The latitudinal gradient in plant height
Quantifying global-scale patterns in ecological traits and processes and understanding how environmental variables shape these patterns is an important goal for ecologists, both for developing our understanding of species’ ecological strategies and in terms of the current concern about the potential effects of climate change on the earth’s biota (if we do not know how climate affects present-day patterns in ecology, then it will be very difficult indeed to predict the likely impact of climate change). Ecologists have spent a great deal of time studying the latitudinal gradient in diversity (many taxa, including bats, plants, fish, mammals, termites and fossil foraminifera have been shown to be more diverse in the tropics, Rosenzweig 1995). There have also been many investigations of the latitudinal gradient in body size in animal taxa (Bergmann’s Rule, which tends to apply to endotherms, but not necessarily to exotherms, Mousseau 1997; Ashton 2002, 2004; Adams & Church 2008). We also have data on large-scale, cross-species patterns in some plant traits, including latitudinal gradients in wood density (higher at low latitudes, Swenson & Enquist 2007), seed mass (higher at low latitudes, Moles et al. 2007), extrafloral nectaries (more nearer the equator, Pemberton 1998) and some leaf traits (e.g. leaf N and P are higher at high latitudes, and leaf margins become more dissected and toothed at low temperatures (and thus away from the equator), Reich & Oleksyn 2004; Royer et al. 2005). There is also some evidence that rates of herbivory are higher in the tropics (Coley & Aide 1991; Coley & Barone 1996; Swihart & Bryant 2001). However, latitudinal gradients in many other ecologically important traits and processes remain undescribed. This study provides the first global, cross-species quantification of the latitudinal gradient in plant height.
Finding a latitudinal gradient in plant height is not terribly surprising. However, the slope of the relationship between height and latitude was impressive. The average maximum height of plant species growing within 15° of the equator is 29 times greater than the height of plant species growing between 60° and 75° N, and 31 times greater than the height of plant species growing between 45° and 60° S. As plant height is a central component of a species’ ecological strategy, this result suggests that there is a dramatic difference in plant ecological strategy between high and low latitude ecosystems.
The shape of the latitudinal gradient in plant height was somewhat surprising. We initially thought that the different climatic conditions found in the northern and southern hemispheres (especially during the colder northern winters) might lead to a steeper relationship between plant height and latitude in the northern hemisphere. However, we found no evidence for a difference in the latitudinal gradient between the two hemispheres, despite the high statistical power that comes from a large data set. This finding, combined with the relatively low explanatory power (R2) of the variable ‘minimum temperature of the coldest month’ suggests that low winter temperatures and the associated risk of freeze-embolisms are less important in determining maximum plant height than is water availability.
This study provided some support for the idea that there is a drop in plant height at the edge of the tropics, and the magnitude of the drop in plant height (2.4-fold) was substantial and comparable to the 7-fold step in seed mass at the edge of the tropics reported by Moles et al. (2007). Together, these findings suggest that there might be a sudden switch in ecological strategy at the edge of the tropics. The idea that there is a substantial and sudden shift in plant strategy at the edge of the tropics definitely merits further investigation. Once the existence of such a step has been confirmed, there are many important questions to be addressed. The current global data sets for plant height and seed mass do not have sufficient resolution to determine exactly how plant traits change around the edge of the tropics. Is there a sudden step in strategy, or a zone of rapid transition? Is the switch in plant strategy associated with environmental conditions (for instance, one might ask whether it was associated with the band of deserts around these latitudes)? It would also be interesting to ask whether changes in plant height and seed mass go along with changes in correlated life-history traits such as life span and time to first reproduction, or whether the scaling relationships among these life-history traits depend on environmental conditions.
Relationships with environmental variables
The relationships between plant height and environmental variables were all triangular (Fig. 2, Appendix S1). A wide range of plant height strategies was present at sites with low temperature, precipitation and/or productivity. While the driest, coldest, highest and most unproductive sites did lack the very tallest species, species above 10 m were present across most of the range of all of the environmental variables. In contrast, there were relatively few very short species at sites with high temperature, precipitation and/or productivity. The scarcity of very short species in these sites is unlikely to result from a sampling bias (e.g. people focusing on woody species in rain forests), as many entire floras from high-productivity sites were included in this study (including floras of Fiji, Norfolk and Lord Howe Islands, and Barro Colorado Island in Panama). The scarcity of very short species at high-productivity, warm, wet sites seems more likely to result from light attenuation through the canopy reducing light levels nearer the ground to levels below the carbon compensation point for understorey species.
Our finding that there are relatively few small species in highly productive, wet, warm sites is seemingly at odds with Niklas et al.’s (2003) findings from an analysis of Gentry’s world-wide data base on plant communities (Phillips & Miller 2002). Niklas et al. showed that communities in which the majority of the species were found in the smallest size class (but were rarely canopy dominants) were absent at high latitudes, but increased in number towards the equator. There are two possible factors that might explain this discrepancy. First, Gentry’s data are from forest plots, and the smallest size-class included was plants with a d.b.h. of 2.5 cm (Phillips & Miller 2002). The omission of herb- and shrub-dominated communities and the exclusion of small plants is an important difference between this study and Niklas et al. (2003), as small herbaceous species make up the majority of the diversity at high latitudes (Aarssen et al. 2006 and this study), and it is only the very shortest species that are missing from the highly productive sites in this study (Fig. 1). Secondly, Gentry’s data come from 0.1-hectare plots (Phillips & Miller 2002). Because of the negative relationship between plant size and the maximum density a species can attain in a community (Enquist et al. 1998), relatively few canopy individuals will be sampled in each plot, but a great many small understorey individuals would be sampled. Thus, in a diverse tropical forest, many species that are potential canopy species at a larger scale would appear to be present only in small size classes. This bias would be less pronounced in a temperate forest, where there are fewer canopy dominants in the regional species pool. That is, this size-based sampling bias could lead to an appearance of greater understorey diversity relative to canopy diversity in more species-rich plots.
We initially thought that the coldest temperatures experienced at a site would be an important variable, because extremely low temperatures expose plants to risk of freeze embolism (Sperry & Sullivan 1992). One might expect taller plants to be at greater risk of freeze embolism, because they have fewer, wider conduits in their trunks (Preston et al. 2006). However, mean temperature in the coldest quarter of the year and minimum temperature of the coldest month were the fifth and sixth strongest correlates of plant height. There is some suggestion in our data of an absence of very tall species at sites where the mean temperature of the warmest quarter is below 10 °C (Appendix S1), which would be consistent with the literature on temperature and tree line height (Korner 1998) but we do not have enough data from extremely cold places to be certain about this.
Minimum temperature was not the only poor predictor of plant height: other variables that provide information about climatic conditions during the harshest times of the year for growth were also relatively weakly correlated with height. That is, variables that capture information about the quality of the growing season are much more informative than are variables that capture information about difficult times when growth is low or entirely stopped. This makes sense: many species avoid growing at the harshest times of the year (for instance deciduous species at high latitudes and ephemeral species in deserts).
Altitudinal gradients in plant height are well known within species (e.g. Totland & Birks 1996; Fernandez-Calvo & Obeso 2004; Macek & Leps 2008), and increases in altitude are often associated with decreases in plant height within a region (e.g. Kappelle et al. 1995; Wilcke et al. 2008). However, altitude was surprisingly weakly related to plant height at a global scale. Despite being the second-worst correlate of plant height in this study (Table 1), a term for altitude was included in the top 5% of models 62% of the time, and altitude was a term in the model with the lowest RMSE. That is, although altitude explains very little of the variation in plant height at the global scale, the variation it does explain is complementary to that explained by the other environmental variables. Altitude has also proved to be a poor predictor in global studies of seed mass (Moles et al. 2007) and wood density (Swenson & Enquist 2007). Perhaps one reason for this is that the sudden drop in plant height found at the tree line occurs at different altitudes in different parts of the world.
The relationships between plant height and environmental variables were all relatively weak (Table 1). The weak relationships with environmental variables in this study are consistent with those found for seed mass (temperature, precipitation, NPP and LAI each explained <16% of the variation in seed mass, Moles et al. 2005) and leaf traits (climate explained just 18% of the variation along the principal multivariate trait axis, Wright et al. 2005). This simply reflects the fact that a great deal of variation in plant traits is between coexisting species. In this study, about half of the variation in plant height lay within sites, so environmental variables could only possibly explain 50% of the total variation in the data set. Increasing our understanding of the coexistence of such a wide range of height strategies at a single site is an important goal, but this requires a different sort of approach to that used for studies that quantify global patterns in plant traits.
The best model for global patterns in plant height contained just one of our original 22 environmental variables: precipitation in the wettest month. This result surprised us – we had thought that NPP would be the strongest correlate of plant height, because productivity depends on a range of variables, including soil fertility, temperature, available sunlight and precipitation (Krebs 1972). Precipitation at the wettest time of the year is obviously of primary importance in arid and semi-arid regions and in ecosystems with strong seasonality in water availability such as seasonally dry tropical forests. Evidence for a primary influence of water availability has also been found in some mountain systems (Littell et al. 2008). However, in very cold places it seems that temperature, rather than water availability, would be of primary importance (Korner 1998).
The importance of water availability in determining the height plant species reach in different parts of the world is in line with the hydraulic limitation hypothesis (Ryan & Yoder 1997; Ryan et al. 2006). This hypothesis begins from the observation that with increasing plant height the difficulty in supplying leaves with water also increases. To avoid embolisms caused by extremely negative water pressures, plants are forced to close their stomata, thus decreasing the amount of photosynthesis and diminishing the amount of carbon available for further growth.
This study provides the first cross-species quantification of global patterns in plant height, and our investigations of relationships between plant height and environmental variables turned up a range of interesting and surprising results. These are major advances, but there are still many questions we need to address in this field. Investigating the idea that there is a sudden switch in plant ecological strategy at the edge of the tropics seems particularly important. It would also be interesting to weight the species in studies such as this by abundance, so that extremely common species that dominate vegetation (such as black spruce –Picea mariana in boreal forests) receive more weight in analyses than do rare species, and to compare the slope of intraspecific relationships with that of the interspecific relationship. Finally, it will be interesting to go beyond correlations between traits and environment and formally link global patterns in plant strategy with a mechanistic understanding of the processes that affect plant growth and reproduction.