Abstract
Sustainable forest management is hugely dependent on high-quality estimates of forest site productivity, but it is challenging to generate productivity maps over large areas. We present a method for generating site index (a measure of such forest productivity) maps for plantation loblolly pine (Pinus taeda L.) forests over large areas in the southeastern United States by combining airborne laser scanning (ALS) data from disparate acquisitions and Landsat-based estimates of forest age. For predicting canopy heights, a linear regression model was developed using ALS data and field measurements from the Forest Inventory and Analysis (FIA) program of the US Forest Service (n = 211 plots). The model was strong (R2 = 0.84, RMSE = 1.85 m), and applicable over a large area (~208,000 sq. km). To estimate the site index, we combined the ALS estimated heights with Landsat-derived maps of stand age and planted pine area. The estimated bias was low (−0.28 m) and the RMSE (3.8 m, relative RMSE: 19.7%, base age 25 years) was consistent with other similar approaches. Due to Landsat-related constraints, our methodology is valid only for relatively young pine plantations established after 1984. We generated 30 m resolution site index maps over a large area (~832 sq. km). The site index distribution had a median value of 19.4 m, the 5th percentile value of 13.0 m and the 95th percentile value of 23.3 m. Further, using a watershed level analysis, we ranked these regions by their estimated productivity. These results demonstrate the potential and value of remote sensing based large-area site index maps.
Highlights
Explicit maps of forest productivity are compelling to forest stakeholders for several reasons
The objective of the present study is to determine whether site index for loblolly pine could be accurately estimated and mapped using disparate discrete return airborne laser scanning (ALS) datasets combined with Landsat-derived estimates of forest stand age
We presented and evaluated a method to generate site index estimates over large areas in the southeastern U.S, an important region with respect to carbon sequestration and forest plantation activity
Summary
Explicit maps of forest productivity are compelling to forest stakeholders for several reasons. Sustainable forest management is generally highly dependent on site productivity measures as they significantly influence silviculture-related decisions such as treatment and harvesting schedules [1,2]. Forests in the southeastern part of the United States constitute large carbon stocks (estimated at around 36% of conterminous U.S forest carbon [4]) and they absorb a sizable amount (around 13%) of regional greenhouse gas emissions [5]. To monitor forest site productivity (the potential of a site to produce biomass; see [7]) accurately, spatially extensive maps are needed that can be updated over time. Spatially extensive site productivity maps are useful for calibrating models related to land-use and land surface processes [8,9]
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