Fine Root Production across a Primary Successional Ecosystem Chronosequence at Mt. Shasta,California

Estimating changes in belowground biomass and production is essential for understanding fundamental patterns and processes during ecosystem development. We examined patterns of fine root production, aboveground litterfall, and forest floor accumulation during forest primary succession at the Mt. Shasta Mudflows ecosystem chronosequence. Fine root production was measured using the root ingrowth cores method over 1 year, and aboveground litterfall was collected over 2 years. Fine root production increased significantly with ecosystem age, but only the youngest ecosystem was significantly different from all of the older ecosystems. Root production was 44.5 ± 13.3, 168.3 ± 20.6, 190.5 ± 33.8, and 236.3 ± 65.4 g m⁻² y⁻¹ in the 77, 255, 616, and >850-year-old ecosystems, respectively. Generally, aboveground litterfall and forest floor accumulation did not follow the same pattern as root production. The relative contribution of fine root production to total fine detrital production increased significantly with ecosystem age, from 14 to 49%, but only the youngest ecosystem was significantly different from all of the older ecosystems. Fine root production was significantly correlated with some measures of soil fertility but was not correlated with leaf or total litterfall, or forest floor accumulation. It was best predicted by soil N concentration alone, but this relationship may not be causal, as soil N concentration was also correlated with ecosystem age. For the oldest ecosystem, fine root production was also measured using the sequential intact cores/compartment-flow model method, and the difference between the two estimates was not significant. Our study suggests that the relative contribution of fine roots to fine detrital production, and hence to soil organic matter accumulation, may increase during forest primary succession.     

Ecophysiological responses of salt cedar (Tamarix spp. L.) to the northern tamarisk beetle (Diorhabdacarinulata Desbrochers) in a controlled environment

The northern tamarisk beetle (Diorhabda carinulata Desbrochers) was released in several western states as a biocontrol agent to suppress Tamarix spp. L. which has invaded riparian ecosystems; however, effects of beetle herbivory on Tamarix physiology are largely undocumented and may have ecosystem ramifications. Herbivory by this insect produces discoloration of leaves and premature leaf drop in these ecosystems, yet the cause of premature leaf drop and the effects of this leaf drop are still unknown. Insect herbivory may change leaf photosynthesis and respiration and may affect a plant’s ability to regulate water loss and increase water stress. Premature leaf drop may affect plant tissue chemistry and belowground carbon allocation. We conducted a greenhouse experiment to understand how Tamarix responds physiologically to adult beetle and larvae herbivory and to determine the proximate cause of premature leaf drop. We hypothesized that plants experiencing beetle herbivory would have greater leaf and root respiration rates, greater photosynthesis, increased water stress, inefficient leaf nitrogen retranslocation, lower root biomass and lower total non-structural carbohydrates in roots. Insect herbivory reduced photosynthesis rates, minimally affected respiration rates, but significantly increased water loss during daytime and nighttime hours and this produced increased water stress. The proximate cause for premature leaf drop appears to be desiccation. Plants exposed to herbivory were inefficient in their retranslocation of nitrogen before premature leaf drop. Root biomass showed a decreasing trend in plants subjected to herbivory. Stress induced by herbivory may render these trees less competitive in future growing seasons.     

Insect biological control accelerates leaf litter decomposition and alters short‐term nutrient dynamics in a Tamarix‐invaded riparian ecosystem

Insect herbivory can strongly influence ecosystem nutrient dynamics, yet the indirect effects of herbivore‐altered litter quality on subsequent decomposition remain poorly understood. The northern tamarisk beetle Diorhabda carinulata was released across several western states as a biological control agent to reduce the extent of the invasive tree Tamarix spp. in highly‐valued riparian ecosystems; however, very little is currently known about the effects of this biocontrol effort on ecosystem nutrient cycling. In this study, we examined alterations to nutrient dynamics resulting from beetle herbivory in a Tamarix‐invaded riparian ecosystem in the Great Basin Desert in northern Nevada, USA, by measuring changes in litter quality and decomposition, as well as changes in litter quantity. Generally, herbivory resulted in improved leaf litter chemical quality, including significantly increased nitrogen (N) and phosphorus (P) concentrations and decreased carbon (C) to nitrogen (C:N), C:P, N:P, and lignin:N ratios. Beetle‐affected litter decomposed 23% faster than control litter, and released 16% more N and 60% more P during six months of decomposition, as compared to control litter. Both litter types showed a net release of N and P during decomposition. In addition, herbivory resulted in significant increases in annual rates of total aboveground litter and leaf litter production of 82% and 71%, respectively, under the Tamarix canopy. Our finding that increased rates of N and P release linked with an increased rate of mass loss during decomposition resulting from herbivore‐induced increases in litter quality provides new support to the nutrient acceleration hypothesis. Moreover, results of this study demonstrate that the introduction of the northern tamarisk beetle as biological control to a Tamarix‐invaded riparian ecosystem has lead to short‐term stimulation of nutrient cycling. Alterations to nutrient dynamics could have implications for future plant community composition, and thus the potential for restoration of Tamarix‐invaded ecosystems.     

Production of Total Potentially Soluble Organic C, N, and P Across an Ecosystem Chronosequence: Root versus Leaf Litter

Dissolved organic matter (DOM) plays several important roles in forest ecosystem development, undergoing chemical, physical and/or biological reactions that affect ecosystem nutrient retention. Very few studies have focused on gross rates of DOM production, and we know of no study that has directly measured DOM production from root litter. Our objectives were to quantify major sources of total potentially water-soluble organic matter (DOM_tps) production, with an emphasis on production from root litter, to quantify and compare total potentially soluble organic C, N, and P (DOC_tps, DON_tps, and DOP_tps) production, and to quantify changes in their production during forest primary succession and ecosystem development at the Mt. Shasta Mudflows ecosystem chronosequence. To do so, we exhaustively extracted freshly senesced root and leaf and other aboveground litter for DOC_tps, DON_tps, and DOP_tps by vegetation category, and we calculated DOM_tps production (g m⁻² y⁻¹) at the ecosystem level using data for annual production of fine root and aboveground litter. DOM production from throughfall was calculated by measuring throughfall volume and concentration over 2 years. Results showed that DOM_tps production from root litter was a very important source of DOM_tps in the Mount Shasta mudflow ecosystems, in some cases comparable to production from leaf litter for DON_tps and larger than production from leaf litter for DOP_tps. Total DOC_tps and DON_tps production from all sources increased early in succession from the 77- to the 255-year-old ecosystem. However, total DOP_tps production across the ecosystem chronosequence showed a unique pattern. Generally, the relative importance of root litter for total fine detrital DOC_tps and DON_tps production increased significantly during ecosystem development. Furthermore, DOC_tps and DON_tps production were predominantly driven by changes in biomass production during ecosystem development, whereas changes in litter solubility due to changes in species composition had a smaller effect. We suggest that DOM_tps production from root litter may be an important source of organic matter for the accumulation of SOM during forest ecosystem development. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Shauna M. Uselman, Robert G. Qualls, and Juliane Lilienfein conceived of or designed the study and performed research. SMU analyzed data and wrote the article. SMU and RGQ contributed new methods or models.     

A test of a potential short cut in the nitrogen cycle: The role of exudation of symbiotically fixed nitrogen from the roots of a N-fixing tree and the effects of increased atmospheric CO2 and temperature

N-fixing trees facilitate the growth of neighboring trees of other species. These neighboring species benefit from the simple presence of the N fixation symbiosis in their surroundings. Because of this phenomenon, it has been hypothesized that a change in atmospheric CO₂ concentration may alter the role of N-fixing trees in their environment. It is thought that the role of N-fixing trees in ecosystems of the future may be more important since they may help sustain growth increases due to increased CO₂ concentration in nitrogen limited forests. We examined: (1) whether symbiotically fixed N was exuded from roots, (2) whether a doubled atmospheric CO₂ concentration would result in increased organic N exudation from roots, and (3) whether increased temperature or N availability affected N exudation from roots. This study analyzed exudation of dissolved organic N from the roots of seedlings of the N-fixing tree Robinia pseudoacacia L. in a full factorial design with 2 CO₂ (35.0 and 70.0 Pa) × 2 temperature (26 or 30 °C during the day) × 2 N fertilizer (0 and 10.0 mM N concentration) levels. Trees with no other source of N except N fixation exuded about 1% to 2% of the fixed N through their roots as dissolved organic N. Increased atmospheric CO₂ concentrations did not, however, increase N exudation rates on a per gram belowground biomass basis. A 4 °C increase in temperature and N fertilization did, however, significantly increase N exudation rates. These results suggest that exudation of dissolved organic N from roots or nodules of N-fixing trees could be a significant, but minor, pathway of transferring N to neighboring plants in a much more rapid and direct way than cycling through death, decomposition and mineralization of plant residues. And, while exudation rates of dissolved organic N from roots were not significantly affected by atmospheric CO₂ concentration, the previously observed CO₂ fertilization effect on N-fixing trees suggests that N exudation from roots could play a significant but minor role in sustaining increases in forest growth, and thus C storage, in a CO₂ enriched atmosphere.     

Effects of increased atmospheric CO2, temperature,and soil N availability on root exudation of dissolved organic carbon by a N-fixing tree (Robinia pseudoacacia L.)

Root exudation has been hypothesized as one possible mechanism that may lead to increased inputs of organic C into the soil under elevated atmospheric CO₂, which could lead to greater long-term soil C storage. In this study, we analyzed exudation of dissolved organic C from the roots of seedlings of the N-fixing tree Robinia pseudoacacia L. in a full factorial design with 2 CO₂ (35.0 and 70.0 Pa) × 2 temperature (26° and 30 °C during the day) × 2 N fertilizer (0 and 10.0 mM N concentration) levels. We also analyzed the decomposition rates of root exudate to estimate gross rates of exudation. Elevated CO₂ did not affect root exudation of organic C. A 4 °C increase in temperature and N fertilization did, however, significantly increase organic C exudation rates. Approximately 60% of the exudate decomposed relatively rapidly, with a turnover rate of less than one day, while the remaining 40% decomposed more slowly. These results suggest that warmer climates, as predicted for the next century, may accelerate root exudation of organic C, which will probably stimulate rapid C cycling and may make a minor contribution to intermediate to more long-term soil C storage. However, as these losses to root exudation did not exceed 1.2% of the net C fixed by Robinia pseudoacacia, root exudation of organic C appears to have little potential to contribute to long-term soil C sequestration. This revised version was published online in June 2006 with corrections to the Cover Date.