Abstract
While the supply of freshwater is expected to decline in many regions in the coming decades, invasive plant species, often 'high water spenders', are greatly expanding their ranges worldwide. In this study, we quantified the ecohydrological differences between native and invasive trees and also the effects of woody invasive removal on plot-level water use in a heavily invaded mono-dominant lowland wet tropical forest on the Island of Hawaii. We measured transpiration rates of co-occurring native and invasive tree species with and without woody invasive removal treatments. Twenty native Metrosideros polymorpha and 10 trees each of three invasive species, Cecropia obtusifolia, Macaranga mappa and Melastoma septemnervium, were instrumented with heat-dissipation sap-flux probes in four 100 m(2) plots (two invaded, two removal) for 10 months. In the invaded plots, where both natives and invasives were present, Metrosideros had the lowest sap-flow rates per unit sapwood, but the highest sap-flow rates per whole tree, owing to its larger mean diameter than the invasive trees. Stand-level water use within the removal plots was half that of the invaded plots, even though the removal of invasives caused a small but significant increase in compensatory water use by the remaining native trees. By investigating the effects of invasive species on ecohydrology and comparing native vs. invasive physiological traits, we not only gain understanding about the functioning of invasive species, but we also highlight potential water-conservation strategies for heavily invaded mono-dominant tropical forests worldwide. Native-dominated forests free of invasive species can be conservative in overall water use, providing a strong rationale for the control of invasive species and preservation of native-dominated stands.
Highlights
Water conservation has long been of global concern, and invasive plant species are encroaching into and altering theecohydrology of much of the Earth’s terrestrial surface as a direct result of an increasingly mobile human population (Millennium Ecosystem Assessment, 2005)
Some studies have found that introduced tree species are more efficient at using water than native trees (e.g. Wise et al, 2011); it is often not appropriate to extrapolate trends in leaf-level water use to whole-plant transpiration rates
Native vs. invasive tree water use Studies which directly compare co-occurring native vs. invasive tree transpiration rates in tropical forests are few, and results are mixed
Summary
Water conservation has long been of global concern, and invasive plant species are encroaching into and altering theecohydrology of much of the Earth’s terrestrial surface as a direct result of an increasingly mobile human population (Millennium Ecosystem Assessment, 2005). A global meta-analysis found that invasives showed significantly greater levels of water use at the leaf level, but that co-occurring invasive and native species were likely to have higher transpiration rates at the whole-plant scale (Cavaleri and Sack, 2010). This disconnect between results found at the leaf level vs the whole-plant level may be due to artifacts of leaf-chamber measurements, variations in canopy structure, sapwood:leaf area ratios, plant size or age (Cavaleri and Sack, 2010). These deficiencies reveal a need for more information about the water use of invasive and native plant species from the ecosystem perspective
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