Disentangling functional trait variation and covariation in epiphytic lichens along a continent-wide latitudinal gradient.

Characterizing functional trait variation and covariation, and its drivers, is critical to understand the response of species to changing environmental conditions. Evolutionary and environmental factors determine how traits vary among and within species at multiple scales. However, disentangling their relative contribution is challenging and a comprehensive trait-environment framework addressing such questions is missing in lichens. We investigated the variation in nine traits related to photosynthetic performance, water use and nutrient acquisition applying phylogenetic comparative analyses in lichen epiphytic communities on beech across Europe. These poikilohydric organisms offer a valuable model owing to their inherent limitations to buffer contrasting environmental conditions. Photobiont type and growth form captured differences in certain physiological traits whose variation was largely determined by evolutionary processes (i.e. phylogenetic history), although the intraspecific component was non-negligible. Seasonal temperature fluctuations also had an impact on trait variation, while nitrogen content depended on photobiont type rather than nitrogen deposition. The inconsistency of trait covariation among and within species prevented establishing major resource use strategies in lichens. However, we did identify a general pattern related to the water-use strategy. Thus, to robustly unveil lichen responses under different climatic scenarios, it is necessary to incorporate both among and within-species trait variation and covariation.

Changes in tree resistance, recovery and resilience across three successive extreme droughts in the northeast Iberian Peninsula.

Understanding which variables affect forest resilience to extreme drought is key to predict future dynamics under ongoing climate change. In this study, we analyzed how tree resistance, recovery and resilience to drought have changed along three consecutive droughts and how they were affected by species, tree size, plot basal area (as a proxy for competition) and climate. We focused on the three most abundant pine species in the northeast Iberian Peninsula: Pinus halepensis, P. nigra and P. sylvestris during the three most extreme droughts recorded in the period 1951-2010 (occurred in 1986, 1994, and 2005-2006). We cored trees from permanent sample plots and used dendrochronological techniques to estimate resistance (ability to maintain growth level during drought), recovery (growth increase after drought) and resilience (capacity to recover pre-drought growth levels) in terms of tree stem basal area increment. Mixed-effects models were used to determine which tree- and plot-level variables were the main determinants of resistance, recovery and resilience, and to test for differences among the studied droughts. Larger trees were significantly less resistant and resilient. Plot basal area effects were only observed for resilience, with a negative impact only during the last drought. Resistance, recovery and resilience differed across the studied drought events, so that the studied populations became less resistant, less resilient and recovered worse during the last two droughts. This pattern suggests an increased vulnerability to drought after successive drought episodes.

Variation in xylem vulnerability to embolism in European beech from geographically marginal populations.

Climate change is expected to increase the frequency and intensity of droughts and heatwaves in Europe, leading to effects on forest growth and major forest dieback events due to hydraulic failure caused by xylem embolism. Inter-specific variability in embolism resistance has been studied in detail, but little is known about intra-specific variability, particularly in marginal populations. We evaluated 15 European beech populations, mostly from geographically marginal sites of the species distribution range, focusing particularly on populations from the dry southern margin. We found small, but significant differences in resistance to embolism between populations, with xylem pressures causing 50% loss of hydraulic conductivity ranging from -2.84 to -3.55 MPa. Significant phenotypic clines of increasing embolism resistance with increasing temperature and aridity were observed: the southernmost beech populations growing in a warmer drier climate and with lower habitat suitability have higher resistance to embolism than those from Northern Europe growing more favourable conditions. Previous studies have shown that there is little or no difference in embolism resistance between core populations, but our findings show that marginal populations have developed ways of protecting their xylem based on either evolution or plasticity.

Isohydric species are not necessarily more carbon limited than anisohydric species during drought.

Isohydry (i.e., strong regulation of leaf water potential, Ψl) is commonly associated with strict stomatal regulation of transpiration under drought, which in turn is believed to minimize hydraulic risk at the expense of reduced carbon assimilation. Hence, the iso/anisohydric classification has been widely used to assess drought resistance and mortality mechanisms across species, with isohydric species being hypothetically more prone to carbon starvation and anisohydric species more vulnerable to hydraulic failure. These hypotheses and their underlying assumptions, however, have rarely been tested under controlled, experimental conditions. Our objective is to assess the physiological mechanisms underlying drought resistance differences between two co-occurring Mediterranean forest species with contrasting drought responses: Phillyrea latifolia L. (anisohydric and more resistant to drought) and Quercus ilex L. (isohydric and less drought resistant). A total of 100 large saplings (50 per species) were subjected to repeated drought treatments for a period of 3 years, after which Q. ilex showed 18% mortality whereas no mortality was detected in P. latifolia. Relatively isohydric behavior was confirmed for Q. ilex, but higher vulnerability to cavitation in this species implied that estimated embolism levels were similar across species (12-52% in Q. ilex vs ~30% in P. latifolia). We also found similar seasonal patterns of stomatal conductance and assimilation between species. If anything, the anisohydric P. latifolia tended to show lower assimilation rates than Q. ilex under extreme drought. Similar growth rates and carbon reserves dynamics in both species also suggests that P. latifolia was as carbon-constrained as Q. ilex. Increasing carbon reserves under extreme drought stress in both species, concurrent with Q. ilex mortality, suggests that mortality in our study was not triggered by carbon starvation. Our results warn against making direct connections between Ψl regulation, stomatal behavior and the mechanisms of drought-induced mortality in plants.

The role of defoliation and root rot pathogen infection in driving the mode of drought-related physiological decline in Scots pine (Pinus sylvestris L.).

Drought-related tree die-off episodes have been observed in all vegetated continents. Despite much research effort, however, the multiple interactions between carbon starvation, hydraulic failure and biotic agents in driving tree mortality under field conditions are still not well understood. We analysed the seasonal variability of non-structural carbohydrates (NSCs) in four organs (leaves, branches, trunk and roots), the vulnerability to embolism in roots and branches, native embolism (percentage loss of hydraulic conductivity (PLC)) in branches and the presence of root rot pathogens in defoliated and non-defoliated individuals in a declining Scots pine (Pinus sylvestris L.) population in the NE Iberian Peninsula in 2012, which included a particularly dry and warm summer. No differences were observed between defoliated and non-defoliated pines in hydraulic parameters, except for a higher vulnerability to embolism at pressures below -2 MPa in roots of defoliated pines. No differences were found between defoliation classes in branch PLC. Total NSC (TNSC, soluble sugars plus starch) values decreased during drought, particularly in leaves. Defoliation reduced TNSC levels across tree organs, especially just before (June) and during (August) drought. Root rot infection by the fungal pathogen Onnia P. Karst spp. was detected but it did not appear to be associated to tree defoliation. However, Onnia infection was associated with reduced leaf-specific hydraulic conductivity and sapwood depth, and thus contributed to hydraulic impairment, especially in defoliated pines. Infection was also associated with virtually depleted root starch reserves during and after drought in defoliated pines. Moreover, defoliated and infected trees tended to show lower basal area increment. Overall, our results show the intertwined nature of physiological mechanisms leading to drought-induced mortality and the inherent difficulty of isolating their contribution under field conditions.

Carbon reserves and canopy defoliation determine the recovery of Scots pine 4 yr after a drought episode.

• Severe drought may increase physiological stress on long-lived woody vegetation, occasionally leading to mortality of overstory trees. Little is known about the factors determining tree survival and subsequent recovery after drought. • We used structural equation modeling to analyse the recovery of Scots pine (Pinus sylvestris) trees 4 yr after an extreme drought episode occurred in 2004-2005 in north-east Spain. Measured variables included the amount of green foliage, carbon reserves in the stem, mistletoe (Viscum album) infection, needle physiological performance and stem radial growth before, during and after the drought event. • The amount of green leaves and the levels of carbon reserves were related to the impact of drought on radial growth, and mutually correlated. However, our most likely path model indicated that current depletion of carbon reserves was a result of reduced photosynthetic tissue. This relationship potentially constitutes a feedback limiting tree recovery. In addition, mistletoe infection reduced leaf nitrogen content, negatively affecting growth. Finally, successive surveys in 2009-2010 showed a direct association between carbon reserves depletion and drought-induced mortality. • Severe drought events may induce long-term physiological disorders associated with canopy defoliation and depletion of carbon reserves, leading to prolonged recovery of surviving individuals and, eventually, to delayed tree death.

Hydraulic adjustment of Scots pine across Europe.

* The variability of branch-level hydraulic properties was assessed across 12 Scots pine populations covering a wide range of environmental conditions, including some of the southernmost populations of the species. The aims were to relate this variability to differences in climate, and to study the potential tradeoffs between traits. * Traits measured included wood density, radial growth, xylem anatomy, sapwood- and leaf-specific hydraulic conductivity (K(S) and K(L)), vulnerability to embolism, leaf-to-sapwood area ratio (A(L) : A(S)), needle carbon isotope discrimination (Delta13C) and nitrogen content, and specific leaf area. * Between-population variability was high for most of the hydraulic traits studied, but it was directly associated with climate dryness (defined as a combination of atmospheric moisture demand and availability) only for A(L) : A(S), K(L) and Delta13C. Shoot radial growth and A(L) : A(S) declined with stand development, which is consistent with a strategy to avoid exceedingly low water potentials as tree size increases. In addition, we did not find evidence at the intraspecific level of some associations between hydraulic traits that have been commonly reported across species. * The adjustment of Scots pine's hydraulic system to local climatic conditions occurred primarily through modifications of A(L) : A(S) and direct stomatal control, whereas intraspecific variation in vulnerability to embolism and leaf physiology appears to be limited.

Plasticity in hydraulic architecture of Scots pine across Eurasia.

Widespread tree species must show physiological and structural plasticity to deal with contrasting water balance conditions. To investigate these plasticity mechanisms, a meta-analysis of Pinus sylvestris L. sap flow and its response to environmental variables was conducted using datasets from across its whole geographical range. For each site, a Jarvis-type, multiplicative model was used to fit the relationship between sap flow and photosynthetically active radiation, vapour pressure deficit (D) and soil moisture deficit (SMD); and a logarithmic function was used to characterize the response of stomatal conductance (G(s)) to D. The fitted parameters of those models were regressed against climatic variables to study the acclimation of Scots pine to dry/warm conditions. The absolute value of sap flow and its sensitivity to D and SMD increased with the average summer evaporative demand. However, relative sensitivity of G(s) to D (m/G (s,ref), where m is the slope and G(s,ref) is reference G(s) at D = 1 kPa) did not increase with evaporative demand across populations, and transpiration per unit leaf area at a given D increased accordingly in drier/warmer climates. This physiological plasticity was linked to the previously reported climate- and size-related structural acclimation of leaf to sapwood area ratios. G (s,ref), and its absolute sensitivity to D(m), tended to decrease with age/height of the trees as previously reported for other pine species. It is unclear why Scots pines have higher transpiration rates at drier/warmer sites, at the expense of lower water-use efficiency. In any case, our results suggest that these structural adjustments may not be enough to prevent lower xylem tensions at the driest sites.

Plant size, not age, regulates growth and gas exchange in grafted Scots pine trees.

We studied the effect of scion donor-tree age on the physiology and growth of 6- to 7-year-old grafted Scots pine (Pinus sylvestris L.) trees (4 and 5 years after grafting). Physiological measurements included photosynthethetic rate, stomatal conductance, transpiration, whole plant hydraulic conductance, needle nitrogen concentration and carbon isotope composition. Growth measurements included total and component biomasses, relative growth rates and growth efficiency. Scion donor trees ranged in age from 36 to 269 years at the time of grafting. Hydraulic conductance was measured gravimetrically, applying the Ohm's law analogy, and directly, with a high-pressure flow meter. We found no effect of scion donor-tree age on any of the variables measured. There was, however, great variation within scion donor-tree age groups, which was related to the size of the grafted trees. Differences in size may have been caused by variable initial grafting success, but there was no indication that grafting success and age were related. At the stem level, hydraulic conductance scaled with total leaf area so that total conductance per unit leaf area did not vary with crown size. However, leaf specific hydraulic conductance (gravimetric), transpiration, photosynthesis and stomatal conductance declined with increasing total tree leaf area and needle width. We hypothesize that needle width is inversely related to mesophyll conductance. We conclude that canopy and needle size and not scion donor-tree age determined gas exchange in our grafted trees.

Size-mediated ageing reduces vigour in trees.

There is increasing interest in understanding the costs and benefits of increased size and prolonged lifespan for plants. Some species of trees can grow more than 100 m in height and can live for several millennia, however whether these achievements are obtained at the cost of some other physiological functions is currently unclear. As increases in size are usually associated with ageing, it is also unclear whether observed reductions in growth rates and increased mortality rates are a function of size or of age per se. One theory proposes that reduced growth after the start of the reproductive phase is caused by cellular senescence. A second set of theories has focussed instead on plant size and the increased respiratory burdens or excessive height. We report on experimental manipulations to separate the effects of extrinsic factors such as size from those of intrinsic factors such as age for four tree species of contrasting phylogeny and life history. For each species, we measured growth, gas exchange and leaf biochemical properties for trees of different ages and sizes in the field and on propagated material obtained from the same genetic individuals but now all of small similar size in our common gardens. For all species, evidence indicated that size, not cellular senescence, accounted for the observed age-related declines in relative growth rates and net assimilation rates. Two species exhibited evidence of genetic control on leaf characters such as specific leaf area, although size also exerted an independent, and stronger, effect. We found partial support for the theory of hydraulic limitations to tree growth. The lack of a marked separation of soma and germline, an unlimited proliferation potential of meristem cells and the exponential increase in reproductive effort with size all help explain the lack of a senescence-induced decline in trees. It is possible that trees much older than the ones we sampled exhibit senescence symptoms.