Journal of Vegetation Science最新文献

Aims

Geographic variation in species richness along elevational and latitudinal gradients may be controlled by energy, water, and productivity; however, spatial factors such as area and geometric constraints may also contribute. We use large mountainous oceanic islands, which exhibit considerable range in all proposed driver variables, to test established plant diversity models, such as the mid-domain effect (MDE), actual evapotranspiration (AET, energy), water–energy dynamics (WED: precipitation + potential evapotranspiration), and net primary production (NPP).

Location

We used published data comprising complete floras with elevation-specific occurrence information for seven mountainous oceanic islands (> 2000 m asl) in tropical and subtropical zones.

Methods

For each of the seven islands, plant richness was estimated within each 100 m elevation band (interpolated from maximum and minimum elevation). We used generalized linear models to evaluate the effects of area, MDE, AET, NPP (MODIS), and WED on each island and all islands simultaneously. The general WED model used in this study is a two-term model that includes a second-order polynomial function of PET and a linear function of precipitation. We use AIC and the proportion of explained deviance to identify the best model for explaining variation in plant richness along elevational gradients on mountainous islands.

Results

We found remarkably consistent patterns in which AET and MDE failed to offer a good explanation for species richness. WED was the best model when all islands were analyzed simultaneously; however, analyses on separate islands revealed that the precipitation term was not significant or negatively related to richness on five of the seven islands. The second-best model was NPP, whereas the best single predictor of richness was the polynomial expression of PET. The spatial variables, area, and the autocorrelated masl were strongly correlated with the residuals of the weak models.

Conclusions

Based on these observations, WED and NPP are superior in explaining richness on mountainous islands, whereas MDE and AET have low explanatory power. Precipitation has a negative correlation with species richness in five out of seven islands.

Aims

Do the effects of an understory shrub on woody seedlings explain the distributions of relict conifer trees at a regional scale?

Location

Three natural Cedrus libani reserves from Lebanon.

Methods

Using an observational approach, we quantified associations of recruits of woody species at the center and periphery of the spiny shrub Juniperus oxycedrus in forests and gaps along a north–south climatic gradient of both winter rainfall continentality and summer drought. We quantified the associations of woody seedlings with shrubs using the Relative Interactions Index (RII) calculated for recruits of all species together (community scale) and for four groups of functionally different species. We then analyzed the variation of RIIs along different treatments. Vapor Pressure Deficit (VPD) was quantified in each treatment combination, and its relationship with species-group RIIs and abundance was evaluated.

Results

At the community-scale, shrubs showed likely higher negative effects on woody seedlings at their center than at their periphery. In contrast, there were contrasting responses at the species-group level, with overall negative associations for Cedrus, positive associations for a group including Abies cilicica, and intermediate responses for two other groups. Positive associations strongly varied along the drought gradient and with the canopy treatment, while negative associations only slightly increased from forests to gaps. Positive associations varied together with VPD, peaking at an intermediate position along the drought gradient coinciding with the southernmost limit of Abies and declining at the dry extreme of the gradient where Cedrus is still present.

Main Conclusions

VPD appeared as a crucial driver of tree species occurrence and likely facilitation. Although manipulative experiments are needed to confirm the real occurrence of facilitative and competitive effects of the shrub, our study highlights the potential role of facilitation in explaining tree species distribution at regional scales.

Questions

Although communities may be differentially susceptible to establishment of exotics, broad predictions of community invasibility remain elusive. Here we have addressed which abiotic and biotic site characteristics were most strongly related to exotic plant cover in plant communities of the southeastern U.S. Piedmont and how effective these site characteristics were in distinguishing communities with successful persistence of exotic plant species from those without exotic plant species.

Location

Piedmont physiographic region, North Carolina, South Carolina, and Virginia, USA.

Methods

We analyzed 28 unique biotic and abiotic variables potentially associated with exotic species establishment using a dataset of 1363 vegetation plots. We developed logistic and binomial regressions to analyze exotic species cover as a function of predictor variables and a classification and regression tree to determine the relationship between exotic species establishment and community characteristics.

Results

Many soil characteristics (pH, exchange capacity, base saturation, and multiple nutrient variables) were positively and significantly related to exotic species establishment, while elevation and soil iron were negatively related. Many other variables were inconsistently related to exotic cover in these communities. Finally, the classification and regression tree analysis indicated that environmental and community conditions can be better used to explain where establishment was unlikely than where it was likely to occur. However, various combinations of wetland species cover, calcium, phosphorus, stem density, slope, and soil iron were most effective in predicting sites with establishment.

Conclusions

The significant positive relationship between exotic species cover and soil nutrients suggests that exotic species often succeed in fertile sites, and the relationship with wetland species suggests the importance of soil moisture, nutrient deposition, and propagule dissemination by water. Although many clues exist as to the potential for exotic species persistence, these factors do not fully explain the complex community interactions driving, and being driven by, plant invasions, including the potential lag in invasion timelines.

Questions

Change in species composition over time is the result of both interannual variability, that is, year-to-year fluctuations due to weather patterns or demographic processes, and directional change, following succession or changing climatic conditions. Quantifying each component is difficult due to the confounding effects of pseudoturnover (i.e., apparent turnover due to observer error). Can yearly-resolved vegetation plot time series be used to quantify the relative contribution of these components of change, while controlling for pseudoturnover?

Location

A European beech (Fagus sylvatica) forest in Central Apennines, Italy.

Methods

We developed an approach based on matrix decomposition and PERMANOVA to disentangle the effect of pseudoturnover, directional change, and interannual variability across nine permanent vegetation plots resurveyed for thirteen consecutive years, comparing the herb layer in a newly formed canopy gap, at the gap margins, and in the forest interior. We used helical graphs, generalized linear models, and non-metric multidimensional scaling to compare the timing and pace of vegetation change.

Results

Interannual variability and directional change accounted for similar shares of overall variation (26.7% and 28.9%, respectively). While pseudoturnover accounted for a modest 0.4%, ignoring it would result in a substantial overestimation of interannual variability. Overall, the herb layer reacted vigorously to disturbance-triggered changes in light conditions. Species richness increased from 11 to 23.3 in canopy gaps but remained stable at the gap margin and in the forest interior. The rate of change was 3.0 species/year immediately after disturbance and slowed down to 0.3 species/year after 11 years.

Conclusions

The composition of the herb layer varied substantially in the study period and showed a marked year-to-year variation even in the forest interior, where light conditions were relatively stable. A proper estimation of the interannual variability of vegetation, while crucial to benchmark the effects of disturbance in forests, should account for the confounding effect of pseudoturnover.

Aims

Lianas form an important component of tropical forests as they contribute to maintaining biodiversity. Thus, understanding the factors that control them is essential for forest management and biodiversity conservation. However, how topography and logging, as well as their interaction, affect liana community structure is not well understood. Therefore, we sought to evaluate the effects of topography and logging on the structure of liana communities in a tropical upland evergreen forest in Ghana.

Location

Tano Offin Forest Reserve, Ghana.

Methods

We assessed liana community structure by quantifying liana species diversity and composition, abundance, and basal area in nine 20 × 20 m plots, each of three topographic habitats (valley, slope, hilltop) in two land-use types (non-logged and logged-over forests) in the Tano Offin Forest Reserve, Ghana. We measured elevation and slope angle, and quantified host tree abundance and basal area for each plot.

Results

Topographic position influenced liana species diversity, abundance and basal area across the forest land-use types, with the valley habitat supporting a significantly greater values than the other habitats. In the same vein, topographic position was a significant predictor of liana species diversity, abundance and basal area in the logged-over forest, but not the non-logged forest. Selective logging also showed significant negative effects on liana species diversity, abundance and basal area, both across forest land-use types and within slope and hilltop habitats. Similarly, there were significant effects of topographic habitat and logging, and their interaction on liana species composition. Liana species composition was significantly driven by elevation, while liana abundance and basal area were predicted by elevation, slope angle, and tree abundance and basal area.

Conclusion

In view of our findings, forest managers should take into account the heterogeneity of topographic landscapes in their management operations and pay particular attention to areas that support higher plant assemblages, such as valleys. The findings further suggest that logging operations in topographic landscapes should be conducted in a manner that does not disrupt the topographic patterns of plant community structure.

Question

What are the reasons that tree gap dynamics are little found in conifer boreal and subalpine forests in upland small forest gaps (1 to 1.5 gap diameter/tree heights (D/H))? Location: High latitude conifer forests in western Canada.

Methods

We use a sample of 480 from 2103 small gaps created 40 years ago for forest water management to increase snow and delay melt to assess if tree gap regeneration occurs. We then used two published studies: a ray-trace model of solar irradiance into gaps (Musselman et al. 2015) and a Gap Radiation Model (GaRM) (Seyednasrollah and Kumar 2014) to explain the net short and long wave radiation and snow accumulation and melting in small forest gaps, in large forest clearings, and in closed canopy forests.

Results/Discussion

We find that tree regeneration is rare in high latitude conifer forests because small gaps (1–1.5 diameter gap/tree height) accumulate deep snow that persists into spring and prevents regeneration of trees. Besides the shorter growing season, the cause may be several species of parasitic snow fungi since seed sources are nearby, adjacent conifer trees do not reach into gaps, and the forest floor is not the best for tree regeneration. Finally, the short return time of large lightning-caused crown fires sets the existence time of these small gaps.

Conclusion

Low net short wave radiation in gaps at latitudes greater than ~40° North leads to deep accumulation and slow melt in the spring of snow in these small gaps compared to closed canopy conifer forests or larger clearings. The result is little or no tree regeneration and thus little or no tree gap dynamics.

Aims

Root traits are critical for resource acquisition, particularly in nutrient-limited environments such as open savannahs, and any changes in these traits can impact ecological processes. However, it is poorly understood whether invasive species outcompete natives by competitive superiority in nutrient acquisition (‘exploitation competition’) or by interfering with natives by space occupancy (‘interference competition’).

Location

Cerrado, south-eastern Brazil.

Methods

We selected five sites of open savannahs, each consisting of two paired subsites, that is, non-invaded or invaded by Urochloa grass species. We assessed community-level root biomass and depth distribution in the upper 1 m of soil. We measured root biomass and functional traits for fine roots (< 2 mm) classified as absorptive and transport roots in the uppermost soil layers (0–30 cm).

Results

Invaded and non-invaded plant communities differed primarily in root trait and biomass in the upper soil layer (0–10 cm). Invaded communities showed higher root biomass and root length density (RLD) compared to non-invaded communities, evidencing space occupancy through a larger root system. Species in invaded communities had roots with larger diameters, but lower root tissue density (RTD) and lower root dry mass content (RDMC), commonly associated with fast root spreading and expansion, compared to species in non-invaded communities. Contrarily, non-invaded communities presented a higher proportion of absorptive roots.

Conclusions

Functional dissimilarities in traits related to space occupancy indicate that interference competition plays a bigger role than exploitation competition in the invasion of Cerrado by Urochloa species. Invaded savannahs are characterized by root traits and biomass allowing effective neighbor suppression. As changes in root traits such as RTD can cause modifications in ecosystem functioning, our results suggest that invasive species may modify ecosystem processes that hamper the restoration of invaded savannahs.

Questions

A non-random spatial distribution of species in relation to environmental factors is an important mechanism for maintaining high tree diversity in tropical forests. Niche theory predicts that competing species should use the environment differently. However, we have only limited information on the extent to which environmental dependence and the relative importance of different types of environmental factors vary between species. It is also unclear how the environmental dependence differs according to life stage and species.

Location

Here we investigate the environmental variables that determine the spatial distribution of tree species in a 25-ha plot of mixed-dipterocarp tropical rainforest in Sri Lanka.

Methods

We compiled data on the spatial distribution of recruits, saplings, and adults of 57 tree species, as well as topographic and soil variables, and applied methods of spatial point process theory to estimate parametric spatial intensity functions for each life stage of the species as a function of environmental variables.

Results

Most species distributions were significantly associated with at least one environmental variable, with elevation and the first principal component of soil nutrients being the most important ones. With few exceptions, species showed an intermediate strength of environmental dependence, and we observed a striking similarity in the intraspecific environmental dependence between life stages. Finally, only a few species showed for the same life stage strong positive or negative correlations in their intensity functions, while most species pairs showed only weak or no correlations at all.

Conclusions

Taking together, our results indicate that the distribution of most of the more abundant species in our forest plot is influenced by local heterogeneity in environmental conditions, and that their environmental preferences lead to a spatial arrangement where competing species use the environment somewhat differently. Overall, our study provides a nuanced understanding of the complex environmental dependencies that shape tropical rainforest ecosystems at local spatial scales.

Questions

Which components of the fire regime affect plant species diversity? Does pre-fire vegetation physiognomy influence the effect of the fire regime? Which species are favoured by different fire regimes?

Location

Natural Reserve of Monte Catillo (central Italy).

Methods

We sampled vegetation in 58 units across areas that underwent different fire regimes and were originally occupied by different vegetation physiognomies.

We used plot-level species diversity calculated through Hill numbers in (generalised) linear models with components of fire regimes (time since the last fire, fire frequency and severity) and pre-fire physiognomy as explanatory variables.

We identified species related to different levels of fire frequency, severity and time since the last fire through indicator species analysis.

Results

Species richness (q = 0) was negatively related to time since last fire, while Shannon (q = 1) and Simpson (q = 2) diversity were influenced by the severity of last fire, with positive and negative relationships, respectively. Shrubland pre-fire vegetation interacted significantly with severity effects on diversity.

All components of fire regimes favoured annual and chamaephytic species with a Mediterranean distribution, while areas subjected to older, less severe and less frequent fires were characterised by tree species, including deciduous ones, and paleotemperate herb species.

Conclusions

Recent fires increase vascular plant species richness, but only temporarily. Severe fires determine relevant shifts in community dominance. Frequent and severe fires favour the spread of Mediterranean herb species in areas potentially occupied by thermophilous forests dominated by a mix of deciduous and evergreen tree species.