宁夏东部荒漠草原-灌丛地转变过程土壤氮素响应

选取近30年荒漠草原灌丛引入形成的典型草地-灌丛镶嵌体内部的荒漠草地、草地边缘、灌丛边缘、灌丛地为研究样地,对各样地及其微生境(植丛与空斑)相关土壤指标进行测定,以了解荒漠草地向灌丛地转变过程中土壤氮素的响应特征。结果表明: 随草地灌丛化转变,草本与灌丛生物量均增加,其中一年生草本随灌丛引入增加明显;土壤水分、全碳、全氮及微生物生物量均呈降低趋势,微生物数量在草地边缘与灌丛边缘降低,而在灌丛地增加并略高于荒漠草地;从荒漠草地转变至灌丛地,硝态氮显著升高52.3%,最高为28.45 mg N·kg^-1,铵态氮显著降低10.4%,最低为4.81 mg N·kg^-1。微生物生物量氮与土壤水分呈显著正相关,而矿化氮与土壤水分响应关系则随着植被转变呈非线性变化,即硝态氮和铵态氮与土壤水分在荒漠草地和草地边缘呈正相关,而在灌丛边缘和灌丛地呈负相关。荒漠草地向灌丛地近30年的转变过程中,土壤全氮、微生物生物量氮呈降低趋势,而矿化氮显著增加,其中硝态氮尤为明显,表现出从荒漠草地土壤氮素的硝化抑制向灌丛地硝化加速转变。     

宁夏东部荒漠草原灌丛引入对土壤水分动态及亏缺的影响

全球气候变化背景下,荒漠草原人工灌丛引入加速其灌丛化进程,对草原土壤水分产生重要影响。为了解宁夏东部荒漠草原灌丛引入过程中土壤水分动态及亏缺现状,选取了封育草地、放牧草地、不同年限(3a、12a、22a)和间距(40 m、6 m、2 m)灌丛柠条(Caragana korshinskii)地进行土壤水分测定,并利用土壤水分相对亏缺指数(compared soil water deficit index,CSWDI)、样地土壤水分相对亏缺指数(plot compared soil water deficit index,PCSWDI)对土壤水分亏缺进行定量分析。结果表明:灌丛引入过程中不同年限、间距灌丛地0—200 cm土层土壤含水量均显著低于封育草地与放牧地(P0.05);各样地季节动态均表现为春季返潮、夏季消耗、秋季蓄积的季节规律,但不同年限、间距灌丛地表现为春季返潮微弱,土壤含水量仅为7.80%—10.90%,显著低于封育草地和放牧地(11.90%—16.09%);灌丛引入过程中各灌丛地0—100 cm有效储水量(-16.98—18.69 mm)均低于封育草地(34.67 mm),虽在种植22a灌丛地和2 m间距灌丛地略有升高,仍不足20.00 mm。土壤水分相对亏缺量(除封育草地外)为6.69—97.16mm;灌丛引入过程中各样地不同土层CSWDI值呈波动变化,除封育草地各土层无显著的亏缺外,其他样地均存在亏缺,亏缺值为0.03—12.10,PCSWDI值均随着灌丛引入年限和密度的增加呈增大趋势。荒漠草原灌丛引入过程产生土壤水分过度利用,使得土壤水分亏缺,并加剧其深层土壤水分的消耗。     

The resprouting response of co‐occurring temperate woody plant and grass species to elevated [CO2]: An insight into woody plant encroachment of grasslands

The phenomenon of woody plant thickening in grasslands has been observed globally and is likely to have widespread ecological consequences. It has been proposed that woody plant thickening is driven in part by rising atmospheric [CO₂] enhancing the resprouting ability of woody plants relative to grasses so they respond more strongly after disturbances such as herbivory and fire. The aim of this study was to examine the CO₂ effect on the resprouting ability of 16 co‐occurring temperate woody plant and grass species (eight species from each growth form). Plants were grown in a controlled glasshouse experiment under ambient (400 ppm) and elevated [CO₂] (600 ppm) for 14 weeks after which their resprouting ability was measured. Root non‐structural carbohydrate (NSC_mass) and nitrogen (N_mass) storage was used as proxies to measure the resprouting ability of woody plants while for the grasses it was measured directly. We found that both the woody plants (22% on average; P = 0.003) and grasses (20% on average; P = 0.003) produced more biomass under elevated [CO₂]. Despite the woody plants not allocating additional carbon to belowground storage under elevated [CO₂], they had significantly greater root NSC_mass (23% on average; P = 0.007) due to increased root biomass production (8% on average; P = 0.007). In contrast, root N_mass of the woody plants did not differ between CO₂ treatments (P = 0.373). Surprisingly, the resprouting ability of the grasses did not significantly differ between the CO₂ treatments (P = 0.067). These results provide evidence that the differing resprouting response of woody plants and grasses under elevated [CO₂] may be contributing to woody plant encroachment of grasslands worldwide.     

鄂尔多斯高原弃耕农田恢复过程中土壤物理性质和生物结皮的变化

在鄂尔多斯高原软梁、硬梁弃耕农田上选取一系列不同时限的弃耕农田,研究弃耕演替过程中地表凋落物、生物结皮,以及不同土层土壤容重、机械组成、含水量的动态变化.结果表明: 软梁弃耕农田表层(0～10 cm)物理环境随弃耕年限增加明显改善,土壤黏粒含量、地表凋落物随弃耕年限增加明显增加,土壤容重明显降低,土壤含水率略微升高;中层(10～30 cm)土壤物理环境随弃耕年限增加稍显恶化,土壤黏粒含量升高,土壤含水量略微降低;深层(30～50 cm)土壤物理性质变异较大.硬梁弃耕农田土壤表面凋落物盖度、生物结皮盖度、生物结皮厚度随弃耕年限增加而增加,表层(0～10 cm)土壤容重、黏粒含量、含水量随弃耕年限变化总体变化不显著,深层(10～40 cm)土壤物理性质变异较大.地表凋落物和土壤黏粒物质的累积导致软梁弃耕农田土壤水分浅层化,可能是20年内浅根性多年生草本植物群落取代半灌木油蒿群落的关键因素.硬梁弃耕农田在演替过程中未见油蒿入侵,可能与较高的浅层含水量和生物结皮发育有关.     

1982-2015年黄土高原NDVI时空演变及其对气候变化的季节响应

基于GIMMS NDVI、温度和降水数据,利用集合经验模态分解(EEMD)、线性回归分析、偏相关分析等方法分析了1982-2015年黄土高原植被覆盖时空变化及其对气候变化的季节响应。结果表明:年际变化趋势上,1982-2015年黄土高原生长季、春、夏、秋季NDVI均呈显著增长趋势,且各个季节NDVI增加速率逐年升高,尤其以夏季增加速率的变化最为明显;空间上,生长季、春、夏、秋季NDVI均呈由西北向东南递增的趋势,且在大部分地区呈显著上升趋势;线性回归表明,生长季、春、夏、秋季温度均呈显著上升趋势;生长季、秋季降水呈增加趋势,春、夏季降水呈减少趋势。EEMD分析进一步表明,生长季、春、夏、秋季温度均先升高后降低,降水均呈先减少后增加的趋势;空间变化趋势上,温度在生长季、春、夏、秋季大部分地区呈显著上升趋势,降水仅秋季有部分区域呈显著上升趋势;NDVI与温度在黄土高原东北部及西南部地区呈显著正相关,与降水在黄土高原北部及西北部地区呈显著正相关。     

Influence of differently managed bush‐encroached sites on the large herbivore distribution in the Namibian Savannah

Bush encroachment is reported from savannah regions worldwide. Different management strategies are used to rehabilitate these areas. In this context, the mutual interaction between vegetation and large herbivore's distribution is evident. We studied effects of land management on vegetation structure in regard to encroaching species and the subsequent habitat use of two grazing (oryx, Oryx gazella L.; common warthog, Phacochoerus africanusGmelin ) and one browsing (greater kudu, Tragelaphus strepsicerosPallas ) herbivore species. We assumed that (i) cleared areas will be favoured by grazers and (ii) noncleared areas will be favoured by browsers. Specifically, we asked: Which factors determine the habitat use of these different feeding guilds? Consistently with our expectations, we found that warthog favoured sites with high grass cover. For oryx, surprisingly shrubs with a height of 80–150 cm influenced their distribution positively, whereas for kudu, only the interaction of site and grass cover was significant in our models. However, this was related to the occurrence of shrubs of 80–150 cm height. We conclude that the management of encroachers, resulting in differences in vegetation, did not influence herbivore distribution as expected. Other factors like human impact and vegetation cover among others are discussed as additional drivers of habitat use.     

Linkages between woody plant proliferation dynamics and plant physiological traits in southwestern North America

Aims Woody plant encroachments in arid and semiarid ecosystems are widely reported but the physiological mechanisms still need to be further revealed. In the current study, we aim to determine whether differences in leaf physiological traits help explain grassland susceptibility to woody plant encroachment and whether distinctive physiological adaptations allow some shrub species to invade grasslands.Methods We compared physiological traits (photosynthesis, leaf water status, pigment compositions and leaf antioxidant capacities) of six species representing three functional groups: woody encroachers (Prosopis velutina, Larrea tridentata), woody non-encroachers (Acacia greggii, Lycium fremontii) and C4 grasses (Bouteloua curtipendula, Bouteloua barbata) which are naturally growing in a botanical garden in University of Arizona, USA.Important findings We infer that P. velutina (encroacher) but not A. greggii or L. fremontii (non-encroachers) is encroaching in grasslands because the former species has higher water and light utilization efficiencies (instantaneous water use efficiency, instantaneous light use efficiency, and Fv/Fm). The extremely high carotenoid and total antioxidant capacity in its leaves appears to help the shrub L. tridentata (encroacher) survive high ambient oxidative damage caused by both drought and high light stresses in this grassland. The two C₄ grass species, B. curtipendula and B. barbata, grow well in the arid ecosystem but may be susceptible to disturbances.     

草原灌丛化对生态系统多功能性的影响

草原灌丛化现象在干旱半干旱区广泛发生,影响了生态系统的结构、过程和功能。生态系统具有同时提供多种功能的能力,即生态系统多功能性。灌丛化是否会引起草原生态系统多功能性的减少,其内在的作用机制又是什么？这些问题仍有待明晰。理解草原灌丛化对生态系统多功能性的影响,对于促进草原地区"草-畜-人"平衡和实现区域可持续发展至关重要。从响应规律、影响路径和控制因素三个方面总结评述了草原灌丛化对生态系统多功能性影响的研究进展,主要包括:(1)阐明了单一生态系统功能和多种生态系统功能对草原灌丛化的响应特征;(2)从生物路径、非生物路径以及气候变化和人类活动的影响方面探讨了灌丛化对生态系统多功能性的影响路径;(3)从灌丛化物种、灌丛化阶段和草原类型三个方面明晰了草原灌丛化对生态系统多功能性影响的控制因素。在此基础上,针对灌丛化对生态系统多功能性的影响机制,对生产-生态功能权衡的影响等方面对未来研究进行了展望,并面向可持续发展目标探讨了灌丛化生态系统的可持续管理路径。研究可为我国灌丛化草原的恢复和管理提供支撑。     

人工灌丛化对荒漠草原生态系统碳储量的影响

宁夏盐池县从20世纪70年代开始在荒漠草原上人工种植柠条灌木用以防风固沙和生态恢复,这一人为措施极大地改变了区域生态系统的植被结构和碳循环,而定量评估人工灌丛化对荒漠草原生态系统碳储量的影响,不仅能够揭示人类活动的碳循环反馈机制,而且可为地方政府生态治理提供理论指导。结合Biome-BGC模型和Logistics生长模型模拟了1958—2017年间荒漠草原人工灌丛化前后的碳储量变化,定量分析了人工灌丛化对生态系统碳储量和组分的影响。结果表明:(1)结合Biome-BGC模型和Logistics生长模型可以较准确地模拟出荒漠草原人工灌丛化过程中生态系统碳储量的变化。(2)人工灌丛化会快速改变荒漠草原的碳储量累积特征,柠条灌木种植后的快速生长阶段极大增强了生态系统的总碳储量,导致生态系统的碳储量特征由草地型向灌木型转变。(3)人工灌丛化改变了生态系统各类型碳储量的组分结构,其对地上植被和枯落物碳储量的影响非常明显,灌丛化后生态系统的植被和枯落物碳分别增加了6倍和1.76倍;因植被碳向土壤碳转化过程较慢,故人工灌丛化对地下土壤碳储量的影响在短期内较为微弱。以上结果显示,荒漠草原人工灌丛化能显...     

Spectral feature differences between shrub and grass communities and shrub coverage retri- eval in shrub-encroached grassland in Xianghuang Banner,Nei Mongol,China

Aims Shrub-encroached grassland has become an important vegetation type in China’s arid and semi-arid region. Our study objective is to explore the spectral features of shrub and grass communities, as well as their empirical relationships with shrub coverage. The quantitative estimation of shrub cover based on medium-resolution Landsat satellite imagery provides the practical basis for long term retrieval of large areas of shrub expansion in the grassland region. Methods Linear models and Multiple Endmember Spectral Analysis Model (MESMA) based on medium resolution Landsat satellite imagery were developed to quantify the shrub coverage in a shrub-encroached grassland region in Xianghuang Banner, Nei Mongol using the spectral features and their seasonal differences between the shrub and grass communities. Important findings Compared to Leymus chinensis and Stipa krylovii dominated grass communities, Caragana microphylla community had a higher normalized difference vegetation index (NDVI), modified red edge normalized difference vegetation index (mNDVI₇₀₅), and red edge slope. The red edge position of C. microphylla community shifted to longer wavelengths. The average and the maximum shrub coverage was 13% and 25%, respectively, in the shrub-encroached grassland based on both models. The correlation coefficient of determination (R²) and root mean square error (RMSE) of the linear model was 0.31 and 0.05, respectively. We found that the linear model based on seasonal differences of shrub and grass community was more suitable for retrieving shrub coverage in the study area from medium resolution imagery than the MESMA model that is based on mid-summer images.     

Shrub encroachment effect on the evapotranspiration and its component—A numerical simulation study of a shrub encroachment grassland in Nei Mongol,China

Aims Shrub encroachment is a common global change phenomenon occurring in arid and semi-arid regions. Due to the difficulty of partitioning evapotranspiration into shrub plants, grass plants and soil in the field, there are few studies focusing on shrub encroachment effect on the evapotranspiration and its component in China. This study aims to illustrate shrub encroachment effect on evapotranspiration by the numerical modeling method. Methods A two-source model was applied and calibrated with the measured evapotranspiration (ET) by the Bowen ratio system to simulate evapotranspiration and its component in a shrub encroachment grassland in Nei Mongol, China. Based on the calibrated model and previous shrub encroachment investigation, we set three scenarios of shrub encroachment characterized by relative shrub coverage of 5%, 15% and 30%, respectively, and quantified their effects caused by shrub encroachment through localized and calibrated two-source model.Important findings The two-source model can well reconstruct the evapotranspiration characteristics of a shrub encroachment grassland. Sensitivity analysis of the model shows that errors for the input variables and parameters have small influence on the result of partitioning evapotranspiration. The result shows that shrub encroachment has relatively small influence on the total amount of ET, but it has clear influence on the proportion of the components of evapotranspiration (E/ET). With shrub coverage increasing from 5% to 15% and then 30%, the evapotranspiration decreased from 182.97 to 180.38 and 176.72 W·m^-2, decreasing amplitude values of 0.34% and 0.44%, respectively. On average, E/ET rises from 52.9% to 53.9% and 55.5%, increasing amplitude values to 2.04% and 3.25%. Data analysis indicates that shrub encroachment results in smaller soil moisture changes, but clear changes of ecosystem structure (decreasing ecosystem leaf area index while increasing vegetation height) which lead to the decrease of transpiration fraction through decreasing canopy conductance. The research highlights that, with the shrub encroachment, more water will be consumed as soil evaporation which is often regarded as invalid part of evapotranspiration and thus resulting in the decrease of water use efficiency.     

Land use affects rodent communities in Kalahari savannah rangelands

Shrub encroachment caused by overgrazing has led to dramatic changes of savannah landscapes and is considered one of the most threatening forms of rangeland degradation leading to habitat fragmentation. Although changes to plant assemblages are becoming better known, however, our understanding of how shrub encroachment affects rodent communities is low. In this study, we investigated relative abundance of five rodent species in sixteen southern Kalahari rangelands where shrub cover ranged from low (<5%) to high (>25%). Rodent abundance was determined on three trapping grids (40 × 100 m) for each site. Our results show that increasing shrub cover affected rodent species differently. The relative abundance of hairy‐footed gerbil, short‐tailed gerbil and bushveld gerbil declined with increasing shrub cover, whereas highveld gerbil and striped mouse exhibited hump‐shaped relationships with shrub cover. Overall, species richness decreased with increasing shrub cover and a negative impact of high shrub cover above 15% on rodent abundance was congruent for all species. We conclude that our results support the hypothesis that long‐term heavy grazing that results in area wide shrub encroachment, threatens the diversity of arid environments.     

荒漠草原向灌丛转变过程两种优势植物定植土壤水分阈值特征

为探究宁夏东部荒漠草原灌丛转变过程中两种优势种蒙古冰草（Agropyron mongolicum）和柠条锦鸡儿（Caragana korshinskii）植物更新土壤水分需求特征,在前期萌发土壤水势阈值研究的基础上,进一步通过野外观测和室内干旱胁迫试验分析确定转变过程中两种优势植物种的定植土壤水分阈值。结果表明：荒漠草原灌丛近30年人为转变过程中0-200 cm土壤水分呈降低趋势,灌丛地土壤水分含量较荒漠草地显著降低了52.43%（P<0.05）,灌丛转变加速了土壤旱化;柠条锦鸡儿和蒙古冰草幼苗定植过程中叶绿素含量随干旱胁迫程度加深呈先增加后降低趋势,其死亡率逐渐上升。通过渗透势和死亡率的拟合函数以及其对应样地的土壤水势、土壤水分关系分析得出,随样地转变柠条锦鸡儿定植土壤水势、水分阈值均低于蒙古冰草,两者平均土壤水势阈值分别为-9.38—-9.95 kPa、-8.72—-9.28 kPa,平均土壤水分阈值分别为4.93%-5.23%、5.92%-6.50%。与蒙古冰草相比,柠条锦鸡儿更适应灌丛引入下或降雨减少引起的土壤旱化,其旱生条件下的定植成功更易发生。     

荒漠草原人工灌丛化对蒸散发及其组分的影响——以盐池县为例

我国西北防沙治沙工程中大量的种植中间锦鸡儿(Caragana intermedia)会导致荒漠草原发生灌丛化现象,研究人工灌丛化对荒漠草原蒸散发的影响,不仅能够揭示半干旱区人为活动对生态系统水循环的影响机理,还可以指导区域生态治理实践。以宁夏盐池县荒漠草原为例,基于植被的生理生态参数和荒漠草原水热条件,采用生物地球化学模型(Biome Bio-Geochemical Cycles,Biome-BGC)和地球呼吸系统模拟模型(Breathing Earth System Simulator,BESS)结合的方法,模拟荒漠草原生态系统人工灌丛引入前后蒸散发及其组分的变化,定量研究荒漠草原人工灌丛化对区域生态水文循环中蒸散发的影响。结果表明,人工灌丛的引入使植被结构及特征发生了变化,叶面积指数(Leaf Area Index,LAI)年最大值由0.20增加为0.67,改变了植被年内与年际变化特征。荒漠草原人工灌丛化后,生态系统年均蒸散发由251.74 mm增加到了281.42 mm;人工灌丛化对生长季的蒸散发增强明显,8月蒸散发峰值时,日均蒸散发由1.27 mm/d增加到1.56 mm/d。灌丛化过程使生态系统蒸腾量平均增加了1.35倍,蒸发量增加了1.06倍,改变了生态系统蒸散发的组分结构,导致蒸发比例降低、蒸腾比例增高。由此可知,荒漠草原在防沙治沙和生态治理中大量种植灌木的现象,加大了区域生态系统的蒸散发,并改变了水分耗散结构,从而对生态系统地气水汽交换有较大影响,研究结论对荒漠草原生态治理及未来的植被重建有一定的借鉴意义。     

What potential is there for regeneration of native species from the soil seed bank in Coast Tea Tree‐dominated scrub?

Shrub encroachment generally causes the loss of native species in herbaceous‐dominated communities. The ability of the original ecosystem to return to its pre‐encroachment state (i.e. its ecological resilience) will be partially contingent on the capacity of these species to regenerate from soil‐stored seed. Coast Tea Tree (Leptospermum laevigatum) has formed a dense scrub in many areas previously dominated by grassy woodland, and hence, managers need guidance about the effectiveness of strategies designed to recover the pre‐encroachment vegetation. In this context, we ask: what is the potential of species stored in the soil seed bank to return following Tea Tree removal? A germination experiment was undertaken using soil collected from dense stands of Tea Tree that had been long established. Heat/smoke was applied to soils to simulate the effects of a fire on the soil seed bank, while leaf litter treatments were used to mimic both undisturbed stands and stands where shrubs have been slashed where litter creates a physical barrier to emergence. We found the soil seed bank was dominated by exotic forbs (83% of all germinants) and contained few grasses. Heat and smoke decreased total species density but increased species diversity through the suppression of common exotics. Our data suggest that slashing would result in germination being dominated by exotic flora, but using fire would likely reduce that dominance. However, we conclude that recovery by much of the original flora after site occupation by Coast Tea Tree may be contingent on mechanisms other than soil‐stored seeds.     

荒漠草原-灌丛镶嵌体的植被稳态转变特征

以柠条锦鸡儿（Caragana korshinskii）灌丛引入形成荒漠草原-灌丛地镶嵌体为研究对象,选择其内部荒漠草地（DG）、草地边缘（GE）、灌丛边缘（SE）、灌丛地（SL）为试验样地,开展荒漠草原向灌丛地人为稳态转变过程土壤水分与植被变化特征研究。结果如下：各样地0-200 cm土层水分含量随着转变过程呈显著降低趋势（P<0.05）,其中荒漠草地、草地边缘和灌丛边缘样地的土壤水分均在秋季雨水补充期增加,灌丛地由于深层土壤水分过度消耗而愈加降低,且灌丛边缘和灌丛地深层水分显著低于荒漠草地、草地边缘（P<0.05）,垂直动态不显著;地上植被随样地转变除优势度指数表现为灌丛地多样性、均匀度、丰富度指数均显著低于各样地（P<0.05）,多年生优势草本蒙古冰草（Agropyron mongolicum）、短花针茅（Stipa breviflora）逐渐被一年生草本刺藜（Dysphania aristata）、猪毛蒿（Artemisia scoparia）所代替;土壤种子库萌发种类随样地转变逐渐减少,种子库密度则显著升高（P<0.05）,灌丛地以一年生草本植物为主（占总密度的97.51%）,荒漠草地则以多年生草本萌发为主,且各样地土壤萌发种子库及多年生草本多集中于0-5 cm土层（P<0.05）;转变过程荒漠草原和灌丛地植被地上地下相似度分别为0.14和0.19,均显著低于两边缘样地0.35,较两边缘样地群落更为稳定,呈现草原灌丛化过程中草地-灌丛共存的植被双稳态特征。     

灌丛化对黄土高原草地植物群落结构和地上生物量的影响

中国北方草地普遍出现灌丛化现象,灌丛化改变植物群落结构、植物多样性和生产力,直接影响着草地生态保护与可持续利用。该研究以黄土高原灌丛化草地为研究对象,通过植被调查,分析比较不同坡向的灌丛斑块与禾草斑块植物群落结构(物种组成、优势种及物种多样性)和地上生物量的差异。结果发现:(1)灌丛化草地不同坡向对物种多样性及地上生物量均无显著影响(P 0.1),但不同斑块植物群落结构(P=0.001)及地上生物量(P0.001)存在显著差异。(2)灌丛化草地共出现植物29种,其中禾草斑块有27种,灌丛斑块有18种;灌丛化显著改变了植物群落的物种组成,优势种由长芒草(Stipa bungeana)更替为矮脚锦鸡儿(Caragana brachypoda),且灌丛化降低了草地物种丰富度,增加了群落均匀度。(3)灌丛化显著改变了草地地上生物量,其中灌丛斑块地上生物量较禾草斑块地上生物量增加251.2 g·m~(-2),灌丛斑块中灌木/半灌木地上生物量提高了452.1 g·m~(-2),多年生丛生禾草减少了176.5 g·m~(-2),其余功能群植物的地上生物量减少了24.4 g·m~(-2)。(4)灌丛化过程(从禾草斑块—灌丛斑块)中,植物种丢失对地上生物量减少的影响较小,新增物种和群落优势种更替促进了灌木斑块地上生物量增加;虽然灌丛化导致草地地上生物量增加,但植物物种丰富度降低和优势种更替很有可能改变草地多样性和稳定性维持机制。     

Cyanobacterial soil crusts and woody shrub canopies in Kalahari rangelands

Intensive grazing of Kalahari rangelands has led to woody plant encroachment, notably of Acacia mellifera and Grewia flava. The mechanisms causing this process, and the ecological stability of woody plant encroached ecosystems, remain uncertain. Past studies suggest that canopy–soil relations may enhance woody plant competitive dominance. This study aims to investigate one element of this ecological change by examining the spatial distribution of cyanobacterial soil crusts in two vegetation sub‐habitats at sites of different disturbance. Crust burial by litter was also assessed to analyse the dynamics of canopy–crust relations. Our results show there is enhanced cyanobacterial crust cover under A. mellifera canopies and that unlike G. flava canopies, the crust cover remains under A. mellifera even at highly disturbed sites. This canopy–crust association suggests A. mellifera encroachment exhibits intrinsic resilience because of the crusts ability to stabilize the soil surface and increase nutrient retention. Crust burial by litter that accumulates under larger woody plants restricts crust development under canopies. Disturbance restricts crust development in plant interspaces and under G. flava. These two mechanisms combine to restrict crust development to an observed 40% threshold, with nonlinear models required to explain spatial patterns of crust dynamics.     

Detecting shrub encroachment in seminatural grasslands using UAS LiDAR

1. Shrub encroachment in seminatural grasslands threatens local biodiversity unless management is applied to reduce shrub density. Dense vegetation of Cytisus scoparius homogenizes the landscape negatively affecting local plant diversity. Detecting structural change (e.g., biomass) is essential for assessing negative impacts of encroachment. Hence, exploring new monitoring tools to achieve this task is important for effectively capturing change and evaluating management activities.
2. This study combines traditional field‐based measurements with novel Light Detection and Ranging (LiDAR) observations from an Unmanned Aircraft System (UAS). We investigate the accuracy of mapping C. scoparius in three dimensions (3D) and of structural change metrics (i.e., biomass) derived from ultrahigh‐density point cloud data (>1,000 pts/m²). Presence–absence of 12 shrub or tree genera was recorded across a 6.7 ha seminatural grassland area in Denmark. Furthermore, 10 individuals of C. scoparius were harvested for biomass measurements. With a UAS LiDAR system, we collected ultrahigh‐density spatial data across the area in October 2017 (leaf‐on) and April 2018 (leaf‐off). We utilized a 3D point‐based classification to distinguish shrub genera based on their structural appearance (i.e., density, light penetration, and surface roughness).
3. From the identified C. scoparius individuals, we related different volume metrics (mean, max, and range) to measured biomass and quantified spatial variation in biomass change from 2017 to 2018. We obtained overall classification accuracies above 86% from point clouds of both seasons. Maximum volume explained 77.4% of the variation in biomass.
4. The spatial patterns revealed landscape‐scale variation in biomass change between autumn 2017 and spring 2018, with a notable decrease in some areas. Further studies are needed to disentangle the causes of the observed decrease, for example, recent winter grazing and/or frost events.
5. Synthesis and applications: We present a workflow for processing ultrahigh‐density spatial data obtained from a UAS LiDAR system to detect change in C. scoparius. We demonstrate that UAS LiDAR is a promising tool to map and monitor grassland shrub dynamics at the landscape scale with the accuracy needed for effective nature management. It is a new tool for standardized and nonbiased evaluation of management activities initiated to prevent shrub encroachment.

     

Shrub expansion alters forest structure but has little impact on native mammal occurrence

The expansion (or encroachment) of shrubs in forests and woodlands is generally considered a serious threat to biodiversity. The effects of shrub expansion on forest fauna, however, are poorly understood and likely to depend on the availability of key resources in shrub‐encroached forest. Coranderrk Bushland, like many conservation reserves in south‐eastern Australia, is considered threatened by the spread of an indigenous shrub. We investigated the associations between cover of Yarra burgan (Kunzea leptospermoides (Myrtaceae)), vegetation structure and the occurrence of terrestrial native mammals within the reserve, basing our predictions on prior knowledge of burgan growth habits and fauna habitat preferences. We quantified burgan cover and other potentially important habitat attributes using structure surveys, and used motion‐sensing cameras to detect terrestrial mammals. Dense burgan cover was associated with less grass, a sparser understorey, and more cryptogams, dead trees and coarse woody debris. However, there was no evidence that these changes negatively affected native mammals: burgan cover had little influence on the occurrence of any species except swamp wallabies (Wallabia bicolor), which occurred in all areas of the reserve but shifted from sites with high burgan cover during the day to sites with low cover at night. Our findings contrast with those from grassland shrub‐expansion studies, where fauna generally show strong responses to shrub cover. The effects of shrub expansion on forest fauna may be mitigated by the greater pre‐existing structural diversity in forests or the longer time required for structural changes to be fully realized. The large quantities of dead wood in areas with high shrub cover may also provide compensatory resources for small mammals, while the proximity to un‐encroached areas may enable large herbivores to move between dense shelter and forage. Shrub‐encroached forests clearly provide resources for some native fauna, and management strategies need to consider the potential impacts of shrub removal on these taxa.