Sex-specific trends imply that there will be winners and losers in the warmer world of the near future, and male trees may be more resilient than females under recurrent stressful conditions.
As Earth has warmed in the past 40 years, the frequency of males of a species of alpine plants grew at a pace that is rapid enough to affect population growth of the species significantly. This finding, reported in a study in the journal Science published on July 1, suggests that climate change may be affecting sex ratios in a way that can enable predicting the fate of some plant populations.
William K. Petry, a plant ecologist at University of California, worked with an alpine herb Valeriana edulis, or valerian, growing across a range of elevations to draw these conclusions. The advantage of different elevations was that it was accompanied by different climates. “We can simply travel up the mountainside to find a cooler, wetter climate or travel down the mountainside to find a hotter, drier climate,” said Petry in an email.
These contrasting features across altitudes mimic the climatic changes expected with time – the cooler, wetter climates can be used to represent the past and the warmer, drier climates the future – a convenient strategy for climate change scientists called space-for-time substitution.
A lucky encounter
However, Petry and his team team were aware that relying solely on contemporary data across a gradient may not serve as an adequate model for climate change. “Other factors change over elevation in addition to climate; for example, the occurrence of competitors or herbivores, the characteristics of the soil,” he said.
The team’s lucky break came when Petry chanced upon a thirty-year-old Ph.D. dissertation in the library of the field station he was working at, the Rocky Mountain Biological Laboratory. The dissertation was by Judy Soule, who, as a graduate student, had collected sex ratio data of a valerian population. “I contacted her to make sure I was resurveying the correct valerian populations, and that marked the beginning of our collaboration.”
Now armed with historic sex ratio data, Petry & Co. could resurvey the same populations that Soule studied and show much more confidently that climate change, not just over altitude but over time as well, affected sex ratios – increasing the frequency of males by about 6%. “Together these two lines of evidence make a strong case that climate is driving sex ratio change, and they underscore the powerful insights we can draw by pairing different approaches to studying climate change,” said Petry.
Male and female plants
The valerian plant is dioecious. This means it belongs to one of two sexes and requires fertilisation from an individual of the opposite sex, unlike most other flowering plants, which have both male and female organs in the same plants and can self-fertilise.
There is no dearth of studies that compare the responses of different species to climate change but not many have done this between opposite sexes of the same species. Petry and team wanted to fill this gap. They knew that plants from the same species belonging to different sexes have shown varying tolerance to environmental stress factors. “This is most likely because females often spend more energy on reproduction than males,” explained Petry.
Historic climate data over the last four decades showed that the mean temperature increased by 0.21°C per decade while the precipitation decreased by 1.91 mm per decade. Petri’s data reflected how, from 50% male valerian plants at the warm and dry lowest elevation, the number decreased to 22.7% at the cold and wet highest altitudes.
Since the proportion of male plants increases with temperature, climate change has resulted in higher proportions of males at greater altitudes, which were in the past dominated by female trees. Surveys of sex ratio in both 1978 and 2011 confirmed that males have become more frequent across the species’ elevation range at a rate of 1.28% per decade.
This is not the first study describing organisms that show shifts in sex ratio as a result of climate change. In 2012, scientists from the Chinese Academy of Sciences found that the males of a deciduous tree species benefited more from elevated carbon dioxide. They predicted that such sex-specific trends imply that there will be winners and losers in the warmer world of the near future, and males may be more resilient than females under recurrent stressful conditions.
However, Petry points out that the phenomenon is relatively rare in both plants and animals. “We don’t know whether climate-skewed sex ratios and the resulting impacts on seed production will be found in other plants because most studies of biological responses to climate change have focused on the species level (between species not within same species).”
At the same time, there’s no evidence yet that valerian plants can actively control the sex of their offspring in response to environmental conditions. “Biased sex ratios in valerian are more likely to arise from differences in the death rates of males and females rather than differences in the rates at which they are born,” clarified Petry.
Predicting the fate of valerian
The researchers also found that it was possible to predict sex ratios of the valerian plants by using their water use efficiencies as indicator. In general, they observed that males appear to be more tolerant of dry conditions than females.
Such fluctuations in the frequency of the sexes are likely to have a great impact because neither sex can reproduce without the other. “Any valerian population that is 100% female or 100% male cannot produce any seeds. The population may persist for a while without new generations to replace the old – especially in long-lived plants like the valerian. But eventually all individuals will die off and the population will go extinct,” cautioned Petry.
Even if things don’t get that extreme, a rarity of males at high elevations would limit the number of seeds that can be produced. As a result, there would end up being fewer valerian higher up. “If this is holding valerian back from colonising higher elevations, an increased seed yield due to climate change may allow this species to shift its range higher to track climate.”
So what about at lower elevations? “Here, females already have sufficient pollen because males are more frequent. Further increases in male frequency are unlikely to affect the number of seeds each female is able to produce.” Instead, said Petry, the population may face extinction if the few remaining females can’t collectively produce enough seeds to replace the plants that die each year.
Loss of females could have more serious repercussions, extending to higher levels of ecology, warn the authors. In the case of the valerian plant, their females support dramatically higher densities of insects than do males, including several specialist herbivores that depend on them exclusively.
In an article commenting on this study also published in Science, biologists Julie Etterson and Susan Mazer emphasised the need for more sources of historical data like the kind that Petry was lucky to come across. Usually such chances are very rare – for example, ancestral seeds have been found preserved in tundra soils or in seed banks. “They should help to more deﬁnitively answer the knotty question of whether evolution can rescue populations of native species, including the wild relatives of crops, from climate change across their geographic ranges,” say Etterson and Mazer in their article.