Plant communities bank against drought

Many plants shed their young embryos (seeds) into the soil where they may accumulate in a dormant (non-growth) state over years before germinating (resuming growth and development). Ecologists describe this collection of seeds as a seed bank.  Marina LaForgia describes how scientists were able to germinate and grow to maturity some 32,000 year old Silene stenophylla seeds that was stashed, probably by an ancient squirrel, in the permafrost! With increased climatic variation predicted by most climate models, she wanted to know how environmental variability might affect germination of particular groups of species within a community.  In addition, she and her colleagues recognized that most ecological studies investigate community responses to disturbances by looking at the aboveground species.  It stands to reason that we should consider the below-surface seed bank as a window to how a community might respond in the future.

LaForgiaSeedlings

Some seedlings coming up from the seed bank. Credit:Marina LaForgia.

Seed banks can be viewed as a bet-hedging strategy that spreads out germination over several (or many) years to reduce the probability of catastrophic population decline in response to one severe disturbance, such as drought, flood or fire. In some California annual grassland communities, species diversity is dominated by annual forbs – nonwoody flowering plants that are not grasses. Many forbs produce seeds that can lie dormant in the seed banks for several years. Though these forbs are the most diverse group, there are also about 15 species of exotic annual grasses that dominate the landscape in abundance and cover. These grasses dominate because they produce up to 60,000 seeds per m2, they grow very quickly, and they build up a layer of thatch that suppresses native forbs. However, seeds from these grasses cannot lie dormant in the seed bank for very long.

 

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Area of field site dominated by Delphinium (purple flower) and Lasthenia (yellow flower).  Looking closely you can also see some tall grasses rising. Credit Marina LaForgia.

How is drought affecting these two major components of the plant community? LaForgia and her colleagues answered this question by collecting seeds from a northern California grassland at the University of California McLaughlin Natural Reserve in fall 2012 (beginning of the drought) and fall 2014 (near the end of the drought). They used a 5-cm diameter 10-cm deep cylindrical sampler  to collect soil and associated seeds from 80 different plots.  The researchers also used these same plots to estimate aboveground-cover, and to identify the aboveground species that were present. The research team germinated and identified more than 11,000 seeds.

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Plants germinating in the greenhouse. Credit Marina LaForgia.

The researchers knew from previous work on aboveground vegetation that exotic annual grasses declined very sharply in response to drought.  In contrast, the native forbs did relatively well, in part depending on their specific leaf area (SLA) – a measure of relative leaf size, with low SLA plants conserving water more efficiently. It seemed reasonable that these same patterns would be reflected belowground. Recall that most grass seeds are incapable of extended dormancy, while many forbs can remain dormant for several years. Consequently, LaForgia and her colleagues expected that grass abundance in the seed bank would decline more sharply than would forb abundance. In addition, they expected that high SLA forbs would not do as well as low SLA forbs during drought.

The researchers discovered very sharp differences between the two groups over the course of the drought. Exotic annual grasses declined sharply in the seed bank, while native annual forb abundance tripled.  Aboveground cover of grasses declined considerably, while aboveground cover of forbs increased modestly.  Clearly the exotic grasses were suffering from the drought, while the forbs were doing quite well.

LaForgiaFig1

(a) Seed bank abundance of grasses (red circles) and forbs (blue triangles) at beginning of drought (2012) and near end of drought (2014). (b) Percent cover of grasses (red circles) and forbs (blue triangles) at beginning of drought (2012) and near end of drought (2014). Data are based on samples from 80 plots. Error bars indicate one standard error.

We can see these differences on an individual species basis, with most of the grasses declining modestly or sharply in abundance, while most of the forbs increased.

LaForgiaFig2

Mean change in seed bank abundance per species based on 15 exotic grass species and 81 native forb species.

It is not surprising that the grasses do so poorly during the drought.  Presumably, less water causes poorer germination, growth, survival and seed production.  In addition, because grass seeds have a low capacity for dormancy, grass abundance will tend to decrease in the seed bank very quickly with such a low infusion of new seeds.

But why are the forbs actually doing better with less water available to them?  One explanation is that grass abundance and cover declined sharply, causing the forbs to experience reduced competition with grasses that might otherwise inhibit their growth, development and reproductive success. The tripling of native forbs in the seed bank was much greater than the 14% increase in aboveground forb cover.  The researchers reason that the drought caused many of the forb seeds to remain dormant, leading to them building up in the seed bank. This was particularly the case for low SLA forbs, which increased much more than did high SLA forbs in the seed bank.

We can understand exotic grass behavior in the context of their place of origin – the Mediterranean basin, which tends to have wet winters.  In that environment, natural selection favored individuals that germinated quickly, grew fast and made lots of babies. Since their introduction to California in the mid 1800s, 2014 was the driest year on record.  It will be fascinating to see if these exotic grasses can recover when, and if, wetter conditions return.  Can we bank on it?

note: the paper that describes this research is from the journal Ecology. The reference is LaForgia, M.L., Spasojevic, M.J., Case, E.J., Latimer, A.M. and Harrison, S.P., 2018. Seed banks of native forbs, but not exotic grasses, increase during extreme drought. Ecology99 (4): 896-903. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

Saguaro survival: establishing an icon

Having grown up in the New York metropolitan area, my only contact with the saguaro cactus, Carnegiea gigantea, was from several TV westerns, which dubiously placed these mammoth cacti in New Mexico, Texas and Colorado.  In fact, the saguaro is limited to the Sonoran Desert of northwestern Mexico, extreme southeast California and southern and central Arizona. You won’t find these cacti further north, because a freeze lasting more than 24 hours kills them.  I still remember my first real sighting of these cacti; I was amazed at how distinct they seemed in comparison to the other vegetation, and I delighted in their abundance.

Daniel Winkler - Saguaro Photo 1

Dense patch of saguaros. Credit: Daniel Winkler

Many others delight in their abundance as well.  The flowers, fruits and seeds feed many animals (including humans).  They were an important food for the Tohono O’odham and Pima Indians – eaten fresh or converted into numerous products including wine, juice, jam and syrup.

Daniel Winkler - Saguaro Photo 2

Large saguaro with many fruits emanating from the apex of its branches. Credit: Daniel Winkler

Woodpeckers and flickers excavate nests in the saguaro’s trunk, which are subsequently occupied by other animals such as snakes, arthropods and small mammals.

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Saguaro with nest cavity excavated near the top of its trunk. Credit: Daniel Winkler

Daniel Winkler also delighted in the saguaro’s awesomeness. As he describes “I fell in love with answering some basic ecology questions about the saguaro. I was surprised that scientists had been studying this wonderful plant for almost 100 years and there were still many basic questions about the species general biology and ecology that remained unanswered. Thus, I was hooked immediately and became obsessed with saguaro.”

Don Swann - Photo of D. Winkler with young saguaros

Daniel Winkler with young saguaros. Credit: Don Swann

Winkler and his colleagues wanted to know how moisture, temperature and habitat influence the establishment or survival of juvenile saguaro seedlings. Previous research had shown that saguaro height can be used to estimate saguaro age, given knowledge of previous rainfall in a particular area. So buoyed by an army of citizen scientists whom they recruited with the help of social media, student groups from schools and volunteers working at the Saguaro National Park, the research team estimated the age of every saguaro on 36 4-ha plots (1 ha = 10,000 m2).

Going into the study, the researchers knew that rainfall was a very important factor, with saguaros surviving better during wet periods.  But they also knew that historically, some areas located near each other showed different establishment trends, thus they suspected that other variables, particularly land use and other landscape factors, might be important.  They did their research in two different districts within the park: 21 plots in the Rincon Mountain District (RMD) on the east side of the park, and 15 plots in the Tucson Mountain District (TMD) to the west. They classified each plot as a particular habitat type based on slope, elevation and soil-type. Bajada was low elevation, flat and had gravelly porous soils.  Foothills were intermediate elevation and intermediate slope, while sloped habitats had highest elevation, steepest slope, and the coarsest rockiest soils.

Daniel Winkler - Saguaro Photo 4

Panoramic view of Saguaro National Park showing diversity of habitats. Credit: Daniel Winkler.

Winkler and his colleagues calculated the Palmer Drought Severity Index (PDSI) for the years 1950-2003. The PDSI quantifies the water balance between precipitation and evapotranspiration, taking into account not only rainfall but also other factors such as temperature and cloud cover.  The PDSI was estimated by assessing tree ring width for each year in nearby woodlands; wet conditions have wide tree rings (maximum PDSI value = +6), while dry years have narrow tree rings (minimum PDSI value = -6).

The researchers discovered a very strong association between the PDSI and seedling establishment. Low PDSI at the beginning and especially the end of the time frame was associated with low seedling establishment, while high PDSI (particularly in the 1980s was associated with high rates of seedling establishment (top graph below).  But other patterns emerged as well.  For example, establishment was higher in the TMD during the wettest years, but higher in the RMD during the most recent drought (bottom graph below).

WinklerFig1

Top. Total number of saguaros (left Y-axis) established per hectare from 1950-2003 in relation to PDSI (dashed line, right Y-axis). Bottom. Total number of saguaros established per hectare in the Tucson Mountain District (TMD – filled bars) and the Rincon Mountain District (RMD – open bars)  from 1950-2003 in relation to PDSI (dashed line, right Y-axis).

Saguaro establishment increased in all habitats when conditions were relatively wet (more positive PDSI values).  Under drought conditions, slopes had greatest saguaro establishment, while establishment increased more rapidly in foothills (and to a lesser extent in Bajadas) as moisture levels increased.

WinklerFig2

Model projecting number of saguaros established in the three major habitats in relation to PDSI.  Shaded regions are 95% confidence intervals.

The researchers were surprised at how tight the connection was between drought and saguaro establishment. But landscape features are also important.  The TMD is warmer and dryer than the nearby RMD, and had substantially lower establishment during the recent drought. The slopes in the RMD are steeper and rockier than sloped areas of the TMD, and may buffer saguaros from drought by capturing water in rock crevices and holding it for longer periods of time so it can be absorbed by saguaro roots. Nurse trees that provide shade to young saguaros may also be more common on the RMD slopes.

Winkler and his colleagues are concerned about the long-term impacts of climate change on saguaro populations, particularly in the drier areas of the TMD. They urge researchers to explore how long-term management of grazing and invasive species influences saguaro establishment across the landscape.  They also encourage researchers to gather some very basic data about saguaros, such as how they access water and how human water use patterns influence the water’s availability to this iconic species.

note: the paper that describes this research is from the journal Ecology. The reference is Winkler, D. E., Conver, J. L., Huxman, T. E. and Swann, D. E. (2018), The interaction of drought and habitat explain space–time patterns of establishment in saguaro (Carnegiea gigantea). Ecology 99: 621-631. doi:10.1002/ecy.2124. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.