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DETECTING FOREST STRESS AND DECLINE IN RESPONSE TO INCREASING RIVER FLOW IN SOUTHWEST FLORIDA, USA
Citation:
Ford, C. R. AND J R. Brooks. DETECTING FOREST STRESS AND DECLINE IN RESPONSE TO INCREASING RIVER FLOW IN SOUTHWEST FLORIDA, USA. FOREST ECOLOGY AND MANAGEMENT. Elsevier Science Ltd, New York, NY, 160:45-64, (2002).
Description:
Forest stress and decline resulting from increased river flows were investigated in Myakka River State Park (MRSP), Florida, USA. Since 1977, land-use changes around the upper Myakka River watershed have resulted in significant increases in water entering the river, which have caused extensive mortality in the upper watershed. The present study assessed whether similar forest stress and mortality was occurring downstream within the park. Our objectives were to (1) determine if tree die-off and/or stress resulting from increased flows were present in MRSP; and (2) determine the relationship between historical and present flow regimes and growth of actively managed forested stands undergoing restoration located both above and below a dam. We used two methodological approaches. The first was recording indications of tree stress and decline (crown dieback, crown thinning, trunk rot, foliage discoloration, and parasitism) in Fraxinus caroliniana Miller dominated forested wetlands, Pinus elliottii Englem. var. densa Little & Dorman dominated mesic pine flatwoods, and Quercus laurifolia Michaux dominated oak palm hammocks. Our second approach was tree-ring analysis, which allows for more detailed analysis of growth in response to precipitation and river flow in the pine flatwoods stands.
Our results indicate significant stress and decline in some forested wetlands upstream of the dam, significant mortality in wet-mesic pine flatwoods sites close to the river, and significant amounts of relative stress in wet-mesic pine flatwoods sites upstream of the dam. Fraxinus caroliniana Miller sites located upstream of the dam had more individuals with symptoms of stress than those downstream of the dam (67% vs. 43%, P=0.031). In Quercus laurifolia Michaux sites, 70-85% of the trees had evidence of flooding stress and mortality, which is comparable to distributions found in severely disturbed forest in the upper watershed. Pinus elliottii Englem. var. densa Little & Dorman sites located <1000 m from the river had higher mortality than sites located >1050 m from the river (P<0.01), and the stressed trees in sites upstream of the dam had significantly lower growth rates in the 1990is versus the 1960is than those downstream. Although the onset of stress and decline coincided with increasing flows, we found that flow is positively correlated with tree growth, both before and after flow increases in the system. Increasing flow may play an indirect role through increased competition in the stress and decline in wet-mesic pine flatwoods; however, flow seems to be the direct cause for stress and decline found in forested wetland stands.