Post-fire Aleppo pine growth,C and N isotope composition depend on site dryness

Key message

The post-fire growth responses and changes in wood C and N isotope composition depend on site water availability and fire severity in Mediterranean Aleppo pine forests.

Abstract

Mediterranean forests are subjected to recurrent wildfires and summer droughts. Under warmer and drier conditions, it is required to determine how Mediterranean pines recover after wildfires, and how this translates into changes in tree radial growth and function (e.g. intrinsic water-use efficiency—iWUE). We analysed four Aleppo pine areas located in SE Spain affected by 1994 wildfires and subjected to different water availability, ranging from mesic to semi-arid conditions. We combined dendrochronological analyses with δ¹³C and δ¹⁵N wood isotopes to quantify the changes in radial growth (expressed as Basal Area Increment—BAI) and functional responses (iWUE and N cycling) to three fire severities (unburned sites, low and medium severities). We expected that the post-fire release in nutrients and a reduction in competition for water would enhance radial growth. We found that fire did not significantly alter growth patterns at the driest sites, but increased BAI at the wettest sites. δ¹³C was significantly (P ≤ 0.01) more negative only in burned stands located at the wettest site indicating a decreased iWUE and thus improved water availability. However, the δ¹⁵N was higher in severely burned than in unburned plots from all sites but the wettest site, indicating a potential fertilization effect of fire in sites subjected to mild drought severity. Site water availability determined how fire affected subsequent modifications in growth and tree functioning of Aleppo pine forests, that is, changes in iWUE and N cycling. Therefore, site dryness should be explicitly considered to forecast the growth and functioning responses of Mediterranean pine forests to the predicted increasing recurrence of fire events due to global warming.

     

Effectiveness of soil N availability indices in predicting site productivity in the oil sands region of Alberta

Background and aims

Quantitative relationships between soil N availability indices and tree growth are lacking in the oil sands region of Alberta and this can hinder the development of guidelines for the reclamation of the disturbed landscape after oil sands extraction. The aim of this paper was to establish quantitative relationships between soil N availability indices and tree growth in the oil sands region of Alberta.

Methods

In situ N mineralization rates, in situ N availability measured in the field using Plant Root Simulators (PRS? probes), laboratory aerobic and anaerobic soil N mineralization rates, and soil C/N and N content were determined for both the forest floor and the 0–20?cm mineral soil in eight jack pine (Pinus banksiana Lamb.) stands in the oil sands region in northern Alberta. Tree growth rates were determined based on changes in tree ring width in the last 6?years and as mean annual aboveground biomass increment.

Results

Soil N availability indices across those forest stands varied and for each stand it was several times higher in the forest floor than in the mineral soil. The in situ and laboratory aerobic and anaerobic soil N mineralization rates, soil mineralized N, in situ N availability measured using PRS probes, soil C/N ratio and N content in both the forest floor and mineral soil, as well as stand age were linearly correlated with tree ring width of jack pine trees across the selected forest stands, consistent with patterns seen in other published studies and suggesting that N availability could be a limiting factor in the range of jack pine stands studied.

Conclusions

In situ and laboratory aerobic and anaerobic N mineralization rates and soil C/N ratio and N content can be used for predicting tree growth in jack pine forests in the oil sand region. Laboratory based measurements such as aerobic and anaerobic N mineralization rates and soil C/N ratio and N content would be preferable as they are more cost effective and equally effective for predicting jack pine growth.     

Pinyon pine (Pinus edulis) mortality and response to water addition across a three million year substrate age gradient in northern Arizona, USA

Background and aims

Pinyon pine (Pinus edulis Engelm.) is an important tree species in the western United States that has experienced large-scale mortality during recent severe drought. The influence of soil conditions on pinyon pine response to water availability is poorly understood. We investigated patterns of tree mortality and response of tree water relations and growth to experimental water addition at four sites across a three million year soil-substrate age gradient.

Methods

We measured recent pinyon mortality at four sites, and tree predawn water potential, leaf carbon isotope signature, and branch, leaf, and stem radial growth on 12 watered and unwatered trees at each site. Watered trees recieved fifty percent more than growing season precipitation for 6 years.

Results

Substrate age generally had a greater effect on tree water stress and growth than water additions. Pinyon mortality was higher on intermediate-aged substrates (50–55%) than on young (15%) and old (17%) substrates, and mortality was positively correlated with pinyon abundance prior to drought.

Conclusions

These results suggest high soil resource availability and consequent high stand densities at intermediate-age substrates predisposes trees to drought-induced mortality in semi-arid regions. The response of tree water relations to water addition was consistent with the inverse texture hypothesis; watering reduced tree water stress most in young, coarsely textured soil, likely because water rapidly penetrated deep in the soil profile where it was protected from evapotranspiration.     

Drivers of spatial variability in urban plant and soil isotopic composition in the Los Angeles basin

Aims

Plant litter decomposition plays an important role in the storage of soil organic matter in terrestrial ecosystems. Conversion of native vegetation to agricultural lands and subsequent land abandonment can lead to shifts in canopy structure, and consequently influence decomposition dynamics by alterations in soil temperature and moisture conditions, solar radiation exposure, and soil erosion patterns. This study was conducted to assess which parameters were more closely related to short-term decomposition dynamics of two predominant Mediterranean leaf litter types.

Methods

Using the litterbag technique, we incubated leaf litter of Pinus halepensis and Rosmarinus officinalis in two Mediterranean land-uses with different degree of vegetation cover (open forest, abandoned agricultural field).

Results

Fresh local litter lost between 20 and 55% of its initial mass throughout the 20-month incubation period. Rosemary litter decomposed faster than pine litter, showing net N immobilization in the early stages of decomposition, in contrast to the net N release exhibited by pine litter. Parameters related to litter quality (N content or C:N) or land-use/site conditions (ash content, an index of soil deposition on litter) were found to explain the cross-site variability in mass loss rates for rosemary and Aleppo pine litter, respectively.

Conclusions

The results from this study suggest that decomposition drivers may differ depending on litter type in this Mediterranean ecosystem. While rosemary litter was degraded mainly by microbial activity, decomposition of pine litter was likely driven primarily by abiotic processes like soil erosion.     

Mixed Norway spruce (Picea abies [L.] Karst) and European beech (Fagus sylvatica [L.]) stands under drought: from reaction pattern to mechanism

Key message

We review causes of synergies in mixed-species stands, develop guiding hypotheses for revealing their mechanisms and present a rainfall exclusion experiment along with a transect (KROOF) for exploring drought effects.

Abstract

While monocultures have dominated forest research and practice in the past, in face of growing resource scarcity and climate change, mixed-species stands are on the advance. Long-term observations show that mixed-species stands frequently over-yield monocultures, and they further suggest that the over-yielding is often higher on poor than on fertile sites and in low-growth than in high-growth years. However, the underlying causes have not yet been clarified. We start with a review of, among others, hydraulic redistribution, complementary eco-physiological traits, and ectomycorrhizal networks as possible causes behind the observed productivity gains in mixed-species stands. Then, we develop guiding hypotheses for further exploration of synergies in mixed-species stands. Finally, we introduce into the concept and model approach of the rainfall exclusion experiment for examining the role of water supply in mixed versus pure forest stands of spruce and beech. At the Kranzberg site, six plots are equipped with water retention roofs, which only close during rain events. The remaining six plots serve as non-roofed control. Together with the rainfall exclusion experiment, an ecological gradient with five sites extending through SE-Germany forms the “Kranzberg Roof Experiment” (in short KROOF). Kranzberg Forest is a part of this gradient from moist to dry conditions, with each site providing mixed and pure associations each of spruce and beech. The ecophysiological approach will be complemented by tree ring analysis and modelling of productivity of the tree associations under intense drought.     

Elemental and stable isotope composition of Pinus halepensis foliage along a metal(loid) polluted gradient: implications for phytomanagement of mine tailings in semiarid areas

Background and aims

Aleppo pine (Pinus halepensis Mill.) is a widely used species for restoring degraded semiarid areas, but its use for the revegetation of metal(loid) polluted soils has not been thoroughly investigated. The main goal of this research was to study the ecophysiological status and elemental composition of spontaneous populations of Pinus halepensis growing on mine tailings to assess their use in phytomanagement of mine spoils in semiarid climates.

Methods

Edaphic characteristics and the physiological (by stable isotopes) and nutritional status of pine trees were determined on mine tailings, in the metalloid-polluted surroundings and a non-polluted control area.

Results

Low soil phosphorus availability at the tailings was found to be a more important limiting factor for pine physiological performance than high soil metal(lloid)s concentrations. Foliar phosphorus concentrations showed a strong negative correlation with foliar sulphur concentrations along the studied transect. The carbon and oxygen isotopic composition (δ¹³C and δ¹⁸O) of pine needles indicated that trees at the tailings were less water stressed than those in surroundings or control areas. The low foliar δ¹⁵N of pines growing at the tailings was due to low soil fertility and/or a heavy reliance on symbiotic ectomycorrhizal fungi for nitrogen uptake.

Conclusions

The results of this study indicate that Pinus halepensis is a suitable tree species for the phytostablisation of neutral or slightly-alkaline mining wastes in semiarid environments, thanks to its drought hardiness and good adaptation to low soil fertility and salinity.     

Declining hydraulic performances and low carbon investments in tree rings predate Scots pine drought-induced mortality

Key message

The retrospective analysis of wood anatomical features evidences how a long-term deterioration of hydraulic performance and carbon use portend drought-induced mortality in Scots pine.

Abstract

Widespread episodes of drought-induced tree mortality are predicted to become more frequent as climate becomes warmer and drier. Nevertheless, growth trends and their links to changes in wood anatomy before tree dies are still poorly understood. Wood anatomical features provide valuable information that can be extracted to infer the mechanisms leading to tree death. In this study, we characterize drought-induced mortality affecting two Scots pine (Pinus sylvestris) sites (Prades and Arcalís) located in the North Eastern Iberian Peninsula. Co-occurring now-dead and living Scots pine trees were sampled and their wood anatomical features were measured and compared. We aimed to detect differences in anatomical features between living and dead trees, and to infer past physiological performances that might have determined their subsequent death or survival. Now-dead trees showed lower tracheid and resin duct production, and smaller radial lumen diameters than co-occurring living trees. At the more xeric Prades site, these anatomical differences were larger and chronic, i.e. were observed over the three studied decades, whilst they were less pronounced at the other, more mesic Arcalís site, where tree mortality episodes were more recent. This indicates that dead trees’ hydraulic conductivity was severely affected and that carbon investment in xylem formation and resin duct production was constrained prior to tree death. Our findings show that both hydraulic deterioration and low carbon allocation to xylem formation were associated to drought-induced mortality in Scots pine. Nevertheless, the temporal dynamics of these processes differed between populations as a function of site climatic conditions.     

Variation of carbon age of fine roots in boreal forests determined from 14C measurements

Background and aims

The main objectives of this study were to determine how the carbon age of fine root cellulose varies between stands, tree species, root diameter and soil depth. In addition, we also compared the carbon age of fine roots from soil cores of this study with reported values from the roots of the same diameter classes of ingrowth cores on the same sites.

Methods

We used natural abundance of ¹⁴C to estimate root carbon age in four boreal Norway spruce and Scots pine stands in Finland and Estonia.

Results

Age of fine root carbon was older in 1.5–2 mm diameter fine roots than in fine roots with <0.5 mm diameter, and tended to be older in mineral soil than in organic soil. Fine root carbon was older in the less fertile Finnish spruce stands (11–12 years) than in the more fertile Estonian stand (3 and 8 years), implying that roots may live longer in less fertile soil. We further observed that on one of our sites carbon in live fine roots with the 1.5–2 mm diameter was of similar C age (7–12 years) than in the ingrowth core roots despite the reported root age in the ingrowth cores – being not older than 2 years.

Conclusions

From this result, we conclude that new live roots may in some cases use old carbon reserves for their cellulose formation. Future research should be oriented towards improving our understanding of possible internal redistribution and uptake of C in trees.     

Dwarf pine invasion in an alpine tundra of discontinuous permafrost area: effects on fine root and soil carbon dynamics

Key message

Recent invasion of Pinus pumila , a highly productive shrub-like tree species, into alpine tundra does not significantly modify the dynamics of fine root and soil carbon in the tundra.

Abstract

Climate warming may directly and indirectly affect the large carbon stock in discontinuous permafrost soil at high latitudes. In recent decades, Siberian dwarf pine [Pinus pumila (Pall.) Regel] has been invading dry heath alpine tundra in the northern Amur region of Far East Russia. Siberian dwarf pine is known to have high aboveground productivity, comparable to that of tall coniferous trees. We hypothesised that the invasion of Siberian dwarf pine into alpine tundra could increase soil carbon stocks via an increase in fine roots. Contrary to our expectations, the invasion of dwarf pine did not significantly increase the fine root biomass and productivity of the tundra, probably due to the belowground competitive exclusion between the dwarf pine and alpine tundra plants. Furthermore, the invasion of the dwarf pine did not affect soil carbon in the alpine tundra ecosystem. These results show that the recent invasion of Siberian dwarf pine into tundra did not influence the fine root dynamics or the soil carbon stock in the study site. Together, these results implied that (1) it takes a long time for pine invasion to change the belowground ecosystem properties of tundra vegetation to that of pine thickets and therefore (2) the lack of an increase in soil carbon from recent tree invasion should be taken into account when modelling future carbon dynamics in alpine tundra.

     

Increasing phosphorus limitation along three successional forests in southern China

Background and Aims

Phosphorus (P) is commonly one of most limiting nutrients in tropical and subtropical forests, but whether P limitation would be exacerbated during forest succession remains unclear.

Methods

Soil phosphatase activity is often used as an indicator of P limitation. Here we examined soil acid phosphatase activity (APA) underneath tree species in pine forest (PF), mixed pine and broadleaf forest (MF) and monsoon evergreen broadleaf forest (MEBF) which represented the early, middle and late successional stages of subtropical forests in China, respectively. We also analyzed other indicators of P status (soil available P and N and P stoichiometry of the tree species).

Results

APA or APA per unit soil organic carbon under tree species was relatively low in the early successional forest. Different from PF and MF, soil available P beneath the tree species was lower than in the bulk soils in MEBF. Soil APA was closely related to N:P ratios of tree species across all three forests.

Conclusions

Our results imply that P limitation increases during forest succession at our site. The dominant tree species with low soil APAs in MEBF are likely more P-limited than other tree species.     

Effects of tree species composition on the CO2 and N2O efflux of a Mediterranean mountain forest soil

Background and Aims

Tree species composition shifts can alter soil CO₂ and N₂O effluxes. We quantified the soil CO₂ and N₂O efflux rates and temperature sensitivity from Pyrenean oak, Scots pine and mixed stands in Central Spain to assess the effects of a potential expansion of oak forests.

Methods

Soil CO₂ and N₂O effluxes were measured from topsoil samples by lab incubation from 5 to 25 °C. Soil microbial biomass and community composition were assessed.

Results

Pine stands showed highest soil CO₂ efflux, followed by mixed and oak forests (up to 277, 245 and 145 mg CO₂-C m^?2 h^?1, respectively). Despite contrasting soil microbial community composition (more fungi and less actinomycetes in pine plots), carbon decomposability and temperature sensitivity of the soil CO₂ efflux remain constant among tree species. Soil N₂O efflux rates and its temperature sensitivity was markedly higher in oak stands than in pine stands (70 vs. 27 μg N₂O-N m^?2 h^?1, Q₁₀, 4.5 vs. 2.5).

Conclusions

Conversion of pine to oak forests in the region will likely decrease soil CO₂ effluxes due to decreasing SOC contents on the long run and will likely enhance soil N₂O effluxes. Our results present only a seasonal snapshot and need to be confirmed in the field.     

Hydraulic limitation on maximum height of Pinus strobus trees in northern Minnesota,USA

Key message

Using comparisons within and between trees, the authors show evidence for hydraulic limitation of tree height in a humid-climate species that is far from the global maximum tree height.

Abstract

We measured water status and two indicators of drought stress as a function of height within the canopies of four tall (32–35 m) eastern white pines (Pinus strobus) at an old-growth site in northern Minnesota, USA. Pre-dawn and midday xylem pressure potential measured on terminal shoots (Ψ _shoot), needle length, and foliar carbon isotope discrimination (δ ¹³C) all showed within-canopy gradients consistent with increasing drought stress with height. Midday Ψ _shoot near tree tops was ?1.8 MPa, close to values associated with stomatal closure for other temperate conifers. Pre-dawn Ψ _shoot decreased with height at >2× the gradient in gravitational potential. δ ¹³C was strongly correlated with height and weakly correlated with light. Needles were 15–25 % shorter at canopy top compared to the bottom of the canopy. Midday Ψ _shoot and needle length showed significant differences in regression model coefficients from tree to tree. The patterns are consistent with hydraulic constraints on height growth of white pine at this site.     

Effects of soil warming and nitrogen foliar applications on bud burst of black spruce

Key message

In mature black spruce, bud burst process is anticipated by soil warming, while delayed by foliar applications of nitrogen; however, the effects depend on growth conditions at the site.

Abstract

The observation of phenological events can be used as biological indicator of environmental changes, especially from the perspective of climate change. In boreal forests, the onset of the bud burst is a key factor in the length of the growing season. With current climate change, the major factors limiting the growth of boreal trees (i.e., temperature and nitrogen availability) are changing and studies on mature trees are limited. The aim of this study was to investigate the effects of soil warming and increased nitrogen (N) deposition on bud burst of mature black spruce [Picea mariana (Mill.) BSP]. From 2008 onwards, an experimental manipulation of these environmental growth conditions was conducted in two stands (BER and SIM) at different altitudes in the boreal forest of Quebec, Canada. An increase in soil temperature (H treatment) and a canopy application of artificial rain enriched with nitrogen (N treatment) were performed. Observations of bud phenology were made during May–July 2012 and 2013. In BER, H treatment caused an anticipation (estimated as 1–3 days); while N treatment, a delay (estimated as 1–2 days but only in 2012) in bud burst. No treatments effect was significant in SIM. It has been demonstrated that soil temperature and N availability can play an important role in affecting bud burst in black spruce but the effects of these environmental factors on growth are closely linked with site conditions.

     

Massive aerial roots affect growth and form of Dracaena draco

Key message

Large aerial roots grow out from the branches of injured Dracaena draco trees. They integrate with the trunk or cause the branches to break off the tree and deform it.

Abstract

Dracaena draco, the dragon tree, is an iconic monocot of the Canary Islands with a tree-like growth habit and some distinctive features that are unique in the plant kingdom. We report about the massive aerial roots in this tree. They appear on trees that are injured or under environmental stress and affect growth form and the whole life of the plant. We analysed the growth of these roots and tested our findings in experiments on plants. Clusters of these roots emerge from the bases of the lowest branches and growing down they may reach the soil. Descending along the trunk, they cling tightly to the trunk, integrate with it and contribute to its radial growth. This may explain (1) why the trunk of a mature D. draco tree looks fibrous, as if made of many individual strands, and (2) how some trees reach enormous radial dimensions. Alternately, a large, 2–5 m high, multi-segmented branch with aerial roots at its base, may break off the tree and grow on its own, as a mammoth off-cut, perhaps the largest known in the plant kingdom. This detachment would cause a significant reduction in the size of the crown and deform its original, highly organized pattern of branching. In the extreme condition this may result in the destruction of the mother plant.     

No-till reduces global warming potential in a subtropical Ferralsol

Aims

We investigated the link between tree community composition and soil microbial community biomass and structure in central-eastern Spain.

Methods

The effects of the forest stand composition on the soil organic matter dynamics and on the structure and activity of the soil microbial community have been determined using phospholipid fatty acid profiles and soil enzymatic activities.

Results

The soil and litter N and C contents were higher in Pinus nigra Arn. ssp. salzmannii and Quercus ilex mixed forest stands (SBHO) and in long-term unmanaged Pinus nigra Arn. ssp. salzmannii forest stands (SBPC) than in pure Pinus nigra Arn. ssp. salzmannii forest stands (SBPA) and Pinus nigra Arn. ssp. salzmannii and Juniperus thurifera mixed forest stands (SBSJ). The bacterial biomass was significantly higher in SBSJ and SBPA than in SBPC and SBHO. The results show an uncoupling of the soil microbial biomass and its activity. pH is related to microbial biomass and its community structure under a Mediterranean humid climate.

Conclusions

The tree species seem to affect the biomass of the soil microbial community and its structure. The pH, but not the C/N ratio, is a factor influencing the microbial dynamics, biomass, and community structure.     

Jack pine foliar δ15N indicates shifts in plant nitrogen acquisition after severe wildfire and through forest stand development

Background and aims

Natural abundance of the stable nitrogen (N) isotope ¹⁵N can elucidate shifts in plant N acquisition and ecosystem N cycling following disturbance events. This study examined the potential relationship between foliar δ¹⁵N and depth of plant N acquisition (surface organic vs. mineral soil) and nitrification as conifer stands develop following stand-replacing wildfire.

Methods

We measured foliar δ¹⁵N along an 18-site chronosequence of jack pine (Pinus banksiana) stands, 1 to 72 years in age post-wildfire. Foliar δ¹⁵N was compared to total δ¹⁵N of the organic (Oe + Oa) and mineral (0–15 cm) soil horizons, and organic horizon N mineralization and nitrification as functions of total mineralization.

Results

Foliar δ¹⁵N declined with stand age, yet wildfire effects were heterogeneous. Jack pine seedlings on burned, mineral soil patches in the youngest stand were significantly more enriched than those on unburned, organic patches (P?=?0.007). High foliar values in the youngest stands relative to mineral-horizon δ¹⁵N indicate that nitrification also likely contributed to seedling enrichment.

Conclusions

Our results suggest jack pine seedlings on burned patches obtain N from the mineral soil with potentially high nitrification rates, whereas seedlings on unburned patches and increasingly N-limited, mature jack pine acquire relatively more N from organic horizons.     

Variation in wood anatomical structure of Douglas-fir defoliated by the western spruce budworm: a case study in the coastal-transitional zone of British Columbia,Canada

Key message

An outbreak of the western spruce budworm temporarily modifies cellular wood anatomy of stem wood in natural and mature Douglas-fir stands impacting wood quality properties.

Abstract

Western spruce budworm (Choristoneura occidentalis Freeman) is a widespread and destructive defoliator of commercially important coniferous forests in western North America. In British Columbia, Canada, Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] is the most important and widely distributed host. Permanent sample plots were established at a number of locations in southern interior at the beginning of a severe western spruce budworm outbreak in the 1970s. Two of the sites were sampled in 2012 to determine whether modifications had occurred in the anatomical characteristics of stem wood formed during outbreak years. We determined that rings formed during the western spruce budworm 1976–1980 outbreak had a significantly lower proportion of latewood, reduced mean cell wall thickness and smaller radial cell diameters. While the cellular characteristics of the earlywood remained fairly constant, significant reductions in lumen area occurred in 1978 and 1979 at each site. Our study shows that western spruce budworm outbreaks not only reduce annual radial growth, but also temporarily modify cellular characteristics in latewood cells, which has implications for wood density and quality in Douglas-fir.     

Linking above- and belowground phenology of hybrid walnut growing along a climatic gradient in temperate agroforestry systems

Background and aims

Plant phenology is a sensitive indicator of plant response to climate change. Observations of phenological events belowground for most ecosystems are difficult to obtain and very little is known about the relationship between tree shoot and root phenology. We examined the influence of environmental factors on fine root production and mortality in relation with shoot phenology in hybrid walnut trees (Juglans sp.) growing in three different climates (oceanic, continental and Mediterranean) along a latitudinal gradient in France.

Methods

Eight rhizotrons were installed at each site for 21 months to monitor tree root dynamics. Root elongation rate (RER), root initiation quantity (RIQ) and root mortality quantity (RMQ) were recorded frequently using a scanner and time-lapse camera. Leaf phenology and stem radial growth were also measured. Fine roots were classified by topological order and 0–1 mm, 1–2 mm and 2–5 mm diameter classes and fine root longevity and risk of mortality were calculated during different periods over the year.

Results

Root growth was not synchronous with leaf phenology in any climate or either year, but was synchronous with stem growth during the late growing season. A distinct bimodal pattern of root growth was observed during the aerial growing season. Mean RER was driven by soil temperature measured in the month preceding root growth in the oceanic climate site only. However, mean RER was significantly correlated with mean soil water potential measured in the month preceding root growth at both Mediterranean (positive relationship) and oceanic (negative relationship) sites. Mean RIQ was significantly higher at both continental and Mediterranean sites compared to the oceanic site. Soil temperature was a driver of mean RIQ during the late growing season at continental and Mediterranean sites only. Mean RMQ increased significantly with decreasing soil water potential during the late aerial growing season at the continental site only. Mean root longevity at the continental site was significantly greater than for roots at the oceanic and Mediterranean sites. Roots in the 0–1 mm and 1–2 mm diameter classes lived for significantly shorter periods compared to those in the 2–5 mm diameter class. First order roots (i.e. the primary or parents roots) lived longer than lateral branch roots at the Mediterranean site only and first order roots in the 0–1 mm diameter class had 44.5% less risk of mortality than that of lateral roots for the same class of diameter.

Conclusions

We conclude that factors driving root RER were not the same between climates. Soil temperature was the best predictor of root initiation at continental and Mediterranean sites only, but drivers of root mortality remained largely undetermined.

     

Variation in the aboveground stand structure and fine-root biomass of Bornean heath (kerangas) forests in relation to altitude and soil nitrogen availability

Key message

Borneo’s tropical heath (kerangas) forest has limited soil nutrient availability, and high variation in aboveground structure and fine-root biomass. This variation depends on altitude and soil nitrogen availability.

Abstract

To elucidate the biotic and abiotic factors affecting the variation in fine-root biomass (FRB, <2 mm diameter) of trees growing under nutrient-poor environments in Sabah, North Borneo, we investigated FRB in different forests with varying soil nitrogen (N) availability. We selected two study sites at different altitudes: the Maliau Basin (ca. 1000 m asl) and Nabawan (ca. 500 m asl). Both sites included tropical heath (kerangas) forest, on infertile soils (podzols) with a surface organic horizon overlying a bleached (eluviated) mineral horizon, and taller forests on more fertile non-podzolic soils. FRB was obtained from each plot by soil coring (to a depth of 15 cm). FRB increased with decreasing soil inorganic N content (NH₄–N and NO₃–N), tree height, and aboveground biomass. Thus, higher proportions of carbon resources were allocated to fine-roots in stands with lower N availability. FRB was significantly greater at the Maliau Basin than at Nabawan, reflecting lower soil N availability at higher altitude. Our results demonstrate high variation in FRB among the heath forests, and suggest that fine-root development is more prominent under a cooler climate where N availability limits tree growth owing to slower decomposition. The variation in N availability under the same climate (i.e., at the same altitude) appears to be related to the extent of soil podzolization.

     

Interannual variations in primary and secondary growth of Nothofagus pumilio and their relationships with climate

Key message

The aim of this study is to evaluate the relationships between primary and secondary growth as well as the influences of climate variations on both types of growth.

Abstract

The relationships between apical (or primary) and radial (or secondary) growth, and climatic influences on both types of growth, were evaluated for Nothofagus pumilio (Nothofagaceae), the dominant subalpine tree in Patagonia. We measured the spacing and number of nodes of annual shoots developed in the period 2001–2010 in 40 N. pumilio trees growing near the upper treeline in the northern Patagonian Andes (41°S). Variations in ring width at the base of each trunk were also recorded. Interannual variations in primary and secondary growth were significantly related to each other, and to several climate variables. Mean temperatures in winter and early spring (June–October) prior to the period of shoot extension were positively associated with both primary and secondary growth. In addition, total summer precipitation (December–March) was positively related to shoot extension, whereas mean summer temperature during the previous growth season (January–March) was directly related to radial growth. These climatic influences on N. pumilio growth may play a major role in regulating the expressions of preformation and neoformation.