Vegetation Dynamics and Slope Management on the Mountains of the Hawaiian Islands

Lyon's conclusion (cf. 1919) with regard to the stand reduction dieback on the lower wet slope of Haleakala Mountain led to a forest management policy that saw importation of alien tree species as necessary for protecting Hawaiian watersheds. Unfortunately, this also led to a further demise of native vegetation.It is possible that the low-canopy species diversity in Hawaii may increase the rate of paludification and swamp (or bog) formation, under wet climatic conditions, over that in a similar environment with a broader spectrum of successional species. However, a boggy landscape can also be a good watershed. Historic evidence comes from the sugar-cane planters on Maui—the supposed beneficiaries of the exotic tree-planting programme—who had objected to it on empirical grounds (Holt, 1983). A more recent study of a canopy dieback in the Hilo watershed showed that extensive tree mortality had no effect on the rate and quality of the water yield from this area. The forest hydrologist, R.D. Doty (1983), who investigated this relationship, attribut ed the outcome to the vigorous undergrowth which remained intact during the dieback event.Further landscape ageing in Hawaii's shield-shaped mountains involves a process of fluviomorphic dissecting of the gentle slopes lying inland. This results first in amphitheatre-headed valleys and eventually, on the windward sides, in steep residual escarpments with numerous secondary footslopes. Wirthmann & Hüser (1987) derived details of this process from the side-by-side comparison of younger with older Hawaiian mountains. When the waterlogged surfaces of the plateau-like lower wet slopes of shield volcanoes become laterally drained, closed Metrosideros forests can become re-established. That this natural re-establishment process works in the Hawaiian mountains can be seen by going from the shield-shaped slopes of the younger, to the more dissected slopes of the older, high islands. Vigorous Metrosidews forests occur also on the older islands, but they are typically of lower stature than those on the younger mountains.Unfortunately, most of the secondary slopes formed at the base of the steeply-cut windward mountain slopes of the older high islands have been deforested to make way for crop agriculture. Many of these lower slopes have subsequently been abandoned for economic reasons; but instead of reverting to forests, they have become invaded in many areas by alien grasses and shrubs. Particularly damaging has been the introduced pyrophytic Broomsedge (Andropogon virginicus). This grass has retained a temperate-zone phenology, whereby it undergoes partial dormancy during the winter season when rainfall increases (Sorenson, 1980). As such it has contributed to accelerated erosion in two ways: (1) by preventing recirculation of soil water in areas where excessive runoff is a problem, and (2) by attracting repeated fires and preventing succession to a forest, which would provide much-improved protection against the accelerated soil erosion (Mueller-Dombois, 1973) that is such a common feature on the lower windward slopes of the older islands.