(1) The MELNHE study examines nutrient co-limitation of forest productivity across 13 forest stands of different ages located in three sites. Fertilization experiments add nitrogen, phosphorus, or both to test for co-limitation. (2) So far, tree growth has been found to be primarily limited by phosphorus rather than nitrogen or co-limitation. (3) A new study tested whether root growth is directly limited by nitrogen or phosphorus availability, or responds to aboveground nutrient allocation. Fine root growth responded directly to nitrogen addition but not phosphorus addition, and roots foraged more in phosphorus-rich patches.

1. Intro to MELNHE

2. Elser et al. (2007) Ecology Letters
Intro to MELNHE
• Co-limitation: adding 2
resources together
increases productivity
more so than adding
either alone
• Lots of interest; limited
mechanistic
understanding
• MELNHE study
conceived to examine
co-limitation in more
detail

3. Thirteen forest stands:
• 3 sites: Hubbard Brook (HB), Jeffers Brook (JB),
Bartlett (BEF)
• 3 ages: young (~30 yrs), mid (~45 yrs), and mature
(>100 yrs)
• Color-coded
General question:
• Is forest productivity co-limited by N and P?
So far (among many results):
• Tree growth limited primarily by P (2011-2015;
Goswami et al. 2018)
• No obvious co-limitation detected
N:
30 kg/ha/yr
P:
10 kg/ha/yr
N+P
Control
One forest stand: four 50
x 50 m treatment plots
NxP fertilization initiated in 2011

4. Testing limitation
control nutrient addition
Limitation is inferred if added
nutrient increases growth
What about belowground?

5. Testing limitation of root growth
control nutrient addition
Limitation is inferred if added
nutrient increases growth
What about belowground?
(1) Direct nutrient limitation of root
growth

6. Testing limitation of root growth
control nutrient addition
Limitation is inferred if added
nutrient increases growth
What about belowground?
(1) Direct nutrient limitation of root
growth
(2) Allocation response to
aboveground limitation
- alleviating aboveground
limitation reduces growth

7. Testing limitation of root growth
control
nutrient-rich
patch (hotspot)
Limitation is inferred if added
nutrient increases growth
What about belowground?
(1) Direct nutrient limitation of root
growth
(2) Allocation response to
aboveground limitation
- alleviating aboveground
limitation reduces growth
- foraging response to nutrient
heterogeneity

8. Testing limitation of root growth
Question:
Is root growth controlled by
(1) belowground nutrient limitation, or
(2) allocation in response to aboveground
limitation?
(Shan et al. in review)
Used root ingrowth cores in 3 mature
forest stands, 2014.

9. Root growth: 3 mature stands
Fine root growth was
greater in response to N
addition (direct nutrient
limitation)
No response to P addition
(Shan et al. in review)
*

10. Root growth: 3 mature stands
Fine roots foraged for P
(allocation response to
aboveground limitation)
Phosphorus hotspot
Control soil
(Shan et al. in review)
Fine root ingrowth length (cm/core)

11. Conclusions
Fine roots forage for P but
their growth is limited by N
Whole-tree growth may be
co-limited by N and P
Does N limit P acquisition?
Acknowledgements:
Funding from LTER, NSF,
USDA; support from HBEF,
BEF; participation of many
students
P limitation
N limitation