Adaptive Signifi cance of Evergreen vs. Deciduous …
(2002). Adaptive significance of evergreen vs. deciduous leaves: solving the triple paradox. vol. no. article id .
Explain This: deciduous vs evergreen - Evenings - ABC …
A large body of empirical studies has found support for the GLH in a variety of treelines (e.g. , ; ; ; ). Yet, the most important piece of evidence against the GLH was observed in a treeline in situ free-air CO2-enrichment experiment, where and found support for both the CLH in deciduous Larix decidua and for the GLH in evergreen Pinus uncinata. They determined that L. decidua augmented its growth as a response to the addition of CO2, indicating that this species is potentially C limited at the treeline. One possible explanation for this contrasting response to CO2 enrichment might be the different leaf functional types (i.e. evergreen vs. deciduous) of the two species.
We found that evergreen species had higher LMA and CC, lower Wm, Nmass, Pmass, Amass, Rmass and PNUE and longer LLS than deciduous species at each elevation (Table ). Similar results have been reported in evergreen and deciduous species at local, regional and global scale (e.g. ; ; ). Our result therefore confirms that the deciduous habit can be considered as an acquisitive leaf strategy while the evergreen habit is a conservative leaf strategy, consistent with our Prediction (i). The different strategies of leaf habits have different advantages in coping with the environmental conditions. Deciduous species are able to achieve higher carbon gain and ensuing higher growth at a lower leaf dry mass cost, thereby conferring a competitive advantage over evergreen species, especially when being co-existing with evergreen species in an environment where resources are not strongly limited (; ). In contrast, evergreen species have longer LLS at a higher leaf dry mass cost. Longer LLS increases the mean residence time of nutrients in the plant, which improves the overall nutrient-use efficiency and extends the photosynthetic season. However, leaf photosynthetic capacity in evergreen species has been found to be relatively lower (e.g. ; ). Therefore, the main advantage of longer LLS in evergreen species resides in the higher nutrient retention potential in the plant, which enables them to be highly competitive and dominate in infertile habitats where natural selection favours traits such as longer LLS for their positive role in nutrient conservation ().
Adaptive significance of evergreen vs. deciduous …
Difference in MAT, soil total N, soil total P, soil C/N or soil N/P among elevations was tested with one-way analysis of variance after data log10-transformation. All species mean values of leaf economic traits were also log10-transformed prior to analysis to increase the normality of distribution (; ). Principal component analyses (PCAs) were performed using nine leaf traits (). Because of the high percentage of variance explained by the primary PCA axis, the primary axis species score (PASS) was used in the subsequent analyses as a proxy for leaf economics. Leaf economic distance along the primary axis, which was calculated as the absolute value of Species A score minus Species B score, could serve as a proxy for niche distance and reflect the competitive intensity between pairwise species (). In order to illustrate the influence of phylogeny on leaf economics, we firstly used linear regression analysis to test the relationship between PASS distance and phylogenetic distance across all species pairs. However, inter-elevational pairs could rarely co-occur in the same natural environment (e.g. pair of Nothotsuga longibracteata at high elevation vs. Castanopsis fargesii at low elevation; Fig. ) and the comparison of evergreen and deciduous species should be considered with their phylogenetic relatedness (). We then conducted linear regression analysis to test for the relationship between PASS distance and phylogenetic distance using all intra-elevational pairs of evergreen vs. deciduous species. These above analyses were conducted in SPSS 13.0 (SPSS, Chicago, IL, USA).
The patterns in the dominance of evergreen vs. deciduous tree species have intrigued ecologists for centuries, but remain incompletely understood (; ; ; ). Evergreen species tend to dominate sites where climatic seasonality is not distinct or where resources are difficult to obtain while deciduous species appear to be favoured wherever annual variation in temperature or precipitation results in marked favourable vs. unfavourable periods for carbon gain (; ). At the global scale, the relative frequency of evergreen species has a bimodal latitudinal distribution pattern. Evergreen broad-leaved species dominate tropical and subtropical regions, whereas evergreen needle-leaved species tend to inhabit in boreal regions. In contrast, deciduous broad-leaved species characterize temperate forests at mid-latitudes (; ).
In this lesson, we will examine them more closely
We found that PASS decreased with increasing elevation in both evergreen and deciduous species (Table ), indicating that both evergreen and deciduous species in general became more conservative with the increase of elevation. Thus, this finding is contrary to our Prediction (ii). The convergent effect of elevation on leaf economics could largely result from the fact that both evergreen and deciduous species tended to increase LMA and CC and decrease Wm, Amass, Rmass, Pmass and PNUE with increasing elevation. Moreover, the increase of LLS and decrease of Nmass with increasing elevation also contributed to the elevation-induced change of resource conservation in evergreen species (Table ). Note that the increase of resource conservation with increasing elevation in deciduous species in our study is contrary to the findings of and . The difference between our results and those obtained by them may be due to the fact that their measures of leaf traits were confined to a specific forest type where the co-existing species are under the same influence of climate but the length of favourable seasons could decrease with increasing elevation. In a specific forest type, the plastic responses of LLS in deciduous species to the elevation-induced varying length of favourable seasons could be adaptive for maximizing photosynthetic carbon gain (). However, the forest type, climate and soil resource availability along the elevational gradient in our study were complicated. The tendency of increasing resource conservation with increasing elevation in both evergreen and deciduous species in our study lends support to the argument that selection imposed by elevation on linked traits results in trait convergence along similar elevational gradients (). Of course, tree species did not respond to elevation directly but rather to a suite of factors such as temperature and soil resource availability that covary with elevation.
The ecophysiological mechanisms underlying the pattern of bimodal elevational distribution of evergreen tree species remain incompletely understood. Here we used leaf economics spectrum (LES) theory to explain such patterns. We measured leaf economic traits and constructed an LES for the co-existing 19 evergreen and 15 deciduous species growing in evergreen broad-leaved forest at low elevation, beech-mixed forest at middle elevation and hemlock-mixed forest at high elevation in Mao'er Mountain, Guangxi, Southern China (25°50′N, 110°49′E). Leaf economic traits presented low but significant phylogenetic signal, suggesting trait similarity between closely related species. After considering the effects of phylogenetic history, deciduous species in general showed a more acquisitive leaf strategy with a higher ratio of leaf water to dry mass, higher leaf nitrogen and phosphorous contents, higher photosynthetic and respiratory rates and greater photosynthetic nitrogen-use efficiency. In contrast, evergreen species exhibited a more conservative leaf strategy with higher leaf mass per area, greater construction costs and longer leaf life span. With the elevation-induced decreases of temperature and soil fertility, both evergreen and deciduous species showed greater resource conservation, suggesting the increasing importance of environmental filtering to community assembly with increasing elevation. We found close inter-specific correlations between leaf economic traits, suggesting that there are strong genetic constraints limiting the independent evolution of LES traits. Phylogenetic signal increased with decreasing evolutionary rate across leaf economic traits, suggesting that genetic constraints are important for the process of trait evolution. We found a significantly positive relationship between primary axis species score (PASS) distance and phylogenetic distance across species pairs and an increasing average PASS distance between evergreen and deciduous species with increasing elevation, implying that the frequency of distantly related evergreen and deciduous pairs with wide spreading of leaf economic values increases with increasing elevation. Our findings thus suggest that elevation acts as an environmental filter to both select the locally adapted evergreen and deciduous species with sufficient phylogenetic variation and regulate their distribution along the elevational gradient based on their coordinated spreading of phylogenetic divergence and leaf economic variation.
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deciduous trees might be expected to have higher rates of photosynthesis than needle-bearing evergreen trees.
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For all tissues examined in this study, NSC concentrations were always higher in deciduous species. This finding corroborates classical studies (; ; ). More recent studies have also found higher NSC concentrations in deciduous than in evergreen tree species. For example, compared growth rates and functional morphological and physiological traits between N. pumilio and N. betuloides juveniles in a non-treeline mixed forest (550 m a.s.l.) in Patagonia (Chile) and found that the former had higher NSCs than the latter; the difference between them was particularly high for roots (18·6 % for deciduous vs. 11·6 % for evergreen). working in mixed forests in the eastern USA found that deciduous species (Acer rubrum, Quercus rubra and Fagus grandifolia) had NSC concentrations of >24 mg g−1, while evergreen coniferous (Picea rubens and Tsuga canadensis) had concentrations of −1. Finally, also observed that deciduous species (Larix potaninii and Betula platyphylla) had higher levels of NSCs than evergreen species (Abies georgie, Juniperus saltuaria and J. tibetica) in the eastern Himalayas (China).
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Temperate winter deciduous species generally have higher levels of C reserves (NSC concentrations) than evergreens (; ). The classical explanation for the higher C storage pool in winter deciduous species is that these species need important amounts of NSCs to support spring growth and leaf-out, in contrast to evergreens which always have at least one photosynthetically active leaf cohort to provide the necessary C supply (; ; ). It has been found, however, that the higher NSC concentrations (particularly in older tissues) of deciduous species when compared with evergreens are maintained year-round (; ; ). This supports the idea that larger pools of C reserves in deciduous tree species may reflect an adaptation to tolerate stressors by means of replacing leaf area in cases when stressors provoke losses in photosynthetic area (). Compared with evergreens, deciduous tree species produce less robust leaves (shown by their higher specific leaf areas; SLAs), which makes them more prone to mechanical damage and eventual C limitations. What is more, in mountain ecosystems, spring frost and wind are two important causes of leaf area loss, which become more intense and frequent with elevation (). Thus, the limited number of experiments, as well as the bias of existing NSC data towards evergreen taxa, does not allow for a conclusive judgement about the general C supply status of trees at alpine treelines. In light of the ambivalent findings on C relations of deciduous trees at the treeline (; ; ), we may ask if the higher C storage pools and/or the differential seasonal use of C stores in deciduous vs. evergreen trees can lead to different C relations (different net C balances) between these two functional types in response to stressors. Few studies have compared C reserves in deciduous and evergreen treeline trees (; ), and none so far has worked with sympatric, phylogenetically close pairs of deciduous and evergreen species naturally growing next to each other in altitudinal treeline ecotones. Such studies should clarify whether treelines formed by deciduous tree species are determined by C limitation, as suggested by a previous study ().
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