Patricia Brito

Spatial and seasonal variations in stem CO2 efflux of Pinus canariensis at their upper distribution limit

We calculated stem CO2 efflux (ES) of Pinus canariensis at a timberline site in Tenerife, Canary Islands, from March 7, 2008 and February 9, 2009. ES varied markedly throughout the year. Although ES generally followed the seasonal trends in temperature, we observed seasonal adjustment of ES in both ES normalized to temperature (ES10) and the temperature sensitivity (Q10) resulting in lower ES10 values during the warm and dry season as compared during the cold and wet season; the latter corresponding with the period of stem growth. The Q10 by contrast, was higher during the warm and dry summer than during the cold and wet season, an observation suggesting that climate extremes such as summer drought in the Mediterranean may override the observation that Q10 declines with increasing temperature. As concurrent measurements estimating the potential impact of sap flow on ES revealed no interaction in P. canariensis, the Q10 values were used along with ES10 and stem temperature records to predict annual total ES and for partitioning total ES into its maintenance (Em) and a growth (Eg) component.

Operation and regulation of the lutein epoxide cycle in seedlings of Ocotea foetens

Two xanthophyll cycles are present in higher plants: the ubiquitous violaxanthin (V) cycle and the taxonomically restricted lutein epoxide (Lx) cycle. Conversions of V to zeaxanthin (Z) in the first and Lx to lutein (L) in the second happen in parallel under illumination. Unlike the V cycle, in which full epoxidation is completed overnight, in the Lx cycle, this reaction has been described as irreversible on a daily basis in most species (the ‘truncated’ Lx cycle). However, there are some species that display complete restoration of Lx overnight (‘true’ Lx cycle). So far, little is known about the physiological meaning of these two versions of the Lx cycle. Therefore, in the present work, the ‘true’ Lx cycle operation was studied in seedlings of Ocotea foetens (Aiton) Benth. under controlled and field conditions. Complete overnight recovery of the Lx pool in the presence of norfluorazon suggested that the inter-conversions between Lx and L represent a true cycle in this species. Furthermore, Lx responded dynamically to environmental conditions during long-term acclimation. Our data demonstrate the operation of a ‘true’ Lx cycle and, for the first time, its potential involvement in the regulation of non-photochemical quenching in situ. We propose dual regulation of Lx cycle in O. foetens, in which the extent of Lx restoration depends on the intensity and duration of illumination.

Gaseous Exchange Between Forests and the Atmosphere

Abstract Climate change is expected to affect the exchange of gases between forest ecosystems and the atmosphere. In this review, we focus on a few related topics, including the emission of greenhouse gases from the forest floor, and vegetation fires and their impact on air quality and soil CO 2 efflux. In particular, we summarise the current state of knowledge on O 3 deposition in forest ecosystems, both for stomatal uptake and non-stomatal sinks. Based on such summaries, we discuss interactions between forests, atmospheric composition and climate, and finally outline directions for multi- and interdisciplinary research required for mechanistically understanding such interrelationships.

Direct Penman–Monteith parameterization for estimating stomatal conductance and modeling sap flow

Key messageThe novel approach for direct parameterization of the Penman–Monteith equation was developed to compute diurnal courses of stand canopy conductance from sap flow.AbstractThe Penman–Monteith equation of evaporation is often combined with sap flow measurements to describe canopy transpiration and stomatal conductance. The traditional approach involves a two-step calculation. In the first step, stomatal conductance is computed using an inverted form of Penman–Monteith equation. The second step correlates these values with environmental factors. In this work, we present an improved approach for direct parameterization of the Penman–Monteith equation developed to compute diurnal courses of stand canopy conductance (gc) from sap flow. The main advantages of this proposed approach versus using the classical approach are: (1) the calculation process is faster and involves fewer steps, (2) parameterization provides realistic values of canopy conductance, including conditions of low atmospheric vapor pressure deficit (D), whereas the traditional approach tends to yield unrealistic values for low D and (3) the new calculation method does not require enveloping curves to describe dependence of gc on D and thus avoids subjective data selection but it still allows to visualize separable responses of gc to environmental drivers (i.e., global radiation and vapor pressure deficit). The proposed approach was tested to calculate gc and to model the sap flow of a high mountain Pinus canariensis forest. The new calculation method permitted us to describe the stand canopy conductance and stand sap flow in sub-hour resolution for both day and night conditions. Direct parameterization of the Penman–Monteith approach as implemented in this study proved sufficiently sensitive for detecting diurnal variation in gc and for predicting sap flow from environmental variables under various atmospheric evapotranspirative demands and differing levels of 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.

Salt crystal deposition as a reversible mechanism to enhance photoprotection in black mangrove

Mangrove forests are ecosystems made up of several woody plants living in saline coastal sedimentary habitats. In order to deal with the high salinity of the substrate, mangrove trees possess a number of different mechanisms to exclude, sequestrate or excrete the excess of salt. The black mangrove (Avicennia germinans L.), one of the dominant species in Central America, is characterized by high levels of salt excretion through epidermal glands. In this study, our aim was to examine whether, apart from its obvious role in salt tolerance, the formation of salt crystals on the upper leaf surface of black mangrove might represent an unusual and dynamic photoprotection mechanism. For this purpose, the reflection of light and a number of physiological parameters were studied during the dry and rainy seasons in black mangroves growing in the Juan Venado Island Nature Reserve (Nicaragua). Excreted salt increased the reflectance of the leaf surface mainly in the blue and red regions of the spectrum. By removing salt crust from the leaf surface, we demonstrated that during the most stressful periods (dry season at noon), this feature allowed leaves to maintain a higher photochemical efficiency and a lower leaf temperature as compared to uncovered leaves. Furthermore, this mechanism is fully reversible when conditions become more favorable, as salt crystals dissolve, forming drops. Thus, while being a detoxification mechanism developed mainly to avoid osmotic imbalance in the tissues, the excretion of salt through the leaves in black mangroves is an example of “exaptation”, as it has positive collateral effects on the photosynthetic performance of the plant, protecting A. germinans from overheating and photodamage during the harsher periods.

Response of Soil CO2 Efflux to Simulated Precipitation Pulses in a Canary Island Pine Forest at Treeline

Pulse-like rapidly increasing CO2 effluxes occur in soils under seasonally dry climates in response to summer rain events, and these CO2 outbursts can have a marked influence on the ecosystem carbon balance. Two irrigation experiments were conducted at the end of the dry season in a Mediterranean Pinus canariensis forest at its upper distribution limit in Tenerife, Canary Islands. Soil CO2 efflux was discontinuously monitored along with soil temperature and soil water content during and after irrigation. Correlation analysis suggested that soil moisture is the leading environmental factor regulating soil CO2 efflux during summer rain events. Cumulative CO2 efflux following a precipitation pulse was estimated to be 50% higher than the value calculated for the dry soil, which is approximately 3% of the year round soil CO2 efflux. It is concluded that such CO2 effluxes may have implications for the prediction of forest response to increasing dryness combined with an increase in the irregularity of summer rain events in Mediterranean regions.

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.