Forecasted increase drought frequency and severity may drive worldwide declines in forest productivity. Species‐level responses to a drier world are likely to be influenced by their functional ...traits. Here, we analyse forest resilience to drought using an extensive network of tree‐ring width data and satellite imagery. We compiled proxies of forest growth and productivity (TRWi, absolutely dated ring‐width indices; NDVI, Normalized Difference Vegetation Index) for 11 tree species and 502 forests in Spain corresponding to Mediterranean, temperate, and continental biomes. Four different components of forest resilience to drought were calculated based on TRWi and NDVI data before, during, and after four major droughts (1986, 1994–1995, 1999, and 2005), and pointed out that TRWi data were more sensitive metrics of forest resilience to drought than NDVI data. Resilience was related to both drought severity and forest composition. Evergreen gymnosperms dominating semi‐arid Mediterranean forests showed the lowest resistance to drought, but higher recovery than deciduous angiosperms dominating humid temperate forests. Moreover, semi‐arid gymnosperm forests presented a negative temporal trend in the resistance to drought, but this pattern was absent in continental and temperate forests. Although gymnosperms in dry Mediterranean forests showed a faster recovery after drought, their recovery potential could be constrained if droughts become more frequent. Conversely, angiosperms and gymnosperms inhabiting temperate and continental sites might have problems to recover after more intense droughts since they resist drought but are less able to recover afterwards.
In this study, we analysed the resistance and resilience to drought of forests dominated by 11 species across wide climatic and environmental gradients in the Mediterranean basin using proxies of forest productivity (NDVI) and carbon accumulation (ring‐width indices, TRWi) and considering four extreme drought events recorded between 1980 and 2005. Our results indicate that drought intensity is a major driver of forest resilience to drought but that species inhabiting different regions present different strategies to cope with drought and thus they can respond differently to more frequent and severe droughts.
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.
This study addresses relationships between leaf phenology, xylogenesis, and functional xylem anatomy in two ring‐porous oak species, the temperate Quercus robur and the sub‐Mediterranean ...Q. pyrenaica. Earlywood vessel (EV) formation and leaf phenology were monitored in 2012 and 2013. Ten individuals per species were sampled at each of three sites located in NW Iberian Peninsula. EV areas measured on microcore sections were used to calculate the hydraulic tree diameter (Dₕ), in order to model relationships to phenology. Thermal requirements were evaluated using growing degree days (GDD). A species‐specific timing of growth resumption was found. The onset of EV formation and budburst were associated to a particular GDD in each species. The onset and duration of EV enlargement affected Dₕ (and EV size) in Q. robur, but hardly in Q. pyrenaica. The relationship between the timings of EV formation and xylem structure appears to be stronger for the temperate oak, whose larger vessels may result from thermal‐induced earlier resumption. In contrast, the sub‐Mediterranean oak would maintain a more conservative hydraulic architecture under warming conditions.
The negative impacts of drought on forest growth and productivity last for several years generating legacies, although the factors that determine why such legacies vary across sites and tree species ...remain unclear.
We used an extensive network of tree‐ring width (RWI, ring‐width index) records of 16 tree species from 567 forests, and high‐resolution climate and normalized difference vegetation index (NDVI) datasets across Spain during the common period 1982‒2008 to test the hypothesis that climate conditions and growth features modulate legacy effects of drought on forests. Legacy effects of drought were calculated as the differences between detrended‐only RWI and NDVI series (i.e. after removing long‐term growth trends) and pre‐whitened RWI and NDVI series predicted by a model including drought intensity. Superposed Epoch Analysis (SEA) was used to estimate whether legacy effects differed from random. Finally, legacy effects were related to water balance, growth persistence and variability, and tree species identity.
We found a widespread occurrence of drought legacy effects on both RWI and NDVI, but they were seldom significant. According to SEA, first‐year drought legacies were negative and different from random in 9% and 5% of the RWI and NDVI series respectively. The number of significant second‐ and third‐year legacies was substantially lower. Differences between RWI and NDVI legacies indicate that canopy greenness and radial growth responses to drought are decoupled. We found variations in legacies between tree species with gymnosperms presenting larger first‐year drought legacies than angiosperms, which were exposed to less severe droughts. Greater growth variability can explain the presence of first‐year RWI legacies in gymnosperms from dry sites despite that the relationship between growth variability and legacies was complex.
Synthesis. Accounting for species and site responses to drought provides a better understanding of the magnitude and duration of drought legacies on forest growth and productivity. Despite the widespread occurrence of growth reductions in the years during and after drought occurrence, significant legacies were not very common, mostly lasted one year, and were more widespread in gymnosperms. These are relevant factors to be considered in the future when studying the consequences of drought on forest productivity and tree growth.
Legacy effects of drought on tree growth (RWI, ring‐width indices) and forest productivity (NDVI) vary between tree species. Drought legacies are more common for gymnosperms than for angiosperms and usually last for one year. Growth variability partially explains the variation in drought legacies between species despite the fact that this relationship is complex and species‐specific.
Aim
Spatial variations of environmental conditions translate into biogeographical patterns of tree growth. This fact is used to identify the origin of timber by means of dendroprovenancing. Yet, ...dendroprovenancing attempts are commonly only based on ring‐width measurements, and largely neglect additional tree–ring variables. We explore the potential of using wood anatomy as a dendroprovenancing tool, and investigate whether it increases the precision of identifying the origin of oak wood. Since different tree–ring variables hold different information on environmental conditions prevailing at specific times of the growing season—which vary between source regions—we hypothesize that their inclusion allows more precise dendroprovenancing.
Location
Europe, Spain.
Taxon
Quercus robur L., Quercus petraea (Matt.) Liebl., Quercus faginea Lam., Quercus pyrenaica Willd.
Methods
We sampled four oak species across Northern Spain, i.e. from the Basque country and Cantabria and—in the Basque country—from low to high elevation (topographic/latitudinal gradient). We measured multiple tree–ring variables to (a) extract complementary variables; (b) present statistical relations among them; (c) analyse region‐specific variation in their patterns based on time–series of individual trees; and (d) determine underlying climate–growth relationships. Leave‐one‐out analysis was used to test whether a combination of selected variables allowed dendroprovenancing of a randomly selected tree within the area.
Results
A combination of latewood width (LW) and earlywood vessel size was used to pinpoint the origin of oak wood with higher precision than ring width or LW only. Variation in LW pinpointed the wood to east and west areas, whereas variation in vessels assigned wood to locations along a latitudinal/topographic gradient. The climatic triggers behind these gradients are respectively an east–west gradient in June–July temperature and a north–south gradient in winter/spring temperatures. The leave‐one‐out analyses supported the robustness of these results.
Main conclusions
Integration of multiple wood–xylem anatomical variables analysed with multivariate techniques leads to higher precision in the dendroprovenancing of ring‐porous oak species.
In this study, we evaluate the importance of the mean earlywood vessel size of oaks as a potential proxy for climate in mesic areas. The study was conducted in Switzerland at three forest sites ...dominated by oak (Quercus petraea and Q. pubescens). The three sites were in different climatic zones, varying mainly in terms of precipitation regime. Three 50-year-long site chronologies of mean earlywood vessel size and tree-ring widths were obtained at each site and related to monthly meteorological records in order to identify the main variables controlling growth. The responses of mean vessel size to climate were compared with those of the width variables to evaluate the potential climatic information recorded by the earlywood vessels. The results show that the mean vessel size has a different and stronger response to climate than ring-width variables, although its common signal and year-to-year variability are lower. This response is better in particular at mesic sites, where it is linked to precipitation during spring, i.e. at the time of vessel formation, and is probably related to the occurrence of only a few processes controlling vessel growth, whereas radial increment is controlled by multiple and varying factors. The mean earlywood vessel size of oak appears to be a promising proxy for future climate reconstructions of mesic sites, where radial growth is not controlled by a single limiting factor.
Summary
Environmental conditions and the structure of the dormant cambium are assumed to affect seasonal patterns of cambial activity, hence controlling allocation of non‐structural carbohydrates ...(NSC) to growth. However, seasonal dynamics of xylogenesis, and their connections with NSC content and dormant cambium size, have been rarely assessed along an environmental gradient.
We monitored xylogenesis and leaf phenology during 2012 and 2013, and NSC in 2012, for the drought‐sensitive Quercus robur and the drought‐tolerant Quercus pyrenaica along a water‐availability gradient in the north‐western Iberian Peninsula, and analysed dependencies of xylem production and phenology on the number of cells in dormant cambium.
Study oak species showed comparable seasonal fluctuations in cambial activity and NSC content, despite Q. pyrenaica had a shorter growing season and a lower wood production than Q. robur. A sharp drop in spring NSC levels at all study sites evidenced that stored carbohydrates were crucial for earlywood formation. Under drier conditions, both species extended the growing period in spring and autumn, but reduced and even stopped xylogenesis in summer, showing an enhanced NSC accumulation before dormancy. A higher number of cells in dormant cambium of large dominant oaks accounted for their wider xylem increments and longer active periods.
Our study demonstrates that xylogenesis is modulated by predisposing effects of dormant cambium size on xylem production and growing season length. Moreover, the high plasticity of cambial activity in deciduous oaks would confer resistance against recurrent summer drought through the improvement of the NSC status.
A lay summary is available for this article.
Lay Summary
• The aim of this study was to identify the climatic signal contained in the earlywood vessel size of the ring-porous chestnut (Castanea sativa) and the physiological processes involved in the ...underlying mechanisms. • In order to assign the encoded signal to a specific physiological process, bud phenology and vessel formation were monitored along an elevation transect and chronologies of the size of the first row of earlywood vessels were retrospectively correlated with 40 yr of early spring temperatures. • The first vessels appeared in late April to early May, after encoding both a negative temperature signal in February-March (during tree quiescence) and a positive temperature signal in early April (at the time of resumption of shoot growth). • We hypothesize that February and March temperatures affect cambial sensitivity to auxin, preconditioning tree responses later in the season. Furthermore, April temperature is related to tree activation whereby new hormone production fosters vessel expansion.
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