Development of TRIPLEX-Management model for simulating the response of forest growth to pre-commercial thinning

In order to simulate forest growth response to pre-commercial thinning (PCT), TRIPLEX1.0 - a process-based model designed to predict forest growth as well as carbon (C) and nitrogen (N) dynamics - was modified and improved to also simulate managed forest ecosystem thinning practices. A three-parameter Weibull distribution model was integrated to simulate thinning treatments within the newly developed TRIPLEX-Management model. The thinning intensity component within the model allows users to simulate thinning treatments by applying basal area, stand density and volume to quantify thinning intensity. Natural mortality decreased following thinning due to an increase in growing space for residual stems. Predicted litterfall pools also increased after thinning events took place. The TRIPLEX-Management model was tested against published observational data for Jack Pine (Pinus banksiana Lamb.) stands subjected to PCT in Northwestern Ontario, Canada. The coefficients of determination (R²) between the predicted and observed variables including stand density, mean DBH (diameter at breast height), the quadratic mean DBH, total volume and merchantable volume as well as belowground, aboveground, and total biomass ranged from 0.50 to 0.88 (n = 20, P < 0.001) with the exception of mean tree height (R² = 0.25, n = 20, P < 0.05). Overall, the Willmott index of agreement between predicted and observed variables ranged from 0.97 to 1.00. Results show that the TRIPLEX-Management model is generally capable of simulating growth response to PCT for Jack Pine stands.     

Temperature influences carbon accumulation in moist tropical forests

Evergreen broad-leaved tropical forests can have high rates of productivity and large accumulations of carbon in plant biomass and soils. They can therefore play an important role in the global carbon cycle, influencing atmospheric CO2 concentrations if climate warms. We applied meta-analyses to published data to evaluate the apparent effects of temperature on carbon fluxes and storages in mature, moist tropical evergreen forest ecosystems. Among forests, litter production, tree growth, and belowground carbon allocation all increased significantly with site mean annual temperature (MAT); total net primary productivity (NPP) increased by an estimated 0.2-0.7 Mg C x ha(-1) x yr(-1) x degrees C(-1). Temperature had no discernible effect on the turnover rate of aboveground forest biomass, which averaged 0.014 yr(-1) among sites. Consistent with these findings, forest biomass increased with site MAT at a rate of 5-13 Mg C x ha(-1) x degrees C(-1). Despite greater productivity in warmer forests, soil organic matter accumulations decreased with site MAT, with a slope of -8 Mg C x ha(-1) x degrees C(-1), indicating that decomposition rates of soil organic matter increased with MAT faster than did rates of NPP. Turnover rates of surface litter also increased with temperature among forests. We found no detectable effect of temperature on total carbon storage among moist-tropical evergreen forests, but rather a shift in ecosystem structure, from low-biomass forests with relatively large accumulations of detritus in cooler sites, to large-biomass forests with relatively smaller detrital stocks in warmer locations. These results imply that, in a warmer climate, conservation of forest biomass will be critical to the maintenance of carbon stocks in moist tropical forests.     

Long-term aboveground and belowground consequences of red wood ant exclusion in boreal forest

Despite their ubiquity, the role of ants in driving ecosystem processes both aboveground and belowground has been seldom explored, except within the nest. During 1995 we established 16 ant exclusion plots of approximately 1.1 x 1.1 m, together with paired control plots, in the understory layer of a boreal forest ecosystem in northern Sweden that supports high densities of the mound-forming ant Formica aquilonia, a red wood ant species of the Formica rufa group. Aboveground and belowground measurements were then made on destructively sampled subplots in 2001 and 2008, i.e., 6 and 13 years after set-up. While ant exclusion had no effect on total understory plant biomass, it did greatly increase the relative contribution of herbaceous species, most likely through preventing ants from removing their seeds. This in turn led to higher quality resources entering the belowground subsystem, which in turn stimulated soil microbial biomass and activity and the rates of loss of mass and carbon (C) and nitrogen (N) from litter in litterbags placed in the plots. This was accompanied by losses of approximately 15% of N and C stored in the humus on a per area basis. Ant exclusion also had some effects on foliar stable isotope ratios for both C and N, most probably as a consequence of greater soil fertility. Further, exclusion of ants had multitrophic effects on a microbe-nematode soil food web with three consumer trophic levels and after six years promoted the bacterial-based relative to the fungal-based energy channel in this food web. Our results point to a major role of red wood ants in determining forest floor vegetation and thereby exerting wide-ranging effects on belowground properties and processes. Given that the boreal forest occupies 11% of the Earth's terrestrial surface and stores more C than any other forest biome, our results suggest that this role of ants could potentially be of widespread significance for biogeochemical nutrient cycling, soil nutrient capital, and sequestration of belowground carbon.     

The invasive grass Agropyron cristatum doubles belowground productivity but not soil carbon

Root dynamics are among the largest knowledge gaps in determining how terrestrial carbon (C) cycles will respond to environmental change. Increases in productivity accompanying plant invasions and introductions could increase ecosystem C storage, but belowground changes are unknown, even though roots may account for 50-90% of production in temperate ecosystems. We examined whether the introduction of a widespread invasive grass with relatively high shoot production also increased belowground productivity and soil C storage, using a multiyear rhizotron study in 50-year-old stands dominated either by the invasive C3 grass Agropyron cristatum or by largely C4 native grasses. Relative to native vegetation, stands dominated by the invader had doubled root productivity. Soil carbon isotope values showed that the invader had made detectable contributions to soil C. Soil C content, however, was not significantly different between invader-dominated stands (0.42 mg C/g soil) and native vegetation (0.45 mg C/g soil). The discrepancy between enhanced production and lack of soil C changes was attributable to differences in root traits between invader-dominated stands and native vegetation. Relative to native vegetation, roots beneath the invader had 59% more young white tissue, with 80% higher mortality and 19% lower C:N ratios (all P < 0.05). Such patterns have previously been reported for aboveground tissues of invaders, and we show that they are also found belowground. If these root traits occur in other invasive species, then the global phenomenon of increased productivity following biological invasion may not increase soil C storage.     

Urbanization increases grassland carbon pools: effects of landscaping in Colorado's front range.

During the past few decades, urban and suburban developments have grown at unprecedented rates and extents with unknown consequences for ecosystem function. Carbon pools of soil and vegetation on landscaped properties were examined in the Front Range of Colorado, USA, in order to characterize vegetation and soils found in urban green spaces; analyze their aboveground biomass, vegetative C storage, and soil C storage; and compare these suburban ecosystem properties to their counterparts in native grassland and cultivated fields. Anthropogenic activities leave clear signatures on all three C compartments measured. Management level dominates the response of grass production, biomass, and N tissue concentration. This, in turn, influences the amount of C and N both stored in and harvested from sites. The site age dominates the amount of woody biomass as well as soil C and N. Soil texture only secondarily affects total soil carbon and total bulk density. Established urban green spaces harbor larger C pools, more than double in some cases, than native grasslands or agricultural fields on a per-area basis. Lawn grass produces more biomass and stores more C than local prairie or agricultural fields. Introduced woody vegetation comprises a substantial C pool in urban green spaces and represents a new ecosystem feature. After an initial decrease with site development, soil organic carbon (SOC) pools surpass those in grasslands within two decades. In addition to the marked increase of C pools through time, a shift in storage from belowground to aboveground occurs. Whereas grasslands store approximately 90% of C belowground, urban green spaces store a decreasing proportion of the total C belowground in soils through time, reaching approximately 70% 30-40 years after construction. Despite the substantial increase in C pools in this urban area, it is important to recognize that this shift is distinct from C sequestration since it does not account for a total C budget, including increased anthropogenic C emissions from these sites.     

Response of a Reptile Guild to Forest Harvesting

Abstract:  Despite the growing concern over reptile population declines, the effects of modern industrial silviculture on reptiles have been understudied, particularly for diminutive and often overlooked species such as small-bodied snakes. We created 4 replicated forest-management landscapes to determine the response of small snakes to forest harvesting in the Coastal Plain of the southeastern United States. We divided the replicated landscapes into 4 treatments that represented a range of disturbed habitats: clearcut with coarse woody debris removed; clearcut with coarse woody debris retained; thinned pine stand; and control (unharvested second-growth planted pines). Canopy cover and ground litter were significantly reduced in clearcuts, intermediate in thinned forests, and highest in unharvested controls. Bare soil, maximum air temperatures, and understory vegetation all increased with increasing habitat disturbance. Concomitantly, we observed significantly reduced relative abundance of all 6 study species (scarletsnake [Cemophora coccinea] , ring-neck snake [Diadophis punctatus] , scarlet kingsnake [Lampropeltis triangulum] , red-bellied snake [Storeria occipitomaculata] , southeastern crowned snake [Tantilla coronata] , and smooth earthsnake [Virginia valeriae] ) in clearcuts compared with unharvested or thinned pine stands. In contrast, the greatest relative snake abundance occurred in thinned forest stands. Our results demonstrate that at least one form of forest harvesting is compatible with maintaining snake populations. Our results also highlight the importance of open-canopy structure and ground litter to small snakes in southeastern forests and the negative consequences of forest clearcutting for small snakes.     

Morphology, growth and carbohydrate storage of the plant Typha angustifolia at different water depths

Morphological characteristics, growth and carbohydrate storage dynamics of Typha angustifolia in relation to water depth were investigated. The study was based on observations carried out in two stands, namely Akigase and Teganuma, for two growth seasons. The latter stand was about two times deeper than the former. Teganuma stands exhibited significantly higher number of thick shoots compared to those of Akigase. Conversely, the difference in shoot heights between the two stands was insignificant. The same was true for rhizome lengths. Rhizome diameters were significantly bigger for Teganuma compared to Akigase. Furthermore, both the above and belowground biomass were significantly higher for Teganuma compared to those of Akigase. Teganuma stands allocated 55% of the total biomass to the aboveground parts, while Akigase stands allocated 60%. Similarly, Teganuma exhibited significantly higher stocks of total non-structural carbohydrates (TNC) compared to Akigase. The higher number of thick shoots and the relatively bigger rhizome diameters in Teganuma are seemingly important tolerance strategies in maintaining effective aeration into the rhizosphere, by reducing internal resistance. In addition, the higher TNC stocks and biomass help not only to enhance survival under anoxia for longer durations, they also provide firm substrate anchorage in the deeper Teganuma stands.     

Forest Stand Dynamics and Livestock Grazing in Historical Context

Abstract:  Livestock grazing has been implicated as a cause of the unhealthy condition of ponderosa pine forest stands in the western United States. An evaluation of livestock grazing impacts on natural resources requires an understanding of the context in which grazing occurred. Context should include timing of grazing, duration of grazing, intensity of grazing, and species of grazing animal. Historical context, when and under what circumstances grazing occurred, is also an important consideration. Many of the dense ponderosa pine forests and less-than-desirable forest health conditions of today originated in the early 1900s. Contributing to that condition was a convergence of fire, climate, and grazing factors that were unique to that time. During that time period, substantially fewer low-intensity ground fires (those that thinned dense stands of younger trees) were the result of reduced fine fuels (grazing), a substantial reduction in fires initiated by Native Americans, and effective fire-suppression programs. Especially favorable climate years for tree reproduction occurred during the early 1900s. Exceptionally heavy, unregulated, unmanaged grazing by very large numbers of horses, cattle, and sheep during the late nineteenth and early twentieth centuries occurred in most of the U.S. West and beginning earlier in portions of the Southwest. Today, livestock numbers on public lands are substantially lower than they were during this time and grazing is generally managed. Grazing then and grazing now are not the same.     

Comparing measured and modelled forest carbon stocks in high-boreal forests of harvest and natural-disturbance origin in Labrador, Canada

Understanding the effects of disturbance regimes on carbon (C) stocks and stock changes is a prerequisite to estimating forest C stocks and fluxes. Live-tree, dead-tree, woody debris (WD), stump, buried wood, organic layer, and mineral soil C stock data were collected from high-boreal black spruce (Picea mariana (Mill.) B.S.P.) stands of harvest and fire origin and compared to values predicted by the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3); the core model of Canada's National Forest Carbon Monitoring, Accounting and Reporting System. Data comparing the effect of natural and anthropogenic disturbance history on forest C stocks are limited, but needed to evaluate models such as the CBM-CFS3. Results showed that adjustments to the CBM-CFS3 volume-to-biomass conversion and partitioning parameters were required for the non-merchantable and branch C pools to accurately capture live-tree C stocks in the studied black spruce ecosystems. Accuracy of the CBM-CFS3 modelled estimates of dead organic matter and soil C pools was improved relative to regional default parameters by increased snag fall and >10 cm WD base decay rates. The model evaluation process also highlighted the importance of developing a bryophyte module to account for bryophyte C dynamics and the physical burial of woody debris by bryophytes. Modelled mineral soil C estimates were improved by applying a preliminary belowground slow C pool base decay rate optimized for the soil type of the studied sites, Humo-Ferric Podzols.     

Ecology of Pacific Yew (Taxus brevifolia) in Western Oregon and Washington

Taxus brevifolia , a subcanopy tree or shrub in forests of the Pacific Northwest, has been harvested intensively in recent years. With management concerns as an impetus, we examined the distribution and population dynamics of Taxus based on data from the mountains of western Oregon and Washington. Surveys of natural forests, long-term studies of forest recovery following logging, and census data on marked trees in forest stands support the hypothesis that Taxus is a widespread but predominantly late-successional species. Sensitive to fire and slow to recover from disturbance on many sites, Taxus attains maximal basal area and adult stem density in old forests. Colonization of Taxus is often slow in potentially suitable habitats. Conservation of Taxus at the landscape level may require large, unmanaged reserves and maintenance of patches of old forest within managed forests. Long rotations (centuries) between harvest events will enhance the long-term viability of the species. Practices designed to accelerate the development of old-growth forest structure will not benefit Taxus and other species requiring long disturbance-free intervals for recovery.     

不同林龄马尾松（Pinus massoniana）人工林碳氮磷分配格局及化学计量特征

为了解长江上游低山丘陵区马尾松（Pinus massoniana）人工林生态系统的C、N、P分配格局及化学计量特征,本文采用时空互代的方法,在宜宾高县来复林区选取三种不同林龄（5年生幼龄林、14年生中龄林、39年生成熟林）,但立地条件相近、样地情况基本一致的马尾松（Pinus massoniana）人工林作为研究对象,对马尾松针叶、凋落物及土壤中的C、N、P含量及 w（C）？w（N）？w（P）化学计量特征进行测定和分析。结果表明,（1）C、N、P 含量均表现为针叶〉凋落物〉土壤,且在三个库之间差异显著;（2）林龄对针叶、凋落物、土壤的 C、N、P 及 w（C）？w（N）、w（C）？w（P）计量比均有显著影响。（3）土壤 C、N、P含量在成熟林中最高;针叶和凋落物的C含量在成熟林中最低,N、P含量则在中龄林中最高。（4）随林龄增加马尾松对N、P的利用效率降低,针叶、凋落物及土壤的w（C）？w（N）与 w（C）？w（P）均表现为下降。（5）马尾松针叶w（N）？w（P）比值在14.37~15.53之间,说明该地区马尾松人工林受N和P的共同限制,但林龄对N、P养分限制的影响不显著。为提高该区马尾松人工林的生产力,建议在人工林的抚育管理中要适当增加N肥和P肥,同时也可在马尾松人工林引入豆科固氮植物以提高地力。该研究将马尾松针叶、凋落物及土壤结合起来探究随林龄增长C、N、P养分元素的分配格局及化学计量特征的变化,有助于全面、系统地揭示马尾松人工林生态系统的养分循环,对指导马尾松人工林生产,调节和改善林木生长环境,提高系统的养分利用效率及林地生产力具有重要意义。     

东北过伐林区不同林分类型土壤肥力质量评价研究

保持和提高森林土壤肥力质量是实现森林健康的基础。研究不同林型下土壤肥力质量对森林可持续经营具有重要意义。以东北过伐林区金仓林场中的落叶松Lartx gmelinii人工纯林、云杉Picea asperatax紫椴Tilia amurensis天然混交林、紫椴×白桦Betula platyphylla天然混交林、五角槭Acer monox白桦×落叶松天然混交林、五角槭×白桦×紫椴天然混交林、白桦×落叶松天然混交林和五角械×紫椴×青杨Populus pseudo-simonii天然混交林为研究对象,分析和比较了不同林分下的土壤物理和化学性质,并采用主成分分析与聚类分析相结合的方法评价了其土壤肥力质量。结果表明：①林分类型对土壤理化性质影响显著;随土壤深度增加,土壤密度和土壤pH值增大,而土壤含水量、物理性粘粒含量和养分质量分数减少,但其在不同林分下的变化程度不同;②土壤因子之间关系密切,物理性粘粒含量与含水量、全氮、速效钾质量分数呈极显著正相关,而与土壤密度和pH值呈显著负相关;土壤养分质量分数之间具有不同程度的显著正相关关系;③采用主成分分析法对不同林分下0～60cm的土壤肥力质量进行了评价,土壤肥力质量表现为天然混交林高于落叶松人工纯林;采伐降低了土壤的肥力质量;在落叶松人工纯林中,随着林龄的增加,土壤的肥力质量下降。通过对土壤肥力质量综合指标值的聚类分析,将研究区土壤肥力质量分为优、良、中、差4个等级,肥力质量属于中等以上（优、良、中等）的林地所占研究样地的比例为66．7％,研究区森林土壤肥力质量属于中等水平。建议在经营过伐林时,注意调整林分结构和树种组成,控制采伐强度,同时进行长期定位观测和比较,以改善林分整体的肥力状况。本研究结果为该地区林业可持续经营提供直接依据,也为东北地区森林土壤肥力质量评价提供参考。     

Soil responses to management, increased precipitation, and added nitrogen in ponderosa pine forests.

Forest management, climatic change, and atmospheric N deposition can affect soil biogeochemistry, but their combined effects are not well understood. We examined the effects of water and N amendments and forest thinning and burning on soil N pools and fluxes in ponderosa pine forests near Flagstaff, Arizona (USA). Using a 15N-depleted fertilizer, we also documented the distribution of added N into soil N pools. Because thinning and burning can increase soil water content and N availability, we hypothesized that these changes would alleviate water and N limitation of soil processes, causing smaller responses to added N and water in the restored stand. We found little support for this hypothesis. Responses of fine root biomass, potential net N mineralization, and the soil microbial N to water and N amendments were mostly unaffected by stand management. Most of the soil processes we examined were limited by N and water, and the increased N and soil water availability caused by forest restoration was insufficient to alleviate these limitations. For example, N addition caused a larger increase in potential net nitrification in the restored stand, and at a given level of soil N availability, N addition had a larger effect on soil microbial N in the restored stand. Possibly, forest restoration increased the availability of some other limiting resource, amplifying responses to added N and water. Tracer N recoveries in roots and in the forest floor were lower in the restored stand. Natural abundance delta15N of labile soil N pools were higher in the restored stand, consistent with a more open N cycle. We conclude that thinning and burning open up the N cycle, at least in the short-term, and that these changes are amplified by enhanced precipitation and N additions. Our results suggest that thinning and burning in ponderosa pine forests will not increase their resistance to changes in soil N dynamics resulting from increased atmospheric N deposition or increased precipitation due to climatic change. Restoration plans should consider the potential impact on long-term forest productivity of greater N losses from a more open N cycle, especially during the period immediately after thinning and burning.     

抚育间伐对栓皮栎种群空间分布格局的影响

抚育间伐是一种重要的改善林木生长条件的经营措施,对林分结构和动态具有重要影响。为研究抚育间伐对林木种群空间结构与格局影响的内在机制,以间伐和未间伐的栓皮栎人工林为研究对象,通过设置2个100 m×100 m样地并进行每木定位和调查,在采用径级结构代替年龄结构方法将栓皮栎种群划分为幼树(2 cm≤DBH<5 cm)、小树(5 cm≤DBH<13 cm)和大树(DBH≥13 cm)3个生长阶段的基础上,分别采用Ripley’s K函数衍生的g(r)函数和双变量g12(r)函数,对栓皮栎种群空间分布点格局及不同生长阶段栓皮栎种群之间的关联性进行了研究。结果表明,间伐和未间伐样地栓皮栎种群空间分布点格局分别在0-16 m和0-33 m距离尺度内呈聚集分布,而分别在大于16 m和33 m距离尺度内呈随机分布;间伐和未间伐样地栓皮栎幼树、小树和大树的株数比分别为8?741?699和261?1134?683,且间伐样地栓皮栎幼树、小树和大树种群分别在0-14、1-16、0-6 m距离尺度内呈现均匀或聚集分布,而在其他距离尺度上表现为随机分布;栓皮栎幼树、小树和大树之间仅在间伐样地0-6 m距离尺度内呈现一定的相关性,而在未间伐样地更大的距离尺度内有显著的关联性,如栓皮栎幼树和大树之间在6-38 m距离尺度上呈显著正相关。因此,抚育间伐一定程度上使得栓皮栎种群在更大距离尺度上呈现出随机分布状态,并弱化了不同生长阶段的林木栓皮栎种群的关联性,这调整了栓皮栎种群空间竞争关系,有利于大径级林木个体的培育。该研究可以为开展抚育间伐对林木种群的影响的研究提供理论依据,也可以为制定科学合理的抚育技术措施提供参考。     

Frequent fire alters nitrogen transformations in ponderosa pine stands of the inland northwest

Recurrent, low-severity fire in ponderosa pine (Pinus ponderosa)/interior Douglas-fir (Pseudotsuga menziesii var. glauca) forests is thought to have directly influenced nitrogen (N) cycling and availability. However, no studies to date have investigated the influence of natural fire intervals on soil processes in undisturbed forests, thereby limiting our ability to understand ecological processes and successional dynamics in this important ecosystem of the Rocky Mountain West. Here, we tested the standing hypothesis that recurrent fire in ponderosa pine/Douglas-fir forests of the Inland Northwest decreases total soil N, but increases N turnover and nutrient availability. We compared soils in stands unburned over the past 69-130 years vs. stands exposed to two or more fires over the last 130 years at seven distinct locations in two wilderness areas. Mineral soil samples were collected from each of the seven sites in June and July of 2003 and analyzed for pH, total C and N, potentially mineralizable N (PMN), and extractable NH4+, NO3-, PO4(-3), Ca+2, Mg+2, and K+. Nitrogen transformations were assessed at five sites by installing ionic resin capsules in the mineral soil in August of 2003 and by conducting laboratory assays of nitrification potential and net nitrification in aerobic incubations. Total N and PMN decreased in stands subjected to multiple fires. This loss of total N and labile N was not reflected in concentrations of extractable NH4+ and NO3-. Rather, multiple fires caused an increase in NO3 sorbed on ionic resins, nitrification potential, and net nitrification in spite of the burned stands not having been exposed to fire for at least 12-17 years. Charcoal collected from a recent fire site and added to unburned soils increased nitrification potential, suggesting that the decrease of charcoal in the absence of fire may play an important role in N transformations in fire-dependent ecosystems in the long term. Interestingly, we found no consistent effect of fire frequency on extractable P or alkaline metal concentrations. Our results corroborate the largely untested hypothesis that frequent fire in ponderosa pine forests increases inorganic N availability in the long term and emphasize the need to study natural, unmanaged sites in far greater detail.     

间伐对杉木人工林林下植被多样性及其营养元素现存量影响

间伐改变了林分结构,影响林下植被的生长和发育,但长期间伐对杉木Cunninghamia lanceolata林下植被多样性的影响尚缺乏系统研究。在浙江开化通过样地调查,采用Simpson指数、Shannon-Wiener指数和Pielou均匀度指数研究3种间伐处理下杉木人工林林下植被多样性的变化;分析林下植被主要营养元素含量,研究间伐对营养元素现存量的影响。以试验区传统经营方式为对照,即林木生长中期（第14年）进行1次轻度间伐（间伐强度约15%,T1）;中度（总间伐强度约35%, T2）和强度（总间伐强度约50%,T3）间伐均在第7年和第14年进行了2次间伐。间伐15年后的结果表明,3种间伐处理下,林下植物种类数分别为18种（T1）、17种（T2）和20种（T3）,林下植被总密度和盖度均随间伐强度增加而增大;林下植物Simpson指数、Shannon-Wiener指数和Pielou均匀度指数在不同间伐处理之间均没有显著差异（P＞0.05）。林下植被全碳（TC）、全磷（TP）和全钾（TK）含量在不同间伐处理之间差异不显著（P＞0.05）,全氮（TN）含量随间伐强度增加而降低,其中T3处理显著低于T1（P＜0.05）。林下植被生物量随间伐强度增大而增加,林下植被TC、TN、TP和TK现存量在不同间伐处理之间均没有显著差异（P＞0.05）。这些试验结果说明杉木人工林间伐15年后间伐强度不会显著影响林下植被多样性和营养元素的积累。     

Inconsistent definitions of "urban" result in different conclusions about the size of urban carbon and nitrogen stocks

There is conflicting evidence about the importance of urban soils and vegetation in regional C budgets that is caused, in part, by inconsistent definitions of "urban" land use. We quantified urban ecosystem contributions to C stocks in the Boston (Massachusetts, USA) Metropolitan Statistical Area (MSA) using several alternative urban definitions. Development altered aboveground and belowground C and N stocks, and the sign and magnitude of these changes varied by land use and development intensity. Aboveground biomass (live trees, dbh > or = 5 cm) for the MSA was 7.2 +/- 0.4 kg C/m2 (mean +/- SE), reflecting a high proportion of forest cover. Vegetation C was highest in forest (11.6 +/- 0.5 kg C/m2), followed by residential (4.6 +/- 0.5 kg C/m2), and then other developed (2.0 +/- 0.4 kg C/m2) land uses. Soil C (0-10 cm depth) followed the same pattern of decreasing C concentration from forest, to residential, to other developed land uses (4.1 +/- 0.1, 4.0 +/- 0.2, and 3.3 +/- 0.2 kg C/m2, respectively). Within a land use type, urban areas (which we defined as > 25% impervious surface area [ISA] within a 1-km(2) moving window) generally contained less vegetation C, but slightly more soil C, than nonurban areas. Soil N concentrations were higher in urban areas than nonurban areas of the same land use type, except for residential areas, which had similarly high soil N concentrations. When we compared our definition of urban to other commonly used urban extents (U.S. Census Bureau, Global Rural-Urban Mapping Project [GRUMP], and the MSA itself), we found that urban soil (1 m depth) and vegetation C stocks spanned a wide range, from 14.4 +/- 0.8 to 54.5 +/- 3.4 Tg C and from 4.2 +/- 0.4 to 27.3 +/- 3.2 Tg C, respectively. Conclusions about the importance of urban soils and vegetation to regional C and N stocks are very sensitive to the definition of urban used by the investigators. Urban areas, regardless of definition, are rapidly expanding in their extent; a systematic understanding of how our development patterns influence ecosystems is necessary to inform future development choices.     

The importance of age-related decline in forest NPP for modeling regional carbon balances.

We show the implications of the commonly observed age-related decline in aboveground productivity of forests, and hence forest age structure, on the carbon dynamics of European forests in response to historical changes in environmental conditions. Size-dependent carbon allocation in trees to counteract increasing hydraulic resistance with tree height has been hypothesized to be responsible for this decline. Incorporated into a global terrestrial biosphere model (the Lund-Potsdam-Jena model, LPJ), this hypothesis improves the simulated increase in biomass with stand age. Application of the advanced model, including a generic representation of forest management in even-aged stands, for 77 European provinces shows that model-based estimates of biomass development with age compare favorably with inventory-based estimates for different tree species. Model estimates of biomass densities on province and country levels, and trends in growth increment along an annual mean temperature gradient are in broad agreement with inventory data. However, the level of agreement between modeled and inventory-based estimates varies markedly between countries and provinces. The model is able to reproduce the present-day age structure of forests and the ratio of biomass removals to increment on a European scale based on observed changes in climate, atmospheric CO2 concentration, forest area, and wood demand between 1948 and 2000. Vegetation in European forests is modeled to sequester carbon at a rate of 100 Tg C/yr, which corresponds well to forest inventory-based estimates.     

Ecological consequences of carbon substrate identity and diversity in a laboratory study

Plants return a wide range of carbon (C) substrates to the soil system. The decomposition rate of these substrates is determined by their chemical nature, yet few studies have examined the relative ecological role of specific substrates (i.e., substrate identity) or mixtures of substrates. Carbon substrate identity and diversity may alter soil chemistry and soil community composition, resulting in changes in belowground ecosystem functions such as decomposition and nutrient transfer, creating feedbacks that may affect plant growth and the aboveground community. A laboratory experiment was set up in which eight C substrates of varying chemical complexity were added to a base soil singly, in pairs, fours, or with all eight together every four days over a 92-day period. After 92 days these soils were analyzed for changes in chemistry, microbial community structure, and components of ecosystem functioning. The identity of the added C substrates significantly affected soil chemistry, microbial basal and substrate-induced respiration, and soil microbial community structure measured by either the catabolic response profile (CRP) technique or phospholipid fatty acid composition. These belowground changes strongly affected the ability of the soil microflora to decompose cellulose paper, probably because of differential effects of the C substrates on soil energy supplies and enzyme activities. The addition of C substrates to soils also reduced plant growth compared to the unamended control soil, but less so in soils amended with a tannin than those amended with other substrates. Carbon substrate diversity effects saturated at low diversity levels, tended to have neutral or negative effects on ecosystem functions, and depended strongly on which C substrates were added. It increased CRP compound use but had little effect on other measures of the soil microbial community. Overall, results showed that the chemical nature of C substrates added to soil, and sometimes their diversity, can affect the soil microbial community and soil chemistry, which subsequently affect other ecosystem processes such as decomposition and plant growth. The identity and diversity of substrates that plants add to soil may therefore have important consequences for both above- and belowground ecosystem functions.