【本月最绕文章】植物密度分布 中英文解析版

lxd19870226

中文考古
亲身试验证明这篇真的绕死了~~~特别是最后一段！
两个theory.一个是neighborhood-competition，另一个是gap的。前一个主要是说一个specie的生长是由周围因素影响的，比如说其他物种啊，距离其他物种的远近什么的。（有题）gap是说is of one generation to another,even adult of other species（没明白，但是有题问的是什么可以undermine gap）。然后举了个例子具体说明什么的。
做了个试验：blue grass and grounsel（貌似这么写的)他们都是有spring generation和 fall genertion 的。于是分别做实验groundsel 貌似只有一个情况，就是把他们很规矩的分布起来。而bluegrass 就是一组是规矩的分布，另一组就ramdom。在spring 的时候得出的结果证明了neighborhood的理论。然后来了个however，（后面没读明白但是应该说在fall 的时候得出了不一致的结果吧~~）
V7 by aiyasong
blue grass 的文章第一段介绍了两个关于植物繁殖的理论，一个是neighborhood理论，大概是讲植物繁殖的密度除了跟本物种有关，还和临的近的其他物种有关。 一个是gap理论，好像是讲本物种的繁殖密度还和其他复杂因素，例如其他物种的密度有关。第二段讲了一个G植物和bluegrass的例子，大概是两种植物要繁殖两次，第一次好像验证了nieghborhood理论，第二次好像验证了gap理论，好像说bluegrass密度高的地方G就低，因为bluegrass残留的根茎会阻止G植物的根发育，但不会影响bluegrass的根发育。
一个问题是问第二段的主旨，我选的是讲解bluegrass 对G植物的影响。待选项有 G对bluegrass的影响；削弱某个理论等等
一个问题是问什么不会影响植物的繁殖密度，我选的是植物种子的发芽情况 (可能不准）
一个问题是问如果削弱gap理论，我选的是G植物的密度不受bluegrass密度的影响。

英文原文地址
http://cn.bing.com/search?q=bluegrass+groundsel+seedling+density+neighboorhood+gap&go=&qs=n&form=QBRE&pq=bluegrass+groundsel+seedling+density+neighboorhood+gap&sc=0-0&sp=-1&sk=

Competitionbetween Two Weeds
Joy Bergelson. American Scientist. Volume 84.Sigma XI-The Scientific Research Society; Nov/Dec 1996.
Every gardenerknows that plants compete with each other-especially if one of the plants is aweed. Although gardeners help their plants win competitive interactions byremoving or poisoning weeds, plants in nature must fight on their own-findingways to defeat rival plants. The question is: What causes competing plants towin or lose?
Competitionbetween plants represents one of the best-documented interactions betweenbiological species. Hundreds of examples show that plant species affect oneanother through the use of shared resources, including light, nutrients andwater. A tomato seedling in your garden, for instance, may grow poorly in theshade cast by a mature oak tree. Another tomato plant may survive better in theshade, because some individual plants have different competitive abilities. Theoutcome of competition between plants may also depend on the composition of thelocal community and debris or chemicals left by plants from earliergenerations.
Although botanistshave assembled a virtual catalogue of examples of competition, we know verylittle about the mechanisms that govern differences in competitive ability. Ishall describe a series of experiments that reveal an intricate-and somewhatsurprising-web of competitive interactions between two particular plantspecies.
Garden Experiments
One of thebest-studied examples of competitive dynamics involves two weedy annuals:common groundsel and annual bluegrass. These species thrive throughout Eurasiaand the United States, where bluegrass grows more abundantly and persistsadmirably when faced with neighboring groundsel. About a decade ago, I starteda series of experiments to unravelthe factors that drive the interactions between these two species and to testthe population level repercussions of their competition.
（主旨题）
I began bygathering 20 groundsel plants from the Seattle area. I collected seeds fromeach of those plants and established a “common garden” experiment to test theability of the seeds to grow and reproduce with and without intense competitionfrom bluegrass. In this experiment, I planted seeds from each group ofsiblings, or sibship, in a field plot. I planted some seeds alone, and Iencircled others with 12 bluegrass seeds. The results from that experimentshowed that some of the groundsel plants fared better than others when facedwith competition from bluegrass. For instance, some of them produced twice asmany flowers as others. Such differences in performance were not evident amongthe groundsel plants grown alone.
（groundsel在有bluegrass的竞争下，长的更好。）
What caused somegroundsel plants to perform better than others in the presence of competitors?A plant’s genotype could affect its competitive ability. Nevertheless, myinitial experiment did not prove a connection between genetics andcompetitiveness, because the seeds used in that experiment also came fromplants that had experienced different environmental conditions. A number ofstudies indicate that a mother plant’s environmental conditions can influencethe characteristics of progeny-a phenomenon known as the maternal effect.Luckily, maternal effects can be controlled for rather easily, at least in thiscase. Groundsel produces seeds autogamously-meaning that a flower getspollinated by its own pollen-and plants collected in the field can bepropagated in a common environment for subsequent experiments. So I grewsibships from each plant for three generations in a greenhouse, and thenrepeated my original experiment. Again, different groundsel plants exhibitedlarge differences in their ability to endure competition, but no differencesappeared without competitors. In addition, if a field-collected mother plantperformed well against competitors, so did her offspring. These resultsconfirmed that there is a genetic basis for the varied tolerance tocompetition.
Early Winners
What features makea plant more competitive? That question can be addressed through a closer lookat competitively superior and inferior groundsel genotypes. During my experiments,I tracked the date of germination and the height of each groundsel plant, whichprovided data that might explain differences in competitive ability.
Using a techniquecalled path analysis, I assessed the relative importance of differences inearly growth rates, late growth rates and emergence dates. Path analysisunravels the effects of several correlated characteristics, such as growthrates and date of emergence, on a plant’s subsequent performance. That analysissuggested strongly that competitively superior genotypes gain their advantageby emerging early In other words, a rapidly emerging groundsel plant is acompetitive one. Moreover, the emergence date of the plants in this experimentsexplained more than 70 percent of the variation in their success.
That result led toanother experiment: manipulating relative emergence dates by planting seeds atdifferent times. I planted slow-germinating genotypes before fast-germinatingones, so that the seedlings would emerge simultaneously. The resulting plantspossessed comparable competitive ability-confirming my hypothesis thatcompetitive superiority comes from getting a head start on germination. Otherplant systems operate similarly; small differences in emergence dates translateinto large differences in performance. The effects can be dramatic. Forexample, Thomas Miller of Florida State University reported that a three-dayhead start in emergence results in more than a 1,900-fold increase in adultperformance in common lambsquarters.
If emergence datedetermines a seedling’s success, we might expect to see the evolution of traitsthat enable seeds to modulate germination in response to competitors. In fact,seeds from some plant species reduce their probability of germination whenadult plants are already growing in the area, evidently avoiding competitionwith established plants. Seeds also appear responsive to the presence of otherseeds. In both groundsel and bluegrass, my colleagues and I have demonstrated anegative relation between seed densities and the probability that each seedwill germinate, presumably a tactic for preventing high levels of competitionamong seedlings, which often leads to extensive mortality Moreover, seed-seedcommunication appears even more complex than a simple density-dependentgermination. A series of greenhouse experiments showed that groundsel andbluegrass seeds accelerate their rates of germination when they are grown insoil that had previously contained germinating seeds. Apparently, thepreviously germinating seeds excrete chemicals, which have not yet beenisolated, that accelerate the emergence of later seeds.
Neighborhoods and Gaps
In thegroundsel-bluegrass system, competition portrays a race in which smalldifferences in emergence produce large differences in performance, and seedsrespond vigorously to clues that indicate the presence of competitors.
Apparently, thethreat of competition shapes the activity of these seeds. That raises broaderquestions: To what extent does competition influence the population dynamics ofcompetitors, and to what extent does community context influence competitiveinteractions? Although biologists commonly assume that competition affects thedynamics of plant communities, we are just beginning to explore the interplaybetween individual performance, which responds to competition, and populationand community-level factors. Such research provides an important arena fortesting our understanding of the forces that structure plant communities.
At this interfacebetween individuals and communities of plants, spatial patterning receives adisproportionate share of attention. Two distinct theories have been developedthat predict that spatial patterning can exert a strong influence oncompetitive interactions. Since plant populations exhibit very patchyarrangements, as opposed to random distributions, the question arises as towhether patterning drives the dynamics of plant populations in nature.
Stephen Pacala ofPrinceton University and John Silander of the University of Connecticutchampion theneighborhood-competition theory. That theory assumes that plant growthand reproduction depend on the density of nearby competitors and thatcompetitors beyond a designated distance exert no impact on a particular plant.Combining that assumption with an elaborate mathematical theory suggests thatpatchiness in a distribution of competitors can profoundly alter competition atthe population level. Intuitively, this theory can be understood by recognizingthat patchiness leads to considerable variation in the amount of competitionexperienced by any individual plant. Some plants will experience lots ofcompetition and experience little reproduction; other plants will be virtuallyfree from competition and contribute a disproportionately large share to thenext generation. In this way, the neighborhood-competition theory suggests thatthe spatial pattern of plants influences competition through interactionsbetween contemporaneous plants.
（理论1——the neighborhood-competition theory——the spatial pattern通过同代植物的相互作用影响竞争）
An alternativetheory for patchiness, gapcolonization, considers the competitive impact of one generation onanother. Traditionally, this theory has been applied to the dynamics offorests, where the germination and growth of seedlings depends on gaps leftafter trees fall. Nevertheless, it might be equally relevant to the dynamics ofgrassland plants. In nearly all cases, established vegetation can overpowerseedlings. In fact, seedlingemergence and survival can be totally inhibited by the presence of adultplants, or even by the litter left from previous generations. This type ofinhibition can arise from several factors: chemicals, shading or structuralinterference produced by the established plants. In some cases, thesefactors may restrict seedling survival to areas that lack establishedvegetation. Moreover, the gaps in a patchy spatial pattern promote thepersistence of competitively inferior plants. Gap-colonization theory, then,suggests that an area’s spatial pattern will influence competitive interactionsbecause previous generations affect the survival of present seedlings.
（理论2——the gap-colonization theory——the spatial pattern通过隔代植物的相互作用影响竞争）
（Except题）
Competitionbetween common groundsel and annual bluegrass also depends on spatial patterns.Both of these species produce two generations each year-one in the spring andanother in the fall. This rather unusual life cycle proves tremendouslyconvenient for testing the neighborhood-competition and gap-colonizationtheories. In each fall generation, newly emerging seedlings compete with eachother, which provides an opportunity for neighborhood competition, and theyalso interact with remnants from the spring generation, which could lead to gapcolonization.
（groundsel和bluegrass都是春、秋两季作物，非常适合测试这2个理论）
Litter Blockade
To explore therole of patchiness on competition between groundsel and bluegrass, Iestablished artificial plots in the spring in which I created either a randomor a patchy distribution of bluegrass. In the same plots, I also plantedgroundsel, but it was always distributed randomly at one-quarter the density ofbluegrass. That relative abundance resembles natural communities, wheregroundsel is typically less abundant than bluegrass. I let the seedlings growand compete, and then I counted the number of groundsel plants in the fallgeneration. The spatial distribution played a dramatic role in the growth ratesof the groundsel: The fall generation contained nearly four times moregroundsel plants when the bluegrass distribution was patchy rather than random.That experiment represented the first experimental manipulation of plantspatial patterns to assess competitive interactions, and it showedunequivocally that spatial patterning can have an enormous impact.
实验一
（实验组：spring时期播种a patchy distribution of bluegrass + a randomdistribution of 四分之一bluegrass数量的groundsel; ）
（控制组：spring时期播种a random distribution of bluegrass+ a random distributionof 四分之一bluegrass数量的groundsel;）
These results did not arise from neighborhood competition.Regardless of the distribution of the bluegrass, the spring generation ofgroundsel produced essentially the same number of seeds. That is, competitionwithin the spring generation did not generate the differences that appeared inthe fall. Pacala and Silander obtained similar results when exploring howpatchiness affected the growth rates in a competitive system of velvetleaf andpigweed.
（实验一结果：fall时期发现实验组的groundsel数量超出控制组4倍多.However, 这些结果不能证明理论1，因为spring时期实验组和控制组的groundsel数量差不多）
On the other hand,gaps did affect the groundsel’s growth. In the plots, a large fraction of allthe surviving groundsel seedlings grew in areas of low bluegrass density. Sopatchy plots promoted the growth of groundsel populations by providing agreater number of gaps.
（相反，理论2在发挥作用，大部分fall时期发芽的groundsel是在bluegrass密度较低的地区）
The difference ingroundsel’s success in the spring and fall generations suggested that thebluegrass litterdead blades and roots from previous generations—could be afactor. To test that hypothesis, I repeated the above experiment with thepatchy and randomly distributed bluegrass but removed the dead bluegrass fromhalf of the plots and left the bluegrass litter intact in the other half. Iflitter drives the effect of spatial patterning on groundsel, then removing thelitter should remove the effect, which is exactly what I found. With the litterintact, groundsel grew better where the bluegrass distribution was patchy; butwith the litter removed, the groundsel grew about the same regardless of thedistribution of bluegrass. This experiment indicated clearly that competitionbetween generations, not within them, governs the dynamics of the groundsel-bluegrasssystem and explains the importance of the spatial pattern.
（spring时期的bluegrass的残留叶子和根茎可能是一个重要因素——引出实验二）
（实验组/控制组：在一半的试验种植地中，remove掉bluegrass的残留物，在另一半中，保持残留物完好无损；其他条件与实验一相同）
（实验逻辑：如果隔代植物的残留物是影响spatial patterning的因素的话，那么remove掉残留物应该使这种效果消失，而此次实验确实证实了）
（实验结果：在残留物完好的土地中，groundsel在pachy的分布下比在random的分布下长的更好；在残留物去除的土地中，groundsel在2种分布下长的差不多；证明理论2，而非理论1，的重要性）
A series ofgreenhouse experiments revealed that the effect of bluegrass litter comes fromthe dead blades above ground. The presence of grass roots or chemicals thatmight have leached from the bluegrass did not affect the germination orsurvival of groundsel seedlings. Instead, litter inhibits groundsel seedlings,because emerging seedlings get trapped by the litter above them. The seedlingscannot penetrate the litter, which prevents them from capturing light orgrowing, and they die. This structural inhibition between bluegrass litter andgroundsel seedlings provides the crucial competitive interaction. In addition,litter generates little trouble for the relatively slender morphology of abluegrass seedling.
Theseinvestigations illustrate that the spatial pattern of bluegrass produces largeeffects on the success of the competitively inferior groundsel, and that themechanism involves gap colonization, or interactions between generations. Thatconclusion has several additional implications. First, the interaction betweengroundsel and bluegrass includes a time lag-earlier generations affecting laterones. A variety of simple mathematical models illustrate that biologicalsystems with time lags tend to have relatively more complex dynamics thansystems without time lags. The second implication involves succession. Assuccession proceeds, a system’s litter accumulates, which can shift thecompetitive balance from litter-intolerant species to litter-tolerant ones. Inthat way, litter can qualitatively alter the outcome of competition. Again,models support such a conclusion, showing that bluegrass should dominatewhenever litter accumulates, and that groundsel dominates if the litter decomposesquickly.
Modeling Trade-offs
These small-scaleexperiments showed that groundsel grows more successfully in a patchy plot. Inthe simplest terms, one might say that greater amounts of bare ground favorgroundsel, because the plant requires such gaps for establishment. Then onemight ask: Given a particular amount of bare ground, how does its spatialdistribution influence the success of invading weeds? I approached thatquestion in collaboration with Jonathan Newman of Southern Illinois Universityand Ernesto Floresroux, then of the University of Chicago. We performedexperiments on a somewhat larger spatial scale-over a few meters-where we triedto determine how the dispersion of gaps influences how fast groundselprogresses through a field. These experiments reveal the community-levelrepercussions of between-generation competition.
We approached theeffects of gap dispersion with a simple experiment. For each experimental plotin a field of ryegrass, we created six transects that were oriented like spokeson a wheel. On each transect, we created artificial gaps that covered one ofthree areas: 25, 225 or 900 square centimeters. To control the total amount ofgap in a given transect, we created fewer large gaps than small ones. Wedistributed the gaps either uniformly or randomly, based on the distancebetween them. By analogy with the small-scale experiments described earlier, alarge variance in the intergap distance corresponds to a patchy distribution,and equal intergap distances correspond to a uniform distribution. Weintroduced 12 invading groundsel plants in the center and then counted thenumber and position of all seedlings in two subsequent generations-hoping todetermine whether the success of invasion depends on the spatial heterogeneityof the gaps.
One can assess aplant’s success of invasion in two different ways. The number of individualsthat get established provides one index, and the distance between a parent andits offspring-the rate of spread-provides another. In our experiments, largergaps increased the number of established groundsel seedlings, even though wecontrolled for overall gap area. In addition, the invading groundsel spreadfaster with large gaps. For example, large gaps produced nearly three timesmore distance between a parent and its offspring, as compared with small gaps.Moreover, the invading groundsel produced more established seedlings withpatchy gaps than uniform ones, regardless of the size of the gaps. However,offspring traveled farther when the gaps were positioned uniformly, regardlessof the size of the gaps.
We wondered if theway that a plant’s “shower” of seeds would fall on such gaps would lead tosimilar results. One can imagine that a plant produces a seed shadow, whichdepicts the proportion of seeds that fall relative to the distance from theplant. In our transect experiment, some seeds would fall in gaps and germinate,and others would fall in vegetation and not germinate. By knowing a plant’sseed shadow and a transect’s arrangement of gaps, one can predict the expecteddistance between parents and seeds that land in gaps. A simple mathematicalmodel of this scenario produced results that resembled what we found in ourexperiments. In other words, how the seeds disperse and the strong competitivedominance of established grass over seedlings explains what we observed.
These resultspoint to an interesting trade-off: An invading plant can progress faster in afield that contains a uniform distribution of gaps, but fewer seeds landsuccessfully in uniform gaps. This trade off affects models of the persistenceof competitively inferior species in patchy environments. In the past, suchmodels suggested that a competitively inferior species can persist in acommunity by dispersing more effectively than its superior competitor, butmodels of that phenomenon ignore the spatial positioning of gaps. Our results,however, indicate that a competitively inferior species faces a more difficultchallenge, because of the negative relationship between rates of dispersal andthe probability that seeds land in gaps and establish successfully Dispersingseeds that travel a far distance, on average, require uniformly positionedgaps; but patchy gaps lead to more established seedlings. In other words, aplant can either widely disperse its offspring or produce lots of them, but itprobably cannot do both. Future research should address how competitivelyinferior species persist in realistic, spatially heterogeneous environments.
My work with two common weedsgroundsel andbluegrass-shows that competition between these plants depends on many factors.Competition between generations-later groundsel seeds battling establishedbluegrass-is the primary factor that governs the dynamics of this system.Nevertheless, groundsel’s genotype determines largely when a seedling willemerge-a crucial factor in competitive success-and that suggests thatcontemporary plants must compete, as well. Moreover, the result of competitionbetween groundsel and bluegrass also depends on the structure of the localenvironment, including the size and arrangement of gaps. In the future,ecologists hope to develop models and experimental systems that simultaneouslyexamine how these factors contribute to plant competition.

宾主尽欢ing

太牛了顶一个，第一坐沙发

huisy0330

好长。。。好多。。。谢谢LZ

pigman007

牛了×的

憨憨528

NIUBILITY

憨憨528

楼主，是不是后面没有标记的那些地方就不用看了，马上上考场了，求赐教

ballerino1110

太太太牛了!!!!!!!!!!!!!!!!!!!!!

大大的翅膀

厉害厉害...

jhy329

大神现身！！

nevereverbe

花了一个小时多才把大神发的原文给摸下来。。。还是没太看明白实验一结束，为啥结果不能证明NeiTheory。。。。求交流啊