Canarian laurel forest have been analyzed. The following species were studied: Persea indica (L.) Spreng., Laurus azorica (Seub.) Franco, and Erica arborea L. Crown architecture was similar in all study species and corresponded to Rauhs’s model. In all species, leaf area was highly correlated to stem diameter. The vertical distribution of foliage was largely symmetrical and was roughly approximated by a Gaussian-like pattern which shifted upwards in dominant tree and downwards in suppressed tree. When radially viewed, most of the foliage area occurred roughly half the distance between the stem axis and the edge of the crown. However, radial leaf area indices, calculated for different areas of annulus around stems, showed a maximum closer to the stem.

Photostress, photoprotection, and water soluble antioxidants in the canopies of five Canarian laurel forest tree species during a diurnal course in the field

Minimal relative water contents of more than 85% indicated that dehydration was not a stress factor. Stomatal conductances decreased from 150 to 200 mmol H 2 O m –2 s –1 in the morning to about 50 mmol H 2 O m –2 s –1 during the day in all species, but this did not limit CO 2 uptake. De-epoxidation of xanthophylls only occurred in sun leaves of I. canariensis (to more than 50%) and M. faya (more than 60%). Decreases in Fv/Fm were only found in sun leaves of P. indica (from ca. 0.80 in the morning to a minimum of 0.70) and, as a trend, also in L. azorica (from ca. 0.75 to ca. 0.65). I. perado showed neither of those responses. P. indica and L. azorica exhibited the highest photosynthesis rates of about 10 µmol CO2 m –2 s –1 compared to 8 in the other species. The photoprotection strategy of P. indica and L. azorica admitted slow recovery from photoinhibition, did not activate protective energy dissipation through xanthophylls, and allowed highest production under these typical conditions.

Leaf gas exchange characteristics of a Canarian laurel forest tree species [Persea indica (L.) K. Spreng.] under natural conditions

Summary Persea indica (L.) K. Spreng. is a tree species of the Canarian laurel forest. Diurnal courses of gas exchange were measured during the period of one year in fully expanded current-year leaves in the uppermost canopy (sun leaves, 18 m above ground) and in the lower canopy (shade leaves, 12 m above ground) in order to understand the general behaviour of this tree species in relation with the environmental conditions of its habitat. Persea indica showed a great plasticity in gas exchange characteristics with large differences between sun and shade leaves (50 percnt; of maximum carbon assimilation rate (A max ) reduction in the shade) that were modulated by the strong light attenuation and high leaf area index (LAI) of the stand. This species presented low A max , about 11 μmol m −2 s −1 , high maximum transpiration (E, above 8 mmol m −2 s −1 ) and high stomatal conductance (g s , 800 mmol m −2 s −1 ) and therefore very low instantaneous water use efficiency (WUE, mean maximum 1.5 mmol mol −1 ) and A/g s (mean maximum between 20-35 μmol mol −1 ) and high mean minimum C i value (around 237 μmol mol −1 ), measured in sun leaves. All these gas exchange characteristics fit well with a non-conservative strategy of water use efficiency, which is appropriate for species that are subject to mild or short duration droughts. Overall, this species is more similar to subtropical plants of humid regions than to sclerophyllous plants of Mediterranean-type climate. However, under strong sunlight and dry conditions, sun leaves of P. indica show midday stomatal closure and downregulation of photochemical efficiency, effective ways to avoid excess water loss and photodamage of photosynthetic apparatus.

The significance of xanthophylls and tocopherols in photo-oxidative stress and photoprotection of three Canarian laurel forest tree species on a high radiation day

Summary Chloroplast pigments, chlorophyll fluorescence, and tocopherols in sun leaves of the Canarian laurel forest species Myrica faya, Laurus azorica, and Persea indica were characterized during the diurnal course of a high irradiation summer day. All species showed a de-epoxidation of the xanthophyll cycle and a decline of the photochemical efficiency of photosystem II measured as the Fv/Fm fluorescence ratio. Whereas in M. faya, decline and recovery of Fv/Fm was only correlated to zeaxanthin formation, these relations were divergent in L. azorica and P. indica .I nP. indica, Fv/Fm did not fully recover until the next morning. Apparently, the decrease of Fv/Fm during the day reflected only a zeaxanthin-dependent, quickly recovering component in M. faya, but an additional component, probably due to photodamage processes requiring repair activities, was observed in L. azorica and P. indica. In leaves of those two species, α -tocopherol concentrations varied during the diurnal course, probably indicating active oxygen scavenging action. In P. indica leaves, a degradation and restoration of chlorophyll contents were observed during the day. The conclusion is that M. faya is most resistent to high light stress and P. indica most sensitive, which coincides with known ecological traits of these species.

Effect of Dehydration on the Photosynthetic Apparatus of Sun and Shade Leaves of Laurel Forest Trees

In this work we study the effect of reduction in relative water content (RW C) on the chlorophyll fluorescence induction kinetics as well as on the membrane integrity, measured as electrolyte leakage, in sun and shade leaves of three Canarian laurel forest trees. No differences were found among the species and type of leaves, when the slow fluorescence kinetic parameters and electrolyte leakage were analyzed, values deviated from the normal ones at 70% and 40% RWC respectively. On the contrary, the photochemical efficiency of PSII was affected at higher values of RWC in sun leaves (90% and 52% RWC depending on the species) than in shade ones (40% RWC). These results indicate that the susceptibility of PSII to water deficit is different depending on species and environmental conditions where the leaves develop

Foliar cation contents of laurel forest trees on the Canary Islands

Summary The leaf cation contents of 15 different tree species of the laurel forest on the Canary Islands were measured. Different developmental stages of leaves were compared to estimate the effect of leaf age on the cation content. Young expanding leaves showed quite high potassium values when compared with mature leaves from the previous year and with senescing leaves. The opposite trend, accumulation with age, generally occurred with Ca, Mg, Fe, Mn, and AI, but with species-specific intensity. The levels of the investigated elements varied considerably between the species; exceptionally high nutrient levels were found in Viburnum rigidum . The cations in the leaves from laurel forest trees were similar to the reported ion contents in leaves of tropical trees, both being higher than those in leaves of temperate forest trees. In most species of the laurel forest the aluminium contents were between 10 and 30 mmol kg -1 dw. This was far above the Al contents of most other terrestrial plants and pointed to a high Al availability in the soils and a high foliar Al tolerance. Accumulation of Al-and to a lesser extent Fe and Mn as well - could be considered a consequence of primitive traits of the coriaceous leaf type like longevity and poor transpiration control.

Seasonal cycles of sap flow and stem radius variation of Spartocytisus supranubius in the alpine zone of Tenerife, Canary Islands

We analyzed the seasonal patterns of sap flow density (Qs) and stem radius variation (SRV) of Spartocytisus supranubius, a dominant, endemic tree-like shrub of the mountain vegetation at high elevation in Tenerife, Canary Islands. We tested the hypotheses that drought and its effect on water status and on radial growth (RG) is primarily related to cool–wet-season precipitation preceding the current year rather than to the dry summer per se. During 2013 and 2014 Qs and SRV were monitored with Granier-type sap flow sensors and automatic band dendrometers, respectively. Tree water deficit (ΔW) was extracted from SRV, and standard meteorological factors were used to calculate daily reference evapotranspiration (ETr) and soil moisture deficit. In both years investigated Qs was highest during the dry summer coinciding with periods of high ETr, indicating that plants were able to tap water from deep soil layers originating from precipitation prior the current year’s growth. The high RG and low ΔW rates observed throughout both dry summers are consistent with a direct access of tap roots to deep soil water reserves. Seasonal variations in RG of S. supranubius were mainly determined by the course of Qs and thus dependent on temperature and tree water status.

Changes in thermic limits and acclimation assessment for an alpine plant by chlorophyll fluorescence analysis: Fv/Fmvs. Rfd

A comparison between maximum quantum yield of PSII photochemistry (Fv/Fm) and chlorophyll fluorescence decrease ratio (Rfd) for low and high temperature resistance was assessed in a seasonal study of the acclimation in Pterocephalus lasiospermus. Analyzing the regression adjustment of both parameters and the lethal temperatures (LT50), Rfd resulted in being a more sensitive indicator for low and high temperature treatments, since the thermic resistance estimated with Rfd parameter was never higher than those estimated with Fv/Fm. Furthermore, the use of Fv/Fm led to an overestimation of the acclimation phenomena, with 6ºC of a maximum difference between both parameters. Using Rfd as the indicator parameter, P. lasiospermus acclimated to low temperatures but it kept on being a sensitive species (the lowest LT50 values only achieved–9.9 ± 0.3ºC). bserved (LT50 around 43.5ºC). Thus, according to Rfd evaluation of the thermic threshold, this species could be in risk of damage at low temperatures in this alpine ecosystem.