Aggregated and scaled up estimation of net primary production in a seasonal ecosystem
Final Report Abstract
We developed methods and tested a model for predicting whether trees in savannas should be evergreen or deciduous. Whether plants are evergreen or deciduous has large implications for net primary productivity and the landsurface energy budget, since leaves are critical drivers of albedo, sensible and latent heat exchanges. Our study developed and empirically parameterised a leaf level model of the annual carbon balance of the leaves of deciduous and evergreen tree species in a semi-arid savanna. We were able to show that deciduous species had higher annual carbon gains, suggesting that they represent the more competitive strategy in these savannas. However theory would predict that evergreen species should retain their leaves for at least two years, but we found that the leaf longevity of evergreen species barely exceeded one year. This suggested that herbivory and fire may be important selective forces on leaf habit is these savannas. We complemented these field studies with global scale analyses of vegetation greenness. In a first study we explored whether similar leaf phenological strategies were observed in regions with similar climates, but dissimilar evolutionary histories. This study allowed us to test whether there is a global convergence in leaf habit, or whether evolutionary history and niche conservatism had prevented the convergence to a global optimum. We found that leaf phenology was largely convergent in the study area, although the study did identify regions of non-convergence (11 % of the study area). This methodological framework for analyzing global patterns of vegetation greenness was then used to explore the extent to which leaf phenology had changed in recent decades. We found overwhelming evidence for substantial and widespread change. This study provided firm evidence that vegetation is dynamically responding to global changes. This study did however not attribute the causes of the change we observed, something that could be undertaken in future studies. A final application was to use our methods of analyzing global patterns of vegetation greenness to propose a new method for classifying biomes. Textbooks define biomes as broad scale vegetation units defined by their structure and function. Existing applications of this theory have however used structure and climate (the latter often implicitly) and largely ignored functional attributes. We proposed a new scheme for classifying biomes that used a structural element (vegetation height) and two functional elements. The first functional element examined whether vegetation is limited by cold, moisture, both cold and moisture or by neither cold nor moisture. The second functional element estimated vegetation productivity. Using these categories we classified the world into 24 biome types and monitored their change over a 31 year time series. We were able to detect substantial change (13% of the monitored surface had changes over the study period). This study provides a repeatable method for monitoring the worlds ecosystems, but also provides a new classification for ecosystems which may benefit meta-analyses of ecosystem functioning. This study has illustrated the benefits of working on a single issue (leaf phenology and carbon gain) at different scales. We are convinced that this multi-scale approach forced us to think more deeply about the ecological process controlling vegetation activity.
Publications
- (2011), ’Is there a temporal niche separation in the leaf phenology of savanna trees and grasses?’, Journal of Biogeography 38(11), 2165- -2175
Higgins, S. I.; Delgado-Cartay, M. D.; February, E. C. & Combrink, H. J.
- (2014), ’Soil water retention curves for the major soil types of the Kruger National Park’, Koedoe 56(1)
Buitenwerf, R.; Kulmatiski, A. & Higgins, S.
(See online at https://doi.org/10.4102/koedoe.v56i1.1228) - (2015), ’Feedback of trees on nitrogen mineralization to restrict the advance of trees in C4 savannahs’, Biology Letters 11(8)
Higgins, S. I.; Keretetse, M. & February, E. C.
(See online at https://doi.org/10.1098/rsbl.2015.0572) - (2015), ’Three decades of multi-dimensional change in global leaf phenology’, Nature Climate Change 5(4), 364-368
Buitenwerf, R.; Rose, L. & Higgins, S. I.
(See online at https://doi.org/10.1038/nclimate2533) - 2015. Leaf phenology: from physiology to global change. Dissertation, Fachbereich 11 Geowissenschaften / Geographie der Johann Wolfgang Goethe-Universität in Frankfurt am Main
Buitenwerf, Robert
- (2016), ’Convergence among global biogeographical realms in the physiological niche of evergreen and deciduous vegetation’, Global Ecology and Biogeography
Buitenwerf, R. & Higgins, S. I.
(See online at https://doi.org/10.1111/geb.12447) - (2016), ’Defining functional biomes and monitoring their change globally’, Global Change Biology
Higgins, S. I.; Buitenwerf, R. & Moncrieff, G.
(See online at https://doi.org/10.1111/gcb.13367) - (2016), ’Rapid Leaf Deployment Strategies in a Deciduous Savanna’, PLoS ONE 11(6), 1-11
February, E. C. & Higgins, S. I.
(See online at https://doi.org/10.1371/journal.pone.0157833)
