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Knowledge of survival, life expectancy and life span is fundamental for understanding population dynamics (Harper 1977; Silvertown & Lovett Doust 1993) and evolutionary fitness (Silvertown 1991). Unfortunately, relatively few data on these three key demographic traits are available for plant species (Wright & Van Dyne 1976; West et al. 1979; van der Maarel 1996; Silvertown et al. 2001; Roach 2003) due to the complications associated with the demography of plants (Harper 1967, 1977). The lack of empirical demographic data has constrained our ability to predict plant population dynamics or understand the evolution of life-history strategies (Silvertown et al. 2001). Extensive empirical data on demographic parameters for species with different life-history strategies and habitat affinities could make generalization possible. For example, evidence that survival is constant with age for many herbaceous perennial forbs – a Type II survivorship curve (Deevey 1947; Harper 1967) – would greatly simplify estimation and modelling of survival for all species in this group.
One of the few ways to determine demographic parameters for populations of herbaceous plants is by long-term mapping of individuals in permanent plots (Weaver & Clements 1938; Dittberner 1971; Harper 1977; West et al. 1979; Silvertown & Lovett Doust 1993). The difficulty of the data collection and analysis are reasons why such analyses are rare. In the perennial grasslands of the North American Great Plains, Frederic Clements, John Weaver and their students left behind a valuable legacy: decades of annual maps of all individual plants in 1-m2 quadrats at a number of locations throughout the grasslands (Albertson & Tomanek 1965; Wright & Van Dyne 1976). These data have been available for much of the past century, but it is only with the recent development of geographic information systems that it has been feasible to extract the richness of the demographic information they contain (Fair et al. 1999).
The limited age-specific demographic data for herbaceous perennials has motivated the development of size and stage-specific methods (Lefkovitch 1965; Werner & Caswell 1977; Kirkpatrick 1984). Many authors have argued that size is a more important predictor of plant life-history attributes than age (Kirkpatrick 1984; Caswell 1989; Menges et al. 2000). Marba et al. (2007) applied allometric theory to the relationship between size and important life-history parameters and reported that mortality and birth rates scaled as the –0.25 power and life span as the 0.25 power of plant mass from phytoplankton to trees. However, testing the assumption that size may serve as a proxy for age requires both age and size-specific data.
Our objective was to evaluate survival, life expectancy and life span for 11 of the most common native grasses and 29 of the most common native forbs in the central Great Plains of North America. For each of these species, we estimated survival rates and life expectancy as a function of plant age, and constructed full life tables. Our estimates of survival, life expectancy and life span allowed us to make generalizations across species and functional groups for important Great Plains plant communities and to explain patterns of dominance within these communities. Additionally, we used general linear models to test relationships between age and size for 11 grass species and to compare the potential of age and size to explain variation in survival. We used these results to comment on a trend to use size-based analyses to estimate age-related parameters in comparative demographic analyses (Cochran & Ellner 1992; Silvertown et al. 1993, 2001).
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Our demographic analysis of 40 of the most important plant species in central North American Great Plains grasslands revealed several generalizations about survival, life expectancy and life span. First, much of the variation in survival was explained by life form: forbs had much lower survival rates and life expectancies than grasses. However, most grassland plants die young regardless of life form, with only a few individuals approaching each species’ maximum observed life span. Second, we found a relationship among life form, longevity and the shape of survivorship curves. Third, relationships between age and size were very weak at the scale of individual plants, which is the scale commonly observed in the field. These relationships were substantially stronger at the genet scale. Fourth, age was a more useful variable than size in predicting survival from one year to the next. Finally, the relative importance of a species was significantly and positively related to its demographic parameters: first year survival, life expectancy at age 1 and maximum observed life span.
Grasses on average had higher survival rates, longer life expectancies and longer maximum observed life spans than forbs. However, while grasses have the potential to live much longer than forbs, we found that even for the longest lived grasses most individuals die young. For all species (grasses and forbs), the highest life expectancy at age 1 was 6.5 years and the mean was 1.9 years. The average life expectancy at age 1 for grasses was 3.2 years. Only a few individuals escape early death (Appendices 1 and 2). Furthermore, it is almost certain that seedling mortality, occurring early in the growing season before our quadrats were censused, is even higher. Low early survival probabilities are commonly reported for herbs (Dittberner 1971; West et al. 1979; Silvertown & Dickie 1980; Mack & Pyke 1983; Ehrlén & Lehtilä 1998; Garrido et al. 2007).
The relationship between log survival and age provides us with another set of generalizations about survival across species and plant types (Tables 1 and 2). While strictly speaking all of our species had Type III survivorship curves (power exponents < 1), on average the forbs had larger power exponents than the grasses (0.635 vs. 0.534) suggesting they had closer to constant mortality with age than did the grasses. Both Types II and III survivorship curves have been commonly reported for herbs (West et al. 1979; Morris & Doak 1998; Hamann 2001; Silvertown et al. 2001).
Longevity and the shape of survivorship curves are related to life form (forbs vs. grasses), suggesting an evolutionary explanation. Reinforcing this finding is that the age at which life expectancy peaked was also correlated to maximum observed life span (r = 0.71, P ≤ 0.05). As demographic data on more species in these communities becomes available, phylogenetic comparisons could shed light on the evolutionary relationship between longevity and the type of survivorship.
Ehrlén & Lehtilä (1998) asked how perennial were perennial plants and suggested that accurate estimates of life span are crucial to our understanding of evolutionary relationships and population and community dynamics. The demographic literature on herbaceous perennial plants makes clear that there are very few field data on life span; most of the estimates are derived from stage-structured demographic analyses (Cochran & Elner 1992; Ehrlén & Lehtilä 1998; Silvertown et al. 2001). Further, most of the field data used to estimate matrix elements for stage-structured analyses does not distinguish between ramets and genets and therefore the convention is to assume they are the same even when it is known that a plant or a portion of the plants being analyzed are clonal (Silvertown et al. 2001). Our results suggest that the strength of the relationship between age and size is different for ramets than for genets and is much stronger for genets (Table 3). Furthermore, contrary to the assumption that size is a more important predictor of important demographic characteristics, we found that size was an inferior predictor of survival for perennial grasses, many of which are the dominant species in central North American grasslands. At both the plant and genet scales, models including age were more strongly supported by the data in 21 out of 22 cases (Table 4). Models that included both age and size were preferred in only three cases.
Table 4. Akaike's Information Criterion for regression analysis of first year survival for models that included age, size, age2, age × size or age2 × size for 11 species of grasses from Hays, KS, USA. Numbers in bold indicate the model with the most support for each species
|Species||Plant scale||Genet scale|
|Age||Size||Age2||Age × size||Age2 × size||Age||Size||Age2||Age × size||Age2 × size|
Plant communities in central North America that are not dominated by woody plants are dominated by grasses, not forbs. Grasses dominate Great Plains plant communities despite the fact that they often account for fewer than 15% of the herbaceous plant species (Lauenroth 2008). For each of three community types and an ecotone the 10 most important plant species were approximately evenly divided between grasses and forbs (Fig. 1). Yet for all communities, grasses occupied the highest ranks, accounting for an overwhelming portion of total species importance. In addition to their importance, the grasses are more constant in time than the forbs. Adler & Lauenroth (2003) used species–time relationships to show that the high rates of species turnover in these plant communities were driven by the rapid dynamics of the forbs.
Our data suggest that the species with the highest first year survivals, life expectancies at age 1 and maximum observed life spans are the most likely to be the dominant species in the central Great Plains communities we studied. While there is considerable variation around this relationship (Fig. 4), the fact that it exists for 40 species across three plant community types and an ecotone suggests that the pattern is robust. To be sure, a long maximum life span does not guarantee large relative importance. For example, B. hirsuta is one of the longest life span species in our data set (Table 2), but ranks in the top ten most important species in only the Little Bluestem community (Fig. 1). From a statistical perspective, however, a long life span increases the probability of dominance. The link between longevity and species turnover is even stronger, as demonstrated by recent work showing that longer life spans are related to greater species persistence through time (Ozinga et al. 2007). The higher survival, life expectancy and life span of grasses compared to forbs may provide a demographic explanation for community-level differences in the dominance and turnover of these two functional groups.
The relationship between longevity and dominance may depend on disturbance, an important influence on species composition and dominance in plant communities (Pickett & White 1985). The ability of some forbs to rapidly colonize disturbed sites compared to the slow recovery of perennial grasses (Albertson & Tomanek 1965; Coffin et al. 1996) suggests that forbs have much higher recruitment rates and that a survival–recruitment trade-off may exist in these herbaceous perennials. In sites with low disturbance rates such as our permanent plots, the survival–recruitment trade-off would favour the grasses with their high survival rates and long life spans. While survival and life span are commonly considered predictors of dominance in the forest ecology literature (Grime 2001; Lorimer et al. 2001), explanations for dominance in herbaceous communities rarely mention life span as an explanatory variable (Tilman 1988; Grime 2001). Evidence of a relationship between longevity and dominance from additional herbaceous plant communities would imply that basic life-history traits can be important in predicting community structure across a wide range of environments.
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Appendix S1 Life tables for forbs based on Kaplan-Meier survival estimates.
Appendix S2 Life tables for grasses based on Kaplan-Merier survival estimates.
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Please note: Wiley Blackwell is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.