Terrestrial hydrological controls on land surface phenology of African savannas and woodlands

This paper presents a continental-scale phenologica l analysis o f African savannas and woodlands. We app ly an array o f synerg is tic ve g e ta tio n and hyd ro log ica l data records from sate llite rem ote sensing and m ode l s im u la tions to exp lo re the in fluence o f ra iny season t im in g and d u ra tio n on reg iona l land surface ph e no lo g y and ecosystem structu re . We fin d th a t (I) the rainy season onse t precedes and is an e ffec tive p re d ic to r o f the g ro w in g season onse t in A frican grasslands. (11) A frican w ood lands genera lly have early g reen -up before rainy season onse t and have a va riab le de layed senescence pe riod a fte r the rainy season, w ith th is de lay corre la ted non linea rly w ith tree frac tion . These w o o d la n d responses suggest th e ir com plex w ater use mechanisms (either from po tentia l g roundw ate r use by relatively deep roots o r stem -water reserve) to m ain ta in d ry season ac tiv ity , (iii) We em p irica lly fin d th a t th e ra iny season leng th has strong non linea r im pacts on tree frac tiona l cover in th e annual ra infa ll range fro m 600 to 1800 m m /yr, w h ich may lend som e su p p o rt to th e prev ious m od e lin g s tudy th a t g iven the same a m o u n t o f to ta l ra infa ll to the tree frac tio n m ay firs t increase w ith the le n g the n in g o f ra iny season u n til reaching an "o p tim a l ra iny season leng th ," a fte r w h ich tree frac tio n decreases w ith th e fu rth e r le n g the n in g o f ra iny season. This non linear response is resu lted fro m c o m p o u n d m echanism s o f hyd ro log ica l cycle, fire, and o th e r factors. We conc lude th a t A frican savannas and dec iduous w o od lands have d is tin c tive responses in th e ir p h e no logy and ecosystem fu n c tio n in g to rainy season. Further research is needed to address in te ra c tio n be tw een g ro u n d w a te r and trop ica l w o o d la n d as w e ll as to e xp lic it ly cons ide r the eco log ica l s ign ificance o f rainy season leng th under c lim a te change.