Myriad phenological strategies in dry ecosystems

Plant functional types across Amboseli National Park in August 2012 (photo credit: Ryan Nagelkirk).

#E4 award paper #OpenAccess

By Ryan Nagelkirk and Kyla Dahlin

Variety may be the spice of life, but for land surface modelers trying to predict plant phenology across Earth’s surface, variation is a challenge. This is especially true in semi-arid and savanna type ecosystems (SAST), where leaf phenology has been difficult to predict because the timing, and even the number of events, often varies annually. This is a problem because SASTs cover over a fifth of global land surface, are home to over 3 billion people and diverse wildlife assemblages, and play a disproportionately large role in intra-annual global climate variability.

Traditionally, models have assumed that the plant functional types (PFTs) (e.g., deciduous trees and shrubs) in SAST systems behave similarly to that of vegetation at higher latitudes, which follow predictable seasonal cycles. However, these models have not been able to adequately reproduce phenology in SAST systems. Our team investigated whether the cues driving phenology in these systems differ from those of the higher latitude systems by reviewing 188 papers related to SAST phenology. We grouped vegetation into 7 PFTs: tropical and temperate broadleaf trees and shrubs and C3, C4 and mixed grasses. Our drivers of phenology were latitude, temperature, precipitation, continent, and PFT.

To our surprise, latitude was found to be the most significant driver of leaf phenology in SAST ecosystems, suggesting that the plants in these systems are sensitive to relatively small changes in solar radiation. Continent and PFT followed latitude. This might not come as a surprise to some given that the life forms on the continents with some of the largest SAST ecosystems, such as South America, Africa and Australia, have evolved separately since the breakup of Gondwana some 180 million years ago. The continents also have significantly different factors driving their climates, such as the Intertropical Convergence Zone, which crosses some locations twice annually in Africa and South America, but only barely contacts Australia’s north once a year. These factors might have taken the vegetation on these continents down different evolutionary paths that led them to today, when their phenologies cannot be summarized by simple, global PFTs. Our findings suggest that models that incorporate latitude or day length and continent-specific PFTs will outperform those of the past; increasing our confidence in projected changes and their potential impacts on the billions of people that call these places home. 

Distribution of SAST ecosystems and ‘data points’ (study × plant functional type × location). (A) Koppen-Geiger classes that include SAST ecosystems; (B) IGBP 2010 classes included in some SAST definitions (shrub. = shrublands, sav. = savannas, grass. = grasslands); (C) distribution of methods – either field based, remotely sensed, or some combination of approaches; (D) general attribution of green up to a process. Many studies did not list a cause of green up, very few tested these attributions. (E) Plant functional types studied: BDT = broadleaf deciduous tree, BDS = broadleaf deciduous shrub, mixed-grass = a combination of C3 grasses, C4 grasses, and/or forbs; (F) year of study initiation (colors) and duration of study (size of circle).

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