Microorganisms in Small Patterned Ground Features and Adjacent Vegetated Soils along Topographic and Climatic Gradients in the High Arctic, Canada

In this study, we determine differences in total biomass of soil microorganisms and community structure (using the most probable number of bacteria (MPN) and the number of fungal genera) in patterned ground features (PGF) and adjacent vegetated soils (AVS) in mesic sites from three High Arctic islands in order to characterize microbial dynamics as affected by cryoturbation, and a broad bioclimatic gradient. We also characterize total biomass of soil microorganisms and the most probable number of bacteria along a topographic gradient within each bioclimatic subzone to evaluate whether differences in topography lead to differences in microbial dynamics at a smaller scale. We found total microbial biomass C, the most probable number of heterotrophic bacteria, and fungal genera vary along this bioclimatic gradient. Microbial biomass C decreased with increasing latitude. Overall, microbial biomass C, MPN and the number of fungal isolates were higher in AVS than in PGFs. The effects which topographic position had on microbial biomass C varied across the bioclimatic gradient as there was no effect of topographic position in Isachsen (subzone A) and Mould Bay (subzone B), when compared to Green Cabin (subzone C, warmer site).There was no effect of topographic position on MPN counts at Mould Bay and Green Cabin. However, in Isachsen, MPN counts were highest in the wet topographic position as compared to the mesic and dry. In conclusion, PGFs seem to decouple the effect climate that might have on the total biomass of soil microorganisms along the bioclimatic gradient;and influence gets ameliorated as latitude increases. Similarly, the effect of topography on the total microbial biomass is significant at the warmest bioclimatic zone of the gradient. Thus, climate and topographic effects on total microbial biomass increase with warmer climate.

Soil Nutrients, Landscape Age, and <i>Sphagno-Eriophoretum vaginati</i>Plant Communities in Arctic Moist-Acidic Tundra Landscapes

Most research exploring the relationship between soil chemistry and vegetation in Alaskan Arctic tundra landscapes has focused on describing differences in soil elemental concentrations (e.g. C, N and P) of areas with contrasting vegetation types or landscape age. In this work we assess the effect of landscape age on physico-chemical parameters in organic and mineral soils from two long-term research sites in northern Alaska, the Toolik Lake and Imnavait grids. These two sites have contrasting landscape age but similar vegetation composition. We also used correlation analysis to evaluate if differences in any of these parameters were linked with between-site variation in the abundance of growth forms. Our analysis was narrowed to soils in Sphagno-Eriophoretum vaginati plant communities. We found no significant differences between these sites for most parameters evaluated, except for total Ca which was significantly higher in organic soils from Imnavait vs. Toolik and total Na which was significantly higher in mineral horizons from Toolik compared to Imnavait. Moreover, the abundance of non-Sphagnum mosses was positively correlated with total Ca in organic soils, whereas the abundance of forbs, non-Sphagnum mosses and bryophytes was negatively correlated with total Na in mineral soils. We suggest that differences in the concentration of these two elements are most likely tied to landscape age differences between these sites. However, since observed dissimilarity in terms of total Ca in organic soils and total Na in mineral soils is concordant with correlation patterns observed between these elements and the aforementioned growth forms, it is likely that existing differences in vegetation composition between these sites are also influencing the concentration of these elements in soils, particularly that of Ca, since non-Sphagnum mosses are dominant above organic soils and are therefore expected to significantly influence biogeochemical processes at this horizon. Thus, we conclude that except for organic Ca and mineral Na, there is little difference between these sites in terms of their soil physico-chemical properties. We suggest that most of the influence of landscape age on evaluated parameters is masked by factors such as moderate cryoturbation and similarities in terms of vegetation properties and climate. These observations are relevant as they suggest a linkage between soil chemistry and vegetation composition in this tundra region.

北方带、温带和热带森林斑块的森林结构和坠落的木质碎片

森林破碎化丰富了森林类型多样性,增加了森林边缘长度,从而影响了累计可燃物的异质性。这种异质性影响我们如何管理可燃物、火灾和森林。理解沿气候梯度,以及在同一气候带里的木质生物量破碎化的相对重要性,将提高我们管理森林可燃物及预测森林火灾的能力。本研究沿北方带、温带和热带的气候梯度,按不同湿度,即干旱林和潮湿林;不同结构,即开放冠层(年轻的)对郁闭冠层(年老的);不同大小,即小型(10～14hm^2)、中型(33～60hm^2)、大型(100～240hm^2),来评估林段中森林可燃物的特点。沿斑块林边缘,我们由外向内测定了一系列横断面样地的半腐层、枯枝落叶层、细碎和粗糙木质残体、死立木和活体生物量。本研究目的是确定沿森林横断面纵深的森林结构和可燃物的特点有何不同,并是否随温度、湿度、结构和斑块大小的变化而有所不同。结果表明,粗木质残体、细木质残体、死立木和活体树木生物量与年平均温度值之间不存在线性关系。温带的生物量最大。北方带森林的地面可燃物(半腐层和枯枝落叶层)与温度和生物量之间呈线性关系。我们对数据进行5维多因素方差分析后发现,温度、湿度和年龄/结构对森林地面可燃物、落下的木质碎片以及活体树木生物量均有重大影响。斑块大小影响了森林地面燃料生物量和活树生物量。距离森林边缘的远近仅在一些样本体中影响明显。但也有一些例外的情况,即森林边缘和内部没有明显的区别。

湿润及干旱的北方带、温带、热带森林斑块中美洲山杨木(Populus tremuloides Michx.)的腐烂分解

本研究对山杨木的腐烂分解进行了实验:1)量化了湿润及干旱的北方带(美国阿拉斯加和明尼苏达)、温带(美国华盛顿和爱达荷)热带(波多黎各)森林类型中美洲山杨木桩(Populustremuloides)的总量残留百分比、分解常数和性能强度;2)因为斑块大小、森林年龄(和/或结构)以及大中等尺度上的气候均与木材的分解相关,测定了斑块化对木材分解速率的影响。斑块大小代表了某一气候地区的景观变化程度。总的说来,小斑块的平均面积为10～14hm^2,中等斑块的为33～60hm^2,大斑块的为100～240hm^2。结果发现:1)热带山杨木桩分解最快,温带最慢;2)在北方带和温带斑块中,干旱林的总量残留百分比显著高于潮湿林的,而在热带斑块中则相反;3)在北方带、温带干旱和热带潮湿林中,森林斑块大小不影响杨木桩的总量残留百分比;4)北方带、温带和热带斑块的边缘和内部,杨木分解没有显著差异。本研究结论如下:1)在广阔的气候梯度下,水汽条件是木材分解的重要控制因素,而且二者之间关系可能是非线性的;2)特殊有机体的存在(白蚁)可能会比单凭气候因子预测的更明显地改变木材分解速率。对木材分解的生物防治可能更是热带地区木材分解的重要预测指标,而非生物限制似乎是寒冷森林斑块木材分解的重要决定因素。

热带森林土壤-火关系研究中的土壤火烧严重度指数

随着火的频率日益增加、人口及对森林的需求增高,需要有一些方法,来对热带森林中火的影响进行评估。火对土壤的短期和长期影响取决于火前、火中和火后的环境。我们把这些成分放在一个火干扰连续体中,来进行我们的文献分析,并提出了一个综合性的土壤火烧严重度指数。这个土壤火烧严重度指数包含一整套指标,反映了过火之后的存在状态。指数有7个等级,1个未火烧等级和6个描述火后土壤状态的等级。我们认为该指数可用于研究火对森林地表的影响,并且认为随着新信息的获得,该指数可能(像我们预测的那样)会有所修改。