Substrate effects on distribution, biomass allocation, and morphology of forest understory plants

Relationships between rooting substrate and the distribution and performance of forest plants are inadequately understood. We tested whether understory species in a dense coniferous forest were associated with coarse woody debris (CWD) or forest floor. In addition, for three species with differing substrate associations (Vaccinium parvifolium Smith, Tiarella trifoliata L., and Maianthemum dilatatum (Wood) Nels. and Macbr. , we excavated individuals rooted in CWD and forest floor, and compared biomass allocation and plant morphological traits. Substrate samples were also tested for moisture content. Of 29 species tested, 18 (62%) showed positive associations with forest floor and 6 (21%) with CWD. Forest floor is a more predictable and stable substrate; in these forests it also supports lower moss cover that can inhibit seedling establishment. As expected, plants rooted in forest floor (which was drier) allocated greater biomass to below- ground structures. Root-system traits, however, did not suggest plasticity in response to resource availability. Instead, the physical structure of logs may constrain root systems in CWD. In addition, total plant biomass did not differ between sub- strates suggesting that under low light, species may be incapable of responding to differences in belowground resources. Alternatively, substrate associations may develop earlier in the life histories of these plants via differential germination and survival. Resume´ : On comprend mal les relations entre le substrat d'enracinement et la distribution ainsi que la performance des plantes forestieres. Les auteurs ont verifiesi les especes de sous-etage d'une foret dense de coniferes sont associees avec de gros debris ligneux (CWD) ou avec le parterre forestier. Ils ont excaveles racines individuelles enracinees dans les CWD ou le parterre forestier chez trois especes associees ades substrats differents (Vaccinum parviflorum Smith, Tiarella trifoliata L. et Maianthemum dilatatum (Wood) Nels. and Macbr. ), et ils ont comparel'allocation de la biomasse et les ca- racteres morphologiques des plantes. Ils ont egalement analysela teneur en humidited'echantillons de substrats. Des 29 especes considerees, 18 (62 %) montrent une association positive avec le parterre forestier et 6 (21 %) avec les CWD. Le parterre forestier constitue un substrat plus previsible et plus stable; dans ces forets, il supporte egalement un couvert de mousse peu elevequi peut inhiber l'etablissement des plantules. Comme prevu, les plantes enracinees dans le parterre fo- restier (plus sec) allouent une plus grande biomasse aux structures hypogees. Cependant, les caracteres racinaires ne sugge`- rent pas de plasticiteen reaction a la disponibilitedes ressources. Au contraire, la structure physique des billes peut contraindre les systemes racinaires, dans les CWD. De plus, la biomasse totale des plantes ne differe pas selon les subs- trats suggerant que sous de faibles intensites lumineuses, les especes pourraient ne pas reagir aux differentes ressources hy- pogees. En echange, les associations aux substrats pourraient se developper plus tot dans le cycle vital de ces plantes via une germination et une survie differentielles.