The knowledge about the distribution of herbaceous angiosperms along elevational gradients worldwide is still scarce as only a limited number of extensive studies have been conducted. For example, in a transcontinental comparison, Cicuzza et al. (2013) studied the distributional patterns of tropical herbaceous angiosperms. They found that the local difference of herbaceous angiosperms species richness is influenced by factors such as temperature, elevation, and actual evapotranspiration. Additionally, they found that elevation promotes the herbaceous angiosperm richness but there was no relation with precipitation. They did not found a clear explanation for that pattern but suggested that apparently the effect of higher moisture in highlands, as well as evolutionary legacies, could explain it. In general, the treeline marks the limit of herbaceous angiosperm richness with a decrease close to and beyond this area (Wesche et al. 2008).
Desalegn & Beierkuhnlein (2010) investigated at the landscape scale the mechanism that drives the structure of herbaceous diversity in the Southwest Ethiopian mountains. They found that herbs presented a cumulative increase tendency from 1100 to 1500 m, a decrease at mid-elevations (1500-2000 m) and then a constant increase to upper elevations. They attributed that the herbs did not show a hump-shaped pattern due to the effect of an incomplete gradient. Therefore, from whole explicated variance, elevation had explained a low proportion of the variance (16%) and the change in bedrock, as an indicator of the nutrient gradient, explained 27% of the variance for herbaceous diversity.
In a global meta-analysis, Nogués-Bravo et al. (2008) analyzed 37 elevational species richness gradients in a range from 0 to 6000 m of terrestrial plants finding that the most common pattern of diversity is the hump-shaped (76%), followed by decreasing diversity with elevation (16%). In the case of the hump-shaped pattern, the peak of diversity was in average at 1364 m (SD ± 531 m).
Along an elevational gradient at the Mt. Kilimanjaro, Hemp (2005) recorded 858 herb species (70% of the total vascular plant richness). The peak of species richness was found
Chapter 1: General introduction
7 at 1700 m and most of the variance was explained by elevation (r= 0.91), mean annual temperature (r= 0.87), mean annual minimum temperature (r= 0.86) and pH (r= 0.81).
There are few studies realized in the Neotropics (Fig. 1.1), for example in Ecuador, along with a gradient from 1850 to 3000 m, where Homeier et al. (2013) studied the factors that are involved in the diversity patterns. They found 552 species at the lowest part of the gradient (ca. 1850 m), which was the site with highest species concentration. The authors explain that soil nutrient concentration and geographical gradients of some abiotic factors such edaphic, precipitation and landform conditions appear to be the major factors determining differences in elevational locations.
Figure 1.1 Elevational patterns of herbaceous angiosperm diversity in the Neotropics based on the following studies: Bolivia (Kessler et al. 2000), Ecuador (Homeier et al. 2013), Northern Mexico (Encina-Domínguez et al. 2007), Eastern Mexico (Krömer et al. 2013), Western Mexico (Vázquez
& Givnish 1998), and Venezuela (Márquez et al. 2004). The black line marks the general pattern of the six combined transects.
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In Venezuela, Márquez et al. (2004) studied the distribution of grasses along an elevation gradient between 2500 and 4200 m. They found 47 grass species along the gradient, with a richness peak at ca. 3500 m (Fig. 1.1). They attributed the elevational pattern to the phytogeographical origin and abiotic processes, such as low temperatures, high incoming radiation, water stress and slope aspect.
In Bolivia, Kessler et al. (2000) studied selected plant groups along an elevational gradient from 500 to 2450 m. They found 32 species of herbaceous angiosperms and a linear decrease of species richness with elevation (Fig. 1.1). They attributed the decrease of species richness to the high frequency of night frosts.
In western Mexico, Vázquez & Givnish (1998) studied several plant groups along an elevational gradient from 1500 to 2500 m. They found 181 terrestrial herbs with a peak of 42 species at 2000 m (Fig. 1.1). Understorey herbs were negatively correlated with elevation, the authors hypothesized this pattern due that at drier and lower elevations the habitats have more totally deciduous canopies and are more exposed to disturb. Likewise, elevation affects soil fertility and anti-herbivore defenses which can drop the diversity with elevation.
Encina-Domínguez et al. (2007) along with a gradient in Northern Mexico from 1590 to 3140 m recorded 171 herbs. The authors found a high diversity in mid-elevations (2100 m) due to the border effect of the transition areas between plant communities (Fig. 1.1).
Also, they attribute the high herb richness to the migrant effect, which is explained by the immigration of species between two different forest types. Finally, they also found a high beta-diversity attributed to a high environmental (climatic and edaphic) heterogeneity associated with the relief.
In the state of Veracruz, Eastern Mexico, Krömer et al. (2013) studied the effect of elevational locations area and climate on herb richness. They studied five taxonomical assemblages along three elevational gradients finding 50 herbaceous angiosperms. The elevational pattern depended on the taxonomical group, with specific groups that presented important variations related with elevation: Araceae presented a decrease with elevation, whereas there was an increase of Orchidaceae and Piperaceae (Fig. 1.1). The authors suggest to separate different groups of terrestrial herbaceous angiosperms in
Chapter 1: General introduction
9 order to obtain more clear elevational patterns and that herb layer is more related to bryophyte cover and precipitation.
All of these studies related the diversity and distribution patterns of herbaceous angiosperms with environmental variables (climate or precipitation) present along the elevational gradients. There are few works that include the impact of human influence on the biotic communities, as done by Kessler et al. (2001), which verified that herbaceous species are negatively affected by human disturbance. As well, Jácome-Reyes (2005) related the structure and composition of high montane herbs with temperature and elevation along an elevational gradient in Bolivia. They found that climate change and human disturbance in these zones might lead to variations in the dominance arrangements and an increase of invasive species from neighboring zones.
In studies concerning the anthropic impact on herb diversity (Cicuzza et al. 2011), those studies found that the systems with a high disturbance degree present an increase in total species richness and showed a considerable increase in richness in less disturbed forest-use categories. However, in more disturbed forest-forest-use categories, the richness widespread species was increased. Therefore, herbs were influenced by traits that control their range sizes in response to anthropological events (Lozada et al. 2008). Under extremely degraded environment circumstances, widespread herbs tend to display strong competitiveness, which is the reason why there is a dominance of such herbs in extremely interfered forest-use categories and the most sensitive group to degradation are the species with narrow ranges form (Kessler 2001). It is also known that human-intervened agroecosystems preserve many narrow-ranged species (endemics) and can contribute considerably to general species richness (Lozada et al. 2008).
The above-mentioned works give an overview of the arrangements of herb diversity along elevational gradients in the tropics; however, the available information is not concluding due to the high variation in different organism groups and several world regions (Cicuzza et al. 2013), therefore it is actually not possible to determine a general elevation pattern of herbs (Fig. 1.1). Even as shown in figure 1.1 there are contradictory patterns on elevational gradients, on one hand, there is an increase in species richness with elevation and on the other hand, the contrary occurs on other elevational gradients. Furthermore,
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10
there is even less information about the impact of human disturbance on the diversity and distribution patterns of herbaceous angiosperms under different biotic and abiotic conditions presents along an elevational gradient.
1.2 Deforestation as driver of species richness loss due to forest use