Ecohydrologic mechanisms of Mopane in the Kruger National Park photo: AK Knapp Jesse B Nippert & Anthony M Swemmer
Predicting the FUTURE Savanna types and distributions are susceptible to climate change because temperature and rainfall are key drivers of this ecosystem. Changes may impact latitudinal ranges of varying savanna types Climate and soil processes are not deterministic for savanna distribution: interplay of local site drivers and evolutionary history is key.
National Parks provide a refugia for species, but with global change the distribution of species / ecosystem types within parks is likely to change. Environmental gradient in KNP provides a novel platform to assess tradeoffs between water use, flux and drought tolerance among species and savanna types.
Biology of Colophospermum mopane Mopane savanna typically has lower alpha and beta diversity than Acacia / Combretum savanna Mopane savanna covers ~ 500,000 km 2 across southern Africa, with tree density ranging between 2-5,000 individuals/ha Irregularly drought deciduous species Commonly found on heavy soils across wide gradients of nutrient availability Classified as drought tolerant, and yet water limitation is the most common limiting resource for growth. Roots concentrated in surface soils, but high lateral branching and taproots common. We know lots about belowground form / biology, but we know comparatively little about ecological differences in water consumption, transport, and flux vary in relation to coexisting dominant species (e.g., Acacia nigrescens, Combretum apiculatum)
Historical dynamics Paleo-ecological evidence suggests that Mopane savanna size is dynamic over time (past 700 years). Mopane pollen abundance is highest during dry-cool climate periods, lowest during wet-warm. The forecast dry-warm climate represents a no-analog environmental condition. Anecdotal evidence over the 20 th century suggests Mopane is getting denser, suggesting direct or indirect correlations with modern climate changes Gillson and Ekblom 2010 Landscape Ecol.
A dynamic modern ecotone Photo: Alan Knapp
Questions (Q1) Does the physiology/morphology of Mopane provide an ecohydrological advantage compared to other savanna tree species, either by quickly utilizing available soil moisture or by being the most drought tolerant species? (Q2) How do source-water dynamics vary between Mopane and the other common KNP savannah species? (Q3) Can we use spatial environmental gradients in Southern Africa to predict savanna productivity, water flux, and cover, both now and in the future?
Sample collection Source-water samples: 2009 Ecotone south of Olyphants River 2011 Malopeni Leaf carbon isotopes 2011 Phalaborwa Gate to Mopani Camp Leaf gas exchange stem morphology 2011 Malopeni, near Phalaborwa Gate.
-1-2 -3 Granite PM mopane other tree spp. -1-2 -3 Basalt PM mopane other tree spp. 18 O ( ) -4-5 -6 18 O ( ) -4-5 -6-7 -7-8 -8-9 A / C ecotone Mopane Basalt PM -9 A / C ecotone Mopane Ecosystem-type sampled During the dry season (winter), Mopane is using water from deeper in the soil profile than coexisting trees (Euclea divinorum / natalensis, Combretum imberbe / heroense / apiculatum, Maurea parvifolia, Phylenoptera violocea, Acacia tortillis / nilatica)
-1 Source water differences among mature Mopane shrubs, seedlings and grasses. -2 δ 18 O adult -5.6 to -2.9 seedling -6.5 to -3.5 grass -3.5 to -1.1
When wet, Mopane has a higher capacity for water flux compared to other coexisting species
Jan. 2011 Phalaborwa
March 2011 Phalaborwa -26 Leaf samples - 13 C ( ) -27-28 -29-30 -31-32 Acacia exuvialis Acacia nigrescens Cissus cornifolia Colophospermum mopane Combretum apiculatum Combretum hereroense Comifera africana Comrbetum mosambicens Dalbergia melanoxylon Dichrostachys cinerea Euclea divinorum Flueggea virosa Grewia bi-color Lonchocarpus capasssa Peltophorum africanum Rhigozum zambesiacum Sclerocarya birrea Terminalia prunoides Terminalia sericea
Total root mass (kg) Scaling water-source use and flux on the landscape using allometeric predictions of root mass and rooting depth ~lateral roots to 16m from stem ~tap roots to 3m depth Mopane 45 40 35 30 25 20 15 10 5 0 0 100 200 300 400 Total basal area of all stems (cm2)
Conclusions ( so far) Appears Mopane uses multiple ecohydrologic strategies compared to co-occurring woody species Quick consumption and flux of water when surface soils are wet Utilize deep-soil water with high stomatal regulation when soils are dry to avoid water stress A warmer and drier future climate is likely to alter woody species distributions in KNP, with the potential for Mopane spread further south.
knobthorn Mopane Proposed Next Steps Greater source-water sampling More detailed analysis of structural traits among species Physiological and morphological Assessment of plant responses to varying site factors
Acknowledgements NSF-LTER Nature Conservancy Division of Biology Alan Knapp Gene Kelly K-State Eco-Phys Lab Teall Culbertson, Jeff Hartman, Gracie Orozco, Troy Ocheltree, Zak Ratajczak