Retranslocation of calcium and magnesium at the heartwood-sapwood boundary of Atlantic white cedar

Ecology, March, 1995 by Jeffrey A. Andrews, Thomas G. Siccama

Supply of Ca and Mg to growing tissues is further limited by slow transport up the plant stem. Divalent cations are transported by ion exchange, binding to successive exchange sites on the inside walls of functioning tracheids (Bell and Biddulph 1963). Particularly immobile in plant tissues (Bollard 1960, Zimmerman 1960), calcium is required in the plant primarily in the formation of cell walls. Excess Ca in the cell is accumulated as insoluble salts such as Caoxalate and stored in cell vacuoles (Hepler and Wayne 1985). Magnesium is required in relatively high concentrations in living cells, where it is utilized for enzymatic reactions and ribosome formation (Bidwell 1979).

A preliminary chemical analysis of Atlantic white cedar bolewood from a Connecticut swamp-forest showed that Ca and Mg concentrations in the inner sapwood were notably higher than those in the immediately adjacent heartwood. This concentration pattern indicates a process by which Ca and Mg are translocated to the younger, functional sapwood as heartwood is formed. The objectives of this study were (1) to verify the patterns of Ca and Mg concentrations that were identified in the preliminary study of cedar bolewood, (2) to establish the trend as a general phenomenon, and (3) to estimate the amounts of Ca and Mg retranslocated in the current year's growth of trees in one study site.

Site Description

Cedar bolewood samples for chemical and ring-width analysis were obtained from four sites. The North Madison Cedar Swamp, located in North Madison, Connecticut (41 [degrees] 21[minutes] 50[seconds]N, 72 [degrees] 38[minutes]40 [seconds]W), is a late-successional bog-forest that occupies a glacial lake basin formed by till blockage of pre-glacial drainage ways. The canopy is dominated by Atlantic white cedar with scattered red maple (Acer rubrum L.) in the subcanopy and high understory. The remaining understory is composed of ericaceous shrubs and Ilex verticillata. The Bethany Bog, Bethany, Connecticut (41 [degrees] 36[minutes] 40[seconds]N, 73 [degrees] 59[minutes] 50[seconds]W), lies [approximately equal to]30 km west of North Madison in a similar geologic setting. Associated species are similar to those found in the North Madison Swamp, with the addition of scattered white pine (Pinus strobus L.) in the canopy. The Eppley Bog, located in South Kingston, Rhode Island (41 [degrees] 31 [minutes] 00[seconds]N, 71 [degrees] 36[minutes] 55[seconds]W), occupies a kettle depression in sandy outwash sediments. Clear-cut in the early 1900s, Eppley Bog is presently dominated by Atlantic white cedar with scattered red maple and white pine associates and a continuous understory of Rhododendron maxima. The southern New England sites are ombrotrophic peat swamps, depending on atmospheric deposition for nutrient inputs. Peat underlies the cedar to depths of at least 2-5 m, and standing water is generally found in the low areas of microtopography throughout the growing season. No logging or water-level manipulations are known to have occurred since the establishment of the present stands.

The fourth site is a streamside swamp system in Pomona, New Jersey (39 [degrees] 27[minutes] 40[seconds]N, 74 [degrees] 36[minutes] 00[seconds]W), [approximately equal to]24 km northwest of Atlantic City in the New Jersey Pine Barrens. Red maple is a minor associate and pitch pine (Pinus rigida Mill.) grows on the fringes of this cedar forest. Nutrient input to and output from the sand and peat soil occurs via stream flooding in this location.

Methods

Two tree cores, 0.52 cm and 0.45 cm in diameter, were removed from each of 11-20 canopy cedar trees in each swamp during the winter and spring of 1993. Cores were taken at breast height, with a vertical separation of 2-3 cm, and tree diameter at breast height (dbh) was measured. Cores were packed in plastic straws and refrigerated within a few hours of extraction to preserve the integrity and moisture of the wood. The larger diameter core was cut at the heartwood-sapwood boundary with a stainless steel blade. The separate heartwood and sapwood were then cut into [approximately equal to]1-cm sections at a growth-increment boundary. Each section [TABULAR DATA FOR TABLE 1 OMITTED] was cut longitudinally to accurately count the number of years of growth represented, usually 4-10 yr.

Thereafter, the core sections were dried at 80 [degrees] C, weighed, and ashed for [approximately equal to]12 h at 475 [degrees] C. The resulting ash was dissolved in 10 mL of 6mol/L HN[O.sub.3] and the solution diluted to 40-50 mL. Exact solution volume was determined by mass. Solution Ca and Mg concentrations were determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Linear regression equations relating Ca and Mg concentration to year of wood formation (the average year of each 1-cm section) were determined for the heartwood. Calcium and Mg concentrations in the sapwood were summarized by the mean concentration of the 1-cm core section weighted by year.