Impacts of forest fragmentation on the climate sensitivity of tree transpiration

Background/Question/Methods

In temperate forests, fragmentation results in elevated tree growth relative to the forest interior. Forest edges also experience hotter and drier microclimates than interior forests, which puts into question whether the growth enhancement at edges will continue with ongoing climatic changes. The strong coupling between carbon uptake during photosynthesis and water loss via tree transpiration suggests impacts of edge effects on both the carbon and water cycles of temperate forests. Here we investigate the climate sensitivity of transpiration in response to fragmentation and among trees growing along two types of edges –adjacent to a meadow versus a road. We hypothesize that: 1) transpiration is greater at the forest edge than interior, corresponding to higher growth rates observed at edges, and 2) the climate sensitivity of transpiration is greater at the edge than interior. Using sap flow sensors we quantified transpiration rates in similarly sized red maple (Acer rubrum) canopy trees growing at either meadow or road adjacent edges as well as on red maples in adjacent interior forest. Sites were located at Harvard Forest in Petersham, MA. We examined the climate sensitivity of transpiration to changes in temperatures and vapor pressure deficit using linear mixed effect models.

Results/Conclusions

The impact of forest fragmentation on transpiration varied depending on the type of forest edge. At the meadow site, rates were 18% higher during the growing season at the edge relative to the interior, while at the road site rates declined by 23% at the edge. Similarly, climate sensitivity varied between edge and interior trees based on the type of adjacent land use. At the meadow site, the climate sensitivity of transpiration was not significantly different between the edge and interior (p=0.60). There were similar, strong responses in transpiration to changes in VPD (Edge: slope=11.6, r2=0.70; Interior:slope=12.1 ,r2=0.87) and temperatures (Edge: slope=1.1, r2=0.42; Interior: 1.04, r2=0.38). In contrast, at the road site we find significantly lower climate sensitivity at the edge compared to the interior in response to changes in VPD (Edge: slope=4.5 , r2=0.15; Interior: slope=11.25 , r2=0.39) and temperatures (Edge: slope=0.46, r2=0.04; Interior: slope=1.04, r2=0.19). These findings highlight the need for understanding how different types of forest edges influence water and carbon cycles in temperate forests.