Autumn Colors Research Articles

Autumn colours and the nutrient retranslocation hypothesis: A theoretical assessment

The adaptive value of the bright colours of leaves in autumn is still debated. It is possible that autumn colours are an adaptation to protect the tree against photoinibition and photooxidation, which allows a more efficient recovery of nutrients. It has been proposed that the preference of aphids for trees that retranslocate nitrogen more efficiently can explain the high diversity of aphids on tree species with bright autumn colours. This scenario however does not take into account the impact of insects on the fitness of the trees and has not been analysed theoretically. Its assumptions and predictions, therefore, remain uncertain. I show with a model of insect–tree interaction that the system can actually evolve under particular conditions. I discuss the differences with the coevolution theory of autumn colours, available evidence and possible tests.

An assessment of the drought tolerance of Fraxinus genotypes for urban landscape plantings

The Fraxinus genus consists of ca. 65 deciduous, rarely evergreen trees found mainly throughout Europe, Asia and North America. Fraxinus species possess the aesthetic characteristics desired of urban trees (autumn colour, attractive bark and flowers); however, no scientifically based tolerance ranking exists for many genotypes within this genera. Aims of this study were to identify whether differences in drought tolerance exist within the Fraxinus genus and thereby provide information as to their usefulness for planting in urban landscapes where water stress poses a threat to transplant survival. At the cessation of a 2-week drought stress and subsequent recovery period, marked differences in drought tolerance among genotypes were distinguished based on net photosynthesis, leaf chlorophyll fluorescence and chlorophyll concentration. In addition, measurement of light absorbance, light trapping, electron transport and dissipation fluxes per leaf cross section of photosystem II at the cessation of the drought period provided an insight into mechanisms of drought damage and resilience among the genotypes tested. The results of this investigation indicate that F. excelsior ‘Aurea Pendula’, F. nigra Marsh., F. ornus L., F. angustifolia ‘Raywood’, F. excelsior L. and F. excelsior ‘Jaspidea’ should be considered for urban planting sites, where water supply is limited. Care should be taken when planting F. americana L., F. americana ‘Autumn Purple’ and F. velutina Torr., into urban environments as results indicated these species are sensitive to drought.

Characterization and Subcellular Localization of Chlorophyllase from Ginkgo biloba

Chlorophyllase (Chlase) catalyzes the initial step of chlorophyll (Chl)-degradation, but the physiological significance of this reaction is still ambiguous. Common understanding of its role is that Chlase is involved in de-greening processes such as fruit ripening, leaf senescence, and flowering. But there is a possibility that Chlase is also involved in turnover and homeostasis of Chls. Among the de-greening processes, autumnal coloration is one of the most striking natural phenomena, but the involvement of Chlase during autumnal coloration is not clear. Previously, it was shown that Chlase activity and expression level of the Chlase gene were not increased during autumnal coloration in Ginkgo biloba, indicating that Chlase does not work specially in the de-greening processes in G. biloba. In this study, we characterized the recombinant Chlase and analyzed its subcellular localization to understand the role of the cloned Chlase of G. biloba (GbCLH). GbCLH exhibited its highest activity at pH 7.5, 40 degrees C. Kinetic analysis revealed that GbCLH hydrolyzes pheophytin (Pheo) a and Chl a more rapidly than Pheo b and Chl b. Transient expression analysis of 40 N-terminus amino acids of GbCLH fused with GFP (green fluorescent protein) and subcellular fractionation showed that GbCLH localizes within chloroplasts. Together with our previous results, property of GbCLH and its location within the chloroplasts suggest that GbCLH plays a role in the turnover and homeostasis of Chls in green leaves of G. biloba.

A simple method to visualize the mechanism why Alnus glutinosa remains green during autumn colouration of Sorbus aucuparia

The autumn colouration of two deciduous trees (Sorbus aucuparia and Alnus glutinosa) from northern Finland (65°N) was studied during leaf senescence in autumn 2003. Their leaves were harvested from the end of August until leaf abscission in Sorbus trees at the end of September. The leaves were extracted in ethanol and measured with a spectrophotometer at 663 nm (chlorophyll a), 644 nm (chlorophyll b), 536 (anthocyanins) and 470 nm (carotenoids). The ratios A663/644, A663/470 and A663/536 were calculated to demonstrate the degradation of chlorophyll a in relation to the other studied pigments. The most important results of the present study were as follows: (1) Rapid (within 4 days) visually observed change in Sorbus colour from green to red. (2) Simultaneously occurred significant change in the calculated absorbance ratios in the same species. (3) The distinct difference between the tree species in both variables: Alnus remained green and maintained its absorbance ratios above the threshold ratios of Sorbus throughout the experiment. Sorbus exhibited the reddish colour in a ratio of 0.4 for A663/470 and 4 for A663/536 around the middle of September. The lowest ratios for Alnus remained 0.5 and 6 for A663/470 and for A663/536 (respectively), i.e. the ratios in Alnus never reached the level in Sorbus. Alnus maintained its green colour throughout the studied period, but the Sorbus trees started to turn red in mid-September. Alnus remained green for two reasons: It maintained higher chlorophyll concentrations, and higher ratios of chlorophyll to other pigments.

Phänologische Trends bei den Waldbäumen in der Schweiz | Phenological trends regarding the forest trees in Switzerland

Numerous publications are devoted to plant phenological trends of all trees, shrubs and herbs. In this work we focus on trees of the forest. We take into account the spring season (leaf and needle development) as well as the autumn (colour turning and shedding of leaves) for larch, spruce and beech, and,owing to the lack of further autumn phases, the horse chestnut. The proportion of significant trends is variable, depending on the phenological phase. The strongest trend to early arrival in spring was measured for needles of the larch for the period between 1951 and 2000 with over 20 days. The leaves of the horse chestnut show the earliest trend to turn colour in autumn. Beech leaves have also changed colour somewhat earlier over the past 50 years. The trend for shedding leaves, on the other hand, is slightly later. Regional differences were examined for the growth of needles in the larch where the weakest trends towards early growth are found in Canton Jura and the strongest on the southern side of the Alps. The warming of the climate strongly influences phenological arrival times. Trees in the forest react to this to in a similar way to other plants that have been observed (other trees, shrubs and herbs).

A test of the coevolution theory of autumn colours: colour preference of Rhopalosiphum padi on Prunus padus

According to the coevolution theory of autumn colours, the bright colours of trees evolved as a warning signal towards parasites colonizing the plant in autumn. We monitored colonization of the aphid Rhopalosiphum padi on individual tress of Prunus padus in autumn and observed a strong preference of aphids for trees with green leaves. This is the first direct observation of a key assumption of the theory, that parasites avoid bright colours. Moreover our observations, compared with previous data gathered on the same species, suggest that aphids colonizing trees with green leaves develop better in spring than aphids colonizing trees with bright autumn colours, which is consistent with the second main assumption of the coevolution theory.

Products of isoprene metabolism

This minimap (Fig. 1) is one of two that illustrate different aspects of the same pathway. They are identical biochemically, but the Products of Isoprene Metabolism minimap has a specific purpose, to illustrate something of the fascinating variety of well known products that can arise from multiples of this five carbon compound (C10, C15, C20, C30, and C40) and much bigger. Isoprene itself can be produced by trees and plants and can create a massive emission into the atmosphere that sometimes results in blue hazes, as in the Blue Ridge Mountains of Virginia. At the other end of the scale, natural rubbers can be polymers of thousands of isoprene residues. Terpenes (C10) are products of two isoprene units and can provide color, smell, and flavor to many plant products. Limonene, citronellal, menthol, pinene, and camphene all speak for themselves. Geraniol contributes to the perfume of roses and geraniums and to the flavor of a wide variety of fruits. Plant and insect hormones and the side-chains of many vitamins and of chlorophyll involve varying multiples of isoprene units. Diterpenes (C20) are fundamentally important. They include phytol, the side-chain of chlorophyll, and retinal, which helps convert light into vision, as well as powerful plant hormones such as gibberellins. Tetraterpenes (C40) include the carotenoids, which provide not only color to tomatoes and carrots but also the light harvesting pigments for photosynthesis and autumn color of trees when the green has gone. The other pathway diverges at the C15 stage and in the historic past led to sterol-like compounds, the hopanoids, the little known pentacyclic compounds present in microorganisms and in sediments and crude oils. They are said to be the most abundant organic molecules in the biosphere, exceeding even that of cellulose. Sterol synthesis itself takes place via cholesterol and produces a wide variety of hormones including estrogens and testosterones with all their implications. Many of the sensual (and sexual) pleasures of life can thus be seen to derive from the same simple five carbon compound, isoprene. An altogether different aspect of isoprene metabolism is illustrated at the southeast corner of the minimap and shows the clinical value of drugs that act by inhibiting enzyme activity. Many breast tumors are sensitive to estrogen, which is metabolized from testosterone by the enzyme aromatase. Drugs such as Letrozol can inhibit the activity of the enzyme and thus block the production of estrogen in some tissues. Enlarged prostate is a common complaint of older men (such as me!), but Finaseride and other drugs can shrink the gland by inhibiting testosterone dehydrogenase, the enzyme that converts testosterone to its active reduced form. REAL-LIFE METABOLISM! Products of isoprene metabolism. Two minimaps on Isoprene Metabolism appear on my website, iubmb.org, one of which, Products of Isoprene Metabolism, is shown here. Its purpose is to illustrate something of the fascinating variety of well known products that can arise from multiples of this five carbon compound (C10, C15, C20, C30, and C40).

The coevolution theory of autumn colours.

According to the coevolution theory of autumn colours, the bright colours of leaves in autumn are a warning signal to insects that lay their eggs on the trees in that season. If the colour is linked to the level of defensive commitment of the tree and the insects learn to avoid bright colours, this may lead to a coevolutionary process in which bright trees reduce their parasite load and choosy insects locate the most profitable hosts for the winter. We try to clarify what the theory actually says and to correct some misunderstandings that have been put forward. We also review current research on autumn colours and discuss what needs to be done to test the theory.

Restless Spirits, Part One

FICTION Restless Spirits, Part One Patty Crow HALLIE WAS ON LOAN TO THE TELEPHONE-engineering group in Wartburg, Tennessee, working as a design engineer for a year or more. I love this job—never boring. Always problems to solve,fires to put out, and the occasional difficult person. I don't even mind being the "token Yankee." It's the same work, North or South of the Mason-Dixon line. At least the telephone company gave us a common joke, the unofficial motto, "Dial Tone Is My Life." Don't even miss Denver much. Miss the Rockies, though. She turned off the computer and picked up her briefcase, intending to cram it full of blueprints and papers. No. I am not taking work home this weekend. I need some time out. She grabbed up her purse and slung it over her shoulder, briefcase in her other hand. Hallie changed into sweat pants and shirt after she got home and spent the rest of the evening lounging on the sofa, watching mindless TV. In the morning, she woke up feeling somewhat refreshed, packed a fried egg sandwich, carrots and threw in an almond poppy muffin for good measure. After adding a thermos of coffee and some bottled water, she headed for the door, car keys jingling merrily in her hand. She opened the closet by the front door to grab her jacket and noticed the duffle bag on the shelf. Guess I'd better take it—winter's coming on— might need it. The duffle contained her emergency survival gear, sleeping bag, utility candles, waterproof matches and other necessities. Comfortably settled behind the wheel, she began to drive aimlessly into the mountains. As she drove higher, the air became crisper. No telephone poles, houses, or traffic distracted her from the awesome views and forests. Blue and purple hills tipped with sunlight rose in the distance. The stream close to the road was alive with wood ducks and mallards. J need this— balmfor the soul. Her imagination twisted and turned with the road. The pavement changed to dirt—dusty and lumpy. How many turns have I taken? Left or right? Am I a mite confused or lost? She pulled over, stopped the car, and got out, stretching as she looked around. Well, ifIfollow this path to the top ofthe hill maybe I'llget my bearings. Hallie struggled through overgrown hedges, brambles and trees as she made her way to the top of a hill. When she broke through the 51 brush, she was surprised to see a small cabin, barn and outbuildings in the little valley below. Ramshackle. Looks like an old-timey photo. I have to go see this. She tramped down the hill toward the cabin. Feels like I'm going through a time warp—back a hundred years. Makes my scalp tingle. Densely forested hills surrounded the cabin. No road led here, just a dim track through the undergrowth. The leaves were alive with brilliant autumn colors typical in the rural backcountry of Tennessee. In the distance, pearl and gray storm clouds began to billow. A light wind set free a kaleidoscope of orange, red and rust. Fallen leaves carpeted the damp ground giving off a pungent earthy aroma. She inhaled deeply, savoring the smell of fall. Doesn't look like anyone's lived herefor years. The logs and planks of the cabin had weathered to a shabby soft dove gray. Silver green and drab olive moss covered the sagging roofs. Cracked windowpanes and drunkenly tilted doors beckoned. Shadows from the trees made the cabin look cold and forbidding. Hallie shivered. Wild rosebushes, thick with clusters of bright russet rose hips and a few ragged blossoms, filled what must have been a sizeable vegetable garden. A dilapidated wooden fence that once defined the front yard, now stood broken and forlorn. An overgrown shale path invited visitors to come and stay a while. She spotted about two dozen mold-covered empty quart jars with zinc lids nestled in the weeds. Why are the canning jars scattered around? They aren't even broken. A rope swing hung from an old red oak tree. The tattered ropes and gray splintered seat moved...

The loss of green color during chlorophyll degradation--a prerequisite to prevent cell death?

During plant senescence, chlorophyll (Chi) is degraded to non-fluorescent Chi catabolites (NCCs; Fig. la). These linear tetrapyrroles accumulate in the vacuoles of senescing cells and, in many plant species, represent the final products of Chi catabolism (Matile et al. 1988; Krautler 2003). Despite the billions of tons of Chi dis appearing this way every year and the fascinating autumnal color change of deciduous trees resulting from it, most reactions underlying conversion of Chi to NCC have only recently been elucidated (for recent reviews, see Hortensteiner 1999; Takamiya et al. 2000; Krautler 2003). Chi breakdown is a multi-step pathway (Fig. la) aiming to increase pigment solubility and to abolish the photodynamic properties of Chi by complete disruption of the conjugated 7i-electron system. Taking this into account, Chi breakdown can be apostrophized as Chi detoxification (Hortensteiner 1999). As inferred from the structures of NCCs (Krautler et al. 1991), the most remarkable structural change is the oxygenolytic opening of the porphyrin macrocycle of Chi. This reaction is catalyzed by the joint action of two enzymes, pheophorbide a oxygenase (PaO) and red Chi catabolite reductase (RCCR) converting pheophorbide (pheide) a to a primary fluorescent Chi catabolite (pFCC; Fig. la; Rodoni et al. 1997a). Several lines of evidence suggest that the two enzymes interact during catalysis, thereby channeling the first porphyrin cleavage

Effects of the Time Interval between Arrival of Winged Females and Autumnal Color on Numbers and Reproductive Rate of Winged Females of the Maple Aphid (Periphyllus californiensis).

Effects of the Time Interval between Arrival of Winged Females and Autumnal Color on Numbers and Reproductive Rate of Winged Females of the Maple Aphid (Periphyllus californiensis).

Autumn colouration and herbivore resistance in mountain birch (Betula pubescens)

AbstractWe explored Hamilton and Brown's autumn signalling hypothesis in mountain birch (Betula pubescens). As predicted by the hypothesis, early autumn colour change (i.e. high degree of autumn colouration in September) was negatively correlated with insect damage the following season. Furthermore, as expected, indices of physiological stress (i.e. leaf fluctuating asymmetry) and reproductive investment (i.e. catkin production) were positively correlated with insect damage the following season. Indirectly, we also found support for the idea that the proposed handicap signal (i.e. early autumn senescence) might be associated with an honesty ensuring cost in terms of lost primary production. Further work is, however, required to determine whether the link between autumn colours and insect damage observed in this study is causal.

Bright autumn colours of deciduous trees attract aphids: nutrient retranslocation hypothesis

We propose an alternative hypothesis to the handicap‐signalling hypothesis, to explain the high number of specialist aphids on tree species having bright autumn colour. Since birch aphids actively seek the first yellowing leaves for breeding in autumn, it is obvious that autumn colour of foliage does not repel migrating aphids. We suggest that aphids use bright colours as a cue to detect individual trees and leaves that are good sources of nitrogen in the form of amino acids in autumn. The active formation of bright‐coloured pigments in leaves is needed to protect them from photo inhibition during energy consuming nutrient retranslocation under cold autumn conditions. During nutrient export from leaves, nitrogen is in the form of amino acids in the sieve elements and easily available for aphids. Therefore, bright colours may act as a signal of easily available high‐quality food for viviparous aphid migrants that are selecting suitable trees for their sexual offspring reproduction. The females of sexual generation grown on the better quality food probably can oviposit the over‐wintering eggs to the twigs in higher numbers, which may have an adaptive advantage in competition with conspecific females.

Decline in length of the summer season on the Kola Peninsula, Russia

By analysing records made in the northern taiga forests of the Lapland Reserve (Kola Peninsula, Russia) during 1930–1998, we unexpectedly discovered a decline in the length of the snow-free and ice-free periods by 15–20 days due to both delayed spring and advanced autumn/winter. Respective seasonal temperatures best explained the dates of all phenological phases: 1 °C shift in temperature was approximately equal to 2–5 day shift in phenology. However the phenological shiftsduring the observation period are much larger than could be expected from the slight (0.56 °C) drop in temperatures during August–September, suggesting that the biotic effects of a very slight cooling have been enhanced by one or more unknown factors. Although emissions of sulphur dioxide from the nickel-copper smelter at Monchegorsk may have contributed to the observed trend (via changes in regional radiative budget), we found no evidence of direct pollution impact on dates of birch autumnal coloration or birch leaf fall, which exhibited the largest (22 days) shift between 1930 and 1998. The detected phenological trends agree with an increase in winter (snow) precipitation in the study area by 44%; however, effects of precipitation on any of the investigated phenological phases were far from significant. Our results highlight the importance of phenological records for the assessment of past regional environmental changes, and demonstrates that the prediction of even the simplest biotic responses to the Global Changes requires a profound understanding of the interactive impact of abiotic factors on the ecosystem.

Out on a limb, or a new branch of signalling theory?

Could it be that one of nature's most dazzling displays is just a ‘keep out’ sign to insects? According to Hamilton and Brown's recent analysis 1xAutumn tree colours as a handicap signal. Hamilton, W.D. and Brown, S.P. Proc. R. Soc. London B Biol. Sci. 2001; 268: 1489–1493Crossref | Scopus (120)See all References1 of published data, the autumnal change in leaf colour of temperate deciduous trees is a handicap signal to their insect pests, revealing the commitment of the trees to defence. This is the first costly handicap signal to be proposed in plants 2xThe origin of autumn colours by coevolution. Archetti, M. J. Theor. Biol. 2000; 205: 625–630Crossref | PubMed | Scopus (90)See all References2. In animals, handicap signals are often behavioural: stotting in Thompson's gazelles being the most commonly cited example. The scope for handicap signals in plants is perhaps more limited, at least to our current view, but the area of plant–herbivore interactions is one in which they could evolve.Several conditions for a handicap signalling system between deciduous trees and their insect pests appear to be met. First, leaf colour change is costly. Greater percentages of chlorophylls than of carotenoids are resorbed before abscission, and, combined with the early cessation of photosynthesis and the production of other pigments in autumn leaves, it is difficult to view this process as a mere side effect of senescence. Second, the signal must be received. Some aphid species display a sensory bias towards yellow-green light, apparently preferring a dilute hue of the most attractive colour, which might provide a mechanism for host choice based on relative display intensity. Third, the signal should ultimately confer differential reproductive success. Pre-emptive antixenotic signalling, exploiting a pre-existing sensory bias, should protect the tree from both the effects of herbivory and any potentially damaging pathogens that their insect pests might be carrying.For the signal to remain honest, there must be a link between it and the quality (here defensive commitment) being signalled. In this case, there are two ways in which this might be true. Chemical changes in the leaf that result in the autumn colour display could be associated with the production of inhibitors that prevent germination of seedlings through the leaf litter. Aphids could therefore use leaf brightness as a means of assessing the overall energetic investment of the plant in defence. Alternatively, the colour change could be a visual call to arms: systems in which insect-damaged plants produce signals to attract other predatory insects are known 3xExperimental evidence that plants under attack may benefit from parasitoids. Fritzsche-Hoballah, M.E. and Turlings, T.C.J. Evol. Ecol. Res. 2001; 3: 553–565See all References3. Either way, the fact that a tree can produce a vivid display would provide useful information to insects seeking a host.The key, as always, lies in variation. The authors readily point out that only empirical tests will reveal the truth of their hypothesis, and suggest several potential veins of research at both the intra- and interspecific levels. In addition, some of their assumptions must be validated, particularly that specialist pests are more damaging to hosts than are generalist ones, and that the energetically costly autumn display is produced at the expense of some other function. However, this does not detract from a tantalizing idea that will no doubt spark an explosion of new research. Autumn will never be the same again.

Autumn tree colours as a handicap signal.

Many species of deciduous trees display striking colour changes in autumn. Here, we present a functional hypothesis: bright autumn coloration serves as an honest signal of defensive commitment against autumn colonizing insect pests. According to this hypothesis, individuals within a signalling species show variation in the expression of autumn coloration, with defensively committed trees producing a more intense display. Insects are expected to be averse to the brightest tree individuals and, hence, preferentially colonize the least defensive hosts. We predicted that tree species suffering greater insect damage would, on average, invest more in autumn-colour signalling than less troubled species. Here, we show that autumn coloration is stronger in species facing a high diversity of damaging specialist aphids. Aphids are likely to be an important group of signal receivers because they are choosy, damaging and use colour cues in host selection. In the light of further aspects of insect and tree biology, these results support the notion that bright autumn colours are expensive handicap signals revealing the defensive commitment of individual trees to autumn colonizing insect pests.

A Comparison of Multispectral and Multitemporal Information in High Spatial Resolution Imagery for Classification of Individual Tree Species in a Temperate Hardwood Forest

Multitemporal, small-format 35-mm aerial photographs were combined in a coregistered database to determine the relative value of spectral and phenological information for overstory tree crown classification of digital images of the Eastern Deciduous Forest. A one-hectare study site, located in a second-growth forest 15 km east of Morgantown, West Virginia, USA, was photographed from a light aircraft nine times from May to October 1997 using both true-color and false-color infrared film. Using this imagery, differences in the spectral properties and timing of phenologic events between tree species made it possible to discriminate four deciduous tree species, namely Liriodendron tulipifera, Acer rubrum, Quercus rubra, and Quercus alba, which made up nearly 99% of the trees at this study site. Optimally timed photography acquired during peak autumn colors provided the best single date of imagery, while photography from spring leaf-out was the second best. The best individual image band for tree species discrimination was the blue band. Classifications using all four spectral bands (blue, green, red, and infrared) and four dates (05/23/97, 06/23/97, 10/11/97, and 10/30/97) provided the best classification accuracies. Variable canopy illumination made digital classification of individual trees complex. A Likelihood Ratio test confirmed that the number of spectral bands included in the classification procedure (spectral resolution) and the number of dates (temporal resolution) significantly influenced the ability to identify tree species correctly. This study suggests that although multispectral data appear to be more valuable than multitemporal data, it may be possible to compensate for the limited spectral resolution of planned high-resolution sensors by combining multiple dates of low spectral resolution images.

Studies in Urban Ecology: the First Wave of Biological Invasion by Pistacia chinensis in Armidale, New South Wales, Australia

Biological invasions by deliberately introduced organisms seriously compromise the integrity of ecosystems, but their early stages have seldom been documented, especially in urban contexts. Since the 1940s, the small Asian tree Pistacia chinensis has been widely planted in streets and gardens in Armidale, New South Wales, Australia, where it makes a major contribution to the autumn colours that are exploited to promote the city. Numerous wild plants are now becoming established in and near Armidale. This ongoing biological invasion may have been accelerated by the large size of the initial (planted) population, and by the presence of efficient avian seed dispersers. It is now at the stage of the first wild generation reaching reproductive maturity. While it is probably still possible to halt the invasion, various human factors appear to make this an unlikely prospect.

NAPS 2000 preview

The thirty-second North American Power Symposium (NAPS) will be held this year at the University of Waterloo, Ontario, Canada, on 23 and 24 October, in the midst of the beautiful and unique autumn colors of the Canadian maple trees. The symposium is sponsored by the University of Waterloo in conjunction with the IEEE PES Power Engineering Education Committee (PEEC), and its main purpose is to provide a relaxed environment where university faculty and students together with industrial representatives working in the power engineering area can discuss and present their research ideas and results.

The Origin of Autumn Colours by Coevolution

We lack an adaptive explanation for a striking phenomenon, that of bright colours displayed in autumn by the leaves of many deciduous trees. The usual explanation is that it is simply a non-adaptive secondary effect of leaf senescence. A game-theoretic model of biological signalling provides an adaptive hypothesis for autumn colours showing that they can be the result of a process of coevolution between insects and trees: if leaf colour acts as a warning indicator of the tree's vigour to autumn parasite insects, trees can gain advantage from the reduction of parasite load and insects can gain advantage from location of the most profitable hosts to lay their eggs. The results of the model are consistent with Zahavi's handicap principle. Possible explanations for the origin of the system and evidence from natural history are discussed.