Biological Sciences, Santa Barbara City College pipeline center for sustainability

Biology 130: Methods in Field Biology

Field Technique:  Plant Phenology

Map of the meeting place at Santa Barbara Botanic Garden is here.

Please carpool if possible. Park as far from the entrance as possible to leave spaces for the community to park closer to the entrance.

 

Phenological Change

The study of how the biological world times natural events is called phenology.  Scientists now understand that plants and animals take their cues from their local climate.  Species use the predictable yearly changes in the climate to determine when they start natural events such as breeding or flowering. Food webs are a set of interactions among trophic levels. Timing of biological events can have impacts on these interactions and are often under the influence of evolution. When the timing changes quickly, then evolution may not keep up and the result can be decline in a population or loss of a species.

The following provide significant background on the science of phenology and how Citizen Science has contributed to data collection.

 

sandhill cranes
(Migratory Sandhill cranes wintering in flooded rice fields near Lodi, CA. Photo by Adam Green)

 

Plants and animals have life cycle events that seemingly occur like clockwork every year:

  • Frogs and salamanders breed and their offspring go through metamorphosis
  • Flowers bloom
  • Male ungulates (deer, elk, pronghorns, etc.) grow antlers to begin the rut and breeding season
  • Birds migrate back and forth between their breeding grounds and their wintering grounds
  • The leaves of deciduous trees change color and fall off
  • Some mammals, such as bears, groundhogs and ground-squirrels, hibernate through the winter
  • Butterflies go through metamorphosis

How do Animals and Plants Know When to Start these Natural Events? 

The three main non-biological factors that affect phenology are:

  • Sunlight
  • Temperature
  • Precipitation (rainfall, snowfall, etc.)

These three factors work together to determine the timing of natural events.  For example, birds in the Northern Hemisphere begin their migrations to their breeding grounds each spring.  One of the main cues they use is the amount of available sunlight.  In the spring, the amount of sunlight increases a little each day signaling that summer is right around the corner.  Along with sunlight, birds also use the warming temperatures to determine the time of migration. Note, Global Warming will affect temperature, but not sunlight, so any change in timing of migration will depend on which cue dominates in a particular species.

Frogs mainly depend on temperature and precipitation to determine when to breed.  Wood frogs use temperature changes to signal when to come out of their winter torpor to breed at vernal pools.  Frogs and toads in the Southwestern United States are completely dependent on rainstorms to create pools where they can lay their eggs.   

Plants use all three factors together--temperature, precipitation and available sunlight--to time their yearly blooming.

The timing of one species' phenological events can be very important to the survival of another species. If one year has an unseasonably warm winter, frogs might mistake the high temperatures as a sign of spring.  Therefore, several frog species could breed much earlier in the year than expected.  If a late winter storm passes through, then all of the frogs that emerged from hibernation and their offspring could die.  The drop in the number of frogs will impact the predators that feed on frogs, such as raptors and snakes.  The impact will be felt throughout the food chain and the entire ecosystem.  

Farmers depend on insects to pollinate crops, such as blueberries, apples and squash.  There is a delicate balance between the insects and the crops. The crops need to flower around the same time that the insects finish developing into adults.  If the crops flower too early, then the pollinator insects could still be larvae. This would result in a much lower crop yield.

Phenology and Global Warming

One of the most noticeable ways that global warming is impacting wildlife is by disrupting the timing of natural events.  In essence, the winter season is shortening. With warmer temperatures, flowering plants are blooming earlier in the year and migratory birds are returning from their wintering grounds earlier in the spring. The table below shows the results of some long term studies on the phenology of plants and animals. These studies show that many species are responding to the warming climate. This is both an indicator of the warming and could possibly lead to problems in biological communities if the changes create mispatches in timing with other species.

 

Organism Study period (yrs) Spatial scale Phenological event Phenological shift Reference
Mexican jay 1971-1998 Mountain range in southern Arizona Egg laying 10 days earlier Brown et. al., 1999
Tree swallow 1959-1991 North America Egg laying 9 days earlier Dunn and Winkler, 1999
Various 1930-1990's One farm in Wisconsin Breeding, migration, flowering 7.3 days earlier Bradley et. al., 1999
Amphibians Early 1900's vs 1990's One city in New York state Breeding 10-13 days earlier Gibbs and Breisch, 2001
Plants 1981-1999 Boreal region Greening period 12 days longer Myneni et. al., 1997; Zhou et. al., 2001; Lucht et. al., 2002
Flowering plants 30-yr period Washington D.C. Flowering 4.5 days earlier Abu-Asab et. al., 2001

 

The following is a 250-year index of first flowering dates for 405 plant species in the UK assessing the impact of climate change on plant communities. The estimated community-level index in the most recent 25 years was 2.2–12.7 days earlier than any other consecutive 25-year period since 1760. The index was closely correlated with February–April mean Central England Temperature, with flowering 5.0 days earlier for every 1°C increase in temperature.

 

250-year index of first flowering dates for 405 plant species in the UK assessing the impact of climate change on plant communities

 

The median (red line) and 95% credible intervals (grey area) of the estimated community-level index (day of the year) showing a temporal change in the timing of first flowering shared by 405 plant species observed throughout the UK. The black line indicates the mean for every 25 years and the dotted line that for the most recent 25 years. The years without estimates indicate those without any observation records (1766, 1813, 1814 and 1817). (http://rspb.royalsocietypublishing.org/content/early/2010/04/01/rspb.2010.0291.full)

 

Timing of arrival of migrating birds is also affected by changing climate:

 

Explaining interspecific variation in phenological response:

Explaining interspecific variation in phenological response.

 

(A) Mean arrival date (averaged over both year and longitude) as a function of latitude for 18 bird species, depicting the rate at which various species advance northward during migration. (B) Boxplots showing the variation in the slope of the trend in arrival date with minimum spring temperature for each species, with more negative values reflecting earlier arrival. +, p<0.10; *, p<0.05; **, p<0.01. (C) Relationship between migration time (from (A)) and the median phenological response of arrival date to temperature. (D) Residuals of the phenological response to temperature after controlling for migration time and migration distance as a function of niche breadth.

 

Geographic variation in phenological response and example trajectories of temperature and arrival date

Geographic variation in phenological response and example trajectories of temperature and arrival date.

 

Geographic variation in the mean shift in arrival date per °C change in minimum spring temperature for four bird species (right-hand column). The left-hand column depicts changes in both minimum spring temperature (solid line) and arrival date (dashed line) through time for one example region (indicated by arrow) for each species. Note that the arrival date axis increases towards the bottom. Photo credits: red-eyed vireo, Dario Sanches; scarlet tanager, Steve Maslowski; great-crested flycatcher, Matt Ward; indigo bunting, Kevin Bolton. (Hurlbert AH, Liang Z. Spatiotemporal variation in avian migration phenology: citizen science reveals effects of climate change. PLoS One. 2012;7(2):e31662.)

Another study examined the first arrival dates of 103 migrant birds in New York and Massachusetts and found that, on average, all migrants arrived significantly earlier during the period 1951–1993 than the period 1903–1950. From 1951–1993 birds wintering in the southern United States arrived on average 13 days earlier while birds wintering in South America arrived 4 days earlier. It was believed by the researchers that the difference in the change in arrival dates between the two groups may have been because the species leaving from regions further south were unable to perceive the weather conditions at their destination area, so there was less of a trigger for early departure. The shorter distance migrants were perceiving a climate more similar to their destination and that likely triggered the earlier departure and arrival. These results are consistent with those expected under a scenario of global warming. (http://onlinelibrary.wiley.com/doi/10.1046/j.1474-919X.2003.00193.x/abstract)


Mismatch

Changes in phenology may create mismatch with aspects of an organism’s environment including weather, and food

Migration and reproduction of many avian species are controlled by endogenous (internal) mechanisms that have been under intense selection over time to ensure that arrival to and departure from breeding grounds is synchronized with moderate temperatures, peak food availability and availability of nesting sites. The timing of egg laying is determined, usually by both endogenous clocks and local factors, so that food availability is near optimal for raising young. Climate change is causing mismatches in food supplies, snow cover and other factors that could severely impact successful migration and reproduction of avian populations unless they are able to adjust to new conditions. Resident (non-migratory) birds also face challenges if precipitation and/or temperature patterns vary in ways that result in mismatches of food and breeding.

A central concept when studying the relationship between climate, phenology, and other life history traits is the match vs. mismatch of the climate–phenology response within different components of the ecological system.

 

The environmental cues triggering onset of egg laying change in asynchrony to the environmental conditions prevailing when chicks are reared and when birds' energetic demands are the highest
(http://www.pnas.org/content/99/21/13379/F1.expansion.html)

 

 

 

(A) The environmental cues triggering onset of egg laying change in asynchrony to the environmental conditions prevailing when chicks are reared and when birds' energetic demands are the highest, as shown for Great tit (18).

This shows that if the chicks hatch earlier than the peak in caterpillar abundance then the parents may have difficulty feeding their young. This could increase chick mortality and cause a decline in population.

 

 

(B) The differential climate change between summer and winter ranges may lead to problems in the transition for migratory birds, such as the American robin (21).

 

This is a video explaining phenological shift associated with climate change and the potential problem of mismatch.

 

 

Plants, insects, and birds are not the only groups showing phenological shifts.

 

Fish

Fish species may also suffer from mis-match. A 2015 study looked at fish species, that have been studied less than terrestrial species for phenological shift and mis-match.

The phenology of 43 Eastern Boundary Current Upwelling [the California current off Southern California] fish species was examined over 58 years; 39% of phenological events occurred earlier in recent decades, with faster changes than many terrestrial ecosystems. Zooplankton did not shift their phenology synchronously with most fishes. Fishes that aren’t changing their phenology synchronously with zooplankton may be subject to mismatches with prey, potentially leading to reduced recruitment to fisheries.

It is unknown how much of an impact this will be on these fish species, but the changes are now documented. Like terrestrial species this mis-match may add stress to species already impacted by fishing and pollution.

 

Amphibians

The timing of amphibian breeding is largely driven by environmental cues such as temperature and moisture (Carey and Alexander 2003); because of this, their breeding phenology may be directly affected by global warming.

Amphibians in temperate regions may be even more susceptible to increases in temperatures. Most temperate species spend a large portion of the year inactive, escaping either cold winters or hot summers. Subtle increases in temperature or moisture trigger them to emerge from their hibernacula. Immediately upon emergence, they migrate to ponds or streams to breed. Thus, one hypothesized direct effect of global warming on amphibians is a trend towards early breeding as the average temperatures increase.

If amphibians breed too early in the season they may be more vulnerable to early snowmelt induced floods and early season freezes that are usually less common later in the season.

To test his hypothesis, researchers from Europe and North America have analyzed long-term data sets looking for trends towards earlier breeding. Some amphibians do show a trend towards earlier breeding but not all species do (Beebee 1995, Blaustein et al. 2001, Gibbs and Breisch 2001). In addition, this trend may vary regionally for a single species. For example, the spring peeper (Pseudacris crucifer) is breeding earlier in Ithaca, New York, in the 1990s than it did in 1900s (Gibbs and Breisch 2001) but does not appear to be breeding earlier in Germfask, Michigan (Blaustein et al. 2001).

In the table below, we list the amphibians in North America and the United Kingdom that are showing a trend towards earlier breeding and those that are not breeding earlier based on studies from Beebee 1995, Blaustein et al. 2001 and Gibbs and Breisch 2001 (table modified from Blaustein et al. 2003). (http://amphibiaweb.org/declines/ClimateChange.html)

 

Amphibians breeding earlier

 

Species:

http://amphibiaweb.org/images/bufocalamita.jpg
natterjack toad 
Bufo calamita

© 2000 Arie van der Meijden

http://amphibiaweb.org/images/ranaesculenta.jpg
edible frog 
Rana esculenta

© 2000 Arie van der Meijden

http://amphibiaweb.org/images/tristurushelveticus.jpg
Palmate newt 
Triturus helveticus
 
© 2002 John P. Clare

Where:

Hampshire, England

Sussex, England

Sussex, England

Reference:

(Beebee 1995)

(Beebee 1995)

(Beebee 1995)

Species:

http://amphibiaweb.org/images/tristurusvugaris.jpg
smooth newt 
T. vulgaris
 
© PENSOFT Publishers

http://amphibiaweb.org/images/tristuruscristatus.jpg
great-crested newt 
T. cristatus
 
© PENSOFT Publishers

http://amphibiaweb.org/images/hylaversicolor.jpg
grey treefrog 
Hyla versicolor
 
© 1998 Joyce Gross

Where:

Sussex, England

Sussex, England

Ithaca, New York, USA

Reference:

(Beebee 1995)

(Beebee 1995)

(Gibbs and Breisch 2001)

Species:

http://amphibiaweb.org/images/pseudacriscrucifer.jpg
spring peeper 
Pseudacris crucifer
 
© 2001 John White

http://amphibiaweb.org/images/ranacatesbeiana.jpg
American bullfrog 
R. catesbeiana
 
© 2002 William Flaxington

http://amphibiaweb.org/images/ranasylvatica.jpg
wood frog 
R. sylvatica
 
© 2003 John White

Where:

Ithaca, New York, USA

Ithaca, New York, USA

Ithaca, New York, USA

Reference:

(Gibbs and Breisch 2001)

(Gibbs and Breisch 2001)

(Gibbs and Breisch 2001)

 

Amphibians not breeding earlier

 

Species:

http://amphibiaweb.org/images/bufobufo.jpg
common toad 
Bufo bufo
 
© 2000 Arie van der Meijden

http://amphibiaweb.org/images/ranatemporaria.jpg
common frog 
Rana temporaria
 
© 2003 Twan Leenders

http://amphibiaweb.org/images/bufoamericanus.jpg
American toad 
B. americanus
 
© 2003 Brad Moon

Where:

Purbeck Hills in south Dorest, England

Sussex, England

Ithaca, New York, USA

Reference:

(Reading 1998)

(Beebee 1995)

(Gibbs and Breisch 2001)

Species:

http://amphibiaweb.org/images/bufoboreas.jpg
western toad 
B. boreas
 
© 2000 Joyce Gross

http://amphibiaweb.org/images/bufofowleri.jpg
Fowler’s toad 
B. fowleri 

© 1979 Alan Resetar

http://amphibiaweb.org/images/pseudacriscrucifer.jpg
spring peeper 
Pseudacris crucifer
 
© 2001 John White

Where:

Lost Lake, Three Creeks and Todd Lake, Oregon, USA

Long Point, Ontario, Canada

Germfask, Michigan, USA

Reference:

(Blaustein et al. 2001)

(Blaustein et al. 2001)

(Blaustein et al. 2001)

 

Citizen Science: You can play a part in studying wildlife and global warming by participating in a phenology citizen science program, such as

  • Project Budburst – monitor the first flowering of plants
  • Project Feederwatch – report bird sightings at feeders throughout the winter and spring
  • FrogWatch USA – listen for the calls of frogs and toads during their breeding season
  • Earth Alive – make observations of common phenological events from bird migrations to flowering

Further Readings:

 


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