Impact of sex on aspen distribution and response to abiotic drought stress

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The profound impact of drought on the fitness, physiology and health of plant species has been studied across a wide range of conditions. However, the linkage between abiotic stress and sex, particularly in long-lived clonal tree species has yet to be explored. The long-term objectives of this program are to: 1) understand the role of sex in stress resistance and distribution of aspen (Populus tremuloides Michx.) clones on the landscape, and 2) assess potential trade-offs in ecophysiology, growth and wood quality traits as mechanisms to resist abiotic stress (ie: drought). By studying both a natural forest system where abiotic stress is occurring and through artificial greenhouse experiments using seedlings, I will investigate if drought stress differentially affects males and females at different ages and by measuring clone size in natural stands, determine if there is a trade-off in the cost of reproduction vs clonal expansion and growth under abiotic stress. Differential expansion of one sex vs the other, could lead to a separation of males and females on the landscape and ultimately cause a shift in the distribution of this species. With the recent discovery (2015) of a simple DNA-based marker for sex now available for aspen, we can begin to explore these questions in depth.****The literature tells us that males will outperform females in species where males and females are on different plants, because of a higher cost of reproduction in the female. The evidence to date in aspen is not convincing. Several factors appear to affect clone size including recolonization rates, fire return interval and moisture availability during flowering and seed dispersal. For example, clones tend to be smaller in the north where more recent recolonization has occurred and moisture is less limiting to seedling regeneration. Since sex has previously been determined when trees were flowering, it is unknown what the current sex ratio is across the landscape. A recent decline in aspen health across much of North America, due primarily to drought at the southern boreal forest fringe however, appears to be having a profound impact on the species range and may be having a serious impact on the sex distribution and ultimate persistence of this species on the landscape.****In this Discovery Grant, I will test the hypothesis that males are out-performing females under drought conditions due to a reduced need to expend energy in reproduction, skewing the distribution of males and females on the landscape. A competing hypothesis is that females are equally represented on the landscape with no added cost to reproduction and maternal investment due to the small cost of seed production in aspen (no endosperm). A second component of the program will test the hypothesis that clones with higher wood density, will exhibit greater water use efficiency, greater resistance to damage of the water transport system and ultimately be more drought stress resistant.******