Results observed in this study indicate that local species richness of woody plants is positively related to latitude and inversely related to altitude. Interestingly, these trends were observed along the coastal as well as the Andean range of the Mediterranean region of Chile. In addition, precipitation increased with latitude and minimal temperatures of winter and relative moisture were negatively correlated to altitude in this region, which may explain latitudinal and altitudinal patterns in local richness of woody species. In fact, these and others climate variables were also related to local species richness, which suggest that climate may be an important driver of spatial and in particular latitudinal and altitudinal variability of local richness of woody species in central Chile.
Many studies suggest that spatial and especially geographical variation of woody species richness is strongly modulated by climate (e.g. [5–8, 10, 11, 15–17, 35–37], but see ). However, most of studies relating climate to species richness have used regional scales to measure species richness (geographical quadrants of different latitudinal by longitudinal degrees). It is less known whether climate has also influence on geographical variation of local woody species richness (plots < 1 ha) (e.g. [15, 36, 37]), where other ecological processes, such as biotic interactions, disturbances, and even historical processes may have strong influences on local richness [2, 3, 18, 19]. In fact, important plant-plant interactions modulating species richness and abundance have been documented in central Chile [38–41]. Thus, results observed in this paper suggest that despite local biotic interactions may be playing a role in this region, climate may also be important in structuring spatial variation of local richness of woody species in central Chile.
Despite the global negative relationship between latitude and species richness [6, 42, 43], specific regions such as the Mediterranean region of Chile may have positive relationships between latitude and species richness, which probably contributes to the high variability reported for this global latitudinal pattern (e.g. ). In this study, the only climate variable significantly related to the latitude within central Chile was precipitation. Although the specific regression between precipitation and local species richness was not significant, the stepwise regression showed that precipitation and relative moisture of summer were more related to species richness than other climate variables. Hence, although precipitation is not the main variable explaining the geographical variation in species richness, it may contribute to produce the latitudinal pattern observed in species richness of woody species in central Chile.
On the other hand, many altitudinal patterns of woody species richness reported by different studies have shown negative trends [13–15, 18, 28, 36, 44], which is consistent with results observed in this study. These negative relationships have mainly been documented where there is no water constraints at the bottom of the altitudinal gradients, and decreasing temperature reduces growth and survival of an increasing number of species [14, 18]. Hence, although mean minimal temperature of winter was not the main variable explaining the geographical variation of species richness in this study, the negative relationship between them, along with the decrease of it with altitude, may explain the linear decrease of species richness with altitude on the coastal and Andean range of this region. However, many other studies have also reported unimodal altitudinal patterns of species richness, mainly where there are water constraints at the bottom of the elevation gradient such as in arid and semiarid regions [12, 15, 36]. This increasing water availability with altitude would enhance species richness up to constraints due to low temperatures reduce species richness again in higher altitudinal levels [15, 36]. Despite Mediterranean region of Chile may be considered as a semiarid region, no increment in precipitation with altitude, at least for localities used in the study, was found. In contrast, a reduction in relative moisture with altitude was observed. This probably occurred because no locality from the central valley between Andean and coastal ranges was included, where climate is drier than higher altitudinal levels of both mountain ranges [23, 34].
Greater local richness of woody species in the coastal than andean ranges of this region has also been observed at a regional scale of richness, and for different life forms . Mean minimal temperature of winter and relative moisture of winter and summer were higher on the coastal range. In addition, local species richness was positively related to mean minimal temperature and relative moisture of summer, hence these climate variables may explain this longitudinal variation in local richness of woody species in central Chile.
Regardless latitudinal, altitudinal and longitudinal patterns in species richness reported in this study, results observed here suggest that climate, and in particular water-related variables (precipitation and relative moisture), may be driving local richness of woody species in central Chile, although temperature patterns are also probably influencing distribution and thus richness of woody species in this region. Many other studies carried out in semiarid environments have also highlighted the role of climate and especially water-related variables on plant species distribution and richness (e.g. [15, 16, 20, 35, 45–49]). In central Chile, other vegetation processes and patterns, for example topographic variability, have also been associated to water-related variables. In particular, species composition and plant regeneration strongly differ between north-facing and south-facing slopes [38, 39, 41, 50]. Thus, climate is probably an important factor, not only driving geographical patterns in species richness, but also many other patterns and processes of plant communities in central Chile and other semiarid ecosystems. However, the relative importance of climate and other factors such as biotic interactions and disturbance, should be adressed in future research in central Chile.
The latitudinal variation of local woody plant species richness in the Mediterranean region of Chile described here is very similar to that documented for regional richness of woody species in this region [25, 26]. Similarly, coastal-andean differences in species richness observed in this study at a local scale of richness is similar to that observed at a regional scale . Therefore, it is possible to speculate that local and regional richness of woody species are positively related, which has been documented in some other studies (e.g. [51–53]). This suggests that factors driving patterns of richness at one scale may be the same or similar to those driving patterns of richness at other scales. Then, it is possible that both regional and local richness of woody species are strongly influenced by climate variability within the Mediterranean region of Chile.
Finally, as response to warmer and drier conditions predicted by climate change models for semiarid regions , it is possible to predict that, in the future, local richness of woody species will decrease in higher latitudes due to drier conditions, and increase in higher elevations due to warmer conditions within the Mediterranean region of Chile and probably other Mediterranean climates in the world.