Many species in nutrient poor or water stressed environments have low SLA Check out figure 3, p. 308. Check out figure 4, p. 309. Leaves with more structural tissue have lower RGR, presumably because these leaves allocate less to enzymes and are poorer light harvesters. Plants with a wide range of RGRs have about the same level of photosynthesis. RGR declines with time.
Plants produce more non-productive support tissue as they get larger.
NAR declines with time. Leaf photosynthetic rate declines as leaves age and as new leaves shade lower leaves as plants get bigger. RGR becomes resource-limited and possibly constrained by hydraulic architecture and cavitation as plants get bigger. RGRmax measured on small plants in a favorable environment is the best approximation of RGRmax. Because RGR varies w size, you should measureRGR for different plants at same size or age, not at same time.