Biomass and area ratios between leaves, stems and roots regulate many physiological and ecological processes. The Huber value H-v (sapwood area/leaf area ratio) is central to plant water balance... Show moreBiomass and area ratios between leaves, stems and roots regulate many physiological and ecological processes. The Huber value H-v (sapwood area/leaf area ratio) is central to plant water balance and drought responses. However, its coordination with key plant functional traits is poorly understood, and prevents developing trait-based prediction models. Based on theoretical arguments, we hypothesise that global patterns in H-v of terminal woody branches can be predicted from variables related to plant trait spectra, that is plant hydraulics and size and leaf economics. Using a global compilation of 1135 species-averaged H-v, we show that H-v varies over three orders of magnitude. Higher H-v are seen in short small-leaved low-specific leaf area (SLA) shrubs with low K-s in arid relative to tall large-leaved high-SLA trees with high K-s in moist environments. All traits depend on climate but climatic correlations are stronger for explanatory traits than H-v. Negative isometry is found between H-v and K-s, suggesting a compensation to maintain hydraulic supply to leaves across species. This work identifies the major global drivers of branch sapwood/leaf area ratios. Our approach based on widely available traits facilitates the development of accurate models of above-ground biomass allocation and helps predict vegetation responses to drought. Show less
Cailleret, M.; Jansen, S.; Robert, E.M.R.; Desoto, L.; Aakala, T.; Antos, J.A.; ... ; Martinez-Vilalta, J. 2017
Tree mortality is a key factor influencing forest functions and dynamics, but our understanding of the mechanisms leading to mortality and the associated changes in tree growth rates are still... Show moreTree mortality is a key factor influencing forest functions and dynamics, but our understanding of the mechanisms leading to mortality and the associated changes in tree growth rates are still limited. We compiled a new pan-conti- nental tree-ring width database from sites where both dead and living trees were sampled (2970 dead and 4224 living trees from 190 sites, including 36 species), and compared early and recent growth rates between trees that died and those that survived a given mortality event. We observed a decrease in radial growth before death in ca. 84% of the mortality events. The extent and duration of these reductions were highly variable (1–100 years in 96% of events) due to the complex interactions among study species and the source(s) of mortality. Strong and long-lasting declines were found for gymnosperms, shade- and drought-tolerant species, and trees that died from competition. Angiosperms and trees that died due to biotic attacks (especially bark-beetles) typically showed relatively small and short-term growth reductions. Our analysis did not highlight any universal trade-off between early growth and tree longevity within a species, although this result may also reflect high variability in sampling design among sites. The intersite and interspecific variability in growth patterns before mortality provides valuable information on the nature of the mortality process, which is consistent with our understanding of the physiological mechanisms leading to mortality. Abrupt changes in growth immediately before death can be associated with generalized hydraulic failure and/or bark-beetle attack, while long-term decrease in growth may be associated with a gradual decline in hydraulic performance coupled with depletion in carbon reserves. Our results imply that growth-based mortality algorithms may be a powerful tool for predicting gymnosperm mortality induced by chronic stress, but not necessarily so for angiosperms and in case of intense drought or bark-beetle outbreaks. Show less
The evolution of lignified xylem allowed for the efficient transport of water under tension,but also exposed the vascular network to the risk of gas emboli and the spread of gas betweenxylem... Show moreThe evolution of lignified xylem allowed for the efficient transport of water under tension,but also exposed the vascular network to the risk of gas emboli and the spread of gas betweenxylem conduits, thus impeding sap transport to the leaves. A well-known hypothesis proposesthat the safety of xylem (its ability to resist embolism formation and spread) should trade offagainst xylem efficiency (its capacity to transport water). We tested this safety–efficiency hypothesis in branch xylem across 335 angiosperm and 89gymnosperm species. Safety was considered at three levels: the xylem water potentials where12%, 50% and 88% of maximal conductivity are lost. Although correlations between safety and efficiency were weak (r2 < 0.086), no species hadhigh efficiency and high safety, supporting the idea for a safety–efficiency tradeoff. However,many species had low efficiency and low safety. Species with low efficiency and low safetywere weakly associated (r2 < 0.02 in most cases) with higher wood density, lower leaf- to sap-wood-area and shorter stature. There appears to be no persuasive explanation for the considerable number of species withboth low efficiency and low safety. These species represent a real challenge for understandingthe evolution of xylem. Show less
