Tree encroachment into savannas alters soil microbiological and chemical properties facilitating forest expansion

Abstract Forests have been expanding over typical savanna sites for the past 3000 years in the Neotropics. Such invasion can produce a series of environmental modifications on typical savanna; however, it remains unclear how modifications in soil properties, caused by the encroachment of woody species, facilitate the expansion of forest ecosystems under dystrophic conditions. Here we examined chemical and microbiological changes associ- ated with tree encroachment in oxisols of a Neotropical Savanna at Assis Ecological Station, Southeastern Brazil. We predicted that tree encroachment caused by typical forest species would cause significant changes in the chemical and microbiological properties of savanna soils. Soils were sampled at Assis Ecological Station, from savanna sites differing in tree encroachment （typical, dense and forested savanna） caused by decades of fire exclusion. We analysed vegetation leaf area index and leaf litter volume deposited in the studied plots and chemical （pH,organic matter, P, K, Ca, Mg, A1, NO3-, NH4＋） and microbiological （microbial C biomass and dehydrogenase activity） properties of soils under distinct encroachment conditions. Most soil chemical properties did not change along the tree encroachment gradient; however, total P, soil organic matter, soil microbial C and dehydrogenase activity increased from typical savanna to forested savanna. The changes in soil organic matter and dehydrogenase activity were correlated with the values of leaf area index and litter volume along the encroachment gradient. Our results demonstrate that forest species can increase carbon and phosphorus supplies in tropical savanna soils.

Post-fire regeneration dynamics in whitebark pine(Pinus albicaulis) forests in Wind River and Absaroka Mountains,Wyoming, USA

Whitebark pine （Pinus albicaulis） populations are in decline across the species＇ range due to historic wildfire exclusion, mountain pine beetle （Dendroctonus ponderosae, MPB） outbreaks, and an invasive fungal pathogen causing the disease white pine blister rust （Cronartium ribicola, WPBR）. Despite reliance on stand- replacing fires, information on whitebark pine regeneration occurrence is limited and the trajectory of future forests is largely unknown in some areas of the range. Regeneration densities were assessed in burned and adjacent non-burned areas at six high elevation locations in northwest Wyoming where stand-replacing fires occurred 8-32 years before thesurveys. In these locations at the eastern extent of the species range, we assessed what site factors were associ- ated with regeneration success. Whitebark pine regen- eration density was greater and seedlings were older in non-burned compared to burned areas. Within burns, north aspects had more regeneration than south aspects. Potential seed source densities and other species＇ regeneration were positively related to whitebark p^ne regeneration densities in burned areas. South facing slopes or grass covered areas may have either delayed or no regeneration of whitebark pine without the help of artificial planting.

荒漠草原向灌丛地转变过程中两种优势植物种子萌发及阈值特征

选取宁夏荒漠草原人为灌丛地转变过程中的两种优势种沙芦草(Agropyron mongolicum)和柠条锦鸡儿(Caragana korshinskii)为对象,利用不同浓度PEG条件下的种子萌发特性来探究其在干旱胁迫下植被更新萌发特征,结果表明:柠条锦鸡儿平均初始萌发时间早于沙芦草14 h,且柠条锦鸡儿吸水率低于沙芦草,分别为201.21%、293.43%;柠条锦鸡儿和沙芦草种子萌发率和萌发速率均随渗透势的升高呈先上升后下降趋势,且均在-0.02 MPa时萌发率最高,分别为28.67%、44.67%,在-1.2 MPa时最低,分别为6.67%、1.33%;两者萌发速率均在-0.02 MPa时达到最大,分别为12.84%、9.52%,且柠条锦鸡儿起始萌发天数少于沙芦草1—4 d;通过S型生长曲线和线性模型模拟种子萌发水势和萌发率关系,分析得出柠条锦鸡儿萌发水势阈值范围为-0.3—-0.86 MPa、沙芦草为-0.3—-0.65 MPa,灌木柠条锦鸡儿种子萌发水势阈值范围低于多年生禾草沙芦草。以上结果表明灌丛引入下的荒漠草原在未来干旱条件下,其灌丛柠条锦鸡儿种子在植被更新中较沙芦草更具萌发优势。

论自招侵害与正当防卫

自招侵害能否成立正当防卫,存在权利滥用说、社会相当性说、原因中的违法行为说等不同主张。除了防卫挑拨本身是借防卫之名行侵害之实,因此不具有防卫意图不能成立正当防卫之外,其他一般自招侵害的场合均应承认有成立正当防卫的余地。承认自招防卫不仅契合罪刑法定主义的基本精神,彰显人权保障的刑法品格,而且也没有超出正当防卫基本构成要件射程,根本上有助于实现案件实质公正。