by Claire Morin | 14 September 2022
Published in Agricultural and Forest Meteorology 323: 109092. https://doi.org/10.1016/j.agrformet.2022.109092
Sapwood characteristics, such as sapwood area as well as thermal and hydraulic conductivity, are linked to species-specific hydraulic function and resource allocation to water transport tissues (xylem). These characteristics are often unknown and thus a major source of uncertainty in sap flow data processing and transpiration estimates because bulk rather than species-specific values are usually applied. Here, we analyzed the sapwood characteristics of fifteen common tree species in eastern North America from different taxonomic (i.e., angiosperms and gymnosperms) and xylem porosity groups (i.e., tracheid-bearing, diffuse- or ring-porous species) and we assessed how uncertainties in sapwood characteristics involved in sap flow calculations are propagated in tree water use estimates. We quantified their sapwood area changes with stem diameter (allometric scaling) and thermal conductivity. We combined these measurements with species-specific values of wood density and hydraulic conductivity found in the literature and assessed the role of wood anatomy in orchestrating their covariation. Using an example sap flow dataset from tree species with different xylem porosity, we assessed the sensitivity of tree water use estimates to sapwood characteristics and their interactions. Angiosperms (ring- and diffuse-porous species), with specialized vessels for water transport, showed a steeper relationship (scaling) between tree stem diameter and sapwood area in comparison to gymnosperms (tracheid-bearing species). Gymnosperms (angiosperms) were characterized by lower (higher) wood density and higher (lower) sapwood moisture content, resulting in non-significant differences in sapwood thermal conductivity between taxonomic and xylem porosity groups. Clustering of species sapwood characteristics based on taxonomic or xylem porosity groups and constraining these parameters could facilitate more accurate sap flow calculations and tree water use estimates. When combined with an increasing number of sap flow observations, these findings should improve tree- and landscape-level transpiration estimates, leading to more robust partitioning of terrestrial water fluxes.
by Claire Morin | 14 September 2022
Published in Journal of Hydrology 604: 127196. https://doi.org/10.1016/j.jhydrol.2021.127196
Transpiration, a key component of the hydrological cycle, contributes greatly to the climate system by transferring large amount of water from soils to the atmosphere. Its correct representation within Land Surface Schemes in climate models is crucial to provide accurate and reliable climate projections. In this study, transpiration simulated by the Canadian Land Surface Scheme (CLASS) was compared to long-term observations of sap flow measurements in two boreal forest sites of eastern Canada dominated by balsam fir and black spruce. In general, CLASS adequately models daily transpiration during the growing season for most of the years at both sites. During the tree rehydration period (preceding the growing season), modeled transpiration was greatly underestimated because of overestimating the duration of the snowpack, the latter restricting transpiration. Moreover, CLASS did not capture the impact of extreme events on tree physiology and maintained high transpiration rates during a heat stress and a drought. During both observed and simulated drought events, transpiration modeled using CLASS was overestimated, due to insensitivity to substantial decreases in soil water content; modeled transpiration being strictly controlled by atmospheric variables (vapour pressure deficit and radiations). Thus, we also proposed and implemented a new equation that was able to increase the sensitivity of CLASS to decreasing soil water content. However, this equation needs to be further tested on different sites and tree species.
by Claire Morin | 14 September 2022
Published in Agricultural and Forest Meteorology 323: 109078. https://doi.org/10.1016/j.agrformet.2022.109078
Soil temperature (Tsoil) and soil water (θsoil) are fundamental variables that have an essential role in many processes in forest ecosystems, as well as influencing the tree species distribution and forest composition over time. We tested the Canadian Land Surface Scheme (CLASS) capacity to simulate Tsoil and θsoil in the boreal forest using a sixteen-year data set of daily measurements. Sensitivity analyses were also carried out to evaluate the impact of the thickness of organic layer (TOL), soil texture (percentage sand and clay: PS and PC), drainage parameter (DP), and water freezing point (FP) on simulated Tsoil and θsoil. Finally, the model was also calibrated with a combination of model parameters. CLASS well simulated Tsoil while its performance for θsoil varied by soil horizon and season. In winter particularly, soil liquid water was greatly underestimated because simulated Tsoil was below 0 ◦C. Nevertheless, simulated θsoil seasonal variation corresponded well with observations. Based on sensitivity analyses, TOL had an important effect on both Tsoil and θsoil. Although PS, PC and DP had almost no effect on Tsoil, their effects on θsoil were substantial. Tsoil increased throughout the year and θsoil increased during the winter with decreasing FP, yet the match between modelled θsoil and observations was not substantially improved. In general, Tsoil was well simulated by CLASS, except for the freezing during winter. Model calibration improves greatly both simulated Tsoil and θsoil, especially during winter in all soil layers. However, despite the model calibration, CLASS still requires improvement for modelling Tsoil and θsoil, hence emphasizing the need to review the equations governing these variables in CLASS.
by Claire Morin | 14 September 2022
Published in Science of the Total Environment 853: 158240. https://doi.org/10.1016/j.scitotenv.2022.158240
The widespread increase of dissolved organic carbon (DOC) in northern hemisphere surface waters have been generally attributed to the recovery from acidic deposition and to climatic variations. The long-term responses of DOC to environmental drivers could be better predicted with a better understanding of the mechanisms taking place at the soil level given organic forest soils are the main site of DOC production in forested watersheds. Here, we assess the long-term variation (25 years) of DOC concentration in the solution leaching from the soil organic layer (DOCOL)of a temperate forest. Our results show that DOCOL increased by 32 % (p < 0.001) during the period of study while the lake outlet DOC concentration did not show any changes. Weekly and annual models based on a simple set of explicative variables including throughfall DOC, throughfall precipitation, temperature, litterfall amounts and organic layer leachate calcium concentration (CaOL, taken as a proxy for soil solution ionic strength) explain between 17 and 58 % of the variance in DOCOL depending on model structures and temporal scales. Throughfall DOC and CaOL were both positively related to DOCOL in the models describing its variations at the weekly and annual scale. Temperature was positively correlated to DOCOL, probably due to increased microbial activity, while precipitation had a negative effect onDOCOL (only at the weekly scale), most probably due to a dilution effect. Contrary to our expectations, annual litterfall inputs had no impacts on annual DOCOL variations. Overall, the results shows that DOCOL control is a complex process implicating a set of environmental factors that are acting in different ways while no single variable alone can explain a large part of the variation in both, weekly or annual DOCOL variations.
by Claire Morin | 9 September 2022
Published in Annals of Forest Science 79(39). https://doi.org/10.1186/s13595-022-01152-w
Context: In temperate hardwood forests, increased intensity of soil and canopy disturbances tends to increase species richness due to the establishment of numerous early-successional plant species. However, while competitive pioneer species from early stages of succession can become recalcitrant and alter patterns of natural regeneration, very few studies have examined longer-term effects of these treatments on plant biodiversity.
Aims: In this study, we investigated mid-term (ca. 20 years) effects of different regeneration treatments with varying soil and canopy disturbance intensities. We compared understory plant communities in temperate hardwood forests from all the South of Quebec (Canada).
Methods: Using circular experimental plots of 1962.5 m2 (radius = 25 m), we measured taxonomic and functional diversity indices and soil properties using four levels of disturbance intensity in six temperate hardwood forests of Quebec distributed along a longitudinal gradient. Reference forests, i.e. control forests with no silvicultural treatment known for ≥ 80 years, were compared to 20-year-old single-tree selection cuts, group-selection cuts and group-selection cuts with soil scarification.
Results: Species richness in both group-selection treatments was higher than that in reference forests. Plant equitability and beta diversity among sites in both group-selection treatments were lower than in single-tree selection cuts and control forests. More intense treatments contributed to the mid-term persistence of recalcitrant competitor species (e.g. Rubus idaeus L., Prunus pensylvanica L.f.) whereas soil scarification appears to have negative sustained effects on species known to be sensitive to regeneration treatments (e.g. Monotropa uniflora L., Dryopteris spinulosa Kuhn).
Conclusions: In temperate hardwood forests of Southern Quebec, silvicultural treatments of higher intensities resulted in detrimental effects on soil properties, especially in the surface horizon, 20 years after disturbance. This legacy, in turn, affected the composition and diversity of understory plant communities. The more intense silvicultural treatments contributed to the persistence of pioneer species better adapted to a wider range of environmental conditions and resulted in a decrease in understory plant community heterogeneity among sites. Conversely, singletree selection cutting appeared to be the most appropriate silvicultural treatment for maintaining soil functions and heterogeneity of understory plant communities after 20 years; composition and structure being similar to long-undisturbed forests.
