Paula F. Murakami

Seasonal dynamics and age of stemwood nonstructural carbohydrates in temperate forest trees.

Nonstructural carbohydrate reserves support tree metabolism and growth when current photosynthates are insufficient, offering resilience in times of stress. We monitored stemwood nonstructural carbohydrate (starch and sugars) concentrations of the dominant tree species at three sites in the northeastern United States. We estimated the mean age of the starch and sugars in a subset of trees using the radiocarbon ((14) C) bomb spike. With these data, we then tested different carbon (C) allocation schemes in a process-based model of forest C cycling. We found that the nonstructural carbohydrates are both highly dynamic and about a decade old. Seasonal dynamics in starch (two to four times higher in the growing season, lower in the dormant season) mirrored those of sugars. Radiocarbon-based estimates indicated that the mean age of the starch and sugars in red maple (Acer rubrum) was 7-14 yr. A two-pool (fast and slow cycling reserves) model structure gave reasonable estimates of the size and mean residence time of the total NSC pool, and greatly improved model predictions of interannual variability in woody biomass increment, compared with zero- or one-pool structures used in the majority of existing models. This highlights the importance of nonstructural carbohydrates in the context of forest ecosystem carbon cycling.

Distribution and mixing of old and new nonstructural carbon in two temperate trees

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.

Association of red coloration with senescence of sugar maple leaves in autumn

We evaluated the association of red coloration with senescence in sugar maple (Acer saccharum Marsh.) leaves by assessing differences in leaf retention strength and the progression of the abscission layer through the vascular bundle of green, yellow, and red leaves of 14 mature open-grown trees in October 2002. Computer image analysis confirmed visual categorization of leaves as predominantly green, yellow or red, and chemical quantification of leaf pigment concentrations verified that leaf color reflected underlying differences in leaf biochemistry. Significantly lower chlorophyll concentrations within red and yellow leaves indicated that senescence was more advanced in leaves from these color categories relative to green leaves. Among leaf types, only red leaves contained high concentrations of anthocyanins. There were significant differences in leaf retention capacity among color categories, with the petioles of green leaves being the most firmly attached to twigs, followed by red and then yellow leaves. Microscopic analysis indicated that yellow leaves had the most advanced extension of the abscission layer through the vasculature, with green and red leaves having significantly less abscission layer progression than yellow. A more limited progression of the abscission layer through vascular bundles may be evidence of delayed leaf senescence that could extend resorption of mobile leaf constituents. Together, results from this study suggest an association between leaf anthocyanin content and functional delays in senescence.

Cold in the common garden: comparative low-temperature tolerance of boreal and temperate conifer foliage

Because they maintain green foliage throughout the winter season, evergreen conifers may face special physiological challenges in a warming world. We assessed the midwinter low-temperature (LT) tolerance of foliage from eight temperate and boreal species in each of the genera Abies, Picea, and Pinus growing in an arboretum in Trondheim, Norway, using relative electrolyte leakage (REL) as an index of cell injury. Relatively LT sensitive species came from temperate coastal and Mediterranean environments and displayed a well-defined sigmoidal response to LT stress, with LT50 ranging from −27 to −38°C. Species originating from boreal regions were not lethally stressed by slow freezing to temperatures as low as −80°C, while species from temperate mountains and continental interiors displayed intermediate responses, with LT50s ranging from −33 to −44°C. Further evaluation of one sensitive and one insensitive species in each genus showed that boreal species can survive quenching in liquid nitrogen at −196°C provided they are first slowly cooled to −30°C or lower. Quantitative image analysis of color changes resulting from LT stress followed by exposure to light showed that foliage from nonlethally stressed boreal species developed mild to moderate chlorosis while more sensitive species developed a mixture of chlorosis and necrosis, with significant necrosis occurring mainly at temperatures resulting in REL of 50% or more. Sensitive and insensitive trees differed significantly in total raffinose, sucrose, and total sugar concentrations, and raffinose and sucrose correlated significantly with LT50 within the sensitive group.

An instructional guide for leaf color analysis using digital imaging software

Digital color analysis has become an increasingly popular and cost-effective method utilized by resource managers and scientists for evaluating foliar nutrition and health in response to environmental stresses. We developed and tested a new method of digital image analysis that uses Scion Image or NIH image public domain software to quantify leaf color. This publication provides instructions for using this software to measure the percentage green and red in leaves, colors of particular importance for the assessment of plant health. Comparisons of results from digital analyses of 326 scanned images of leaves and concurrent spectrophotometric measures of chlorophyll a, chlorophyll b, and anthocyanins verify that image analysis provides a reliable quantitative measure of leaf color and the relative concentrations of underlying plant pigments.

Effects of soil calcium and aluminum on the physiology of balsam fir and red spruce saplings in northern New England

We examined the influence of calcium (Ca) and aluminum (Al) nutrition on the foliar physiology of red spruce (Picea rubens Sarg.) and balsam fir [Abies balsamea (L.) Mill.] in northern New England, USA. At the Hubbard Brook Experimental Forest (NH, USA), spruce and fir saplings were sampled from control, Al-, and Ca-supplemented plots at a long-established nutrient perturbation (NuPert) study in fall 2008. Measurements included cation concentrations (roots and foliage), dark-adapted chlorophyll fluorescence (Fv/Fm), soluble sugar concentrations, and ascorbate peroxidase (APX) and glutathione reductase (GR) activity in current-year foliage. Additional untreated saplings were sampled from base-rich Sleepers River (VT) and base-poor Jeffers Brook (NH) for Fv/Fm and foliar nutrient concentrations. At NuPert, there were significantly greater Ca concentrations and Ca:Al ratios in roots from the Ca end vs. the Al end of the Al-control-Ca addition gradient. There were also trends toward greater foliar Ca and Ca:Al ratios and lower Al concentrations across the treatment gradient at NuPert and for foliage at Sleepers River vs. Jeffers Brook. At NuPert, Fv/Fm and APX activity increased across the treatment gradient, and red spruce was higher in these measures than balsam fir. These patterns were also observed when Jeffers Brook and Sleepers River were compared. Increased Ca availability appeared to enhance the ability of red spruce and balsam fir to repair oxidative stress damage, including photooxidation. Our findings support work indicating a greater contemporary level of stress for balsam fir relative to red spruce, which is surprising considering the well-documented regional decline of spruce.

Observations on the relationship between above- and below-ground anthocyanin production in Galax urceolata (Poir.) Brummitt growing in sun-exposed and shaded locations

Abstract Galax urceolata (Diapensiaceae) is a common evergreen herb of southern Appalachian forests. During the fall and winter, leaves of plants in high light produce substantial amounts of anthocyanins. Oddly, rhizomes in these plants also accumulate anthocyanins. The purpose of this observational study was to identify seasonal trends in anthocyanin production in above- and below-ground tissues of Galax. We measured anthocyanins and chlorophyll in Galax using standard extraction and spectrophotometric procedures from plants in sun-exposed and shaded locations; one population at Mount Jefferson State Natural Area in fall 2007 and two at Grandfather Mountain in fall/winter 2008–09. Rhizome carbohydrates (soluble sugars and starch) were measured from the Grandfather populations using high performance liquid chromatography. We found significantly more anthocyanins in leaves and rhizomes of plants from sun-exposed locations compared to plants from shaded locations, but no differences in carbohydrate concentrations. Starch levels declined significantly through the fall/winter of 2008–09, while soluble sugars, such as sucrose, raffinose, and fructose, increased. Rhizomal anthocyanins were distributed throughout the entire cross-section except for the vascular tissues, whereas in petioles and leaves, they were restricted to the epidermal or subepidermal layers. Rhizomal anthocyanins often concentrated around lateral roots as they penetrated the cortex. These results contradict the paradigm that light is always required for anthocyanin production, and suggest the possibility of some form of communication between leaves and rhizomes with respect to anthocyanin content, although the nature of that signal is unknown. At this time, the adaptive significance of below-ground anthocyanins in Galax remains unresolved.