Species mixture effects and climate influence growth, recruitment and mortality in Interior West U.S.A. Populus tremuloides - conifer communities

Tree-species mixture effects (e.g., complementarity and facilitation) have
been found to increase individual-tree productivity, lessen mortality, and
improve recruitment in forests worldwide. By promoting more efficient and
complete resource use, mixture effects may also lessen
individual-tree-level water stress, thus improving drought-resistance. We
investigated the influence of mixture effects on tree productivity,
mortality, and recruitment across broad compositional and moisture
gradients in high-elevation Interior West US mixed-conifer communities,
where Populus tremuloides (trembling aspen) is the major contributor to
functional diversity. Our goal was to provide a more complete scientific
foundation for managing these drought-prone, fire-excluded systems under
an uncertain climate. We used landscape-scale national forest inventory
data to examine mixture effects on P. tremuloides and the major associated
conifer species, Pseudotsuga menziesii, Pinus contorta, Abies lasiocarpa,
and Picea engelmannii. Using generalized linear mixed modeling, we
isolated the influences of P. tremuloides relative density and climate on
tree-level (stems ≥ 12.7 cm DBH) growth, mortality, and stand-level
recruitment (presence/absence of new trees). Cold-season precipitation
(PPT) and warm-season vapor pressure deficit (VPD) served to represent
soil moisture supply and demand, respectively. Populus tremuloides growth
declined as interspecific density increased. In contrast, Pinus contorta
and A. lasiocarpa growth increased with P. tremuloides density. For all
species except A. lasiocarpa and P. menziesii, growth increased under
higher PPT and VPD. Populus tremuloides mortality increased under high VPD
but not with interspecific relative density. We found limited evidence
that A. lasiocarpa mortality decreased as P. tremuloides density
increased. Populus tremuloides recruitment declined steeply above 25%
interspecific relative density. We found a decline in conifer recruitment
odds as P. tremuloides density increased, ranging from strong in P.
contorta to insubstantial in P. engelmannii. Synthesis. Our findings have
implications for sustaining mixed-conifer communities impacted by climate
change and historical fire exclusion. Mixtures of P. tremuloides and
conifers may improve conifer growth while adversely impacting P.
tremuloides growth relative to pure stands. Higher conifer productivity
combined with lower P. tremuloides recruitment odds at conifer relative
density may accelerate succession.