Environmental variation and moose Alces alces density as determinants of spatio-temporal heterogeneity in browsing

Understanding temporal variation in habitat selection and browsing intensity by large herbivores is fundamental because of their large impact on the ecosystems. I studied the annual variation in winter browsing pressure on young trees and habitat selection by moose Alces alces over a ten year period. Specifically, the relationships between browsing pressure on Scots pine Pinus sylvestris and two birch species ( Betula ssp.) and three explanatory variables – 1) availability of forage, 2) moose density (estimated by pellet group counts) and 3) snow cover was studied. At a larger spatial scale (forest stand level) the relationship between moose habitat selection between three different habitat types (forest <30 yr, forest>30 yr and mire) and two explanatory variables, 1) snow condition and 2) moose density, were studied. Browsing pressure on Scots pine, the dominating food plant, was related to forage availability, moose density and snow condition. No significant relationships between any of the three explanatory variables and browsing pressure on the two birch species were found. Moose selection for certain habitats varied between years and was affected by number of days with >0.10 m of snow.
Habitat selection was not significantly related to moose density and the relationship between overall moose density and habitat specific moose densities was proportional within the studied density range. These findings have implications for understanding varying browsing patterns – and will affect both the ability to predict herbivores' effect on the forest ecosystem. A snow dependent browsing pattern also indicates that one can expect a long term decrease in browsing pressure on the tree and shrub layer as a consequence of the ongoing large-scale climate change.     

Postfire Vegetation Recovery in Highland Pine Forests of the Dominican Republic

We surveyed postfire vegetation at five sites at high elevations (> 2000 m) in the Cordillera Central, Dominican Republic. Highlands of the Cordillera Central are dominated by a single pine species, Pinus occidentalis, but plant communities are rich with endemics and conservation and fire management efforts in these systems are ongoing. The burns were 2–7 yr in age and had consumed nearly all shrub crowns. Pines suffered high mortality (> 50%, all sites combined), but shrubs resprouted at high rates (88%, N = 957) after fire. All shrub taxa produced basal resprouts; eight of 11 shrub taxa measured had resprouting rates > 90 percent, while Baccharis myrsinites had the lowest (56%). Most taxa grew to prefire height quickly (within 5–7 yr), with regrowth of stem diameters lagging behind. Patterns and rates of shrub recovery resembled those documented in high elevation shrublands in Costa Rica and Brazil. Pinus occidentalis does not resprout, but larger individuals can survive fire. Survival increases dramatically when trees attain > 13-cm dbh, when bark becomes thick enough to protect cambial tissue. Overall, pines are regenerating much more slowly than shrubs, but seedling establishment varied considerably between sites. Frequent fires may cause a decline in pines and an increase in shrub- or grass-dominated communities. Succession in these high elevation fire-dependent pine forests favors taxa already present in the preburn vegetation, with woody composition changing little after fire, in contrast to lower-elevation cloud forest, where postfire vegetation has been shown to bear little resemblance to mature forest even after several years.     

Long-term benefits to the growth of ponderosa pines from controlling southwestern pine tip moth (Lepidoptera: Tortricidae) and weeds

The southwestern pine tip moth, Rhyacionia neomexicana (Dyar) (Lepidoptera: Tortricidae), is a native forest pest that attacks seedlings and saplings of ponderosa pine, Pinus ponderosa Dougl. ex Laws, in the southwestern United States. Repeated attacks can cause severe deformation of host trees and significant long-term growth loss. Alternatively, effective control of R. neomexicana, vegetative competition, or both in young pine plantations may increase survival and growth of trees for many years after treatments are applied. We test the null hypothesis that 4 yr of R. neomexicana and weed control with insecticide, weeding, and insecticide plus weeding would not have any residual effect on survival and growth of trees in ponderosa pine plantation in northern Arizona 14 yr post-treatment, when the trees were 18 yr old. Both insecticide and weeding treatment increased tree growth and reduced the incidence of southwestern pine tip moth damage compared with the control. However, weeding alone also significantly increased tree survival, whereas insecticide alone did not. The insecticide plus weeding treatment had the greatest tree growth and survival, and the lowest rate of tip moth damage. Based on these results, we rejected our null hypothesis and concluded that there were detectable increases in the survival and growth of ponderosa pines 14 yr after treatments applied to control R. neomexicana and weeds.     

Stability of pre- and post-fire spatial structure of pine trees in Aleppo pine forest

The study reported here describes for the first time the similarity between pre- and post-fire spatial patterns of the trees in a Mediterranean pine forest demonstrating that the pre-fire ancestor microsite is occupied also by the next generation. Although Aleppo pine Pinus halepensis Mill, is an obligatory post-fire seeder, it is adapted to regenerate in its pre-fire growing microsite. thus keeping suitable growing sites from generation to generation. We studied the effect of the dead burned adult pines on the density and size of their recruited saplings 2, 5, 11 and 20 yr after fire. A comparison of pine sapling density and size was made between the "near" zone (under the former effect of the burned canopy) and the'far'zone (beyond the former effect of the burned canopy).
In the site 2 yr after fire, seedling density was 56% higher in the "far" zone than in the'near'zone, but seedling .size was similar. However in the site 20 yr after fire, densities were similar in both zones, but the size was bigger by 89% in the "near" zone. Thus, population recruitment after fire seems to peak near the burned pine trees rather than at u distance from them, in contrast to Janzen's original'distance hypothesis' model suggested for undisturbed rainforest. Mere we present a new hypothetical model for the spatial pattern of post-fire regeneration of obligate seeder tree species forming open forests. It is proposed that in such trees the microsites which were kept by the burned adult trees, which are killed by the fire, are also the favorable regeneration microsite for the post-fire generation.     

Effects of browsing in relation to vegetation cover on common yew (<Emphasis Type="Italic">Taxus baccata</Emphasis> L.) recruitment in Mediterranean environments

Common yew (Taxus baccata L.) stands are recognized as prioritary habitats for biodiversity conservation within the European Union. The effects of browsing on the regeneration capacity and spatial dispersal of T. baccata recruits at the European southern limit of the species in the Mediterranean Basin have been herein studied. The efficacy of T. baccata recruitment has been evaluated at six localities in the Northern Sardinia mountains, which have similar altitude, climate, soil, and vegetation but have different types of uses (three were grazed by livestock and three were not). At each site, five habitats have been identified for T. baccata seed dispersal: reproductive female T. baccata canopy, reproductive female Ilex aquifolium canopy, non-fleshy-fruited tree canopy, fleshy-fruited shrubs, and open areas. The density of seedlings was found to be greater under fleshy-fruited trees (reproductive female T. baccata and I. aquifolium) than under shrubs, whereas the sapling density was higher in shrubby habitats, especially at grazed sites due to the mechanical protection afforded by the spiny shrubs against herbivores. Land use (LU) has been found to be the most important factor in determining the spatial distribution of seedlings and saplings in relation to forest habitats. Although browsers had an ephemeral but positive effect on seed germination through their trampling and the resultant scarification, this process eventually became ineffective as was shown by the occurrence of the lowest density of saplings in those habitats where the density of seedlings was the highest. The ultimate and most important effect of browsing was the sharp decrease in the density of saplings, and their almost complete extinction, in non-shrubby habitats. This study highlights the result that, in Mediterranean ecosystems, browsing constitutes the main negative factor on T. baccata seedling-sapling transition and furthermore confirms the necessity to preserve shrubby patches in the vicinity of reproductive female T. baccata and I. aquifolium to permit the regeneration of T. baccata in the presence of livestock. Moreover, at ungrazed sites, T. baccata is able to colonize non-shrubby shady habitats. The application of different management strategies to ungrazed and grazed sites should therefore be the main direction in the management and preservation of T. baccata stands in the Mediterranean region.     

Holocene altitudinal shifts in vegetation belts and environmental changes in the Sierra Madre Occidental, Northwestern Mexico, based on modern and fossil pollen data

A modern pollen rain study was performed in a 300 km-long altitudinal transect (~ 28° N latitude) from 300 to 2300 m elevation. The higher elevation modern communities: epithermal oak–pines, pine–oak forest, pine forest, and mixed conifer forest were easy to distinguish from their pollen content. In contrast, lower elevation subtropical communities: thornscrub and tropical deciduous forest were difficult to separate, because they share many pollen taxa. Nevertheless we identify high frequencies of Bursera laxiflora as an important component of the tropical deciduous forest.Additionally, fossil pollen was analyzed at three sites located between 1700 and 1950 m altitude at ~ 28° latitude north in the Sierra Madre Occidental of northwestern Mexico. The sites were in pine–oak (Pinus–Quercus), pine, and mixed-conifer forests respectively. Shifts in the altitudinal distribution of vegetation belts were recorded for the last 12,849 cal yr BP, and climate changes were inferred. The lowest site (pine–oak forest) was surrounded by pine forest between 12,849 and 11,900 cal yr BP, suggesting a cold and relatively dry Younger Dryas period. The early Holocene was also cold but wetter, with mixed conifer forest with Abies (fir) growing at the same site, at 1700 m elevation, 300 m lower than today. After 9200 cal yr BP, a change to warmer/drier conditions caused fir migration to higher elevations and the expansion of Quercus at 1700 m. At 5600 cal yr BP Abies was growing above 1800 m and Picea (spruce) that is absent today, was recorded at 1950 m elevation. Fir and spruce disappeared from the 1950 m site and reached their present distribution (scattered, above 2000 m) after 1000 cal yr BP; we infer an episodic Holocene migration rate to higher elevations for Abies of 23.8 m/1000 cal yr and for Picea of 39.2 m/1000 cal yr. The late Holocene reflects frequent climate oscillations, with variations in the representation of forest trees. A tendency towards an openness of the forest is recorded for the last 2000 yrs, possibly reflecting human activities along with short-term climate change.     

Browse biomass production an.regrowth capacity after biomass loss in deciduous and coniferous trees: responses to moose browsing along a productivity gradient

To understand how large herbivores interact with their food resources and the ecosystem, it is necessary to understand plant growth responses to biomass loss. Tolerance to biomass loss, and the ability to regrow new photosynthetic tissue after herbivore attack, is one such important mechanism. Here we experimentally studied how year-round browsing corresponding to four different levels of moose, Alces alces , population density affected total annual browse biomass production, which also serves as a proxy for production of new photosynthetic tissue, in birch ( Betula pubescens and B. pendula ) and Scots pine, Pinus sylvestris , along a habitat productivity gradient. Our results revealed that habitat productivity index was the most important variable determining annual browse biomass production for birch, and that a high moose density and biomass removals of 25–40% annually actually could stimulate birch above-ground productivity in more productive sites. Contrary to birch, pine was not affected by habitat productivity index, and appeared to have much lower regrowth capacity than birch. For pine, percent biomass removed was the most important factor affecting annual browse biomass production, which decreased with increasing levels of biomass removal. We also tested if birch and pine responded to biomass removal by allocating more resources to browse biomass production at in the lower portion of the stem (0–0.5 m), which was protected from biomass removal in winter. Birch did not allocate more resources to biomass production in the lower portion of the stem, whereas pines subjected to high biomass removal had more biomass in the lower portion of the stem than those protected from losses. Our study thus shows that plant responses can be linear or non-linear along the range of herbivore densities we studied, and that responses can change from negative to positive along underlying habitat productivity gradients.     

Winter Habitat Selection by Canada Lynx in Maine: Prey Abundance or Accessibility?

Abstract: We related winter habitat selection by Canada lynx (Lynx canadensis), relative abundance of snowshoe hares (Lepus americanus), and understory stem densities to evaluate whether lynx select stands with the greatest snowshoe hare densities or the greatest prey accessibility. Lynx (3 F, 3 M) selected tall (4.4-7.3 m) regenerating clear-cuts (11-26 yr postharvest) and established partially harvested stands (11-21 yr postharvest) and selected against short (3.4-4.3 m) regenerating clear-cuts, recent partially harvested stands (1-10 yr), mature second-growth stands (>40 yr), and roads and their edges (30 m on either side of roads). Lynx selected stands that provided intermediate to high hare density and intermediate cover for hares (i.e., prey access) but exhibited lower relative preference for stand types with highest hare densities where coniferous saplings exceeded 14,000 stems/ha.     

Coarse woody debris can reduce mammalian browsing damage of woody plant saplings in box‐gum grassy woodlands

The critically endangered box‐gum grassy woodlands of south‐east Australia face numerous threats including the failure of woody plant regeneration caused by over‐browsing. In the Australian Capital Territory, over‐browsing of tree and shrub saplings is likely caused by dense populations of Eastern Grey Kangaroo (Macropus giganteus) found in many nature reserves free of livestock. One possible way to protect these saplings is using coarse woody debris (CWD) as a browsing deterrent. We tested this idea by planting palatable Red Stemmed Wattle (Acacia rubida) saplings among manually applied CWD, among naturally fallen CWD, and in the open, in five woodland reserves. We recorded the proportion of saplings browsed, the number of weeks to first browsing and the browsing severity (sapling height lost). Applied CWD protected saplings from being browsed only at relatively low‐to‐moderate kangaroo browsing pressure (as measured by faecal pellet counts). At relatively high browsing pressure, the probability of a sapling being browsed among applied CWD was 100%, similar to the probability in the open treatment (no CWD). Natural CWD, in contrast, provided some protection even at high browsing pressures. Time to browsing was most affected by browsing pressure, although CWD cover also had an influence. Browsing severity was similar between the three treatments and was only affected by browsing pressure. These results indicate that without protection, palatable woody plant saplings have a high chance of being browsed by kangaroos in woodland reserves, and therefore, some protection is needed for successful regeneration. The CWD being applied to reserves has a limited capacity to protect regenerating saplings. If more protection is wanted a CWD structure more resembling natural fallen timber should be used. This could be done by artificially placing branches around plantings. However, the most important action to facilitate regeneration is to manage kangaroo populations to reduce overall browsing pressure.     

Biomass allocation and growth responses of Scots pine saplings to simulated herbivory depend on plant age and light availability

This study experimentally analyses the response to simulated herbivory of juvenile Scots pine of two different ages in contrasting abiotic scenarios, focusing on the potential dual role of browsing ungulates: negative, by removing aerial biomass, and positive, by stimulating compensation capacity and providing nutrients by depositing their excrement. Compensation against herbivory was investigated by experimentally clipping a set of Scots pine (Pinus sylvestris L. nevadensis) juveniles, grown under different levels of light and nutrient availability. The responses analysed were survival, trunk-diameter growth, leader-shoot growth, increment in number of meristems, RGR, biomass of needles, shoots, root and whole plant, and root-to-shoot ratio. Clipping consistently resulted in a worse survival and performance of pines with respect to unclipped ones. From the factors analysed, light availability was responsible mainly for the variations in plant performance, while the addition of nutrients was much less important. Age was also important, with older pines showing in general better performance after clipping. Overall, clipping invariably had a negative effect on Scots pine, since none of the combinations of abiotic factors used resulted in overcompensation. However, the intensity on this negative effect proved quite variable, from almost an exact compensation in clipped older pines under full sunlight availability to very poor performance and high death probability in younger pines in shade. Scots pine cannot overcompensate after clipping, but, depending of the environmental conditions, the negative result of clipping varies from severe undercompensation to almost exact compensation. Also, small differences in sapling age can promote significant differences in sapling response to clipping and light environment.     

Ungulate browsing in winter reduces the growth of <Emphasis Type="Italic">Fraxinus</Emphasis> and <Emphasis Type="Italic">Acer</Emphasis> saplings in subsequent unbrowsed years

Browsing by ungulates has become a hotly debated issue in many European mountain forests in the past century. Acer pseudoplatanus L. and Fraxinus excelsior L. are broadleaved tree species that are preferentially browsed by roe deer (Capreolus capreolus) in central Europe. We analyzed growth data from shaded saplings of both tree species to quantify the extent to which height growth after game browsing is reduced in subsequent, unbrowsed years in forest stands. Sixty saplings of F. excelsior and A. pseudoplatanus from forest stands at Albisriederberg (Switzerland) were available to us that had been dissected into pieces that then were split in the middle for counting tree rings and assessing ungulate damage. We fitted the von Bertalanffy growth equation to these height growth data and included a reduction factor for winter browsing. Both tree species showed significantly reduced height growth in unbrowsed years after one to several browsing events in winter, and this effect increased with the number of browsing events. Saplings with a high growth rate showed a higher growth reduction. After winter browsing, height growth of A. pseudoplatanus saplings was less affected in unbrowsed years than that of F. excelsior saplings. We conclude that browsed saplings of these species in forest stands are not able to compensate browsing-induced height loss, but that height differences between browsed and unbrowsed saplings probably increase over time. A comparison between our analysis and the parameters estimated using equations published by Eiberle for predicting age at 130 cm height suggests that our parameter values are rather conservative estimates of the growth reduction effect after winter browsing. Neither F. excelsior nor A. pseudoplatanus show a distinct pattern in browsing-induced growth reduction with respect to soil moisture, nutrient level, and altitude. We thus conclude that our results are likely to be valid for a wide range of forested sites.     

Threats to pine stands by the pine shoot beetles Tomicus piniperda (L.) and Tomicus minor (Hart.) (Col., Scolytidae) around a sawmill in southern Poland

Forest stands growing around sawmills are especially exposed to damage due to the feeding by the pine shoot beetles Tomicus piniperda (L.) and Tomicus minor (Hart.) (Col., Scolytidae) in the pine shoots, causing a considerable loss of increment. The purpose of this study was to determine the relation of the distance zone of a stand from a sawmill to the density of the pine shoot fall, width of annual rings, and pine stand quality. The investigations were carried out between 1992 and 1996 in a Pinus silvestris – Abies alba stand adjacent to a sawmill in the Zagnańsk Forest District. In the investigated stand, eight zones situated at different distances from the sawmill were marked out, in which the fallen pine shoots were collected and dendrometric measurements taken. The study results showed that the pine shoot fall in the area up to 300 m away from the sawmill was significantly greater than the fall in zones situated further away. In addition, the mean current annual and 10-year radial increments at breast height were significantly smaller, and the pine stand quality class was lower within that area. The radial increment at breast height in the area up to 300 m away from the sawmill was almost twice as low as that in the control area, and about 40% lower than the increment in the area situated 300–900 m away. The increment pattern of pine in this area differed from that of fir, suggesting that the incremental loss and lower pine stand quality are the result of long-term feeding of the pine shoot beetles in the pine shoots. These results indicate that other causes, such as differences in site class, climatic factors or air pollution, can be excluded.     

Comparative root system structure of post-fire Pinus halepensis Mill. and Cistus monspeliensis L saplings.

To describe root system topology of Pinus halepensis and Cistus monspeliensis saplings co-inhabiting natural post-fire sites, 55 P. halepensis and 26 C. monspeliensis saplings were extracted by the total excavation method from a burnt pine stand. Seedlings were individually labelled when emerging after fire and extracted three years later, at the sapling phase. In order to evaluate the effect of inter-specific competition of C. monspeliensis on P. halepensis root system, a stratified sampling was carried out according to density and height of the saplings. Topological parameters considered in the analysis were magnitude, total external pathlength, and altitude of the root systems. Weight and length of roots were also measured in order to estimate the specific root length, an index commonly used in morphological studies. Results clearly evidenced greater variability in root system topology of P. halepensis than C. monspeliensis saplings. Herringbone architecture (i.e., the most ordered pattern possible, with branching confined to the main axis) characterised small pine saplings, regardless of competition from C. monspeliensis, which changed to random branching in large saplings. In medium sized saplings, the root system was affected by inter-specific competition, which delayed changes in root branching. In contrast, C. monspeliensis invariably adopted randomly branched architecture, regardless of intra-specific competition. It is concluded that such different topological patterns make C. monspeliensis more competitive during the early stages of post-fire succession, because its root system is much more transport-efficient in the nutrient-rich environment. Those pines which finally branch roots by random pattern will reach higher stem height and magnitude, a factor which allows them to successfully compete with C. monspeliensis for soil nutrients and water. The morphological analysis showed a significant increase in the specific root length with competition, both in P. halepensis and C. monspeliensis saplings, which could be interpreted as a consequence of the reduction of root diameter in response to nutrient depletion.     

Colonization by Pinus sylvestris of a former middle-geolittoral habitat on the Umeälven river in northern Sweden, following river regulation for hydro-electric power

The colonization by Scots pine of a former middle-geolittoral habitat on the Umeälven river, northern Sweden, laid almost completely dry in 1958, was compared with that of a similar habitat, subject to natural changes in water-level, on the Vindelälven river. The geographical position, altitude, exposition, slope and type of vegetation were similar at both sites. The percentage cover of field and bottom layers of the middle geolittoral are considered to have decreased after the fall in water-level of the Umeälven river. The pines were dated by making annual ring counts. During the period 1962–67 a significant increase in pine colonization took place at the Umeälven site above the new, regulated, high-water level. The tree densities (pines m^-2) in 1978 on middle geolittoral sites of the Umeälven and Vindelälven rivers were 4.9 and 1.4, respectively.     

The impact of simulated moose densities on conifer aphids along a productivity gradient

Large herbivores have a significant impact on boreal forest ecosystems. The modification of resources through their feeding induces changes in trophic dynamics and affects the direction of interactions in a community. Moose Alces alces may decrease the available plant biomass for herbivorous insects on one of their main winter forage species in Fennoscandia, Scots pine Pinus sylvestris, and indirectly alter the abundance of invertebrates through exploitative competition. Moose browsing can also induce chemical, morphological, and phenological changes in plants, changing their nutritive value to insect herbivores such as aphids. Habitat productivity may further modify the responses of aphids to moose browsing. We studied the responses of the gray pine aphid Schizolachnus pineti to different moose densities, and their relations to habitat productivity by sampling pine branches and measuring the number of aphids on pine needles. The experimental setup consisted of 8 exclosures along a productivity gradient, where the feeding, defecation, and urination of 4 densities of moose had been simulated for 7 yr. We here show that high levels of simulated browsing decrease the amount of gray pine aphids in areas with high productivity. In areas with low productivity, however, simulated browsing had no such effect. Habitat productivity should therefore be considered as an important factor that may determine the strength of an areas buffering capacity to high moose densities. Low resource environments appear to be favourable to specialist conifer aphids on pines under high browsing pressure, but the performance of generalist insect herbivores might be lowered.     

秦岭山地锐齿栎次生林幼苗更新特征

分析了秦岭山地锐齿栎次生林群落乔木及木本幼苗物种特征值、更新生态位宽度及不同影响因子下（包括林分密度、坡向、海拔等）乔木更新动态。结果表明,乔木层物种有25种,锐齿栎占有明显地优势,重要值为149.18%。其次为华山松(Pinus armandii)和千斤榆(Carpinus cordata)等;更新层共有木本植物41种,其中乔木种有28种,占所有木本植物总数量的68%,优势种有青蛙皮槭(Acer grosseri)、木姜子(Litsea pungens)等。林下有丰富的幼苗库,径级及龄级比较小。更新方式以实生为主;分析了10种更新优势乔木种群的生态位宽度,同种幼树种群生态位宽度均大于幼苗种群生态位宽度。幼苗种群生态位宽度从大到小依次为榛子(Corylus heterophylla)、锐齿栎、青蛙皮槭等。幼树种群生态位宽度从大到小依次为榛子、青蛙皮槭、锐齿栎等;林分密度对林下乔木幼苗和幼树数量影响不同。当林分密度（株?hm-2）从720增加到1460时,幼苗密度逐渐增大。随后随着林分密度的增加,又呈现出减少的趋势。而幼树密度随着林分密度增加一直呈现出减少的趋势。坡向对锐齿栎次生林林下乔木幼苗和幼树的影响各异,阳坡（南偏西3）均利于幼苗和幼树的更新。南偏西45林分内,幼苗密度急剧减少。随着坡向转为阴坡,幼苗密度又逐渐增加。而幼树密度逐渐减少。当海拔从1083 m增加到1547 m时,幼树密度逐渐增加。随后,随着海拔增加到1882 m,幼树密度则逐渐减小。而幼苗密度随着海拔的增高一直呈现减小的趋势。     

The conservation ecology ofIris rossii Baker (Iridaceae), a threatened plant in rural Japan

Spatial distribution, size structure and reproductive activities of a population ofIris rossii were examined in managed secondary grassland with scattered pines and hardwoods. Size structure and fecundity patterns among individuals were different between the three sites, which were an open area, under pine canopy, and under hardwood canopy. Growth and reproductive parameters of the species were significantly different at each site. In the open area, mean shoot number of individuals was 9.17, and it was 6.37 under the pine canopy and 5.63 under the hardwood canopy. Fruit set ratio was 26.8% in the open area, 21.1% under the pine canopy and 12.1% under the hardwood canopy. Six seedlings were found in the open area and one under the pine canopy, while no seedlings occurred under hardwood canopy. Most of the individuals distributed in the sites where the height of herbaceous layer was low. These results suggest thatI. rossii can not grow in the closed, especially hardwood, canopies or tall herbaceous layer. Therefore, human interventions such as annual mowing for the restriction of the growth of dominant grasses and tree saplings are essential for the persistence of the population of the species.     

利用大面积无人机航拍影像研究松材线虫病病死树林间分布规律

本研究使用固定翼无人机拍摄4 200 ha林地,从中选取了广东省河源市和平县阳明镇、紫金县紫城镇、东源县义合镇共3个样地的3 500 ha林地的航拍影像进行分析,用以探究松材线虫Bursaphelenchus xylophilus病死树的空间分布情况,及不同立地因子对疫情的影响,为松材线虫病监测预报提供解决途径。通过Pix4Dmapper软件对航拍的图像进行拼接生成正射影像图(DOM)等成果,然后使用eCognition(易康)软件对影像成果进行分割、分类和信息提取,最后借助ArcGIS平台进行病死树数量统计并获取方位、坡向、坡度、海拔等立地因子信息。结果表明,松材线虫病死树分布均呈聚集分布。使用双对角线法、平行线法、“Z”字法、五点法等不同抽样方法调查发现,仅五点法所得平均数与总体平均数无明显差异(P<0.05)。松材线虫病死树在不同立地因子下均有差异：主要分布在西坡、南坡和东南坡,西坡最多为25.94%,其次是南坡23.57%;主要分布在半阳坡和阳坡,半阳坡占36.54%,阳坡占34.09%;主要分布在凸坡,但随着疫情的发展,凹坡病死树数量逐渐超过凸坡;主要分布海拔区间在30...     

Fatal interactions between Scots pine and Sphagnum mosses in bog ecosystems

In this study, we explore how Sphagnum mosses and Scots pine, Pinus sylvestris , interact on different spatial and temporal scales in a boreal bog ecosystem. We were particularly interested in relationships between the occurrence of Sphagnum- dominated habitats and the occurrence of Scots pines of different age and size.
Juvenile and adult pines occurred in different habitats. While juveniles mainly occurred in Sphagnum- dominated habitats, predominantly with Sphagnum rubellum , adult pines were found in habitats dominated by lichens, or with a sparse vegetation cover. Examination of surface peat cores sampled close to adult pines revealed that almost all pines (97%) had established in a Sphagnum -dominated environment and that the habitat had changed since pine establishment. Scots pine is thus capable of changing and exterminating the Sphagnum -dominated environment preferred for germination and establishment. Pines impede Sphagnum growth and peat accumulation significantly once they have reached a stem diameter of approximately 20 mm. It takes from 30 to 90 yr for a pine to reach that size.
Our results show the importance of interactions between Scots pine and Sphagnum mosses in bog ecosystems. We conclude that interactions between trees and Sphagnum mosses are important driving forces behind the vegetation change that has characterised boreal bogs during the Holocene.     

REPRODUCTION IN MALE ATLANTIC WALRUSES (ODOBENUS ROSMARUS ROSMARUS) FROM THE NORTH WATER (N BAFFIN BAY)

Age at sexual maturity and timing of the mating season were determined in male Atlantic walruses ( Odobenus rosmarus rosmarus , L.) from the "North Water" subpopulation in northern Baffin Bay. Testes and epididymides of 174 male walruses (between 0 and 30 yr old) from NW Greenland (1987–1990) were studied macroscopically and a subset of 57 specimens was analyzed microscopically. In physically mature bulls ( i.e. , ≥12 yr old), sperm or apparently ripe spermatids were found between 9 November and 12 July. In younger walruses these signs of fertility were found in a few specimens (7–11 yr old) collected between 9 January and 28 May. The mating season seems to peak in January—April. The youngest sexually mature individual was 7 yr old and the oldest apparently immature individual was 13 yr old. Average age of sexual maturity was 10.9 yr (95% C.I.: 9–6–12.2 yr) and all were sexually mature by the time they were 14 yr old. The non-spermiogenetic testes and epididymides showed accelerated growth between about the 5–6th and about the 12–15th year of life, indicating that sexual maturation occurs during these years. The length of the baculum increased gradually until about 12–15 yr of age, when physical maturity was reached.