Sap flow measurements reveal influence of temperature and stand structure on water use of Eucalyptus regnans forests

We examined water use by maturing Eucalyptus regnans, growing with or without an mid-storey stratum of Acacia spp. (Acacia dealbata or A. melanoxylon), for >180 consecutive days. Study sites were located in the Upper Yarra catchment area in south-eastern Australia. Depending on their contribution to stand basal area, mid-storey Acacia spp. increased total stand water use by up to 30%. Monthly water use in such stands reached more than 640,000 L ha⁻¹ (compared to 545,000 L ha⁻¹ in stands where acacias were absent) in early spring. Water use was curvilinearly related to sapwood area of Acacia spp. and logistically related to sapwood area of E. regnans. Water use of all three species showed a strong relation to daily maximum air temperatures. Distinct and simple relationships provide clear guides to the likely impacts of climate change and forest management on water yield. We compared a traditional up-scaling approach, from individual tree water use to stand water use, to a new approach that incorporates variation in temperature. Development of this approach can lead to greater precision of stand water use estimates – and in turn catchment water yield – under current climate change scenarios, which predict a rise in air temperatures of 0.6–2.5 °C by 2050 for the study area. Our temperature-dependent approach suggests that under conditions of non-limiting water availability, stand water use will rise by 2% for every 0.25 °C increase in maximum air temperatures during winter, and possibly more than that during summer.     

Transpiration and hydraulic traits of old and regrowth eucalypt forest in southwestern Australia

We tested the hypothesis that overstorey of eucalypt forest dominated by tall, large diameter trees uses less water than regrowth stands in the high rainfall zone (>1100 mm year⁻¹) of the northern jarrah (Eucalyptus marginata) forest in southwestern Australia. We measured leaf area, cover, sapwood area and sapwood density at three paired old and regrowth stands. We also measured sapflow velocity at one paired stand (Dwellingup) from June 2007 to October 2008. Old stands had more basal area but less foliage cover, less leaf area and slightly thinner sapwood. The ratio of sapwood area to basal area decreased markedly as tree size increased. Sapwood area of the regrowth forest stands (6.6 ± 0.30 m² ha⁻¹) was nearly double that of the old stands (3.4 ± 0.17 m² ha⁻¹), despite larger basal area at the old stands. Leaf area index of the regrowth stands (2.1 ± 0.26) was only one-third larger than that at the old stands (1.5 ± 0.15); hence, the ratio of leaf area to sapwood area was larger in old stands than in regrowth stands (0.45 ± 0.022 m² cm⁻² versus 0.32 ± 0.045 m² cm⁻²). Our results are consistent with theories that trees have evolved to optimize carbon gain rather than maintain stomatal conductance. Neither sapwood density (540–650 kg m⁻³) nor sap velocity differed greatly between regrowth and old stands. At the old forest site, daily transpiration rose from 0.5 mm day⁻¹ in winter to 0.9 mm day⁻¹ in spring–summer, compared to 0.9 mm day⁻¹ and 1.8 mm day⁻¹ at the regrowth site. Annual water use by the overstorey trees was estimated to be ∼230 mm year⁻¹ for the old stand and ∼500 mm year⁻¹ at the regrowth stand, or 20% and 44% of annual rainfall. The overwhelming role of stand sapwood area in determining stand water use, combined with the marked changes in the ratio of sapwood area to basal area with tree age and size, suggest that stand overstorey structure can be managed to alter overstorey water use and catchment water yield. Silviculture to promote old-forest-like attributes may be a viable means of delivering multiple water and conservation benefits.     

Simulating age-related changes in carbon storage and allocation in a Chinese fir plantation growing in southern China using the 3-PG model

Chinese fir [(Cunninghamia lanceolata (Lamb.) Hook (Taxodiaceae)] plantations are helping to meet China's increasing demands for timber, while, at the same time, sequestering carbon (C) above and belowground. The latter function is important as a means of slowing the rate that CO₂ is increasing in the atmosphere. Available data are limited, however, and even if extensive, would necessitate consideration of future changes in climatic conditions and management practices. To evaluate the contribution of Chinese fir plantations under a range of changing conditions a dynamic model is required. In this paper, we report successful outcome in parameterizing a process-based model (3-PG) and validating its predictions with recent and long-term field measurements acquired from different ages of Chinese fir plantations at the Huitong National Forest Ecosystem Research Station. Once parameterized, the model performed well when simulating leaf area index (LAI), net primary productivity (NPP), biomass of stems (W_S), foliage (W_F) and roots (W_R), litterfall, and shifts in allocation over a period of time. Although the model does not specifically include heterotrophic respiration, we made some attempts to estimate changes in root C storage and decomposition rates in the litterfall pool as well as in the total soil respiration. Total C stored in biomass increased rapidly, peaking at age 21 years in unthinned stands. The predicted averaged above and belowground NNP (13.81 t ha⁻¹ a⁻¹) of the Chinese fir plantations between the modeling period (from 4 to 21-year-old) is much higher than that of Chinese forests (4.8–6.22 t ha⁻¹ a⁻¹), indicating that Chinese fir is a suitable tree species to grow for timber while processing the potential to act as a C sequestration sink. Taking into account that maximum LAI occurs at the age of 15 years, intermediate thinning and nutrient supplements should, according to model predictions, further increase growth and C storage in Chinese fir stands. Predicted future increases (approximately 0–2 °C) in temperature due to global warming may increase plantation growth and reduce the time required to complete a rotation, but further increases (approximately 2–6 °C) may reduce the growth rate and prolong the rotational age.     

Carbon dioxide exchange of a larch forest after a typhoon disturbance

A typhoon event catastrophically destroyed a 45-year-old Japanese larch plantation in southern Hokkaido, northern Japan in September 2004, and about 90% of trees were blown down. Vegetation was measured to investigate its regeneration process and CO₂ flux, or net ecosystem production (NEP), was measured in 2006–2008 using an automated chamber system to investigate the effects of typhoon disturbance on the ecosystem carbon balance. Annual maximum aboveground biomass (AGB) increased from 2.7 Mg ha⁻¹ in 2006 to 4.0 Mg ha⁻¹ in 2007, whereas no change occurred in annual maximum leaf area index (LAI), which was 3.7 m² m⁻² in 2006 and 3.9 m² m⁻² in 2007. Red raspberry (Rubus idaeus) had become dominant within 2 years after the typhoon disturbance, and came to account for about 60% and 50% of AGB and LAI, respectively. In comparison with CO₂ fluxes measured by the eddy covariance technique in 2001–2003, for 4.5 months during the growing season, the sum of gross primary production (GPP) decreased on average by 739 gC m⁻² (64%) after the disturbance, whereas ecosystem respiration (RE) decreased by 501 gC m⁻² (51%). As a result, NEP decreased from 159 ± 57 gC m⁻² to −80 ± 30 gC m⁻², which shows that the ecosystem shifted from a carbon sink to a source. Seasonal variation in RE was strongly correlated to soil temperature. The interannual variation in the seasonal trend of RE was small. Light-saturated GPP (P_max) decreased from 30–45 μmol m⁻² s⁻¹ to 8–12 μmol m⁻² s⁻¹ during the summer season through the disturbance because of large reduction in LAI.     

Photosynthetic characteristics of Fagus sylvatica and Quercus robur established for stand conversion from Picea abies

Efforts in Europe to convert Norway spruce (Picea abies) plantations to broadleaf or mixed broadleaf-conifer forests could be bolstered by an increased understanding of how artificial regeneration acclimates and functions under a range of Norway spruce stand conditions. We studied foliage characteristics and leaf-level photosynthesis on 7-year-old European beech (Fagus sylvatica) and pedunculate oak (Quercus robur) regeneration established in open patches and shelterwoods of a partially harvested Norway spruce plantation in southwestern Sweden. Both species exhibited morphological plasticity at the leaf level by developing leaf blades in patches with an average mass per unit area (LMA) 54% greater than of those in shelterwoods, and at the plant level by maintaining a leaf area ratio (LAR) in shelterwoods that was 78% greater than in patches. However, we observed interspecific differences in photosynthetic capacity relative to spruce canopy openness. Photosynthetic capacity (A₁₆₀₀, net photosynthesis at a photosynthetic photon flux density of 1600 μmol photons m⁻² s⁻¹) of beech in respect to the canopy gradient was best related to leaf mass, and declined substantially with increasing canopy openness primarily because leaf nitrogen (N) in this species decreased about 0.9 mg g⁻¹ with each 10% rise in canopy openness. In contrast, A₁₆₀₀ of oak showed a weak response to mass-based N, and furthermore the percentage of N remained constant in oak leaf tissues across the canopy gradient. Therefore, oak photosynthetic capacity along the canopy gradient was best related to leaf area, and increased as the spruce canopy thinned primarily because LMA rose 8.6 g m⁻² for each 10% increase in canopy openness. These findings support the premise that spruce stand structure regulates photosynthetic capacity of beech through processes that determine N status of this species; leaf N (mass basis) was greatest under relatively closed spruce canopies where leaves apparently acclimate by enhancing light harvesting mechanisms. Spruce stand structure regulates photosynthetic capacity of oak through processes that control LMA; LMA was greatest under open spruce canopies of high light availability where leaves apparently acclimate by enhancing CO₂ fixation mechanisms.     

The effect of land use type on net nitrogen mineralization on abandoned agricultural land: Silver birch stand versus grassland

The effect of land use type on the dynamics and annual rate of net nitrogen mineralization (NNM) in a naturally generated silver birch stand and in a grassland, both on abandoned agricultural land, was assessed in situ in the upper 0–20 cm soil layer using the method of buried polyethylene bags. Annual NNM rate in the birch stand (156 kg N ha⁻¹ year⁻¹) was higher than in the grassland (102 kg N ha⁻¹ year⁻¹); in both cases NNM covered a major part of the plants annual nitrogen demand. The rate of NNM in the upper 0–10 cm soil layer in the birch stand (99 kg N ha⁻¹ year⁻¹) exceeded the respective rate of NNM in the grassland (51 kg N ha⁻¹ year⁻¹) roughly two times. In the grassland the rates of NNM in the 0–10 and 10–20 cm layers were equal; in the birch stand NNM in the 0–10 cm layer was 1.7 times higher than in deeper 10–20 cm layer. The intensity of daily NNM in the upper 0–10 cm soil layer in the birch stand was the highest in June and in the grassland in May, 776 and 528 mg kg⁻¹ N day⁻¹, respectively. In our study no significant correlation was found between NNM and the environmental factors monthly mean soil temperature, soil moisture content and pH.     

Influences of forest tending works on carbon distribution and cycling in a Pinus densiflora S. et Z. stand in Korea

This study was conducted to determine carbon (C) dynamics following forest tending works (FTW) which are one of the most important forest management activities conducted by Korean forest police and managers. We measured organic C storage (above- and below-ground biomass C, forest floor C, and soil C at 50 cm depth), soil environmental factors (soil CO₂ efflux, soil temperature, soil water content, soil pH, and soil organic C concentration), and organic C input and output (litterfall and litter decomposition rates) for one year in FTW and non-FTW (control) stands of approximately 40-year-old red pine (Pinus densiflora S. et Z.) forests in the Hwangmaesan Soopkakkugi model forest in Sancheonggun, Gyeongsangnam-do, Korea. This forest was thinned in 2005 as a representative FTW practice. The total C stored in tree biomass was significantly lower (P < 0.05) in the FTW stand (40.17 Mg C ha⁻¹) than in the control stand (64.52 Mg C ha⁻¹). However, C storage of forest floor and soil layers measured at four different depths was not changed by FTW, except for that at the surface soil depth (0–10 cm). The organic C input due to litterfall and output due to needle litter decomposition were both significantly lower in the FTW stand than in the control stand (2.02 Mg C ha⁻¹ year⁻¹ vs. 2.80 Mg C ha⁻¹ year⁻¹ and 308 g C kg⁻¹ year⁻¹ vs. 364 g C kg⁻¹ year⁻¹, respectively, both P < 0.05). Soil environmental factors were significantly affected (P < 0.05) by FTW, except for soil CO₂ efflux rates and organic C concentration at soil depth of 0–20 cm. The mean annual soil CO₂ efflux rates were the same in the FTW (0.24 g CO₂ m⁻² h⁻¹) and control (0.24 g CO₂ m⁻² h⁻¹) stands despite monthly variations of soil CO₂ efflux over the one-year study period. The mean soil organic C concentration at a soil depth of 0–20 cm was lower in the FTW stand (81.3 g kg⁻¹) than in the control stand (86.4 g kg⁻¹) but the difference was not significant (P > 0.05). In contrast, the mean soil temperature was significantly higher, the mean soil water content was significantly lower, and the soil pH was significantly higher in the FTW stand than in the control stand (10.34 °C vs. 8.98 °C, 48.2% vs. 56.4%, and pH 4.83 vs. pH 4.60, respectively, all P < 0.05). These results indicated that FTW can influence tree biomass C dynamics, organic C input and output, and soil environmental factors such as soil temperature, soil water content and soil pH, while soil C dynamics such as soil CO₂ efflux rates and soil organic C concentration were little affected by FTW in a red pine stand.     

Growth and productivity of black spruce in even- and uneven-aged stands at the limit of the closed boreal forest

The increasing commercial interest and advancing exploitation of new remote territories of the boreal forest require deeper knowledge of the productivity of these ecosystems. Canadian boreal forests are commonly assumed to be evenly aged, but recent studies show that frequent small-scale disturbances can lead to uneven-aged class distributions. However, how age distribution affects tree growth and stand productivity at high latitudes remains an unanswered question. Dynamics of tree growth in even- and uneven-aged stands at the limit of the closed black spruce (Picea mariana) forest in Quebec (Canada) were assessed on 18 plots with ages ranging from 77 to 340 years. Height, diameter and age of all trees were measured. Stem analysis was performed on the 10 dominant trees of each plot by measuring tree-ring widths on discs collected each meter from the stem, and the growth dynamics in height, diameter and volume were estimated according to tree age. Although growth followed a sigmoid pattern with similar shapes and asymptotes in even- and uneven-aged stands, trees in the latter showed curves more flattened and with increases delayed in time. Growth rates in even-aged plots were at least twice those of uneven-aged plots. The vigorous growth rates occurred earlier in trees of even-aged plots with a culmination of the mean annual increment in height, diameter and volume estimated at 40–80 years, 90–110 years earlier than in uneven-aged plots. Stand volume ranged between 30 and 238 m³ ha⁻¹ with 75% of stands showing values lower than 120 m³ ha⁻¹ and higher volumes occurring at greater dominant heights and stand densities. Results demonstrated the different growth dynamics of black spruce in single- and multi-cohort stands and suggested the need for information on the stand structure when estimating the effective or potential growth performance for forest management of this species.     

Impact of stand density on water status and leaf gas exchange in Quercus ilex

Tree thinning reduces tree-to-tree competition and likely contributes to the improvement of tree water status and productivity in water-limited systems. In this study, we examined the importance of competition for water among Quercus ilex trees in open woodlands by comparing the water consumption and physiological status of trees located along stand density gradients which ranged from 10% (low density; LD) to 100% (high density; HD) of canopy cover. The study was carried out at two sites which differed in mean annual rainfall (506 and 816 L m⁻²; D_site and W_site, respectively). Predawn and midday leaf water potential (ψ_d and ψ_m, respectively) and CO₂ assimilation rate (A) were measured every two weeks from mid May to mid September, in eight trees located along a stand density gradient at each site. Sap flow and soil moisture were measured only at D_site. Sap flow was continuously recorded by sap flowmeters (constant heating method) installed in 12 trees along two stand density gradients. Soil moisture (?) was measured every 20 cm for the first meter and then every 50 cm up to 250 cm. Measurements were conducted in 18 soil profiles, 6 located in HD and 12 in LD (six beneath and six out the canopy). At W_site, differences among stand densities for ψ and A were very small and emerged only at the end of the dry season. At D_site, ψ (both predawn and midday), A, ?, and sap flow density were significantly higher in LD trees than in HD ones. At D_site, some water remained unused in the soil at the end of the dry season beyond the canopy in the LD areas, and trees did not experienced such an acute water deficit (ψ_d > −1 MPa) as the HD trees did (ψ_d < −3 MPa). Summer tree transpiration at the stand level (E_stand) tended to saturate with the increase of canopy cover. E_stand increases by 32% when canopy cover goes from 50% to 100%. Results confirmed that the increase of tree-to-tree competition with stand density was much more significant at dry sites. In these sites, tree thinning is recommended as a way to maintain tree functioning.     

Energy and water balance of two contrasting loblolly pine plantations on the lower coastal plain of North Carolina,USA

During 2005–2007, we used the eddy covariance and associated hydrometric methods to construct energy and water budgets along a chronosequence of loblolly pine (Pinus taeda) plantations that included a mid-rotation stand (LP) (i.e., 13–15 years old) and a recently established stand on a clearcut site (CC) (i.e., 4–6 years old) in Eastern North Carolina. Our central objective was to quantify the differences in both energy and water balances between the two contrasting stands and understand the underlining mechanisms of environmental controls. We found that the LP site received about 20% more net radiation (R_n) due to its lower averaged albedo (α) of 0.25, compared with that at the CC (α = 0.34). The mean monthly averaged Bowen ratios (β) at the LP site were 0.89 ± 0.7, significantly (p = 0.02) lower than at the CC site (1.45 ± 1.2). Higher net radiation resulted in a 28% higher (p = 0.02) latent heat flux (LE) for ecosystem evapotranspiration at the LP site, but there was no difference in sensible heat flux (H) between the two contrasting sites. The annual total evapotranspiration (ET) at the LP site and CC site was estimated as 1011–1226 and 755–855 mm year⁻¹, respectively. The differences in ET rates between the two contrasting sites occurred mostly during the non-growing seasons and/or dry periods, and they were small during peak growing seasons or wet periods. Higher net radiation and biomass in LP were believed to be responsible to the higher ET. The monthly ET/Grass Reference ET ratios differed significantly across site and season. The annual ET/P ratio for the LP and CC were estimated as 0.70–1.13 and 0.60–0.88, respectively, indicating higher runoff production from the CC site than the LP site. This study implied that reforestation practices reduced surface albedos and thus increased available energy, but they did not necessarily increase energy for warming the atmosphere in the coastal plain region where soil water was generally not limited. This study showed the highly variable response of energy and water balances to forest management due to climatic variability.     

Predicting growth and sequestration of carbon by plantations growing in regions of low-rainfall in southern Australia

In regions of Australia of low–medium rainfall (500–800 mm/year), there is growing community and land-owner support for re-planting trees to achieve multiple environmental objectives, particularly amelioration of soil salinity. Sequestration of carbon by newly established trees is not only another important environmental benefit, but also a potential commercial benefit. To obtain estimates of carbon sequestered by species of commercial potential in such regions, we calibrated the carbon (C) accounting model FullCAM to Eucalyptus cladocalyx and Corymbia maculata plantations. This was achieved by harvesting trees of a range in sizes to determine the allometric relationships that most accurately predict biomass and stem density from measures of stem diameter. Predictions of stem diameter were obtained from a forest growth model (3-PG) previously calibrated for these two species. By applying these predictions of changes in stem diameter as the stand matures in our allometric relationships, we estimated changes in partitioning of biomass (between stem, branches, bark, foliage and roots) and stem wood density as the stand matures under scenarios of 500, 600 and 750 mm mean annual rainfall. We found that for both species, regardless of annual rainfall, throughout the rotation 37–50% of carbon sequestered in the total tree biomass was in the stem, 18–27% in both branches and roots, and the remainder in foliage or bark. However, rate of accumulation of carbon was dependent on annual rainfall, with average annual rate of sequestration of carbon in tree biomass and litter during the first rotation of E. cladocalyx (or C. maculata) increasing from 3.68 (or 4.17) to 4.72 (or 4.86) Mg C ha⁻¹ yr⁻¹ as annual rainfall increased from about 500 to 750 mm. Although it was predicted that decomposition negated any accumulation of debris between successive rotations, carbon was predicted to accumulate in sawlog products, given that assumed rates of product decomposition were slightly less than their rate of accumulation. This resulted in a slight increase (<8 Mg C ha⁻¹) in predicted total sequestration of carbon between successive rotations.     

Heartwood/sapwood variation of western redcedar as influenced by cultural treatments and position in tree

We studied heartwood and sapwood variation in western redcedar (Thuja plicata) at three sites, including a 95-year-old naturally regenerated, unmanaged stand, a 35-year-old planted spacing trial, and a 30-year-old naturally regenerated stand to which thinning and fertilization treatments had been applied. In the 95-year-old stand, we studied within-tree variation in heartwood and sapwood. In the thinning/fertilization trial and the planted spacing trial, we studied effects of cultural practices and growth rate on heartwood and sapwood. In the trees that we studied, sapwood width was generally fairly narrow, rarely exceeding 3.5 cm. Heartwood formation in western redcedar appeared to begin at a relatively small stem diameter (7 cm) and at a young age, probably 10–15 years. The amount and proportion of heartwood increased with distance downward from the top of the tree, with the implication that older trees will contain a greater proportion of heartwood than younger trees. For any given age, it appears that cultural treatments that favor rapid growth will result in stems with greater amounts of both sapwood and heartwood, and a greater proportion of heartwood.     

Validation of plantation transpiration in south-eastern Australia estimated using the 3PG+ forest growth model

Forest plantations for wood production are an increasingly important land use in southern Australia, and there are potentially important hydrologic consequences of what is mostly a change in land use from agriculture to silviculture. An ability to predict, with some degree of accuracy, the impact of plantation expansion on surface water and groundwater resources is essential. A validated process-based modelling approach, integrating the many interacting environmental and management factors which may influence plantation growth and transpiration, can be used for this purpose. The 3PG forest growth model has been evaluated for a number of species from widely differing climate and site conditions. While growth predictions have been validated, little attention has been given to testing the accuracy of the transpiration predictions or the model's representation of the water balance. We enhanced the 3PG forest growth model (known as 3PG+) and then integrated it into the Catchment Analysis Tool (CAT), so that it now interfaces with a more detailed multi-layered, daily time step representation of the soil water balance. Simulated transpiration using 3PG+ in CAT was compared with field measurements in 30 plots (across 15 sites) representing 5 common plantation species (Eucalyptus globulus, E. nitens, E. grandis, E. regnans and Pinus radiata) across ages 2–31 years. Mean daily plot transpiration during the measurement periods ranged between 0.4 and 4.2 mm day⁻¹ (average 2.0 mm day⁻¹). Simulated mean daily plot transpiration using 3PG+ in CAT for Eucalyptus was good (coefficient of efficiency = 0.80; R² = 0.81). While the model tended to slightly under-predict transpiration at higher measured rates (>3.5 mm day⁻¹), predictions at monthly timescales had acceptable accuracy. The integration of 3PG+ into CAT resulted in an improvement in accuracy and applicability of CAT, and provides for the spatial application of 3PG+ across diverse and mixed land use catchments for investigation into carbon and water movement in forest systems.     

Enrichment planting as a silvicultural option in the eastern Amazon: Case study of Fazenda Cauaxi

Liana-dominated forest patches constitute 15–20% of old-growth forests in the Eastern Amazon but are generally excluded from management for timber production. Here we ask if liana-dominated patches may be brought into production by clearing lianas and conducting enrichment planting (EP) of native timber species. We present growth results from 8 years of such EP trials. Rapid growth and low mortality of all species in this study suggest that EP in cleared liana patches can contribute to timber stocks in second and third harvests of managed forests. The most vigorous individuals of Parkiagigantocarpa and Schizolobium amazonicum in each enrichment site grew more than 1 cm diameter per year (rates were initially >2 cm yr⁻¹), and attained dominant canopy positions and diameters equal to those of small canopy trees in the surrounding forest within 8 years of planting (mean dbh ∼18 cm and ∼20 cm, respectively, at year 8). Limited data on Ceiba pentandra plantings indicate a similar trajectory for this species (dbh ∼40 cm in 8 years). The most vigorous Swietenia macrophylla grew at least 1 cm per year in enrichment plots (mean dbh ∼10 cm in 8 years), but take longer to attain dominant positions. Tabebuia serratifolia may take much longer to reach the canopy than other species tested (rates <1 m yr⁻¹). We attribute the excellent performance to light availability; planting in intact soil with minimal compaction and abundant organic material; and low competition rates maintained by periodic thinning of competing vegetation.     

Forest stand dynamics and sudden oak death: Mortality in mixed-evergreen forests dominated by coast live oak

Sudden oak death (SOD), caused by the recently discovered non-native invasive pathogen, Phytophthora ramorum, has already killed tens of thousands of native coast live oak and tanoak trees in California. Little is known of potential short and long term impacts of this novel plant–pathogen interaction on forest structure and composition. Coast live oak (Quercus agrifolia) and bay laurel (Umbellularia californica) form mixed-evergreen forests along the northern California coast. This study measured tree mortality over a gradient of disease in three time periods. Direct measurements of current mortality were taken during 2004, representing a point-in-time estimate of present and ongoing mortality. Past stand conditions, c. 1994, were estimated using a stand reconstruction technique. Future stand conditions, c. 2014, were calculated by assuming that, given a lack of host resistance, live trees showing signs of the disease in 2004 would die. Results indicate that coast live oaks died at a rate of 4.4–5.5% year⁻¹ between 1994 and 2004 in highly impacted sites, compared with a background rate of 0.49% year⁻¹, a ten-fold increase in mortality. From 2004 to 2014, mortality rates in the same sites were 0.8–2.6% year⁻¹. Over the entire period, in highly impacted sites, a 59–70% loss of coast live oak basal area was predicted, and coast live oak decreased from 60% to 40% of total stand basal area, while bay laurel increased from 22% to 37%. Future stand structures will likely have greater proportions of bay laurel relative to coast live oak.     

Effects of [CO2] and nitrogen on morphological and biomass traits of white birch (Betula papyrifera) seedlings

To investigate the interactive effects of CO₂ concentration ([CO₂]) and nitrogen supply on the growth and biomass of boreal trees, white birch seedlings (Betula papyrifera) were grown under ambient (360 μmol mol⁻¹) and elevated [CO₂] (720 μmol mol⁻¹) with five nitrogen supply regimes (10, 80, 150, 220, and 290 μmol mol⁻¹) in greenhouses. After 90 days of treatment, seedling height, root-collar diameter, biomass of different organs, leaf N concentration, and specific leaf area (SLA) were measured. Significant interactive effects of [CO₂] and N supply were found on height, root-collar diameter, leaf biomass, stem biomass and total biomass, stem mass ratio (SMR), and root mass ratio (RMR), but not on root mass, leaf mass ratio (LMR), leaf to root ratio (LRR), or leaf N concentration. The CO₂ elevation generally increased all the growth and biomass parameters and the increases were generally greater at higher levels of N supply or higher leaf N concentration. However, the CO₂ elevation significantly reduced SLA (13.4%) and mass-based leaf N concentration but did not affect area-based leaf N concentration. Increases in N supply generally increased the growth and biomass parameters, but the relationships were generally curvilinear. Based on a second order polynomial model, the optimal leaf N concentration was 1.33 g m⁻² for height growth under ambient [CO₂] and 1.52 g m⁻² under doubled [CO₂]; 1.48 g m⁻² for diameter under ambient [CO₂] and 1.64 g m⁻² under doubled [CO₂]; 1.29 g m⁻² for stem biomass under ambient [CO₂] and 1.43 g m⁻² under doubled [CO₂]. The general trend is that the optimal leaf N was higher at doubled than ambient [CO₂]. However, [CO₂] did not affect the optimal leaf N for leaf and total biomass. The CO₂ elevation significantly increased RMR and SMR but decreased LMR and LRR. LMR increased and RMR decreased with the increasing N supply. SMR increased with increase N supply up to 80 μmol mol⁻¹ and then leveled off (under elevated [CO₂]) or stated to decline (under ambient [CO₂]) with further increases in N supply. The results suggest that the CO₂ elevation increased biomass accumulation, particularly stem biomass and at higher N supply. The results also suggest that while modest N fertilization will increase seedling growth and biomass accumulation, excessive application of N may not stimulate further growth or even result in growth decline.     

Effect of species and spacing of fast-growing nurse trees on growth of an indigenous tree, Hopea odorata Roxb., in northeast Thailand

We tested the effects of species and spacing of nurse trees on the growth of Hopea odorata, a dipterocarp tree indigenous to Southeast Asia, in a two-storied forest management system in northeast Thailand. Eucalyptus camaldulensis, Acacia auriculiformis, and Senna siamea were planted as nurse trees in 1987 at spacings of 4 m × 8 m, 2 m × 8 m, 4 m × 4 m, and 2 m × 4 m in the Sakaerat Silvicultural Research Station of the Royal Forest Department, Thailand. Seedlings of H. odorata were planted in the nurse tree stands at a uniform spacing of 4 m × 4 m and in control plots (no nurse trees) in 1990. Stem numbers of some nurse trees were thinned by half in 1994. The stem diameter and height of all trees were measured annually until 1995 and again in 2007. The mean annual increment (MAI) in volume was estimated as 8.2–10.1 m³ ha⁻¹ year⁻¹ for E. camaldulensis and 0.9–1.2 m³ ha⁻¹ year⁻¹ for S. siamea, smaller than reported elsewhere. This suggests that the site properties were not suitable for them. The MAI of A. auriculiformis was 7.9–9.8 m³ ha⁻¹ year⁻¹, within the reported range. Survival rates of H. odorata in the S. siamea stands and the control plots decreased rapidly during the first 2 years but then stayed constant from 1992. In contrast, survival rates of H. odorata in the E. camaldulensis and A. auriculiformis stands were initially high (>70%), but then decreased after 1995. Stem diameter, tree height, and stand basal area of H. odorata were large in both the S. siamea stands and the control plots from then. The growth of H. odorata was largest in the 2 m × 8 m S. siamea stands. In contrast, it was restricted in the E. camaldulensis and A. auriculiformis stands owing to strong shading by their canopies. Thinning by 50% tended to facilitate the growth of H. odorata temporarily in the E. camaldulensis and A. auriculiformis stands. The stand basal areas of nurse trees and of H. odorata showed a trade-off. These results suggest that the growth of H. odorata was maximized in the S. siamea stands. We assume, however, that the growth of H. odorata could be improved even in the E. camaldulensis and A. auriculiformis stands by frequent or heavy thinning.     

Carbon accumulation along a stand development sequence of Nothofagus antarctica forests across a gradient in site quality in Southern Patagonia

Above- and below-ground C pools were measured in pure even-aged stands of Nothofagusantarctica (Forster f.) Oersted at different ages (5–220 years), crown and site classes in the Patagonian region. Mean tissue C concentration varied from 46.3% in medium sized roots of dominant trees to 56.1% in rotten wood for trees grown in low quality sites. Total C concentration was in the order of: heartwood > rotten wood > sapwood > bark > small branches > coarse roots > leaves > medium roots > fine roots. Sigmoid functions were fitted for total C accumulation and C root/shoot ratio of individual trees against age. Total C accumulated by mature dominant trees was six times greater than suppressed trees in the same stands, and total C accumulated by mature dominant trees grown on the best site quality was doubled that of those on the lowest site quality. Crown classes and site quality also affected the moment of maximum C accumulation, e.g. dominant trees growing on the worse site quality sequestered 0.73 kg C tree⁻¹ year⁻¹ at 139 years compared to the best site where 1.44 kg C tree⁻¹ year⁻¹ at 116 years was sequestered. C root/shoot ratio decreased over time from a maximum value of 1.3–2.2 at 5 years to a steady-state asymptote of 0.3–0.7 beyond 60 years of age depending on site quality. Thus, root C accumulation was greater during the regeneration phase and for trees growing on the poorest sites. The equations developed for individual trees have been used to estimate stand C accumulation from forest inventory data. Total stand C content ranged from 128.0 to 350.9 Mg C ha⁻¹, where the soil C pool represented 52–73% of total ecosystem C depending on age and site quality. Proposed equations can be used for practical purposes such as estimating the impact of silvicultural practices (e.g. thinning or silvopastoral systems) on forest C storage or evaluating the development of both above- and below-ground C over the forest life cycle for different site qualities for accurate quantification of C pools at regional scale.     

Variability and grouping of leaf traits in multi-species reforestation (Leyte,Philippines)

Conventional reforestation in the tropics often results in stands with low tree species and functional diversities. A different approach to reforestation, the so-called rainforestation, has been developed in the Philippines. It emphasizes mixed stands and the preferential use of native species supplemented by fruit trees. In such stands, we studied several functional leaf traits (stomatal conductance for water vapour, leaf morphology and chemistry) with the objectives (1) of assessing the species-specific variation of leaf traits and in particular that of maximal leaf stomatal conductance (g_smax), (2) of determining relationships between g_smax and other tree variables, and (3) of assessing whether leaf traits group the species studied. Sixteen broad-leaved species were studied, using five individual trees per species and ten fully expanded sunlit leaves per individual tree. Species-specific g_smax differed fivefold (165–772 mmol m⁻² s⁻¹). Among studied leaf traits, only the carbon isotope ratio δ¹³C exhibited a simple linear correlation with g_smax. A separate analysis for dipterocarp species indicated a strong negative relationship between g_smax and specific leaf area (SLA) (r² = 0.96, P < 0.001, n = 5). For all 16 species, a multiple linear regression with the combinations leaf size/tree height and leaf size/canopy projection area also resulted in significant relationships, which partly explained the variability in g_smax. A multivariate approach (principal component analysis) combining the leaf traits provided an explanation of 75% of the variability along the first two axes. All native dipterocarps species, a native Guttiferae and the durian tree (Durio zibethinus) were associated with more depleted δ¹³C, small leaves and a low leaf width to length ratio. Two exotic species frequently used for reforestation (Gmelina arborea and Swietenia macrophylla) and the native early successional Terminalia microcarpa were differentiated by their high SLA and high leaf nitrogen content per leaf area (N_area). Both species of Artocarpus (A. blancoi and A. odoratissima) were also differentiated and had large leaves with low SLA and low N_area. These associations of species with leaf traits as variables indicate that species have different leaf investment strategies, which may imply that there are differences in whole plant performance. We conclude that rainforestation creates substantial variation in leaf traits, which is based on the combination of species with different leaf trait groupings. This can be seen as an important step towards – partly – restoring the functional diversity which characterizes many natural tropical rainforests.     

Dynamics of wood recruitment in streams of the northeastern US

Wood is an important component of forested stream ecosystems, and stream restoration efforts often incorporate large wood. In most cases, however, stream restoration projects are implemented without information regarding the amount of wood that historically occurred or the natural rates of wood recruitment. This study uses a space-for-time analysis to quantify large wood loading to 28 streams in the northeastern US with a range of in-stream and riparian forest characteristics. We document the current volume and frequency of occurrence of large wood in streams with riparian forests varying in their stage of stand development as well as stream size and gradient. Linear models relating stream wood characteristics to stream geomorphic and forest characteristics were compared using Akaike's Information Criterion (AIC) model selection. The AIC analysis indicated that the volume and frequency of large wood and wood accumulations (wood jams) in streams was most closely associated with the age of the dominant canopy trees in the riparian forest (best models: log₁₀(large wood volume (m³ 100 m⁻¹)) = (0.0036 × stand age) − 0.2281, p < 0.001, r² = 0.80; and large wood frequency (number per 100 m) = (0.1326 × stand age) + 7.3952, p < 001, r² = 0.63). Bankfull width was an important factor accounting for wood volume per unit area (m³ ha⁻¹) but not the volume of wood per length of stream (100 m⁻¹). The empirical models developed in this study were unsuccessful in predicting wood loading in other regions, most likely due to difference in forest characteristics and the legacy of forest disturbance. However, these models may be applicable in other streams in the northeastern US or in streams with comparable riparian forests, underlying geology, and disturbance regimes—factors that could alter long-term wood loading dynamics. Our results highlight the importance of understanding region-specific processes when planning stream restoration and stream management projects.