Forest encroachment into a Californian grassland: examining the simultaneous effects of facilitation and competition on tree seedling recruitment

Competition and facilitation are both considered major factors affecting the structure of plant assemblages, yet few studies have quantified positive, negative, and net effects simultaneously. In this study, we investigated the positive, negative, and net effects of tree saplings on the encroachment of two tree species, Douglas fir (Pseudotsuga menziesii) and tanoak (Lithocarpus densiflora), into a coastal California grassland. The study involved three components: sampling the spatial distributions of P. menziesii and L. densiflora in the grasslands, a field experiment examining seedling survival in different grassland environments, and a greenhouse experiment examining the effects of soil moisture on early seedling performance. The field experiment was conducted over a 2-year period, using Pseudotsuga in 2002 and both species in 2003. Seedlings were separated into four treatment groups: those planted in open grassland, in shaded grassland, under artificial (plastic) conifer saplings, and under natural Pseudotsuga saplings. Air temperature, relative humidity, soil moisture, incident radiation levels and fog water inputs were measured for each treatment group in 2003. In the greenhouse experiment, Pseudotsuga and Lithocarpus seedlings were grown for 13 weeks in watering treatments simulating the summer soil moisture conditions of the open grasslands and under Pseudotsuga saplings. Surveys of naturally established seedlings found that Lithocarpus occurred only under Pseudotsuga saplings, while most Pseudotsuga seedlings were located near but not directly under conspecific saplings. In the field experiment, positive effects of tree saplings were much larger than negative effects, resulting in strong net facilitation of seedling establishment. Survival for both species was always higher under the plastic and live trees than in the open or shade plots. The primary mechanism facilitating seedling survival appeared to be increased soil moisture caused by input of fog precipitation coupled with reduced microsite evaporation. The greenhouse experiment further showed that soil moisture strongly affected seedling performance, with both species having much higher photosynthetic rates in the higher moisture treatment. In the lower moisture treatment, Pseudotsuga seedlings had higher photosynthetic rates and stomatal conductance than Lithocarpus, suggesting they may be able to better tolerate the environmental conditions found in the open grasslands. Our combined results suggest that rate and patterning of woody plant encroachment can be strongly influenced by facilitation and that fog precipitation may play a key role in plant interactions.     

Fog in the California redwood forest: ecosystem inputs and use by plants

Fog has been viewed as an important source of moisture in many coastal ecosystems, yet its importance for the plants which inhabit these ecosystems is virtually unknown. Here, I report the results of a 3-year investigation of fog inputs and the use of fog water by plants inhabiting the heavily fog inundated coastal redwood (Sequoia sempervirens) forests of northern California. During the study period, 34%, on average, of the annual hydrologic input was from fog drip off the redwood trees themselves (interception input). When trees were absent, the average annual input from fog was only 17%, demonstrating that the trees significantly influence the magnitude of fog water input to the ecosystem. Stable hydrogen and oxygen isotope analyses of water from fog, rain, soil water, and xylem water extracted from the dominant plant species were used to characterize the water sources used by the plants. An isotopic mixing model was employed to then quantify how much fog water each plant used each month during the 3-year study. In summer, when fog was most frequent, ～19% of the water within S.sempervirens, and ～66% of the water within the understory plants came from fog after it had dripped from tree foliage into the soil; for S.sempervirens, this fog water input comprised 13–45% of its annual transpiration. For all plants, there was a significant reliance on fog as a water source, especially in summer when rainfall was absent. Dependence on fog as a moisture source was highest in the year when rainfall was lowest but fog inputs normal. Interestingly, during the mild El Niño year of 1993, when the ratio of rainfall to fog water input was significantly higher and fog inputs were lower, both the proportion and coefficient of variation in how much fog water was used by plants increased. An explanation for this is that while fog inputs were lower than normal in this El Niño year, they came at a time when plant demand for water was highest (summer). Therefore, proportional use of fog water by plants increased. The results presented suggest that fog, as a meteorological factor, plays an important role in the water relations of the plants and in the hydrology of the forest. These results demonstrate the importance of understanding the impacts of climatic factors and their oscillations on the biota. The results have important implications for ecologists, hydrologists, and forest managers interested in fog-inundated ecosystems and the plants which inhabit them.     

夜间低温对不同光强下生长的两种沟谷雨林树苗荧光参数的影响

 于雾凉季测定了叶片叶绿素荧光参数,探讨了4～6 ℃夜间低温对4种相对光强下生长的两种西双版纳沟谷雨林树苗光系统Ⅱ（PSⅡ）活性的影响及雾对植物的可能保护机制。随夜间低温处理时间延长,不同光强下生长的团花树（Anthocephalus chinensis）和玉蕊（Barringtonia macrostachya）叶片日间和长期光抑制,以及PSⅡ反应中心的可逆失活或破坏加剧,生长环境光越强夜间低温的效应越明显,弱光下其效应不显著。间接表明雾使光强减弱利于缓解自然夜温降低对本区热带植物的影响。中光强下玉蕊对照植株发生了胁迫诱导的光抑制;相同处理条件下玉蕊的光抑制程度均比团花树重,表明玉蕊对夜间低温引起的光抑制更敏感。夜间低温处理后,中等和低光强下团花树的热耗散多于玉蕊,表明其光保护作用较强。夜间低温处理期间两种植物的光抑制与热耗散增多和PSⅡ反应中心的可逆失活或破坏的加剧有关。     

Rosette scrub occurrence and fog availability in arid mountains of Mexico

Abstract. Large succulent leaf rosettes are a characteristic life form in many deserts. In certain areas they become the dominant life form, creating a vegetation type indicated as rosette scrub. The large number of rosette species suggests a close relationship between form and environment. Rosettes are excellent harvesters of low‐intensity rains and fogs. We propose that some rosette‐dominated formations of the Mexican mountains, namely the montane rosette scrub, occur in altitudinal belts around mountains where fog is abundant. We sampled four altitudinal gradients in mountains with different flora recording the abundance and richness of plants. At one site, the Tehuacán Valley, we also measured the quantity of fog along the gradient, below, above and in the rosette scrub for one year. We found that the abundance and richness of succulent rosette species are strongly associated with altitude, showing maximum values in the well‐defined elevational belts where the montane rosette scrub occurs. Other life forms, such as stem succulent cacti or woody shrubs, do not show this mid‐elevation pattern. The altitudinal ranges where the montane rosette scrub occurs usually coincide with areas where clouds and fog form. Our micrometeorological measurements indicate that rosette plants growing within a cloud belt can increase their water supply by 10–100% by harvesting fog. Outside these belts fog harvest is negligible. Desert rosettes constitute one of the most common fog‐harvesting growth forms.     

The dendroecological potential of shrubs in north Iranian semi-deserts

In deserts and semi-deserts such as in the Irano-Turanian region in northern Iran, forest vegetation is scarce but shrubs are dominant. For this floristic province, placed in a biodiversity hotspot with a cold and dry climate, we provide the first climate-growth study on shrubs. From stems of three wide-spread shrubs (Astragalus, Rhamnus and Ephedra species) annual rings were identified and their widths measured.On average, around 40-year long annual-ring series per stem were obtained, cross-dated and related to meteorological variables. Astragalus and Rhamnus reflected a clear regional climate signal in their ring widths whereas Ephedra showed an only weak association with climate variables. While above-average air humidity in combination with low temperatures in spring and summer favored shrub growth, precipitation had surprisingly only a weak effect on growth. From the abundance of fog events in this area, we concluded that the extreme moisture dependency of the shrubs before and throughout the growing season may have been relieved by the uptake of fog drip through the foliage.As projected by climate models, the deficit in humidity will intensify and temperature will continue to rise in this region. So, the ability of the Irano-Turanian endemic shrubs to infiltrate into neighboring regions could become limited and their current distribution range may be confined to higher elevations which provide a moister and cooler environment.     

西双版纳热带季节雨林内雾特征研究

 利用4年（1999～2002）的雾观测资料,对西双版纳热带季节雨林内雾特征进行了观测研究。结果表明,雾首先形成于最上林冠层,林下雾是由上层雾变浓、下沉而来。夜间,雾形成前,气温高于叶表温;雾形成后,气温则低于叶表温。热带雨林内各季节雾日数和雾日频率均高于无林地。热带雨林内平均全年雾日数可达258 d,其中雾季和干热季共占154 d（59.6%）,而雾季的雾日频率高达90%。雾日数的季节变化与各季节雨量呈明显的负相关。雾季,雾在23∶00左右生成,比干热季、雨季分别提前0.7、2.3 h,而消散时间则分别推迟0.8、2.2 h。雾生成和消散时间呈现出较明显的负相关。雾季雾的持续时间达12.2 h·d-1,比干热季、雨季分别长1.5、4.6 h·d-1。全年雾总持续时间占全年时间的39.7%,而雾季的相应值为50.8%。雾的形成不仅凝结了水汽进入森林（全年89.4 mm）,同时也对森林起到了一定的保温作用,这对热带雨林的生存和发展具有至关重要的作用。     

西双版纳地区附生与非附生植物叶片对雾水的吸收

采用蒸馏水喷雾(模拟雾)法,测定了西双版纳地区干季中10种附生植物和非附生植物叶片水势(Φ)、相对含水量(RWC)和吸水量的变化,探讨了不同类型植物叶片的吸收雾水的能力.结果表明,随喷雾时间的延长,植物叶片Φ、RWC和吸水量均升高,说明附生植物和非附生植物叶片都能吸收雾水,但附生植物叶片吸水后Φ升高明显快于非附生植物.附生植物附着实蕨和爬树龙叶片吸水快、RWC变化大,表明其叶片吸收雾水的能力强;贝母兰和掌唇兰叶片吸水能力低于非附生植物中的穿鞘花和野靛稞,但高于其它4种非附生植物.傍晚雾生之前附生植物叶片Φ显著低于清晨,表明夜间附生植物叶片吸收了雾水;而非附生植物傍晚叶片Φ与清晨水势差异不显著,夜间几乎不吸收雾水.除贝母兰外,附生植物叶生物量分数高于非附生植物,利于其吸收雾水.由于西双版纳地区干季多雾,该区植物叶片最低水势均在-0.8 MPa以上,水分胁迫不严重.     

A security mechanism based on evolutionary game in fog computing

Fog computing is a distributed computing paradigm at the edge of the network and requires cooperation of users and sharing of resources. When users in fog computing open their resources, their devices are easily intercepted and attacked because they are accessed through wireless network and present an extensive geographical distribution. In this study, a credible third party was introduced to supervise the behavior of users and protect the security of user cooperation. A fog computing security mechanism based on human nervous system is proposed, and the strategy for a stable system evolution is calculated. The MATLAB simulation results show that the proposed mechanism can reduce the number of attack behaviors effectively and stimulate users to cooperate in application tasks positively.     

The influence of summertime fog and overcast clouds on the growth of a coastal Californian pine: a tree-ring study

The coast of California is home to numerous rare, endemic conifers and other plants that are limited in distribution by drought sensitivity and the summer-dry climate that prevails across most of the state. Ecologists have long assumed that some coastal plant populations survived the early Pleistocene transition to a warmer and drier environment because they benefit from frequent fog and stratus clouds that provide water and shade during the rainless summer. One such population is that of Torrey pine (Pinus torreyana ssp. Insularis) on Santa Rosa Island in Channel Islands National Park. Here we report that the tree-ring width record from this population indicates strong growth sensitivities to summer fog drip and cloud shading. We quantified the effects of summer cloud cover by comparing ring-width indices to coastal airport cloud-frequency records (1944–2004). For the first time observed, summertime cloud frequency correlated positively with ring-width indices, regardless of whether the effect of rainfall was first removed from the ring-width record. The effect of ground-level fog was strongest in July early mornings (03:00 PST, R ² = 0.262, P < 0.0002). The effect of clouds high enough to provide shade but not fog water was also strongest in July, but climbed steadily throughout the day before becoming strongest in late afternoon (16:00–18:00 PST, R ² = 0.148, P < 0.004). Correlations were substantially stronger in years with higher soil moisture, suggesting that growth response to summer clouds is strongly affected by pre-summer rainfall. A change in the height and/or timing of coastal cloud formation with climate change would likely affect this and other populations of California’s coastal vegetation.