Linking growth strategies to long‐term population dynamics in a guild of desert annuals

Abstract

Summary1 Combining long-term observational studies with comparative physiological ecology can yield a deeper understanding of the contribution of individual function to population and community dynamics. Sonoran Desert winter annuals exhibit striking yearto-year variation in population dynamics that is driven by variable precipitation, but species differ in the strength of demographic response to precipitation and hence in the degree of temporal variance in population dynamics. To understand the physiological mechanisms of differing population dynamic responses to environmental variation, we investigated interspecific differences in functional traits that mediate responsiveness to precipitation. 2 We conducted sequential harvests throughout the growing season to examine relative growth rate and biomass allocation patterns. We then related growth parameters to leaflevel carbon isotope discrimination (a time-integrated measure of water-use efficiency) and long-term demographic variation. 3 We hypothesized that water-use efficiency should trade-off with rapid growth rates. Furthermore, we hypothesized that species having efficient water use should have buffered population dynamics in dry years but sacrifice high growth and fecundity in wet years, resulting in low long-term variance in demographic success. Conversely, species with high growth capacity should be very responsive to infrequent periods of high precipitation and thus exhibit high temporal variance. 4 Species differed in seasonal relative growth rate and allocation patterns. Species with the highest relative growth rates rapidly deployed large leaf area displays following mid-season rainfall. Species with intermediate relative growth rates exhibited high biomass assimilation rates per unit leaf area. Species with low relative growth rates exhibited low leaf area ratios and low assimilation rates per unit leaf area. 5 Relative growth rate was positively related to leaf carbon isotope discrimination, consistent with a trade-off between growth rate and water-use efficiency. 6 Seasonal relative growth rate did not predict long-term demographic variance. However, leaf area plasticity in response to precipitation was positively related to longterm demographic variance. Our results illustrate how morphological and physiological traits influence demographic tracking of environmental variability and demonstrate how species differences in functional strategies determine population and community dynamics.

Functional tradeoffs determine species coexistence via the storage effect

“…Such a tradeoff has been described across life forms, but we find the tradeoff within one functional guild ( Fig. 1) (23). Sonoran Desert winter annuals are arrayed along a tradeoff between relative growth rate (RGR) and a measure of intrinsic water-use efficiency (WUE), carbon isotope discrimination [⌬, where lower ⌬ indicate higher intrinsic WUE (24)] (Fig.…”

“…Species with high RGR exhibit low WUE, whereas species with high WUE have low RGR. Our prior work has identified the key morphological and physiological traits that underlie growth capacity and lowwater tolerance in these species (23,25). Species that display high growth capacity allocate a large fraction of biomass to photosynthetic surfaces and have the ability to rapidly deploy large leaf area displays to maximize growth after infrequent, large rainfall events (23).…”

“…Our prior work has identified the key morphological and physiological traits that underlie growth capacity and lowwater tolerance in these species (23,25). Species that display high growth capacity allocate a large fraction of biomass to photosynthetic surfaces and have the ability to rapidly deploy large leaf area displays to maximize growth after infrequent, large rainfall events (23). Conversely, species that display high intrinsic WUE invest a large fraction of leaf nitrogen in the photosynthetic processes that become limiting at low temperatures characteristic of postrainfall periods, which optimizes carbon assimilation for the short windows of time after small but relatively frequent rain events (25).…”

Contrasting trait responses in plant communities to experimental and geographic variation in precipitation

“…For example, drought tolerance is related to more conservative growth and resource use strategies (e.g. long leaf life span; Wright et al, 2004;Angert et al, 2007), water-conserving leaf traits (including high leaf N concentration; Wright et al, 2001) and larger seed masses (Baker, 1972;Leishman & Westoby, 1994;Pakeman et al, 2008). If these traits predict species' responses to precipitation, changes in community composition (including both relative abundance and species presence ⁄ absence) caused by altered precipitation should correspond with changes in community mean trait values.…”

Predicting Evolutionarily Stable Strategies from Functional Responses of Sonoran Desert Annuals to Precipitation

“…We expect these functional traits to trade-off, such that the ability to tolerate stress is negatively associated with growth capacity under favorable conditions. Indeed, Sonoran Desert annual plants exhibit among-species trade-offs in functional traits related to stress tolerance, such as water use efficiency, and those related to growth capacity, such as relative growth rate [14][15][16]. One approach to assess the fitness consequences of such trade-offs is through relating resource availability to two quantities: probability of reproductive success and yield upon reproductive success.…”