Effects of the clear-cutting of a Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantation on chemical soil fertility

? Stand harvesting and regeneration were usually considered to be a critical phase for the sustainability of forest soils. The present study concerned the effects on soil chemical fertility of the clear-cutting of a highly productive Douglas-fir stand aged 67 years that was clear-cut with no disturbance.

? Results showed that soil changes were rapid in the three-year period following the cutting. The forest floor mass considerably decreased and the mineral soil showed a limited but real acidification. Soil losses represented 4% of the available nutrients over a depth of 60 cm for N, 22% for K, 25% for Ca and 32% for Mg. Only P increased by 11%. Due to the spatial variability of forest soils, and despite regular re-sampling, confidence intervals were large and difficult to reduce.

? The reversibility of the effects of the clear-cutting and its consequences on soil functions depend on the element: it should not be a problem for C, N and K, which would recover when the biological cycle is re-established once again in the young stand. Phosphorus is not at issue since it changes form in the soil. The depletion of Ca, and to a lesser extent of Mg, is of some concern as a result of limited soil reserves, the limited flux of cations from the mineral changes in the soil, the relatively large part of Ca and Mg in the forest floor, and the negative input-output budgets for those elements.

? The duration of the impact of the clear-cutting on soil requires medium-term observations because it cannot be deduced from the current knowledge of this ecosystem.

     

Biogeochemical cycles of nutrients in tropical Eucalyptus plantations: Main features shown by intensive monitoring in Congo and Brazil

The sustainability of fast-growing tropical Eucalyptus plantations is of concern in a context of rising fertilizer costs, since large amounts of nutrients are removed with biomass every 6–7 years from highly weathered soils. A better understanding of the dynamics of tree requirements is required to match fertilization regimes to the availability of each nutrient in the soil. The nutrition of Eucalyptus plantations has been intensively investigated and many studies have focused on specific fluxes in the biogeochemical cycles of nutrients. However, studies dealing with complete cycles are scarce for the Tropics. The objective of this paper was to compare these cycles for Eucalyptus plantations in Congo and Brazil, with contrasting climates, soil properties, and management practices.The main features were similar in the two situations. Most nutrient fluxes were driven by crown establishment the two first years after planting and total biomass production thereafter. These forests were characterized by huge nutrient requirements: 155, 10, 52, 55 and 23 kg ha^?1 of N, P, K, Ca and Mg the first year after planting at the Brazilian study site, respectively. High growth rates the first months after planting were essential to take advantage of the large amounts of nutrients released into the soil solutions by organic matter mineralization after harvesting. This study highlighted the predominant role of biological and biochemical cycles over the geochemical cycle of nutrients in tropical Eucalyptus plantations and indicated the prime importance of carefully managing organic matter in these soils. Limited nutrient losses through deep drainage after clear-cutting in the sandy soils of the two study sites showed the remarkable efficiency of Eucalyptus trees in keeping limited nutrient pools within the ecosystem, even after major disturbances. Nutrient input–output budgets suggested that Eucalyptus plantations take advantage of soil fertility inherited from previous land uses and that long-term sustainability will require an increase in the inputs of certain nutrients.     

柳杉人工林皆伐后初期土壤有机碳和微生物量碳动态

本文研究了华西雨屏区柳杉人工林皆伐后1年内土壤有机碳和微生物量碳动态。结果表明:柳杉人工林皆伐林地土壤平均有机碳含量比对照(未皆伐林地)减小2.01 gC.kg-1,但差异不显著,而土壤平均有机碳储量及微生物量碳分别比对照减少20.97 tC.hm-2、6.68 mg.kg-1(P0.05);皆伐林地土壤有机碳含量及微生物量碳均随季节的变化而逐渐降低,但有机碳储量随季节的变化无明显减少趋势;皆伐林地土壤四季的有机碳含量、碳储量和微生物量碳差异不显著。皆伐对柳杉人工林土壤有机碳储量的影响主要表现在0~20 cm土层(P0.05);皆伐林地和对照在0~40 cm土层的微生物量碳和有机碳含量都表现出显著相关性(P0.05),但对照的相关性高于皆伐林地。总之,柳杉人工林转变为采伐迹地后,其初期土壤有机碳储量和微生物量碳都明显减少。     

皆伐与不同迹地清理方式对我国南方林地土壤温度的影响

采用土壤温度测试仪分别对火烧法和散铺法2种南方常用迹地清理方式清理后的林地表层（0-20 cm）土壤温度的变化情况以及皆伐前后土壤温度的变化情况进行实测。结果表明：皆伐后的林地表层土壤温度明显高于皆伐前的土壤温度,且两者差别大小与迹地清理方式有关。不同迹地清理方式,林地表层土壤温度有明显的差异,在相同条件下,火烧法的土壤温度高于散铺法,尤其在0-5 cm的土层温度差别最显著,最大相差5-10℃。随着土层深度的增大,土壤温度变化的差异逐渐减小。     

Biomass and nutrient allocation in Douglas-fir and amabilis fir seedlings: influence of growth rate and nutrition

Allocation of biomass and nutrients to shoots and roots was followed for three years in fast and slow growing populations of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), a fast growing pioneer species, and amabilis fir (Abies amabilis Dougl. ex J. Forbes), a slow growing shade-tolerant species. Seedlings were grown for three seasons in five nutrient treatments containing varying proportions of nitrogen and phosphorus (N:P). In both species, growth was greatest in the 250:20 N:P treatment followed by the 100:60 and 100:20 treatments. Vector analysis showed that, in both species, relative to the 100:20 treatment, seedlings in the 20:20 treatment were N deficient and seedlings in the 100:4 treatment were P deficient, i.e., where deficiency is defined to mean that an increase in nutrient supply increases nutrient content, nutrient concentration and plant dry weight. Seedlings in the 100:60 treatment had a higher P content than seedlings in the 100:20 treatment but the same dry weight, indicative of what Timmer and Armstrong (1987) termed luxury consumption. No nutrient retranslocation was observed in either species until the third growing season. In Douglas-fir, the greatest percentage of nutrients was exported from one-year-old shoots between May and July of the third growing season to support new growth. The total amount and percent of nutrients retranslocated was higher in Douglas-fir than in amabilis fir. Amabilis fir seedlings also exported N and P from older shoots, but this was later partially replenished. In both species, P retranslocation was greatest in treatments with a high N:P ratio. Nitrogen retranslocation was greatest in amabilis fir seedlings in treatments with a low N:P ratio, and greatest in Douglas-fir seedlings in the 250:20 and 100:60 treatments. Potassium retranslocation was correlated with seedling size. Douglas-fir retranslocated more of its shoot N reserves into new growth at the expense of older needles when soil fertility was high and sinks were strong. Otherwise, both species preferentially translocated the elements in short supply. Thus, retranslocation varied with the ecological characteristics of species, the relative availability of soil nutrients and sink strength.     

Eight-year performance of bareroot Douglas-fir and bareroot and plug western larch Seedlings following herbicide applications,northeast Oregon,USA

Plantation success in northeastern Oregon can be limited by poor survival for western larch (Larix occidentalis Nutt.) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. glauca (Beissn.) Franco) seedlings. Vegetation management treatments may increase soil moisture availability during the dry summer months, thereby increasing the probability of seedling survival. Chemical site preparation treatments were applied in spring 2007, summer 2007, and spring 2008 to determine if such treatments would improve survival and growth of bareroot Douglas-fir and bareroot and plug western larch seedlings on three sites near Elgin, Oregon. Although some herbicide treatments significantly increased survival, results were not consistent among sites. Overall, Douglas-fir had greater survival after 7–8 years than larch seedlings. Seedling size 7–8 years after planting was also highly variable among sites and treatments with larch seedlings being taller and larger than Douglas-fir seedlings, on the average. Size for all stock types was negatively correlated with vegetative cover and positively correlated with first-year soil moisture. Macro- and microclimatic conditions, including topography and soil depth, were likely contributors to differences in survival and growth.     

杉木人工林采伐迹地更新群落结构特征的研究

从杉木人工林科学经营和自然资源管理保护出发，根据定位研究和群落样地调查结果，分析和比较了湖南杉木中心产区会同杉木人工林不同采伐方式迹地上更新群落的组成结构和数量特征．结果表明：迹地群落中共有维管植物85科164属254种，其中乔木、灌木、藤本和草本分别为53种、85种、39种和77种；皆伐和间伐后物种多样性指数高；皆伐8a后迹地群落与皆伐2a后群落相似性较小，而与间伐8a后群落相似性较大；杉木人工林采伐迹地种类成分变更快．提出了合理经营和保护杉木林生态系统多样性和稳定性的措施．     

Clear-cutting reduces nitrate leaching in a pine plantation of high natural N status

Following the tree harvest, the biogeochemistry of a catchment is modified by changes in soil temperature and moisture, and nutrient cycling. We monitored soil-solution and stream-water chemistry, and soil properties in a Pinus radiata D. Don plantation in New Zealand before and after clear-cutting and replanting in 1997. The annual rainfall during the study was 1440–1860 mm. The soil was a 1800-year-old pumice soil of high natural N status; the catchment had received large inputs of volcanic N in rain, probably over the 1800 years since the pumice had been deposited. The leaching loss of nitrate-N was 28 kg ha⁻¹ yr⁻¹ in 1996, and then decreased sharply after clear-cutting to 3 kg ha⁻¹ yr⁻¹ in 1998 and <1 kg ha⁻¹ yr⁻¹ in 1999. Weed growth and soil microbial biomass increased during this time, and would have removed much of the N from soil solution in the upper soil layers. Although the catchment was small (8.7 ha), there was a 2-year lag until N decreased in stream-water; the losses of dissolved organic N to stream-water were low. There was no change in soil pH over the 4 years, but spring-water pH appeared to increase, which was consistent with the increase in bicarbonate that accompanied grass/weed growth. The export of cations (mmol_c l⁻¹) in the spring-water was Na>Ca>Mg=K as expected for rhyolitic pumice, and the total concentration was probably controlled by the accompanying anions. The export of anions was NO₃=Cl>SO₄=HCO₃ before harvest and HCO₃=Cl>SO₄=NO₃ after harvest.     

采伐强度对小兴安岭低质林分土壤碳通量的影响

针对择伐后的小兴安岭低质林分,采用LI-8100碳通量自动监测系统在春、夏、秋、冬4个季节对低质林分土壤碳通量进行观测,运用统计分析方法,分析采伐强度对土壤碳通量、土壤温度与湿度的影响。结果表明:采伐区的土壤碳通量高于对照区,随采伐强度的增加土壤碳通量呈现波动性,从采伐强度22%～47%,碳通量逐渐减小,之后趋于平稳,春、夏、秋、冬4个季节土壤碳通量的最大值出现在低度和中度采伐强度林分条件下;随采伐强度增加土壤温度增加,土壤湿度减少,土壤碳通量与距地表10cm处的土壤温度的关系符合指数模型,土壤碳通量与土壤湿度的关系符合一元二次模型;从标准化回归方程看出,土壤碳通量主要由土壤温度与湿度共同影响,且土壤湿度对碳通量的影响大于土壤温度的影响。采伐强度与土壤碳通量具有负相关性,二者的关系为双曲线。     

皆伐方式对小兴安岭低质林土壤呼吸的影响

针对小兴安岭低质林分,采用不同的皆伐方式,利用LI-8100碳通量自动监测系统在春、夏、秋、冬4个季节对低质林分土壤呼吸进行观测,运用统计分析的方法,分析皆伐方式对土壤呼吸的影响和不同皆伐方式的土壤呼吸产生差异的原因.结果显示:试验区土壤呼吸夏季最大,冬季最低.春季和秋季因土壤温度和湿度差异不大使土壤呼吸相差较小;对于水平带同一条皆伐带,土壤呼吸并无显著性差异,并且不同的带宽对土壤呼吸的影响也不显著;垂直带随海拔升高土壤呼吸呈现波动性,总体趋势随海拔升高土壤呼吸逐渐降低,但是差距较小;水平带、垂直带和林窗的年土壤呼吸量分别为1.184,1.426,1.179 kgCO2·m-a-1,垂直带最高,水平带次之,林窗最低.影响垂直带土壤呼吸的关键因素是土壤温度,林窗则是土壤湿度,而水平带土壤温度和湿度的条件则介于垂直带和林窗之间.这说明在不同的皆伐方式条件下,影响土壤呼吸的关键因素并不完全相同.     

Relative importance of water and nutrients on the growth of coast Douglas fir in the Pacific Northwest

The Douglas-fir region in northwestern North America is characterized by abundant moisture supply during winter, extended dry periods during the growing-season and significant differences in water availability. Many soils have low fertility and native tree species respond to nitrogen fertilization, especially Pseudotsuga menziesii (Mirb.) Franco (coast Douglas fir). Although irrigation of commercial forests in this region is currently impractical, questions arising about the relative importance of water and nutrients were examined using long-term growth data from three studies.

At Pack Forest, fertilization within irrigation doubled growth rates, and no positie growth responses were measured from irrigation. Short-term (5 years) irrigation with sewage effluent containing many nutrients resulted in a six-fold increase in biomass production for poplar and three-fold for Douglas fir as compared to irrigation with equal volumes of river water.

Volume growth in 12- to 65-year-old stands in southwestern Oregon was increased by fertilization at about 70% of the location; annual gain averaged 2.73 m³ ha⁻¹ for 5–12 years. Response was not related to annual precipitation, which ranhed from 81 to 279 cm, nor other moisture-related variables. Absolute and relative volume response showed highest correlation with soil carbon: nitrogen ratio.

Compared with nutrition, moisture does not seem to be a major limiting factor for growth in the Douglas-fir region of the Pacific northwest.     

Effect of clear-cutting on the understory vegetation,soil and diversity of litter beetles in scots pine-dominated forest

Currently, the aim of modern forest management is not only timber production but also the protection of biodiversity. The initial effects of clear-cut logging on forest understory vegetation, soil and the diversity of litter beetles have been studied. We examined unstudied effects of clear-cutting disturbance on understory vascular plant species abundance, forest soils, and understory vegetation nutrients as well as beetle diversity one year after clear-cutting. Substantial changes in the prominence values and above-ground mass of forest-related vascular plant and moss species were detected. Clear-cuttings resulted in fast appearance of new light-loving plant species. The significant decrease in understory plant biomass influenced the reduction of nutrient pools in clear-cut areas after one year. The clear-cut logging negatively influenced the richness of species and the number of individuals of stenobiontic forest-related beetles in the forest litter. However, forest clear-cuttings were beneficial for eurytopic and open land species.     

Nutrient cycling in a clonal stand of Eucalyptus and an adjacent savanna ecosystem in Congo: 2. Chemical composition of soil solutions

The dynamics of nutrients were compared over three years in a clonal Eucalyptus plantation and in a native savanna in Congo. This paper focuses on the changes in the chemical composition of solutions during their transfer through the soil in both ecosystems. The main characteristics of the soil (Ferralic Arenosol) were similar in the two ecosystems, and the low inter-stand variability allowed reliable comparisons of the influence of afforestation on the soil solution chemistry.

Rainfall amounted to about 1400 mm per year during the experimental period. In both ecosystems, an enrichment was observed for most elements during the transfer of solutions through the foliage, but N uptake occurred. Concentrations of H⁺ and dissolved organic carbon (DOC) in solutions increased through the litter layer in both stands. In the Eucalyptus ecosystem, a quick uptake of nutrients by a dense root mat inside the forest floor likely explained why the concentrations of gravitational waters were not enhanced markedly for ‘base cations’, despite the mineralization of high amounts of nutrients during the litter decay.

Soil solutions were collected by zero tension lysimeters (ZTL) at a depth of 15 cm, but these lysimeters were inefficient at collecting gravitational solutions beyond this depth. By contrast, tension lysimeters (TL) maintained at a suction of −60 kPa, collected soil solutions at the depths of 15, 50 cm, 1, 2, 3, 4 m in both ecosystems and 6 m in the plantation. In the topsoil of both stands, the nutrient concentration decreased sharply when the time of residence of solutions increased. This pattern highlighted the crucial role of the inputs by throughfall, stemflow and mineralization of the litter layer for the nutrition of these stands.

A combination of high nutrient requirements of the stands and low availability of exchangeable cations in this highly weathered soil might account for the extremely low nutrient concentrations in solutions collected by TL, regardless of the depth.     

毛竹伐蔸根系养分含量、抗氧化能力与伐后年数的关系

【目的】揭示毛竹伐蔸根系养分含量和抗氧化能力的年际变化规律,为毛竹林伐蔸促腐技术研究提供理论参考。【方法】以6年生活立竹的蔸根为对照,分析比较了笋材两用毛竹林伐后2 a、4 a、6 a的伐蔸根系C、N、P、丙二醛(MDA)、可溶性蛋白质含量和根系活力、抗氧化酶活性。【结果】表明随着毛竹立竹伐后年限的增加,伐蔸根系的细胞膜脂过氧化程度提高,渗透调节能力降低,根系活力明显降低,抗氧化能力逐渐下降。但毛竹立竹伐后,甚至伐后6 a时,伐蔸根系仍有较强的生理活性和生长更新能力,具有较高的C、N、P含量,尤其是C含量明显提高,主要养分化学计量比产生适应性调节,N/P相对稳定,C/N、C/P趋于升高。【结论】毛竹伐后6 a,蔸根仍具有较强的养分吸收能力,也能通过与竹鞭的联系从活立竹转运利用有机C,这是毛竹伐蔸难以腐解的重要机制。     

Responses of polypore fungi following disturbance-emulating harvesting treatments and deadwood creation in boreal Norway spruce dominated forests

ABSTRACT

The emulation of natural disturbances in harvesting has become a widely accepted approach to reach ecologically sustainable forest management. The purpose of this study was to examine the responses of polypore fungi (species richness and composition) on four different cutting methods (selection, gap, patch, and clear-cutting) plus uncut controls, and the creation of high stumps and cut logs in combination with harvesting. Polypores were inventoried one year before, and two and seven years after cutting. A total of 71 polypore species were found, but no red-listed species were recorded seven years after cutting. Irrespective of the treatment, the variation in polypore richness was best explained by the number of deadwood objects in the stands. Species richness per deadwood object was the highest on natural logs. Species richness per volume of spruce deadwood was the highest on small-diameter deadwood and stumps, intermediate on natural logs and lower on cut logs and high stumps. These results indicate that cut logs created in harvesting cannot compensate the loss of deadwood in cutting and energy wood harvesting. Our results demonstrate the difficulty in rapidly creating habitats for red-listed polypores in commercially managed forests.     

皆伐前后油松人工林下植物多样性变化及与土壤水分的关系

【目的】探讨油松人工林皆伐前后林下植物多样性的变化及与土壤水分的关系,为在油松林皆伐后植被恢复和发育阶段维持生物多样性、保持林分结构稳定、发挥更高生态效益等方面提供科学依据和经营措施建议。【方法】采用时序研究法,在河北平泉地区选择立地一致的不同林龄油松人工林(32年中龄林、40年近熟林、53年成熟林)和皆伐后不同时间(5、10、24 a)的天然更新林作为研究对象,分析不同生长发育阶段油松人工林下植物物种组成与多样性的变化规律及与土壤水分间的相关性。【结果】1)皆伐前后6种林分样地共出现灌木植物21种,草本植物65种,不同样地林下植物组成存在差异。胡枝子在各林分样地均有出现,且重要值均> 20%,在灌木层优势地位明显;草本层中菊科植物种类最多,有18种,其次是蔷薇科和禾本科,分别为6种和4种。2)不同林分样地林下植物多样性指数均表现为草本层>灌木层,皆伐前随着林龄增加林下植物多样性逐渐增加;皆伐后灌木层植物多样性表现为先下降后增加再下降,且变化幅度明显,伐后10 a灌木层Margalef丰富度指数、Simpson多样性指数、Shannon-Wiener多样性指数和Pielou均匀度指数最大,分别为1.40、0.61、1.24和0.74,与53年样地差异不显著。皆伐后不同时间的天然更新林草本层植物多样性均与伐前53年样地的差异不显著,皆伐对灌木层植物的影响较大。3)不同林地土壤水分变化规律较一致,表现为伐前随林龄增加而逐渐增加,伐后5~10 a仍保持较高水平,伐后24 a出现下降,经相关分析植物多样性指数与土壤水分之间有显著的正相关关系,与草本层的相关性达极显著水平。【结论】伐前林下植物多样性变化主要与林分密度降低、光照条件改善有关,伐后主要与油松天然更新及种间竞争有关。依靠油松天然更新恢复的林地保持了较高的生物多样性,但在皆伐10 a后出现下降的趋势,建议在此时对油松更新苗进行人工抚育、降低密度以维持植物多样性。土壤水分是影响植物多样性的关键生态因子,良好的土壤水分条件和林下植被的恢复和发展可以相互促进。     

Nutrient cycles and successional changes following shifting cultivation practice in moist semi-deciduous forests in Nigeria

The nutrient contents of moist semi-deciduous forest, successional regrowth and cultivated soils were studied along with successional changes following clear-cutting and burning. Water-soluble N, P, Ca, Mg and K were measured in the primary forest, successional regrowth, cultivated soils and soils of secondary forests ranging from 1 to 6 years after cessation of cultivation. Clearing and burning of the forest releases stored essential nutrients to soils under cultivation. K and N seem to be lost more rapidly in soils under cultivation than in soils under successional regrowth.It was observed that under shifting cultivation essential nutrients build up temporarily; P and Ca rapidly build up in cultivated soils to values much higher than their original values in the forest soils. Six years after the cessation of cultivation, the soil nutrient level of the successional regrowth was still far from being restored to the level of the primary forest soils.Seedlings and resprouts were equally important in the vegetation recovery of burnt and unburnt clear-cut forest. Seedlings of pioneer woody species established themselves in the burnt and unburnt plots within 4 months. Secondary regrowth in the burnt plot appeared better than in the unburnt plot, although resprouts played a more significant role in the unburnt plot.     

Comparison of 5th- and 14th-year Douglas-fir and understory vegetation responses to selective vegetation removal

The effects of early vegetation management on the survival and growth of Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] were examined 5 and 15 years after planting in the Oregon Coast Range. Our first objective was to document the effects of vegetation species competition upon key ecosystem properties. The second objective was to document the effects of vegetation removal during early Douglas-fir stand establishment upon long-term tree growth and on biomass production by vegetation components. Seven levels of manual vegetation removal were maintained for the first 5 years after planting: 0%, 25%, 50%, 75%, and 100% shrub removal; and 100% shrub removal combined with 50% or 100% herbaceous vegetation removal. Shrub and herb removal did not affect Douglas-fir survival at year five, but treatments providing less than 75% shrub removal significantly reduced Douglas-fir survival by year 15. Removing shrubs and herbs completely (100S + 100H) during the 5 years following tree planting allowed successful tree establishment, with a 366% increase in biomass accumulation per hectare for Douglas-fir in that treatment at the end of 14 years of growth. At 15 years stand age, even with shrub removal alone, a 304% gain in tree biomass per hectare was obtained compared to no vegetation removal (NVR). By stand age 15 years, any increase in the degree of understory removal beyond 75% did not contribute significantly to additional tree survival and growth. The understory vegetation on NVR treatment plots and the herbaceous vegetation on 100% shrub removal (100S) treatment plots, contained >90% and >80% of aboveground biomass N at 5 years, respectively, indicating possible competition for soil N. Soil moisture was not different among treatments at 5 years. Complete vegetation removal (100S + 100H) for 5 years resulted in a significant increase in soil bulk density (P < 0.05), a significant decrease in total soil C (P < 0.05) and no change in total soil N in the upper 15 cm of the mineral soil. By 14 years, however, only the soil bulk density remained greater (P < 0.05) on the 100S + 100H treatment. We conclude that greater tree survival and growth occurred with at least 75% shrub removal. Our results suggest that managers may have substantial flexibility in maintaining a partial understory component suitable for ecosystem productivity, canopy cover and wildlife habitat, while maintaining forests productive for timber resources.     

Ecosystem production in Douglas-fir plantations: Interaction of red alder and site fertility

Pure stands of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) and mixed stands of Douglas-fir and naturally established red alder (Alnus rubra Bong.) were examined on two different sites for soil properties, tree growth and nutrition, and aboveground ecosystem biomass and net primary production. One site (Mt. Benson, Nanaimo, B.C.) was nitrogen (N)-deficient and had a low site index (expected Douglas-fir height of 24 m at 50 years). The other site (Skykomish, western Washington) was N-rich and had a site index of 45 m at 50 years. Soil N accretion on the red alder units was estimated at 65 (Mt. Benson) and 42 (Skykomish) kg ha^?1 year^?1 for 23 years to a soil depth of 50 cm. At the current stage of plantation development, presence of red alder at the infertile Mt. Benson site increased average Douglas-fir diameter but did not affect its basal area and basal area growth rate; including alder stem biomass increased total stand basal area and basal area growth 2.5 fold. Presence of red alder at the fertile Skykomish site decreased average diameter, basal area, and basal area growth of Douglas-fir; including alder biomass left total stand basal area and basal area growth unchanged. Douglas-fir foliar N concentrations on Mt. Benson increased from 0.93 without alder to 1.41% on the red alder unit but were 1.55% for both units at Skykomish. Although alder did not affect Douglas-fir aboveground biomass and net primary production on Mt. Benson, total ecosystem biomass doubled and production tripled when alder biomass was included. Conversely, at Skykomish, Douglas-fir biomass and production decreased, and total ecosystem values were essentially unchanged. Mixing red alder and Douglas-fir seems to have great potential for increasing Douglas-fir growth and ecosystem production on infertile, N-deficient sites but probably has limited value on fertile, N-rich sites.     

Hydraulic redistribution of soil water during summer drought in two contrasting Pacific Northwest coniferous forests

The magnitude of hydraulic redistribution of soil water by roots and its impact on soil water balance were estimated by monitoring time courses of soil water status at multiple depths and root sap flow under drought conditions in a dry ponderosa pine (Pinus ponderosa Dougl. ex Laws) ecosystem and in a moist Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) ecosystem. The fate of deuterated water applied to small plots to create a strong horizontal soil water potential gradient was also monitored to assess the potential for horizontal redistribution of water and utilization of redistributed water by co-occurring shallow-rooted plants. In a 20-year-old Douglas-fir stand, approximately 28% of the water removed daily from the upper 2 m of soil was replaced by nocturnal hydraulic redistribution during late August. In an old-growth ponderosa pine stand, approximately 35% of the total daily water utilization from the upper 2 m of soil appeared to be replaced by hydraulic redistribution during July and August. By late September, hydraulic redistribution in the ponderosa pine stand was no longer apparent, even though total water use from the upper 2 m of soil was nearly identical to that observed earlier. Based on these results, hydraulic redistribution would allow 21 and 16 additional days of stored water to remain in the upper soil horizons in the ponderosa pine and Douglas-fir stands, respectively, after a 60-day drought. At both sites, localized applications of deuterated water induced strong reversal of root sap flow and caused soil water content to cease declining or even temporarily increase at locations too distant from the site of water application to have been influenced by movement of water through the soil without facilitation by roots. Xylem water deuterium values of ponderosa pine seedlings suggested utilization of redistributed water. Therefore, hydraulic redistribution may enhance seedling survival and maintain overstory transpiration during summer drought. These first approximations of the extent of hydraulic redistribution in these ecosystems suggest that it is likely to be an important process in both wet and dry forests of the Pacific Northwest.