Summary In the light of all that has been discovered about the mechanism of evolution it has become tempting to follow Darwin's lead and to “see no limit to this power“. Yet a careful examination of situations in which evolution is known to be occurring in some species, shows complete absence of evolution in others. This not because these species have not had the opportunity; in many situations there may even be uncolonised bare space. The explanation must lie in the supply of appropriate variation. A tacit assumption of evolution by natural selection is that the necessary variation is always available. Yet there is no a priori justification for this. Evidence from populations in nature, particularly of species which are potential colonists of old metal mine workings and similar metal contaminated habitats, shows that the species that successfully colonise these habitats, by the evolution of metal tolerant populations, possess within their normal populations the necessary variation. But those species which fail to colonise these habitats, despite the opportunity, do not possess this variation. This applies also at the level of the population, in the replicated evolutionary situations occurring under electricity pylons. Such evidence, together with arguments from theory, suggests that the failures of evolution have been as important as its successes in moulding the living world as we see it today, and that the reasons for failure must be sought at the molecular level in limitations to the origin of new variation. 相似文献
Background: The dwarf shrub Vaccinium myrtillus – with high cover, height, and fruit production – benefits capercaillie (Tetrao urogallus).
Aims: Our aim was to quantify landscape (e.g. elevation, geographic location, and precipitation), site (e.g. overstorey cover and stoniness) and very fine scale factors (e.g. spatial associations in the understorey) that affect cover, height, and fruit production of V. myrtillus in subalpine forests in thePyrenees, with understorey usually dominated by Rhododendron ferrugineum.
Methods: We sampled 155 plots (0.5 m × 5 m) in six sites. For each plot, in the understorey layer, we assessed species cover, height for R. ferrugineum and V. myrtillus, number of total fruits in V. myrtillus, and spatial associations among V. myrtillus and the remaining cover types.
Results: Overstorey cover negatively influenced V. myrtillus cover, its height, and particularly, the number of fruits, which was also negatively influenced by R. ferrugineum cover. Associations between R. ferrugineum and V. myrtillus were site dependent, while V. myrtillus showed mostly positive associations with grasses and mosses.
Conclusions: Reducing overstorey and R. ferrugineum cover has the strongest positive effect on increasing V. myrtillus fruit production, but with additional positive effects on V. myrtillus cover and height. Increases in grass and moss coverage could favour V. myrtillus. 相似文献
Reducing forest stand density through silvicultural thinning has demonstrated potential to mitigate drought impacts on growth; however, less has been studied on how changes in stand structure created by different thinning methods influence forest growth responses to drought. This research examined the growth responses to drought of natural-origin red pine in a long-term study contrasting thinning methods. Dendrochronological methods were used to examine growth responses during several drought events among stands where different thinning methods have been applied since 1950. Growth responses to drought were expressed as resistance (maintaining growth during drought), and resilience (regaining pre-drought growth). Results indicate that periodic thinning from above, which resulted in smaller diameters, has the potential to moderate drought-induced growth reductions. Larger tree diameters negatively influenced tree-level resistance and resilience across all treatments; however, the proportion of dominant trees in a stand had contrasting effects on stand-level drought responses. Stands thinned from above exhibited more complex vertical structure and increased stand-level resistance and resilience to drought-induced growth declines because competition is more stratified among smaller diameter trees. Opposite trends were observed in stands thinned from below, where the larger diameters and monolayered structure create greater competition among trees of similar size and crown position. The results of this study highlight the utility in managing for greater structural diversity to mitigate the negative effects of drought in red pine forest ecosystems. 相似文献
Temperature, water availability and photoperiod are the primary drivers of tree phenological processes. However, there is lack of information how the spring temperature and summer drought affect phenology of both cambium and needles. We evaluated the interplay between xylem and needle development of mature Scots pine trees in three consecutive years differing in the spring temperatures and onset and duration of summer drought. Cambial activity began on the day of a year (DOY) 83–87, while the bud break occurred on DOY 113–119, when also the secondary cell wall of tracheids started its formation. While the timing of bud break correlated with the sum of effective temperatures, the beginning of cambial zone activity did not. The needles were fully unfolded around DOY 170, which corresponded to the transition between earlywood and latewood tracheids. Summer drought did not affect needle development, but it changed the rate of production and morphology of latewood tracheids. Latewood tracheids from the year 2015 when the drought was longest (i.e. until the second half of August in 2015 compared to the duration from June until the middle of July in the two other years) were 32% narrower and they had 34% thinner cell walls than in 2014 and 2016. The improvement of tree water status in July resulted in the formation of intra-annual density fluctuations (IADF). The interplay between needle and cambium phenology provided an insight into how the tree allocates the resources with varying temperature and soil water availability. 相似文献
Rapid global climate change is resulting in novel abiotic and biotic conditions and interactions. Identifying management strategies that maximize probability of long‐term persistence requires an understanding of the vulnerability of species to environmental changes. We sought to quantify the vulnerability of Kirtland's Warbler (Setophaga kirtlandii), a rare Neotropical migratory songbird that breeds almost exclusively in the Lower Peninsula of Michigan and winters in the Bahamian Archipelago, to projected environmental changes on the breeding and wintering grounds. We developed a population‐level simulation model that incorporates the influence of annual environmental conditions on the breeding and wintering grounds, and parameterized the model using empirical relationships. We simulated independent and additive effects of reduced breeding grounds habitat quantity and quality, and wintering grounds habitat quality, on population viability. Our results indicated the Kirtland's Warbler population is stable under current environmental and management conditions. Reduced breeding grounds habitat quantity resulted in reductions of the stable population size, but did not cause extinction under the scenarios we examined. In contrast, projected large reductions in wintering grounds precipitation caused the population to decline, with risk of extinction magnified when breeding habitat quantity or quality also decreased. Our study indicates that probability of long‐term persistence for Kirtland's Warbler will depend on climate change impacts to wintering grounds habitat quality and contributes to the growing literature documenting the importance of considering the full annual cycle for understanding population dynamics of migratory species. 相似文献
The Mediterranean region is recognized as a global biodiversity hotspot. However, over the last decades, the cessation of traditional farming in the north part of the Mediterranean basin has given way to strong afforestation leading to occurrence of abandoned agricultural lands colonized by pioneer expansionist species like Pinus halepensis. This pine species is known to synthesize a wide range of secondary metabolites, and previous studies have demonstrated strong allelopathic potentialities of its needle and root leachates. Pinus halepensis is also recognized to release significant amounts of volatile organic compounds (VOC) with potential allelopathic effects that have never been investigated. In this context, the objectives of the present study were to improve our knowledge about the VOC released from P. halepensis needles and roots, determine if these VOC affect the seed germination and root growth of two herbaceous target species (Lactuca sativa and Linum strictum), and evaluate if soil microorganisms modulate the potential allelopathic effects of these VOC. Thirty terpenes were detected from both, needle and root emissions with β‐caryophyllene as the major volatile. Numerous terpenes, such as β‐caryophyllene, δ‐terpinene, or α‐pinene, showed higher headspace concentrations according to the gradient green needles < senescent needles < needle litter. Seed germination and root growth of the two target species were mainly reduced in presence of P. halepensis VOC. In strong contrast with the trend reported with needle leachates in literature, we observed an increasing inhibitory effect of P. halepensis VOC with the progress of needle physiological stages (i.e., green needle < senescent needle < needle litter). Surprisingly, several inhibitory effects observed on filter paper were also found or even amplified when natural soil was used as a substrate, highlighting that soil microorganisms do not necessarily limit the negative effects of VOC released by P. halepensis on herbaceous target species. 相似文献