The Pine Barrens of West Hampton Beach, Long Island, NY contain a unique ecosystem with pitch pine trees existing in two distinct forms: tall and dwarf. Several theories exist to explain why the dwarf form exists, but none have been rigorously tested. Recent theory and research suggest that height growth is linked to hydraulic conductance properties of tree stems. Thus, dwarf trees could potentially be stunted due to a restricted ability to conduct water through their stems. In this study, we tested whether dwarf and tall pitch pines have different hydraulic conductances using Granier style sapflux probes. Three growth forms were selected for study: dwarf trees (n=5), medium height trees growing with the dwarf trees (n=6), and tall trees growing nearby (n=3). Sapflux data were collected in the late summer of 2005. Leaf area and sapwood area data were collected in 2007 and 2009 and standardized to 2005 conditions. Tests showed that the ratio of sapwood area to total basal area did not change between 2005 and 2009, and leaf area to sapwood area ratio did not change between 2007 and 2009 for the trees we measured. Crown conductance was estimated using sapflux, leaf area to sapwood area ratio, and VPD.
Our results show that crown conductance was greater in the dwarf pines and least in the tall pines with the medium pines having intermediate values. There was no significant relationship between tree height and crown conductance. However, leaf area to sapwood area ratios were greater in the tall pines and smaller in the dwarf pines and the relationship between crown conductance and leaf area to sapwood area ratio was significant. Again, the medium pines had intermediate values. An analysis of a measure of resistance to embolisms showed that all trees were operating below critical exposure to embolisms, but dwarf pines had more sensitive stomata to VPD than tall pines. We conclude that the tall pines have access to more nutrients than the dwarf and medium pines and thus are able to have higher leaf area to sapwood area ratios despite xeric conditions. Dwarf pines are not hydraulically limited. Instead, they appear to be genetically limited in their ability to add leaf area and height. The consistent intermediate responses of medium pines suggest that they are genetically similar to the tall pines, but they are physiologically limited by the dwarf pine site conditions.