Mixed-power scaling of whole-plant respiration from seedlings to giant trees.

The scaling of respiratory metabolism with body mass is one of the most pervasive phenomena in biology. Using a single allometric equation to characterize empirical scaling relationships and to evaluate alternative hypotheses about mechanisms has been controversial. We developed a method to directly measure respiration of 271 whole plants, spanning nine orders of magnitude in body mass, from small seedlings to large trees, and from tropical to boreal ecosystems. Our measurements include the roots, which have often been ignored. Rather than a single power-law relationship, our data are fit by a biphasic, mixed-power function. The allometric exponent varies continuously from 1 in the smallest plants to 3/4 in larger saplings and trees. Therefore, our findings support the recent findings of Reich et al. [Reich PB, Tjoelker MG, Machado JL, Oleksyn J (2006) Universal scaling of respiratory metabolism, size, and nitrogen in plants. Nature 439:457-461] and West, Brown, and Enquist [West GB, Brown JH, Enquist BJ (1997) A general model for the origin of allometric scaling laws in biology. Science 276:122 -126.]. The transition from linear to 3/4-power scaling may indicate fundamental physical and physiological constraints on the allocation of plant biomass between photosynthetic and nonphotosynthetic organs over the course of ontogenetic plant growth.

The self-thinning process in mangrove Kandelia obovata stands.

The self-thinning process was monitored in crowded Kandelia obovata Sheue, Liu & Yong stands over four years. The frequency distribution of tree phytomass was an L-shape, which was kept over the experimental period. Spearman's rank correlation coefficient for phytomass decreased as the time span of the comparison became longer, a result which indicates that the rank of phytomass changes as stands grow. Death of trees resulted from one-sided competition, i.e., death occurred in lower-rank trees. Surviving trees continued to grow. Whatever the current spatial distribution of the trees, death occurred randomly and the spatial distribution gradually became close to random as stands grew. The self-thinning exponent was 1.46, which can be regarded as evidence in favor of the 3/2 power law of self-thinning. Relative growth rate, RGR, decreased in proportion to decreasing relative mortality rate, RMR, with a proportionality constant of 1.57, which was not significantly different from the slope of the self-thinning exponent. This experimental result probably justifies the assumption that the ratio of RGR to RMR in the mean phytomass-density trajectory for any self-thinning population with different densities becomes constant as the growth stage progresses.

Stand stratification and woody species diversity of a subtropical forest in limestone habitat in the northern part of Okinawa Island.

Stand stratification and woody species diversity were studied in a subtropical forest in limestone habitat in the northern part of Okinawa Island, Japan. The forest stand consisted of four architectural layers. Cinnamomum japonicum Sieb. was the most dominant species as it had the highest importance value in all layers, especially in the lower three layers. Although Rhus succedanea L. was a major top-layer species, it might disappear because of its absence in the lower two layers. The lower two layers had similar floristic composition, while the composition of the top layer differed greatly from that of the lower two layers. The value of H' in each layer increased from the bottom to the second layer and then decreased, and the value of J' in each layer consistently increased from the bottom layer upward. The values of Shannon's index H' and Pielou's index J' were 4.27 bit and 0.74, respectively, at the stand level. The trends of H' and J' based on the cumulative height range from the top layer downward were, respectively, the same as those in each layer for the present forest in limestone habitat. However, these trends were opposite from those of a nearby subtropical forest in silicate habitat, where the values of H' and J' based on the cumulative height range decreased steadily from the bottom layer upward.

Changes in the relationship between tree size and aboveground respiration in field-grown hinoki cypress (Chamaecyparis obtusa) trees over three years.

Respiration measurements of aerial parts of 18-year-old hinoki cypress (Chamaecyparis obtusa (Sieb. et Zucc.) Endl.) trees were made under field conditions over three years to study changing relationships with tree age between respiration and phytomass, phytomass increment, and leaf mass. The relationship between annual respiration (r(a)) and phytomass (w(T)) was approximated by a proportional function (r(a) = aw(T)), where the proportional constant (a) decreased year by year. The effect of time on the relationship between annual respiration and phytomass of each sample tree was fitted by a power function. Respiration of the tree suppressed by the canopy decreased year by year, but respiration of the other trees increased slightly with age. The relationship between annual respiration and leaf mass was also approximated by a generalized power function. Excluding the suppressed tree, the relationship between annual respiration (r(a)) and the annual increment of aboveground phytomass (Deltaw(T)) was described by a proportional function (r(a) = 2.27Deltaw(T)), where the proportional constant, 2.27, was independent of sample tree and year, indicating that about 2.3 times of the annual aboveground phytomass increment equivalent was respired annually. For any tree, the time constant relationships between annual respiration and leaf mass and phytomass increment for different-sized trees were similar to the corresponding time continuum relationships. In contrast, the time continuum relationship between annual respiration and phytomass differed from the time constant relationship, indicating that respiration of less active woody tissue contributed significantly to aboveground respiration. Based on the relationship between tree size and annual respiration, annual aboveground stand respiration was estimated to be 25.0, 26.9, and 25.8 Mg(dm) ha(-1) year(-1) for the three consecutive years, respectively, and the corresponding aboveground stand biomass was 60.0, 69.0, and 76.8 Mg(dm) ha(-1).

Carbon balance of the aerial parts of a young hinoki cypress (Chamaecyparis obtusa) stand.

The aerial carbon balance of a 9-year-old hinoki cypress (Chamaecyparis obtusa (Sieb. et Zucc.) Endl.) stand with an aerial biomass of 24.6 Mg(DW) ha(-1) was studied over a 1-year period. Various components that constitute gross production were measured on the basis of the summation method. Respiration of the aerial parts of six sample trees was measured at monthly intervals by an enclosed standing-tree method. The aerial respiration of sample trees was partitioned into growth and maintenance respiration by a two-component model. The growth coefficient varied between 1.2791 and 1.7957 g CO(2) g(DW) (-1) giving a mean value of 1.5223 +/- 0.0729 (SE) g CO(2) g(DW) (-1), whereas the maintenance coefficient ranged between 0.0200 and 0.0373 g CO(2) g(DW) (-1) month(-1) with a mean value of 0.0299 +/- 0.0031 (SE) g CO(2) g(DW) (-1) month(-1). Growth and maintenance respiration of the stand were estimated to be 8.62 and 10.52 Mg CO(2) ha(-1) year(-1), respectively. An open-top cloth trap method was employed to measure the death of the parts of five sample trees. The death of leaves and total organs in the stand were assessed to be 6.26 and 7.60 Mg(DW) ha(-1) year(-1), respectively. Tree mortality and biomass increment were 1.47 and 4.64 Mg(DW) ha(-1) year(-1), respectively. In terms of carbon, respiration, death, grazing and biomass increment were equivalent to 5.22, 4.53, 0.04 and 2.32 Mg C ha(-1) year(-1), respectively. Net production and gross production were estimated to be 6.89 and 12.11 Mg C ha(-1) year(-1), respectively. Biomass accumulation ratio (biomass/net production) and overall efficiency (net production/gross production) were 1.8 and 0.57, respectively.