Tree Growth In Temperate Forests Research Articles

Modelling short-term CO 2 fluxes and long-term tree growth in temperate forests with ASPECTS

The net ecosystem exchange (NEE) of CO 2 between temperate forests and the atmosphere governs both carbon removal from the atmosphere and forest growth. In recent years, many experiments have been conducted to determine temperate forest NEE. These data have been used by forest modellers to better understand the processes that govern CO 2 fluxes, and estimate the evolution of these fluxes under changing environmental conditions. Nevertheless, it is not clear whether models capable of handling short-term processes, which are mostly source-driven, can provide an accurate estimate of long-term forest growth, which is potentially more influenced by sink- and phenology-related processes. To analyse the interactions between short- and long-term processes, we developed the ASPECTS model, which predicts long-term forest growth by integrating, over time, hourly NEE estimates. Validation data consisting of measurements of NEE by eddy-covariance and forest carbon reservoir estimates were obtained from mixed deciduous and evergreen experimental forests located in Belgium. ASPECTS accurately estimated both: (1) the NEE fluxes for several years of data; and (2) the amount of carbon contained in stems, branches, leaves, fine and coarse roots. Our simulations demonstrated that: (1) NEE measurements in Belgian forests are compatible with forest growth over the course of the 20th century; and (2) that forest history and long-term processes need to be considered for accurate simulation of short-term CO 2 fluxes.

FOREST LITTER PRODUCTION, CHEMISTRY, AND DECOMPOSITION FOLLOWING TWO YEARS OF FREE-AIR CO2ENRICHMENT

Increases in tree biomass may be an important sink for CO 2 as the atmospheric concentration continues to increase. Tree growth in temperate forests is often limited by the availability of soil nutrients. To assess whether soil nutrient limitation will constrain forest productivity under high atmospheric CO 2 , we studied the changes in forest litter production and nutrient cycling in a maturing southern U.S. loblolly pine–hardwood forest during two years of free-air CO 2 enrichment. The objective of this paper is to present data on the chemistry of green leaves and leaf litter, nutrient-retranslocation efficiency, aboveground litter production, whole-system nutrient-use efficiency, decomposition, and N availability in response to forest growth under elevated CO 2 . The chemical composition of green leaves and leaf litter was largely unaffected by elevated CO 2 . Green-leaf nitrogen (N) and phosphorus (P) concentrations were not significantly lower under elevated CO 2 . N and P retranslocation from green leaves did not increase under elevated CO 2 ; therefore, leaf litter N and P concentrations were not significantly lower under elevated CO 2 . The concentrations of carbon, lignin, and total nonstructural carbohydrates in litter were not significantly different under elevated CO 2 . Total aboveground litterfall increased significantly with CO 2 fumigation. The increase in litterfall was due to significant increases in loblolly pine leaf litter and bark production. The mass of leaves from deciduous species did not increase with CO 2 fumigation. Whole-system nutrient-use efficiency (aboveground litterfall/nutrient content of litterfall) did not increase as a consequence of forest growth under elevated CO 2 , but N and P fluxes from vegetation to the forest floor increased significantly. During the second year of CO 2 fumigation, the flux of N and P to the forest floor in litterfall increased by 20% and 34%, respectively. The rate of mass loss during one year of decomposition was unaffected by “litter type” (whether the litter was produced under ambient or elevated CO 2 ), nor by the “site” of decomposition (whether the litter was decomposed in the ambient or elevated CO 2 plots). N was immobilized in litter during decomposition, whereas P was mineralized. There was no consistent effect of litter type or site on nutrient dynamics in decomposing litter. There was no significant effect of elevated CO 2 on the pool size of inorganic N (NH 4 + and NO 3 − ) in the top 7.5 cm of mineral soil. The rate of net N mineralization and nitrification in mineral soil was not significantly different between treatment and control plots. Identifying the source of the nutrients lost in litterfall is critical to the long-term potential growth stimulation of forests under elevated CO 2 . If the nutrients lost from biomass come from storage (e.g., the movement of nutrients from wood to leaves), then the increase in litter production should decrease over time as slowly replenished nutrient reserves are exhausted. If the nutrients lost in plant litter are replaced by uptake from soils, then it is possible (1) that trees acquire soil nutrients at a rate commensurate with growth stimulated by elevated CO 2 ; and (2) that forest productivity will be stimulated by elevated CO 2 in the long term.