Journal of Ecology
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An Investigation of Two Benthic Algal Communities in Malham Tarn, Yorkshire
Journal of Ecology - Tập 41 Số 1 - Trang 174 - 1953
Effects of crown architecture and stand structure on light absorption in mixed and monospecific<i>Fagus sylvatica</i>and<i>Pinus sylvestris</i>forests along a productivity and climate gradient through Europe Abstract When tree‐species mixtures are more productive than monocultures, higher light absorption is often suggested as a cause. However, few studies have quantified this effect and even fewer have examined which light‐related interactions are most important, such as the effects of species interactions on tree allometric relationships and crown architecture, differences in vertical or horizontal canopy structure, phenology of deciduous species or the mixing effects on tree size and stand density. In this study, measurements of tree sizes and stand structures were combined with a detailed tree‐level light model (Maestra) to examine the contribution of each light‐related interaction on tree‐ and stand‐level light absorption at 21 sites, each of which contained a triplet of plots including a mixture and monocultures ofFagus sylvatica andPinus sylvestris (63 plots). These sites were distributed across the current distribution of these species within Europe. Averaged across all sites, the light absorption of mixtures was 14% higher than the mean of the monocultures. At the whole community level, this positive effect of mixing on light absorption increased as canopy volume or site productivity increased, but was unrelated to climate. At the species population or individual tree levels, the mixing effect on light absorption resulted from light‐related interactions involving vertical canopy structure, stand density, the presence of a deciduous species (F. sylvatica ), as well as the effects of mixing on tree size and allometric relationships between diameter and height, crown diameter and crown length. The mixing effects on light absorption were only correlated with the mixing effects on growth forP. sylvestris , suggesting that the mixing effects on this species were driven by the light‐related interactions, whereas mixing effects onF. sylvatica or whole community growth were probably driven by non‐light‐related interactions. Synthesis . The overall positive effect of mixing on light absorption was the result of a range of light‐related interactions. However, the relative importance of these interactions varied between sites and is likely to vary between other species combinations and as stands develop.
Journal of Ecology - Tập 106 Số 2 - Trang 746-760 - 2018
Forest productivity increases with evenness, species richness and trait variation: a global meta‐analysis Summary 1. Although there is ample support for positive species richness–productivity relationships in planted grassland experiments, a recent 48‐site study found no diversity–productivity relationship (DPR) in herbaceous communities. Thus, debate persists about diversity effects in natural versus planted systems. Additionally, current knowledge is weak regarding the influence of evenness on the DPRs, how DPRs are affected by the variation in life‐history traits among constituent species in polycultures and how DPRs differ among biomes. The impacts of these factors on DPRs in forest ecosystems are even more poorly understood.2. We performed a meta‐analysis of 54 studies to reconcile DPRs in forest ecosystems. We quantified the net diversity effect as log effect size [ln(ES)], the log ratio of the productivity in polycultures to the average of those in monocultures within the same type of mixture, site condition and stand age of each study. The first use of a boosted regression tree model in meta‐analysis, a useful method to partition the effects of multiple predictors rather than relying on vote‐counting of individual studies, unveiled the relative influences of individual predictors.3. Global average ln(ES) was 0.2128, indicating 23.7% higher productivity in polycultures than monocultures. The final model explained 21% of the variation in ln(ES). The predictors that substantially accounted for the explained variation included evenness (34%), heterogeneity of shade tolerance (29%), richness (13%) and stand age (15%). In contrast, heterogeneity of nitrogen fixation and growth habits, biome and stand origin (naturally established versus planted) contributed negligibly (each ≤ 4%). Log effect size strongly increased with evenness from 0.6 to 1 and with richness from 2 to 6. Furthermore, it was higher with heterogeneity of shade tolerance and generally increased with stand age.4. Synthesis. Our analysis is, to our knowledge, the first to demonstrate the critical role of species evenness, richness and the importance of contrasting traits in defining net diversity effects in forest polycultures. While testing the specific mechanisms is beyond the scope of our analysis, our results should motivate future studies to link richness, evenness, contrasting traits and life‐history stage to the mechanisms that are expected to produce positive net biodiversity effects such as niche differentiation, facilitation and reduced Janzen–Connell effects.
Journal of Ecology - Tập 100 Số 3 - Trang 742-749 - 2012
Preventing crown collisions increases the crown cover and leaf area of maturing lodgepole pine Summary
Crown collisions induced by tree sway are hypothesized to reduce crown closure and leaf area in maturing cold temperate forests. These declines are thought to lead to the decline in productivity when a stand ages. We tethered groups of lodgepole pine (Pinus contorta Dougl. Ex Loud. Var. latifolia Engelm.) trees in a web pattern at 10 m height, in four 15‐m tall stands in western Alberta, Canada, to determine whether preventing crown collisions would increase crown cover and leaf area. The stands all had less than 65% crown closure at the beginning of study. Photographs of the canopy were taken in each control and webbed plot in 1998 and at the same point in 2004. Six years after webbing, crown cover had increased by 14.4%, compared to a 2.1% increase for the control plots. Webbing also resulted in significant increases in mean branch length, leaf area per branch and foliage density of individual branches from top and middle sections of the crown. Polishing of branches, caused by chronic contact with adjacent trees, was three times as common on control trees compared to webbed trees. The mean leaf area per tree was larger for the webbed trees. Crowns of webbed trees were more symmetrical than those of control trees. Trees from webbed plots, however, had a decline in leaf area density. The branches of control trees were typically curved upward with twigs pointed inward, making the crowns more compact compared to the outwardly expanding crowns of trees from the webbed plots. The fact that crowns expanded laterally after webbing, despite little change in light regime, provides strong evidence against the hypothesis that loss of crown closure in maturing stands is caused by a lack of light. The study indicates that the decline in crown closure and leaf area in maturing and tall stands is at least partly related to wind‐induced sway of trees abrading the edges of crowns.
Journal of Ecology - Tập 94 Số 3 - Trang 681-686 - 2006
Climate modulates the effects of tree diversity on forest productivity Summary
Despite growing evidence that, on average, diverse forests tend to be more productive than species‐poor ones, individual studies often report strongly contrasting relationships between tree species richness and above‐ground wood production (AWP ). In the attempt to reconcile these apparently inconsistent results, we explored whether the strength and shape of AWP –diversity relationships shifts along spatial and temporal environmental gradients in forests across Europe. We used tree ring data from a network of permanent forest plots distributed at six sites across Europe to estimate annual AWP over a 15‐year period (1997–2011). We then tested whether the relationship between tree species richness and AWP changes (i) across sites as a function of large‐scale gradients in climatic productivity and tree packing density and (ii) among years within each sites as a result of fluctuating climatic conditions. AWP –species richness relationships varied markedly among sites. As predicted by theory, the relationship shifted from strongly positive at sites where climate imposed a strong limitation on wood production and tree packing densities were low, to weakly negative at sites where climatic conditions for growth were most suitable. In contrast, we found no consistent effect of interannual fluctuations in climate on the strength of AWP –species richness relationships within sites.Synthesis . Our results indicate that the shape and strength of the relationship between tree diversity and forest productivity depends critically on environmental context. Across Europe, tree diversity shows the greatest potential to positively influence forest productivity at either end of the latitudinal gradient, where adverse climatic conditions limit productivity and lead to the development of less densely packed stands.
Journal of Ecology - Tập 104 Số 2 - Trang 388-398 - 2016
Tree species diversity increases fine root productivity through increased soil volume filling Summary
Although fine roots (< 2 mm in diameter) account for a major share of the production of terrestrial ecosystems, diversity effects on fine root productivity and their mechanisms remain unclear. We hypothesized that: (i) fine root productivity increases with tree species diversity, (ii) higher fine root productivity is a result of greater soil volume filling due to species‐specific patterns of root placement and proliferation, and (iii) differences in fine root productivity and soil volume filling associated with tree species diversity are more pronounced in summer when plants are physiologically active and demand for water and nutrients is at its greatest. We investigated the effects of tree species diversity on fine root productivity and soil volume filling of boreal forest stands that have grown naturally for 85 years on similar sites. Annual fine root production was 19–83% higher in evenly mixed‐ than single‐species‐dominated stands, and increased with tree species evenness, but not tree species richness. Fine root biomass was higher in evenly mixed‐ than single‐species‐dominated stands in summer months, but not in spring or fall. Higher fine root productivity in evenly mixed‐ than single‐species‐dominated stands was realized by filling more soil volume horizontally and vertically in the forest floor in the mixtures of deep‐ and shallow‐rooted species vs. the deeper mineral soil in the mixtures of deep‐rooted species. Synthesis . Our results provide some of the first direct evidence for below‐ground species complementarity in heterogeneous natural forests, by demonstrating that tree species evenness increases fine root productivity by filling/exploiting the soil environment more completely in space and time, driven by differences in the inherent rooting traits of the component species and variations of root growth within species.
Journal of Ecology - Tập 101 Số 1 - Trang 210-219 - 2013
Overyielding in mixed forests decreases with site productivity Summary
There is a rising interest in the role of species diversity in ecosystem functioning and services, including productivity. Yet, how the diversity–productivity relationship depends on species identity and abiotic conditions remains a challenging issue. We analysed mixture effects on species productivity along site productivity gradients, calculated from a set of abiotic factors, in two biogeographic contexts (highlands and lowlands). We compared the productivity of 5 two‐species mixtures (i.e. 10 cases of mixed species) with that of monocultures of the same species. Five main E uropean tree species were considered: sessile oak (Q uercus petraea L iebl.), Scots pine (P inus sylvestris L .), E uropean beech (F agus sylvatica L .), silver fir (A bies alba M ill.) and N orway spruce (P icea abiesL .) H . K arst). Our data set was compiled from the 2006 to 2010 F rench N ational F orest Inventory data base and covers 2361 plots including pure and mixed stands. Overall productivity of mixtures in highlands, that is E uropean beech–N orway spruce, E uropean beech–silver fir and to a lesser extent, silver fir–N orway spruce, was found to be higher than expected from the productivity of corresponding monospecific stands. Overyielding was mainly due to E uropean beech for the first two mixtures and to silver fir for the third one. No effect of mixture was found for sessile oak–S cots pine and sessile oak–E uropean beech stands in lowlands. Overyielding of sessile oak mixed with S cots pine was not strong enough to significantly increase overall stand productivity. Overyielding of E uropean beech was balanced by an underyielding of sessile oak. The mixture effect changed along site productivity gradients for six cases out of the 10 studied, with a stronger and positive effect on sites with low productivity. The magnitude of this change along site productivity gradients varied up to 89% depending on the tree species. Synthesis . The nature of species interaction in mixtures with regard to productivity changes with species assemblage and abiotic conditions. Overyielding is strongest when species grow in highlands on less productive sites. A negative link between mixture effect and site productivity was found, in line with the stress‐gradient hypothesis.
Journal of Ecology - Tập 103 Số 2 - Trang 502-512 - 2015
Competition for light and water play contrasting roles in driving diversity–productivity relationships in Iberian forests Summary
Mixed‐species forests generally sequester and store more carbon in above‐ground woody biomass compared to species‐poor systems. However, the mechanisms driving the positive relationship between diversity and above‐ground wood production (AWP ) remain unclear.
We investigate the role of competition for light and water as possible sources of complementarity among Iberian pine and oak species. Using tree core data from permanent plots, we test the hypotheses that (i) contrasting abilities of pines and oaks to tolerate shade will promote AWP in mixtures, while (ii) drought stress results in less room for complementarity.
We found that pine species receive more light, develop larger crowns and grow 138–155% faster when in mixture with oaks. However, this positive effect of species mixing on growth was severely reduced under drought conditions due to increased competition for water with neighbouring oaks. In contrast to pines, oak trees were less responsive to mixing, primarily as a result of their ability to tolerate shade and water shortage.
Mixed pine‐oak forests produce an average 48% more above‐ground woody biomass compared to monocultures each year. However, the magnitude of the diversity effect on AWP fluctuates with time, decreasing noticeably in strength during drought years.
Synthesis . Complementary light use strategies among neighbouring trees are critical in explaining why above‐ground wood production (AWP ) increases in mixed‐species stands. In contrast, drought causes trees in mixture to compete more fiercely for below‐ground resources, leaving less room for complementarity and causing positive diversity effects to lessen in strength. Together, these two mechanisms provide much needed context for AWP –diversity relationships in Mediterranean forests. Whether or not managing for mixed pine‐oak forests proves to be beneficial for AWP is likely to depend on how climate changes in the Iberian Peninsula.
Journal of Ecology - Tập 102 Số 5 - Trang 1202-1213 - 2014
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