Photosynthetic Performance in Pinus canariensis at Semiarid Treeline: Phenotype Variability to Cope with Stressful Environment

Abstract

Low temperatures represent the most important environmental stress for plants at the treeline ecotone; however, drought periods at the semiarid treeline could modify photosynthetic performance patterns. Gas exchange, chlorophyll fluorescence, photosynthetic pigments, and α-tocopherol were measured in a Pinus canariensis forest located at a semiarid treeline forest at 2070 m altitude over a whole year. The level of summer drought, caused by an extended period without rain and very low previous rainfall, was remarkable during the study. Furthermore, the cold season showed extraordinarily low temperatures, which persisted for five months. All of these factors combined made the study period an extraordinary opportunity to improve our understanding of photosynthetic performance in a drought-affected treeline ecotone. A high dynamism in all the measured parameters was detected, showing robust changes over the year. Maximum photosynthesis and optimal values were concentrated over a short period in spring. Beyond that, fine regulation in stomatal closure, high WUEi with a great plasticity, and changes in pigments and antioxidative components prevented dehydration during drought. In winter, a strong chronic photoinhibition was detected, and α-tocopherol and β-carotene acquired a main role as protective molecules, accompanied by morphological variations as changes in specific leaf areas to avoid freezing. The recovery in the next spring, i.e., after these extreme environmental conditions returned to normal, showed a strategy based on the breakdown of pigments and lower photosynthetic functions during the winter, and rebuilding and regreening. So, a high level of plasticity, together with some structural and physiological adaptations, make P. canariensis able to cope with stresses at the treeline. Nevertheless, the carbon gain was more limited by drought than by low temperatures and more extended droughts predicted in future climate change scenarios may strongly affect this forest.

Coexistent Heteroblastic Needles of Adult Pinus canariensis C.Sm. ex DC. in Buch Trees Differ Structurally and Physiologically

“…This resembles the lack of diurnal fluctuations in the content of pigments described by Porcar-Castell et al (2012) for P. sylevestris [58]. Seasonally, Chl a+b can slightly decrease towards winter in conifers [52,59,60] but generally remains constant between summer and autumn [61], as was also the case of P. canariensis in our study and in previous studies with this species [52]. Overall, higher photochemical efficiency but lower Chl content were obtained in secondary needles of P. canariensis.…”

“…The obtained values of net carbon assimilation of 2-11 µmol CO 2 m −2 s −1 fall within the expected range for this species and habitat. As an example, data obtained in the same arid tree-line ecotone at Teide Mountain and covering a whole year and a wide variety of physiological conditions (including drought episodes) ranged between −1 and +18 µmol CO 2 m −2 s −1 [52]. We did not find significant differences in A N between primary and secondary needles.…”

Photosynthetic Pigments in Siberian Pine and Fir under Climate Warming and Shift of the Timberline

“…In A. sibirica however, the ratio has a pronounced peak in middle elevations. The For other species of the Pinus genus, in particular, for P. canariensis, a decrease in the chlorophyll content in the treeline zone in winter was also found, which reduced the risk of overexcitation photoinhibition and photodamage, making rebuilding of photosynthetic activity possible in spring [23].…”

“…(sampling points 2 to 4) the amount of chlorophyll a in the needles remains constant; then, from 1637 m upward the chlorophyll content is reduced (Table 1). For other species of the Pinus genus, in particular, for P. canariensis, a decrease in the chlorophyll content in the treeline zone in winter was also found, which reduced the risk of overexcitation photoinhibition and photodamage, making rebuilding of photosynthetic activity possible in spring [23]. The data obtained show that at the beginning of the growing season the higher the altitude the smaller the amount of chlorophylls in the needles of P. sibirica, while the amounts of carotenoids increase (Figure 3).…”

Unexpected Vulnerability to High Temperature in the Mediterranean Alpine Shrub Erysimum scoparium (Brouss. ex Willd.) Wettst

“…These values indicates a risk of damage even when maximum air temperature in April is around +20 °C, because the elevated irradiation at this time of the year can induce a strong decoupling between leaf and air temperatures. As illustrative examples, the canopy temperature of antarctic mosses can exceed in 15 °C the temperature of the air at irradiances ≥ 800 μmol m −2 s −1 [ 46 ]; while leaf temperatures ≥ +50 °C have been measured in alpine plants during summer-months [ 10 ]. Both cases are comparable to April in Teide, in terms of irradiance, considering its low latitude, and that irradiance can easily overpass 1000 μmol m −2 s −1 during sunny days [ 31 , 47 ].…”