We investigated the common assumption that severing stems and petioles under water preserves the hydraulic continuity in the xylem conduits opened by the cut when the xylem is under tension. In red maple and white ash, higher percent loss of conductivity (PLC) in the afternoon occurred when the measurement segment was excised under water at native xylem tensions, but not when xylem tensions were relaxed prior to sample excision. Bench drying vulnerability curves in which measurement samples were excised at native versus relaxed tensions showed a dramatic effect of cutting under tension in red maple, a moderate effect in sugar maple, and no effect in paper birch. We also found that air injection of cut branches (red and sugar maple) at pressures of 0.1 and 1.0 MPa resulted in PLC greater than predicted from vulnerability curves for samples cut 2 min after depressurization, with PLC returning to expected levels for samples cut after 75 min. These results suggest that sampling methods can generate PLC patterns indicative of repair under tension by inducing a degree of embolism that is itself a function of xylem tensions or supersaturation of dissolved gases (air injection) at the moment of sample excision. Implications for assessing vulnerability to cavitation and levels of embolism under field conditions are discussed.
Summary Despite the importance of nonstructural carbohydrates (NSC) for growth and survival in woody plants, we know little about whole‐tree NSC storage. The conventional theory suggests that NSC reserves will increase over the growing season and decrease over the dormant season. Here, we compare storage in five temperate tree species to determine the size and seasonal fluctuation of whole‐tree total NSC pools as well as the contribution of individual organs. NSC concentrations in the branches, stemwood, and roots of 24 trees were measured across 12 months. We then scaled up concentrations to the whole‐tree and ecosystem levels using allometric equations and forest stand inventory data. While whole‐tree total NSC pools followed the conventional theory, sugar pools peaked in the dormant season and starch pools in the growing season. Seasonal depletion of total NSCs was minimal at the whole‐tree level, but substantial at the organ level, particularly in branches. Surprisingly, roots were not the major storage organ as branches stored comparable amounts of starch throughout the year, and root reserves were not used to support springtime growth. Scaling up NSC concentrations to the ecosystem level, we find that commonly used, process‐based ecosystem and land surface models all overpredict NSC storage.
We know surprisingly little about whole-tree nonstructural carbon (NSC; primarily sugars and starch) budgets. Even less well understood is the mixing between recent photosynthetic assimilates (new NSC) and previously stored reserves. And, NSC turnover times are poorly constrained. We characterized the distribution of NSC in the stemwood, branches, and roots of two temperate trees, and we used the continuous label offered by the radiocarbon (carbon-14, 14C) bomb spike to estimate the mean age of NSC in different tissues. NSC in branches and the outermost stemwood growth rings had the 14C signature of the current growing season. However, NSC in older aboveground and belowground tissues was enriched in 14C, indicating that it was produced from older assimilates. Radial patterns of 14C in stemwood NSC showed strong mixing of NSC across the youngest growth rings, with limited ‘mixing in’ of younger NSC to older rings. Sugars in the outermost five growth rings, accounting for two-thirds of the stemwood pool, had a mean age < 1 yr, whereas sugars in older growth rings had a mean age > 5 yr. Our results are thus consistent with a previously-hypothesized two-pool (‘fast’ and ‘slow’ cycling NSC) model structure. These pools appear to be physically distinct.
We surveyed and wounded forest-grown sugar maple (Acer saccharum Marsh.) trees in a long-term, replicated Ca manipulation study at the Hubbard Brook Experimental Forest in New Hampshire, USA. Plots received applications of Ca (to boost Ca availability above depleted ambient levels) or Al (to compete with Ca uptake and further reduce Ca availability). We found significantly greater total foliar and membrane-associated Ca in foliage of trees in plots fertilized with Ca when compared with trees from Al-addition and control plots (P = 0.005). Coinciding with foliar Ca differences, trees exhibited a significant difference in crown vigor and in percent branch dieback among treatments (P < 0.05), with a trend towards improved canopy health as Ca levels increased. Annual basal area increment growth for the years following treatment initiation (1998)(1999)(2000)(2001)(2002)(2003)(2004) was significantly greater in trees subjected to Ca addition compared with trees in control and Al treatments. Treatment-related improvements in growth were particularly evident after overstory release following a 1998 ice storm. The amount of wound closure was also greatest for trees in Ca-addition plots relative to Al-addition and control plots (P = 0.041). These findings support evidence that ambient Ca depletion is an important limiting factor regarding sugar maple health and highlight the influence of Ca on wound closure and growth following release from competition.Résumé : Nous avons recensé et blessé des tiges d'érable à sucre (Acer saccharum Marsh.) qui croissent en forêt dans le cadre d'une expérience à long terme dans laquelle des traitements visant à modifier la teneur en Ca dans le sol ont été ré-pétés à la forêt expérimentale de Hubbard Brook dans l'É tat du New Hampshire, aux É tats-Unis. Les parcelles ont reçu des applications de Ca (pour augmenter la disponibilité de Ca au-dessus des niveaux ambiants déficients) ou de Al (pour compétionner avec l'absorption de Ca et réduire davantage la disponibilité de Ca). Nous avons observé significativement plus de Ca total associé aux feuilles et aux membranes dans le feuillage des arbres situés dans les parcelles fertilisées avec Ca comparativement aux arbres situés dans les parcelles témoins ou traitées avec Al (P = 0,005). Parallèlement aux différ-ences dans le statut foliaire de Ca, la vigueur de la cime et le pourcentage de mort en cime étaient significativement différ-ents selon le traitement (P < 0,05); l'état de santé de la canopée avait tendance à s'améliorer avec l'augmentation du niveau de Ca. L'accroissement annuel en surface terrière pendant les années qui sont suivi le traitement (1998)(1999)(2000)(2001)(2002)(2003)(2004) était significativement plus élevé chez les arbres qui avaient reçu un supplément de Ca comparativement à ceux qui avaient été traités avec Al ou aux arbres témoins. L'amélioration de la croissance reliée aux traitements était particulièrement évidente après le dégagement survenu dans l'étage dominant à la suite du verglas de 1998. La fermeture des blessu...
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