The reasons that clonal plants shift between sexual and clonal reproduction have persisted as a knowledge gap in ecological literature. We hypothesized that clonal plants’ shifts between sexual and clonal reproduction in different environments are driven by the relative costs of sexual and clonal reproduction. Moreover, we hypothesized plants prioritize sexual reproduction over clonal reproduction. To test these hypotheses, we determined the costs of sexual and clonal reproduction, and proportions of sexual and clonal reproduction of Caragana stenophylla along a climatic aridity gradient (semi-arid, arid, very arid and intensively arid zones) in the Inner Mongolia Steppe using several complementary field experiments. The cost of sexual reproduction increased while the cost of clonal reproduction decreased as climatic drought stress increased from the semi-arid to the intensively arid zones. The changes in the costs of these reproductive modes drove a shift in the reproductive mode of C. stenophylla from more sexual reproduction in the semi-arid zone to more clonal propagation in the intensively arid zone. However, because of the evolutionary advantages of sexual reproduction, sexual reproduction still held priority over clonal production in C. stenophylla, with the priority of sexual reproduction gradually increasing from the semi-arid to the intensively arid zones. Our study suggested that sexual reproduction has relatively high priority in propagation of C. stenophylla. However, if the costs of sexual reproduction are too high, C. stenophylla likely chooses clonal reproduction, and the ratio between sexual and clonal reproduction could be mediated by reproductive cost. These reproductive strategies reflect optimal resource utilization, and allow the persistence of both reproductive modes across stressful conditions depending on their evolutionary advantages.

Caragana species are commonly distributed in desert regions. We hypothesized that the photosynthetic traits of desert Caragana species could acclimated well to the desert environments, which are characterized by high temperature, intense radiation, and low moisture. To test our hypothesis, we studied the responses of photosynthesis to light, temperature and humidity, and fluorescence parameters of four Caragana species from desert region and one Caragana species from steppe region in Inner Mongolian Plateau. The desert Caragana species had higher light compensation point, light saturation point, optimum temperature for photosynthesis, and lower optimum relative air humidity for photosynthesis than the steppe Caragana species, and their net photosynthesis rates (Pn) were higher under high temperature, intense sunlight, and low humidity. The nonphotochemical quenching efficiency (NPQ) and minimal fluorescence (Fo) of the desert Caragana species were higher, and the values of maximum fluorescence (Fm), maximum quantum yield (Fv/Fm), relative quantum yield (Φ_PSII), and photochemical quenching coefficient (qP) were lower, except for C. korshinskii. The electron transport rate (ETR) of the desert Caragana species was greater or similar to that of C. microphylla. In summary, the photosynthetic traits of the desert Caragana species acclimated well to desert environment, although such acclimation might be still not sufficient. The order of photosynthetic acclimation of the four species to desert environment is: C. korshinskii > C. stenophylla > C. tibetica > C. roborovskyi.

Plant can alter reproductive strategies for adaptation to different environments. However, alterations in flowering strategies and sexual allocation for the same species growing in different environments still remain unclear. We examined the sexual reproduction parameters ofCaragana stenophyllaacross four climatic zones from semi-arid, arid, very arid, to intensively arid zones in the Inner Mongolia Steppe, China. Under the relatively favorable climatic conditions of semi-arid zone,C. stenophyllatook aK-strategy for flowering (fewer but bigger flowers and higher seed set). In contrast, under the harsher climatic conditions of intensively arid zone,C. stenophyllatook anr-strategy for flowering (more but smaller flowers and lower seed set). In arid and very arid zones,C. stenophyllaexhibited intermediate flowering strategies betweenK-andr-strategies. In semi-arid, arid and very arid zones, sexual allocation and sexual allocation efficiency (SAE) ofC. stenophyllawere high and the population recruitment might be mainly through sexual reproduction; in intensively arid zone, however, sexual allocation and SAE were very low, seed production was very limited and clonal reproduction might compensate for the decrease in sexual reproduction. Our results suggested thatC. stenophyllaadapted to the climatic aridity gradient by alterations in flowering strategies and reproductive allocation.