Soil CO2 respiration of forest floor after whole-tree harvesting of spruce stands

201

Global warming will affect the maximum potential abundance of boreal plant species

Forecasting the impact of future global warming on biodiversity requires understanding how temperature limits the distribution of species. Here we rely on Liebig's Law of Minimum to estimate the effect of temperature on the maximum potential abundance that a species can attain at a certain location. We develop 95%‐quantile regressions to model the influence of effective temperature sum on the maximum potential abundance of 25 common understory plant species of Finland, along 868 nationwide plots sampled in 1985. Fifteen of these species showed a significant response to temperature sum that was consistent in temperature‐only models and in all‐predictors models, which also included cumulative precipitation, soil texture, soil fertility, tree species and stand maturity as predictors. For species with significant and consistent responses to temperature, we forecasted potential shifts in abundance for the period 2041–2070 under the IPCC A1B emission scenario using temperature‐only models. We predict major potential changes in abundance and average northward distribution shifts of 6–8 km yr−1. Our results emphasize inter‐specific differences in the impact of global warming on the understory layer of boreal forests. Species in all functional groups from dwarf shrubs, herbs and grasses to bryophytes and lichens showed significant responses to temperature, while temperature did not limit the abundance of 10 species. We discuss the interest of modelling the ‘maximum potential abundance’ to deal with the uncertainty in the predictions of realized abundances associated to the effect of environmental factors not accounted for and to dispersal limitations of species, among others. We believe this concept has a promising and unexplored potential to forecast the impact of specific drivers of global change under future scenarios.202

Stem emissions of monoterpenes, acetaldehyde, and methanol from Scots pine (Pinus sylvestris L.) affected by tree water relations and cambial growth

Abstract Tree stems are an overlooked source of volatile organic compounds (VOCs). Their contribution to ecosystem processes and total VOC fluxes is not well studied, and assessing it requires better understanding of stem emission dynamics and their driving processes. To gain more mechanistic insight into stem emission patterns, we measured monoterpene, methanol, and acetaldehyde emissions from the stems of mature Scots pines (Pinus sylvestris L.) in a boreal forest over three summers. We analysed the effects of temperature, soil water content, tree water status, transpiration, and growth on the VOC emissions, and used generalized linear models to test their relative importance in explaining the emissions. We show that Scots pine stems are considerable sources of monoterpenes, methanol, and acetaldehyde, and their emissions are strongly regulated by temperature. However, even small changes in water availability affected the emission potentials: increased soil water content increased the monoterpene emissions within a day, whereas acetaldehyde and methanol emissions responded within two to four days. This lag corresponded to their transport time in the xylem sap from the roots to the stem. Moreover, the emissions of monoterpenes, methanol, and acetaldehyde were influenced by the cambial growth rate of the stem with six- to ten-day lags. This article is protected by copyright. All rights reserved.Peer reviewe

Calibration of time domain reflectometry for forest soil humus layers

Time domain reflectometry (TDR) has become a widely used method for determining the volumetric water content of soils. However, due to the differences in bulk density and surface area, the relationship between the dielectric constant and water content in organic soils is very different from that in mineral soil. It is therefore impossible to have a universal calibration suitable for all soil types. In this article we describe the relationship between the apparent dielectric constant (Ka) and volumetric water content ([theta]v) for homogenized and undisturbed humus layers (Of + Oh) from forested soil using three empirical models. There was a clear relationship between the Ka and [theta]v and this relationship was best described (R2 = 0.968) with a logarithmic equation of the form [theta]v = aln(Ka) – b. Accurate determination of sample volume was the main source of variation in the calibration, having a greater effect on the calibration results than differences in bulk density

Cation-exchange reactions involving aluminium ions in podzols disturbed by deep tilling

The stratified structure of a podzolic soil is changed during deep tilling of forest soils. Here, we assessed whether (i) cation-exchange properties and (ii) cation-exchange reactions in podzolic soil were altered when the illuvial (B) horizon was subject to environmental conditions prevailing on the soil surface and the organic (O) horizon was buried within the mineral soil. The samples were taken from undisturbed and disturbed podzolic soil profiles at two forest sites that had been ploughed 17 and 31 years ago and planted with Norway spruce seedlings. An increase in soil organic C and effective cation-exchange capacity (CECe) was observed in the Bs horizons exposed to the soil surface. The accumulation of soil C was mainly due to organic matter produced by the planted Norway spruce. The Al3+-Ca2+ cation-exchange equilibria were similar in both the disturbed and undisturbed O and Bs horizons. This indicates that similar reactions between Al3+ and Ca2+ took place in different horizons of podzols irrespective of the disturbance. A positive correlation of CECe in the Bs horizons with soil C indicated that soil organic matter played a major role in the cation-exchange reactions. The importance of exchangeable cations in determining pH variation in the O horizon was additionally supported by the success in describing the relationship between soil solution pHs and base saturation using the extended Henderson-Hasselbach equation in which exchangeable Ale had nonacidic properties similar to those of base cations. The results of this study suggest that organic matter, accumulated within the soil profile during soil formation and upon ploughing, had an important role in determining the cation-exchange properties and reactions in the studied podzols

Modelling effects of soil acidification on tree growth and nutrient status

Understanding the effects of soil acidification on tree growth requires understanding the nutrient relations of trees and stands, notably the uptake of nutrients by the roots in relation to soil conditions. Although a substantial amount of research has been carried out on nutrient relationships, both on plant and stand scale, changes in nutrient uptake as a result of soil acidification are hard to predict. This poses serious problems for attempts to model nutrient uptake by roots in relation to changes in soil chemistry induced by acidification and nitrogen enrichment. Very detailed mechanistic models of root uptake have been developed, but the extrapolation of rhizosphere models developed under controlled, laboratory conditions to field situations is cumbersome. On the other hand, general models of nutrient dynamics very often lack the sensitivity that is required to describe the reaction to gradually changing site conditions. This renders difficult the linkage between critical loads derived from soil criteria, and tree and stand reactions such as allocation and growth. In the models applied to Solling, most emphasis is on soil chemistry, with only few models accounting for feedback mechanisms between soil conditions and tree growth. From the model results presented during the workshop, it would appear that nitrogen and magnesium are the key elements in Solling, but such conclusion is biased as much the same assumption also underlies the guiding concepts on which the models are based. From the models presented at the workshop, no clear consensus emerged on the predictions of the consequences of changes in soil chemistry. At this stage, there seems to be a clear need for additional experimental results on nutrient transport in soil, on decomposition under changed soil conditions, and on nutrient uptake in the case of competition between different ions. In addition, more detailed information on the response of uptake kinetics and biomass allocation in case of reduced nutrient supply, would improve deterministic models of nutrient relations of trees. From such experimental information, theoretical understanding can be derived, and perspectives for generalization and modelling can be drawn