Early snowmelt projected to cause population decline in a subalpine plant
New Publication
article written by MRI
19.06.19 | 01:06

Although negative impacts of climate change will ultimately occur by driving populations to extinction, we know remarkably little about such impacts on plant demography. Most long-term research focuses instead on shifts to early blooming. The present paper shows that climate change is expected to cause negative population growth in a plant population within a few decades.

Early snowmelt is associated with reduced vital rates, with the effects on seedling establishment and seed production especially important to population dynamics. The negative impact is expected even without the changes in floral display so evident in other plant species in the same subalpine community. Thus, these mountain plant communities are at risk from declining snowpack.

 A critical challenge in predicting the biological impacts of climate change is to understand how the new environmental conditions will influence decline or growth of a population. Climate change has reduced the extent of snow cover in the Northern Hemisphere since the mid-20th century (e.g., by 11.7% per decade in June) and is projected to further reduce it as the average global temperature rises. In mountainous areas of the western United States, the snowpack water equivalent has declined, and, in at least some of those regions, the average date of snowmelt in the spring has advanced. That earlier snowmelt has been associated with shifts to earlier phenology in a large number of species, including blooming times in flowering plants. Whereas phenological shifts of plants in response to climate change are well documented, rarely is the impact of the phenological shift on reproduction known. In general, few studies examine associations of climatic variables with the demography of individual plant populations. However, it is ultimately through changes in demographic vital rates that a population will persist or not in the face of environmental change.

An understanding of how climate influences plant demography can be separated into two problems: (i) how climate influences a vital rate and (ii) how that vital rate influences population growth. Studies of the climate sensitivity for any vital rate are relatively rare for plant populations, but that gap is most striking for the rate of reproduction through seed production, as reproduction is the vital rate that would be impacted by a shift in blooming time. Studies of how climate influences seed production have often focused on long-lived tree species, especially those exhibiting masting behavior. However, for tree species, there are only a handful of studies assessing seed limitation to population growth. In contrast, for herbaceous species that lend themselves more easily to studies of seed limitation, studies of trends in fecundity that last longer than 10 y are extremely rare, especially in comparison with the large literature on masting trees. Only one study as it stands today, has combined these two types of information. Short-term experimental manipulations of climatic factors, including drought, CO2, and temperature, have generally found larger effects on viability than fecundity components of fitness, but long-term consequences need not necessarily reflect responses seen in short-term experiments.

Here the paper reports on a long-term study that integrated (i) the responses of vital rates to environmental conditions and (ii) the impact of those vital rates on population growth in the same plant species. It includes 15 y of data beginning in 2001 on how annual snowmelt date in the spring influences fecundity in the herbaceous subalpine plants Ipomopsis aggregata and Ipomopsis tenuituba (Polemoniaceae) and their hybrids. Plants were studied at two sites in the Colorado Rocky Mountains, a site at 2,900 m a.s.l. where I. aggregata grows and another at 3,050 m a.s.l. in a natural hybrid zone. Near these sites, the date of snowmelt has advanced by 1.34 d per decade over the years of 1935–2016. For these plant species, there are also 25 y of demographic data on how survival and reproductive success influence the finite rate of increase for a population. Common garden data were analyzed with integral projection models incorporating the dependence of vital rates on snowmelt and then projecting when early snowmelt is likely to drive the populations into decline.

Impacts on Population Growth.

Demographic impacts of snowmelt date were determined in two ways: (i) examining the impact of the changes in seed production by themselves, assuming no changes in other vital rates; and (ii) by using integral projection modeling (IPM) to incorporate impacts of changes in all vital rates. For the first approach, seed production required for positive population growth (i.e., finite rate of increase or λ > 1) was estimated from an age-structured model for an independent common garden experiment conducted between 1994 and 2006 at these same sites. For I. aggregata at the I. aggregata site, and assuming no changes in growth or survival, 77 undamaged seeds per reproductive plant were needed to maintain a stable population size. The analogous requirement was 74 undamaged seeds per plant at the hybrid site, using information for hybrids with I. tenuituba as the maternal parent, as natural hybrids almost always have that cytoplasmic genetic background. Mean seed production fell below that estimated replacement value for a stable population in 7 of 14 y at the I. aggregata site. At the hybrid site, mean seed production fell below replacement in 9 of 15 y, with SEs not overlapping the critical value in 8 of those years. Incorporating the percentage by which seed production differed in each year from values used in the original demographic model, and regressing the resultant value for λ on snowmelt date, the populations are predicted to be below replacement whenever snowmelt occurs before day 134 (14 May in nonleap years; SE = 20 d) or day 146 (25 May; SE = 20 d) at the two sites, respectively. Over the past 84 y, snow has melted 0.138 d earlier each year (SE = 0.051, linear-regression P < 0.01). Projecting a linear trend from the historical data on snowmelt date, the critical snowmelt day would be reached in 2035 at the I. aggregata site, with the hybrid site already past that point, although there is high variance around the historical trend line.

An important finding of this study is that the reductions in seed production with early snowmelt and reduction in snowpack water equivalent were sufficient to expect populations to decline with climate change. The population at the center of the hybrid zone appears already to be below replacement, as are nearby populations of I. tenuituba, as a result of scarcity of hawkmoth pollinators, even without taking into account trends in snowmelt timing or how the pollinators themselves might respond to climate change

The population of I. aggregata is predicted to fall below replacement within two to four decades unless the ecological loss in fecundity is outpaced by evolutionary adaptation, which would have to be very rapid. Uncertainty in these estimates is relatively high, and seedling establishment had to be estimated from a different site. The prognosis is for even more rapid decline if projections use only the more rapid observed snowmelt from the past four decades, although the impact of the high variance around the trendline was not explored, and variability in climate can have large demographic impacts depending on longevity. That uncertainty underscores the need for more long-term data of this sort to refine our ability to predict the impacts of climate change. However, sufficient demographic data even to make a prediction about average date of population decline, as done here, are very rare. Temporal studies of plant responses to climate have tended instead to focus on phenological changes in blooming, changes in distribution, or, more rarely, change in sex ratio. Phenology of flowering, in particular, is one of the most visible signs of climate change worldwide and has received much attention. There are abundant data on blooming date over four decades for 60 other plant species in this region of the Colorado Rocky Mountains, providing one of the longest data sets in the world on plant phenology, but it is rarely known whether the observed changes in blooming impact vital rates sufficiently to cause a population to decline. To understand the expected impacts of climate change, it will be important in future studies to address directly the impacts on population dynamics.


CITATION

 

Diane R. Campbell ‘Early snowmelt projected to cause population decline in a subalpine plant’, (2019) https://www.pnas.org/content/early/2019/06/04/1820096116?ct