Testing the generality of above-ground biomass allometry across plant functional types at the continent scale.

Accurate ground-based estimation of the carbon stored in terrestrial ecosystems is critical to quantifying the global carbon budget. Allometric models provide cost-effective methods for biomass prediction. But do such models vary with ecoregion or plant functional type? We compiled 15 054 measurements of individual tree or shrub biomass from across Australia to examine the generality of allometric models for above-ground biomass prediction. This provided a robust case study because Australia includes ecoregions ranging from arid shrublands to tropical rainforests, and has a rich history of biomass research, particularly in planted forests. Regardless of ecoregion, for five broad categories of plant functional type (shrubs; multistemmed trees; trees of the genus Eucalyptus and closely related genera; other trees of high wood density; and other trees of low wood density), relationships between biomass and stem diameter were generic. Simple power-law models explained 84-95% of the variation in biomass, with little improvement in model performance when other plant variables (height, bole wood density), or site characteristics (climate, age, management) were included. Predictions of stand-based biomass from allometric models of varying levels of generalization (species-specific, plant functional type) were validated using whole-plot harvest data from 17 contrasting stands (range: 9-356 Mg ha(-1) ). Losses in efficiency of prediction were <1% if generalized models were used in place of species-specific models. Furthermore, application of generalized multispecies models did not introduce significant bias in biomass prediction in 92% of the 53 species tested. Further, overall efficiency of stand-level biomass prediction was 99%, with a mean absolute prediction error of only 13%. Hence, for cost-effective prediction of biomass across a wide range of stands, we recommend use of generic allometric models based on plant functional types. Development of new species-specific models is only warranted when gains in accuracy of stand-based predictions are relatively high (e.g. high-value monocultures).

Changes in sapwood permeability and anatomy with tree age and height in the broad-leaved evergreen species Eucalyptus regnans.

Increases in plant size and structural complexity with increasing age have important implications for water flow through trees. Water supply to the crown is influenced by both the cross-sectional area and the permeability of sapwood. It has been hypothesized that hydraulic conductivity within sapwood increases with age. We investigated changes in sapwood permeability (k) and anatomy with tree age and height in the broad-leaved evergreen species Eucalyptus regnans F. Muell. Sapwood was sampled at breast height from trees ranging from 8 to 240 years old, and at three height positions on the main stem of 8-year-old trees. Variation in k was not significant among sampling height positions in young trees. However, k at breast height increased with tree age. This was related to increases in both vessel frequency and vessel diameter, resulting in a greater proportion of sapwood being occupied by vessel lumina. Sapwood hydraulic conductivity (the product of k and sapwood area) also increased with increasing tree age. However, at the stand level, there was a decrease in forest sapwood hydraulic conductivity with increasing stand age, because of a decrease in the number of trees per hectare. Across all ages, there were significant relationships between k and anatomy, with individual anatomical characteristics explaining 33-62% of the variation in k. There was also strong agreement between measured k and permeability predicted by the Hagen-Poiseuille equation. The results support the hypothesis of an increase in sapwood permeability at breast height with age. Further measurements are required to confirm this result at other height positions in older trees. The significance of tree-level changes in sapwood permeability for stand-level water relations is discussed.

Nutrient cycling in forests.

Studies of nutrient cycling in forests span more than 100 yr. In earlier years, most attention was given to the measurement of the pools of nutrients in plants and soil and of the return of nutrients from plant to soil in litterfall. The past 20 yr or so have seen a major concentration on the processes of nutrient cycling, with particular emphasis on those processes by which the supply of nutrients to the growing forest is sustained. In the more highly productive forests, up to 10 tonnes of litter of low nutritional quality is deposited annually on the forest floor. The decomposition of this litter, the mineralization of the nutrients it holds, and the uptake of nutrients by tree roots in the carbon-rich environment which results are the themes of this review. Studies of decomposition of litter in forests have been dominated by the role of nitrogen as a limiting factor, a domination which reflects the preponderance of studies of temperate forests in the Northern Hemisphere. For many forests of the world growing on soils of considerable age, it seems more probable that growth and nutrient cycling are limited by phosphorus (or some other element). There is increasing evidence for a number of forests that phosphorus is immobilized in the first stages of decomposition to a significantly greater extent than is nitrogen. Advances in research will depend, as with studies of soil organic matter, in denning and developing analytical techniques for studying biologically active forms of potentially limiting nutrients, rather than total elemental concentrations. The availability of phosphorus in forests is sustained by phosphorus cycling. More than 50% of the total phosphorus in the surface soils is in organic forms and much of the more labile phosphorus is in the form of diesters. Phosphorus availability is determined by competition between biological and geochemical sinks, and it is clear that the sinks in the rhizosphere (plant roots, microorganisms, soil mineral and organic components) are extensively modified by active processes (e.g. production of exudates, nutrient storage in a variety of organic or polymeric forms and nutrient transport away from sites of uptake). There is abundant evidence that roots of many species exude compounds which have the ability to solubilize sources of phosphorus of otherwise low availability. The significance of root exudates (for example, phosphatases, organic acids) in the functioning of perennial ecosystems has yet to be quantified and there are conflicting reports as to the effects of simple organic acids on phosphorus availability. The distribution of phosphorus sinks and their relative competitiveness and their modification are topics of fundamental importance for future research. In contrast to the mineralization of phosphorus, our knowledge of transformations and availability of nitrogen in forest soils is well-developed. Net nitrogen mineralization rates approximate rates of nitrogen return in litterfall but the contribution of nitrification is variable. Nitrification is not inhibited by the low pH of many forest soils and there is increasing evidence of nitrate immobilization by microorganisms and of increased diversity and better competitiveness for NH4 + of nitrifying microorganisms than has previously been accepted. Variability in rates of nitrification is often interpreted as being due to allelopathy. Hypotheses invoking allelopathy are more or less untestable, and it seems likely that new techniques using 15 N in situ will lead to a more fundamental understanding of nitrogen transformations in forest soils. Recent studies in coniferous forest soils have highlighted the short (< 1 d) turnover time of NH4 + . Finally, it seems that forest soils are resistant to major changes in patterns of nitrogen mineralization (and certainly, because of the large number of sinks, in patterns of phosphorus mineralization) following disturbance by natural events such as wind-throw and fire, and by man-made events such as logging and fertilizing. The long-term disturbance by acid rain is a more complex matter since forest ecosystems are not adequate buffers for nitrate. Contents Summary 561 I. Introduction 562 II. Linking nutrient cycling to nutrient availability - Setting the themes 563 III. The nature of soil organic matter 566 IV. Tree roots and the availability of nutrients 566 V. The decomposition of forest litter 569 VI. Mineralization of organically-bound nutrients 571 Acknowledgements 576 References 576.

Effects of fire and harvesting on nitrogen transformations and ionic mobility in soils of Eucalyptus regnans forests of south-eastern Australia.

Effects of fire and forest harvesting on inorganic-N in the soil, on net N-mineralization, and on the leaching of NO inf3sup- -N and metallic cations were measured in forests of Eucalyptus regnans following a severe wildfire in 1983. E. regnans regenerates profusely by seed after fire, and this study compared unburnt forest with forests burnt at varying intensities (surface fire and crown fire), and with logged and burnt forest (slash fire). Total inorganic-N in soil (0-5 cm) increased with increasing fire intensity to a maximum of 158 µg g-1 in the slash fire plot (compared with 51 µg g-1 in the unburnt forest) over the first 205 days after fire. Total inorganic-N returned to a concentration equal to that in the unburnt forest after 485 days at the slash fire plot, and after only 205 days at the surface fire plot. Studies of net mineralization in situ and of NO inf3sup- -N in soil solution support the hypothesis that inorganic-N was immobilized in all of the burnt forests; microbial immobilization after fire is identified as a key process in N-conservation, limiting the substrate available for nitrification and thereby limiting the loss of N from the system by leaching. The concentrations of NO inf3sup- -N and metallic cations in soil solution increased with increasing fire intensity. For the first 318 days after the fire, [NO inf3sup- -N] in soil solution at 10 cm averaged 0.6 µg ml-1 in the unburnt forest, 9.7 mg l-1 in the surface fire plot, 26 mg l-1 in the crown fire plot, and 70 mg l-1 in the slash fire plot. The concentration of metallic cations in soil solution was significantly correlated with [NO inf3sup- -N], the observed order of mobility being Ca2+>Mg2+>K+>Na+. Processes which limit the production and persistence of NO inf3sup- -N in soil solution following disturbance will significantly reduce nutrient losses or redistribution.

Interactions between carbon and nutrients in the forest ecosystem.

"There is little understanding of how improved plant nutrition increases the production of dry matter" (Linder and Rook 1984). Much of the work with forest trees has been at the broad level of concentration of nutrients, rather than with detailed physiological studies of nutrient response functions, of factors controlliing 'efficiency' of nutrient utilization, and of the redistribution of nutrients from aging to developing tissues. The sustained supply of nutrients in forest ecosystems depends on processes by which nutrients are cycled from plant (in organic combinations) to soil and back to plant (in simple inorganic form). Studies of the key processes of decomposition and mineralization, and of equilibria determining nutrient availability have been hampered by lack both of appropriate chemical methods and of methods that distinguish among fractions of organic matter of varying nutritional quality. The root systems of forests must also be studied more intensively. In particular, mechanisms by which nutrients in short supply are taken up (for example, the role of mycorrhizae and of specialized systems such as proteoid roots) and the redistribution of nutrients associated with turnover of the fine root system are fields for future research.