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Nevertheless, looking at xylem anatomy of conifers with elongated tracheids for axial transport and very few pits connecting tracheids radially suggests that radial conductance must be much lower than axial conductance and appears to be mainly conducted through rays[7]. If the axial conductance is 20 times higher, losing half of the conductive area at 1.8 m results in a total reduction of sap flow of 7% and if axial conductance is 10 or 30 times higher, total flow would be reduced by 6% or 9%, respectively (Supplementary Table S1C). Because radial conductance is very low and not relevant for the re-distribution of sap flow, the simplified model is two-dimensional with axial to tangential conductance 20:1 (see text for details). The resistance of the xylem to this embolism formation is thought to be an important aspect of plant drought resistance[3] and the total conductance of the transport system must be sufficient to maintain water flow without the water potential becoming too negative[4]. [Extracted from the article]

Due to massive energetic investments in woody support structures, trees are subject to unique physiological, mechanical, and ecological pressures not experienced by herbaceous plants. Despite a wealth of studies exploring trait relationships across the entire plant kingdom, the dominant traits underpinning these unique aspects of tree form and function remain unclear. Here, by considering 18 functional traits, encompassing leaf, seed, bark, wood, crown, and root characteristics, we quantify the multidimensional relationships in tree trait expression. We find that nearly half of trait variation is captured by two axes: one reflecting leaf economics, the other reflecting tree size and competition for light. Yet these orthogonal axes reveal strong environmental convergence, exhibiting correlated responses to temperature, moisture, and elevation. By subsequently exploring multidimensional trait relationships, we show that the full dimensionality of trait space is captured by eight distinct clusters, each reflecting a unique aspect of tree form and function. Collectively, this work identifies a core set of traits needed to quantify global patterns in functional biodiversity, and it contributes to our fundamental understanding of the functioning of forests worldwide. Understanding patterns in woody plant trait relationships and trade-offs is challenging. Here, by applying machine learning and data imputation methods to a global database of georeferenced trait measurements, the authors unravel key relationships in tree functional traits at the global scale. [ABSTRACT FROM AUTHOR]

Tropical rainforests harbor exceptionally high biodiversity and store large amounts of carbon in vegetation biomass. However, regional variation in plant species richness and vegetation carbon stock can be substantial, and may be related to the heterogeneity of topoedaphic properties. Therefore, aboveground vegetation carbon storage typically differs between geographic forest regions in association with the locally dominant plant functional group. A better understanding of the underlying factors controlling tropical forest diversity and vegetation carbon storage could be critical for predicting tropical carbon sink strength in response to projected climate change. Based on regionally replicated 1-ha forest inventory plots established in a region of high geomorphological heterogeneity we investigated how climatic and edaphic factors affect tropical forest diversity and vegetation carbon storage. Plant species richness (of all living stems >10 cm in diameter) ranged from 69 to 127 ha−1 and vegetation carbon storage ranged from 114 to 200 t ha−1. While plant species richness was controlled by climate and soil water availability, vegetation carbon storage was strongly related to wood density and soil phosphorus availability. Results suggest that local heterogeneity in resource availability and plant functional composition should be considered to improve projections of tropical forest ecosystem functioning under future scenarios. [ABSTRACT FROM AUTHOR]

Disturbances affect almost all terrestrial ecosystems, but it has been difficult to identify general principles regarding these influences. To improve our understanding of the long-term consequences of disturbance on terrestrial ecosystems, we present a conceptual framework that analyzes disturbances by their biogeochemical impacts. We posit that the ratio of soil and plant nutrient stocks in mature ecosystems represents a characteristic site property. Focusing on nitrogen (N), we hypothesize that this partitioning ratio (soil N: plant N) will undergo a predictable trajectory after disturbance. We investigate the nature of this partitioning ratio with three approaches: (1) nutrient stock data from forested ecosystems in North America, (2) a process-based ecosystem model, and (3) conceptual shifts in site nutrient availability with altered disturbance frequency. Partitioning ratios could be applied to a variety of ecosystems and successional states, allowing for improved temporal scaling of disturbance events. The generally short-term empirical evidence for recovery trajectories of nutrient stocks and partitioning ratios suggests two areas for future research. First, we need to recognize and quantify how disturbance effects can be accreting or depleting, depending on whether their net effect is to increase or decrease ecosystem nutrient stocks. Second, we need to test how altered disturbance frequencies from the present state may be constructive or destructive in their effects on biogeochemical cycling and nutrient availability. Long-term studies, with repeated sampling of soils and vegetation, will be essential in further developing this framework of biogeochemical response to disturbance. [ABSTRACT FROM AUTHOR]

Key message: Understanding the mobility and distribution of chemical elements in wood is necessary to apply dendrochemistry. Crystals are likely stable and could be used to analyze changes in nutrient supply. Abstract: Dendrochemistry uses the variation in wood chemical composition to infer about past environmental conditions and possible effects on tree growth. Elemental or isotopic variation might also help to identify annual growth where tree rings are anatomically not distinct. However, most elements are-to a certain degree-mobile within wood and may be related to anatomical structures. Therefore, understanding what affects elemental distribution is important to make use of and critically assess the potential of dendrochemistry. We studied the variation of wood density and elements at high spatial resolution in wood of six species with anatomically distinct to rather indistinct tree rings from a Thai monsoon forest. Many elements had a higher concentration in parenchyma than in fiber cells, and the co-variation of elements differed strongly between elements and also between species. Strong wood density changes along the ring boundary were found only in Melia azedarach. In all species, the X-ray images showed crystals. EDX spectra showed that these consist of calcium or silicon (in Chukrasia tabularis) as major elemental components. A high concentration of heavy metals (Fe, Cu and Zn) was found in Vitex peduncularis. We conclude that at least for the species studied the radial variation of elemental concentration is unlikely to reveal annual rings that anatomy could not. However, if elements in crystals are more stable than in cell walls or living protoplasts, analyzing the distribution of elements present in crystals may show environmental conditions that, in turn, influence crystal formation and are little known. [ABSTRACT FROM AUTHOR]

Tropical montane forests suffer from increasing fragmentation and replacement by other types of land-use such as coffee plantations. These processes are known to affect gene flow and genetic structure of plant populations. Epiphytes are particularly vulnerable because they depend on their supporting trees for their entire life-cycle. We compared population genetic structure and genetic diversity derived from AFLP markers of two epiphytic fern species differing in their ability to colonize secondary habitats. One species, Pleopeltis crassinervata, is a successful colonizer of shade trees and isolated trees whereas the other species, Polypodium rhodopleuron, is restricted to forests with anthropogenic separation leading to significant isolation between populations. By far most genetic variation was distributed within rather than among populations in both species, and a genetic admixture analysis did not reveal any clustering. Gene flow exceeded by far the benchmark of one migrant per generation to prevent genetic divergence between populations in both species. Though populations are threatened by habitat loss, long-distance dispersal is likely to support gene flow even between distant populations, which efficiently delays genetic isolation. Consequently, populations may rather be threatened by ecological consequences of habitat loss and fragmentation. [ABSTRACT FROM AUTHOR]

Understanding the responses of tropical trees to increasing [CO] and climate change is important as tropical forests play an important role in carbon and hydrological cycles. We used stable carbon isotopes (δC) in tree rings to study the physiological responses of a tropical dry forest tree species in southern Mexico, Mimosa acantholoba to changes in atmospheric [CO] and variation in climate. Based on annual records of tree ring δC, we calculated intrinsic water use efficiency ( W) and intercellular [CO] ( c). Our results showed that trees responded strongly to the increase in atmospheric [CO] over the last four decades; W increased dramatically by 40%, while c remained largely constant. The maintenance of a constant c indicates that photosynthetic rates are unlikely to have increased in response to higher [CO], and that improvements in W are probably due to a reduction in stomatal conductance. This may have large consequences for the hydrological cycle. Inter-annual variation in c was strongly correlated with total annual rainfall ( r = 0.70), and not influenced by temperature, solar radiation or cloud cover. Our results show that δC in tree rings of tropical dry forest trees may be a powerful tool to evaluate long-term responses of trees to increasing [CO] and to variation in climate. [ABSTRACT FROM AUTHOR]

Nutrients obtained by epiphytes may either be of atmospheric origin or from organic matter in the canopy, which decomposes to form canopy soil on large branches. We hypothesised that the N supply for epiphytes on small branches was lower, and a larger proportion provided by rainwater, than for epiphytes rooting in canopy soil. We tested this by measuring the N concentration and isotopic composition in terrestrial and canopy soil and in various canopy compartments of a Costa Rican cloud forest. In general, epiphytes on small branches without canopy soil had lower N foliar concentrations and δ15N signals than plants rooted in canopy soil, suggesting that the former receive a higher proportion of N directly from the rain. Epiphytes on small branches also had less negative δ13C values, indicating more frequent water stress. Epiphytes had lower δ15N values (–3.9±2.3‰) than ground-rooted trees (–1.1±1.6‰), and canopy soil had lower values (0.7±1.2‰) than terrestrial soil (3.8±0.7‰). Assuming that the isotopic effect of terrestrial and canopy soil organic matter formation is similar, our findings support earlier results showing that canopy soil is derived mainly from epiphytes, with only minor inputs from host tree matter. Thus, the epiphyte N cycle appears to be largely detached from the tree-soil cycle. Epiphylls on leaves of understorey shrubs had higher δ15N signals than cryptogams in the upper canopy, as a result of either 15N accumulation in throughfall or increased N2 fixation. The correlation between epiphyll and understorey host leaf δ15N suggests some exchange of N between epiphylls and host leaves. Differences between epiphyte groups also appear to be related to uptake of N through mycorrhizas or N2 fixation. Thus, the source and quantity of N supply is highly variable, depending on the systematic group and canopy position. [ABSTRACT FROM AUTHOR]

A letter to the editor is presented in response to the article "Pushing for Power," by Ad Lagendijk that was published in a previous issue.

Human societies depend on an Earth system that operates within a constrained range of nutrient availability, yet the recent trajectory of terrestrial nitrogen (N) availability is uncertain. Examining patterns of foliar N concentrations and isotope ratios (δ 15 N) from more than 43,000 samples acquired over 37 years, here we show that foliar N concentration declined by 9% and foliar δ 15 N declined by 0.6-1.6‰. Examining patterns across different climate spaces, foliar δ 15 N declined across the entire range of mean annual temperature and mean annual precipitation tested. These results suggest declines in N supply relative to plant demand at the global scale. In all, there are now multiple lines of evidence of declining N availability in many unfertilized terrestrial ecosystems, including declines in δ 15 N of tree rings and leaves from herbarium samples over the past 75-150 years. These patterns are consistent with the proposed consequences of elevated atmospheric carbon dioxide and longer growing seasons. These declines will limit future terrestrial carbon uptake and increase nutritional stress for herbivores.