Effects of stand age, wildfire and clearcut harvesting on forest floor in boreal mixedwood forests

Carbon (C) in the forest floor (FF) of the boreal region is an important reservoir of terrestrial C. We examined the effects of stand age and disturbance type (clearcutting vs. wildfire) on quantity and quality of organic C of FF in a boreal mixedwood forest of central Canada. Forest floor samples were collected from 6 post-fire (2- to 203-year-old) and 3 post-harvest age classes (2- to 28-year-old) on mesic sites, each randomly replicated three times. Samples were analyzed to determine the physical and chemical properties and the C quality was assessed by quantifying C fractions as easily labile, moderately labile and recalcitrant. Bulk density, total organic C concentration, N concentration and the cation exchange capacity increased with stand age and peaked at 85-year-old sites. Soil pH and concentration of P and K decreased with stand age. In post-fire stands, the depth of FF, total organic C, and labile C fractions increased with stand age in the 2- to 85-year-old stands, while recalcitrant C was lower in 2-year-old stands than older stands. In stands ≤28 years old, post-harvest sites had significantly higher concentration of total organic C and the three C fractions than post-fire sites in 2-year-old stands. No or marginal difference occurred between the two stand origins in 10- and 28-year-old stands. The relative proportions of C fractions did not differ with stand age or stand origin. Our results showed that the quantity of organic C in FF of boreal mixedwoods increased with stand development till 85 years and then slightly decreased in older stands, and post-harvest stands had a higher amount of organic C than post-fire stands immediately after disturbance, but the effect of two disturbances on C in FF converged shortly (within 10 years). The quality of organic C remains the same through stand development and between the two studied stand origins.     

Fine root dynamics in lodgepole pine and white spruce stands along productivity gradients in reclaimed oil sands sites

Open‐pit mining activities in the oil sands region of Alberta, Canada, create disturbed lands that, by law, must be reclaimed to a land capability equivalent to that existed before the disturbance. Re‐establishment of forest cover will be affected by the production and turnover rate of fine roots. However, the relationship between fine root dynamics and tree growth has not been studied in reclaimed oil sands sites. Fine root properties (root length density, mean surface area, total root biomass, and rates of root production, turnover, and decomposition) were assessed from May to October 2011 and 2012 using sequential coring and ingrowth core methods in lodgepole pine (Pinus contorta Dougl.) and white spruce (Picea glauca (Moench.) Voss) stands. The pine and spruce stands were planted on peat mineral soil mix placed over tailings sand and overburden substrates, respectively, in reclaimed oil sands sites in Alberta. We selected stands that form a productivity gradient (low, medium, and high productivities) of each tree species based on differences in tree height and diameter at breast height (DBH) increments. In lodgepole pine stands, fine root length density and fine root production, and turnover rates were in the order of high > medium > low productivity sites and were positively correlated with tree height and DBH and negatively correlated with soil salinity (P < 0.05). In white spruce stands, fine root surface area was the only parameter that increased along the productivity gradient and was negatively correlated with soil compaction. In conclusion, fine root dynamics along the stand productivity gradients were closely linked to stand productivity and were affected by limiting soil properties related to the specific substrate used for reconstructing the reclaimed soil. Understanding the impact of soil properties on fine root dynamics and overall stand productivity will help improve land reclamation outcomes.     

The effect of dense understory dwarf bamboo (Sasa senanensis) on soil respiration before and after clearcutting of cool temperate deciduous broad-leaved forest

To evaluate the effect of understory dwarf bamboo (Sasa senanensis) on soil respiration in forest ecosystems, we compared soil respiration rates between four deciduous broad-leaved forest sites representing two levels of understory Sasa (with and without) and two levels of forest stand age (50-year-old stand and 1-year-old stand after clearcut). The understory Sasa enhances the soil respiration rate both before and after the clearcutting of deciduous broad-leaved forest. The Sasa sites had larger total belowground biomass compared with the non-Sasa sites, which could be attributed to Sasa presence. Our results also suggest that clearcutting decreases temperature-normalized soil respiration rates (R ₁₅) and temperature sensitivity (Q ₁₀) in both Sasa and non-Sasa ecosystems. Clearcutting significantly reduced the fine root biomass of trees and Sasa. The fine roots of trees and Sasa had high specific respiration rates compared with larger roots and rhizomes at Sasa and non-Sasa sites, respectively. Therefore, we hypothesize that the loss of fine roots after clearcutting is responsible for the reduction in soil respiration rate. A comparison with other studies revealed a positive linear relationship between total (tree and Sasa) fine root biomass and R ₁₅, suggesting that fine root biomass controls soil respiration at the landscape scale. The Q ₁₀ value is also likely to be related to fine root biomass, although the relationship was not significant. We conclude that understory Sasa increases belowground biomass, especially fine roots, and the spatial variation in soil respiration at the landscape scale.     

Effects of Forest Type and Disturbance on Diversity of Coarse Woody Debris in Boreal Forest

Coarse woody debris (CWD) volume and diversity are vital attributes of forest ecosystems. However, despite their importance, their long-term dynamics associated with fire- or logging-origin and overstory type have not been examined in boreal forest. We hypothesize that (1) CWD compositional diversity increases with stand development whereas CWD volume follows a U-shaped pattern. Furthermore, we attempted to test if (2) CWD volume and compositional diversity converge for postlogged and postfire stands through stand development, and (3) mixedwoods have more CWD volume and greater compositional diversity than conifer or broadleaf overstory types. We sampled 72 stands ranging in age from 7 to 201 years in fire-origin stands and 7–31 years in managed stands with conifer, mixedwood, and broadleaf overstory types in central boreal Canada. For fire-origin stands, snag volume was 100–260 m³/ha in 7-year-old stands, 5–20 m³/ha in 25-year-old stands, and 25–60 m³/ha in older stands; downed woody debris (DWD) volume decreased from 7 to 72–90 year-old stands, increased in 124- to 139-year-old stands, then either decreased or increased in 201-year-old stands depending on overstory type. CWD diversity increased from 25 to 124–139 year-old and plateaued, but in 7-year-old stands, CWD diversity was as high as that in the 124 and up year-old age classes. Logging resulted in a smaller amount and lower size variability of CWD in 7-year-old stands, with a larger portion being fast-decomposing Betula papyrifera. Most CWD characteristics had not converged by approximately 30 years since disturbance between the two stand origins. More diverse CWD occurred in mixedwoods, but conifer stands contained the greatest CWD volume except in 7 year-old postfire stands. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. B. W. Brassard collected and analyzed data and wrote the paper. H. Y. H. Chen conceived and designed the study, analyzed data, and critiqued earlier drafts of the paper.     

The direct regeneration hypothesis in northern forests

Question: Can the direct regeneration hypothesis (DRH) be used to predict post‐disturbance regeneration after fire, wind disturbance, and clearcutting in northern forests? Do life‐history traits such as regeneration strategy and shade tolerance influence post‐disturbance regeneration success of tree species? Location: Northern forests in North America. Methods: A meta‐analysis was conducted by collecting published data on pre‐ and post‐disturbance stand compositional characteristics in the northern forests. For each tree species, compositional difference (CD) was calculated as the difference between basal area proportions of the post‐ and pre‐disturbance stands, but for post‐disturbance stands <25 years of age, post‐disturbance proportions were calculated based on relative stem density. Results: Species response to disturbances was best explained by regeneration strategy, while disturbance type had no effect on CD. The proportion of broadleaf trees with either strong or weak vegetative reproduction ability increased after all disturbances. Serotinous species had CD values not significantly different from zero after fire, while CD for semi‐serotinous species was negative. The post‐disturbance proportions of non‐serotinous conifers decreased after all forms of disturbance. Conclusions: All disturbances promote broadleaf trees, regardless of regeneration strategy (suckering, sprouting, or seeding). The DRH is supported for conifers with serotinous cones after fire. Fire causes local extinction of non‐serotinous conifers, while wind and clearcutting only decrease the proportion of non‐serotinous conifers because of partial survival of seed sources and advanced regeneration. This study suggests that increasing stand‐replacing disturbances associated with global climate change will promote broadleaf trees in northern forests.     

Succession of boreal forest spider assemblages following wildfire and harvesting

To test whether spider succession following harvest differed from succession following wildfire, spiders were collected by pitfall trapping and sweep netting over two years in aspen-dominated boreal forests. Over 8400 individuals from 127 species of spiders were identified from 12 stands representing three age-classes (stand origin in 1995, 1982, and 1968) and two disturbance types (wildfire and harvesting). The diversity of spider assemblages tended to be higher in fire-origin stands than in harvest-origin stands; the youngest fire-origin stands also supported more even distributions of spider species. Spider assemblages responded quickly to wildfire and harvesting as open habitat specialists colonized stands within one year after disturbance. Many web-building species common to older forests either survived harvesting, or re-colonized harvest-origin stands more rapidly than they re-colonized fire-origin stands. Cluster analyses and DCA ordination show faunal convergence by ca 30 years after wildfire and harvesting; trajectories in re-colonization, however, differed by disturbance type as the succession of spider assemblages from fire-origin stands lagged behind spider succession in harvest-origin stands. Comparison with cluster analyses using vegetation data and abiotic site conditions suggests spider assemblages recover from harvesting and fire more rapidly than do a variety of other site characteristics. Several spider species (e.g. Gnaphosa borea Kulezyński, Pirata bryantae Kurata, Arctosa alpigena (Doleschall)) appear dependent on some of the conditions associated with wildfires as they were absent or rarely collected in harvest-origin stands.     

Landscape-Scale Disturbances Modified Bird Community Dynamics in Successional Forest Environment

Ecosystem-based forest management strives to develop silvicultural practices that best emulate natural disturbances such as wildfire to conserve biodiversity representative of natural forest ecosystems. Yet, current logging practices alter forest structure and reduce the proportion of old-growth forest and, consequently, can exert long-term effects on the dynamics of forest biota. The stand- and landscape-scale factors driving bird community dynamics in post-disturbance environment remain poorly understood. In this study, we examined bird community dynamics along successional gradients in boreal ecosystems originating from fire and logging in landscapes dominated by old-growth forest. We tested if bird species richness and community compositions in clear-cutting stands became comparable to those in natural stands after 70 years, and identified the relative contributions of stand- and landscape-scale forest attributes in bird community dynamics. Based on records of bird occurrences at 185 field sites in natural and clearcutting stands, we demonstrate that (1) both forest structures and bird communities underwent evident changes along successional gradients in post-clearcutting environment; (2) bird species richness and community composition in 60- to 70-years-old clearcutting stands still differed from those in 50- to 79-years-old natural stands, in spite of the fact that most forest attributes of clearcutting stands became comparable to those of natural stands after 40 years; and (3) landscape disturbances contributed more than stand characteristics in explaining the lack of convergence of mature forest species, residents, and short-distance migrants in post-clearcutting environment. Our study points out that more regards should be paid to improve the landscape configuration of the managed forests, and implies that old-growth forest retention within logged areas, combined with selection cutting and prolonged logging rotations, can better emulate fire and alleviate forest harvesting effects on bird community assemblages typical of natural boreal ecosystem.     

The legacy of harvest and fire on ecosystem carbon storage in a north temperate forest

Forest harvesting and wildfire were widespread in the upper Great Lakes region of North America during the early 20th century. We examined how long this legacy of disturbance constrains forest carbon (C) storage rates by quantifying C pools and fluxes after harvest and fire in a mixed deciduous forest chronosequence in northern lower Michigan, USA. Study plots ranged in age from 6 to 68 years and were created following experimental clear‐cut harvesting and fire disturbance. Annual C storage was estimated biometrically from measurements of wood, leaf, fine root, and woody debris mass, mass losses to herbivory, soil C content, and soil respiration. Maximum annual C storage in stands that were disturbed by harvest and fire twice was 26% less than a reference stand receiving the same disturbance only once. The mechanism for this reduction in annual C storage was a long‐lasting decrease in site quality that endured over the 62‐year timeframe examined. However, during regrowth the harvested and burned forest rapidly became a net C sink, storing 0.53 Mg C ha⁻¹ yr⁻¹ after 6 years. Maximum net ecosystem production (1.35 Mg C ha⁻¹ yr⁻¹) and annual C increment (0.95 Mg C ha⁻¹ yr⁻¹) were recorded in the 24‐ and 50‐year‐old stands, respectively. Net primary production averaged 5.19 Mg C ha⁻¹ yr⁻¹ in experimental stands, increasing by < 10% from 6 to 50 years. Soil heterotrophic respiration was more variable across stand ages, ranging from 3.85 Mg C ha⁻¹ yr⁻¹ in the 6‐year‐old stand to 4.56 Mg C ha⁻¹ yr⁻¹ in the 68‐year‐old stand. These results suggest that harvesting and fire disturbances broadly distributed across the region decades ago caused changes in site quality and successional status that continue to limit forest C storage rates.     

Stand fine root biomass and fine root morphology in old-growth beech forests as a function of precipitation and soil fertility

Only very limited information exists on the plasticity in size and structure of fine root systems, and fine root morphology of mature trees as a function of environmental variation. Six northwest German old-growth beech forests (Fagus sylvatica L.) differing in precipitation (520 – 1030 mm year^–1) and soil acidity/fertility (acidic infertile to basic fertile) were studied by soil coring for stand totals of fine root biomass (0–40 cm plus organic horizons), vertical and horizontal root distribution patterns, the fine root necromass/biomass ratio, and fine root morphology (root specific surface area, root tip frequency, and degree of mycorrhizal infection). Stand total of fine root biomass, and vertical and horizontal fine root distribution patterns were similar in beech stands on acidic infertile and basic fertile soils. In five of six stands, stand fine root biomass ranged between 320 and 470 g m^–2; fine root density showed an exponential decrease with soil depth in all profiles irrespective of soil type. An exceptionally small stand fine root biomass (<150 g m^–2) was found in the driest stand with 520 mm year^–1 of rainfall. In all stands, fine root morphological parameters changed markedly from the topsoil to the lower profile; differences in fine root morphology among the six stands, however, were remarkably small. Two parameters, the necromass/biomass ratio and fine root tip density (tips per soil volume), however, were both much higher in acidic than basic soils. We conclude that variation in soil acidity and fertility only weakly influences fine root system size and morphology of F. sylvatica, but affects root system structure and, probably, fine root mortality. It is hypothesized that high root tip densities in acidic infertile soils compensate for low nutrient supply rates, and large necromasses are a consequence of adverse soil chemical conditions. Data from a literature survey support the view that rainfall is another major environmental factor that influences the stand fine root biomass of F. sylvatica.     

Vertical patterns of fine root biomass, morphology and nitrogen concentration in a subalpine fir-wave forest

To clarify the nutrient acquisition strategies for below-ground resources in a subalpine Abies forest with shallow soils, we examined the vertical patterns of fine root biomass, morphology, nitrogen concentration of fine root tissue and soil chemical characteristics in nine quadrats of sapling, young and mature stands in a subalpine fir-wave forest, central Japan. The community characteristics changed with stand development, but stand development did not influence the vertical pattern of fine root characteristics. Fine root biomass decreased with soil depth. Specific root length did not differ among soil depths, and neither average diameter nor tissue density of fine roots changed vertically. The nitrogen concentration of fine roots differed significantly among soil depths, and was higher in surface soils than in deeper soils. Moreover, soil pH, soil electrical conductivity and soil nitrogen concentration were higher in surface layers than deeper layers. Therefore, we suggest that the subalpine Abies community has a nutrient acquisition strategy that allows uptake of more nutrients near the surface in shallow soils due to the larger investment in biomass and more active metabolism, but not due to phenotypic plasticity in fine root morphology. In addition, we observed that fine root biomass changed with stand development, where specific root length was greater in sapling stands than in older stands.     

落叶松和水曲柳人工林细根生长、死亡和周转

 细根周转是陆地生态系统碳分配格局与过程的核心环节,而细根周转估计的关键是了解细根的生长和死亡动态。该研究以18年生落叶松(Larix gmelinii)和水曲柳(Fraxi nus mandshurica)人工林为对象,采用微根管（Minirhizotron）技术对两树种0～40 cm深度的细根生长和死亡动态进行了为期1年的观测,研究了两树种细根在不同土层深度的生长与死亡动态、细根周转以及与土壤有效氮含量、土壤温度、大气温度和降水的关系。结果表明：1） 落叶松平均细根生长（Root length density production, RLDP）0.0045 mm•cm^－2•d^－1）明显低于水曲柳RLDP（0.0077 mm•cm^－2•d^－1）。两个树种细根平均RLDP在表层（0～10 cm）最大,而底层（30～40 cm）最小 ,两树种平均细根死亡（Root length density mortality, RLDM）也表现同样规律 。水曲柳春季生长的细根占41.7%,夏季占39.7%,而落叶松细根生长分别是24.0%和51.2%,水曲柳细根死亡主要发生在春季（34.3%） 和夏季（34.0%）,而落叶松细根死亡主要发生在夏季和秋季（分别占28.5%和32.3%）,两 树种细根生长与死亡在冬季均较小;2）落叶松细根年生长量（0.94 mm•cm^－2•a^－1）和年死亡量（0.72 mm•cm^－2•a^－1）明显低于水曲柳（1.52和1.21 mm•cm^－2•a^－1）,两树种细根表层年生长量和年死亡量均最高,底层最低。落叶松细根年周转为3.1次•a^－1（按年生长量计算）和2.4次•a^－1（按年死亡量计算）,相比较,水曲柳细根年周转分别为2.7次•a^－1和2.2次•a^－1;3）土壤有效氮含量、土壤温度、大气温度和降水综合作用影响细根生长和死亡动态,可以解释细根生长80%的变异和细根死亡95%以上的变异。     

Estimating Fine Root Turnover in Tropical Forests along an Elevational Transect using Minirhizotrons

Growth and death of fine roots represent an important carbon sink in forests. Our understanding of the patterns of fine root turnover is limited, in particular in tropical forests, despite its acknowledged importance in the global carbon cycle. We used the minirhizotron technique for studying the changes in fine root longevity and turnover along a 2000-m-elevational transect in the tropical mountain forests of South Ecuador. Fine root growth and loss rates were monitored during a 5-mo period at intervals of four weeks with each 10 minirhizotron tubes in three stands at 1050, 1890, and 3060 m asl. Average root loss rate decreased from 1.07 to 0.72 g/g/yr from 1050 to 1890 m, indicating an increase in mean root longevity with increasing elevation. However average root loss rate increased again toward the uppermost stand at 3060 m (1.30 g/g/yr). Thus, root longevity increased from lower montane to mid-montane elevation as would be expected from an effect of low temperature on root turnover, but it decreased further upslope despite colder temperatures. We suggest that adverse soil conditions may reduce root longevity at high elevations in South Ecuador, and are thus additional factors besides temperature that control root dynamics in tropical mountain forests.     

Allometry of fine roots in forest ecosystems

Theoretical predictions regarding fine root production are needed in many ecosystem models but are lacking. Here, we expand the classic pipe model to fine roots and predict isometric scaling relationships between leaf and fine root biomass and among all major biomass production components of individual trees. We also predict that fine root production scales more slowly against increases in leaf production across global forest ecosystems at the stand level. Using meta‐analysis, we show fine root biomass scales isometrically against leaf biomass both at the individual tree and stand level. However, despite isometric scaling between stem and coarse root production, fine root production scales against leaf production with a slope of about 0.8 at the stand level, which probably results from more rapid increase of turnover rate in leaves than in fine roots. These analyses help to improve our understandings of allometric theory and controls of belowground C processes.     

Contrasting patterns of tree mortality in late‐successional Picea abies stands in two areas in northern Fennoscandia

Question: What are tree mortality rates and how and why do they vary in late‐successional Picea abies‐dominated forests? Do observed tree mortality patterns allow comparative assessment of models of long‐term stand development? Location: Northern boreal Fennoscandia. Methods: We measured stand structure in 10 stands in two different areas. We determined age distributions and constructed a chronology of tree deaths by cross‐dating the years of death of randomly sampled dead trees. Results: The stands in the two areas had contrasting tree age distributions, despite similar live tree structure. In one area, stands were relatively even‐aged and originated following a stand‐replacing fire 317 years earlier. The stands in the second area had an uneven age structure and virtually no signs of past fires, suggesting a very long period since the last major disturbance. The younger stands were characterized by a high mortality rate and inter‐annual variation, which we attributed to senescence of the relatively even‐aged stands approaching the maximum age of P. abies. In contrast, the tree mortality rates in the older stands were low and relatively stable. Conclusions: Patterns of tree mortality were, to a large extent, dependent on the time since the last stand‐replacing disturbance, suggesting that northern boreal P. abies stands eventually reach a shifting mosaic state maintained through small‐scale dynamics, but the time needed to reach this state appears to be lengthy; even 300 years after a forest fire stands showed changes in patterns of tree mortality that were related to the developmental stage of the stands.     

Effects of fire and spruce beetle outbreak legacies on the disturbance regime of a subalpine forest in Colorado

Aim There is increasing research attention being given to the role of interactions among natural disturbances in ecosystem processes. We studied the interactions between fire and spruce beetle (Dendroctonus rufipennis Kirkby) disturbances in a Colorado subalpine forest. The central questions of this research were: (1) How does fire history influence stand susceptibility to beetle outbreak? And conversely, (2) How does prior occurrence of a beetle outbreak influence stand susceptibility to subsequent fire? Methods We reconstructed the spatial disturbance history in a c. 4600 ha area by first identifying distinct patches in the landscape on aerial photographs. Then, in the field we determined the disturbance history of each patch by dating stand origin, fire scars, dates of mortality of dead trees, and releases on remnant trees. A geographical information system (GIS) was used to overlay disturbance by fire and spruce beetle. Results and main conclusions The majority of stands in the study area arose following large, infrequent, severe fires occurring in c. 1700, 1796 and 1880. The study area was also affected by a severe spruce beetle outbreak in the 1940s and a subsequent low‐severity fire. Stands that originated following stand‐replacing fire in the late nineteenth century were less affected by the beetle outbreak than older stands. Following the beetle outbreak, stands less affected by the outbreak were more affected by low‐severity fire than stands more severely affected by the outbreak. The reduced susceptibility to low‐severity fire possibly resulted from increased moisture on the forest floor following beetle outbreak. The landscape mosaic of this subalpine forest was strongly influenced by the interactions between fire and insect disturbances.     

Controls of fine root dynamics across a gradient of gap sizes in a pine woodland

Controls of fine dynamics were investigated in a Pinus palustris Mill. (longleaf pine) woodland subjected to two understory vegetation treatments (control versus removed) and four overstory treatments (no gap control, and canopy gaps of three sizes with constant total gap area per stand). Fine root (<2 mm diameter) dynamics were measured over 11 months using ingrowth cores (all treatments) and minirhizotrons (understory removed in no gap control and large gap treatments only). At the fine (microsite) spatial scale, pine and non-pine root mass production responded negatively to each other (P=0.033). Each life form was significantly (P< or =0.028) related to nearby overstory density, and pine root production compensated for reductions in non-pine roots if understory vegetation was removed. Soil moisture and NO(3) mineralization rate were negatively related to pine root mass production (ingrowth cores; P<0.001 and P=0.052) and positively related to pine root length production, mortality and turnover (minirhizotrons; P from <0.001 to 0.078). Temperature variance was negatively related to pine root lifespan P<0.001) and positively related to pine root turnover (P=0.003). At the ecosystem scale, pattern of overstory disturbance (gap size and number) had no significant effect on non-pine, pine, or total root production. However, the presence of gaps (versus the no-gap control) increased non-pine root mass production (ANOVA, P=0.055) in natural understory conditions, and reduced pine root mass production (P=0.035) where the understory was removed. Ecosystem-wide pine root length production, mortality and turnover were positively related to weekly soil temperature (P< or =0.02). In natural systems, fine root dynamics are highly variable and strongly affected by biotic factors. Roots quickly close belowground gaps because one life form (pine or non-pine) compensates for the absence of the other. When understory vegetation is removed, however, pine roots respond to the local abiotic environment, particularly moisture and NO(3).     

Belowground drought response of European beech: fine root biomass and carbon partitioning in 14 mature stands across a precipitation gradient

How tree root systems will respond to increased drought stress, as predicted for parts of Central Europe, is not well understood. According to the optimal partitioning theory, plants should enhance root growth relative to aboveground growth in order to reduce water limitations. We tested this prediction in a transect study with 14 mature forest stands of European beech (Fagus sylvatica L.) by analysing the response of the fine root system to a large decrease in annual precipitation (970–520 mm yr⁻¹). In 3 years with contrasting precipitation regimes, we investigated leaf area and leaf biomass, fine root biomass and necromass (organic layer and mineral soil to 40 cm) and fine root productivity (ingrowth core approach), and analysed the dependence on precipitation, temperature, soil nutrient availability and stand structure. In contrast to the optimal partitioning theory, fine root biomass decreased by about a third from stands with >950 mm yr⁻¹ to those with <550 mm yr⁻¹, while leaf biomass remained constant, resulting in a significant decrease, and not an increase, in the fine root/leaf biomass ratio towards drier sites. Average fine root diameter decreased towards the drier stands, thereby partly compensating for the loss in root biomass and surface area. Both δ¹³C‐signature of fine root mass and the ingrowth core data indicated a higher fine root turnover in the drier stands. Principal components analyses (PCA) and regression analyses revealed a positive influence of precipitation on the profile total of fine root biomass in the 14 stands and a negative one of temperature and plant‐available soil phosphorus. We hypothesize that summer droughts lead to increased fine root mortality, thereby reducing root biomass, but they also stimulate compensatory fine root production in the drier stands. We conclude that the optimal partitioning theory fails to explain the observed decrease in the fine root/leaf biomass ratio, but is supported by the data if carbon allocation to roots is considered, which would account for enhanced root turnover in drier environments.     

Vertical distribution and seasonal changes of fine and coarse root mass in Pinus kesiya Royle Ex.Gordon forest of three different ages

Seasonal changes and vertical distribution of fine (< 2 mm diameter) and coarse (2-10 mm diameter) root mass of Pinus kesiya and fine root and rhizome mass of herbaceous species, and root production were studied in the 6-, 15- and 23-year old Pinus kesiya forest stands at Shillong, in the Meghalaya state of north-east India. Maximum fine and coarse root mass of P. kesiya, and fine root and rhizome mass of the ground vegetation were recorded during the rainy season. The contribution of the tree fine roots in 0-10 cm soil layer declined from 51% in the 6-year old stand to about 33% in the older stands. The major proportion (63-88%) of herbaceous fine root and rhizome mass was concentrated in this soil layer in all the three stands. The majority (36-57%) of tree coarse roots were present in the 10-20 cm layer in all the stands. The biomass and necromass values in the case of fine roots were more or less equal in a given stand, but the coarse roots had 5 to 9 times more live than the dead mass. The proportion of herbaceous fine root mass to the total fine root mass declined from 54% in the 6-year old stand to 30-32% in the 15- and 23-year old stands. The mean total fine root mass (pine + herbaceous species) decreased from 417 g m^–2 in the 6-year old stand to 302 in 15-year and 322 g m^–2 in the 23-year old stand. Annual fine root production showed a marked decrease from 1055 g m^–2 in the 6-year old stand to 743 g m^–2 in the 23-year old stand, but coarse root production increased from 169 g m^–2 in the 6-year to 466 g m^–2  in the 23-year old stand; the total root production thus remained approximately constant.     

Stand age-related effects on soil respiration in a first rotation Sitka spruce chronosequence in central Ireland

The effect of stand age on soil respiration and its components was studied in a first rotation Sitka spruce chronosequence composed of 10‐, 15‐, 31‐, and 47‐year‐old stands established on wet mineral gley in central Ireland. For each stand age, three forest stands with similar characteristics of soil type and site preparation were used. There were no significant differences in total soil respiration among sites of the same age, except for the case of a 15‐year‐old stand that had lower soil respiration rates due to its higher productivity. Soil respiration initially decreased with stand age, but levelled out in the older stands. The youngest stands had significantly higher respiration rates than more mature sites. Annual soil respiration rates were modelled by means of temperature‐derived functions. The average Q ₁₀ value obtained treating all the stands together was 3.8. Annual soil respiration rates were 991, 686, 556, and 564 g C m^?2 for the 10‐, 15‐, 31‐, and 47‐year‐old stands, respectively. We used the trenching approach to separate soil respiration components. Heterotrophic respiration paralleled soil organic carbon dynamics over the chronosequence, decreasing with stand age to slightly increase in the oldest stand as a result of accumulated aboveground litter and root inputs. Root respiration showed a decreasing trend with stand age, which was explained by a decrease in fine root biomass over the chronosequence, but not by nitrogen concentration of fine roots. The decrease in the relative contribution of autotrophic respiration to total soil CO₂ efflux from 59.3% in the youngest stand to 49.7% in the oldest stand was explained by the higher activity of the root system in younger stands. Our results show that stand age should be considered if simple temperature‐based models to predict annual soil respiration in afforestation sites are to be used.     

Fine root dynamics and soil carbon accretion under thinned and un-thinned Cupressus lusitanica stands in, Southern Ethiopia

Background and aims

Forest management activities influences stand nutrient budgets, belowground carbon allocation and storage in the soil. A field experiment was carried out in Southern Ethiopia to investigate the effect of thinning on fine root dynamics and associated soil carbon accretion of 6-year old C. lusitanica stands.

Methods

Fine roots (≤2 mm in diameter) were sampled seasonally to a depth of 40 cm using sequential root coring method. Fine root biomass and necromass, vertical distribution, seasonal dynamics, annual turnover and soil carbon accretion were quantified.

Results

Fine root biomass and necromass showed vertical and temporal variations. More than 70 % of the fine root mass was concentrated in the top 20 cm soil depth. Fine root biomass showed significant seasonal variation with peaks at the end of the major rainy season and short rainy season. Thinning significantly increased fine root necromass, annual fine root production and turnover. Mean annual soil carbon accretion, through fine root necromass, in the thinned stand was 63 % higher than that in the un-thinned stand.

Conclusions

The temporal dynamics in fine roots is driven by the seasonality in precipitation. Thinning of C. lusitanica plantation would increase soil C accretion considerably through increased fine root necromass and turnover.