Ruth D. Yanai

The ecology of root lifespan

Publisher Summary This chapter discusses the competing theories of root lifespan and reviews the evidence available to support them. New methods of root observation and analysis produce data appropriate to testing these theories, but the results till date are few and often conflicting. Tentative generalizations include a suggestion that small diameter roots with low tissue density tend to have short lifespan. Root lifespan appears to be longest in cold environments, but data are lacking for tropical species. There is a strong seasonal variation in lifespan, with roots produced in the fall surviving longest, at least in temperate climates. Species differences are difficult to quantify because of seasonal and interannual variation, but root lifespan of deciduous fruit crops seems to be shorter than those of temperate deciduous forest trees or citrus, a broadleaf evergreen. The current model of root efficiency omits some important factors that may exert control over root lifespan. Fine roots have other functions in addition to absorption, including transport of water and nutrients. Furthermore, seasonality of climate and the need for carbon and nutrient storage could constrain the root lifespan that optimizes plant fitness to differ from that which maximizes root efficiency.

Soil Carbon Dynamics after Forest Harvest: An Ecosystem Paradigm Reconsidered

In one of the most influential studies in the recent history of forest ecology, W. W. Covington (1981) described a pattern in organic matter storage in the forest floors of northern hardwood stands as a function of date of harvest. We review the history of the use and misuse of Covington’s curve, describe the studies that tested and failed to support early interpretations of the curve, and provide some alternate interpretations. The curve suggested that forest floor organic matter declines by 50% within 20 years after harvest, and this decline was attributed to accelerated decomposition and changes in litter inputs after harvest. Subsequent studies showed that decomposition rates of surface litter generally decrease after clear-cutting, but accelerated decomposition remains possible in the Oe and Oa horizons. Changes in litter inputs are still difficult to evaluate, because the rate at which woody debris enters the forest floor is unknown. Although Covington attempted to minimize variation due to mechanical disturbance during logging, a reasonable alternative explanation for low organic matter in the forest floor of young stands is that surface material is mixed into mineral soil during harvesting operations. The pattern of forest floor organic matter in stands of different ages may be partly due to changes over time in logging technology and the intensity of biomass removal, in addition to successional effects. It is important to distinguish between mechanisms that release carbon to the atmosphere and those that transfer it to the mineral soil before making inferences about nutrient cycling and carbon sequestration.

Challenges of measuring forest floor organic matter dynamics:: Repeated measures from a chronosequence

Accurate estimates of the retention of carbon in forest soils following forest disturbances are essential to predictions of global carbon cycling. The belief that 50% of soil carbon is lost in the first 20 years after clearcutting is largely based on a chronosequence study of forest floors in New Hampshire northern hardwoods (Covington, 1981). We resampled forest floors in 13 stands in a similar chronosequence after an interval of 15 years. The three youngest stands, which were predicted to lose organic matter over this time, did not exhibit the 40‐50% losses predicted by the original chronosequence. The oldest stands had about twice as much organic mass in the forest floor as those cut recently, but this pattern could be explained equally well by historical changes in the nature of forest harvest as by the age of the stands. For example, mechanized logging probably causes more mechanical disturbance to the forest floor than horse logging, burying more organic matter into the mineral soil. Markets for forest products and the intensity of harvest removals have also changed over time, possibly contributing to lower organic matter in the forest floor in young stands. In any chronosequence study, effects of change in the nature of the treatment over time can easily be misinterpreted as change with time since treatment. Repeated sampling of the chronosequence provides controls for some of these effects. In the case of forest floor organic matter, however, high spatial variation makes it difficult to distinguish whether the observed variation is more likely due to changes in treatment over time or to differences in time since treatment. Because of the large amounts of carbon involved, small changes in rates of soil organic matter storage may be quite important in global climate change, but they will remain difficult to detect, even with very intensive sampling. # 2000 Elsevier Science B.V. All rights reserved.

Phosphorus budget of a 70-year-old northern hardwood forest

Recent measurements have made it possible to revise and improve the phosphorus budget of the Hubbard Brook Experimental Forest, including partitioning P uptake by vegetation from the forest floor and mineral soil and estimating net P mineralization in the forest floor. Both living biomass and forest floor are accumlating P (at rates of 1.3 and 0.16 kg P ha-1 yr-1 respectively) in this 70-yr old northern hardwood forest. About 61% of the P taken up by the vegetation each year comes from the forest floor (5.9 kg P ha-1 yr-1 of a total 9.6 kg P ha-1 yr-1), even though the P content of this pool is just 5% of that in mineral soil. The turnover rate of P in the forest floor is 7% yr-1, while that of the mineral soil is 0.3% yr-1. Recycling of P in the forest floor is very efficient; of the 5.6 kg P ha-1 yr-1 net mineralization in the forest floor, only 0.3 kg P ha-1 leaches into the mineral soil; the rest is taken up by plants. This tight recycling of P is important because P is less readily available in the mineral soil than in the forest floor.

ACCUMULATION AND DEPLETION OF BASE CATIONS IN FOREST FLOORS IN THE NORTHEASTERN UNITED STATES

Loss of base cations from forest soils can be accelerated by acid rain, by forest regrowth following harvest removals, and by declining inputs of base cations from atmospheric deposition. Calcium losses from forest floors have been reported at several sites in the northeastern United States. To test for loss of base cations from forest floors at the Hubbard Brook Experimental Forest in New Hampshire (USA), we analyzed samples collected on seven dates between 1976 and 1997. Calcium and magnesium contents of the forest floor did not decline significantly; a change >0.9%/yr would have been detectable. Concentrations of Ca were 40% higher in 1969-1970 than in the current study, but the difference is partly due to changes in collection methods. Magnesium concentrations were too variable to detect a loss of <47% over the 21-yr interval. To determine whether base- cation losses were associated with forest growth, we resampled a chronosequence of north- ern hardwood stands in the White Mountains of New Hampshire. The 13 stands did not show consistent changes in Ca, Mg, and potassium over the 15-yr interval. Losses of these cations were most pronounced in stands logged more than 25 yr earlier. Younger stands, contrary to our expectation that rapid forest growth should cause cation depletion, all gained base cations in the forest floor. Early in stand development these forest floors appeared to accumulate biomass along with living vegetation, rather than serving as a net source of nutrients. Finally, in a regional survey of 28 mature stands in the northeastern United States, some lost significant forest-floor Ca and Mg between 1980 and 1990, while others gained. The average change in Ca and Mg content was not significant; a loss of 1.4%/yr would have been detectable. Forest floors in the region are not currently experiencing rapid losses of base cations, though losses may have preceeded the onset of these three studies.

The effect of whole-tree harvest on phosphorus cycling in a northern hardwood forest

A small watershed ecosystem at the Hubbard Brook Experimental Forest in New Hampshire was whole-tree harvested in the dormant season; all trees greater than 10 cm diameter were removed. This harvest removed 50 kg P/ha, five times more P than the bole-only clearcut of an adjacent watershed. The P content of branches and twigs was nearly twice that of stem wood and bark, which contributed to the intensity of P removal. The amount of P removed in harvest was small compared to total P in the mineral soil (1600 kg/ha) but large compared to the pools of P in the living vegetation (70 kg/ha) and the forest floor (85 kg/ha). Negligible P was lost in streamwater and sediment (0.2 kg P/ha over 3 years), although export of other nutrients increased dramatically. Although leaching of P from the forest floor to the mineral soil in the first 2 years after logging was higher in harvested sites than in undisturbed forest, the increase in P leaching (0.7 kg P/(ha yr)) was much less than the estimated decrease in P uptake from the forest floor (4.8 kg/(ha yr)), suggesting a 70% decline in net P mineralization in the forest floor. Even in areas where P uptake by regrowing vegetation was quite high, calculated net P mineralization was 40% lower in the first 2 years of growth than in the undisturbed forest. Revegetation was rapid: in the first 2 years after logging, P in biomass had accumulated to 3% of the uncut forest; uptake was 12% of that in the uncut forest. Whether intensive biomass removals could induce P deficiency in future rotations is unknown.

Biotic Control of Calcium Cycling in Northern Hardwood Forests: Acid Rain and Aging Forests

Observations of declining base saturation in soils and declining calcium (Ca) in streamwater have contributed to concerns that prolonged exposure to acid rain threatens forest health and productivity. We suggest that these changes could be caused, in part, by aging of the forests. To test this possibility, we characterized Ca cycling in previously harvested, variously aged northern hardwood stands over 15–18 years. The Ca content and concentrations in the forest floor and the density of snails, which require Ca for growth, increased in young stands (less than 30 years old) and decreased in older stands (more than 30 years old) over the measurement period. Similarly, the concentrations of Ca in litterfall decreased with stand age, and hydrologic export of Ca from a young stand was higher than that from an old stand. Ecosystem budgets suggest that the supply of Ca from the mineral soil to other parts of the ecosystem is large (3.3–4.7 g Ca m 2 y 1 ) in young forest stands but negligible or negative in older stands (–1 g Ca m 2 y 1 ). This difference in Ca mobilization between young and old stands is large compared to the changes in soil Ca that can reasonably be attributed to acid precipitation (less than 1gm 2 y 1 ). We conclude that changes in soil and streamwater Ca in maturing forests do not necessarily indicate an important loss of bioavailable Ca. Trace amounts of apatite in the mineral soil may be the source of Ca needed for forest regrowth.

Multi-dimensional sensitivity analysis and ecological implications of a nutrient uptake model

Mechanistic models of nutrient uptake are essential to the study of plant-soil interactions. In these models, uptake rates depend on the supply of the nutrient through the soil and the uptake capacity of the roots. The behaviour of the models is complex, although only six to ten parameters are used. Our goal was to demonstrate a comprehensive and efficient method of exploring a steady-state uptake model with variation in parameters across a range of values described in the literature. We employed two analytical techniques: the first a statistical analysis of variance, and the second a graphical representation of the simulated response surface. The quantitative statistical technique allows objective comparison of parameter and interaction sensitivity. The graphical technique uses a judicious arrangement of figures to present the shape of the response surface in five dimensions. We found that the most important parameters controlling uptake per unit length of root are the average dissolved nutrient concentration and the maximal rate of nutrient uptake. Root radius is influential if rates are expressed per unit root length; on a surface area basis, this parameter is less important. The next most important parameter is the effective diffusion coefficient, especially in the uptake of phosphorus. The interactions of parameters were extremely important and included three and four dimensional effects. For example, limitation by maximal nutrient influx rate is approached more rapidly with increasing nutrient solution concentration when the effective diffusion coefficient is high. We also note the ecological implications of the response surface. For example, in nutrient-limited conditions, the rate of uptake is best augmented by extending root length; when nutrients are plentiful increasing uptake kinetics will have greater effect.

Estimating nutrient uptake by mature tree roots under field conditions: challenges and opportunities

Nutrient uptake by roots of mature trees is difficult to measure accurately under field conditions using existing methods. In this review, we discuss current techniques for measuring uptake at the root surface including excised roots, isotopic tracers, autoradiography, depletion, and lysimeters. Although these methods have provided many insights, each has drawbacks. Estimates of uptake are affected by the sampling scheme, experimental conditions, whether roots are excised or not, concentrations of ions, and the rate of efflux of ions. Microbes and mycorrhizas can also affect estimates of uptake. A greater focus on methods development is critical to advancing our understanding of nutrient uptake of mature trees under conditions representative of those in the field.

From missing source to missing sink: Long-term changes in the nitrogen budget of a northern hardwood forest

Biogeochemical monitoring for 45 years at the Hubbard Brook Experimental Forest in New Hampshire has revealed multiple surprises, seeming contradictions, and unresolved questions in the long-term record of ecosystem nitrogen dynamics. From 1965 to 1977, more N was accumulating in living biomass than was deposited from the atmosphere; the “missing” N source was attributed to biological fixation. Since 1992, biomass accumulation has been negligible or even negative, and streamwater export of dissolved inorganic N has decreased from ∼4 to ∼1 kg of N ha–1 year–1, despite chronically elevated atmospheric N deposition (∼7 kg of N ha–1 year–1) and predictions of N saturation. Here we show that the ecosystem has shifted to a net N sink, either storing or denitrifying ∼8 kg of N ha–1 year–1. Repeated sampling over 25 years shows that the forest floor is not detectably accumulating N, but the C:N ratio is increasing. Mineral soil N has decreased nonsignificantly in recent decades, but the variability of these measurements prevents detection of a change of <700 kg of N ha–1. Whether the excess N is accumulating in the ecosystem or lost through denitrification will be difficult to determine, but the distinction has important implications for the local ecosystem and global climate.