A. M. Gonzalez-Rodriguez

Influence of canopy position, needle age and season on the foliar gas exchange of Pinus canariensis

We investigated the seasonal variation in the gas exchange of current and 1-year-old needles in the upper sun and lower shade crown of adult Pinus canariensis trees. In general, current year needles displayed lower gas exchange rates than the 1-year-old needles. In both needle age classes, gas exchange was significantly lower in the shade than in the sun crown. However irrespective of crown position and needle age, maximum daily net photosynthesis, transpiration, and stomatal conductance for water vapour were generally higher during the wet and cold winter as compared to the dry and hot summer. These higher gas exchange values obtained during the cold and wet season can mainly be explained by higher soil-water availability and lower evaporative demand as compared to the warm and dry seaon. In addition, we also observed a displacement in the temperature optimum of net photosynthesis towards lower temperatures during the cold and wet season as compared to the warm and dry season. The observed gas exchange characteristics indicate a conservative water saving strategy and thus allowing P. canariensis needles to maintain a positive carbon gain even at periods of high evaporative demand and low soil-water availability.

Water availability drives stem growth and stem water deficit of Pinus canariensis in a drought-induced treeline in Tenerife

Tree growth of Pinus canariensis at treeline in Tenerife, Canary Islands, is thought to be primarily controlled by wet season precipitation (P) prior to the current year´s growth. Therefore, we investigated the inter-annual variations in stem water deficit (ΔW) and radial growth (RG) during two consecutive years differing in wet season P. ΔW was extracted from stem circumference variations, and the influence of environmental variables was evaluated by Pearson correlation statistics. Wet season P was considerably lower in 2008 than in 2009; despite this difference in P between both years, shallow soil water availability was almost exhausted during both summers. However, the effect of shallow soil drought showing a clear seasonality of ΔW and RG was only detected in 2008. In summer 2009, RG rates were highest during the summer indicating that P. canariensis was able to tap water from deep soil layers originating from P prior to the current year´s growth. The ability to use deep soil water during extended periods of shallow soil water deficit was also reflected in a close positive correlation between RG and whole-tree water use. In our study, the effect of only one hydrological dry year resulted in a severe reduction in annual RG. Thus, when wet season P is low for a number of years, chronic drought may have negative implications for tree growth at treeline in Tenerife.

Canary Island Pine (Pinus canariensis), an Evergreen Species in a Semiarid Treeline

Canary Island pine (Pinus canariensis) is an endemic conifer of the Canary Archipelago where it forms the treeline in Tenerife and La Palma at 2,000–2,100 m a.s.l. Due to climatic and edaphic drought and immature soils, the treeline in the Canary Islands is 1,000–1,900 m lower than in continental mountains at similar latitude. This review summarizes the present knowledge on the ecophysiology of P. canariensis growing at treeline where the climate is typically semiarid with high winter precipitation and summer drought. Studies on needle anatomy together with specific root patterns, allowing to search for water, suggest that P. canariensis is able to withstand climatic and edaphic drought. At the treeline in Tenerife, drought relates to the quantity of winter precipitation. Treeline trees are able to tap water from deep soil water reserves originating from ample winter precipitation prior to a dry summer. Winter precipitation also influences growth and determines whether forests at treeline are carbon sinks or carbon sources. Topsoil desiccation, however, impedes seedling establishment, a prerequisite for regeneration and potential treeline migration.