Influences of gap microheterogeneity on the regeneration of Nothofagus pumilio in a xeric old-growth forest of northwestern Patagonia, Argentina

We experimentally examined the influences of within-gap environmental heter ogeneity on regeneration patterns of Nothofagus pumilio (Poepp. & Endl.) Kr asser near the xeric limit of its distribution in northern Patagonia, Argen tina. Results from this xeric old-growth forest are compared with patterns previously described for the same species in mesic forests. Survival of N. pumilio seedlings beneath tree-fall gaps in this relatively xeric forest ap pears to be strongly influenced by moisture availability. Seedlings and sap lings that have survived this demographic bottleneck are found at microsite s where soil water potentials are higher, such as in the shady northern edg es of tree-fall gaps (Psi = -0.46 MPa compared with less than -0.6 MPa in o ther gap positions) and on coarse woody debris (Psi = -0.29 MPa, compared w ith -0.51 MPa on the forest floor). Although gap creation in this dry N. pu milio forest is favorable to tree regeneration by releasing light resources , decreased water resources may switch the system from a light- to a water- limited system in some positions of the gap. This may explain the lack of r egeneration of N. pumilio often observed after creation of large gaps towar ds the xeric end of its range and needs to be considered in the management of this important timber species.