Ignacio García-González

Studying global change through investigation of the plastic responses of xylem anatomy in tree rings

Variability in xylem anatomy is of interest to plant scientists because of the role water transport plays in plant performance and survival. Insights into plant adjustments to changing environmental conditions have mainly been obtained through structural and functional comparative studies between taxa or within taxa on contrasting sites or along environmental gradients. Yet, a gap exists regarding the study of hydraulic adjustments in response to environmental changes over the lifetimes of plants. In trees, dated tree-ring series are often exploited to reconstruct dynamics in ecological conditions, and recent work in which wood-anatomical variables have been used in dendrochronology has produced promising results. Environmental signals identified in water-conducting cells carry novel information reflecting changes in regional conditions and are mostly related to short, sub-annual intervals. Although the idea of investigating environmental signals through wood anatomical time series goes back to the 1960s, it is only recently that low-cost computerized image-analysis systems have enabled increased scientific output in this field. We believe that the study of tree-ring anatomy is emerging as a promising approach in tree biology and climate change research, particularly if complemented by physiological and ecological studies. This contribution presents the rationale, the potential, and the methodological challenges of this innovative approach.

Climatic control of intra-annual wood density fluctuations of Pinus pinaster in NW Spain

Tree rings of Pinus pinaster often contain intra-annual density fluctuations (IADFs), which have been attributed to the succession of dry and rainy periods typical of Mediterranean climate, but their formation has not been studied yet under Atlantic climate. We analyzed the occurrence and climatic significance of replicated IADFs in ten monospecific stands in NW Spain. The frequency of IADFs was higher than previously reported for this species under Mediterranean conditions and consistently decreased with increasing elevation. The formation of bands of latewood-like tracheids within the earlywood was favored by dry previous August, cold previous winter and dry April. Bands of earlywood-like tracheids within the early latewood were also favored by low winter temperatures. However, their occurrence was geographically heterogeneous, with two groups of stands being defined by their distances to the shoreline. In coastal stands, cold May–August triggered IADFs formation, while in inland stands their formation was favored by dry May–July. Regional winter temperatures and April water balance were strongly related to the East Atlantic (EA) pattern, which greatly conditioned the occurrence of IADFs in the earlywood and the early latewood. By contrast, the presence of bands of earlywood-like tracheids in the late latewood was independent of the EA pattern, being strongly related to warm conditions in spring and especially to a wet October. The link between regional climate and the EA pattern strongly controlled the physiological processes that determine intra-annual growth dynamics and short-term cell enlargement of P. pinaster in NW Spain.

Differential tree-growth responses to local and large-scale climatic variation in two Pinus and two Quercus species in northwest Spain

Abstract: We assessed the seasonal climatic response of 2 Pinus and 2 Quercus tree species in a littoral island in northwest Spain, analyzing earlywood, latewood, and total ring widths. We used bootstrapped response functions to identify the variables controlling growth, and we estimated the pure and shared effects of local climate components (temperature and water balance) and a large-scale atmospheric pattern (North Atlantic Oscillation, NAO) on tree growth using a variance partitioning approach. For all species, the response functions showed stronger climatic signals in latewood and total ring than in earlywood. The pure effects of temperature, water balance, and NAO on tree growth were significant. However, a null shared effect between local climate and NAO was evidenced due to the absence of a direct link between NAO in the previous autumn and the other significant climatic variables. Pinus pinaster was the species most sensitive to local climate, mainly to water availability during its active period as a consequence of higher water use efficiency. Quercus growth was negatively influenced by winter temperature, and growth of P. pinea and both Quercus species was inversely related to NAO in the previous autumn. The obtained results suggest that Q. robur and Q. pyrenaica have similar growth behaviour and climatic sensitivity. However, both P. pinea and P. pinaster showed contrasting growth dynamics and sensitivity in this Atlantic littoral island, despite their common Mediterranean origin. Nomenclature : Castroviejo et al., 1986–1989.

Environmental control of vessel traits in Quercus ilex under Mediterranean climate: relating xylem anatomy to function

Time series of tree-ring anatomical features are important to understand the functional role of xylem plasticity over the life span of trees, and thus to reconstruct past ecological and climatic conditions. Holm oak (Quercus ilex L.) is a drought-tolerant tree widely distributed in the Mediterranean Basin. Chronologies of tree-ring width (TRW), vessel lumen area [maximum (MAX) and mean (MVA)] and vessel density (VD) were developed for the period 1942–2001. Each ring was divided into three sections to compare the intra-annual variation of vessel features with the climate conditions during the growing season. The common variability of tree-growth and vessel features was analyzed using a principal component analysis (PCA). Vessel lumen area (MAX and MVA) and TRW loaded positively on the first axis (PC1), whereas VD from the first and second part of the ring (VD1 and VD2) loaded negatively, suggesting that these variables share a common variance. On the other hand, VD in the last third of the ring (VD3) loaded positively on second axis (PC2). PC1 showed a strong positive correlation with precipitation during the hydrological year (prior October–September) and a negative correlation with temperature in spring (April–May), while PC2 showed a negative correlation with precipitation in June. Our results showed that TRW and vessel lumen area were mainly dependent on moisture conditions along the growing season, while vessel density probably plays an important role in the balance between hydraulic conductivity and safety to embolism to better adjust the hydraulic system to water availability.

Earlywood vessels of the sub-Mediterranean oak Quercus pyrenaica have greater plasticity and sensitivity than those of the temperate Q. petraea at the Atlantic–Mediterranean boundary

Key messageEarlywood vessel features indicate different adaptations ofQuercus petraeaandQ. pyrenaica, which are probably related with their corresponding Atlantic and sub-Mediterranean ecological requirements.AbstractWe studied the climatic signal of the earlywood anatomy of a temperate [Quercus petraea (Mattuschka) Liebl.] and a sub-Mediterranean (Quercus pyrenaica Willd.) oak species growing under similar climatic conditions in a transitional area between the Atlantic and Mediterranean regions of the Iberian Peninsula. We hypothesized that both species react differently in their wood anatomy due to their contrasting ecological requirements, and we test the usefulness of earlywood anatomical features to study the behaviour of these ring-porous oaks upon climate. For this, we measured the earlywood vessels, and obtained annual series of several anatomical variables for the period 1937–2006 using dendrochronological techniques, considering whether the vessels belonged to the first row or not. After optimizing the data set by principal component analysis and progressive filtering of large vessels, we selected maximum vessel area and total number of vessels as they resulted to be the optimal variables to describe vessel size and number, respectively. Vessel size of Q. pyrenaica was dependent on precipitation along the previous growing season, whereas it did not show any clear climatic response for Q. petraea. On the contrary, vessel number was related to winter temperature for both species. These relationships observed between climate and anatomy appeared to be stable through time. The results obtained reinforce the utility of earlywood vessel features as potential climate proxies.

Structure and function of intra–annual density fluctuations: Mind the gaps

Tree rings are natural archives of climate and environmental information with a yearly resolution. Indeed, wood anatomical, chemical, and other properties of tree rings are a synthesis of several intrinsic and external factors, and their interaction during tree growth. In particular, Intra-Annual Density Fluctuations (IADFs) can be considered as tree-ring anomalies that can be used to better understand tree growth and to reconstruct past climate conditions with intra-annual resolution. However, the ecophysiological processes behind IADF formation, as well as their functional impact, remain unclear. Are IADFs resulting from a prompt adjustment to fluctuations in environmental conditions to avoid stressful conditions and/or to take advantage from favorable conditions? In this paper we discuss: (1) the influence of climatic factors on the formation of IADFs; (2) the occurrence of IADFs in different species and environments; (3) the potential of new approaches to study IADFs and identify their triggering factors. Our final aim is to underscore the advantages offered by network analyses of data and the importance of high-resolution measurements to gain insight into IADFs formation processes and their relations with climatic conditions, including extreme weather events.

Effects of thinning intensity on radial growth patterns and temperature sensitivity in Pinus canariensis afforestations on Tenerife Island, Spain

Abstract• ContextThe suitability of thinning to prevent forest growth decline as a result of global warming has not been tested extensively in Macaronesian Canary pine (Pinus canariensis Sweet ex Spreng.). • AimsThis study aimed to answer the following questions: (1) are tree growth patterns modified by the aspect and thinning intensity? (2) Is sensitivity to climate modulated by thinning? (3) Do trees of different crown classes respond differentially to thinning intensity and climate? • MethodsWe used tree-ring series from dominant, codominant, and overtopped trees to study the effects of thinning intensity on basal area increments (BAI) and climate sensitivity on windward (wet) and leeward (dry) slopes on Tenerife, Canary Islands. Three replicated blocks of control, light thinning, and heavy thinning stands were set on each slope in 1988, and cores were extracted in 2007. • ResultsHeavy thinning induced growth release and increased BAI, mainly on dominant and codominant trees, whereas light thinning effects were negligible; their impacts were more intense on the windward side. Temperature sensitivity was hardly affected by thinning on leeward, where climate control was stronger. On windward, thinning enhanced the influence of summer temperatures. Upper crown classes were more sensitive overall, but overtopped trees responded better in summer. Thinning intensity and aspect greatly influence growth on Canary pine afforestations, but individual responses are highly dependent on crown classes. In addition, thinning may be less effective to modify growth conditions on leeward slopes, at least if not intense.

Contrasting effects of wildfire and climate on radial growth of Pinus canariensis on windward and leeward slopes on Tenerife, Canary Islands

Little is known concerning the effects of wildfires on tree radial growth and their climatic response under contrasting regimes of fog water inputs on oceanic islands. On Tenerife, Canary Islands, windward slopes are humid with high-fog frequency due to influence of wet trade winds, while climate on leeward slopes is more arid. We used tree-ring records of Pinus canariensis Sweet ex Spreng. to quantify the effects of a fire of known date on radial growth and determine the main limiting climatic factors for growth. Radial growth patterns and their responsiveness to fire severity and climatic variation differed between windward and leeward slopes. Surface fire did not significantly impact growth, while crown fire caused short-term growth reduction, and even cessation, more pronounced on the windward slope. Growth rates, tree-ring common signal, and climate sensitivity were smaller on the windward slope, with cold winters, and summer water stress limiting growth. On the leeward slope, climate explained a greater amount of growth variation mainly due to negative effects of high October–December sea-level pressures causing dry winter conditions. Contrasting growth dynamics on both slopes may result from diverging physiological effects of water inputs and reduced radiation caused by fog drip. Our findings suggest that dating growth suppressions and absent rings are useful to date past high-severity crown fires in P. canariensis forests, in addition to ordinary fire scars dating indicative of low-severity surface fires.

Non-stationary influence of El Niño-Southern Oscillation and winter temperature on oak latewood growth in NW Iberian Peninsula.

The properties of El Niño-Southern Oscillation (ENSO), such as period, amplitude, and teleconnection strength to extratropical regions, have changed since the mid-1970s. ENSO affects the regional climatic regime in SW Europe, thus tree performance in the Iberian Peninsula could be affected by recent ENSO dynamics. We established four Quercus robur chronologies of earlywood and latewood widths in the NW Iberian Peninsula. The relationship between tree growth and the Southern Oscillation Index (SOI), the atmospheric expression of ENSO, showed that only latewood growth was correlated negatively with the SOI of the previous summer–autumn–winter. This relationship was non-stationary, with significant correlations only during the period 1952–1980; and also non-linear, with enhanced latewood growth only in La Niña years, i.e. years with a negative SOI index for the previous autumn. Non-linear relationship between latewood and SOI indicates an asymmetric influence of ENSO on tree performance, biassed towards negative SOI phases. During La Niña years, climate in the study area was warmer and wetter than during positive years, but only for 1952–1980. Winter temperatures became the most limiting factor for latewood growth since 1980, when mean regional temperatures increased by 1°C in comparison to previous periods. As a result, higher winter respiration rates, and the extension of the growing season, would probably cause an additional consumption of stored carbohydrates. The influence of ENSO and winter temperatures proved to be of great importance for tree growth, even at lower altitudes and under mild Atlantic climate in the NW Iberian Peninsula.

Contrasting effects of water availability on Pinus pinaster radial growth near the transition between the Atlantic and Mediterranean biogeographical regions in NW Spain

Summer water stress is the main limiting factor for Pinus pinaster growth under Mediterranean climate, but no information on limiting physiographic and climatic factors under Atlantic conditions is available for this species. We hypothesize that water availability during the active period limits P. pinaster growth, with stands nearby the Atlantic–Mediterranean boundary being more sensitive to summer drought stress. We analyzed the inter- and intra-annual climatic response of this species at ten monospecific stands in a transitional area between the Atlantic and Mediterranean biogeographical regions in Galicia, NW Spain. Mean sensitivity of radial growth consistently decreased with increasing elevation, and growth variation was quite similar for the earlywood, latewood or total ring widths, suggesting a strong intra-annual coherency of radial growth. Growth patterns were site dependent and geographically structured, with three groups of stands showing different climatic responses. As expected for sites suffering from summer drought stress, water availability enhances growth in western and southern Galicia, showing negative responses to maximum temperatures and positive to water availability. In northern Galicia, away from the Atlantic-Mediterranean boundary, water surplus in the rainy seasons negatively influenced growth. This was probably due to the combined effects of seasonal water-logging stress and the reduction of solar radiation associated with cloudiness, which would limit photosynthetic rates in winter and spring. Local variations of water availability strongly controlled the physiological processes that determine growth dynamics of P. pinaster in NW Spain, contributing to its geographical structure and contrasting sensitivity.